Non-Destructive Testing (NDT) Study Materiallibrary.abes.ac.in/E-Books/NDT Notes.pdf2015-04-29 · Segregation Segregation is ... assuring sufficient superheat in the poured metal

Non-Destructive Testing (NDT)

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Introduction to Non-Destructive Testing Techniques

Defectology Page 1 of 12

Defectology

The aim of non-destructive inspection is to determine if the object being inspected is to be accepted or rejected. During the inspection, the inspector looks for discontinuities in the object and idenefies their nature and size. Then, those discontiouities are evaluated according to an acceptance criterion to determine if they are considered to be defects (the presence of defects mans that the object will be rejected).

A Discontinuity is defined as an imperfection or interruption in the normal physical characteristics or structure of an object (crack, porosity, inhomogeneity, etc.). On the other hand, a Defect is defined as a flaw or flaws that by nature or accumulated effect render a part or product unable to meet minimum applicable acceptance standards or specifications (defect designates rejectability).

It should be clear that a discontinuity is not necessarily a defect. Any imperfection that is found by the inspector is called a discontinuity until it can be identified and evaluated as to the effect it will have on the service of the part or to the requirements of the specification. A certain discontinuity may be considered to be a defect in some cases and not a defect in some other cases because the defenetion of defect changes with the type of component, its construction, its materials and the specifications or codes being used.

Types of Discontinuities

Discontinuities are generally categorized according to the stage of the manufacturing or use in which they initiate.

Therefore, discontinuities are categorized in four groups which are:

Inherent discontinuities

Primary processing discontinuities

Secondary procession discontinuities

Service discontinuities

INHERENT DISCONTINUITIES

This group refers to the discontinuities that originate during the initial casting process (when the metal is casted into ingots for further processing) and also it includes the discontinuities that are produced when metal is casted as parts of any given shape. The initial casting discontinuities are usually removed by chopping the ingots but some of

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them remain and further change their shape and nature during the subsequent manufacturing operations.

Cold Shut

Cold shut occurs usually during the casting of parts because of imperfect fusion between two streams of molten metal that converged together. It could be on the surface or subsurface. It could be attributed to sluggish molten metal, surging or interruption in pouring, or any factor that prevents the fusion of two meeting streams.

Pipe

During solidification of molten material it shrinks causing an inverted-cone shaped cavity in the top of the ingot. It could be on the surface or subsurface. If this defected region is not cut out completely before further processing (rolling or forging) it will show up in the final product as an elongated subsurface discontinuity. Also, pipe could occur during extrusion when the oxidized surface of the billet flows inwards toward the center of the extruded bar.

Shrinkage Cavities

Shrinkage cavities are subsurface discontinuities that are found in casted parts. They are caused by the lack of enough molten metal to fill the space created by shrinkage (similar to pipe in an ingot).

Micro-shrinkage Cavities

Micro-shrinkage cavities are aggregates of subsurface discontinuities that are found in casted parts. They are usually found close to the gate and they occur if metal at the gate solidifies while some of the metal beneath is still molten. Also, micro-shrinkage could be found deeper in the part when molten metal enters from the light section into heavy section where metal could solidify in the light section before the heavy section.

Hot Tears

Hot tears occurs when low melting point materials segregate during solidification and thus when they try to shrink during solidification

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cracks and tears will develop because the surrounding material has already solidified.

Also, hot tears occur at the joining of thin sections with larger sections because of the difference of the cooling rate and thus solidification.

Blowholes and Porosity

Blowholes and porosity are small rounded cavities found at the surface or near surface of castings and they are caused by the entrapped gasses that could not escape during solidification. Blowholes are caused by gases released from the mold itself (external gases) while porosity is caused by gases entrapped in the molten material (internal gases). During subsequent manufacturing operations these gas pockets get flattened or elongated or fused shut.

Nonmetallic Inclusions

Nonmetallic (or slag) inclusions are usually oxides, sulfides or silicates that remained with the molten metal during original casting. The properties of those inclusions are different from the metal and usually they have irregular shapes and discontinuous nature therefore they serve as stress raisers that limit the ability of the material to withstand stresses.

Segregation

Segregation is localized differences in material composition (and thus mechanical properties) caused by the concentration of some alloying elements in limited areas. These compositional differences may be equalized during subsequent hot working processes but some still remain.

PRIMARY PROCESSING DISCONTINUITIES

This group refers to the discontinuities that originate during hot or cold forming processes (extrusion, forging, rolling, drawing, welding, etc.). Also, some of the inherent discontinuities in the material could propagate and become significant.

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Seams

Seams are elongated surface discontinuities that occur in bars during rolling or drawing operations. They result due to under-filled areas that are closed shut during rolling passes. Those under-filled areas may result because of blowholes or cracks in the material. Also seams may result from the use of faulty, poorly lubricated or oversized dies.

Lamination

Laminations are thin flat subsurface separations that are parallel to the surface of plates. They may result from inherent discontinuities (pipe, inclusions, porosity, etc.) that are flattened during the rolling process.

Stringers

Stringers are elongated subsurface discontinuities that are found in bars (they run in the axial direction). They result from the flattening and lengthening of nonmetallic inclusions during the rolling process.

Cupping

Cupping is a subsurface discontinuity that may occur in bars during extrusion or sever cold drawing. It is a series of cone-shaped internal ruptures that happen because the interior of the material cannot flow as fast as the surface where that causes stress buildup and thus rupture.

Cooling Cracks

Cooling cracks may occur on the surface of bars after rolling operations due to stresses developed by uneven cooling. They run in the axial direction (similar to seams) but unlike seams, they do not have surface oxidation.

Forging and Rolling Laps

Laps are elongated surface discontinuities that occur during rolling or forging operations due to the presence of some excessive material (fin) that is folded over. They may result because of oversized blanks or improper handling of the material in the die.

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Internal or External Bursts

Internal bursts are found in bars and forgings formed at excessive temperatures due to presence of inherent discontinuities that are pulled apart by the tensile forces developed during the forming operation.

External bursts occur when the forming section is too severe or the sections are thin.

Slugs

Slugs are surface discontinuities found on the inner surface of seamless (extruded) tubes. They occur when some metallic pieces that are stuck on the mandrel, are torn and fused back on the inner surface of the tube.

Gouging

Gouging is surface tearing found on the inner surface of seamless (extruded) tubes and it is caused by excessive friction between the mandrel and the inner surface of the tube.

Hydrogen Flakes

Hydrogen is available during manufacturing operations (from decomposition of water vapor or hydrocarbons “oil”, atmosphere, etc.) and it dissolves in material at temperatures above 200° C. Hydrogen flakes are thin subsurface discontinuities that develop during cooling of large size parts produced by forging or rolling because of the entrapment of hydrogen resulting from rapid cooling.

Welding Discontinuities

Several types of discontinuities result from welding operations. Only the discontinuities associated with fusion welding processes (arc welding, gas welding, etc.) are presented here.

Cold Cracks

Cold cracks, also known as delayed cracks, are hydrogen induced surface or subsurface cracks that appear in the heat affected zone or the weld

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metal during cooling or after a period of time (hours or even days). The sources of hydrogen which leads to this type of cracks may include moisture in the electrode shielding, the shielding gas or base metal surface, or contamination of the base metal with hydrocarbon (oil or grease).

Hot Cracks

Hot cracks include several types of cracks that occur at elevated temperatures in the weld metal or heat affected zone. In general, hot cracks are usually associated with steels having high sulfur content. The common types of hot cracks include:

Solidification Cracks: This type occurs near the solidification temperature of the weld metal. They are caused by the presence of low melting point constituents (such as iron sulfides) that segregate during solicitation then the shrinkage of the solidified material causes cracks to open up.

Centerline Crack is a longitudinal crack along the centerline of the weld bead. It occurs because the low melting point impurities move to the center of the wield pool as the solidification progresses from the weld toe to the center, then shrinkage stresses of the solidified material causes cracking along the centerline. The likelihood of centerline cracking increases when the travel speed is high or the depth-to-width ratio is high.

Crater Crack which occurs in the crater formed at the termination of the weld pass. Crater cracks are mostly star shaped and they are caused by three dimensional shrinkage stresses. The likelihood of crater cracks increases when welding is terminated suddenly.

Liquidation Cracks: This type, also known as hot tearing, occurs in the heat affected zone when the temperature in that region reaches to the melting temperature of low melting point constituents causing them to liquidate and segregate at grain boundaries. As the weld cools down, shrinkage stresses causes the formation of small micro-scale cracks which later might link up due to applied stresses to form a continuous surface or subsurface crack.

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Lamellar Tearing

Lamellar tearing is a subsurface discontinuity that occurs in rolled plates having high content of nonmetallic inclusions. Those inclusions have low strength and they are fattened during roiling, thus they can be torn underneath the welds because of shrinkage stresses in the through thickness direction.

Lack of Fusion

Lack of fusion is the failure of the filler metal to fuse with the adjacent base metal (or weld metal from previous pass) because the surface of base metal did not reach to melting temperature during welding. This typically occurs when welding large components that could dissipate heat rapidly especially when it is at a relatively low temperature before welding. Lack of fusion is often seen at the beginning of the first pass and in such case it is commonly called a cold start. Also, lack of fusion could occur when the surface a previous pass is not properly cleaned from slag where slag reduces the heating of the under-laying surface.

Lack of Penetration

Lack of penetration is insufficient (less than specified) penetration of the weld metal into the root of the joint. This is mostly caused by improper welding parameters such as; low amperage, oversized electrode or improper angle, high travel speed, or inadequate surface pre-cleaning. Also, lack of penetration could happen when the root face is too large, the root opening is too narrow, or the bevel angle is too small.

Porosity

Porosity is small cavities or bores, which mostly have spherical shape, that are found on the surface of the weld or slightly below surface. Porosity occurs when some constituents of the molten metal vaporize causing small gas pockets that get entrapped in the metal as it solidifies. These small bores could have a variety of shapes but mostly they have a spherical shape. The distribution of bores in weld metal could be linear (linear porosity) or they could be

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clustered together (cluster porosity). In general, porosity can result from the presence of dirt, rust or moisture on the surface of base or filler metal. Also, it could result from high sulfur content in the base metal or excessive arc length.

Inclusions

Inclusions refer to the presence of some material, that is not supposed to be present, in the weld metal.

Slag Inclusions: This type of inclusions mostly happens in shielded metal arc welding (SMAW) and it occurs when the slag cannot float to the surface of the molten metal and get entrapped in the weld metal during solidification. This could happen when; the solidification rate is high, the weld pool viscosity is high, an oversized electrode is used, or slag on the previous pass was not properly removed.

Tungsten Inclusions: This type of inclusions can be found in weld metal deposited by gas tungsten arc welding (GTAW) as a result of allowing the tungsten electrode to come in contact with the molten metal.

Oxide Inclusions: This type of inclusions results from the presence of high melting point oxides on the base metal which mixes with the molten material during welding.

Undercut

Undercut is a reduction in the base metal thickness at the weld toe. This is caused by an oversized molten weld pool which may result from excessive amperage or oversized electrode.

Overlap

Overlap is the protrusion of the weld metal over the weld toe (due to lack of fusion). This may be caused by insufficient amperage or travel speed.

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SECONDARY PROCESSING DISCONTINUITIES

This group refers to the discontinuities that originate during grinding, machining, heat treating, plating and related finishing operations.

Grinding Cracks

Grinding cracks develop at locations where there is a localized heating of the base metal and they are usually shallow and at right angle to the grinding direction. Such cracks might be caused by the use of glazed wheels, inadequate coolant, excessive feed or grinding depth.

Pickling Cracks

Pickling is chemical surface cleaning operation (using acids) used to remove unwanted scale. Picking cracks are hydrogen induced cracks caused by the diffusion of the hydrogen generated at the surface into the base metal. Such cracks mostly occur in materials having high residual stresses such as hardened or cold worked metals.

Heat Treatment (Quenching) Cracks

Heat treatment cracks mostly occur during quenching especially when harsh media is used for quenching (such as cold water, oil quenching is less harsh). During quenching the material at the surface cools immediately upon contacting the liquid while the material inside take relatively longer time. This difference in cooling rate causes residual stresses in the component and could also result in cracks at the surface if the residual tensile stress is higher than the strength of the material. In steels, austenite is transferred into ferrite and martensite upon cooling. This transformation results in volume increase and thus causes tensile stresses at the surface layer since the material at the surface transformed and solidified before material at the core.

Machining Tears

Machining tears result from the use of machining tools having dull or chipped cutting edges. Such discontinuities serve as stress raisers and can lead to premature failure of a component especially when it is subjected to fatigue loading.

Plating Cracks

Plating cracks are surface discontinuities that can develop due to the penetration of hydrogen or hot plating material into the base metal. Also, some plating materials (such as chromium, copper and nickel) produce residual tensile stress which can reduce the fatigue strength of a component.

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SERVICE DISCONTINUITIES

This group refers to the discontinuities that originate or develop while the component is in service. The service conditions (loading, mechanical and chemical environment, maintenance) of a component affect its expected life. Although most of service discontinuities might look somehow similar but they are caused by different failure mechanisms.

Fatigue Cracks

When a component is subjected to fatigue stress (cyclically applied stress), fatigue cracks can develop and grow and that will eventually lead to failure (even if the magnitude of the stress is smaller than the ultimate strength of the material). Fatigue cracks normally originate at the surface but in some cases can also initiate below surface. Fatigue cracks initiate at location with high stresses such as discontinuities (hole, notch, scratch, sharp corner, porosity, crack, inclusions, etc.) and can also initiate at surfaces having rough surface finish or due to the presence of tensile residual stresses.

According to Linear-Elastic Fracture Mechanics (LEFM), fatigue failure develops in three stages:

- Stage 1: development of one or more micro cracks due to the cyclic local plastic deformation at a location having high stress concentration.

- Stage 2: the cracks progress from micro cracks to larger cracks (macro cracks) and keep growing making a smooth plateau-like fracture surfaces which usually have beach marks that result from variation in cyclic loading. The geometry and orientation of the beach marks can help in determining the location where the crack originated and the progress of crack growth. The direction of the crack during this stage is perpendicular to the direction of the maximum principal stress.

- Stage 3: occurs during the final stress cycle where the remaining material cannot support the load, thus resulting in a sudden fracture.

The presence of the crack can (and should) be detected during the crack growth stage (stage 2) before the component suddenly fails.

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Creep Cracks

When a metal is at a temperature greater than 0.4 to 0.5 of its absolute melting temperature and is subjected to a high enough value of stress (lower than the yield strength at room temperature but it is actually higher than the yield strength at the elevated temperature), it will keep deforming continuously until it finally fractures. Such type of deformation is called creep and it is caused by the continuous initiation and healing of slipping dislocation inside the grains of the material.

According to the rate of progress of the deformation, three stages of creep deformation can be distinguished:

- Initial stage (or primary creep): the strain rate is relatively high but slows with increasing time due to work hardening.

- Second stage (or steady-state creep): the strain rate reaches a minimum and becomes steady due to the balance between work hardening and annealing (thermal softening). The characterized "creep strain rate" typically refers to the rate in this secondary stage.

- Third stage (or tertiary creep): the strain rate exponentially increases with stress because of necking phenomena and finally the component ruptures.

Creep cracks usually develop at the end of the second stage (the beginning of third stage) and they eventually lead to failure. However, when a component reaches to the third stage, its useful life is over and thus creep should be detected (by monitoring the deformation) during the second stage which takes the longest time period of the three stages. For steels, adding some alloying elements such as molybdenum and tungsten can enhance creep resistance. Also, heat treatments that produce coarse grains (such as annealing) can also increase life under creep conditions.

Stress Corrosion Cracks

Stress corrosion cracks are small sharp and usually branched cracks that result from the combined effect of a “static” tensile stress and a corrosive environment. The stress can either be resulting from an applied load or a residual stress. Stress corrosion cracks

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can lead to a sudden failure of ductile materials without any previous plastic deformation. The cracks usually initiate at the surface due to the presence of preexisting discontinuity or due to corrosive attack on the surface. Once the cracks initiate at the surface, corrosive material enters the cracks and attacks the material inside forming corrosion products. The formation of the corrosion products (which have a larger volume than original metal) inside the tight cracks causes a wedging action which increase the stress at the crack tip and causes the crack to grow. The corrosive environment varies from material to material; for example saltwater is corrosive to aluminum and stainless steel, ammonia is corrosive to copper alloys, and sodium hydroxide is corrosive to mild steel. The resistance to corrosion can be improved by plating the surface of a component by appropriate material which does not react with the environment.

Hydrogen Cracks

Hydrogen cracking, also known as hydrogen embrittlement, results from the presence of hydrogen medium and usually occurs in conjunction with the presence of applied tensile stress or residual stress. Hydrogen can be already present in the metal due to previous processes such as electroplating, pickling, welding in moist atmosphere or the melting process itself. Also, hydrogen can come from the presence of hydrogen sulfides, water, methane or ammonia in the work environment of a component. Hydrogen can diffuse in the metal and initiate very small cracks at subsurface cites (usually at the grain boundaries) subjected to high values of stress. The presence of such cracks at several locations causes ductile materials to show brittle fracture behavior.

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Stringers defect:

http://www.nasaspaceflight.com/2010/11/sts-133-structural-defectcrack-found-on-et-

137/

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http://www.nasaspaceflight.com/2010/11/sts-133-structural-defectcrack-found-on-et-137/


Welding

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http://nptel.ac.in/courses/112107144/welding/lecture13.htm

Welding Defects

The defects in the weld can be defined as irregularities in the weld metal produced due to incorrect welding parameters or wrong welding procedures or wrong combination of filler metal and parent metal.

Weld defect may be in the form of variations from the intended weld bead shape, size and desired quality. Defects may be on the surface or inside the weld metal. Certain defects such as cracks are never tolerated but other defects may be acceptable within permissible limits. Welding defects may result into the failure of components under service condition, leading to serious accidents and causing the loss of property and sometimes also life.

Various welding defects can be classified into groups such as cracks, porosity, solid inclusions, lack of fusion and inadequate penetration, imperfect shape and miscellaneous defects.

1. Cracks

Cracks may be of micro or macro size and may appear in the weld metal or base metal or base metal and weld metal boundary. Different categories of cracks are longitudinal cracks, transverse cracks or radiating/star cracks and cracks in the weld crater. Cracks occur when localized stresses exceed the ultimate tensile strength of material. These stresses are developed due to shrinkage during solidification of weld metal.

Fig 13.1: Various Types of Cracks in Welds

Cracks may be developed due to poor ductility of base metal, high sulpher and carbon contents, high arc travel speeds i.e. fast cooling rates, too concave or convex weld bead and high hydrogen contents in the weld metal.

2. Porosity

Porosity results when the gases are entrapped in the solidifying weld metal. These gases are generated from the flux or coating constituents of the electrode or shielding gases used during welding or from absorbed moisture in the coating. Rust, dust, oil and grease present on the surface of work pieces or on electrodes are also source of gases during welding. Porosity may be easily prevented if work pieces are properly cleaned from rust, dust, oil and grease.Futher, porosity can also be controlled if excessively high welding currents, faster welding speeds and long arc lengths are avoided flux and coated electrodes are properly baked.

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Fig 13.2: Different Forms of Porosities

3. Solid Inclusion

Solid inclusions may be in the form of slag or any other nonmetallic material entrapped in the weld metal as these may not able to float on the surface of the solidifying weld metal. During arc welding flux either in the form of granules or coating after melting, reacts with the molten weld metal removing oxides and other impurities in the form of slag and it floats on the surface of weld metal due to its low density. However, if the molten weld metal has high viscosity or too low temperature or cools rapidly then the slag may not be released from the weld pool and may cause inclusion.

Slag inclusion can be prevented if proper groove is selected, all the slag from the previously deposited bead is removed, too high or too low welding currents and long arcs are avoided.

Fig 13.3: Slag Inclusion in Weldments

4. Lack of Fusion and Inadequate or incomplete penetration:

Lack of fusion is the failure to fuse together either the base metal and weld metal or subsequent beads in multipass welding because of failure to raise the temperature of base metal or previously deposited weld layer to melting point during welding. Lack of fusion can be avoided by properly cleaning of surfaces to be welded, selecting proper current, proper welding technique and correct size of electrode.

Fig 13.4: Types of Lack of Fusion

Incomplete penetration means that the weld depth is not upto the desired level or root faces have not reached to melting point in a groove joint. If either low currents or larger arc lengths or large root face or small root gap or too narrow groove angles are used then it results into poor penetration.

Fig 13.5: Examples of Inadequate Penetration

5. Imperfect Shape

Imperfect shape means the variation from the desired shape and size of the weld bead. Library

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During undercutting a notch is formed either on one side of the weld bead or both sides in which stresses tend to concentrate and it can result in the early failure of the joint. Main reasons for undercutting are the excessive welding currents, long arc lengths and fast travel speeds.

Underfilling may be due to low currents, fast travel speeds and small size of electrodes. Overlap may occur due to low currents, longer arc lengths and slower welding speeds.

Fig 13.6: Various Imperfect Shapes of Welds

Excessive reinforcement is formed if high currents, low voltages, slow travel speeds and large size electrodes are used. Excessive root penetration and sag occur if excessive high currents and slow travel speeds are used for relatively thinner members.

Distortion is caused because of shrinkage occurring due to large heat input during welding.

6. Miscellaneous Defects

Various miscellaneous defects may be multiple arc strikes i.e. several arc strikes are one behind the other, spatter, grinding and chipping marks, tack weld defects, oxidized surface in the region of weld, unremoved slag and misalignment of weld beads if welded from both sides in butt welds.

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Casting Defects http://nptel.ac.in/courses/112107144/26

The following are the major defects, which are likely to occur in sand castings

Gas defects Shrinkage cavities Molding material defects Pouring metal defects Mold shift

Gas Defects

A condition existing in a casting caused by the trapping of gas in the molten metal or by mold gases evolved during the pouring of the casting. The defects in this category can be classified into blowholes and pinhole porosity. Blowholes are spherical or elongated cavities present in the casting on the surface or inside the casting. Pinhole porosity occurs due to the dissolution of hydrogen gas, which gets entrapped during heating of molten metal.

Causes

The lower gas-passing tendency of the mold, which may be due to lower venting, lower permeability of the mold or improper design of the casting. The lower permeability is caused by finer grain size of the sand, high percentage of clay in mold mixture, and excessive moisture present in the mold.

Metal contains gas Mold is too hot Poor mold burnout

Shrinkage Cavities

These are caused by liquid shrinkage occurring during the solidification of the casting. To compensate for this, proper feeding of liquid metal is required. For this reason risers are placed at the appropriate places in the mold. Sprues may be too thin, too long or not attached in the proper location, causing shrinkage cavities. It is recommended to use thick sprues to avoid shrinkage cavities.

Molding Material Defects

The defects in this category are cuts and washes, metal penetration, fusion, and swell.

Cut and washes

These appear as rough spots and areas of excess metal, and are caused by erosion of molding sand by the flowing metal. This is caused by the molding sand not having enough strength and the molten metal flowing at high velocity. The former can be taken care of by the proper choice of molding sand and the latter can be overcome by the proper design of the gating system.

Metal penetration

When molten metal enters into the gaps between sand grains, the result is a rough casting surface. This occurs because the sand is coarse or no mold wash was applied on the surface of the mold. The coarser the sand grains more the metal penetration.

Fusion

This is caused by the fusion of the sand grains with the molten metal, giving a brittle, glassy appearance on the casting surface. The main reason for this is that the clay or the sand particles are of lower refractoriness or that the pouring temperature is too high.

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Under the influence of metallostatic forces, the mold wall may move back causing a swell in the dimension of the casting. A proper ramming of the mold will correct this defect.

Inclusions

Particles of slag, refractory materials, sand or deoxidation products are trapped in the casting during pouring solidification. The provision of choke in the gating system and the pouring basin at the top of the mold can prevent this defect.

Pouring Metal Defects

The likely defects in this category are

Mis-runs and Cold shuts.

A mis-run is caused when the metal is unable to fill the mold cavity completely and thus leaves unfilled cavities. A mis-run results when the metal is too cold to flow to the extremities of the mold cavity before freezing. Long, thin sections are subject to this defect and should be avoided in casting design.

A cold shut is caused when two streams while meeting in the mold cavity, do not fuse together properly thus forming a discontinuity in the casting. When the molten metal is poured into the mold cavity through more-than-one gate, multiple liquid fronts will have to flow together and become one solid. If the flowing metal fronts are too cool, they may not flow together, but will leave a seam in the part. Such a seam is called a cold shut, and can be prevented by assuring sufficient superheat in the poured metal and thick enough walls in the casting design.

The mis-run and cold shut defects are caused either by a lower fluidity of the mold or when the section thickness of the casting is

very small. Fluidity can be improved by changing the composition of the metal and by increasing the pouring temperature of the metal.

Mold Shift

The mold shift defect occurs when cope and drag or molding boxes have not been properly aligned.

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http://nptel.ac.in/courses/Webcourse-contents/IIT-ROORKEE/strength%20of%20materials/lects%20&%20picts/image/lect11/lecture11.htm

MECHANICAL PROPERTIES

LECTURE 1

Mechanical Properties:

In the course of operation or use, all the articles and structures are subjected to the action of external forces, which create stresses that inevitably cause deformation. To keep these stresses, and, consequently deformation within permissible limits it is necessary to select suitable materials for the Components of various designs and to apply the most effective heat treatment. i.e. a Comprehensive knowledge of the chief character tics of the semi-finished metal products & finished metal articles (such as strength, ductility, toughness etc) are essential for the purpose.

For this reason the specification of metals, used in the manufacture of various products and structure, are based on the results of mechanical tests or we say that the mechanical tests conducted on the specially prepared specimens (test pieces) of standard form and size on special machines to obtained the strength, ductility and toughness characteristics of the metal.

The conditions under which the mechanical test are conducted are of three types

(1) Static: When the load is increased slowly and gradually and the metal is loaded by tension, compression, torsion or bending.

(2) Dynamic: when the load increases rapidly as in impact

(3) Repeated or Fatigue: (both static and impact type) . i.e. when the load repeatedly varies in the course of test either in value or both in value and direction Now let us consider the uniaxial tension test.

[ For application where a force comes on and off the structure a number of times, the material cannot withstand the ultimate stress of a static tool. In such cases the ultimate strength depends on no. of times the force is applied as the material works at a particular stress level. Experiments one conducted to compute the number of cycles requires to break to specimen at a particular stress when fatigue or fluctuating load is acting. Such tests are known as fatigue tests ]

Uniaxial Tension Test: This test is of static type i.e. the load is increased comparatively slowly from zero to a certain value.

Standard specimen's are used for the tension test.

There are two types of standard specimen's which are generally used for this purpose, which have been shown below:

Specimen I:

This specimen utilizes a circular X-section.

Specimen II:

This specimen utilizes a rectangular X-section.

lg = gauge length i.e. length of the specimen on which we want to determine the mechanical properties.The uniaxial tension test is carried out on tensile testing machine and the following steps are performed to conduct this test.

(i) The ends of the specimen's are secured in the grips of the testing machine.

(ii) There is a unit for applying a load to the specimen with a hydraulic or mechanical drive.

(iii) There must be a some recording device by which you should be able to measure the final output in the form of Load or stress. So the testing machines are often equipped with Library

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the pendulum type lever, pressure gauge and hydraulic capsule and the stress Vs strain diagram is plotted which has the following shape.

A typical tensile test curve for the mild steel has been shown below

Nominal stress – Strain OR Conventional Stress – Strain diagrams:

Stresses are usually computed on the basis of the original area of the specimen; such stresses are often referred to as conventional or nominal stresses.

True stress – Strain Diagram:

Since when a material is subjected to a uniaxial load, some contraction or expansion always takes place. Thus, dividing the applied force by the corresponding actual area of the specimen at the same instant gives the so called true stress.

SALIENT POINTS OF THE GRAPH:

(A) So it is evident form the graph that the strain is proportional to strain or elongation is proportional to the load giving a st.line relationship. This law of proportionality is valid upto a point A.

or we can say that point A is some ultimate point when the linear nature of the graph ceases or there is a deviation from the linear nature. This point is known as the limit of proportionality or the proportionality limit.

(B) For a short period beyond the point A, the material may still be elastic in the sense that the deformations are completely recovered when the load is removed. The limiting point B is termed as Elastic Limit .

(C) and (D) - Beyond the elastic limit plastic deformation occurs and strains are not totally recoverable. There will be thus permanent deformation or permanent set when load is removed. These two points are termed as upper and lower yield points respectively. The stress at the yield point is called the yield strength.

A study a stress – strain diagrams shows that the yield point is so near the proportional limit that for most purpose the two may be taken as one. However, it is much easier to locate the former. For material which do not posses a well define yield points, In order to find the yield point or yield strength, an offset method is applied.

In this method a line is drawn parallel to the straight line portion of initial stress diagram by off setting this by an amount equal to 0.2% of the strain as shown as below and this happens especially for the low carbon steel.

(E) A further increase in the load will cause marked deformation in the whole volume of the metal. The maximum load which the specimen can with stand without failure is called the load at the ultimate strength.

The highest point ‘E' of the diagram corresponds to the ultimate strength of a material.

u = Stress which the specimen can with stand without failure & is known as Ultimate Strength or Tensile Strength.

u is equal to load at E divided by the original cross-sectional area of the bar. Library Study Mate


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(F) Beyond point E, the bar begins to forms neck. The load falling from the maximum until fracture occurs at F.

[ Beyond point E, the cross-sectional area of the specimen begins to reduce rapidly over a relatively small length of bar and the bar is said to form a neck. This necking takes place whilst the load reduces, and fracture of the bar finally occurs at point F ]

Note: Owing to large reduction in area produced by the necking process the actual stress at fracture is often greater than the above value. Since the designers are interested in maximum loads which can be carried by the complete cross section, hence the stress at fracture is seldom of any practical value.

Percentage Elongation: ' ':

The ductility of a material in tension can be characterized by its elongation and by the reduction in area at the cross section where fracture occurs.

It is the ratio of the extension in length of the specimen after fracture to its initial gauge length, expressed in percent.

lI = gauge length of specimen after fracture(or the distance between the gage marks at fracture)

lg= gauge length before fracture(i.e. initial gauge length)

For 50 mm gage length, steel may here a % elongation of the order of 10% to 40%.

Elastic Action:

The elastic is an adjective meaning capable of recovering size and shape after deformation. Elastic range is the range of stress below the elastic limit.

Many engineering materials behave as indicated in Fig(a) however, some behaves as shown in figures in (b) and (c) while in elastic range. When a material behaves as in (c),

the vs is not single valued since the strain corresponding to any particular ‘ ' will depend upon loading history.

Fig (d): It illustrates the idea of elastic and plastic strain. If a material is stressed to level (1) and then relased the strain will return to zero beyond this plastic deformation remains.

If a material is stressed to level (2) and then released, the material will recover the

amount (2 2p ), where 2p is the plastic strain remaining after the load is removed.

Similarly for level (3) the plastic strain will be3p.

Ductile and Brittle Materials:

Based on this behaviour, the materials may be classified as ductile or brittle materials

Ductile Materials:

It we just examine the earlier tension curve one can notice that the extension of the materials over the plastic range is considerably in excess of that associated with elastic loading. The Capacity of materials to allow these large deformations or large extensions without failure is termed as ductility. The materials with high ductility are termed as ductile materials. Library

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Brittle Materials:

A brittle material is one which exhibits a relatively small extensions or deformations to fracture, so that the partially plastic region of the tensile test graph is much reduced.

This type of graph is shown by the cast iron or steels with high carbon contents or concrete.

Conditions Affecting Mechanical Properties:

The Mechanical properties depend on the test conditions

(1) It has been established that lowering the temperature or increasing the rate of deformation considerably increases the resistance to plastic deformation. Thus, at low temperature (or higher rates of deformation), metals and alloys, which are ductile at normal room temperature may fail with brittle fracture.

(2) Notches i.e. sharp charges in cross sections have a great effect on the mechanical properties of the metals. A Notch will cause a non – uniform distribution of stresses. They will always contribute lowering the ductility of the materials. A notch reduces the ultimate strength of the high strength materials. Because of the non – uniform distribution of the stress or due to stress concentration.

(3) Grain Size: The grain size also affects the mechanical properties.

Hardness:

Hardness is the resistance of a metal to the penetration of another harder body which does not receive a permanent set.

Hardness Tests consists in measuring the resistance to plastic deformation of layers of metals near the surface of the specimen i.e. there are Ball indentation Tests.

Ball indentation Tests:

iThis method consists in pressing a hardened steel ball under a constant load P into a specially prepared flat surface on the test specimen as indicated in the figures below :

After removing the load an indentation remains on the surface of the test specimen. If area of the spherical surface in the indentation is denoted as F sq. mm. Brinell Hardness number is defined as :

Bhn = P / F

F is expressed in terms of D and d

D = ball diameter

d = diametric of indentation and Brinell Hardness number is given

by

Then is there is also Vicker's Hardness Number in which the ball is of conical shape.

Goto Home

LECTURE 2

Compression Test: Machines used for compression testing are basically similar to those used for tensile testing often the same machine can be used to perform both tests. Library

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http://nptel.ac.in/courses/Webcourse-contents/IIT-ROORKEE/strength%20of%20materials/main.htm


Shape of the specimen: The shapes of the specimen to be used for the different materials

are as follows:

(i) For metals and certain plastics: The specimen may be in the form of a cylinder

(ii) For building materials: Such as concrete or stone the shape of the specimen may be in the form of a cube.

Shape of stress stain diagram

(a) Ductile materials: For ductile material such as mild steel, the load Vs compression diagram would be as follows

(1) The ductile materials such as steel, Aluminum, and copper have stress – strain diagrams similar to ones which we have for tensile test, there would be an elastic range which is then followed by a plastic region.

(2) The ductile materials (steel, Aluminum, copper) proportional limits in compression test are very much close to those in tension.

(3) In tension test, a specimen is being stretched, necking may occur, and ultimately fracture fakes place. On the other hand when a small specimen of the ductile material is compressed, it begins to bulge on sides and becomes barrel shaped as shown in the figure

above. With increasing load, the specimen is flattened out, thus offering increased resistance to further shortening ( which means that the stress – strains curve goes upward ) this effect is indicated in the diagram.

Brittle materials ( in compression test )

Brittle materials in compression typically have an initial linear region followed by a region in which the shortening increases at a higher rate than does the load. Thus, the compression stress – strain diagram has a shape that is similar to the shape of the tensile diagram.

However, brittle materials usually reach much higher ultimate stresses in compression than in tension.

For cast iron, the shape may be like this

Brittle materials in compression behave elastically up to certain load, and then fail suddenly by splitting or by craking in the way as shown in figure. The brittle fracture is performed by separation and is not accompanied by noticeable plastic deformation.

Hardness Testing:

The term ‘hardness' is one having a variety of meanings; a hard material is thought of as one whose surface resists indentation or scratching, and which has the ability to indent or cut other materials.

Hardness test: The hardness test is a comparative test and has been evolved mainly from the need to have some convenient method of measuring the resistance of materials to scratching, wear or in dentation this is also used to give a guide to overall strength of a Library

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materials, after as an inspection procedure, and has the advantage of being a non – destructive test, in that only small indentations are lift permanently on the surface of the specimen.

Four hardness tests are customarily used in industry namely

(i) Brinell

(ii) Vickers

(iii) Rockwell

(vi) Shore Scleroscopy

The most widely used are the first two.

In the Brinell test the indenter is a hardened steel ball which is pressed into the surface using a known standard load. The diameter of resulting indentation is than measured using a microscope & scale.

Units:

The units of Brinell Hardness number in S.I Unit would have been N/mm2 or Mpa

To avoid the confusion which would have been caused of her wise Hardness numbers are quotes as kgf / mm

2

Brinell Hardness test:

In the Brinell hardness test, a hardened steel ball is pressed into the flat surface of a test piece using a specified force. The ball is then removed and the diameter of the resulting indentation is measured using a microscope.

The Brinell Hardness no. ( BHN ) is defined as

BHN = P / A

Where P = Force applied to the ball.

A = curved area of the indentation

It may be shown that

D = diameter of the ball,

d = the diameter of the indentation.

In the Brinell Test, the ball diameter and applied load are constant and are selected to suit the composition of the metal, its hardness, and selected to suit the composition of the metal, its hardness, the thickness etc. Further, the hardness of the ball should be at least 1.7 times than the test specimen to prevent permanent set in the ball.

Disadvantage of Brinell Hardness Test: The main disadvantage of the Brinell Hardness test is that the Brinell hardness number is not independent of the applied load. This can be realized from. Considering the geometry of indentations for increasing loads. As the ball is pressed into the surface under increasing load the geometry of the indentation charges.

Here what we mean is that the geometry of the impression should not change w.r.t. load, however the size it impression may change.

Vickers Hardness test:

The Vicker's Hardness test follows a procedure exactly a identical with that of Brinell test, but uses a different indenter. The steel ball is replaced by a diamond, having the from of a square – based pyramid with an angle of 136

0 between opposite faces. This is pressed

into the flat surface of the test piece using a specified force, and the diagonals of the resulting indentation measured is using a microscope. The Hardness, expressed as a Vicker's pyramid number is defined as the ratio F/A, where F is the force applied to the diamond and A is the surface area of the indentation.

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It may be shown that

In the Vicker Test the indenters of pyramidal or conical shape are used & this overcomes the disadvantage which is faced in Brinell test i.e. as the load increases, the geometry of the indentation's does not change

The Variation of Hardness number with load is given below.

Advantage: Apart from the convenience the vicker's test has certain advantages over the Brinell test.

(i) Harder material can be tested and indentation can be smaller & therefore less obtrusive or damaging.

Upto a 300 kgf /mm2 both tests give the same hardness number but above too the Brinell

test is unreliable.

Rockwell Hardness Test :

The Rockwell Hardness test also uses an indenter when is pressed into the flat surface of the test piece, but differs from the Brinell and Vicker's test in that the measurement of hardness is based on the depth of penetration, not on the surface area of indentation. The indenter may be a conical diamond of 120

0 included angle, with a rounded

apex. It is brought into contact with the test piece, and a force F is applied. Library Study Mate


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Advantages :

Rockwell tests are widely applied in industry due to rapidity and simplicity with which they may be performed, high accuracy, and due to the small size of the impressions produced on the surface.

IMPACT STRENGTH

Static tension tests of the un-notched specimen's do not always reveal the susceptibility of metal to brittle fracture. This important factor is determined in impact tests. In impact tests we use the notched specimen's.

This specimen is placed on its supports on anvil so that blow of the striker is opposite to the notch the impact strength is defined as the energy A, required to rupture the specimen,

Impact Strength = A / f

Where f = the cross – section area of the specimen in cm2 at fracture & obviously at notch.

The impact strength is a complex characteristic which takes into account both toughness and strength of a material. The main purpose of notched – bar tests is to study the simultaneous effect of stress concentration and high velocity load application

Impact test are of the severest type and facilitate brittle friction. Impact strength values cannot be as yet is used for design calculations but these tests as rule provided for in specifications for carbon & alloy steels. Further, it may be noted that in impact tests fracture may be either brittle or ductile. In the case of brittle fracture, fracture occurs by separation and is not accompanied by noticeable plastic deformation as occurs in the case of ductile fracture.

Impact testing:

In an ‘impact test' a notched bar of material, arranged either as a cantilever or as a simply supported beam, is broken by a single blow in such a way that the total energy required to fracture it may be determined.

The energy required to fracture a material is of importance in cases of “shock loading' when a component or structure may be required to absorb the K.E of a moving object.

Often a structure must be capable of receiving an accidental ‘shock load' without failing completely, and whether it can do this will be determined not by its strength but by its ability to absorb energy. A combination of strength and ductility will be required, since large amounts of energy can only be absorbed by large amounts of plastic deformation. The ability of a material to absorb a large amount of energy before breaking is often referred as toughness, and the energy absorbed in an impact test is an obvious indication of this property.

Impact tests are carried out on notched specimens, and the notches must not be regarded simply as a local reduction in the cross – sectional area of the specimen, Notches – and , in fact, surface irregularities of many kind – give rise to high local stresses, and are in practice, a potential source of cracks.

The specimen may be of circular or square cross – section arranged either as a cantilever or a simply supported beam. Library

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Toughness: It is defined as the ability of the material to withstand crack i.e to prevent the transfer or propagation of cracks across its section hence causing failures. Cracks are propagated due to stress concentraction.

Creep: Creep is the gradual increase of plastic strain in a material with time at constant load. Particularly at elevated temperatures some materials are susceptible to this phenomena and even under the constant load, mentioned strains can increase continually until fractures. This form of facture is particularly relevant to the turbines blades, nuclear rectors, furnaces rocket motors etc.

The general from of strain versus time graph or creep curve is shown below.

The general form of Vs t graph or creep curve is shown below for two typical operation conditions, In each case the curve can be considered to exhibit four principal features

(a) An initial strain, due to the initial application of load. In most cases this would be an elastic strain. (b) A primary creep region, during which he creep rate ( slope of the graph ) dimensions. (c) A secondary creep region, when the creep rate is sensibly constant. (d) A tertiary creep region, during which the creep rate accelerate to final fracture. It is obvious that a material which is susceptible to creep effects should only be subjected to stresses which keep it in secondary (st.line) region throughout its service life. This enables the amount of creep extension to be estimated and allowed for in design.

Practice Problems: PROB 1: A standard mild steel tensile test specimen has a diameter of 16 mm and a gauge length of 80 mm such a specimen was tested to destruction, and the following results obtained. Load at yield point = 87 kN

Extension at yield point = 173 x 166 m

Ultimate load = 124 kN Total extension at fracture = 24 mm Diameter of specimen at fracture = 9.8 mm Cross - sectional area at fracture = 75.4 mm

2

Cross - sectional Area ‘A' = 200 mm2

Compute the followings: (i) Modulus of elasticity of steel (ii) The ultimate tensile stream (iii) The yield stress (iv) The percentage elongation (v) The Percentage reduction in Area. PROB 2: A light alloy specimen has a diameter of 16mm and a gauge Length of 80 mm. When tested in tension, the load extension graph proved linear up to a load of 6kN, at which point the extension was 0.034 mm. Determine the limits of proportionality stress and the modulus of elasticity of material. Note: For a 16mm diameter specimen, the Cross – sectional area A = 200 mm

2

This is according to tables Determine the limit of proportion try stream & the modulus of elasticity for the material.

Ans: 30 MN /m2 , 70.5 GN /m

2

solution:

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Visual Inspection

(NDT)

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Visual Inspection (VT)

Visual is the most common inspection method

Basic principle: – illuminate the test specimen with light –

examine the specimen with the eye.

VT reveals spatter, excessive buildup, incomplete slag

removal, cracks, heat distortion, undercutting, & poor

penetration

Simple, easy to apply, quickly carried out and usually low

in cost.

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Visual Inspection Equipment

Magnifying Glass

The eye can not focus sharply on objects closer than

approximately 250 mm.

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Magnifying Mirror

Fillet gauges / Weld gauge

Fillet gauges measure

The “Legs“ of the weld

Convexity

(weld rounded outward)

Concavity

(weld rounded inward)

Flatness

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VT:

Uses of Weld Gauge

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Microscope

Bore-scope – endoscopes or endoprobes

Endoscope:

https://www.youtube.com/watch?v=9pv5Eg1PwLE

End probe:

https://www.youtube.com/watch?v=4OVWq6wG3Ic

Flexible Fiber Optic Borescope – working lengths are

normally 60 to 365 cm with diameters from 3 to 12.5 mm

Video Image-scope

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Borescope

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Endoscope

An endoscope can consist of:

a rigid or flexible tube.

a light delivery system to illuminate the object under

inspection. The light source is normally outside the object

and the light is typically directed via an optical

fiber system.

a lens system transmitting the image from the objective

lens to the viewer.

an eyepiece. Modern instruments may be videoscopes,

with no eyepiece, a camera transmits image to a screen

for image capture.

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8-1

components with stated hourly operating limitations is normally accomplished during the calendar inspection falling nearest the hourly limitation.

In some instances, a flight hour limitation is established to limit the number of hours that may be flown during the calendar interval.

Aircraft operating under the flight hour system are inspected when a specified number of flight hours are accumulated. Components with stated hourly operating limitations are normally replaced during the inspection that falls nearest the hourly limitation.

Basic Inspection Techniques/PracticesBefore starting an inspection, be certain all plates, access doors, fairings, and cowling have been opened or removed and the structure cleaned. When opening inspection plates and cowling and before cleaning the area, take note of any oil or other evidence of fluid leakage.

PreparationIn order to conduct a thorough inspection, a great deal of paperwork and/or reference information must be accessed and studied before actually proceeding to the aircraft to conduct the inspection. The aircraft logbooks must be reviewed to provide background information and a maintenance history of the particular aircraft. The appropriate checklist or checklists must be utilized to ensure that no items will be forgotten or overlooked during the inspection. Also, many addi-tional publications must be available, either in hard copy or in electronic format to assist in the inspections. These additional publications may include information provided by the aircraft and engine manufacturers, appliance manufacturers, parts venders, and the Federal Aviation Administration (FAA).

Inspections are visual examinations and manual checks to determine the condition of an aircraft or component. An aircraft inspection can range from a casual walk-around to a detailed inspection involving complete disassembly and the use of complex inspection aids.

An inspection system consists of several processes, including reports made by mechanics or the pilot or crew flying an aircraft and regularly scheduled inspec-tions of an aircraft. An inspection system is designed to maintain an aircraft in the best possible condition. Thorough and repeated inspections must be considered the backbone of a good maintenance program. Irregu-lar and haphazard inspection will invariably result in gradual and certain deterioration of an aircraft. The time spent in repairing an abused aircraft often totals far more than any time saved in hurrying through routine inspections and maintenance.

It has been proven that regularly scheduled inspections and preventive maintenance assure airworthiness. Operating failures and malfunctions of equipment are appreciably reduced if excessive wear or minor defects are detected and corrected early. The importance of inspections and the proper use of records concerning these inspections cannot be overemphasized.

Airframe and engine inspections may range from preflight inspections to detailed inspections. The time intervals for the inspection periods vary with the models of aircraft involved and the types of operations being conducted. The airframe and engine manufacturer’s instructions should be consulted when establishing inspection intervals.

Aircraft may be inspected using flight hours as a basis for scheduling, or on a calendar inspection system. Under the calendar inspection system, the appropriate inspection is performed on the expiration of a speci-fied number of calendar weeks. The calendar inspec-tion system is an efficient system from a maintenance management standpoint. Scheduled replacement of Li

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Aircraft Logs“Aircraft logs,” as used in this handbook, is an inclu-sive term which applies to the aircraft logbook and all supplemental records concerned with the aircraft. They may come in a variety of formats. For a small aircraft, the log may indeed be a small 5" × 8" logbook. For larger aircraft, the logbooks are often larger, in the form of a three-ring binder. Aircraft that have been in service for a long time are likely to have several logbooks.

The aircraft logbook is the record in which all data con-cerning the aircraft is recorded. Information gathered in this log is used to determine the aircraft condition, date of inspections, time on airframe, engines and propellers. It reflects a history of all significant events occurring to the aircraft, its components, and acces-sories, and provides a place for indicating compliance with FAA airworthiness directives or manufactur-ers’ service bulletins. The more comprehensive the logbook, the easier it is to understand the aircraft’s maintenance history.

When the inspections are completed, appropriate entries must be made in the aircraft logbook certifying that the aircraft is in an airworthy condition and may be returned to service. When making logbook entries, exercise special care to ensure that the entry can be clearly understood by anyone having a need to read it in the future. Also, if making a hand-written entry, use good penmanship and write legibly. To some degree, the organization, comprehensiveness, and appearance of the aircraft logbooks have an impact on the value of the aircraft. High quality logbooks can mean a higher value for the aircraft.

ChecklistsAlways use a checklist when performing an inspection. The checklist may be of your own design, one provided by the manufacturer of the equipment being inspected, or one obtained from some other source. The checklist should include the following:

1. Fuselage and hull group. a. Fabric and skin—for deterioration,

distortion, other evidence of failure, and defective or insecure attachment of fittings.

b. Systems and components—for proper installation, apparent defects, and satisfactory operation.

c. Envelope gas bags, ballast tanks, and related parts—for condition.

2. Cabin and cockpit group. a. Generally—for cleanliness and loose

equipment that should be secured. b. Seats and safety belts—for condition and

security. c. Windows and windshields—for deterioration

and breakage. d. Instruments—for condition, mounting,

marking, and (where practicable) for proper operation.

e. Flight and engine controls—for proper installation and operation.

f. Batteries—for proper installation and charge. g. All systems—for proper installation, general

condition, apparent defects, and security of attachment.

3. Engine and nacelle group. a. Engine section—for visual evidence of

excessive oil, fuel, or hydraulic leaks, and sources of such leaks.

b. Studs and nuts—for proper torquing and obvious defects.

c. Internal engine—for cylinder compression and for metal particles or foreign matter on screens and sump drain plugs. If cylinder compression is weak, check for improper internal condition and improper internal tolerances.

d. Engine mount—for cracks, looseness of mounting, and looseness of engine to mount.

e. Flexible vibration dampeners—for condition and deterioration.

f. Engine controls—for defects, proper travel, and proper safetying.

g. Lines, hoses, and clamps—for leaks, condition, and looseness.

h. Exhaust stacks—for cracks, defects, and proper attachment.

i. Accessories—for apparent defects in security of mounting.

j. All systems—for proper installation, general condition defects, and secure attachment.

k. Cowling—for cracks and defects. l. Ground runup and functional check—check

all powerplant controls and systems for correct response, all instruments for proper operation and indication.Li

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c. Anti-icing devices—for proper operation and obvious defects.

d. Control mechanisms—for proper operation, secure mounting, and travel.

8. Communication and navigation group. a. Radio and electronic equipment—for proper

installation and secure mounting. b. Wiring and conduits—for proper routing,

secure mounting, and obvious defects. c. Bonding and shielding—for proper

installation and condition. d. Antennas—for condition, secure mounting,

and proper operation. 9. Miscellaneous.

a. Emergency and first aid equipment—for general condition and proper stowage.

b. Parachutes, life rafts, flares, and so forth—inspect in accordance with the manufacturer’s recommendations.

c. Autopilot system—for general condition, security of attachment, and proper operation.

PublicationsAeronautical publications are the sources of informa-tion for guiding aviation mechanics in the operation and maintenance of aircraft and related equipment. The proper use of these publications will greatly aid in the efficient operation and maintenance of all air-craft. These include manufacturers’ service bulletins, manuals, and catalogs; FAA regulations; airworthiness directives; advisory circulars; and aircraft, engine and propeller specifications.

Manufacturers’ Service Bulletins/InstructionsService bulletins or service instructions are two of sev-eral types of publications issued by airframe, engine, and component manufacturers.

The bulletins may include: (1) purpose for issuing the publication, (2) name of the applicable airframe, engine, or component, (3) detailed instructions for service, adjustment, modification or inspection, and source of parts, if required and (4) estimated number of manhours required to accomplish the job.

Maintenance ManualThe manufacturer’s aircraft maintenance manual contains complete instructions for maintenance of all systems and components installed in the aircraft. It contains information for the mechanic who normally

4. Landing gear group. a. All units—for condition and security of

attachment. b. Shock absorbing devices—for proper oleo

fluid level. c. Linkage, trusses, and members—for undue or

excessive wear, fatigue, and distortion. d. Retracting and locking mechanism—for

proper operation. e. Hydraulic lines—for leakage. f. Electrical system—for chafing and proper

operation of switches. g. Wheels—for cracks, defects, and condition of

bearings. h. Tires—for wear and cuts. i. Brakes—for proper adjustment. j. Floats and skis—for security of attachment

and obvious defects. 5. Wing and center section.

a. All components—for condition and security. b. Fabric and skin—for deterioration, distortion,

other evidence of failure, and security of attachment.

c. Internal structure (spars, ribs compression members)—for cracks, bends, and security.

d. Movable surfaces—for damage or obvious defects, unsatisfactory fabric or skin attachment and proper travel.

e. Control mechanism—for freedom of movement, alignment, and security.

f. Control cables—for proper tension, fraying, wear and proper routing through fairleads and pulleys.

6. Empennage group. a. Fixed surfaces—for damage or obvious

defects, loose fasteners, and security of attachment.

b. Movable control surfaces—for damage or obvious defects, loose fasteners, loose fabric, or skin distortion.

c. Fabric or skin—for abrasion, tears, cuts or defects, distortion, and deterioration.

7. Propeller group. a. Propeller assembly—for cracks, nicks, bends,

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works on components, assemblies, and systems while they are installed in the aircraft, but not for the over-haul mechanic. A typical aircraft maintenance manual contains:

• A description of the systems (i.e., electrical, hydraulic, fuel, control)

• Lubrication instructions setting forth the frequency and the lubricants and fluids which are to be used in the various systems,

• Pressures and electrical loads applicable to the various systems,

• Tolerances and adjustments necessary to proper functioning of the airplane,

• Methods of leveling, raising, and towing,• Methods of balancing control surfaces, • Identification of primary and secondary structures,• Frequency and extent of inspections necessary to

the proper operation of the airplane,• Special repair methods applicable to the airplane,• Special inspection techniques requiring x-ray,

ultrasonic, or magnetic particle inspection, and• A list of special tools.

Overhaul ManualThe manufacturer’s overhaul manual contains brief descriptive information and detailed step by step instructions covering work normally performed on a unit that has been removed from the aircraft. Simple, inexpensive items, such as switches and relays on which overhaul is uneconomical, are not covered in the overhaul manual.

Structural Repair ManualThis manual contains the manufacturer’s information and specific instructions for repairing primary and sec-ondary structures. Typical skin, frame, rib, and stringer repairs are covered in this manual. Also included are material and fastener substitutions and special repair techniques.

Illustrated Parts CatalogThis catalog presents component breakdowns of struc-ture and equipment in disassembly sequence. Also included are exploded views or cutaway illustrations for all parts and equipment manufactured by the aircraft manufacturer.

Code of Federal Regulations (CFRs) The CFRs were established by law to provide for the safe and orderly conduct of flight operations and to prescribe airmen privileges and limitations. A knowl-edge of the CFRs is necessary during the performance of maintenance, since all work done on aircraft must comply with CFR provisions.

Airworthiness DirectivesA primary safety function of the FAA is to require correction of unsafe conditions found in an aircraft, aircraft engine, propeller, or appliance when such con-ditions exist and are likely to exist or develop in other products of the same design. The unsafe condition may exist because of a design defect, maintenance, or other causes. Title 14 of the Code of Federal Regulations (14 CFR) part 39, Airworthiness Directives, defines the authority and responsibility of the Administra-tor for requiring the necessary corrective action. The Airworthiness Directives (ADs) are published to notify aircraft owners and other interested persons of unsafe conditions and to prescribe the conditions under which the product may continue to be operated.

Airworthiness Directives are Federal Aviation Regu-lations and must be complied with unless specific exemption is granted.

Airworthiness Directives may be divided into two categories: (1) those of an emergency nature requiring immediate compliance upon receipt and (2) those of a less urgent nature requiring compliance within a rela-tively longer period of time. Also, ADs may be a one-time compliance item or a recurring item that requires future inspection on an hourly basis (accrued flight time since last compliance) or a calendar time basis.

The contents of ADs include the aircraft, engine, pro-peller, or appliance model and serial numbers affected. Also included are the compliance time or period, a description of the difficulty experienced, and the nec-essary corrective action.

Type Certificate Data SheetsThe type certificate data sheet (TCDS) describes the type design and sets forth the limitations prescribed by the applicable CFR part. It also includes any other limitations and information found necessary for type certification of a particular model aircraft.

Type certificate data sheets are numbered in the upper right-hand corner of each page. This number is the same as the type certificate number. The name of the type certificate holder, together with all of the approved Li

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models, appears immediately below the type certificate number. The issue date completes this group. This information is contained within a bordered text box to set it off.

The data sheet is separated into one or more sections. Each section is identified by a Roman numeral followed by the model designation of the aircraft to which the section pertains. The category or categories in which the aircraft can be certificated are shown in parenthe-ses following the model number. Also included is the approval date shown on the type certificate.

The data sheet contains information regarding:

1. Model designation of all engines for which the aircraft manufacturer obtained approval for use with this model aircraft.

2. Minimum fuel grade to be used. 3. Maximum continuous and takeoff ratings of the

approved engines, including manifold pressure (when used), rpm, and horsepower (hp).

4. Name of the manufacturer and model designation for each propeller for which the aircraft manufacturer obtained approval will be shown together with the propeller limits and any operating restrictions peculiar to the propeller or propeller engine combination.

5. Airspeed limits in both mph and knots. 6. Center of gravity range for the extreme loading

conditions of the aircraft is given in inches from the datum. The range may also be stated in percent of MAC (Mean Aerodynamic Chord) for transport category aircraft.

7. Empty weight center of gravity (CG) range (when established) will be given as fore and aft limits in inches from the datum. If no range exists, the word “none” will be shown following the heading on the data sheet.

8. Location of the datum. 9. Means provided for leveling the aircraft. 10. All pertinent maximum weights. 11. Number of seats and their moment arms. 12. Oil and fuel capacity. 13. Control surface movements. 14. Required equipment. 15. Additional or special equipment found necessary

for certification. 16. Information concerning required placards.

It is not within the scope of this handbook to list all the items that can be shown on the type certificate data sheets. Those items listed above serve only to acquaint aviation mechanics with the type of information gener-ally included on the data sheets. Type certificate data sheets may be many pages in length. Figure 8-1 shows a typical TCDS.

When conducting a required or routine inspection, it is necessary to ensure that the aircraft and all the major items on it are as defined in the type certificate data sheets. This is called a conformity check, and verifies that the aircraft conforms to the specifications of the aircraft as it was originally certified. Sometimes altera-tions are made that are not specified or authorized in the TCDS. When that condition exists, a supplemental type certificate (STC) will be issued. STCs are considered a part of the permanent records of an aircraft, and should be maintained as part of that aircraft’s logs.

Routine/Required InspectionsFor the purpose of determining their overall condition, 14 CFR provides for the inspection of all civil aircraft at specific intervals, depending generally upon the type of operations in which they are engaged. The pilot in command of a civil aircraft is responsible for determin-ing whether that aircraft is in condition for safe flight. Therefore, the aircraft must be inspected before each flight. More detailed inspections must be conducted by aviation maintenance technicians at least once each 12 calendar months, while inspection is required for others after each 100 hours of flight. In other instances, an aircraft may be inspected in accordance with a system set up to provide for total inspection of the aircraft over a calendar or flight time period.

To determine the specific inspection requirements and rules for the performance of inspections, refer to the CFR, which prescribes the requirements for the inspection and maintenance of aircraft in various types of operations.

Preflight/Postflight InspectionsPilots are required to follow a checklist contained within the Pilot’s Operating Handbook (POH) when operating aircraft. The first section of a checklist includes a section entitled Preflight Inspection. The preflight inspection checklist includes a “walk-around” section listing items that the pilot is to visually check for general condition as he or she walks around the airplane. Also, the pilot must ensure that fuel, oil and other items required for flight are at the proper levels

(Continued on page 8-12)

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Figure 8-1. Type certificate data sheet.

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Figure 8-1. Type certificate data sheet. (continued)

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and not contaminated. Additionally, it is the pilot’s responsibility to review the airworthiness certificate, maintenance records, and other required paperwork to verify that the aircraft is indeed airworthy. After each flight, it is recommended that the pilot or mechanic conduct a postflight inspection to detect any problems that might require repair or servicing before the next flight.

Annual/100-Hour InspectionsTitle 14 of the Code of Federal Regulations (14 CFR) part 91 discusses the basic requirements for annual and 100-hour inspections. With some exceptions, all aircraft must have a complete inspection annually. Aircraft that are used for commercial purposes and are likely to be used more frequently than noncommercial aircraft must have this complete inspection every 100 hours. The scope and detail of items to be included in annual and 100-hour inspections is included as appen-dix D of 14 CFR part 43 and shown as Figure 8-2.

A properly written checklist, such as the one shown earlier in this chapter, will include all the items of appendix D. Although the scope and detail of annual and 100-hour inspections is identical, there are two significant differences. One difference involves persons authorized to conduct them. A certified airframe and powerplant maintenance technician can conduct a 100-hour inspection, whereas an annual inspection must be conducted by a certified airframe and powerplant maintenance technician with inspection authorization (IA). The other difference involves authorized over-flight of the maximum 100 hours before inspection. An aircraft may be flown up to 10 hours beyond the 100-hour limit if necessary to fly to a destination where the inspection is to be conducted.

Progressive InspectionsBecause the scope and detail of an annual inspection is very extensive and could keep an aircraft out of service for a considerable length of time, alternative

(a) Each person performing an annual or 100-hour inspection shall, before that inspection, remove or open all necessary inspection plates, access doors, fairing, and cowling. He shall thoroughly clean the aircraft and aircraft engine.

(b) Each person performing an annual or 100-hour inspection shall inspect (where applicable) the following components of the fuselage and hull group: (1) Fabric and skin—for deterioration,

distortion, other evidence of failure, and defective or insecure attachment of fittings.

(2) Systems and components—for improper installation, apparent defects, and unsatisfactory operation.

(3) Envelope, gas bags, ballast tanks, and related parts—for poor condition.

(c) Each person performing an annual or 100-hour inspection shall inspect (where applicable) the following components of the cabin and cockpit group: (1) Generally—for uncleanliness and loose

equipment that might foul the controls. (2) Seats and safety belts—for poor condition

and apparent defects.

Appendix D to Part 43—Scope and Detail of Items (as Applicable to the Particular Aircraft) To Be Included in Annual and 100-Hour Inspections

Figure 8-2. Scope and detail of annual and 100-hour inspections.

(3) Windows and windshields—for deterioration and breakage.

(4) Instruments—for poor condition, mounting, marking, and (where practicable) improper operation.

(5) Flight and engine controls—for improper installation and improper operation.

(6) Batteries—for improper installation and improper charge.

(7) All systems—for improper installation, poor general condition, apparent and obvious defects, and insecurity of attachment.

(d) Each person performing an annual or 100-hour inspection shall inspect (where applicable) components of the engine and nacelle group as follows: (1) Engine section—for visual evidence of

excessive oil, fuel, or hydraulic leaks, and sources of such leaks.

(2) Studs and nuts—for improper torquing and obvious defects.

(3) Internal engine—for cylinder compression and for metal particles or foreign matter on screens and sump drain plugs. If there is Li

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weak cylinder compression, for improper internal condition and improper internal tolerances.

(4) Engine mount—for cracks, looseness of mounting, and looseness of engine to mount.

(5) Flexible vibration dampeners—for poor condition and deterioration.

(6) Engine controls—for defects, improper travel, and improper safetying.

(7) Lines, hoses, and clamps—for leaks, improper condition, and looseness.

(8) Exhaust stacks—for cracks, defects, and improper attachment.

(9) Accessories—for apparent defects in security of mounting.

(10) All systems—for improper installation, poor general condition, defects, and insecure attachment.

(11) Cowling—for cracks and defects. (e) Each person performing an annual or 100-hour

inspection shall inspect (where applicable) the following components of the landing gear group: (1) All units—for poor condition and

insecurity of attachment. (2) Shock absorbing devices—for improper

oleo fluid level. (3) Linkages, trusses, and members—for

undue or excessive wear fatigue, and distortion.

(4) Retracting and locking mechanism—for improper operation.

(5) Hydraulic lines—for leakage. (6) Electrical system—for chafing and

improper operation of switches. (7) Wheels—for cracks, defects, and

condition of bearings. (8) Tires—for wear and cuts. (9) Brakes—for improper adjustment. (10) Floats and skis—for insecure attachment

and obvious or apparent defects.

Figure 8-2. Scope and detail of annual and 100-hour inspections. (continued)

(f) Each person performing an annual or 100-hour inspection shall inspect (where applicable) all components of the wing and center section assembly for poor general condition, fabric or skin deterioration, distortion, evidence of failure, and insecurity of attachment.

(g) Each person performing an annual or 100-hour inspection shall inspect (where applicable) all components and systems that make up the com-plete empennage assembly for poor general condition, fabric or skin deterioration, distortion, evidence of failure, insecure attachment, improper component installation, and improper component operation.

(h) Each person performing an annual or 100-hour inspection shall inspect (where applicable) the following components of the propeller group:

(1) Propeller assembly—for cracks, nicks, binds, and oil leakage.

(2) Bolts—for improper torquing and lack of safetying.

(3) Anti-icing devices—for improper operations and obvious defects.

(4) Control mechanisms—for improper operation, insecure mounting, and restricted travel.

(i) Each person performing an annual or 100-hour inspection shall inspect (where applicable) the following components of the radio group:

(1) Radio and electronic equipment—for improper installation and insecure mounting.

(2) Wiring and conduits—for improper routing, insecure mounting, and obvious defects.

(3) Bonding and shielding—for improper installation and poor condition.

(4) Antenna including trailing antenna—for poor condition, insecure mounting, and improper operation.

(j) Each person performing an annual or 100-hour inspection shall inspect (where applicable) each installed miscellaneous item that is not otherwise covered by this listing for improper installation and improper operation.

Appendix D to Part 43—Scope and Detail of Items (as Applicable to the Particular Aircraft) To Be Included in Annual and 100-Hour Inspections (continued)

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inspection programs designed to minimize down time may be utilized. A progressive inspection program allows an aircraft to be inspected progressively. The scope and detail of an annual inspection is essentially divided into segments or phases (typically four to six). Completion of all the phases completes a cycle that satisfies the requirements of an annual inspection. The advantage of such a program is that any required segment may be completed overnight and thus enable the aircraft to fly daily without missing any revenue earning potential. Progressive inspection programs include routine items such as engine oil changes and detailed items such as flight control cable inspection. Routine items are accomplished each time the aircraft

comes in for a phase inspection and detailed items focus on detailed inspection of specific areas. Detailed inspections are typically done once each cycle. A cycle must be completed within 12 months. If all required phases are not completed within 12 months, the remain-ing phase inspections must be conducted before the end of the 12th month from when the first phase was completed.

Each registered owner or operator of an aircraft desiring to use a progressive inspection program must submit a written request to the FAA Flight Standards District Office (FSDO) having jurisdiction over the area in which the applicant is located. Title 14 of the Code of Federal Regulations (14 CFR) part 91, §91.409(d)

Figure 8-3. 14 CFR §91.409(d) Progressive inspection.

(d) Progressiveinspection. Each registered owner or operator of an aircraft desiring to use a progressive inspection program must submit a written request to the FAA Flight Standards district office having jurisdiction over the area in which the applicant is located, and shall provide—

(1) A certificated mechanic holding an inspection authorization, a certificated airframe repair station, or the manufacturer of the aircraft to supervise or conduct the progressive inspection;

(2) A current inspection procedures manual available and readily understandable to pilot and maintenance personnel containing, in detail— (i) An explanation of the progressive

inspection, including the continuity of inspection responsibility, the making of reports, and the keeping of records and technical reference material;

(ii) An inspection schedule, specifying the intervals in hours or days when routine and detailed inspections will be performed and including instructions for exceeding an inspection interval by not more than 10 hours while en route and for changing an inspection interval because of service experience;

(iii) Sample routine and detailed inspection forms and instructions for their use; and

(iv) Sample reports and records and instructions for their use;

(3) Enough housing and equipment for necessary disassembly and proper inspection of the aircraft; and

(4) Appropriate current technical information for the aircraft.

The frequency and detail of the progressive inspection shall provide for the complete inspection of the aircraft within each 12 calendar months and be consistent with the manufacturer's recommendations, field service experience, and the kind of operation in which the aircraft is engaged. The progressive inspection schedule must ensure that the aircraft, at all times, will be airworthy and will conform to all applicable FAA aircraft specifications, type certificate data sheets, airworthiness directives, and other approved data. If the progressive inspection is discontinued, the owner or operator shall immediately notify the local FAA Flight Standards district office, in writing, of the discontinuance. After the discontinuance, the first annual inspection under §91.409(a)(1) is due within 12 calendar months after the last complete inspection of the aircraft under the progressive inspection. The 100-hour inspection under §91.409(b) is due within 100 hours after that complete inspection. A complete inspection of the aircraft, for the purpose of determining when the annual and 100-hour inspections are due, requires a detailed inspection of the aircraft and all its components in accordance with the progressive inspection. A routine inspection of the aircraft and a detailed inspection of several components is not considered to be a complete inspection.

§91.409 Inspections.

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establishes procedures to be followed for progressive inspections and is shown in Figure 8-3.

Continuous InspectionsContinuous inspection programs are similar to pro-gressive inspection programs, except that they apply to large or turbine-powered aircraft and are therefore more complicated.

Like progressive inspection programs, they require approval by the FAA Administrator. The approval may be sought based upon the type of operation and the CFR parts under which the aircraft will be operated. The maintenance program for commercially operated aircraft must be detailed in the approved operations specifications (OpSpecs) of the commercial certificate holder.

Airlines utilize a continuous maintenance program that includes both routine and detailed inspections. However, the detailed inspections may include differ-ent levels of detail. Often referred to as “checks,” the A-check, B-check, C-check, and D-checks involve increasing levels of detail. A-checks are the least com-prehensive and occur frequently. D-checks, on the other hand, are extremely comprehensive, involving major disassembly, removal, overhaul, and inspection of systems and components. They might occur only three to six times during the service life of an aircraft.

Altimeter and Transponder InspectionsAircraft that are operated in controlled airspace under instrument flight rules (IFR) must have each altimeter and static system tested in accordance with procedures described in 14 CFR part 43, appendix E, within the preceding 24 calendar months. Aircraft having an air traffic control (ATC) transponder must also have each transponder checked within the preceding 24 months. All these checks must be conducted by appropriately certified individuals.

ATA iSpec 2200In an effort to standardize the format for the way in which maintenance information is presented in aircraft maintenance manuals, the Air Transport Association of America (ATA) issued specifications for Manufac-turers Technical Data. The original specification was called ATA Spec 100. Over the years, Spec 100 has been continuously revised and updated. Eventually, ATA Spec 2100 was developed for electronic docu-mentation. These two specifications evolved into one document called ATA iSpec 2200. As a result of this standardization, maintenance technicians can always

find information regarding a particular system in the same section of an aircraft maintenance manual, regardless of manufacturer. For example, if you are seeking information about the electrical system on any aircraft, you will always find that information in section (chapter) 24.

The ATA Specification 100 has the aircraft divided into systems, such as air conditioning, which covers the basic air conditioning system (ATA 21). Number-ing in each major system provides an arrangement for breaking the system down into several subsystems. Late model aircraft, both over and under the 12,500 pound designation, have their parts manuals and maintenance manuals arranged according to the ATA coded system.

The following abbreviated table of ATA System, Subsys-tem, and Titles is included for familiarization purposes.

ATA Specification 100 Systems

Sys. Sub. Title 21 AIR CONDITIONING 21 00 General 21 10 Compression 21 20 Distribution 21 30 Pressurization Control 21 40 Heating 21 50 Cooling 21 60 Temperature Control 21 70 Moisture/Air Contaminate Control

The remainder of this list shows the systems and title with subsystems deleted in the interest of brevity. Con-sult specific aircraft maintenance manuals for a com-plete description of the subsystems used in them.

22 AUTO FLIGHT 23 COMMUNICATIONS 24 ELECTRICAL POWER 25 EQUIPMENT/FURNISHINGS 26 FIRE PROTECTION 27 FLIGHT CONTROLS 28 FUEL 29 HYDRAULIC POWER 30 ICE AND RAIN PROTECTION 31 INDICATING/RECORDING SYSTEMS 32 LANDING GEAR 33 LIGHTS 34 NAVIGATION

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35 OXYGEN 36 PNEUMATIC 37 VACUUM/PRESSURE 38 WATER/WASTE 39 ELECTRICAL/ELECTRONIC PANELS AND

MULTIPURPOSE COMPONENTS 49 AIRBORNE AUXILIARY POWER 51 STRUCTURES 52 DOORS 53 FUSELAGE 54 NACELLES/PYLONS 55 STABILIZERS 56 WINDOWS 57 WINGS 61 PROPELLERS 65 ROTORS 71 POWERPLANT 72 (T) TURBINE/TURBOPROP 72 (R) ENGINE RECIPROCATING 73 ENGINE FUEL AND CONTROL 74 IGNITION 75 BLEED AIR 76 ENGINE CONTROLS 77 ENGINE INDICATING 78 ENGINE EXHAUST 79 ENGINE OIL 80 STARTING 81 TURBINES (RECIPROCATING ENG) 82 WATER INJECTION 83 REMOTE GEAR BOXES (ENG DR)

Keep in mind that not all aircraft will have all these systems installed. Small and simple aircraft have far fewer systems than larger more complex aircraft.

Special InspectionsDuring the service life of an aircraft, occasions may arise when something out of the ordinary care and use of an aircraft might happen that could possibly affect its airworthiness. When these situations are encountered, special inspection procedures should be followed to determine if damage to the aircraft structure has occurred. The procedures outlined on the following pages are general in nature and are intended to acquaint the aviation mechanic with the areas which

should be inspected. As such, they are not all inclusive. When performing any of these special inspections, always follow the detailed procedures in the aircraft maintenance manual. In situations where the manual does not adequately address the situation, seek advice from other maintenance technicians who are highly experienced with them.

Hard or Overweight Landing InspectionThe structural stress induced by a landing depends not only upon the gross weight at the time but also upon the severity of impact. However, because of the difficulty in estimating vertical velocity at the time of contact, it is hard to judge whether or not a landing has been sufficiently severe to cause structural damage. For this reason, a special inspection should be performed after a landing is made at a weight known to exceed the design landing weight or after a rough landing, even though the latter may have occurred when the aircraft did not exceed the design landing weight.

Wrinkled wing skin is the most easily detected sign of an excessive load having been imposed during a land-ing. Another indication which can be detected easily is fuel leakage along riveted seams. Other possible loca-tions of damage are spar webs, bulkheads, nacelle skin and attachments, firewall skin, and wing and fuselage stringers. If none of these areas show adverse effects, it is reasonable to assume that no serious damage has occurred. If damage is detected, a more extensive inspection and alignment check may be necessary.

Severe Turbulence Inspection/Over “G” When an aircraft encounters a gust condition, the airload on the wings exceeds the normal wingload supporting the aircraft weight. The gust tends to accelerate the aircraft while its inertia acts to resist this change. If the combination of gust velocity and airspeed is too severe, the induced stress can cause structural damage.

A special inspection should be performed after a flight through severe turbulence. Emphasis should be placed upon inspecting the upper and lower wing surfaces for excessive buckles or wrinkles with permanent set. Where wrinkles have occurred, remove a few rivets and examine the rivet shanks to determine if the rivets have sheared or were highly loaded in shear.

Through the inspection doors and other accessible openings, inspect all spar webs from the fuselage to the tip. Check for buckling, wrinkles, and sheared attachments. Inspect for buckling in the area around Li

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the nacelles and in the nacelle skin, particularly at the wing leading edge.

Check for fuel leaks. Any sizeable fuel leak is an indi-cation that an area may have received overloads which have broken the sealant and opened the seams.

If the landing gear was lowered during a period of severe turbulence, inspect the surrounding surfaces carefully for loose rivets, cracks, or buckling. The interior of the wheel well may give further indications of excessive gust conditions. Inspect the top and bottom fuselage skin. An excessive bending moment may have left wrinkles of a diagonal nature in these areas.

Inspect the surface of the empennage for wrinkles, buckling, or sheared attachments. Also, inspect the area of attachment of the empennage to the fuselage. The above inspections cover the critical areas. If exces-sive damage is noted in any of the areas mentioned, the inspection should be continued until all damage is detected.

Lightning StrikeAlthough lightning strikes to aircraft are extremely rare, if a strike has occurred, the aircraft must be care-fully inspected to determine the extent of any damage that might have occurred. When lightning strikes an aircraft, the electrical current must be conducted through the structure and be allowed to discharge or dissipate at controlled locations. These controlled locations are primarily the aircraft’s static discharge wicks, or on more sophisticated aircraft, null field dis-chargers. When surges of high voltage electricity pass through good electrical conductors, such as aluminum or steel, damage is likely to be minimal or nonexistent. When surges of high voltage electricity pass through non-metallic structures, such as a fiberglass radome, engine cowl or fairing, glass or plastic window, or a composite structure that does not have built-in electri-cal bonding, burning and more serious damage to the structure could occur. Visual inspection of the structure is required. Look for evidence of degradation, burning or erosion of the composite resin at all affected struc-tures, electrical bonding straps, static discharge wicks and null field dischargers.

Fire DamageInspection of aircraft structures that have been sub-jected to fire or intense heat can be relatively simple if visible damage is present. Visible damage requires repair or replacement. If there is no visible damage, the structural integrity of an aircraft may still have been compromised. Since most structural metallic

components of an aircraft have undergone some sort of heat treatment process during manufacture, an exposure to high heat not encountered during normal operations could severely degrade the design strength of the structure. The strength and airworthiness of an aluminum structure that passes a visual inspection but is still suspect can be further determined by use of a conductivity tester. This is a device that uses eddy cur-rent and is discussed later in this chapter. Since strength of metals is related to hardness, possible damage to steel structures might be determined by use of a hard-ness tester such as a Rockwell C hardness tester.

Flood DamageLike aircraft damaged by fire, aircraft damaged by water can range from minor to severe, depending on the level of the flood water, whether it was fresh or salt water and the elapsed time between the flood occurrence and when repairs were initiated. Any parts that were totally submerged should be completely disassembled, thoroughly cleaned, dried and treated with a corrosion inhibitor. Many parts might have to be replaced, particularly interior carpeting, seats, side panels, and instruments. Since water serves as an electrolyte that promotes corrosion, all traces of water and salt must be removed before the aircraft can again be considered airworthy.

SeaplanesBecause they operate in an environment that acceler-ates corrosion, seaplanes must be carefully inspected for corrosion and conditions that promote corrosion. Inspect bilge areas for waste hydraulic fluids, water, dirt, drill chips, and other debris. Additionally, since seaplanes often encounter excessive stress from the pounding of rough water at high speeds, inspect for loose rivets and other fasteners; stretched, bent or cracked skins; damage to the float attach fitting; and general wear and tear on the entire structure.

Aerial Application AircraftTwo primary factors that make inspecting these aircraft different from other aircraft are the corrosive nature of some of the chemicals used and the typical flight pro-file. Damaging effects of corrosion may be detected in a much shorter period of time than normal use aircraft. Chemicals may soften the fabric or loosen the fabric tapes of fabric covered aircraft. Metal aircraft may need to have the paint stripped, cleaned, and repainted and corrosion treated annually. Leading edges of wings and other areas may require protective coatings or tapes. Hardware may require more frequent replacement. Li

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During peak use, these aircraft may fly up to 50 cycles (takeoffs and landings) or more in a day, most likely from an unimproved or grass runway. This can greatly accelerate the failure of normal fatigue items. Landing gear and related items require frequent inspections. Because these aircraft operate almost continuously at very low altitudes, air filters tend to become obstructed more rapidly.

Special Flight PermitsFor an aircraft that does not currently meet airworthi-ness requirements because of an overdue inspection, damage, expired replacement times for time-limited parts or other reasons, but is capable of safe flight, a special flight permit may be issued. Special flight permits, often referred to as ferry permits, are issued for the following purposes:

• Flying the aircraft to a base where repairs, alterations, or maintenance are to be performed, or to a point of storage.

• Delivering or exporting the aircraft.• Production flight testing new production

aircraft.• Evacuating aircraft from areas of impending

danger.• Conducting customer demonstration flights in

new production aircraft that have satisfactorily completed production flight tests.

Additional information about special flight permits may be found in 14 CFR part 21. Application forms for special flight permits may be requested from the nearest FAA Flight Standards District Office (FSDO).

Nondestructive Inspection/TestingThe preceding information in this chapter provided general information regarding aircraft inspection. The remainder of this chapter deals with several methods often used on specific components or areas on an air-craft when carrying out the more specific inspections. They are referred to as nondestructive inspection (NDI) or nondestructive testing (NDT). The objective of NDI and NDT is to determine the airworthiness of a component without damaging it, which would render it unairworthy. Some of these methods are simple, requiring little additional expertise, while others are highly sophisticated and require that the technician be highly trained and specially certified.

Additional information on NDI may be found by referring to chapter 5 of FAA Advisory Circular (AC) 43.13-1B, Acceptable Methods, Techniques, and Prac-tices — Aircraft Inspection and Repair. Information regarding training, qualifications, and certification of NDI personnel may be found in FAA Advisory Circular (AC) 65-31A, Training, Qualification and Certification of Non-destructive Inspection (NDI) Personnel.

General TechniquesBefore conducting NDI, it is necessary to follow preparatory steps in accordance with procedures spe-cific to that type of inspection. Generally, the parts or areas must be thoroughly cleaned. Some parts must be removed from the aircraft or engine. Others might need to have any paint or protective coating stripped. A complete knowledge of the equipment and procedures is essential and if required, calibration and inspection of the equipment must be current.

Visual InspectionVisual inspection can be enhanced by looking at the suspect area with a bright light, a magnifying glass, and a mirror (when required). Some defects might be so obvious that further inspection methods are not required. The lack of visible defects does not neces-sarily mean further inspection is unnecessary. Some defects may lie beneath the surface or may be so small that the human eye, even with the assistance of a mag-nifying glass, cannot detect them.

BorescopeInspection by use of a borescope is essentially a visual inspection. A borescope is a device that enables the inspector to see inside areas that could not otherwise be inspected without disassembly. An example of an area that can be inspected with a borescope is the inside of a reciprocating engine cylinder. The borescope can be inserted into an open spark plug hole to detect damaged pistons, cylinder walls, or valves. Another example would be the hot section of a turbine engine to which access could be gained through the hole of a removed igniter or removed access plugs specifically installed for inspection purposes.

Borescopes are available in two basic configurations. The simpler of the two is a rigid type of small diameter telescope with a tiny mirror at the end that enables the user to see around corners. The other type uses fiber optics that enables greater flexibility. Many borescopes provide images that can be displayed on a computer or video monitor for better interpretation of what is being viewed and to record images for future reference. Most Li

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borescopes also include a light to illuminate the area being viewed.

Liquid Penetrant InspectionPenetrant inspection is a nondestructive test for defects open to the surface in parts made of any nonporous material. It is used with equal success on such metals as aluminum, magnesium, brass, copper, cast iron, stainless steel, and titanium. It may also be used on ceramics, plastics, molded rubber, and glass.

Penetrant inspection will detect such defects as surface cracks or porosity. These defects may be caused by fatigue cracks, shrinkage cracks, shrinkage poros-ity, cold shuts, grinding and heat treat cracks, seams, forging laps, and bursts. Penetrant inspection will also indicate a lack of bond between joined metals.

The main disadvantage of penetrant inspection is that the defect must be open to the surface in order to let the penetrant get into the defect. For this reason, if the part in question is made of material which is magnetic, the use of magnetic particle inspection is generally recommended.

Penetrant inspection uses a penetrating liquid that enters a surface opening and remains there, making it clearly visible to the inspector. It calls for visual examination of the part after it has been processed, increasing the visibility of the defect so that it can be detected. Visibility of the penetrating material is increased by the addition of one of two types of dye, visible or fluorescent.

The visible penetrant kit consists of dye penetrant, dye remover emulsifier, and developer. The fluores-cent penetrant inspection kit contains a black light assembly, as well as spray cans of penetrant, cleaner, and developer. The light assembly consists of a power transformer, a flexible power cable, and a hand-held lamp. Due to its size, the lamp may be used in almost any position or location.

Briefly, the steps for performing a penetrant inspec-tion are:

1. Thorough cleaning of the metal surface. 2. Applying penetrant. 3. Removing penetrant with remover emulsifier or

cleaner. 4. Drying the part. 5. Applying the developer. 6. Inspecting and interpreting results.

InterpretationofResultsThe success and reliability of a penetrant inspection depends upon the thoroughness with which the part was prepared. Several basic principles applying to penetrant inspection are:

1. The penetrant must enter the defect in order to form an indication. It is important to allow sufficient time so the penetrant can fill the defect. The defect must be clean and free of contaminating materials so that the penetrant is free to enter.

2. If all penetrant is washed out of a defect, an indication cannot be formed. During the washing or rinsing operation, prior to development, it is possible that the penetrant will be removed from within the defect, as well as from the surface.

3. Clean cracks are usually easy to detect. Surface openings that are uncontaminated, regardless of how fine, are seldom difficult to detect with the penetrant inspection.

4. The smaller the defect, the longer the penetrating time. Fine crack-like apertures require a longer penetrating time than defects such as pores.

5. When the part to be inspected is made of a material susceptible to magnetism, it should be inspected by a magnetic particle inspection method if the equipment is available.

6. Visible penetrant-type developer, when applied to the surface of a part, will dry to a smooth, even, white coating. As the developer dries, bright red indications will appear where there are surface defects. If no red indications appear, there are no surface defects.

7. When conducting the fluorescent penetrant-type inspection, the defects will show up (under black light) as a brilliant yellow-green color and the sound areas will appear deep blue-violet.

8. It is possible to examine an indication of a defect and to determine its cause as well as its extent. Such an appraisal can be made if something is known about the manufacturing processes to which the part has been subjected.

The size of the indication, or accumulation of pen-etrant, will show the extent of the defect and the bril-liance will be a measure of its depth. Deep cracks will hold more penetrant and will be broader and more bril-liant. Very fine openings can hold only small amounts of penetrants and will appear as fine lines. Figure 8-4 shows some of the types of defects that can be located using dry penetrant.Li

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FalseIndicationsWith the penetrant inspection, there are no false indi-cations in the sense that they occur in the magnetic particle inspection. There are, however, two condi-tions which may create accumulations of penetrant that are sometimes confused with true surface cracks and discontinuities.

The first condition involves indications caused by poor washing. If all the surface penetrant is not removed in the washing or rinsing operation following the pen-etrant dwell time, the unremoved penetrant will be vis-ible. Evidences of incomplete washing are usually easy to identify since the penetrant is in broad areas rather than in the sharp patterns found with true indications. When accumulations of unwashed penetrant are found on a part, the part should be completely reprocessed. Degreasing is recommended for removal of all traces of the penetrant.

False indications may also be created where parts press fit to each other. If a wheel is press fit onto a shaft, penetrant will show an indication at the fit line. This is perfectly normal since the two parts are not meant to be welded together. Indications of this type are easy to identify since they are regular in form and shape.

Eddy Current InspectionElectromagnetic analysis is a term which describes the broad spectrum of electronic test methods involving the intersection of magnetic fields and circulatory currents. The most widely used technique is the eddy current.

Eddy currents are composed of free electrons under the influence of an induced electromagnetic field which are made to “drift” through metal.

Eddy current is used in aircraft maintenance to inspect jet engine turbine shafts and vanes, wing skins, wheels, bolt holes, and spark plug bores for cracks, heat or frame damage. Eddy current may also be used in repair of aluminum aircraft damaged by fire or excessive heat. Different meter readings will be seen when the same metal is in different hardness states. Readings in the affected area are compared with identical materials in known unaffected areas for comparison. A difference in readings indicates a difference in the hardness state of the affected area. In aircraft manufacturing plants, eddy current is used to inspect castings, stampings, machine parts, forgings, and extrusions. Figure 8-5 shows a technician performing an eddy current inspec-tion on an aluminum wheel half.

BasicPrinciplesWhen an alternating current is passed through a coil, it develops a magnetic field around the coil, which in turn induces a voltage of opposite polarity in the coil and opposes the flow of original current. If this coil is placed in such a way that the magnetic field passes

Pits of porosity Tight crack or partially welded lap Crack or similar opening

Figure 8-4. Types of defects.

Figure 8-5. Eddy current inspection of wheel half.

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ing surface and subsurface corrosion, pots and heat treat condition.

Ultrasonic InspectionUltrasonic detection equipment makes it possible to locate defects in all types of materials. Minute cracks, checks, and voids too small to be seen by x-ray can be located by ultrasonic inspection. An ultrasonic test instrument requires access to only one surface of the material to be inspected and can be used with either straight line or angle beam testing techniques.

Two basic methods are used for ultrasonic inspection. The first of these methods is immersion testing. In this method of inspection, the part under examination and the search unit are totally immersed in a liquid couplant, which may be water or any other suitable fluid.

The second method is called contact testing, which is readily adapted to field use and is the method discussed in this chapter. In this method, the part under exami-nation and the search unit are coupled with a viscous material, liquid or a paste, which wets both the face of the search unit and the material under examination.

There are three basic ultrasonic inspection meth-ods: (1) pulse echo; (2) through transmission; and (3) resonance.

through an electrically conducting specimen, eddy currents will be induced into the specimen. The eddy currents create their own field which varies the original field’s opposition to the flow of original current. The specimen’s susceptibility to eddy currents determines the current flow through the coil. [Figure 8-6]

The magnitude and phase of this counter field is depen-dent primarily upon the resistance and permeability of the specimen under consideration, and which enables us to make a qualitative determination of various physical properties of the test material. The interaction of the eddy current field with the original field results is a power change that can be measured by utilizing electronic circuitry similar to a Wheatstone bridge.

The specimen is either placed in or passed through the field of an electromagnetic induction coil, and its effect on the impedance of the coil or on the voltage output of one or more test coils is observed. The process, which involves electric fields made to explore a test piece for various conditions, involves the transmission of energy through the specimen much like the transmission of x-rays, heat, or ultrasound.

Eddy current inspection can frequently be performed without removing the surface coatings such as primer, paint, and anodized films. It can be effective in detect-

Oscillator Amplifier

Probe

Meter

Sample part

Figure 8-6. Eddy current inspection circuit.Libra

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PulseEchoFlaws are detected by measuring the amplitude of sig-nals reflected and the time required for these signals to travel between specific surfaces and the discontinuity. [Figure 8-7]

The time base, which is triggered simultaneously with each transmission pulse, causes a spot to sweep across the screen of the cathode ray tube (CRT). The spot sweeps from left to right across the face of the scope 50 to 5,000 times per second, or higher if required for high speed automated scanning. Due to the speed of the cycle of transmitting and receiving, the picture on the oscilloscope appears to be stationary.

A few microseconds after the sweep is initiated, the rate generator electrically excites the pulser, and the pulser in turn emits an electrical pulse. The transducer converts this pulse into a short train of ultrasonic sound waves. If the interfaces of the transducer and the specimen are properly oriented, the ultrasound will be reflected back to the transducer when it reaches the internal flaw and the opposite surface of the speci-men. The time interval between the transmission of the initial impulse and the reception of the signals from within the specimen are measured by the timing circuits. The reflected pulse received by the transducer is amplified, then transmitted to and displayed on the instrument screen. The pulse is displayed in the same relationship to the front and back pulses as the flaw is

in relation to the front and back surfaces of the speci-men. [Figure 8-8]

Pulse-echo instruments may also be used to detect flaws not directly underneath the probe by use of the angle-beam testing method. Angle beam testing differs from straight beam testing only in the manner in which the ultrasonic waves pass through the material being tested. As shown in Figure 8-9, the beam is projected into the material at an acute angle to the surface by means of a crystal cut at an angle and mounted in plastic. The beam or a portion thereof reflects successively from the surfaces of the material or any other discontinuity, including the edge of the piece. In straight beam test-ing, the horizontal distance on the screen between the initial pulse and the first back reflection represents the thickness of the piece; while in angle beam testing, this distance represents the width of the material between the searching unit and the opposite edge of the piece.

ThroughTransmissionThrough transmission inspection uses two transducers, one to generate the pulse and another placed on the opposite surface to receive it. A disruption in the sound path will indicate a flaw and be displayed on the instru-ment screen. Through transmission is less sensitive to small defects than the pulse-echo method.

ResonanceThis system differs from the pulse method in that the frequency of transmission may be continuously varied. The resonance method is used principally for thickness measurements when the two sides of the material being tested are smooth and parallel and the backside is inac-cessible. The point at which the frequency matches the resonance point of the material being tested is the thickness determining factor.

RF pulser

Rate generator

Timing circuit

Specimen

Flaw

12

3

Transducer

Amplifier

Cathode ray oscilloscope

1 2 3

Figure 8-7. Block diagram of basic pulse-echo system.

Flaw

Cathode ray tube

F

T

Transducer

Flaw

Speciman

B

T‘ F’ B’

Figure 8-8. Pulse-echo display in relationship to flaw detection.

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It is necessary that the frequency of the ultrasonic waves corresponding to a particular dial setting be accurately known. Checks should be made with standard test blocks to guard against possible drift of frequency.

If the frequency of an ultrasonic wave is such that its wavelength is twice the thickness of a specimen (funda-mental frequency), then the reflected wave will arrive back at the transducer in the same phase as the original transmission so that strengthening of the signal will occur. This results from constructive interference or a resonance and is shown as a high amplitude value on the indicating screen. If the frequency is increased such that three times the wavelength equals four times the thickness, the reflected signal will return completely out of phase with the transmitted signal and cancella-tion will occur. Further increase of the frequency causes the wavelength to be equal to the thickness again and gives a reflected signal in phase with the transmitted signal and a resonance once more.

By starting at the fundamental frequency and gradually increasing the frequency, the successive cancellations and resonances can be noted and the readings used to check the fundamental frequency reading. [Figure 8-10]

In some instruments, the oscillator circuit contains a motor driven capacitor which changes the frequency of the oscillator. [Figure 8-11] In other instruments, the frequency is changed by electronic means.

The change in frequency is synchronized with the horizontal sweep of a CRT. The horizontal axis thus represents a frequency range. If the frequency range contains resonances, the circuitry is arranged to pres-ent these vertically. Calibrated transparent scales are then placed in front of the tube, and the thickness can be read directly. The instruments normally operate

Coaxial cable

Material

Quartz crystal Defect

Figure 8-9. Pulse-echo angle beam testing.

F = F1 (Fundamental frequency)

Transducerincident wave Reflective wave

Reflectingsurface

Material under test

T =

T = W F = 2F1 (2nd Harmonic)

T = 11⁄2W F = 3F1 (3rd Harmonic)

T = 2W F = 4F1 (4th Harmonic)

Wavelength2

A

B

C

D

Figure 8-10. Conditions of ultrasonic resonance in a metal plate.Li

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between 0.25 millicycle (mc) and 10 mc in four or five bands.

The resonance thickness instrument can be used to test the thickness of such metals as steel, cast iron, brass, nickel, copper, silver, lead, aluminum, and magnesium. In addition, areas of corrosion or wear on tanks, tubing, airplane wing skins, and other structures or products can be located and evaluated.

Direct reading dial-operated units are available that measure thickness between 0.025 inch and 3 inches with an accuracy of better than ±1 percent.

Ultrasonic inspection requires a skilled operator who is familiar with the equipment being used as well as the inspection method to be used for the many different parts being tested. [Figure 8-12]

Figure 8-12. Ultrasonic inspection of a composite structure.

Acoustic Emission InspectionAcoustic emission is an NDI technique that involves the placing of acoustic emission sensors at various locations on an aircraft structure and then applying a load or stress. The materials emit sound and stress waves that take the form of ultrasonic pulses. Cracks and areas of corrosion in the stressed airframe structure emit sound waves which are registered by the sensors. These acoustic emission bursts can be used to locate flaws and to evaluate their rate of growth as a func-tion of applied stress. Acoustic emission testing has an advantage over other NDI methods in that it can detect and locate all of the activated flaws in a struc-ture in one test. Because of the complexity of aircraft structures, application of acoustic emission testing to aircraft has required a new level of sophistication in testing technique and data interpretation.

Magnetic Particle InspectionMagnetic particle inspection is a method of detecting invisible cracks and other defects in ferromagnetic materials such as iron and steel. It is not applicable to nonmagnetic materials.

In rapidly rotating, reciprocating, vibrating, and other highly stressed aircraft parts, small defects often develop to the point that they cause complete failure of the part. Magnetic particle inspection has proven extremely reliable for the rapid detection of such defects located on or near the surface. With this method of inspection, the location of the defect is indicated and the approximate size and shape are outlined.

The inspection process consists of magnetizing the part and then applying ferromagnetic particles to the surface area to be inspected. The ferromagnetic par-ticles (indicating medium) may be held in suspension in a liquid that is flushed over the part; the part may be immersed in the suspension liquid; or the particles, in dry powder form, may be dusted over the surface of the part. The wet process is more commonly used in the inspection of aircraft parts.

If a discontinuity is present, the magnetic lines of force will be disturbed and opposite poles will exist on either side of the discontinuity. The magnetized particles thus form a pattern in the magnetic field between the opposite poles. This pattern, known as an “indica-tion,” assumes the approximate shape of the surface projection of the discontinuity. A discontinuity may be defined as an interruption in the normal physical structure or configuration of a part, such as a crack, forging lap, seam, inclusion, porosity, and the like.

CRT

Pulseamplifier

Horizontaltime-basegenerator

Contacts

Transducer

Material

MotorTuning

capacitor

H. F.Oscillator

Figure 8-11. Block diagram of resonance thickness measuring system.

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A discontinuity may or may not affect the usefulness of a part.

DevelopmentofIndicationsWhen a discontinuity in a magnetized material is open to the surface, and a magnetic substance (indicating medium) is available on the surface, the flux leak-age at the discontinuity tends to form the indicating medium into a path of higher permeability. (Perme-ability is a term used to refer to the ease with which a magnetic flux can be established in a given magnetic circuit.) Because of the magnetism in the part and the adherence of the magnetic particles to each other, the indication remains on the surface of the part in the form of an approximate outline of the discontinuity that is immediately below it.

The same action takes place when the discontinuity is not open to the surface, but since the amount of flux leakage is less, fewer particles are held in place and a fainter and less sharply defined indication is obtained.

If the discontinuity is very far below the surface, there may be no flux leakage and no indication on the sur-face. The flux leakage at a transverse discontinuity is shown in Figure 8-13. The flux leakage at a longitudinal discontinuity is shown in Figure 8-14.

TypesofDiscontinuitiesDisclosedThe following types of discontinuities are normally detected by the magnetic particle test: cracks, laps, seams, cold shuts, inclusions, splits, tears, pipes, and voids. All of these may affect the reliability of parts in service.

Cracks, splits, bursts, tears, seams, voids, and pipes are formed by an actual parting or rupture of the solid metal. Cold shuts and laps are folds that have been formed in the metal, interrupting its continuity.

Inclusions are foreign material formed by impurities in the metal during the metal processing stages. They may

consist, for example, of bits of furnace lining picked up during the melting of the basic metal or of other foreign constituents. Inclusions interrupt the continuity of the metal because they prevent the joining or welding of adjacent faces of the metal.

PreparationofPartsforTestingGrease, oil, and dirt must be cleaned from all parts before they are tested. Cleaning is very important since any grease or other foreign material present can produce nonrelevant indications due to magnetic par-ticles adhering to the foreign material as the suspension drains from the part.

Grease or foreign material in sufficient amount over a discontinuity may also prevent the formation of a pat-tern at the discontinuity. It is not advisable to depend upon the magnetic particle suspension to clean the part. Cleaning by suspension is not thorough and any foreign materials so removed from the part will contaminate the suspension, thereby reducing its effectiveness.

In the dry procedure, thorough cleaning is absolutely necessary. Grease or other foreign material will hold the magnetic powder, resulting in nonrelevant indica-tions and making it impossible to distribute the indicat-ing medium evenly over the part’s surface.

All small openings and oil holes leading to internal passages or cavities should be plugged with paraffin or other suitable nonabrasive material.

Coatings of cadmium, copper, tin, and zinc do not interfere with the satisfactory performance of magnetic particle inspection, unless the coatings are unusually heavy or the discontinuities to be detected are unusu-ally small.

Chromium and nickel plating generally will not inter-fere with indications of cracks open to the surface of the base metal but will prevent indications of fine discontinuities, such as inclusions.

Figure 8-13. Flux leakage at transverse discontinuity. Figure 8-14. Flux leakage at longitudinal discontinuity.

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In longitudinal magnetization, the magnetic field is produced in a direction parallel to the long axis of the part. This is accomplished by placing the part in a solenoid excited by electric current. The metal part then becomes the core of an electromagnet and is mag-netized by induction from the magnetic field created in the solenoid.

In longitudinal magnetization of long parts, the solenoid must be moved along the part in order to magnetize it. [Figure 8-17] This is necessary to ensure adequate field strength throughout the entire length of the part.

Solenoids produce effective magnetization for approxi-mately 12 inches from each end of the coil, thus accom-modating parts or sections approximately 30 inches in length. Longitudinal magnetization equivalent to that obtained by a solenoid may be accomplished by wrapping a flexible electrical conductor around the part. Although this method is not as convenient, it has

Because it is more strongly magnetic, nickel plating is more effective than chromium plating in preventing the formation of indications.

EffectofFluxDirectionTo locate a defect in a part, it is essential that the mag-netic lines of force pass approximately perpendicular to the defect. It is therefore necessary to induce magnetic flux in more than one direction since defects are likely to exist at any angle to the major axis of the part. This requires two separate magnetizing operations, referred to as circular magnetization and longitudinal magne-tization. The effect of flux direction is illustrated in Figure 8-15.

Circular magnetization is the induction of a magnetic field consisting of concentric circles of force about and within the part which is achieved by passing electric current through the part. This type of magnetization will locate defects running approximately parallel to the axis of the part. Figure 8-16 illustrates circular magnetization of a camshaft.

Figure 8-15. Effect of flux direction on strength of indication.

Figure 8-16. Circular magnetization of a camshaft. Figure 8-17. Longitudinal magnetization of crankshaft

(solenoid method).

Longitudinal magnetization

A

Attraction of particles at defects

B

Circular magnetization

Attraction of particles at defects

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an advantage in that the coils conform more closely to the shape of the part, producing a somewhat more uniform magnetization.

The flexible coil method is also useful for large or irregularly shaped parts for which standard solenoids are not available.

EffectofFluxDensityThe effectiveness of the magnetic particle inspection also depends on the flux density or field strength at the surface of the part when the indicating medium is applied. As the flux density in the part is increased, the sensitivity of the test increases because of the greater flux leakages at discontinuities and the resulting improved formation of magnetic particle patterns.

Excessively high flux densities may form nonrelevant indications; for example, patterns of the grain flow in the material. These indications will interfere with the detection of patterns resulting from significant discontinuities. It is therefore necessary to use a field strength high enough to reveal all possible harmful discontinuities but not strong enough to produce con-fusing nonrelevant indications.

MagnetizingMethodsWhen a part is magnetized, the field strength in the part increases to a maximum for the particular magnetiz-ing force and remains at this maximum as long as the magnetizing force is maintained.

When the magnetizing force is removed, the field strength decreases to a lower residual value depending on the magnetic properties of the material and the shape of the part. These magnetic characteristics determine whether the continuous or residual method is used in magnetizing the part.

In the continuous inspection method, the part is mag-netized and the indicating medium applied while the magnetizing force is maintained. The available flux density in the part is thus at a maximum. The maximum value of flux depends directly upon the magnetizing force and the permeability of the material of which the part is made.

The continuous method may be used in practically all circular and longitudinal magnetization procedures. The continuous procedure provides greater sensitivity than the residual procedure, particularly in locating subsurface discontinuities. The highly critical nature of aircraft parts and assemblies and the necessity for

subsurface inspection in many applications have resulted in the continuous method being more widely used.

Inasmuch as the continuous procedure will reveal more nonsignificant discontinuities than the residual procedure, careful and intelligent interpretation and evaluation of discontinuities revealed by this procedure are necessary.

The residual inspection procedure involves magne-tization of the part and application of the indicating medium after the magnetizing force has been removed. This procedure relies on the residual or permanent magnetism in the part and is more practical than the continuous procedure when magnetization is accom-plished by flexible coils wrapped around the part.

In general, the residual procedure is used only with steels which have been heat treated for stressed applications.

IdentificationofIndicationsThe correct evaluation of the character of indications is extremely important but is sometimes difficult to make from observation of the indications alone. The principal distinguishing features of indications are shape, buildup, width, and sharpness of outline. These characteristics are more valuable in distinguishing between types of discontinuities than in determining their severity. Careful observation of the character of the magnetic particle pattern should always be included in the complete evaluation of the significance of an indicated discontinuity.

The most readily distinguished indications are those produced by cracks open to the surface. These discon-tinuities include fatigue cracks, heat treat cracks, shrink cracks in welds and castings, and grinding cracks. An example of a fatigue crack is shown in Figure 8-18.

MagnagloInspectionMagnaglo inspection is similar to the preceding method but differs in that a fluorescent particle solution is used and the inspection is made under black light. Efficiency of inspection is increased by the neon-like glow of defects allowing smaller flaw indications to be seen. This is an excellent method for use on gears, threaded parts, and aircraft engine components. The reddish brown liquid spray or bath that is used consists of Magnaglo paste mixed with a light oil at the ratio of 0.10 to 0.25 ounce of paste per gallon of oil.

After inspection, the part must be demagnetized and rinsed with a cleaning solvent.Li

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Magnetizing Equipment

Fixed(Nonportable)GeneralPurposeUnitA fixed general purpose unit is shown in Figure 8-19. This unit provides direct current for wet continuous or residual magnetization procedures. Circular or longitudinal magnetization may be used and it may be powered with rectified alternating current (ac), as well as direct current (dc). The contact heads provide the electrical terminals for circular magnetization. One head is fixed in position with its contact plate mounted on a shaft surrounded by a pressure spring, so that the plate may be moved longitudinally. The plate is maintained in the extended position by the spring until pressure transmitted through the work from the movable head forces it back.

The motor driven movable head slides horizontally in longitudinal guides and is controlled by a switch. The spring allows sufficient overrun of the motor driven head to avoid jamming it and also provides pressure on the ends of the work to ensure good electrical contact.

Figure 8-18. Fatigue crack in a landing gear.

Main gear outer cylinder

Fatigue crack

Torsion link lugs

Figure 8-19. Fixed general-purpose magnetizing unit.

Movable headAmmeter Pressure spring

Contact plate

Solenoid

Nozzle

Contact plate

Fixed head

Movable head switch

Push button

Pump switch

Rheostat

Short-circuiting switch

Solenoid switch Circulating

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A plunger operated switch in the fixed head cuts out the forward motion circuit of the movable head motor when the spring has been properly compressed.

In some units the movable head is hand operated, and the contact plate is sometimes arranged for operation by an air ram. Both contact plates are fitted with vari-ous fixtures for supporting the work.

The magnetizing circuit is closed by depressing a pushbutton on the front of the unit. It is set to open automatically, usually after about one-half second.

The strength of the magnetizing current may be set manually to the desired value by means of the rheostat or increased to the capacity of the unit by the rheostat short circuiting switch. The current utilized is indicated on the ammeter.

Longitudinal magnetization is produced by the sole-noid, which moves in the same guide rail as the mov-able head and is connected in the electrical circuit by means of a switch.

The suspension liquid is contained in a sump tank and is agitated and circulated by a pump. The suspension is applied to the work through a nozzle. The suspen-sion drains from the work through a nonmetallic grill into a collecting pan that leads back to the sump. The circulating pump is operated by a pushbutton switch.

PortableGeneralPurposeUnitIt is often necessary to perform the magnetic particle inspection at locations where fixed general purpose equipment is not available or to perform an inspection on members of aircraft structures without removing them from the aircraft. It is particularly useful for inspecting landing gear and engine mounts suspected of having developed cracks in service. Portable units supply both alternating current and direct current magnetization.

This unit is only a source of magnetizing and demag-netizing current and does not provide a means for supporting the work or applying the suspension. It operates on 200 volt, 60 cycle, alternating current and contains a rectifier for producing direct current when required. [Figure 8-20]

The magnetizing current is supplied through the flex-ible cables. The cable terminals may be fitted with prods, as shown in the illustration, or with contact clamps. Circular magnetization may be developed by using either the prods or clamps.

Longitudinal magnetization is developed by wrapping the cable around the part.

The strength of the magnetizing current is controlled by an eight point tap switch, and the time duration for which it is applied is regulated by an automatic cutoff similar to that used in the fixed general purpose unit.

This portable unit also serves as a demagnetizer and supplies high amperage low voltage alternating current for this purpose. For demagnetization, the alternat-ing current is passed through the part and gradually reduced by means of a current reducer.

In testing large structures with flat surfaces where cur-rent must be passed through the part, it is sometimes impossible to use contact clamps. In such cases, contact prods are used.

Prods can be used with the fixed general purpose unit as well as the portable unit. The part or assembly being tested may be held or secured above the standard unit and the suspension hosed onto the area; excess suspension drains into the tank. The dry procedure may also be used.

Prods should be held firmly against the surface being tested. There is a tendency for a high amperage cur-rent to cause burning at contact areas, but with proper care, such burning is usually slight. For applications where prod magnetization is acceptable, slight burning is normally acceptable.

Figure 8-20. Portable general purpose unit.

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IndicatingMediumsThe various types of indicating mediums available for magnetic particle inspection may be divided into two general material types: wet and dry. The basic require-ment for any indicating medium is that it produce acceptable indications of discontinuities in parts.

The contrast provided by a particular indicating medium on the background or part surface is particu-larly important. The colors most extensively used are black and red for the wet procedure and black, red, and gray for the dry procedure.

For acceptable operation, the indicating medium must be of high permeability and low retentivity. High per-meability ensures that a minimum of magnetic energy will be required to attract the material to flux leakage caused by discontinuities. Low retentivity ensures that the mobility of the magnetic particles will not be hindered by the particles themselves becoming mag-netized and attracting one another.

DemagnetizingThe permanent magnetism remaining after inspection must be removed by a demagnetization operation if the part is to be returned to service. Parts of operat-ing mechanisms must be demagnetized to prevent magnetized parts from attracting filings, grindings, or chips inadvertently left in the system, or steel particles resulting from operational wear.

An accumulation of such particles on a magnetized part may cause scoring of bearings or other working parts. Parts of the airframe must be demagnetized so they will not affect instruments.

Demagnetization between successive magnetizing operations is not normally required unless experience indicates that omission of this operation results in decreased effectiveness for a particular application. Demagnetization may be accomplished in a number of different ways. A convenient procedure for aircraft parts involves subjecting the part to a magnetizing force that is continually reversing in direction and, at the same time, gradually decreasing in strength. As the decreasing magnetizing force is applied first in one direction and then the other, the magnetization of the part also decreases.

StandardDemagnetizingPracticeThe simplest procedure for developing a reversing and gradually decreasing magnetizing force in a part involves the use of a solenoid coil energized by alter-nating current. As the part is moved away from the

alternating field of the solenoid, the magnetism in the part gradually decreases.

A demagnetizer whose size approximates that of the work should be used. For maximum effectiveness, small parts should be held as close to the inner wall of the coil as possible.

Parts that do not readily lose their magnetism should be passed slowly in and out of the demagnetizer sev-eral times and, at the same time, tumbled or rotated in various directions. Allowing a part to remain in the demagnetizer with the current on accomplishes very little practical demagnetization.

The effective operation in the demagnetizing procedure is that of slowly moving the part out of the coil and away from the magnetizing field strength. As the part is withdrawn, it should be kept directly opposite the opening until it is 1 or 2 feet from the demagnetizer.

The demagnetizing current should not be cut off until the part is 1 or 2 feet from the opening as the part may be remagnetized if current is removed too soon.

Another procedure used with portable units is to pass alternating current through the part being demagne-tized, while gradually reducing the current to zero.

RadiographicX and gamma radiations, because of their unique ability to penetrate material and disclose discontinuities, have been applied to the radiographic (x-ray) inspection of metal fabrications and nonmetallic products.

The penetrating radiation is projected through the part to be inspected and produces an invisible or latent image in the film. When processed, the film becomes a radiograph or shadow picture of the object. This inspection medium and portable unit provides a fast and reliable means for checking the integrity of air-frame structures and engines. [Figure 8-21]

Radiation

Void

Specimen

Film

Blackarea After processingBlack

areaWhitearea

Whitearea

Grayarea

Figure 8-21. Radiograph.

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Radiographic InspectionRadiographic inspection techniques are used to locate defects or flaws in airframe structures or engines with little or no disassembly. This is in marked contrast to other types of nondestructive testing which usually require removal, disassembly, and stripping of paint from the suspected part before it can be inspected. Due to the radiation risks associated with x-ray, extensive training is required to become a qualified radiographer. Only qualified radiographers are allowed to operate the x-ray units.

Three major steps in the x-ray process discussed in subsequent paragraphs are: (1) exposure to radiation, including preparation, (2) processing of film, and (3) interpretation of the radiograph.

PreparationandExposureThe factors of radiographic exposure are so interde-pendent that it is necessary to consider all factors for any particular radiographic exposure. These factors include but are not limited to the following:

• Material thickness and density• Shape and size of the object• Type of defect to be detected• Characteristics of x-ray machine used• The exposure distance• The exposure angle• Film characteristics• Types of intensifying screen, if used

Knowledge of the x-ray unit’s capabilities should form a background for the other exposure factors. In addition to the unit rating in kilovoltage, the size, portability, ease of manipulation, and exposure particu-lars of the available equipment should be thoroughly understood.

Previous experience on similar objects is also very helpful in the determination of the overall exposure techniques. A log or record of previous exposures will provide specific data as a guide for future radiographs.

FilmProcessingAfter exposure to x-rays, the latent image on the film is made permanently visible by processing it successively through a developer chemical solution, an acid bath, and a fixing bath, followed by a clear water wash.

RadiographicInterpretationFrom the standpoint of quality assurance, radiographic interpretation is the most important phase of radiog-raphy. It is during this phase that an error in judgment can produce disastrous consequences. The efforts of the whole radiographic process are centered in this phase; the part or structure is either accepted or rejected. Conditions of unsoundness or other defects which are overlooked, not understood, or improperly interpreted can destroy the purpose and efforts of radiography and can jeopardize the structural integrity of an entire air-craft. A particular danger is the false sense of security imparted by the acceptance of a part or structure based on improper interpretation.

As a first impression, radiographic interpretation may seem simple, but a closer analysis of the problem soon dispels this impression. The subject of interpretation is so varied and complex that it cannot be covered adequately in this type of document. Instead, this chapter gives only a brief review of basic requirements for radiographic interpretation, including some descrip-tions of common defects.

Experience has shown that, whenever possible, radio-graphic interpretation should be conducted close to the radiographic operation. When viewing radiographs, it is helpful to have access to the material being tested. The radiograph can thus be compared directly with the material being tested, and indications due to such things as surface condition or thickness variations can be immediately determined.

The following paragraphs present several factors which must be considered when analyzing a radiograph.

There are three basic categories of flaws: voids, inclu-sions, and dimensional irregularities. The last category, dimensional irregularities, is not pertinent to these discussions because its prime factor is one of degree, and radiography is not exact. Voids and inclusions may appear on the radiograph in a variety of forms ranging from a two-dimensional plane to a three-dimensional sphere. A crack, tear, or cold shut will most nearly resemble a two-dimensional plane, whereas a cavity will look like a three-dimensional sphere. Other types of flaws, such as shrink, oxide inclusions, porosity, and so forth, will fall somewhere between these two extremes of form.

It is important to analyze the geometry of a flaw, espe-cially for items such as the sharpness of terminal points. For example, in a crack-like flaw the terminal points appear much sharper in a sphere-like flaw, such as a Li

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gas cavity. Also, material strength may be adversely affected by flaw shape. A flaw having sharp points could establish a source of localized stress concentra-tion. Spherical flaws affect material strength to a far lesser degree than do sharp pointed flaws. Specifica-tions and reference standards usually stipulate that sharp pointed flaws, such as cracks, cold shuts, and so forth, are cause for rejection.

Material strength is also affected by flaw size. A metal-lic component of a given area is designed to carry a certain load plus a safety factor. Reducing this area by including a large flaw weakens the part and reduces the safety factor. Some flaws are often permitted in com-ponents because of these safety factors; in this case, the interpreter must determine the degree of tolerance or imperfection specified by the design engineer. Both flaw size and flaw shape should be considered carefully, since small flaws with sharp points can be just as bad as large flaws with no sharp points.

Another important consideration in flaw analysis is flaw location. Metallic components are subjected to numerous and varied forces during their effective ser-vice life. Generally, the distribution of these forces is not equal in the component or part, and certain critical areas may be rather highly stressed. The interpreter must pay special attention to these areas. Another aspect of flaw location is that certain types of discon-tinuities close to one another may potentially serve as a source of stress concentrations creating a situation that should be closely scrutinized.

An inclusion is a type of flaw which contains entrapped material. Such flaws may be of greater or lesser den-sity than the item being radiographed. The foregoing discussions on flaw shape, size, and location apply equally to inclusions and to voids. In addition, a flaw containing foreign material could become a source of corrosion.

RadiationHazardsRadiation from x-ray units and radioisotope sources is destructive to living tissue. It is universally recognized that in the use of such equipment, adequate protection must be provided. Personnel must keep outside the primary x-ray beam at all times.

Radiation produces changes in all matter through which it passes. This is also true of living tissue. When radia-tion strikes the molecules of the body, the effect may be no more than to dislodge a few electrons, but an excess of these changes could cause irreparable harm. When a complex organism is exposed to radiation, the degree

of damage, if any, depends on which of its body cells have been changed.

Vital organs in the center of the body that are penetrated by radiation are likely to be harmed the most. The skin usually absorbs most of the radiation and reacts earli-est to radiation.

If the whole body is exposed to a very large dose of radiation, death could result. In general, the type and severity of the pathological effects of radiation depend on the amount of radiation received at one time and the percentage of the total body exposed. Smaller doses of radiation could cause blood and intestinal disorders in a short period of time. The more delayed effects are leukemia and other cancers. Skin damage and loss of hair are also possible results of exposure to radiation.

Inspection of CompositesComposite structures should be inspected for delamina-tion, which is separation of the various plies, debonding of the skin from the core, and evidence of moisture and corrosion. Previously discussed methods including ultrasonic, acoustic emission, and radiographic inspec-tions may be used as recommended by the aircraft manufacturer. The simplest method used in testing composite structures is the tap test.

Tap TestingTap testing, also referred to as the ring test or coin test, is widely used as a quick evaluation of any acces-sible surface to detect the presence of delamination or debonding. The testing procedure consists of lightly tapping the surface with a light hammer (maximum weight of 2 ounces), a coin or other suitable device. The acoustic response or “ring” is compared to that of a known good area. A “flat” or “dead” response indicates an area of concern. Tap testing is limited to finding defects in relatively thin skins, less than 0.080" thick. On honeycomb structures, both sides need to be tested. Tap testing on only one side would not detect debonding on the opposite side.

Electrical ConductivityComposite structures are not inherently electrically conductive. Some aircraft, because of their relatively low speed and type of use, are not affected by electrical issues. Manufacturers of other aircraft, such as high-speed high-performance jets, are required to utilize various methods of incorporating aluminum into their structures to make them conductive. The aluminum is imbedded within the plies of the lay-ups either as a Li

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thin wire mesh, screen, foil, or spray. When damaged sections of the structure are repaired, care must be taken to ensure that the conductive path be restored. Not only is it necessary to include the conductive material in the repair, but the continuity of the electrical path from the original conductive material to the replacement conductor and back to the original must be maintained. Electrical conductivity may be checked by use of an ohmmeter. Specific manufacturer’s instructions must be carefully followed.

Inspection of WeldsA discussion of welds in this chapter will be confined to judging the quality of completed welds by visual means. Although the appearance of the completed weld is not a positive indication of quality, it provides a good clue about the care used in making it.

A properly designed joint weld is stronger than the base metal which it joins. The characteristics of a properly welded joint are discussed in the following paragraphs.

A good weld is uniform in width; the ripples are even and well feathered into the base metal, which shows no burn due to overheating. [Figure 8-22] The weld has good penetration and is free of gas pockets, porosity, or inclusions. The edges of the bead illustrated in Figure 8-22 (B) are not in a straight line, yet the weld is good since penetration is excellent.

Penetration is the depth of fusion in a weld. Thorough fusion is the most important characteristic contributing to a sound weld. Penetration is affected by the thickness of the material to be joined, the size of the filler rod, and how it is added. In a butt weld, the penetration should be 100 percent of the thickness of the base metal. On a fillet weld, the penetration requirements are 25 to 50 percent of the thickness of the base metal. The width and depth of bead for a butt weld and fillet weld are shown in Figure 8-23.

To assist further in determining the quality of a welded joint, several examples of incorrect welds are discussed in the following paragraphs.

The weld shown in Figure 8-24 (A) was made too rapidly. The long and pointed appearance of the

A B

Figure 8-22. Examples of good welds.

Leg 2 to 3 T

25 to 50% T

Throat 1 1⁄3 to 1 1⁄2 TBead width

3 to 5 T

100%Penetration

A B

Reinforcement 1⁄4 to 1⁄2 T

T

Approx. 1⁄2 T

Figure 8-23. (A) Butt weld and (B) fillet weld, showing width and depth of bead.

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ripples was caused by an excessive amount of heat or an oxidizing flame. If the weld were cross-sectioned, it would probably disclose gas pockets, porosity, and slag inclusions.

Figure 8-24 (B) illustrates a weld that has improper penetration and cold laps caused by insufficient heat. It appears rough and irregular, and its edges are not feathered into the base metal.

A

C D

B

Figure 8-24. Examples of poor welds.

The puddle has a tendency to boil during the welding operation if an excessive amount of acetylene is used. This often leaves slight bumps along the center and craters at the finish of the weld. Cross-checks are appar-ent if the body of the weld is sound. If the weld were cross-sectioned, pockets and porosity would be visible. Such a condition is shown in Figure 8-24 (C).

A bad weld with irregular edges and considerable varia-tion in the depth of penetration is shown in D of Figure 8-24. It often has the appearance of a cold weld.

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Liquid Penetrant Testing Page 1 of 20

Liquid Penetrant Testing

Liquid penetrant testing is one of the oldest and simplists NDT methods where its earliest versions (using kerosene and oil mixture) dates back to the 19th century. This method is used to reveal surface discontinuities by bleedout of a colored or fluorescent dye from the flaw. The technique is based on the ability of a liquid to be drawn into a "clean" surface discontinuity by capillary action. After a period of time called the "dwell time", excess surface penetrant is removed and a developer applied. This acts as a blotter that draws the penetrant from the discontinuity to reveal its presence.

The advantage that a liquid penetrant inspection offers over an unaided visual inspection is that it makes defects easier to see for the inspector where that is done in two ways:

It produces a flaw indication that is much larger and easier for the eye to detect than the flaw itself. Many flaws are so small or narrow that they are undetectable by the unaided eye (a person with a perfect vision can not resolve features smaller than 0.08 mm).

It improves the detectability of a flaw due to the high level of contrast between the indication and the background which helps to make the indication more easily seen (such as a red indication on a white background for visable penetrant or a penetrant that glows under ultraviolate light for flourecent penetrant).

Liquid penetrant testing is one of the most widely used NDT methods. Its popularity can be attributed to two main factors: its relative ease of use and its flexibility. It can be used to inspect almost any material provided that its surface is not extremely rough or porous. Materials that are commonly inspected using this method include; metals, glass, many ceramic materials, rubber and plastics.

However, liquid penetrant testing can only be used to inspect for flaws that break the surface of the sample (such as surface cracks, porosity, laps, seams, lack of fusion, etc.).

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Steps of Liquid Penetrant Testing

The exact procedure for liquid penetrant testing can vary from case to case depending on several factors such as the penetrant system being used, the size and material of the component being inspected, the type of discontinuities being expected in the component and the condition and environment under which the inspection is performed. However, the general steps can be summarized as follows:

1. Surface Preparation: One of the most critical steps of a liquid penetrant testing is the surface preparation. The surface must be free of oil, grease, water, or other contaminants that may prevent penetrant from entering flaws. The sample may also require etching if mechanical operations such as machining, sanding, or grit blasting have been performed. These and other mechanical operations can smear metal over the flaw opening and prevent the penetrant from entering.

2. Penetrant Application: Once the surface has been thoroughly cleaned and dried, the penetrant material is applied by spraying, brushing, or immersing the part in a penetrant bath.

3. Penetrant Dwell: The penetrant is left on the surface for a sufficient time to allow as much penetrant as possible to be drawn from or to seep into a defect. Penetrant dwell time is the total time that the penetrant is in contact with the part surface. Dwell times are usually recommended by the penetrant producers or required by the specification being followed. The times vary depending on the application, penetrant materials used, the material, the form of the material being inspected, and the type of discontinuity being inspected for. Minimum dwell times typically range from five to 60 minutes. Generally, there is no harm in using a longer penetrant dwell time as long as the penetrant is not allowed to dry. The ideal dwell time is often determined by experimentation and may be very specific to a particular application.

4. Excess Penetrant Removal: This is the most delicate part of the inspection procedure because the excess penetrant must be removed from the surface of the sample while removing as little penetrant as possible from defects.

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Depending on the penetrant system used, this step may involve cleaning with a solvent, direct rinsing with water, or first treating the part with an emulsifier and then rinsing with water.

5. Developer Application: A thin layer of developer is then applied to the sample to draw penetrant trapped in flaws back to the surface where it will be visible. Developers come in a variety of forms that may be applied by dusting (dry powders), dipping, or spraying (wet developers).

6. Indication Development: The developer is allowed to stand on the part surface for a period of time sufficient to permit the extraction of the trapped penetrant out of any surface flaws. This development time is usually a minimum of 10 minutes. Significantly longer times may be necessary for tight cracks.

7. Inspection: Inspection is then performed under appropriate lighting to detect indications from any flaws which may be present.

8. Clean Surface: The final step in the process is to thoroughly clean the part surface to remove the developer from the parts that were found to be acceptable.

Advantages and Disadvantages

The primary advantages and disadvantages when compared to other NDT methods are:

Advantages

High sensitivity (small discontinuities can be detected).

Few material limitations (metallic and nonmetallic, magnetic and nonmagnetic, and conductive and nonconductive materials may be inspected).

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Rapid inspection of large areas and volumes.

Suitable for parts with complex shapes.

Indications are produced directly on the surface of the part and constitute a visual representation of the flaw.

Portable (materials are available in aerosol spray cans)

Low cost (materials and associated equipment are relatively inexpensive)

Disadvantages

Only surface breaking defects can be detected.

Only materials with a relatively nonporous surface can be inspected.

Pre-cleaning is critical since contaminants can mask defects.

Metal smearing from machining, grinding, and grit or vapor blasting must be removed.

The inspector must have direct access to the surface being inspected.

Surface finish and roughness can affect inspection sensitivity.

Multiple process operations must be performed and controlled.

Post cleaning of acceptable parts or materials is required.

Chemical handling and proper disposal is required.

Penetrants

Penetrants are carefully formulated to produce the level of sensitivity desired by the inspector. The penetrant must possess a number of important characteristics:

- spread easily over the surface of the material being inspected to provide complete and even coverage.

- be drawn into surface breaking defects by capillary action. - remain in the defect but remove easily from the surface of the part. - remain fluid so it can be drawn back to the surface of the part through the

drying and developing steps. - be highly visible or fluoresce brightly to produce easy to see indications. - not be harmful to the material being tested or the inspector.

Penetrant materials are not designed to perform the same. Penetrant manufactures have developed different formulations to address a variety of inspection applications. Some applications call for the detection of the smallest defects possible while in other

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applications, the rejectable defect size may be larger. The penetrants that are used to detect the smallest defect will also produce the largest amount of irrelevant indications.

Standard specifications classify penetrant materials according to their physical characteristics and their performance.

Penetrant materials come in two basic types:

Type 1 - Fluorescent Penetrants: they contain a dye or several dyes that fluoresce when exposed to ultraviolet radiation.

Type 2 - Visible Penetrants: they contain a red dye that provides high contrast against the white developer background.

Fluorescent penetrant systems are more sensitive than visible penetrant systems because the eye is drawn to the glow of the fluorescing indication. However, visible penetrants do not require a darkened area and an ultraviolet light in order to make an inspection.

Penetrants are then classified by the method used to remove the excess penetrant from the part. The four methods are:

Method A - Water Washable: penetrants can be removed from the part by rinsing with water alone. These penetrants contain an emulsifying agent (detergent) that makes it possible to wash the penetrant from the part surface with water alone. Water washable penetrants are sometimes referred to as self-emulsifying systems.

Method B - Post-Emulsifiable, Lipophilic: the penetrant is oil soluble and interacts with the oil-based emulsifier to make removal possible.

Method C - Solvent Removable: they require the use of a solvent to remove the penetrant from the part.

Method D - Post-Emulsifiable, Hydrophilic: they use an emulsifier that is a water soluble detergent which lifts the excess penetrant from the surface of the part with a water wash.

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Penetrants are then classified based on the strength or detectability of the indication that is produced for a number of very small and tight fatigue cracks. The five sensitivity levels are:

Level ½ - Ultra Low Sensitivity

Level 1 - Low Sensitivity

Level 2 - Medium Sensitivity

Level 3 - High Sensitivity

Level 4 - Ultra-High Sensitivity

The procedure for classifying penetrants into one of the five sensitivity levels uses specimens with small surface fatigue cracks. The brightness of the indication produced is measured using a photometer.

Developers

The role of the developer is to pull the trapped penetrant material out of defects and spread it out on the surface of the part so it can be seen by an inspector. Developers used with visible penetrants create a white background so there is a greater degree of contrast between the indication and the surrounding background. On the other hand, developers used with fluorescent penetrants both reflect and refract the incident ultraviolet light, allowing more of it to interact with the penetrant, causing more efficient fluorescence.

According to standards, developers are classified based on the method that the developer is applied (as a dry powder, or dissolved or suspended in a liquid carrier). The six standard forms of developers are:

Form a - Dry Powder

Form b - Water Soluble

Form c - Water Suspendable

Form d - Nonaqueous Type 1: Fluorescent (Solvent Based)

Form e - Nonaqueous Type 2: Visible Dye (Solvent Based)

Form f - Special Applications

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Dry Powder

Dry powder developers are generally considered to be the least sensitive but they are inexpensive to use and easy to apply. Dry developers are white, fluffy powders that can be applied to a thoroughly dry surface in a number of ways; by dipping parts in a container of developer, by using a puffer to dust parts with the developer, or placing parts in a dust cabinet where the developer is blown around. Since the powder only sticks to areas of indications since they are wet, powder developers are seldom used for visible inspections.

Water Soluble

As the name implies, water soluble developers consist of a group of chemicals that are dissolved in water and form a developer layer when the water is evaporated away. The best method for applying water soluble developers is by spraying it on the part. The part can be wet or dry. Dipping, pouring, or brushing the solution on to the surface is sometimes used but these methods are less desirable. Drying is achieved by placing the wet but well drained part in a recirculating, warm air dryer with the temperature 21°C. Properly developed parts will have an even, pale white coating over the entire surface.

Water Suspendable

Water suspendable developers consist of insoluble developer particles suspended in water. Water suspendable developers require frequent stirring or agitation to keep the particles from settling out of suspension. Water suspendable developers are applied to parts in the same manner as water soluble developers then the parts are dried using warm air.

Nonaqueous

Nonaqueous developers suspend the developer in a volatile solvent and are typically applied with a spray gun. Nonaqueous developers are commonly distributed in aerosol spray cans for portability. The solvent tends to pull penetrant from the indications by solvent action. Since the solvent is highly volatile, forced drying is not required.

Special Applications

Plastic or lacquer developers are special developers that are primarily used when a permanent record of the inspection is required.

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Preparation of Part

One of the most critical steps in the penetrant inspection process is preparing the part for inspection. All coatings, such as paints, varnishes, plating, and heavy oxides must be removed to ensure that defects are open to the surface of the part. If the parts have been machined, sanded, or blasted prior to the penetrant inspection, it is possible that a thin layer of metal may have smeared across the surface and closed off defects. Also, some cleaning operations, such as steam cleaning, can cause metal smearing in softer materials. This layer of metal smearing must be removed before inspection.

Penetrant Application and Dwell Time

The penetrant material can be applied in a number of different ways, including spraying, brushing, or immersing the parts in a penetrant bath. Once the part is covered in penetrant it must be allowed to dwell so the penetrant has time to enter any defect that

is present.

There are basically two dwell mode options:

- Immersion-dwell: keeping the part immersed in the penetrant during the dwell period.

- Drain-dwell: letting the part drain during the dwell period (this method gives better sensitivity).

Penetrant Dwell Time

Penetrant dwell time is the total time that the penetrant is in contact with the part surface. The dwell time is important because it allows the penetrant the time necessary to seep or be drawn into a defect. Dwell times are usually recommended by the penetrant producers or required by the specification being followed. The time required to fill a flaw depends on a number of variables which include:

The surface tension of the penetrant.

The contact angle of the penetrant.

The dynamic shear viscosity of the penetrant.

The atmospheric pressure at the flaw opening.

The capillary pressure at the flaw opening.

The pressure of the gas trapped in the flaw by the penetrant.

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The radius of the flaw or the distance between the flaw walls.

The density or specific gravity of the penetrant.

Microstructural properties of the penetrant.

The ideal dwell time is often determined by experimentation and is often very specific to a particular application. For example, the table shows the dwell time requirements for steel parts according to some of the commonly used specifications.

Penetrant Removal Process

The penetrant removal procedure must effectively remove the penetrant from the surface of the part without removing an appreciable amount of entrapped penetrant from the discontinuity. If the removal process extracts penetrant from the flaw, the flaw indication will be reduced by a proportional amount. If the penetrant is not effectively removed from the part surface, the contrast between the indication and the background will be reduced.

Removal Method

As mentioned previously, penetrant systems are classified into four types according to the method used for excess penetrant removal.

- Method A: Water-Washable - Method B: Post-Emulsifiable, Lipophilic - Method C: Solvent Removable

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- Method D: Post-Emulsifiable, Hydrophilic

Method C, Solvent Removable, is used primarily for inspecting small localized areas. This method requires hand wiping the surface with a cloth moistened with the solvent remover, and is, therefore, too labor intensive for most production situations.

Method A, Water-Washable, is the most economical to apply of the different methods and it is easy to use. Water-washable or self-emulsifiable penetrants contain an emulsifier as an integral part of the formulation. The excess penetrant may be removed from the object surface with a simple water rinse.

When removal of the penetrant from the defect due to over-washing of the part is a concern, a post-emulsifiable penetrant system can be used. The post-emulsifiable methods are generally only used when very high sensitivity is needed. Post-emulsifiable penetrants require a separate emulsifier to breakdown the penetrant and make it water washable. The part is usually immersed in the emulsifier but hydrophilic emulsifiers may also be sprayed on the object. Brushing the emulsifier on to the part is not recommended either because the bristles of the brush may force emulsifier into discontinuities, causing the entrapped penetrant to be removed. The emulsifier is allowed sufficient time to react with the penetrant on the surface of the part but not given time to make its way into defects to react with the trapped penetrant. Controlling the reaction time is of essential importance when using a post-emulsifiable system. If the emulsification time is too short, an excessive amount of penetrant will be left on the surface, leading to high background levels. If the emulsification time is too long, the emulsifier will react with the penetrant entrapped in discontinuities, making it possible to deplete the amount needed to form an indication.

The hydrophilic post-emulsifiable method (Method D) is more sensitive than the lipophilic post-emulsifiable method (Method B). The major advantage of hydrophilic emulsifiers is that they are less sensitive to variation in the contact and removal time.

When using an emulsifiable penetrant is used, the penetrant inspection process includes the following steps (extra steps are underlined): 1. pre-clean part, 2. apply penetrant and allow to dwell, 3. pre-rinse to remove first layer of penetrant, 4. apply hydrophilic emulsifier and allow contact for specified time, 5. rinse to remove excess penetrant, 6. dry part, 7. apply developer and allow part to develop, and 8. inspect.

Rinse Method and Time for Water-Washable Penetrants

The method used to rinse the excess penetrant from the object surface and the time of the rinse should be controlled so as to prevent over-washing. It is generally

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recommended that a coarse spray rinse or an air-agitated, immersion wash tank be used. When a spray is being used, it should be directed at a 45° angle to the part surface so as to not force water directly into any discontinuities that may be present. The spray or immersion time should be kept to a minimum through frequent inspections of the remaining background level.

Hand Wiping of Solvent Removable Penetrants

When a solvent removable penetrant is used, care must also be taken to carefully remove the penetrant from the part surface while removing as little as possible from the flaw. The first step in this cleaning procedure is to dry wipe the surface of the part in one direction using a white, lint-free, cotton rag. One dry pass in one direction is all that should be used to remove as much penetrant as possible. Next, the surface should be wiped with one pass in one direction with a rag moistened with cleaner. One dry pass followed by one damp pass is all that is recommended. Additional wiping may sometimes be necessary; but keep in mind that with every additional wipe, some of the entrapped penetrant will be removed and inspection sensitivity will be reduced.

Use and Selection of a Developer

The use of developer is almost always recommended. The output from a fluorescent penetrant is improved significantly when a suitable powder developer is used. Also, the use of developer can have a dramatic effect on the probability of detection of an inspection.

Nonaqueous developers are generally recognized as the most sensitive when properly applied. However, if the thickness of the coating becomes too great, defects can be masked. The relative sensitivities of developers and application techniques as ranked in Volume II of the Nondestructive Testing Handbook are shown in the table below.

Ranking 1 2 3 4 5 6 7 8 9 10

Developer Form Nonaqueous, Wet Solvent

Plastic Film Water-Soluble

Water-Suspendable Water-Soluble

Water-Suspendable Dry Dry Dry Dry

Method of Application Spray Spray Spray Spray

Immersion Immersion

Dust Cloud (Electrostatic) Fluidized Bed

Dust Cloud (Air Agitation) Immersion (Dip)

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The following table lists the main advantages and disadvantages of the various developer types.

Developer Advantages Disadvantages

Dry Indications tend to remain brighter and more distinct over time

Easily to apply

Does not form contrast background so cannot be used with visible systems

Difficult to assure entire part surface has been coated

Soluble Ease of coating entire part

White coating for good contrast can be produced which work well for both visible and fluorescent systems

Coating is translucent and provides poor contrast (not recommended for visual systems)

Indications for water washable systems are dim and blurred

Suspendable Ease of coating entire part

Indications are bright and sharp


Indications weaken and become diffused after time

Nonaqueous Very portable

Easy to apply to readily accessible surfaces


Indications show-up rapidly and are well defined

Provides highest sensitivity

Difficult to apply evenly to all surfaces

More difficult to clean part after inspection

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Quality & Process Control

Quality control of the penetrant inspection process is essential to get good and consistent results. Since several steps and materials are involved in the inspection process, there are quality control procedures for each of them.

Temperature Control

The temperature of the penetrant materials and the part being inspected can have an effect on the results. Temperatures from 27 to 49°C are reported in the literature to produce optimal results. Many specifications allow testing in the range of 4 to 52°C. Raising the temperature beyond this level will significantly raise the speed of evaporation of penetrants causing them to dry out quickly.

Since the surface tension of most materials decrease as the temperature increases, raising the temperature of the penetrant will increase the wetting of the surface and the capillary forces. Of course, the opposite is also true, so lowering the temperature will have a negative effect on the flow characteristics.

Penetrant Quality Control

The quality of a penetrant inspection is highly dependent on the quality of the penetrant materials used. Only products meeting the requirements of an industry specification, such as AMS 2644, should be used. Deterioration of new penetrants primarily results from aging and contamination. Virtually all organic dyes deteriorate over time, resulting in a loss of color or fluorescent response, but deterioration can be slowed with proper storage. When possible, keep the materials in a closed container and protect from freezing and exposure to high heat.

Contamination can occur during storage and use. Of course, open tank systems are much more susceptible to contamination than are spray systems. Regular checks must be performed to ensure that the material performance has not degraded. When the penetrant is first received from the manufacturer, a sample of the fresh solution should be collected and stored as a standard for future comparison. The standard specimen should be stored in a sealed, opaque glass or metal container. Penetrants that are in-use should be compared regularly to the standard specimen to detect any changes in properties or performance.

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Dwell Quality Control

Dwell times are usually recommended by the penetrant producer or required by the specification being followed. The only real quality control required in the dwell step of the process is to ensure that a minimum dwell time is reached. There is no harm in allowing a penetrant to dwell longer than the minimum time as long as the penetrant is not allowed to dry on the part.

Emulsifier Path Quality Control

Quality control of the emulsifier bath is important and it should be performed per the requirements of the applicable specification.

Lipophilic Emulsifiers

Lipophilic emulsifiers mix with penetrants but when the concentration of penetrant contamination in the emulsifier becomes too great, the mixture will not function effectively as a remover. Standards require that lipophilic emulsifiers be capable of 20% penetrant contamination without a reduction in performance. When the cleaning action of the emulsifier becomes less than that of new material, it should be replaced.

Hydrophilic Emulsifiers

Hydrophilic emulsifiers have less tolerance for penetrant contamination. The penetrant tolerance varies with emulsifier concentration and the type of contaminating penetrant. In some cases, as little as 1% (by volume) penetrant contamination can seriously affect the performance of an emulsifier.

Emulsifier Concentration and Contact Time

The optimal emulsifier contact time is dependent on a number of variables that include the emulsifier used, the emulsifier concentration, the surface roughness of the part being inspected, and other factors. Usually some experimentation is required to select the proper emulsifier contact time.

Wash Quality Control

The wash temperature, pressure and time are three parameters that are typically

controlled in penetrant inspection process specification. A coarse spray or an

immersion wash tank with air agitation is often used. When the spray method is used,

the water pressure is usually limited to 276 kPa. The temperature range of the water is

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usually specified as a wide range (e.g., 10 to 38°C). The wash time should only be as

long as necessary to decrease the background to an acceptable level. Frequent visual

checks of the part should be made to determine when the part has been adequately

rinsed.

Drying Process Quality Control

The temperature used to dry parts after the application of an aqueous wet developer or prior to the application of a dry powder or a nonaqueous wet developer, must be controlled to prevent drying in the penetrant in the flaw. To prevent harming the penetrant material, drying temperature should be kept to less than 71°C. Also, the drying time should be limited to the minimum length necessary to thoroughly dry the component being inspected.

Developer Quality Control

The function of the developer is very important in a penetrant inspection. In order to accomplish its functions, a developer must adhere to the part surface and result in a uniform, highly porous layer with many paths for the penetrant to be moved due to capillary action. Developers are either applied wet or dry, but the desired end result is always a uniform, highly porous, surface layer. Since the quality control requirements for each of the developer types is slightly different, they will be covered individually.

Dry Powder Developer

A dry powder developer should be checked daily to ensure that it is fluffy and not caked. It should be similar to fresh powdered sugar and not granulated like powdered soap. It should also be relatively free from specks of fluorescent penetrant material from previous inspection. This check is performed by spreading a sample of the developer out and examining it under UV light.

When using the developer, a light coat is applied by immersing the test component or dusting the surface. After the development time, excessive powder can be removed by gently blowing on the surface with air not exceeding 35 kPa.

Wet Soluble/Suspendable Developer

Wet soluble developer must be completely dissolved in the water and wet suspendable developer must be thoroughly mixed prior to application. The concentration of powder in the carrier solution must be controlled in these developers.

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The concentration should be checked at least weekly using a hydrometer to make sure it meets the manufacturer's specification. To check for contamination, the solution should be examined weekly using both white light and UV light. Some specifications require that a clean aluminum panel be dipped in the developer, dried, and examined for indications of contamination by fluorescent penetrant materials.

These developers are applied by spraying, flowing or immersing the component. They should never be applied with a brush. Care should be taken to avoid a heavy accumulation of the developer solution in crevices and recesses.

Solvent Suspendable

Solvent suspendable developers are typically supplied in sealed aerosol spray cans. Since the developer solution is in a sealed vessel, direct check of the solution is not possible. However, the way that the developer is dispensed must be monitored. The spray developer should produce a fine, even coating on the surface of the part. Make sure the can is well shaken and apply a thin coating to a test article. If the spray produces spatters or an uneven coating, the can should be discarded.

When applying a solvent suspendable developer, it is up to the inspector to control the thickness of the coating. With a visible penetrant system, the developer coating must be thick enough to provide a white contrasting background but not heavy enough to mask indications. When using a fluorescent penetrant system, a very light coating should be used. The developer should be applied under white light and should appear evenly transparent.

Development Time

Parts should be allowed to develop for a minimum of 10 minutes and no more than 2 hours before inspecting.

Lighting Quality Control

Proper lighting is of great importance when visually inspecting a surface for a penetrant indication. Obviously, the lighting requirements are different for an inspection conducted using a visible dye penetrant than they are for an inspection conducted using a fluorescent dye penetrant.

Lighting for Visible Dye Penetrant Inspections

When using a visible penetrant, the intensity of the white light is of principal importance. Inspections can be conducted using natural lighting or artificial lighting.

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However, since natural daylight changes from time to time, the use of artificial lighting is recommended to get better uniformity. Artificial lighting should be white whenever possible (halogen lamps are most commonly used). The light intensity is required to be 100 foot-candles at the surface being inspected.

Lighting for Fluorescent Penetrant Inspections

Fluorescent penetrant dyes are excited by UV light of 365nm wavelength and emit visible light somewhere in the green-yellow range between 520 and 580nm. The source of ultraviolet light is often a mercury arc lamp with a filter. The lamps emit many wavelengths and a filter is used to remove all but the UV and a small amount of visible light between 310 and 410nm. Visible light of wavelengths above 410nm interferes with contrast, and UV emissions below 310nm include some hazardous wavelengths.

Standards and procedures require verification of filter condition and light intensity. The black light filter should be clean and the light should never be used with a cracked filter. Most UV light must be warmed up prior to use and should be on for at least 15 minutes before beginning an inspection. Since fluorescent brightness is linear with respect to ultraviolet excitation, a change in the intensity of the light (from age or damage) and a change in the distance of the light source from the surface being inspected will have a direct impact on the inspection. For UV lights used in component evaluations, the normally accepted intensity is 1000 µW/cm2 at 38cm distance from the filter face. The required check should be performed when a new bulb is installed, at startup of the inspection cycle, if a change in intensity is noticed, or every eight hours of continuous use.

When performing a fluorescent penetrant inspection, it is important to keep white light to a minimum as it will significantly reduce the inspector’s ability to detect fluorescent indications. Light levels of less than 2 foot-candles are required by most procedures. When checking black light intensity a reading of the white light produced by the black light may be required to verify white light is being removed by the filter.

Light Measurement

Light intensity measurements are made using a radiometer (an instrument that transfers light energy into an electrical current). Some radiometers have the ability to measure both black and white light, while others require a separate sensor for each measurement. Whichever type is used, the sensing area should be clean and free of any materials that could reduce or obstruct light reaching the sensor. Radiometers are

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relatively unstable instruments and readings often change considerable over time. Therefore, they should be calibrated at least every six months.

System Performance Check

A system performance check is typically required daily, at the reactivation of a system after maintenance or repairs, or any time the system is suspected of being out of control. System performance checks involve processing a test specimen with known defects to determine if the process will reveal discontinuities of the size required. The specimen must be processed following the same procedure used to process production parts. The ideal specimen is a production item that has natural defects of the minimum acceptable size. As with penetrant inspections in general, results are directly dependent on the skill of the operator and, therefore, each operator should process a test specimen.

There are some universal test specimens that can be used if a standard part is not available. The most commonly used test specimen is the TAM or PSM panel which is used for fluorescent penetrant systems. These panels are usually made of stainless steel that has been chrome plated on one half and surfaced finished on the other half to produce the desired roughness. The chrome plated section is impacted from the back side to produce a starburst set of cracks in the chrome. There are five impacted areas with a range of different crack sizes corresponding to the five levels of sensitivity.

Care of system performance check specimens is critical. Specimens should be handled carefully to avoid damage. They should be cleaned thoroughly between uses and storage in a solvent is generally recommended. Before processing a specimen, it should be inspected under UV light to make sure that it is clean and not already producing an indication.

Nature of the Defect

The nature of the defect can have a large effect on sensitivity of a liquid penetrant

inspection. Sensitivity is defined as the smallest defect that can be detected with a high

degree of reliability. Typically, the crack length at the sample surface is used to define

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size of the defect. However, the crack length alone does not determine whether a flaw

will be seen or go undetected. The volume of the defect is likely to be the more

important feature. The flaw must be of sufficient volume so that enough penetrant will

bleed back out to a size that is detectable by the eye or that will satisfy the

dimensional thresholds of fluorescence. The figure shows an example of fluorescent

penetrant inspection probability of detection (POD) curve as a function of crack length.

In general, penetrant testing is more effective at finding:

Small round defects than small linear defects.

Deeper flaws than shallow flaws.

Flaws with a narrow opening at the surface than wide open flaws.

Flaws on smooth surfaces than on rough surfaces.

Flaws with rough fracture surfaces than smooth fracture surfaces.

Flaws under tensile or no loading than flaws under compression loading.

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Health and Safety Precautions

When proper health and safety precautions are followed, liquid penetrant inspection operations can be completed without harm to inspection personnel. However, there is a number of health and safety related issues that need to be taken in consideration. The most common of those are discussed here.

Chemical Safety

Whenever chemicals must be handled, certain precautions must be taken. Before working with a chemical of any kind, it is highly recommended that the material safety data sheets (MSDS) be reviewed so that proper chemical safety and hygiene practices can be followed. Some of the penetrant materials are flammable and, therefore, should be used and stored in small quantities. They should only be used in a well ventilated area and ignition sources avoided. Eye protection should always be worn to prevent contact of the chemicals with the eyes. Gloves and other protective clothing should be worn to limit contact with the chemicals.

Ultraviolet Light Safety

Ultraviolet (UV) light has wavelengths ranging from 180 to 400 nanometers. These wavelengths place UV light in the invisible part of the electromagnetic spectrum between visible light and X-rays. The most familiar source of UV radiation is the sun and is necessary in small doses for certain chemical processes to occur in the body. However, too much exposure can be harmful to the skin and eyes. The greatest threat with UV light exposure is that the individual is generally unaware that the damage is occurring. There is usually no pain associated with the injury until several hours after the exposure. Skin and eye damage occurs at wavelengths around 320 nm and shorter which is well below the 365 nm wavelength, where penetrants are designed to fluoresce. Therefore, UV lamps sold for use in penetrant testing are almost always filtered to remove the harmful UV wavelengths. The lamps produce radiation at the harmful wavelengths so it is essential that they be used with the proper filter in place and in good condition.

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MAGNETIC PARTICLE TESTING

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Introduction

• This module is intended to present information on the widely used method of magnetic particle inspection.

• Magnetic particle inspection can detect both production discontinuities (seams, laps, grinding cracks and quenching cracks) and in-service damage (fatigue and overload cracks).

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Outline

• Magnetism and Ferromagnetic Materials

• Introduction of Magnetic Particle Inspection

• Basic Procedure and Important Considerations

1. Component pre-cleaning 2. Introduction of magnetic field 3. Application of magnetic media 4. Interpretation of magnetic particle indications

• Examples of MPI Indications

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Magnetic lines of force

around a bar magnet

Opposite poles attracting Similar poles repelling

Introduction to Magnetism

Magnetism is the ability of matter to attract other matter to itself. Objects that possess the property of magnetism are said to be magnetic or magnetized and magnetic lines of force can be found in and around the objects. A magnetic pole is a point where the a magnetic line of force exits or enters a material.

Magnetic field lines: • Form complete loops. • Do not cross. • Follow the path of least

resistance. • All have the same strength. • Have a direction such that

they cause poles to attract or repel.

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How Does Magnetic Particle

Inspection Work?

A ferromagnetic test specimen is magnetized with a strong magnetic field created by a magnet or special equipment. If the specimen has a discontinuity, the discontinuity will interrupt the magnetic field flowing through the specimen and a leakage field will occur.

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How Does Magnetic Particle

Inspection Work? (Cont.)

Finely milled iron particles coated with a dye pigment are applied to the test specimen. These particles are attracted to leakage fields and will cluster to form an indication directly over the discontinuity. This indication can be visually detected under proper lighting conditions.

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Basic Procedure

Basic steps involved:

1. Component pre-cleaning

2. Introduction of magnetic field

3. Application of magnetic media

4. Interpretation of magnetic particle indications

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Pre-cleaning

When inspecting a test part with the magnetic particle method it is essential for the particles to have an unimpeded path for migration to both strong and weak leakage fields alike. The part’s surface should be clean and dry before inspection.

Contaminants such as oil, grease, or scale may not only prevent particles from being attracted to leakage fields, they may also interfere with interpretation of indications.

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Introduction of the Magnetic Field

The required magnetic field can be introduced into a component in a number of different ways.

1. Using a permanent magnet or an electromagnet that contacts the test piece

2. Flowing an electrical current through the specimen

3. Flowing an electrical current through a coil of wire around the part or through a central conductor running near the part.

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Direction of the Magnetic Field

Two general types of magnetic fields (longitudinal and circular) may be established within the specimen. The type of magnetic field established is determined by the method used to magnetize the specimen.

• A longitudinal magnetic field has magnetic lines of force that run parallel to the long axis of the part.

• A circular magnetic field has magnetic lines of force that run circumferentially around the perimeter of a part. Librar

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Importance of Magnetic Field Direction

Being able to magnetize the part in two directions is important because the best detection of defects occurs when the lines of magnetic force are established at right angles to the longest dimension of the defect. This orientation creates the largest disruption of the magnetic field within the part and the greatest flux leakage at the surface of the part. An orientation of 45 to 90 degrees between the magnetic field and the defect is necessary to form an indication.

Since defects may occur in various and unknown directions, each part is normally magnetized in two directions at right angles to each other.

Flux Leakage

No Flux Leakage

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Question

? From the previous slide regarding the optimum test sensitivity, which kinds of defect are easily found in the images below?

Longitudinal (along the axis) Transverse (perpendicular the axis)

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Producing a Longitudinal Magnetic Field

Using a Coil

A longitudinal magnetic field is usually established by placing the part near the inside or a coil’s annulus. This produces magnetic lines of force that are parallel to the long axis of the test part.

Coil on Wet Horizontal Inspection Unit

Portable Coil

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Producing a Longitudinal Field Using

Permanent or Electromagnetic Magnets

Permanent magnets and electromagnetic yokes are also often used to produce a longitudinal magnetic field. The magnetic lines of force run from one pole to the other, and the poles are positioned such that any flaws present run normal to these lines of force.

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Circular Magnetic Fields

Circular magnetic fields are produced by passing current through the part or by placing the part in a strong circular magnet field.

A headshot on a wet horizontal test unit and the use of prods are several common methods of injecting current in a part to produce a circular magnetic field. Placing parts on a central conductors carrying high current is another way to produce the field.

Magnetic Field

Electric

Current

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Application of Magnetic Media (Wet Versus Dry)

MPI can be performed using either dry particles, or particles suspended in a liquid.

With the dry method, the particles are lightly dusted on to the surface. With the wet method, the part is flooded with a solution carrying the particles.

The dry method is more portable. The wet method is generally more sensitive since the liquid carrier gives the magnetic particles additional mobility. Librar

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Dry Magnetic Particles

Magnetic particles come in a variety of colors. A

color that produces a high level of contrast against the background should be used.

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Wet Magnetic Particles

Wet particles are typically supplied as visible or fluorescent.

Visible particles are viewed under normal white light and;

fluorescent particles are viewed under black light.

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Interpretation of Indications

After applying the magnetic field, indications that

form must interpreted. This process requires that the inspector distinguish between relevant and non-relevant indications.

The following series of images depict relevant indications produced from a variety of components inspected with the magnetic particle method.

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Crane Hook with

Service Induced Crack

Fluorescent, Wet Particle Method

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Gear with



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Drive Shaft with

Heat Treatment Induced Cracks


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Splined Shaft with

Service Induced Cracks


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Threaded Shaft with



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Large Bolt with



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Crank Shaft with

Service Induced Crack Near Lube Hole


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Lack of Fusion in SMAW Weld

Visible, Dry Powder Method

Indication

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Toe Crack in SMAW Weld

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Throat and Toe Cracks in

Partially Ground Weld

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Demagnetization

• Parts inspected by the magnetic particle method may sometimes have an objectionable residual magnetic field that may interfere with subsequent manufacturing operations or service of the component.

• Possible reasons for demagnetization include:

– May interfere with welding and/or machining operations

– Can effect gauges that are sensitive to magnetic fields if placed in close proximity.

– Abrasive particles may adhere to components surface and cause and increase in wear to engines components, gears, bearings etc.

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Demagnetization (Cont.)

• Demagnetization requires that the residual magnetic field is reversed and reduced by the inspector.

• This process will scramble the magnetic domains and reduce the strength of the residual field to an acceptable level.

Demagnetized Magnetized

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Advantages of

Magnetic Particle Inspection

• Can detect both surface and near sub-surface defects.

• Can inspect parts with irregular shapes easily.

• Pre-cleaning of components is not as critical as it is for some other inspection methods. Most contaminants within a flaw will not hinder flaw detectability.

• Fast method of inspection and indications are visible directly on the specimen surface.

• Considered low cost compared to many other NDT methods.

• Is a very portable inspection method especially when used with battery powered equipment.

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Limitations of

Magnetic Particle Inspection

• Cannot inspect non-ferrous materials such as aluminum, magnesium or most stainless steels.

• Inspection of large parts may require use of equipment with special power requirements.

• Some parts may require removal of coating or plating to achieve desired inspection sensitivity.

• Limited subsurface discontinuity detection capabilities. Maximum depth sensitivity is approximately 0.6” (under ideal conditions).

• Post cleaning, and post demagnetization is often necessary.

• Alignment (angle) between magnetic flux and defect is important.

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Glossary of Terms

• Black Light: ultraviolet light which is filtered to produce a wavelength of approximately 365 nanometers.

Black light will cause certain materials to fluoresce.

• Central conductor: an electrically conductive bar usually made of copper used to introduce a circular magnetic field in to a test specimen.

• Coil: an electrical conductor such a copper wire or cable that is wrapped in several or many loops that are brought close to one another to form a strong longitudinal magnetic field.

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Glossary of Terms

• Discontinuity: an interruption in the structure of the material such as a crack.

• Ferromagnetic: a material such as iron, nickel and cobalt or one of it’s alloys that is strongly attracted to a magnetic field.

• Heads: electrical contact pads on a wet horizontal magnetic particle inspection machine. The part to be inspected is clamped and held in place between the heads and shot of current is sent through the part from the heads to create a circular magnetic field in the part.

• Leakage field: a disruption in the magnetic field. This disruption must extend to the surface of the part for particles to be attracted.

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Glossary of Terms

• Non-relevant indications: indications produced due to some intended design feature of a specimen such

• a keyways, splines or press fits.

• Prods: two electrodes usually made of copper or aluminum that are used to introduce current in to a test part. This current in turn creates a circular magnetic field where each prod touches the part. (Similar in principal to a welding electrode and ground clamp).

• Relevant indications: indications produced from something other than a design feature of a test specimen. Cracks, stringers, or laps are examples of relevant indications.

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Glossary of Terms

• Suspension: a bath created by mixing particles with either oil or water.

• Yoke: a horseshoe magnet used to create a longitudinal magnetic field.

• Yokes may be made from permanent magnets or electromagnets.

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Magnetic Particle Testing Page 1 of 34

Magnetic Particle Testing

Magnetic particle testing is one of the most widely utilized NDT methods since it is fast and relatively easy to apply and part surface preparation is not as critical as it is for some other methods. This mithod uses magnetic fields and small magnetic particles (i.e.iron filings) to detect flaws in components. The only requirement from an inspectability standpoint is that the component being inspected must be made of a ferromagnetic material (a materials that can be magnetized) such as iron, nickel, cobalt, or some of their alloys.

The method is used to inspect a variety of product forms including castings, forgings, and weldments. Many different industries use magnetic particle inspection such as structural steel, automotive, petrochemical, power generation, and aerospace industries. Underwater inspection is another area where magnetic particle inspection may be used to test items such as offshore structures and underwater pipelines.

Basic Principles

In theory, magnetic particle testing has a relatively simple concept. It can be considered as a combination of two nondestructive testing methods: magnetic flux leakage testing and visual testing. For the case of a bar magnet, the magnetic field is in and around the magnet. Any place that a magnetic line of force exits or enters the magnet is called a “pole” (magnetic lines of force exit the magnet from north pole and enter from the south pole).

When a bar magnet is broken in the center of its length, two complete bar magnets with magnetic poles on each end of each piece will result. If the magnet is just cracked but not broken completely in two, a north and south pole will form at each edge of the crack. The magnetic field exits the north pole and reenters at the south pole. The magnetic field spreads out when it encounters the small air gap created by the crack because the air cannot support as much magnetic field per unit volume as the magnet can. When the field spreads out, it appears to leak out of the material and, thus is called a flux leakage field.

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If iron particles are sprinkled on a cracked magnet, the particles will be attracted to and cluster not only at the poles at the ends of the magnet, but also at the poles at the edges of the crack. This cluster of particles is much easier to see than the actual crack and this is the basis for magnetic particle inspection.

The first step in a magnetic particle testing is to magnetize the component that is to be inspected. If any defects on or near the surface are present, the defects will create a leakage field. After the component has been magnetized, iron particles, either in a dry or wet suspended form, are applied to the surface of the magnetized part. The particles will be attracted and cluster at the flux leakage fields, thus forming a visible indication that the inspector can detect.



Advantages

High sensitivity (small discontinuities can be detected).

Indications are produced directly on the surface of the part and constitute a visual representation of the flaw.

Minimal surface preparation (no need for paint removal)

Portable (small portable equipment & materials available in spray cans)

Low cost (materials and associated equipment are relatively inexpensive)

Disadvantages

Only surface and near surface defects can be detected.

Only applicable to ferromagnetic materials.

Relatively small area can be inspected at a time.

Only materials with a relatively nonporous surface can be inspected.

The inspector must have direct access to the surface being inspected.

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Magnetism

The concept of magnetism centers around the magnetic field and what is known as a dipole. The term "magnetic field" simply describes a volume of space where there is a change in energy within that volume. The location where a magnetic field exits or enters a material is called a magnetic pole. Magnetic poles have never been detected in isolation but always occur in pairs, hence the name dipole. Therefore, a dipole is an object that has a magnetic pole on one end and a second, equal but opposite, magnetic pole on the other. A bar magnet is a dipole with a north pole at one end and south pole at the other.

The source of magnetism lies in the basic building block of all matter, the atom. Atoms are composed of protons, neutrons and electrons. The protons and neutrons are located in the atom's nucleus and the electrons are in constant motion around the nucleus. Electrons carry a negative electrical charge and produce a magnetic field as they move through space. A magnetic field is produced whenever an electrical charge is in motion. The strength of this field is called the magnetic moment.

When an electric current flows through a conductor, the movement of electrons through the conductor causes a magnetic field to form around the conductor. The magnetic field can be detected using a compass. Since all matter is comprised of atoms, all materials are affected in some way by a magnetic field; however, materials do not react the same way to the magnetic field.

Reaction of Materials to Magnetic Field

When a material is placed within a magnetic field, the magnetic forces of the material's electrons will be affected. This effect is known as Faraday's Law of Magnetic Induction. However, materials can react quite differently to the presence of an external magnetic field. The magnetic moments associated with atoms have three origins: the electron motion, the change in motion caused by an external magnetic field, and the spin of the electrons.

In most atoms, electrons occur in pairs where these pairs spin in opposite directions. The opposite spin directions of electron pairs cause their magnetic fields to cancel each other. Therefore, no net magnetic field exists. Alternately, materials with some unpaired

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electrons will have a net magnetic field and will react more to an external field.

According to their interaction with a magnetic field, materials can be classified as:

Diamagnetic materials which have a weak, negative susceptibility to magnetic fields. Diamagnetic materials are slightly repelled by a magnetic field and the material does not retain the magnetic properties when the external field is removed. In diamagnetic materials all the electrons are paired so there is no permanent net magnetic moment per atom. Most elements in the periodic table, including copper, silver, and gold, are diamagnetic.

Paramagnetic materials which have a small, positive susceptibility to magnetic fields. These materials are slightly attracted by a magnetic field and the material does not retain the magnetic properties when the external field is removed. Paramagnetic materials have some unpaired electrons. Examples of paramagnetic materials include magnesium, molybdenum, and lithium.

Ferromagnetic materials have a large, positive susceptibility to an external magnetic field. They exhibit a strong attraction to magnetic fields and are able to retain their magnetic properties after the external field has been removed. Ferromagnetic materials have some unpaired electrons so their atoms have a net magnetic moment. They get their strong magnetic properties due to the presence of magnetic domains. In these domains, large numbers of atom's moments are aligned parallel so that the magnetic force within the domain is strong (this happens during the solidification of the material where the atom moments are aligned within each crystal ”i.e., grain” causing a strong magnetic force in one direction). When a ferromagnetic material is in the unmagnetized state, the domains are nearly randomly organized (since the crystals are in arbitrary directions) and the net magnetic field for the part as a whole is zero. When a magnetizing force is applied, the domains become aligned to produce a strong magnetic field within the part. Iron, nickel, and cobalt are examples of ferromagnetic materials. Components made of these materials are commonly inspected using the magnetic particle method.

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Magnetic Field Characteristics

Magnetic Field In and Around a Bar Magnet

The magnetic field surrounding a bar magnet can be seen in the magnetograph below. A magnetograph can be created by placing a piece of paper over a magnet and sprinkling the paper with iron filings. The particles align themselves with the lines of magnetic force produced by the magnet. It can be seen in the magnetograph that there are poles all along the length of the magnet but that the poles are concentrated at the ends of the magnet (the north and south poles).

Magnetic Fields in and around Horseshoe and Ring Magnets

Magnets come in a variety of shapes and one of the more common is the horseshoe (U) magnet. The horseshoe magnet has north and south poles just like a bar magnet but the magnet is curved so the poles lie in the same plane. The magnetic lines of force flow from pole to pole just like in the bar magnet. However, since the poles are located closer together and a more direct path exists for the lines of flux to travel between the poles, the magnetic field is concentrated between the poles.

General Properties of Magnetic Lines of Force

Magnetic lines of force have a number of important properties, which include:

They seek the path of least resistance between opposite magnetic poles (in a single bar magnet shown, they attempt to form closed loops from pole to pole).

They never cross one another.

They all have the same strength.

Their density decreases with increasing distance from the poles.

Their density decreases (they spread out) when they move from an area of higher permeability to an area of lower permeability.

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They are considered to have direction as if flowing, though no actual movement occurs.

They flow from the south pole to the north pole within a material and north pole to south pole in air.

Electromagnetic Fields

Magnets are not the only source of magnetic fields. The flow of electric current through a conductor generates a magnetic field. When electric current flows in a long straight wire, a circular magnetic field is generated around the wire and the intensity of this magnetic field is directly proportional to the amount of current carried by the wire. The strength of the field is strongest next to the wire and diminishes with distance. In most conductors, the magnetic field exists only as long as the current is flowing. However, in ferromagnetic materials the electric current will cause some or all of the magnetic domains to align and a residual magnetic field will remain.

Also, the direction of the magnetic field is dependent on the direction of the electrical current in the wire. The direction of the magnetic field around a conductor can be determined using a simple rule called the “right-hand clasp rule”. If a person grasps a conductor in one's right hand with the thumb pointing in the direction of the current, the fingers will circle the conductor in the direction of the magnetic field.

Note: remember that current flows from the positive terminal to the negative terminal (electrons flow in the opposite direction).

Magnetic Field Produced by a Coil

When a current carrying wire is formed into several loops to form a coil, the magnetic field circling each loop combines with the fields from the other loops to produce a concentrated field through the center of the coil (the field flows along the longitudinal axis and circles back around the outside of the coil).

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When the coil loops are tightly wound, a uniform magnetic field is developed throughout the length of the coil. The strength of the magnetic field increases not only with increasing current but also with each loop that is added to the coil. A long, straight coil of wire is called a solenoid and it can be used to generate a nearly uniform magnetic field similar to that of a bar magnet. The concentrated magnetic field inside a coil is very useful in magnetizing ferromagnetic materials for inspection using the magnetic particle testing method.

Quantifying Magnetic Properties

The various characteristics of magnetism can be measured and expressed quantitatively. Different systems of units can be used for quantifying magnetic properties. SI units will be used in this material. The advantage of using SI units is that they are traceable back to an agreed set of four base units; meter, kilogram, second, and Ampere.

The unit for magnetic field strength H is ampere/meter (A/m). A magnetic field strength of 1 A/m is produced at the center of a single circular conductor with a 1 meter diameter carrying a steady current of 1 ampere.

The number of magnetic lines of force cutting through a plane of a given area at a right angle is known as the magnetic flux density, B. The flux density or magnetic induction has the Tesla as its unit. One Tesla is equal to 1 Newton/(A/m). From these units, it can be seen that the flux density is a measure of the force applied to a particle by the magnetic field.

The total number of lines of magnetic force in a material is called magnetic flux, ɸ. The strength of the flux is determined by the number of magnetic domains that are aligned within a material. The total flux is simply the flux density applied over an area. Flux carries the unit of a weber, which is simply a Tesla-meter2.

The magnetization M is a measure of the extent to which an object is magnetized. It is a measure of the magnetic dipole moment per unit volume of the object. Magnetization carries the same units as a magnetic field A/m.

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Quantity SI Units (Sommerfeld)

SI Units (Kennelly)

CGS Units (Gaussian)

Field (Magnetization Force)

H A/m A/m oersteds

Flux Density (Magnetic Induction)

B Tesla Tesla gauss

Flux ɸ Weber Weber maxwell Magnetization M A/m - erg/Oe-cm3

The Hysteresis Loop and Magnetic Properties

A great deal of information can be learned about the magnetic properties of a material by studying its hysteresis loop. A hysteresis loop shows the relationship between the induced magnetic flux density (B) and the magnetizing force (H). It is often referred to as the B-H loop. An example hysteresis loop is shown below.

The loop is generated by measuring the magnetic flux of a ferromagnetic material while the magnetizing force is changed. A ferromagnetic material that has never been previously magnetized or has been thoroughly demagnetized will follow the dashed line as H is increased. As the line demonstrates, the greater the amount of current applied (H+), the stronger the magnetic field in the component (B+). At point "a"

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almost all of the magnetic domains are aligned and an additional increase in the magnetizing force will produce very little increase in magnetic flux. The material has reached the point of magnetic saturation. When H is reduced to zero, the curve will move from point "a" to point "b". At this point, it can be seen that some magnetic flux remains in the material even though the magnetizing force is zero. This is referred to as the point of retentivity on the graph and indicates the level of residual magnetism in the material (Some of the magnetic domains remain aligned but some have lost their alignment). As the magnetizing force is reversed, the curve moves to point "c", where the flux has been reduced to zero. This is called the point of coercivity on the curve (the reversed magnetizing force has flipped enough of the domains so that the net flux within the material is zero). The force required to remove the residual magnetism from the material is called the coercive force or coercivity of the material.

As the magnetizing force is increased in the negative direction, the material will again become magnetically saturated but in the opposite direction, point "d". Reducing H to zero brings the curve to point "e". It will have a level of residual magnetism equal to that achieved in the other direction. Increasing H back in the positive direction will return B to zero. Notice that the curve did not return to the origin of the graph because some force is required to remove the residual magnetism. The curve will take a different path from point "f" back to the saturation point where it with complete the loop.

From the hysteresis loop, a number of primary magnetic properties of a material can be determined:

1. Retentivity - A measure of the residual flux density corresponding to the saturation induction of a magnetic material. In other words, it is a material's ability to retain a certain amount of residual magnetic field when the magnetizing force is removed after achieving saturation (The value of B at point b on the hysteresis curve).

2. Residual Magnetism or Residual Flux - The magnetic flux density that remains in a material when the magnetizing force is zero. Note that residual magnetism and retentivity are the same when the material has been magnetized to the saturation point. However, the level of residual magnetism may be lower than the retentivity value when the magnetizing force did not reach the saturation level.

3. Coercive Force - The amount of reverse magnetic field which must be applied to a magnetic material to make the magnetic flux return to zero (The value of H at point c on the hysteresis curve).

4. Permeability, µ - A property of a material that describes the ease with which a magnetic flux is established in the material.

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5. Reluctance - Is the opposition that a ferromagnetic material shows to the establishment of a magnetic field. Reluctance is analogous to the resistance in an electrical circuit.

Permeability

As previously mentioned, permeability (µ) is a material property that describes the ease with which a magnetic flux is established in a component. It is the ratio of the flux density (B) created within a material to the magnetizing field (H) and is represented by the following equation:

µ = B/H

This equation describes the slope of the curve at any point on the hysteresis loop. The permeability value given in letrature for materials is usually the maximum permeability or the maximum relative permeability. The maximum permeability is the point where the slope of the B/H curve for the unmagnetized material is the greatest. This point is often taken as the point where a straight line from the origin is tangent to the B/H curve.

The shape of the hysteresis loop tells a great deal about the material being magnetized. The hysteresis curves of two different materials are shown in the graph.

Relative to other materials, a material with a wider hysteresis loop has: - Lower Permeability - Higher Retentivity - Higher Coercivity - Higher Reluctance - Higher Residual Magnetism

Relative to other materials, a material with a narrower hysteresis loop has: - Higher Permeability - Lower Retentivity - Lower Coercivity - Lower Reluctance

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- Lower Residual Magnetism

In magnetic particle testing, the level of residual magnetism is important. Residual magnetic fields are affected by the permeability, which can be related to the carbon content and alloying of the material. A component with high carbon content will have low permeability and will retain more magnetic flux than a material with low carbon content.

Magnetic Field Orientation and Flaw Detectability

To properly inspect a component for cracks or other defects, it is important to understand that the orientation of the crack relative to the magnetic lines of force determinies if the crack can or cannot be detected. There are two general types of magnetic fields that can be established within a component.

A longitudinal magnetic field has magnetic lines of force that run parallel to the long axis of the part. Longitudinal magnetization of a component can be accomplished using the longitudinal field set up by a coil or solenoid. It can also be accomplished using permanent magnets or electromagnets.

A circular magnetic field has magnetic lines of force that run circumferentially around the perimeter of a part. A circular magnetic field is induced in an article by either passing current through the component or by passing current through a conductor surrounded by the component.

The type of magnetic field established is determined by the method used to magnetize the specimen. Being able to magnetize the part in two directions is important because the best detection of defects occurs when the lines of magnetic force are established at right angles to the longest dimension of the defect. This orientation creates the largest disruption of the magnetic field within the part and the greatest flux leakage at the surface of the part. If the magnetic field is parallel to the defect, the field will see little disruption and no flux leakage field will be produced. Libr

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An orientation of 45 to 90 degrees between the magnetic field and the defect is necessary to form an indication. Since defects may occur in various and unknown directions, each part is normally magnetized in two directions at right angles to each other. If the component shown is considered, it is known that passing current through the part from end to end will establish a circular magnetic field that will be 90 degrees to the direction of the current. Therefore, defects that have a significant dimension in the direction of the current (longitudinal defects) should be detectable, while transverse-type defects will not be detectable with circular magnetization.

Magnetization of Ferromagnetic Materials

There are a variety of methods that can be used to establish a magnetic field in a component for evaluation using magnetic particle inspection. It is common to classify the magnetizing methods as either direct or indirect.

Magnetization Using Direct Induction (Direct Magnetization)

With direct magnetization, current is passed directly through the component. The flow of current causes a circular magnetic field to form in and around the conductor. When using the direct magnetization method, care must be taken to ensure that good electrical contact is established and maintained between the test equipment and the test component to avoid damage of the the component (due to arcing or overheating at high resistance ponts).

There are several ways that direct magnetization is commonly accomplished.

- One way involves clamping the component between two electrical contacts in a special piece of equipment. Current is passed through the component and a circular magnetic field is established in and around the component. When the magnetizing current is stopped, a residual magnetic field will remain within the component. The strength of the induced magnetic field is proportional to the amount of current passed through the component.

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- A second technique involves using clamps or prods, which are attached or placed in contact with the component. Electrical current flows through the component from contact to contact. The current sets up a circular magnetic field around the path of the current.

Magnetization Using Indirect Induction (Indirect Magnetization)

Indirect magnetization is accomplished by using a strong external magnetic field to establish a magnetic field within the component. As with direct magnetization, there are several ways that indirect magnetization can be accomplished.

- The use of permanent magnets is a low cost method of establishing a magnetic field. However, their use is limited due to lack of control of the field strength and the difficulty of placing and removing strong permanent magnets from the component.

- Electromagnets in the form of an adjustable horseshoe magnet (called a yoke) eliminate the problems associated with permanent magnets and are used extensively in industry. Electromagnets only exhibit a magnetic flux when electric current is flowing around the soft iron core. When the magnet is placed on the component, a magnetic field is established between the north and south poles of the magnet.

- Another way of indirectly inducting a magnetic field in a material is by using the magnetic field of a current carrying conductor. A circular magnetic field can be established in cylindrical components by using a central conductor. Typically, one or more cylindrical components are hung from a solid copper bar running through the inside diameter. Current is passed through the copper bar and the resulting circular magnetic field establishes a magnetic field within the test components.

- The use of coils and solenoids is a third method of indirect magnetization. When the length of a component is several times larger than its diameter, a longitudinal

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magnetic field can be established in the component. The component is placed longitudinally in the concentrated magnetic field that fills the center of a coil or solenoid. This magnetization technique is often referred to as a "coil shot".

Types of Magnetizing Current

As mentioned previously, electric current is often used to establish the magnetic field in components during magnetic particle inspection. Alternating current (AC) and direct current (DC) are the two basic types of current commonly used. The type of current used can have an effect on the inspection results, so the types of currents commonly used are briefly discussed here.

Direct Current

Direct current (DC) flows continuously in one direction at a constant voltage. A battery is the most common source of direct current. The current is said to flow from the positive to the negative terminal, though electrons flow in the opposite direction. DC is very desirable when inspecting for subsurface defects because DC generates a magnetic field that penetrates deeper into the material. In ferromagnetic materials, the magnetic field produced by DC generally penetrates the entire cross-section of the component.

Alternating Current

Alternating current (AC) reverses in direction at a rate of 50 or 60 cycles per second. Since AC is readily available in most facilities, it is convenient to make use of it for magnetic particle inspection. However, when AC is used to induce a magnetic field in ferromagnetic materials, the magnetic field will be limited to a thin layer at the surface of the component. This phenomenon is known as the "skin effect" and it occurs because the changing magnetic field generates eddy currents in the test object. The eddy currents produce a magnetic field that opposes the primary field, thus reducing the net magnetic flux below the surface. Therefore, it is recommended that AC be used only when the inspection is limited to surface defects.

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Rectified Alternating Current

Clearly, the skin effect limits the use of AC since many inspection applications call for the detection of subsurface defects. Luckily, AC can be converted to current that is very much like DC through the process of rectification. With the use of rectifiers, the reversing AC can be converted to a one directional current. The three commonly used types of rectified current are described below.

Half Wave Rectified Alternating Current (HWAC)

When single phase alternating current is passed through a rectifier, current is allowed to flow in only one direction. The reverse half of each cycle is blocked out so that a one directional, pulsating current is produced. The current rises from zero to a maximum and then returns to zero. No current flows during the time when the reverse cycle is blocked out. The HWAC repeats at same rate as the unrectified current (50 or 60 Hz). Since half of the current is blocked out, the amperage is half of the unaltered AC. This type of current is often referred to as half wave DC or pulsating DC. The pulsation of the HWAC helps in forming magnetic particle indications by vibrating the particles and giving them added mobility where that is especially important when using dry particles. HWAC is most often used to power electromagnetic yokes.

Full Wave Rectified Alternating Current (FWAC) (Single Phase)

Full wave rectification inverts the negative current to positive current rather than blocking it out. This produces a pulsating DC with no interval between the pulses. Filtering is usually performed to soften the sharp polarity switching in the rectified

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current. While particle mobility is not as good as half-wave AC due to the reduction in pulsation, the depth of the subsurface magnetic field is improved.

Three Phase Full Wave Rectified Alternating Current

Three phase current is often used to power industrial equipment because it has more favorable power transmission and line loading characteristics. This type of electrical current is also highly desirable for magnetic particle testing because when it is rectified and filtered, the resulting current very closely resembles direct current. Stationary magnetic particle equipment wired with three phase AC will usually have the ability to magnetize with AC or DC (three phase full wave rectified), providing the inspector with the advantages of each current form.

Magnetic Fields Distribution and Intensity

Longitudinal Fields

When a long component is magnetized using a solenoid having a shorter length, only the material within the solenoid and about the same length on each side of the solenoid will be strongly magnetized. This occurs because the magnetizing force diminishes with increasing distance from the solenoid. Therefore, a long component must be magnetized and inspected at several locations along its length for complete inspection coverage.

Circular Fields

When a circular magnetic field is forms in and around a conductor due to the passage of electric current through it, the following can be said about the distribution and intensity of the magnetic field:

- The field strength varies from zero at the center of the component to a maximum at the surface.

- The field strength at the surface of the conductor decreases as the radius of the conductor increases (when the current strength is held constant).

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- The field strength inside the conductor is dependent on the current strength, magnetic permeability of the material, and if magnetic, the location on the B-H curve.

- The field strength outside the conductor is directly proportional to the current strength and it decreases with distance from the conductor.

The images below show the magnetic field strength graphed versus distance from the center of the conductor when current passes through a solid circular conductor.

In a nonmagnetic conductor carrying DC, the internal field strength rises from zero at the center to a maximum value at the surface of the conductor.

In a magnetic conductor carrying DC, the field strength within the conductor is much greater than it is in the nonmagnetic conductor. This is due to the permeability of the magnetic material. The external field is exactly the same for the two materials provided the current level and conductor radius are the same.

When the magnetic conductor is carrying AC, the internal magnetic field will be

concentrated in a thin layer near the surface of the conductor (skin effect). The

external field decreases with increasing distance from the surface same as with DC.

The magnetic field distribution in and around a solid conductor of a nonmagnetic material carrying direct current.

The magnetic field distribution in and around a solid conductor of a magnetic material carrying direct current.

The magnetic field distribution in and around a solid conductor of a magnetic material carrying alternating current.

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In a hollow circular conductor there is no magnetic field in the void area. The magnetic field is zero at the inner surface and rises until it reaches a maximum at the outer surface.

Same as with a solid conductor, when DC current is passed through a magnetic conductor, the field strength within the conductor is much greater than in nonmagnetic conductor due to the permeability of the magnetic material. The external field strength decreases with distance from the surface of the conductor. The external field is exactly the same for the two materials provided the current level and conductor radius are the same.

When AC current is passed through a hollow circular magnetic conductor, the skin effect concentrates the magnetic field at the outside diameter of the component.

The magnetic field distribution in and around a hollow conductor of a nonmagnetic material carrying direct current.

The magnetic field distribution in and around a hollow conductor of a magnetic material carrying direct current.

The magnetic field distribution in and around a hollow conductor of a magnetic material carrying alternating current.

As can be seen from these three field distribution images, the field strength at the inside surface of hollow conductor is very low when a circular magnetic field is established by direct magnetization. Therefore, the direct method of magnetization is not recommended when inspecting the inside diameter wall of a hollow component for shallow defects (if the defect has significant depth, it may be detectable using DC since the field strength increases rapidly as one moves from the inner towards the outer surface).

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A much better method of magnetizing hollow components for inspection of the ID and OD surfaces is with the use of a central conductor. As can be seen in the field distribution image, when current is passed through a nonmagnetic central conductor (copper bar), the magnetic field produced on the inside diameter surface of a magnetic tube is much greater and the field is still strong enough for defect detection on the OD surface.

Demagnetization

After conducting a magnetic particle inspection, it is usually necessary to demagnetize the component. Remanent magnetic fields can:

- affect machining by causing cuttings to cling to a component. - interfere with electronic equipment such as a compass. - create a condition known as "arc blow" in the welding process. Arc blow may

cause the weld arc to wonder or filler metal to be repelled from the weld. - cause abrasive particles to cling to bearing or faying surfaces and increase wear.

Removal of a field may be accomplished in several ways. The most effective way to demagnetize a material is by heating the material above its curie temperature (for instance, the curie temperature for a low carbon steel is 770°C). When steel is heated above its curie temperature then it is cooled back down, the the orientation of the magnetic domains of the individual grains will become randomized again and thus the component will contain no residual magnetic field. The material should also be placed with it long axis in an east-west orientation to avoid any influence of the Earth's magnetic field.

However, it is often inconvenient to heat a material above its curie temperature to demagnetize it, so another method that returns the material to a nearly unmagnetized state is commonly used.

Subjecting the component to a reversing and decreasing magnetic field will return the dipoles to a nearly random orientation throughout the material. This can be accomplished by pulling a component out and away from a coil with AC passing

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through it. With AC Yokes, demagnetization of local areas may be accomplished by placing the yoke contacts on the surface, moving them in circular patterns around the area, and slowly withdrawing the yoke while the current is applied. Also, many stationary magnetic particle inspection units come with a demagnetization feature that slowly reduces the AC in a coil in which the component is placed.

A field meter is often used to verify that the residual flux has been removed from a component. Industry standards usually require that the magnetic flux be reduced to less than 3 Gauss (3x10-4 Tesla) after completing a magnetic particle inspection.

Measuring Magnetic Fields

When performing a magnetic particle inspection, it is very important to be able to determine the direction and intensity of the magnetic field. The field intensity must be high enough to cause an indication to form, but not too high to cause nonrelevant indications to mask relevant indications. Also, after magnetic inspection it is often needed to measure the level of residual magnetezm.

Since it is impractical to measure the actual field strength within the material, all the devices measure the magnetic field that is outside of the material. The two devices commonly used for quantitative measurement of magnetic fields n magnetic particle inspection are the field indicator and the Hall-effect meter, which is also called a gauss meter.

Field Indicators

Field indicators are small mechanical devices that utilize a soft iron vane that is deflected by a magnetic field. The vane is attached to a needle that rotates and moves the pointer for the scale. Field indicators can be adjusted and calibrated so that quantitative information can be obtained. However, the measurement range of field indicators is usually small due to the mechanics of the device (the one shown in the image has a range from plus 20 to minus 20 Gauss). This limited range makes them best suited for measuring the residual magnetic field after demagnetization.

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Hall-Effect (Gauss/Tesla) Meter

A Hall-effect meter is an electronic device that provides a digital readout of the magnetic field strength in Gauss or Tesla units. The meter uses a very small conductor or semiconductor element at the tip of the probe. Electric current is passed through the conductor. In a magnetic field, a force is exerted on the moving electrons which tends to push them to one side of the conductor. A buildup of charge at the sides of the conductors will balance this magnetic influence, producing a measurable voltage between the two sides of the conductor. The probe is placed in the magnetic field such that the magnetic lines of force intersect the major dimensions of the sensing element at a right angle.

Magnetization Equipment for Magnetic Particle Testing

To properly inspect a part for cracks or other defects, it is important to become familiar with the different types of magnetic fields and the equipment used to generate them. As discussed previously, one of the primary requirements for detecting a defect in a ferromagnetic material is that the magnetic field induced in the part must intercept the defect at a 45 to 90 degree angle. Flaws that are normal (90 degrees) to the magnetic field will produce the strongest indications because they disrupt more of the magnet flux. Therefore, for proper inspection of a component, it is important to be able to establish a magnetic field in at least two directions.

A variety of equipment exists to establish the magnetic field for magnetic particle testing. One way to classify equipment is based on its portability. Some equipment is designed to be portable so that inspections can be made in the field and some is designed to be stationary for ease of inspection in the laboratory or manufacturing facility.

Portable Equipment

Permanent Magnets

Permanent magnets can be used for magnetic particle inspection as the source of magnetism (bar magnets or horseshoe magnets). The use of industrial magnets is not popular because they are very strong (they require significant strength to remove them

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from the surface, about 250 N for some magnets) and thus they are difficult and sometimes dangerous to handle. However, permanent magnets are sometimes used by divers for inspection in underwater environments or other areas, such as explosive environments, where electromagnets cannot be used. Permanent magnets can also be made small enough to fit into tight areas where electromagnets might not fit.

Electromagnetic Yokes

An electromagnetic yoke is a very common piece of equipment that is used to establish a magnetic field. A switch is included in the electrical circuit so that the current and, therefore, the magnetic field can be turned on and off. They can be powered with AC from a wall socket or by DC from a battery pack. This type of magnet generates a very strong magnetic field in a local area where the poles of the magnet touch the part being inspected. Some yokes can lift weights in excess of 40 pounds.

Prods

Prods are handheld electrodes that are pressed against the surface of the component being inspected to make contact for passing electrical current (AC or DC) through the metal. Prods are typically made from copper and have an insulated handle to help protect the operator. One of the prods has a trigger switch so that the current can be quickly and easily turned on and off. Sometimes the two prods are connected by any insulator, as shown in the image, to facilitate one hand operation. This is referred to as a dual prod and is commonly used for weld inspections.

However, caution is required when using prods because electrical arcing can occur and cause damage to the component if proper contact is not maintained between the prods and the component surface. For this reason, the use of prods is not allowed when inspecting aerospace and other critical components. To help prevent arcing, the

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prod tips should be inspected frequently to ensure that they are not oxidized, covered with scale or other contaminant, or damaged.

Portable Coils and Conductive Cables

Coils and conductive cables are used to establish a longitudinal magnetic field within a component. When a preformed coil is used, the component is placed against the inside surface on the coil. Coils typically have three or five turns of a copper cable within the molded frame. A foot switch is often used to energize the coil.

Also, flexible conductive cables can be wrapped around a component to form a coil. The number of wraps is determined by the magnetizing force needed and of course, the length of the cable. Normally, the wraps are kept as close together as possible. When using a coil or cable wrapped into a coil, amperage is usually expressed in ampere-turns. Ampere-turns is the amperage shown on the amp meter times the number of turns in the coil.

Portable Power Supplies

Portable power supplies are used to provide the necessary electricity to the prods, coils or cables. Power supplies are commercially available in a variety of sizes. Small power supplies generally provide up to 1,500A of half-wave DC or AC. They are small and light enough to be carried and operate on either 120V or 240V electrical service.

When more power is necessary, mobile power supplies can be used. These units come with wheels so that they can be rolled where needed. These units also operate on 120V or 240V electrical service and can provide up to 6,000A of AC or half-wave DC.

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Stationery Equipment

Stationary magnetic particle inspection equipment is designed for use in laboratory or production environment. The most common stationary system is the wet horizontal (bench) unit. Wet horizontal units are designed to allow for batch inspections of a variety of components. The units have head and tail stocks (similar to a lathe) with electrical contact that the part can be clamped between. A circular magnetic field is produced with direct magnetization.

Most units also have a movable coil that can be moved into place so the indirect magnetization can be used to produce a longitudinal magnetic field. Most coils have five turns and can be obtained in a variety of sizes. The wet magnetic particle solution is collected and held in a tank. A pump and hose system is used to apply the particle solution to the components being inspected. Some of the systems offer a variety of options in electrical current used for magnetizing the component (AC, half wave DC, or full wave DC). In some units, a demagnetization feature is built in, which uses the coil and decaying AC.

Magnetic Field Indicators

Determining whether a magnetic field is of adequate strength and in the proper direction is critical when performing magnetic particle testing. There is actually no easy-to-apply method that permits an exact measurement of field intensity at a given point within a material. Cutting a small slot or hole into the material and measuring the leakage field that crosses the air gap with a Hall-effect meter is probably the best way to get an estimate of the actual field strength within a part. However, since that is not practical, there are a number of tools and methods that are used to determine the presence and direction of the field surrounding a component.

Hall-Effect Meter (Gauss Meter)

As discussed earlier, a Gauss meter is commonly used to measure the tangential field strength on the surface of the part. By placing the probe next to the surface, the meter

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measures the intensity of the field in the air adjacent to the component when a magnetic field is applied. The advantages of this device are: it provides a quantitative measure of the strength of magnetizing force tangential to the surface of a test piece, it can be used for measurement of residual magnetic fields, and it can be used repetitively. The main disadvantage is that such devices must be periodically calibrated.

Quantitative Quality Indicator (QQI)

The Quantitative Quality Indicator (QQI) or Artificial Flaw Standard is often the preferred method of assuring proper field direction and adequate field strength (it is used with the wet method only). The QQI is a thin strip (0.05 or 0.1 mm thick) of AISI 1005 steel with a specific pattern, such as concentric circles or a plus sign, etched on it. The QQI is placed directly on the surface, with the itched side facing the surface, and it is usually fixed to the surface using a tape then the component is then magnetized and particles applied. When the field strength is adequate, the particles will adhere over the engraved pattern and provide information about the field direction.

Pie Gage

The pie gage is a disk of highly permeable material divided into four, six, or eight sections by non-ferromagnetic material (such as copper). The divisions serve as artificial defects that radiate out in different directions from the center. The sections are furnace brazed and copper plated. The gage is placed on the test piece copper side up and the test piece is magnetized. After particles are applied and the excess removed, the indications provide the inspector the orientation of the magnetic field. Pie gages are mainly used on flat surfaces such as weldments or steel castings where dry powder is used with a yoke or prods. The pie gage is not recommended for precision parts with complex shapes, for wet-method applications, or for proving field magnitude. The gage should be demagnetized between readings.

Slotted Strips

Slotted strips are pieces of highly permeable ferromagnetic material with slots of different widths. These strips can be used with the wet or dry method. They are placed

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on the test object as it is inspected. The indications produced on the strips give the inspector a general idea of the field strength in a particular area.

Magnetic Particles

As mentioned previously, the particles that are used for magnetic particle inspection are a key ingredient as they form the indications that alert the inspector to the presence of defects. Particles start out as tiny milled pieces of iron or iron oxide. A pigment (somewhat like paint) is bonded to their surfaces to give the particles color. The metal used for the particles has high magnetic permeability and low retentivity. High magnetic permeability is important because it makes the particles attract easily to small magnetic leakage fields from discontinuities, such as flaws. Low retentivity is important because the particles themselves never become strongly magnetized so they do not stick to each other or the surface of the part. Particles are available in a dry mix or a wet solution.

Dry Magnetic Particles

Dry magnetic particles can typically be purchased in red, black, gray, yellow and several other colors so that a high level of contrast between the particles and the part being inspected can be achieved. The size of the magnetic particles is also very important. Dry magnetic particle products are produced to include a range of particle sizes. The fine particles have a diameter of about 50 µm while the course particles have a diameter of 150 µm (fine particles are more than 20 times lighter than the coarse particles). This makes fine particles more sensitive to the leakage fields from very small discontinuities. However, dry testing particles cannot be made exclusively of the fine particles where coarser particles are needed to bridge large discontinuities and to reduce the powder's dusty nature. Additionally, small particles easily adhere to surface contamination, such as remnant dirt or moisture, and get trapped in surface roughness features. It should also be recognized that finer particles will be more easily blown away by the wind; therefore, windy conditions can reduce the sensitivity of an inspection. Also, reclaiming the dry particles is not recommended because the small particles are less likely to be recaptured and the "once used" mix will result in less sensitive inspections.

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The particle shape is also important. Long, slender particles tend align themselves along the lines of magnetic force. However, if dry powder consists only of elongated particles, the application process would be less than desirable since long particles lack the ability to flow freely. Therefore, a mix of rounded and elongated particles is used since it results in a dry powder that flows well and maintains good sensitivity. Most dry particle mixes have particles with L/D ratios between one and two.

Wet Magnetic Particles

Magnetic particles are also supplied in a wet suspension such as water or oil. The wet magnetic particle testing method is generally more sensitive than the dry because the suspension provides the particles with more mobility and makes it possible for smaller particles to be used (the particles are typically 10 µm and smaller) since dust and adherence to surface contamination is reduced or eliminated. The wet method also makes it easy to apply the particles uniformly to a relatively large area.

Wet method magnetic particles products differ from dry powder products in a number of ways. One way is that both visible and fluorescent particles are available. Most non-fluorescent particles are ferromagnetic iron oxides, which are either black or brown in color. Fluorescent particles are coated with pigments that fluoresce when exposed to ultraviolet light. Particles that fluoresce green-yellow are most common to take advantage of the peak color sensitivity of the eye but other fluorescent colors are also available.

The carrier solutions can be water or oil-based. Water-based carriers form quicker indications, are generally less expensive, present little or no fire hazard, give off no petrochemical fumes, and are easier to clean from the part. Water-based solutions are usually formulated with a corrosion inhibitor to offer some corrosion protection. However, oil-based carrier solutions offer superior corrosion and hydrogen embrittlement protection to those materials that are prone to attack by these mechanisms.

Also, both visible and fluorescent wet suspended particles are available in aerosol spray cans for increased portability and ease of application.

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Dry Particle Inspection

In this magnetic particle testing technique, dry particles are dusted onto the surface of the test object as the item is magnetized. Dry particle inspection is well suited for the inspections conducted on rough surfaces. When an electromagnetic yoke is used, the AC current creates a pulsating magnetic field that provides mobility to the powder.

Dry particle inspection is also used to detect shallow subsurface cracks. Dry particles with half wave DC is the best approach when inspecting for lack of root penetration in welds of thin materials.

Steps for performing dry particles inspection:

Surface preparation - The surface should be relatively clean but this is not as critical as it is with liquid penetrant inspection. The surface must be free of grease, oil or other moisture that could keep particles from moving freely. A thin layer of paint, rust or scale will reduce test sensitivity but can sometimes be left in place with adequate results. Specifications often allow up to 0.076 mm of a nonconductive coating (such as paint) or 0.025 mm of a ferromagnetic coating (such as nickel) to be left on the surface. Any loose dirt, paint, rust or scale must be removed. o Some specifications require the surface to be coated with a thin layer of white

paint in order to improve the contrast difference between the background and the particles (especially when gray color particles are used).

Applying the magnetizing force - Use permanent magnets, an electromagnetic yoke, prods, a coil or other means to establish the necessary magnetic flux.

Applying dry magnetic particles - Dust on a light layer of magnetic particles.

Blowing off excess powder - With the magnetizing force still applied, remove the excess powder from the surface with a few gentle puffs of dry air. The force of the air needs to be strong enough to remove the excess particles but not strong enough to remove particles held by a magnetic flux leakage field.

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Terminating the magnetizing force - If the magnetic flux is being generated with an electromagnet or an electromagnetic field, the magnetizing force should be terminated. If permanent magnets are being used, they can be left in place.

Inspection for indications - Look for areas where the magnetic particles are clustered.

Wet Suspension Inspection

Wet suspension magnetic particle inspection, more commonly known as wet magnetic particle inspection, involves applying the particles while they are suspended in a liquid carrier. Wet magnetic particle inspection is most commonly performed using a stationary, wet, horizontal inspection unit but suspensions are also available in spray cans for use with an electromagnetic yoke.

A wet inspection has several advantages over a dry inspection. First, all of the surfaces of the component can be quickly and easily covered with a relatively uniform layer of particles. Second, the liquid carrier provides mobility to the particles for an extended period of time, which allows enough particles to float to small leakage fields to form a visible indication. Therefore, wet inspection is considered best for detecting very small discontinuities on smooth surfaces. On rough surfaces, however, the particles (which are much smaller in wet suspensions) can settle in the surface valleys and lose mobility, rendering them less effective than dry powders under these conditions.

Steps for performing wet particle inspection:

Surface preparation - Just as is required with dry particle inspections, the surface should be relatively clean. The surface must be free of grease, oil and other moisture that could prevent the suspension from wetting the surface and preventing the particles from moving freely. A thin layer of paint, rust or scale will reduce test sensitivity, but can sometimes be left in place with adequate results. Specifications often allow up to 0.076 mm of a nonconductive coating (such as paint) or 0.025 mm of a ferromagnetic coating (such as nickel) to be left on the surface. Any loose dirt, paint, rust or scale must be removed. o Some specifications require the surface to be coated with a thin layer of white

paint when inspecting using visible particles in order to improve the contrast

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difference between the background and the particles (especially when gray color particles are used).

Applying suspended magnetic particles - The suspension is gently sprayed or flowed over the surface of the part. Usually, the stream of suspension is diverted from the part just before the magnetizing field is applied.

Applying the magnetizing force - The magnetizing force should be applied immediately after applying the suspension of magnetic particles. When using a wet horizontal inspection unit, the current is applied in two or three short busts (1/2 second) which helps to improve particle mobility.

Inspection for indications - Look for areas where the magnetic particles are clustered. Surface discontinuities will produce a sharp indication. The indications from subsurface flaws will be less defined and lose definition as depth increases.

Quality & Process Control

Particle Concentration and Condition

Particle Concentration

The concentration of particles in the suspension is a very important parameter and it is checked after the suspension is prepared and regularly monitored as part of the quality system checks. Standards require concentration checks to be performed every eight hours or at every shift change.

The standard process used to perform the check requires agitating the carrier for a minimum of thirty minutes to ensure even particle distribution. A sample is then taken in a pear-shaped 100 ml centrifuge tube having a graduated stem (1.0 ml in 0.05 ml increments for fluorescent particles, or 1.5 ml in 0.1 ml increments for visible particles). The sample is then demagnetized so that the particles do not clump together while settling. The sample must then remain undisturbed for a period of time (60 minutes for a petroleum-based carrier or 30 minutes for a water-based carrier). The volume of settled particles is then read. Acceptable ranges are 0.1 to 0.4 ml for fluorescent particles and 1.2 to

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2.4 ml for visible particles. If the particle concentration is out of the acceptable range, particles or the carrier must be added to bring the solution back in compliance with the requirement.

Particle Condition

After the particles have settled, they should be examined for brightness and agglomeration. Fluorescent particles should be evaluated under ultraviolet light and visible particles under white light. The brightness of the particles should be evaluated weekly by comparing the particles in the test solution to those in an unused reference solution that was saved when the solution was first prepared. Additionally, the particles should appear loose and not lumped together. If the brightness or the agglomeration of the particles is noticeably different from the reference solution, the bath should be replaced.

Suspension Contamination

The suspension solution should also be examined for contamination which may come from inspected components (oils, greases, sand, or dirt) or from the environment (dust). This examination is performed on the carrier and particles collected for concentration testing. Differences in color, layering or banding within the settled particles would indicate contamination. Some contamination is to be expected but if the foreign matter exceeds 30 percent of the settled solids, the solution should be replaced. The liquid carrier portion of the solution should also be inspected for contamination. Oil in a water bath and water in a solvent bath are the primary concerns.

Water Break Test

A daily water break check is required to evaluate the surface wetting performance of water-based carriers. The water break check simply involves flooding a clean surface similar to those being inspected and observing the surface film. If a continuous film forms over the entire surface, sufficient wetting agent is present. If the film of suspension breaks (water break) exposing the surface of the component, insufficient wetting agent is present and the solution should be adjusted or replaced.

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Electrical System Checks

Changes in the performance of the electrical system of a magnetic particle inspection unit can obviously have an effect on the sensitivity of an inspection. Therefore, the electrical system must be checked when the equipment is new, when a malfunction is suspected, or every six months. Listed below are the verification tests required by active standards.

Ammeter Check

It is important that the ammeter provide consistent and correct readings. If the meter is reading low, over magnetization will occur and possibly result in excessive background "noise." If ammeter readings are high, flux density could be too low to produce detectable indications. To verify ammeter accuracy, a calibrated ammeter is connected in series with the output circuit and values are compared to the equipment's ammeter values. Readings are taken at three output levels in the working range. The equipment meter is not to deviate from the calibrated ammeter more than ±10 percent or 50 amperes, whichever is greater. If the meter is found to be outside this range, the condition must be corrected.

Shot Timer Check

When a timer is used to control the shot duration, the timer must be calibrated. Standards require the timer be calibrated to within ± 0.1 second. A certified timer should be used to verify the equipment timer is within the required tolerances.

Magnetization Strength Check

Ensuring that the magnetization equipment provides sufficient magnetic field strength is essential. Standard require the magnetization strength of electromagnetic yokes to be checked prior to use each day. The magnetization strength is checked by lifting a steel block of a standard weight using the yoke at the maximum pole spacing to be used (10 lb weight for AC yokes or 40 lb weight for DC yokes).

Lighting

Magnetic particle inspection predominately relies on visual inspection to detect any indications that form. Therefore, lighting is a very important element of the inspection process. Obviously, the lighting requirements are different for an inspection conducted

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using visible particles than they are for an inspection conducted using fluorescent particles.

Light Requirements When Using Visible Particles

Visible particles inspections can be conducted using natural lighting or artificial lighting. However, since natural daylight changes from time to time, the use of artificial lighting is recommended to get better uniformity. Artificial lighting should be white whenever possible (halogen lamps are most commonly used). The light intensity is required to be 100 foot-candles (1076 lux) at the surface being inspected.

Light Requirements When Using Fluorescent Particles

Ultraviolet Lighting

When performing a magnetic particle inspection using fluorescent particles, the condition of the ultraviolet light and the ambient white light must be monitored. Standards and procedures require verification of lens condition and light intensity. Black lights should never be used with a cracked filter as the output of white light and harmful black light will be increased. Also, the cleanliness of the filter should also be checked regularly. The filter should be checked visually and cleaned as necessary before warming-up the light. Most UV light must be warmed up prior to use and should be on for at least 15 minutes before beginning an inspection.

For UV lights used in component evaluations, the normally accepted intensity is 1000 µW/cm2 at 38cm distance from the filter face. The required check should be performed when a new bulb is installed, at startup of the inspection cycle, if a change in intensity is noticed, or every eight hours of continuous use.

Ambient White Lighting

When performing a fluorescent magnetic particle inspection, it is important to keep white light to a minimum as it will significantly reduce the inspector’s ability to detect fluorescent indications. Light levels of less than 2 foot-candles (22 lux) are required by most procedures. When checking black light intensity a reading of the white light produced by the black light may be required to verify white light is being removed by the filter.

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White Light for Indication Confirmation

While white light is held to a minimum in fluorescent inspections, procedures may require that indications be evaluated under white light. The white light requirements for this evaluation are the same as when performing an inspection with visible particles. The minimum light intensity at the surface being inspected must be 100 foot-candles (1076 lux).

Light Measurement

Light intensity measurements are made using a radiometer (an instrument that transfers light energy into an electrical current). Some radiometers have the ability to measure both black and white light, while others require a separate sensor for each measurement. Whichever type is used, the sensing area should be clean and free of any materials that could reduce or obstruct light reaching the sensor. Radiometers are relatively unstable instruments and readings often change considerable over time. Therefore, they should be calibrated at least every six months.

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Chapter 6 Quality Control

of Electroless Nickel Deposits Phillip Stapleton

Electroless nickel deposits are smooth, hard nickel-phosphorus alloy coatings produced by a chemical reduction reaction. The properties of these coatings are dependent upon the substrate, pretreatment, plating process, composition of deposits, and post treatments. This interdependency of processes and materials on the performance of the coating makes quality control and process control key aspects of the manufacturing system.

In most applications, electroless nickel is used because of aggressive corrosion or wear conditions on the base material. In these applications, the coating performance can be critical to the overall system, and the potential loss to society if the coating fails significant.

Changes in coating performance, likesurface finish and porosity, are affected by the manufacturing process before plating and bath formulation, while thickness, adhesion, and hardness are dependent on the plating process. Many times a combination of effects will influence the performance of the coating.

Electroless nickel deposit properties can also vary significantly between types of solutions. This is a result of the electropotential in the application environment and differences in the coating structure.

These chemical and physical changes in the deposit can be related to the phosphorus content and the number of lattice defects in the structure. By design, some types of solutions have greater numbers of lattice defects than others. Generally, as the lattice defects in the alloy increase, the deposit becomes more brittle and harder with a reduction in the modulus and increase in tensile strength.

The control of these properties is accomplished by maintaining the plating solution within a narrow operating range and controlling trace elements such as sulfur, antimony, cadmium, and bismuth, which are generally attributed with having the greatest effect on the structure and composition of the deposit.

QUALITY METHOD

Developing a method of controlling the factors that affect quality requires an overall scheme. This scheme must identify the quality level, analyze the process system to identify critical conditions, modify the manufacturing process, and

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170 ELECTROLESS PLATING

identify the new quality level. Many methods have been developed to describe the relationship of quality to manufacturing process. One that works well for metal finishing is based on the work of Taguchi (Fig. 6.1).

According to his method, the goal is to develop a more robust manufacturing system that has no performance variation of the product, while experiencing a higher level of noise or variations within the processes.

The term noise is used to describe process variations from a mean. These variations can be from the wear of machines, operator training, contamination of materials, as well as many other factors. The use of the term inner noise refers to variations that are being monitored and can be controlled, while outer noise refers to variations that are not being controlled.

In Fig. 6.1, the block “Off-line Quality Control” is generally provided by the suppliers of processes, while “Transfer Design” and “On-line Quality Control” are performed by the producing facility.

When these basic concepts are transferred to a shop floor system, several familiar shop functions are revealed. The first item is the specification, which describes the requirements by acceptance and qualification tests that must be completed to verify the quality of the coating. In addition to the specification, a shop traveler is used to transfer the designs to the processor. This document includes the sequence, conditions, times, and other critical information needed to produceaquality part. A phaseofthedesign includes thefrequencyoftesting of process conditions, which allows the process engiiieer to determine the Process Capability Index.

By following the shop traveler and collecting the process information, studies on the coating performance can be related to process conditions, different operators, base materials, and more. From these studies, the frequency of testing can be modified, new, more robust conditions can be selected, and the quality can be brought under control.

Fig. 6.1-Quallty control and process management.

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Quality Control of Elecfroless Nickel Deposits 171

Experience and the pursuit of these methodologies has provided the information to build a series of cause and effect charts covering the electroless nickel process (see Charts 6.1-6.4). These charts describe the relationship of a coating failure to the processes.

Figure 6.2 shows the three areas that require control by operators: (a) process analysis and history; (b) qualification tests; and (c) acceptance tests.

Process Analysis and Management In order to maintain the quality of the electroless nickel deposit, the electroless solution must be maintained at optimum chemical and physical condition.

Constant filtration of the plating solution is recommended. The solution should be filtered more than 10 times the volume of the process every hour. At this rate of filtration, the filter will be able to remove most of the solids and prevent the formation of roughness on the parts.

An important aspect in the design of the filter system is the pump dynamics and how the system will operate with a high specific gravity and temperature. The system should be sized for an operating temperature of 195" F and a specific gravity of 1.310.

The maintenance of a relatively narrow pH range will insure a constant plating rate and phosphorus content in the alloy. A pH change of 0.1 points may cause a variation of 0.1 mil/hr and affect the final product performance and cost. For some solutions, a change in pH of 0.5 points will cause a 1 percent change in phosphorus content and significantly affect the properties of the subsequent deposit.

Maintenance of the chemical composition of the plating solution in general, and the nickel/hypophosphite ratio in particular, will ensure a uniform plating rate and phosphorus content. Air agitation and low loading will cause oxidation of the available hypophosphite and may lead to a lower phosphorus content in the deposit and slower plating rates. To overcome this, periodic titrations for sodium hypophosphite and additions in the form of replenisher should be made to the plating solution.

The concentration of trace metals within the plating solution will also affect the deposit properties. There are several elements that will directly affect the performance in all solutions. These include sulfur, iron, cadmium, bismuth, antimony, mercury, lead, and zinc. There are several others that may affect the deposit properties, and which are primarly controlled by atomic adsorption, inductively coupled plasma, or poloragraphy. The presence of some of these materials may cause porosity in thin deposits and high stress. Some trace metals increase the propensity to pitting in theenvironment by setting upactive cells on the surface between nickel and the trace element. Excessive concentration of trace elements in the solution may also produce a condition called step plating, in which edges are not plated or areas have low thicknesses.

The control of trace elements starts with keeping them out the solution. If they are present, several techniques can be tried to reduce their effect. Dilution is the first choice, followed by dummying, and addition of secondary reducing agents. Generally a combination of treatments will reduce the effect of the trace elements and bring the solution back to a productive condition.

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flg. 6.2-Putting a quallty control rystem together.

Organic contamination within the solution will also cause deposit properties to be compromised. The source of the organic materials may be from the part, facilities and tank, or masking. The problem is manifested by lack of adhesion, porosity, and stress.

Some common sources of organic contamination include maskants, organic materials, oils from within the substrate, plasticizers from hoses and liners, airborne organics, drippage from overhead, and contaminated make-up water. Silicates, though not organic in nature, can adversely contaminate the bath through drag-in from the pretreatment cycle.

Activated carbon treatment of the plating solution at operating temperature can often help reduce or eliminate some types of contaminants. If carbon treatment is used on the solution to remove organic contaminants, care should be taken to replace any desired organic control additives and that the carbon is clean and is not a source of contamination.

Another type of contaminant is oxidizers. These materials, such as hydrogen peroxide and nitric acid, will change the deposition potential within the solution and cause black or streaked deposits to be produced. With low levels of volatile oxidizers, heating the solution may help. With higher levels, and with non- volatile oxidizers, the solution must be discarded.

Other conditions that require control are agitation and loading. These two factorsaffect thediffusion of nickel ions in the reduction reaction. High agitation and either high or low loading can cause step plating and a low plating rate. Generally, the plating solution will produce coatings when the velocity of the solution is less than 4 Wsec. This value will be dependent on the solution chemistry and solution operating conditions. At extremely low loads, the

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Quality Control of Electroless Nickel Deposits 177

deposition potential may be low enough to prevent the reduction reaction from proceeding. This problem may be observed on sharp edges, such as needles, where the sharp point will not plate. In most cases, this problem can be corrected by selection of solutions that can operate at higher velocities (7 to 10 Wsec).

Testing of Deposit Properties The selection of requirements for the electroless nickel deposit are generally made by the purchaser of the coating. These requirements are established in specifications. Based on the sampling requirements, a part or specimen must be tested to identify any variation in performance.

The choice of tests that will characterize performance variation specific to the application of the part is important. Two types of tests are employed: acceptance tests and qualification tests. These categories can be used to distinguish tests that will be performed on an individual lot, and those that might be run on a regular basis, such as weekly or monthly.

Acceptance tests include:

0 Thickness 0 Appearance

Tolerance Adhesion Porosity

Qualification tests include:

Corrosion resistance Wear resistance Alloy composition Internal stress Hydrogen embrittlement Microhardness

The following list of requirements and test methods have been provided to offer the user of electroless nickel coatings an overview of the options available. The actual organization of requirements and test methods have been published in MIL-C-26074C and ASTM 8733 and should be used to maintain a national standard for ordering and performance.

TEST METHODS

Appearance The coating surface shall have a uniform, metallic appearance without visible defects such as blisters, pits, pimples, and cracks.

Imperfections that arise from surface conditions of the substrate and persist in the coating shall not be cause for rejection. Also, discoloration that results from heat treatment shall not be cause for rejection.

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I 7a ELECTROLESS PLATiNG

Thickness Measurement The thickness shall be measured at any place on the significant surface designated by the purchaser, and the measurement shall be made with an accuracy of better than 5 percent by a method selected by the purchaser. Examples of common measuring devices are shown in Figs. 6.3 and 6.4.

Fig. 6.3-X-ray fluorescence device measures thickness by analyzing the mass per unit area of the eieciroless nickel deposit according lo ASTM 8568.

Fig. 6.4-Beta backscatter device also measures thickness by analyzing the mass per unit area of the electroless nickel deposit. There are some limitations to this method, but the cost and availability of instruments makes it an excellent choice for many applications. ASTM 8567 can be used to standardize and peiform the measurements with this instrument.

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Qualify Control of Electroless Nickel Deposifs 179

Weigh, Plate, Weigh Method Using a similar substrate material, weigh to the nearest milligram before and after plating, ensuring that the part is at the same temperature for each measurement. Calculate the thickness from the increase in weight, surface area, and density of the coating.

NOTE: The density of the coating will vary with the weight percentage of phosphorus in the coating. For a 9-percent-P alloy, the density is 8 glcm'.

Example-A coupon made of mild steel has a weight of 3198 mg with an area of 19.736 cm' before plating. After plating with electroless nickel, the coupon weighs 3583 mg. Calculation for thickness is as follows:

3583 mg (after) - 3198 mg (before) T = + 19.736 cm-'

8.01 g/cm' x 1000 mg/g

T = 0.00244 cm x 10,000 pm/cm T = 24.4 pm

Table 6.1 8ubstrate Densities

For Weigh, Plate, Weigh Method

Steel, mild 1020 Stainless steel 316 Aluminum 2024 Aluminum 6061 Copper

7.86 8.02 2.79 2.70 8.91

Metallographic Sectioning Plate a specimen of similar composition and metallurgical condition to the article being plated, or use a sample from the lot; mount and polish at 90" to the surface. Using a Vernier Calibrated Microscope, examine the thickness of the deposit and average over 10 readings.

NOTE: Accurate microscopic metallographic sectioning is very dependent on the sample preparation. Backing springs are recommended to reduce the smearing effects of the polishing step.

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Micrometer Method Measure a part of the test coupon in a specific spot before and after plating, usin'g a suitable micrometer. Ensure that the part is at the same temperature for each measurement and that the surface measured is smooth.

Beta Backscatter Method Thecoating thickness can be measured by the use of a beta backscatter device. The use of beta backscatter is restricted to base metals that have an atomic number of less than 18 or greater than 40. The actual phosphorus content of the coating shall be taken into consideration; consequently, the measuring device shall be calibrated using specimens of the same substrate having the same phosphorus content as the articles to be tested.

Magnetic Method A magnetic thickness detector is applicable to magnetic substrates plated with autocatalytic nickel deposits that contain more than 10 weight percent phosphorus (non-magnetic) and that have not been heat treated. The instrument must be calibrated with deposits plated in the same bath on steel and whose thickness has been determined by the microscopic method.

X-ray Spectrometry The coating thickness can be measured by X-ray spectrometry. This technique will measure the mass per unit area of the coating applied over the substrate. X-ray spectrometry equipment should be calibrated according to ASTM 8568 with standards of known phosphorus and thickness. This method is nondestructive and will produce rapid and accurate results.

Coulometric Method Measure the coating thickness in accordance with ASTM 8504. The solution to be used shall be in accordance with manufacturer's recommendations. The surface of the coating shall be cleaned prior to testing.

Standard thickness specimens shall be calibrated with deposits plated in the same solution under the same conditions.

NOTE: This method is only recommended for deposits in the as-plated condition. The phosphorus content of the coating must be known in order to calculate the thickness of the deposit.

Adhesion Measurement Bend Test The sample specimen shall be bent through 180° over a minimum mandrel of 12 mm in diameter or four times the thickness of the specimen and examined at 4X magnification. No detachment of the coating shall occur. Fine cracks in the coating on the tension side of the bend are not an indication of poor adhesion.

Quench Test Heat a plated article for 1 hr in an oven in accordance with Table 6.2 for the appropriate basis metals within 210" C. Then quench in room temperature water. The appearance of blisters or peeling is evidence of inadequate adhesion.

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Ouality Control of Electroless Nickel Deposits

NOTE: This test procedure may have an adverse effect on the mechanical properties of the articles tested.

181

Table 6.2 Substrate Heat-Treatment

Temperatures for Quench Test

Steel 300" c Zinc 150" C Copper or copper alloy 250" c Aluminum or aluminum alloy 250" C

Punch Test Make several indentations (approximately 5 mm apart) in the coating, using a spring-loaded center punch on which the point has been ground to a 2-mm radius. Blistering or flaking indicates poor adhesion.

File Test By agreement with the purchaser, a file may be applied to the coated article. A non-significant area shall be filed at an angle of 45" to the coating, so that the base rnetaVcoating interface is exposed. No lifting of the coating shall be observed.

Microhardness ASTM 8578 shall be used for Knoop hardness with a test load of 100 g. The instrument shall be verified on calibrated standard test blocks having a hardness similar to that of the deposit under test.

NOTE: For thin (less than 25 pm) deposits using less than 100 g loads, the standard commercial hardness tester produces varied results. This is due to the plastic deformation of the coating and the optical qualities of the instrument.

On thick (75+ pm) deposits, a surface microhardness determination using

Conversion of microhardness (Knoop or Vickers) to Rockwell scale is ASTM E384 is permissible.

inaccurate and therefore inappropriate (see ASTM El40).

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Hydrogen Embrittlement ASTM F519 shall be used once a month to evaluate the plating process for relief of hydrogen embrittlement. A minimum of three V-notch tensile specimens made of AIS1 4340 heat treated to a strength of 260 to 280 ksi shall be plated and loaded at 75 percent or greater of their ultimate notch tensile strength and held for 200 hours. No evidence of fracture or cracks shall exist.

Alloy Determination There are generally three phosphorus ranges for electroless nickel deposits. Specific types of solution formulations will provide the selection of range of phosphorus in the alloy. Specific confirmation of the alloy can be accomplished by analyzing for nickel and phosphorus by one of the methods described below.

Most applications have been developed using a mid-range of 3 to 9.5 percent phosphorus and are considered typical. Confirmation of the alloy for these coatings can be by nickel or phosphorus, while coatings of less than 3 percent P or greater than 9.5 percent P should use both the nickel and phosphorus analysis to determine the alloy.

Preparation of Test Specimens There are two general methods of preparing a foil specimen for this test. The most efficient technique is to plate a stainless steel panel with a 25- to 50-pm- thick deposit, cut the edges and peel the deposit off the panel.

Another way to produce an autocatalytic nickel phosphorus foil is to deposit a 25- to 50-pm-thick coating onto a masked aluminum panel. Then remove the maskant and remove the aluminum by immersing in 10 percent sodium hydroxide solution. When finished, a foil will have been produced that is acceptable for analysis. Although better adhesion is obtained using a zincate treatment, a coherent plate may be obtained by immersing clean aluminum foil in the autocatalytic nickel solution.

Determination of Nickel Content-Dimethylglyoxime Method Reagents:

1:l v/v concentrated nitric acid (specific gravity 1.42) 1 percent solution of dimethylglyoxime

Procedure: Accurately weigh 0.1 g of autocatalytic nickel deposit and transfer to a 400 mL

beaker. Dissolve in 20 mL of 1:l nitric acid, boil to expel nitrous oxide fumes, then cool and dilute to 150 mL with distilled water. Add approximately 1 g of citric or tartaric acid to complex any ion that may be present and neutralize with ammonium hydroxide to pH 8 to 9.

Heat gently to 60 to 70" C, and while stirring, add 30 mL of dimethylglyoxime reagent. Allow to stand at 60 to 70" C for 1 hr, cool to below 20" C, and filter through a clean sintered glass crucible of No. 4 porosity. Wash the precipitate well with distilled water, dry in an oven at 110" C for 1 hr, cool, and weigh the precipitate as nickel dimethylglyoxime.

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Qualify Confrol of Electroless Nickel Deposifs 183

O/O Ni = (weight of precipitate x 0.2032 x 100)/sample weight

Determination of Phosphorus Content The percentage of phosphorus is determined after dissolution of the deposit in acid, either colorimetrically or volumetrically.

Reagents: For dissolution and oxidation 40 percent v/v concentrated nitric acid (specific gravity 1.42) 2 percent sodium nitrate solution Approximately 0.1 N potassium permanganate solution

For colorimetric phosphorus analysis Molybdate vanadate reagent-Dissolve separately in hot water, 20 g

ammonium molybdate and 1 g ammonium vanadate, then mix the two solutions. Add 200 mL of concentrated nitric acid (specific gravity 1.42) and dilute to 1 L.

For volumetric phosphorus analysis Solution A: Dissolve 15 g of ammonium molybdate in 80 mL distilled water.

Solution B: Dissolve 24 g ammonium nitrate in 60 mL distilled water. Add 33 Add 6 mL ammonium solution (specific gravity 0.880) and dilute to 100 mL.

mL concentrated nitric acid and dilute to 100 mL.

Procedure for dissolution and oxidation:

50 mL of 40 percent v/v nitric acid solution.

fumes.

approximately 0.1 N potassium permanganate solution.

1. Dissolve 0.1 9 to 0.21 g (weigh accurately) of autocatalytic nickel deposit in

2. Heat gently until the deposit is fully dissolved. Then boil to remove brown

3. Dilute to approximately 100 mL, bring to the boil, and add 20 mL of

4. Boil for 5 minutes. 5. Add 2 percent solution of sodium nitrite dropwise until the precipitated

6. Boil for 5 minutes, then cool to room temperature. 7. Dilute the solution in a volumetric flask to 250 mL and mix well. At this stage, the phosphorus content may be estimated either colorimetrically

manganese dioxide is dissolved.

or volumetrically, as described below.

Procedure for colorimetric analysis: 1. Transfer 10 mL of the solution from step 7 above to a 100-mL standard flask,

add 50 rnL distilled water, 25 mL molybdate-vanadate reagent, dilute to the mark with water, and mix well.

2. Read the absorption at 420 nm after 5 minutes, using 1 cm glass cells with water in the reference cell. Read off the concentration from a previously prepared calibration curve.

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% P = mg P from graph/weight of sample (mg)

1. Dry potassium dihydrogen orthophosphate at 115O C for 1 hour. 2. Weigh out 0.4392 g. dissolve in water, and make up to 1 L of solution (1 mL =

0.1 mg phosphorus). 3. Prepare a calibration curve by adding 25 mL of reagent to 2 mL, 4 rnL, 6 mL,

8 mL, and 10 rnL liquids of this standard solution in 100 rnL standard flasks and diluting to the mark. Read the absorption of these solutions exactly as for the estimated reading at 420 nrn, as described above. Plot a calibration curve of absorption against rng phosphorus in samples of 0.2 mg, 0.4 mg. etc.. up to 1 .O rng when prepared as above.

Procedure for preparation of calibration curve:

Procedure for volumetric analysis: 1. Transfer 10 mL of the solution from step 7 above to a stoppered flask and

dilute to 100 mL with distilled water. 2. Warm to 40 to 50" C (do not exceed thls temperature) and slowly add 50 rnL

of ammonium molybdate reagent while stirring. 3. Stopper the flask. 4. Agitate the flask vigorously for 10 minutes. 5. Allow the flask to stand for 30 minutes and filter through a Whatrnan No.

542 filter paper. 6. Wash the flask and precipitate with 1 percent potassium nitrate until the

filtrate will not decolorize 1 mL of water containing 1 drop of 0.1N sodium hydroxide and 1 drop of phenolphthalein. This will require about 100 mL of the washing liquid.

7. Place the paper and precipitate in the original flask, add 50 mL water, and shake well.

8. Add 10 mL of 0.1 N sodium hydroxide solution and shake well to dissolve the precipitate.

9. Add phenolphthalein indicator and back-titrate with 0.1 N hydrochloric acid. Let " X rnL be the titration.

YO P = [25 x (10-X) x 0.01349]/weight of sample

Spectra Analysis A suitable method using emission spectra produced by Inductively Coupled Plasma (ICP) would be acceptable for analysis in nickel, phosphorus, and trace elements.

The following lines have been found to have low interferences when using argon ICP techniques. AA standards should be used for this analysis. Phosphorus standards should be made weekly to ensure accuracy.

Ni 216.10 nm AI 202.55 nrn Cr 284.32 nm Pb 283.30 nm P 215.40 nrn Cd 214.44 nm Cu 324.75 nm Sn 189.94 nrn P 213.62 nm Co 238.34 nm Fe 238.20 nm Zn 206.20 nm

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Quality Confrol of Electroless Nickel Deposits 185

Porosity Measurement The test to be applied shall be decided by the purchaser in agreement with the plater.

Porosity Tests for Ferrous Substrates Ferroxyl Test The test solution is prepared by dissolving 25 g of potassium ferrocyanide and 15 g of sodium chloride in 1 L solution. The part is cleaned and immersed for 5 sec in the test solution at 25" C, followed by water rinsing and air drying. Blue spots visible to the unaided eye will form at pore sites. Their allowable number should be specified.

Alternately, strips of suitable paper (e.g., "wet strength" filter paper) are first immersed in a warm (about 35" C) solution containing 50 g/L of sodium chloride and 50 g/L of white gelatin and then allowed to dry. Just before use, they are immersed in a solution containing 50 g/L sodium chloride and 1 g/L of a non-ionic wetting agent, and pressed firmly to make satisfactory contrast onto the cleaned nickel surface to be tested and allowed to remain for 30 minutes.

If papers should become dry during the test, they should be moistened again, in place, with the sodium chloride solution. The papers are then removed and introduced at once into a solution containing 10 g/L potassium ferrocyanide to produce sharply defined blue markings on the papers, wherever the basis metal was exposed by discontinuities in the coating, leading to attack by the sodium chloride and transference of the ion components to the paper. If necessary, the same area may be retested.

Hot Water Test Immerse the part to be tested in a beaker filled with aerated water at room temperature. Apply heat to the beaker at such a rate that the water begins to boil in not less than 15 minutes, and not more than 20 minutes. Then remove the part from the water and air dry. Examine the part for rust spots, which will indicate pores.

Aerated water is prepared by bubbling clean compressed air through a reservoir of distilled water by means of a glass diffusion disc for at least 24 hours. The pH of the aerated water should be 6.7 k0.5. The test parts should be covered by at least 30 +5 mm of aerated water.

Neutral Salt Spray Testing in accordance with ASTM 8117 shall be conducted monthly on the plating process. Coat a 4 x 6 x 0.20 AIS1 4130 steel panel with 0.0015 in. of deposit. Wash edges and expose in salt spray chamber for a minimum of 240 hours. This panel shall have a rating of 9.5 or better in accordance with ASTM 8537. NOTE: This test can be made more aggressive by reducing the plating thickness and increasing the exposure time (e.g., 0.0005 in. for 1000 hours). page 186

Hot Chloride Porosity Test Immerse a steel part in 50 percent reagent HCI for3 hoursat 83" C. After testing, the acid shall not be significantly discolored or the part will have exfoliation or

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blisters. Equipment needed to perform this test: a glass beaker, hot plate, thermometer, and a timer.

The purposeof this test is to locate pore sites in thecoating when the porecell potential is greater than 150 mV. Rapid corrosion will occur, causing significant failure of the coating and substrate. A polyethylene cover affixed to the beaker with a large rubber band can be used to reduce the vapor and evaporation of the acid. This test should be performed under a fume hood to remove acid vapor.

Porosity Test for Aluminum Substrates Wipe the specimens with a 10 percent solution of sodium hydroxide. After3 min, rinse and apply a solution of sodium alizarin sulfonate (9,10-anthraquinon-l,2- dihydroxy-3-sulfonate, sodium salt) to the specimen. This solution is prepared by dissolving 1.5 g methyl cellulose in 90 mL boiling water to which, after cooling, a solution of 0.1 g sodium alizarin sulfonate dissolved in 5 mL ethanol is added.

After 4 min, apply glacial acetic acid at ambient temperature until the violet color disappears. Any red spots remaining indicate pores.

Porosity lest for Copper Substrates Wipe the specimen with glacial acetic acid at ambient temperature. After 3 min, apply to the specimen surface a solution of potassium ferricyanide, prepared by dissolving 1 g of potassium ferricyanideand 1.5 g of methyl cellulose in 90 mL of boiling deionized water. After 2 to 3 min, the appearance of brown spots will indicate pores.

Standard Method for Measuring Corrosion Rate Scope This method establishes the apparatus, specimen preparation, test procedure, evaluation, and reporting of the corrosion rate of autocatalytic nickel deposits. The results are determined by using linear polarization and potentiodynamic techniques, and can be used to rank the coatings in order of corrosion resistance.

This test method is applicable to electroless nickel deposits applied to specimens of G5 specification and corroded in an artificial environment.

This test method is provided for use in an interlaboratory corrosion procedure for evaluating electroless nickel deposits.

Applicable Documents ASTM standards:

Measurements in Corrosion Testing

static and Potentiodynamic Anodic Polarization Measurements

G3 Standard Practice for Conventions Applicable to Electrochemical

G5 Standard Practice for Standard Reference Method for Making Potentio-

G15 Definition of Terms Relating to Corrosion and Corrosion Testing G59 Practice for Conducting Potentiodynamic Polarization Resistance

Measurements

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Quahty Control of Electroless Nickel Deposits 187

Summary of Method The specimen (working electrode) is prepared by plating in an electroless nickel solution a steel G5 plug. The plug is then cleaned and mounted in an electrochemical cell of G5 design. The cell is charged with a test solution at ambient temperature and purged with nitrogen. The cell is then pressurized with the gas in question and heated to the desired temperature. Measurements are then taken after 1000 sec to find the corrosion potential (E,,,,,). Polarization measurements are then made to determine the instantaneous corrosion rate for the applied pressure and temperature. The curves from various conditions and alloys can then be compared and the relative corrosion resistance can be established.

Significance and Use The significance of these tests is that they can be used to rank electroless nickel deposits in order of their corrosion resistance. These tests can then be used to develop a comprehensive corrosion monitoring program to evaluate the substrate pretreatment and electroless nickel deposit.

Results from these tests produce quantitative values as to the corrosive nature of the environments and the protection afforded by specific electroless nickel deposits under laboratory conditions.

Apparatus The following required equipment is described in ASTM G5:

1. Potentiostat calibrated according to ASTM G5 2. Potential measuring device 3. Current measuring device 4. Saturated calomel electrode 5. Salt bridge probe The specimen (working electrode) to be tested is plated with electroless nickel

and post-treated with the desired process. The specimen can be made from any material tothefollowing physical dimensions: 1/2 in. (12.7 mm) long, 3.8 in. (9.5 rnrn) diameter, with a 3-84 tapped hole at one end. The entire surface shall be 16 rms or better.

NOTE: The specimens used in this ASTM program are made from C7275 steel.

The polarization cell shall be made to the following ASTM G5 specifications: 1. Carbon or platinum counter electrodes shall be used in the cell. 2. The salt bridge shall be adjustable and able to be located near the tip of the

3. The electrolytes shall be agitated constantly with a magnetic stirrer. 4. The cell shall be purged constantly with N, gas. 5. The cell shall be able to operate at 1 atmosphere. 6. The cell shall be able to operate at a temperature of 22" C. 7. The specimen holder shall be designed and maintained to form a tight

working electrode.

connection between the specimen and the teflon gasket of the holder.

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Reagents Corroding Electrolyte I is prepared by dissolving 12.5 g of reagent grade sodium chloride (NaCI) into 900 mL of Type I reagent water.* Adjust pH to 7.8 20.1 with sodium hydroxide. Fill to 1000 mL with Type I reagent water.

Corroding Electrolyte II is prepared by dissolving 40 g of reagent grade sodium chloride (NaCI) into 1000 mL of Type I reagent water.

Specimen Preparation Prepare the specimen by the following steps:

1. Vapor degrease or solvent wash. 2. Caustic clean with appropriate cleaner for steel or aluminum substrate.

3. Rinse with Type I reagent water. 4. Wash with 25 percent sulfuric acid for 60 sec at 25" C. 5. Rinse with Type I reagent water. 6. Mount the specimen on the electrode holder. Care should be exercised not

Note mounting hole will expose substrate alloy to cleaning solution.

to contaminate or disturb the cleaned surface.

Test Parameters Potentiodynamic polarization:

1. Initial potential -100 mV from E,,, 2. Final potential -600 mV 3. Vertex potential +lo00 mV 4. Input anodic tafel constant (ATC) 0.145; cathodic tafel constant (CTC)

5. Scan rate 2.0 mV/sec 0.150

Linear polarization: 1. Initial potential -30 mV €(,,, 2. Final potential 30 mV E,,,, 3. Anodic and cathodic tafel constant from potentiodynamic 4. Scan rate 0.2 mV/sec

Specimen physical parameters: 1. Area 4.285 cm' 2. Density 7.95 g/cm' 3. Equivalent weight 28.51 mols/electron

Wear Analysis Abrasion Resistance Abrasion resistance can be measured by a Taber Abrader using a CS-10 wheel. Wear specimens should be dressed for the first 1000 cycles and then weighed. TheCS-10 wheelsshould beredressedfor50cyclesforeach 1OOOcyclesof wear on thespecimen. Typical testsaretaken to 10,OOOcycleswith a rangeof 15 to30

'Type I reagent water is defined in ASTM D1193 as 16.67 M ohm-cm resistivity.

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Qualify Control of Electroless Nickel Deposifs

mg/l000 cycles for as-plated deposits, and 6 to 18 mg/l000 Cycles for precipitation-hardened deposits. The test is not precise, but can be used to characterize the differences between coatings when several specimens are tested.

189

Adhesive Wear Resistance Adhesive wear resistance can be tested by several means. These include pin on disc, block on ring, and pin on notch. Each of these types of adhesive wear tests simulate slight differences in metal-to-metal wear. To evaluate the specific wear for a particular application, the correct wear test method must be selected. Conditions such as temperature, lubrication, load, vibration, wear scar, velocity, and others will affect the results requiring careful study of the test methods before a test can be selected.

The Alpha-LFW1 is a block-on-ring test that has been found to provide wear information on electroless nickel coatings. The Falex tester is a pin-on-notch device that provides information on high load wear. Both of these tests can be run dry or lubricated, and can be used to characterize the differences between coatings.

Stress Analysis The intrinsic stress of the deposit can be determined using ASTM 8636. This test uses a spiral contractometer, plating the spiral and measuring the amount of movement in the helix while the spiral is plating. The amount of intrinsic stress in the deposit is determined by the rate of swing in a needle attached to the spiral. The movement is magnified by a factor of IO, with readings being taken while the process is operating. After the measurement of intrinsic stress has been taken at plating temperature, the helix can be removed and cooled. A second reading can be taken, providing the total stress, and then the thermal component can be calculated:

Spiral total stress = intrinsic stress + thermal stress

The results can be used to predict when adhesion may be compromised on aluminum and other alloys, as well as the preference for certain types of corrosion.

Other methods of analysis for total stress have been developed using a rigid strip. Strips are first plated on both sides, then one side is removed with a stripper. The thickness of the strip and the coating are measured, and then the amount of bow in the strip is measured. Calculations can be made from this information, and the total stress in the part can be selected.

POST-TREATMENTS

The quality control of an electroless nickel deposit implies that certain post- treatments have been completed. These are generally used to improve the

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deposit adhesion or increase the hardness of the deposit by precipitation of phosphorus to nickel phosphide.

In most specifications, the requirements for heat treatment are described. Generally there are no tests for these processes, and self-certification must be used. In some cases, a test may exist that is destructive, and therefore not usable.

MIL-C-26074C and ASTM 8733 both use the same classification system to describe the post-heat treatment of the deposit. These documents use the term class and establish the steps to reduce the potential for hydrogen embrittlement, increase the adhesion of the coating, improve the fatigue properties of the part@), and increase the wear resistance and hardness of the coating.

Classes of Electroless Nickel Coatings Class 1 As plated, no treatment other than hydrogen embrittlement relief at 190 * lo" C for steels with a tensile strength of 1051 MPa or greater.

Class 2 Heat treated for .hardness, the coating shall be heat treated to a minimum hardness of 850 Knoop (100 g load). This hardness can be produced by heat treating the coating at 400" C for 60 minutes. Higher temperatures for shorter times may be used.

Class 3 Heat treatment at 180 to 200" C for 2 to 4 hours to improve coating adhesion on aluminum.

Class 4 Heat treatment at 120 to 130" C for a minimum of 1 hour to improve adhesion on heat-treatable (age-hardened) aluminum alloys and carburized steels.

Class 5 Heat treatment at 140 to 150" C fora minimum of 1 hour to improve adhesion on non-age-hardened aluminum and beryllium alloys.

The use of the shop traveler to record the completion of the heat treatment, as well as a notation on the chart record of the oven, is a standard practice.

Additional post-treatments such as silicates, water glasses, waxes, and chromates, are sometimes applied to prevent staining and oxidation of the nickel. The analysis of these treatments are complicated and seldom tested. Contact resistance tests are generally used if the treatment requires testing.

QUALITY CONTROL

To conclude, the electroless nickel facility must have a quality control system. The platers need to identify how the parts are to be processed. In addition, they

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Quahty Control of Electroless Nlckel Deposits 191

must maintain the chemistry, qualify the processes, and complete the acceptance tests as required by the specifications.

The system can be operated from a single notebook or from a multiuser computer system. The quality control program can be highly complex, or just cover the essentials.

In the end, it is dedicated people who must build and follow the quality control system. It is people who will produce higher levelsof qualityand make it possible to extend the performance of electroless nickel deposits into new markets and applications.

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Chapter 1Fundamental Properties of X-rays

1.1 Nature of X-rays

X-rays with energies ranging from about 100 eV to 10 MeV are classified as electro-magnetic waves, which are only different from the radio waves, light, and gammarays in wavelength and energy. X-rays show wave nature with wavelength rangingfrom about 10 to 10�3 nm. According to the quantum theory, the electromag-netic wave can be treated as particles called photons or light quanta. The essentialcharacteristics of photons such as energy, momentum, etc., are summarized asfollows.

The propagation velocity c of electromagnetic wave (velocity of photon) withfrequency � and wavelength � is given by the relation.

c D �� .ms�1/ (1.1)

The velocity of light in the vacuum is a universal constant given as c D299792458 m=s (�2:998 � 108 m=s). Each photon has an energy E , which isproportional to its frequency,

E D h� D hc

�.J/ (1.2)

where h is the Planck constant (6:6260 � 10�34 J � s). With E expressed in keV, and� in nm, the following relation is obtained:

E.keV/ D 1:240

�.nm/(1.3)

The momentum p is given by mv, the product of the mass m, and its velocity v.The de Broglie relation for material wave relates wavelength to momentum.

� D h

pD h

mv(1.4)

1

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2 1 Fundamental Properties of X-rays

The velocity of light can be reduced when traveling through a material medium,but it does not become zero. Therefore, a photon is never at rest and so has no restmass me. However, it can be calculated using Einstein’s mass-energy equivalencerelation E D mc2.

E D mer1 �

� v

c

�2c2 (1.5)

It is worth noting that (1.5) is a relation derived from Lorentz transformation in thecase where the photon velocity can be equally set either from a stationary coordi-nate or from a coordinate moving at velocity of v (Lorentz transformation is givenin detail in other books on electromagnetism: for example, P. Cornille, AdvancedElectromagnetism and Vacuum Physics, World Scientific Publishing, Singapore,(2003)). The increase in mass of a photon with velocity may be estimated in thefollowing equation using the rest mass me:

m D mer1 �

� v

c

�2(1.6)

For example, an electron increases its mass when the accelerating voltage exceeds100 kV, so that the common formula of 1

2mv2 for kinetic energy cannot be used. In

such case, the velocity of electron should be treated relativistically as follows:

E D mc2 � mec2 D mer

1 �� v

c

�2c2 � mec

2 (1.7)

v D c �s

1 ��

mec2

E C mec2

�2

(1.8)

The value of me is obtained, in the past, by using the relationship of m D h=.c�/

from precision scattering experiments, such as Compton scattering and me D9:109 � 10�31 kg is usually employed as electron rest mass. This also means that anelectron behaves as a particle with the mass of 9:109 � 10�31 kg, and it correspondsto the energy of E D mc2 D 8:187 � 10�14 J or 0:5109 � 106 eV in eV.

There is also a relationship between mass, energy, and momentum.

�E

c

�2

� p2 D .mec/2 (1.9)

It is useful to compare the properties of electrons and photons. On the one hand,the photon is an electromagnetic wave, which moves at the velocity of light some-times called light quantum with momentum and energy and its energy depends uponLi

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1.2 Production of X-rays 3

the frequency �. The photon can also be treated as particle. On the other hand, theelectron has “mass” and “charge.” It is one of the elementary particles that is aconstituent of all substances. The electron has both particle and wave nature suchas photon. For example, when a metallic filament is heated, the electron inside itis supplied with energy to jump out of the filament atom. Because of the negativecharge of the electron, (e D 1:602 � 10�19 C), it moves toward the anode in anelectric field and its direction of propagation can be changed by a magnetic field.

1.2 Production of X-rays

When a high voltage with several tens of kV is applied between two electrodes,the high-speed electrons with sufficient kinetic energy, drawn out from the cath-ode, collide with the anode (metallic target). The electrons rapidly slow down andlose kinetic energy. Since the slowing down patterns (method of loosing kineticenergy) vary with electrons, continuous X-rays with various wavelengths are gener-ated. When an electron loses all its energy in a single collision, the generated X-rayhas the maximum energy (or the shortest wavelength D �SWL). The value of theshortest wavelength limit can be estimated from the accelerating voltage V betweenelectrodes.

eV � h�max (1.10)

�SWL D c

�maxD hc

eV(1.11)

The total X-ray intensity released in a fixed time interval is equivalent to the areaunder the curve in Fig. 1.1. It is related to the atomic number of the anode target Z

and the tube current i :

Icont D AiZV 2 (1.12)

where A is a constant. For obtaining high intensity of white X-rays, (1.12) suggeststhat it is better to use tungsten or gold with atomic number Z at the target, increaseaccelerating voltage V , and draw larger current i as it corresponds to the numberof electrons that collide with the target in unit time. It may be noted that most ofthe kinetic energy of the electrons striking the anode (target metal) is converted intoheat and less than 1% is transformed into X-rays. If the electron has sufficient kineticenergy to eject an inner-shell electron, for example, a K shell electron, the atom willbecome excited with a hole in the electron shell. When such hole is filled by an outershell electron, the atom regains its stable state. This process also includes productionof an X-ray photon with energy equal to the difference in the electron energy levels.

As the energy released in this process is a value specific to the target metal andrelated electron shell, it is called characteristic X-ray. A linear relation between thesquare root of frequency � of the characteristic X-ray and the atomic number Z ofthe target material is given by Moseley’s law.Li

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Fig. 1.1 Schematicrepresentation of the X-rayspectrum

p� D BM.Z � �M/ (1.13)

Here, BM and �M are constants. This Moseley’s law can also be given in terms ofwavelength � of emitted characteristic X-ray:

1

�D R.Z � SM/2

�1

n21

� 1

n22

�(1.14)

Here, R is the Rydberg constant (1:0973�107 m�1), SM is a screening constant, andusually zero for K˛ line and one for Kˇ line. Furthermore, n1 and n2 represent theprincipal quantum number of the inner shell and outer shell, respectively, involvedin the generation of characteristic X-rays. For example, n1 D 1 for K shell, n2 D 2

for L shell, and n3 D 3 for M shell. As characteristic X-rays are generated whenthe applied voltage exceeds the so-called excitation voltage, corresponding to thepotential required to eject an electron from the K shell (e.g., Cu: 8.86 keV, Mo:20.0 keV), the following approximate relation is available between the intensity ofK˛ radiation, IK, and the tube current, i , the applied voltage V , and the excitationvoltage VK:

IK D BSi.V � VK/1:67 (1.15)

Here, BS is a constant and the value of BS D 4:25�108 is usually employed. As it isclear from (1.15), larger the intensity of characteristic X-rays, the larger the appliedvoltage and current.

It can be seen from (1.14), characteristic radiation is emitted as a photoelec-tron when the electron of a specific shell (the innermost shell of electrons, theK shell) is released from the atom, when the electrons are pictured as orbitingLi

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1.3 Absorption of X-rays 5

the nucleus in specific shells. Therefore, this phenomenon occurs with a specificenergy (wavelength) and is called “photoelectric absorption.” The energy, Eej, ofthe photoelectron emitted may be described in the following form as a differenceof the binding energy (EB) for electrons of the corresponding shell with which thephotoelectron belongs and the energy of incidence X-rays (h�):

Eej D h� � EB (1.16)

The recoil of atom is necessarily produced in the photoelectric absorption pro-cess, but its energy variation is known to be negligibly small (see Question 1.6).Equation (1.16) is based on such condition. Moreover, the value of binding energy(EB) is also called absorption edge of the related shell.

1.3 Absorption of X-rays

X-rays which enter a sample are scattered by electrons around the nucleus of atomsin the sample. The scattering usually occurs in various different directions other thanthe direction of the incident X-rays, even if photoelectric absorption does not occur.As a result, the reduction in intensity of X-rays which penetrate the substance isnecessarily detected. When X-rays with intensity I0 penetrate a uniform substance,the intensity I after transmission through distance x is given by.

I D I0e��x (1.17)

Here, the proportional factor � is called linear absorption coefficient, which isdependent on the wavelength of X-rays, the physical state (gas, liquid, and solid)or density of the substance, and its unit is usually inverse of distance. However,since the linear absorption coefficient � is proportional to density �,.�=�/ becomesunique value of the substance, independent upon the state of the substance. Thequantity of .�=�/ is called the mass absorption coefficient and the specific valuesfor characteristic X-rays frequently-used are compiled (see Appendix A.2). Equa-tion (1.17) can be re-written as (1.18) in terms of the mass absorption coefficient.

I D I0e��

��

��x (1.18)

Mass absorption coefficient of the sample of interest containing two or more ele-ments can be estimated from (1.19) using the bulk density, �, and weight ratio of wj

for each element j.

��

�

�D w1

��

�

�1

C w2

��

�

�2

C � � � DXjD1

wj

��

�

�j

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Fig. 1.2 Wavelength dependences of mass absorption coefficient of X-ray using the La as anexample

Absorption of X-rays becomes small as transmittivity increases with increasingenergy (wavelength becomes shorter). However, if the incident X-ray energy comesclose to a specific value (or wavelength) as shown in Fig. 1.2, the photoelectricabsorption takes place by ejecting an electron in K-shell and then discontinu-ous variation in absorption is found. Such specific energy (wavelength) is calledabsorption edge. It may be added that monotonic variation in energy (wavelength)dependence is again detected when the incident X-ray energy is away from theabsorption edge.

1.4 Solved Problems (12 Examples)

Question 1.1 Calculate the energy released per carbon atom when 1 g ofcarbon is totally converted to energy.

Answer 1.1 Energy E is expressed by Einstein’s relation of E D mc2 where m ismass and c is the speed of light. If this relationship is utilized, considering SI unitthat expresses mass in kg,

E D 1 � 10�3 � .2:998 � 1010/2 D 8:99 � 1013 J

The atomic weight per mole (molar mass) for carbon is 12.011 g from referencetable (for example, Appendix A.2). Thus, the number of atoms included in 1 gcarbon is calculated as .1=12:011/ � 0:6022 � 1024 D 5:01 � 1022 because thenumbers of atoms are included in one mole of carbon is the Avogadro’s numberLi

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1.4 Solved Problems 7

.0:6022 � 1024/. Therefore, the energy release per carbon atom can be estimated as:

.8:99 � 1013/

.5:01 � 1022/D 1:79 � 10�9 J

Question 1.2 Calculate (1) strength of the electric field E , (2), force on theelectron F , (3) acceleration of electron ˛, when a voltage of 10 kV is appliedbetween two electrodes separated by an interval of 10 mm.

Answer 1.2 The work, W , if electric charge Q (coulomb, C) moves under voltage V

is expressed by W D VQ. When an electron is accelerated under 1 V of differencein potential, the energy obtained by the electron is called 1 eV. Since the elementarycharge e is 1:602 � 10�19 (C),

1eV D 1:602 � 10�19 � 1 (C)(V)

D 1:602 � 10�19 (J)

Electric field E can be expressed with E D V=d , where the distance, d , betweenelectrodes and the applied voltage being V . The force F on the electron withelementary charge e is given by;

F D eE (N)

Here, the unit of F is Newton. Acceleration ˛ of electrons is given by the followingequation in which m is the mass of the electron:

˛ D eE

m.m=s2/

.1/ E D 10 .kV/

10 .mm/D 104 .V/

10�2 .m/D 106 .V=m/

.2/ F D 1:602 � 10�19 � 106 D 1:602 � 10�13 .N/

.3/ ˛ D 1:602 � 10�13

9:109 � 10�31D 1:76 � 1017 .m=s2/

Question 1.3 X-rays are generated by making the electrically charged parti-cles (electrons) with sufficient kinetic energy in vacuum collide with cathode,as widely used in the experiment of an X-ray tube. The resultant X-rays canbe divided into two parts: continuous X-rays (also called white X-rays) andcharacteristic X-rays. The wavelength distribution and intensity of continu-ous X-rays are usually depending upon the applied voltage. A clear limit isrecognized on the short wavelength side.Li

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(1) Estimate the speed of electron before collision when applied voltage is30,000 V and compare it with the speed of light in vacuum.

(2) In addition, obtain the relation of the shortest wavelength limit �SWL ofX-rays generated with the applied voltage V , when an electron loses allenergy in a single collision.

Answer 1.3 Electrons are drawn out from cathode by applying the high voltage oftens of thousands of V between two metallic electrodes installed in the X-ray tubein vacuum. The electrons collide with anode at high speed. The velocity of electronsis given by,

eV D mv2

2! v2 D 2eV

m

where e is the electric charge of the electron, V the applied voltage across theelectrodes, m the mass of the electron, and v the speed of the electron before thecollision. When values of rest mass me D 9:110 � 10�31 .kg/ as mass of electron,elementary electron charge e D 1:602 � 1019 .C/ and V D 3 � 104 .V/ are used forcalculating the speed of electron v.

v2 D 2 � 1:602 � 10�19 � 3 � 104

9:110 � 10�31D 1:055 � 1016; v D 1:002 � 108 m=s

Therefore, the speed of electron just before impact is about one-third of the speedof light in vacuum .2:998 � 108 m=s/.

Some electrons release all their energy in a single collision. However, some otherelectrons behave differently. The electrons slow down gradually due to successivecollisions. In this case, the energy of electron (eV) which is released partially andthe corresponding X-rays (photon) generated have less energy compared with theenergy (h�max) of the X-rays generated when electrons are stopped with one colli-sion. This is a factor which shows the maximum strength moves toward the shorterwavelength sides, as X-rays of various wavelengths generate, and higher the inten-sity of the applied voltage, higher the strength of the wavelength of X-rays (seeFig. 1). Every photon has the energy h�, where h is the Planck constant and � thefrequency.

The relationship of eV D h�max can be used, when electrons are stopped in oneimpact and all energy is released at once. Moreover, frequency (�) and wavelength(�) are described by a relation of � D c=�, where c is the speed of light. Therefore,the relation between the wavelength �SWL in m and the applied voltage V may begiven as follows:

�SWL D c=�max D hc=eV D .6:626 � 10�34/ � .2:998 � 108/

.1:602 � 10�19/VD .12:40 � 10�7/

V

This relation can be applied to more general cases, such as the production of electro-magnetic waves by rapidly decelerating any electrically charged particle includingLi

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electron of sufficient kinetic energy, and it is independent of the anode material.When wavelength is expressed in nm, voltage in kV, and the relationship becomes�V D 1:240.

10

8

6

4

2

00.01 0.02 0.05 0.1 0.2 0.4

Wavelength [nm]

Inte

nsity

[a.u

.]100kV

80kV

60kV

20kV

40kV

Fig. 1 Schematic diagram for X-ray spectrum as a function of applied voltage

Question 1.4 K˛1 radiation of Fe is the characteristic X-rays emitted whenone of the electrons in L shell falls into the vacancy produced by knockingan electron out of the K-shell, and its wavelength is 0.1936 nm. Obtain theenergy difference related to this process for X-ray emission.

Answer 1.4 Consider the process in which an L shell electron moves to the vacancycreated in the K shell of the target (Fe) by collision with highly accelerated electronsfrom a filament. The wavelength of the photon released in this process is given by�, (with frequency �). We also use Planck’s constant h of .6:626 � 10�34 Js/ andthe velocity of light c of .2:998 � 108 ms�1/. Energy per photon is given by,

E D h� D hc

�

Using Avogadro’s number NA, one can obtain the energy difference �E related tothe X-ray release process per mole of Fe.

�E D NAhc

�D 0:6022 � 1024 � 6:626 � 10�34 � 2:998 � 108

0:1936 � 10�9

D 11:9626

0:1936� 10�7 D 6:1979 � 108 J=moleLi

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Reference: The electrons released from a filament have sufficient kinetic energy andcollide with the Fe target. Therefore, an electron of K-shell is readily ejected. Thisgives the state of FeC ion left in an excited state with a hole in the K-shell. Whenthis hole is filled by an electron from an outer shell (L-shell), an X-ray photon isemitted and its energy is equal to the difference in the two electron energy levels.This variation responds to the following electron arrangement of FeC.

Before release K1 L8 M14 N2After release K2 L7 M14 N2

Question 1.5 Explain atomic density and electron density.

Answer 1.5 The atomic density Na of a substance for one-component system isgiven by the following equation, involving atomic weight M , Avogadro’s numberNA, and the density �.

Na D NA

M�: (1)

In the SI system, Na .m�3/, NA D 0:6022 � 1024 .mol�1/, �.kg=m3/, andM .kg=mol/, respectively.

The electron density Ne of a substance consisting of single element is given by,

Ne D NA

MZ� (2)

Each atom involves Z electrons (usually Z is equal to the atomic number) and theunit of Ne is also .m�3/.

The quantity Na D NA=M in (1) or Ne D .NAZ/=M in (2), respectively, givesthe number of atoms or that of electrons per unit mass (kg), when excluding den-sity, �. They are frequently called “atomic density” or “electron density.” However,it should be kept in mind that the number per m3 (per unit volume) is completelydifferent from the number per 1 kg (per unit mass). For example, the following val-ues of atomic number and electron number per unit mass (D1kg) are obtained foraluminum with the molar mass of 26.98 g and the atomic number of 13:

Na D 0:6022 � 1024

26:98 � 10�3D 2:232 � 1025 .kg�1/

Ne D 0:6022 � 1024

26:98 � 10�3� 13 D 2:9 � 1026 .kg�1/

Since the density of aluminum is 2:70 Mg=m3 D 2:70 � 103 kg=m3 from referencetable (Appendix A.2), we can estimate the corresponding values per unit volume asNa D 6:026 � 1028 .m�3/ and Ne D 7:83 � 1029 .m�3/, respectively.Li

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Reference: Avogadro’s number provides the number of atom (or molecule) includedin one mole of substance. Since the atomic weight is usually expressed by the num-ber of grams per mole, the factor of 10�3 is required for using Avogadro’s numberin the SI unit system.

Question 1.6 The energy of a photoelectron, Eej, emitted as the result of pho-toelectron absorption process may be given in the following with the bindingenergy EB of the electron in the corresponding shell:

Eej D h� � EB

Here, h� is the energy of incident X-rays, and this relationship has beenobtained with an assumption that the energy accompanying the recoil of atom,which necessarily occurs in photoelectron absorption, is negligible.

Calculate the energy accompanying the recoil of atom in the followingcondition for Pb. The photoelectron absorption process of K shell for Pb wasmade by irradiating X-rays with the energy of 100 keV against a Pb plate andassuming that the momentum of the incident X-rays was shared equally byPb atom and photoelectron. In addition, the molar mass (atomic weight) ofPb is 207.2 g and the atomic mass unit is 1amu D 1:66054 � 10�27 kg D931:5 � 103 keV.

Answer 1.6 The energy of the incident X-rays is given as 100 keV, so that itsmomentum can be described as being 100 keV=c, using the speed of light c. Sincethe atom and photoelectron shared the momentum equally, the recoil energy of atomwill be 50 keV=c. Schematic diagram of this process is illustrated in Fig. 1.

Fig. 1 Schematic diagram for the photo electron absorption process assuming that the momentumof the incident X-rays was shared equally by atom and photoelectron. Energy of X-ray radiation is100 keV

On the other hand, one should consider for the atom that 1amu D 931:5�103 keVis used in the same way as the energy which is the equivalent energy amount ofthe rest mass for electron, me. The molar mass of 207.2 g for Pb is equivalent toLi

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207.2 amu, so that the mass of 1 mole of Pb is equivalent to the energy of 207:2 �931:5 � 103 D 193006:8 � 103 keV=c.

When the speed of recoil atom is v and the molar mass of Pb is MA, its energycan be expressed by 1

2MAv2. According to the given assumption and the momen-

tum described as p D MAv, the energy of the recoil atom, EAr , may be obtained

as follows:

EAr D 1

2MAv2 D p2

2MAD .50/2

2 � .193006:8 � 103/D 0:0065 � 10�3 .keV/

The recoil energy of atom in the photoelectron absorption process shows just avery small value as mentioned here using the result of Pb as an example, althoughthe recoil of the atom never fails to take place.

Reference:

Energy of 1 amu D 1:66054 � 10�27 � .2:99792 � 108/2

1:60218 � 10�19D 9:315 � 108 .eV/

On the other hand, the energy of an electron with rest mass me D 9:109 � 10�31 .kg/

can be obtained in the following with the relationship of 1 .eV/ D 1:602 � 10�19 .J/:

E D mec2 D 9:109 � 10�31 � .2:998 � 108/2

1:602 � 1019D 0:5109 � 106 .eV/

Question 1.7 Explain the Rydberg constant in Moseley’s law with respect tothe wavelength of characteristic X-rays, and obtain its value.

Answer 1.7 Moseley’s law can be written as,

1

�D R.Z � SM/2

�1

n21

� 1

n22

�(1)

The wavelength of the X-ray photon .�/ corresponds to the shifting of an electronfrom the shell of the quantum number n2 to the shell of the quantum number of n1.Here, Z is the atomic number and SM is a screening constant.

Using the elementary electron charge of e, the energy of electron characterizedby the circular movement around the nucleus charge Ze in each shell (orbital) maybe given, for example, with respect to an electron of quantum number n1 shell inthe following form:

En D �2�2me4

h2

Z2

n21

(2)

Here, h is a Planck constant and m represents the mass of electron. The energy ofthis photon is given by,Li

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h� D En2� En1

D �E D 2�2me4

h2Z2

�1

n21

� 1

n22

�(3)

The following equation will also be obtained, if the relationship of E D h� D hc�

is employed while using the velocity of photon, c:

1

�D 2�2me4

ch3Z2

�1

n21

� 1

n22

�(4)

If the value of electron mass is assumed to be rest mass of electron and a compar-ison of (1) with (4) is made, the Rydberg constant R can be estimated. It may benoted that the term of .Z � SM/2 in (1) could be empirically obtained from themeasurements on various characteristic X-rays as reported by H.G.J. Moseley in1913.

R D 2�2me4

ch3D 2 � .3:142/2 � .9:109 � 10�28/ � .4:803 � 10�10/4

.2:998 � 1010/ � .6:626 � 10�27/3

D 109:743 � 103 .cm�1/ D 1:097 � 107 .m�1/ (5)

The experimental value of R can be obtained from the ionization energy (�13.6 eV)of hydrogen (H). The corresponding wave number (frequency) is 109737:31 cm�1,in good agreement with the value obtained from (5). In addition, since Moseley’slaw and the experimental results are all described by using the cgs unit system (gausssystem), 4:803 � 10�10 esu has been used for the elemental electron charge e. Con-version into the SI unit system is given by (SI unit � velocity of light � 10�1) (e.g.,5th edition of the Iwanami Physics-and-Chemistry Dictionary p. 1526 (1985)). Thatis to say, the amount of elementary electron charge e can be expressed as:

1:602 � 10�19Coulomb � 2:998 � 1010 cm=s � 10�1 D 4:803 � 10�10 esu

The Rydberg constant is more strictly defined by the following equation:

R D 2�2�e4

ch3(6)

1

�D 1

mC 1

mp(7)

Here, m is electron mass and mP is nucleus (proton) mass.The detected differenceis quite small, but the value of mP depends on the element. Then, it can be seenfrom the relation of (6) and (7) that a slightly different value of R is obtained foreach element. However, if a comparison is made with a hydrogen atom, there is adifference of about 1,800 times between the electron mass me D 9:109 � 10�31 kgand the proton mass which is mP D 1:67 � 10�27 kg. Therefore, the relationship of(6) is usually treated as � D m, because mP is very large in comparison with me.Li

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Reference: The definition of the Rydberg constant in the SI unit is given in the formwhere the factor of .1=4�0/ is included by using the dielectric constant 0.8:854�10�12 F=m/ in vacuum for correlating with nucleus charge Ze.

R D 2�2�e4

ch3�

�1

4�0

�2

D me4

820ch3

D 9:109 � 10�31 � .1:602 � 10�19/4

8 � .8:854 � 10�12/2 � .2:998 � 108/ � .6:626 � 10�34/3

D 9:109 � .1:602/4 � 10�107

8 � .8:854/2 � .2:998/ � .6:626/3 � 10�118D 1:097 � 107 .m�1/

Question 1.8 When the X-ray diffraction experiment is made for a platesample in the transmission mode, it is readily expected that absorptionbecomes large and diffraction intensity becomes weak as the sample thicknessincreases. Obtain the thickness of a plate sample which makes the diffrac-tion intensity maximum and calculate the value of aluminum for the Cu-K˛

radiation.

Backside

Surfaceside

xdx

t

t-xI0 I

Fig. A Geometry for a case where X-ray penetrates a plate sample

Answer 1.8 X-ray diffraction experiment in the transmission mode includes bothabsorption and scattering of X-rays. Let us consider the case where the samplethickness is t , the linear absorption coefficient �, the scattering coefficient S , andthe intensity of incident X-rays I0 as referred to Fig. A.

Since the intensity of the incident X-rays reaching the thin layer dx which is atdistance of x from the sample surface is given by I0e��x , the scattering intensitydI 0

x from the thin layer dx (with scattering coefficient S ) is given by the followingequation:

dI 0x D SI0e��xdx (1)

The scattering intensity dIx passes through the distance of .t � x/ in the sampleand the absorption during this passage is expressed by the form of e��.t�x/. There-fore, the scattering intensity of the thin layer dx after passing through the samplemay be given in the following form:

dIx D .SI0e��xdx/e��.t�x/ D SI0e��t dx (2)Libr

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The scattering intensity of the overall sample will be equal to the result obtainedby integrating the intensity of the thin layer dx with respect to the sample thicknessfrom zero to t .

I DZ t

0

SI0e��t dx D SI0t � e��t (3)

The maximum value of I is given under the condition of dI=dt D 0.

dI

dtD SI0.e��t � t�e��t / D 0; t� D 1 ! t D 1

�(4)

We can find the values of mass absorption coefficient .�=�/ and density .�/

of aluminum for Cu-K˛ radiation in the reference table (e.g., Appendix A.2). Theresults are .�=�/ D 49:6 cm2=g and � D 2:70 g=cm3, respectively. The linearabsorption coefficient of aluminum is calculated in the following:

� D�

�

�

�� D 49:6 � 2:70 D 133:92 .cm�1/

Therefore, the desired sample thickness t can be estimated as follows:

t D 1

�D

�1

133:92

�D 7:47 � 10�3 .cm/ D 74:7 .m/

Question 1.9 There is a substance of linear absorption coefficient �.

(1) Obtain a simple relation to give the sample thickness x required to reducethe amount of transmitted X-ray intensity by half.

(2) Calculate also the corresponding thickness of Fe-17 mass % Cr alloy.density D 7:76 � 106 g=m3/ for Mo-K˛ radiation, using the relationobtained in (1).

Answer 1.9 Let us consider the intensity of the incident X-rays as I0 and that of thetransmitted X-rays as I . Then,

I D I0e��x (1)

If the condition of I D I0

2is imposed, taken into account, one obtains,

I0

2D I ��x

e (2)

1

2D e��x (3)Li

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When the logarithm of both sides is taken, we obtain log 1 � log 2 D ��x log e.The result is � log 2 D ��x, as they are log1 D 0, and loge D 1. Here, naturallogarithm is used and the required relation is given as follows:

x D log2

�' 0:693

�(4)

The values of mass absorption coefficients of Fe and Cr for the Mo-K˛ radiationare 37:6 and 29:9 cm2=g obtained from Appendix A.2, respectively. The concentra-tion of Cr is given by 17 mass %, so that the weight ratio of two alloy componentscan be set as wFe D 0:83 and wCr D 0:17. Then, the mass absorption coefficient ofthe alloy is expressed in the following:

��

�

�Alloy

D wFe

��

�

�Fe

C wCr

��

�

�Cr

D 0:83 � .37:6/ C 0:17 � .29:9/ D 36:3 .cm2=g/

Next, note that the unit of the density of the Fe–Cr alloy is expressed in cgs,7:76 � 106 g=m3 D 7:76 g=cm3. We obtain,

�Alloy D 36:3 � 7:76 .cm�1/ D 281:7 .cm�1/

x D 0:693

281:7D 0:0025 cm D 0:025 mm D 25 m

Question 1.10 Calculate the mass absorption coefficient of lithium niobate.LiNbO3/ for Cu-K˛ radiation.

Answer 1.10 The atomic weight of Li, Nb, and oxygen (O) and their mass absorp-tion coefficients for Cu-K˛ radiation are obtained from Appendix A.2, as follows:

Atomic weight Mass-absorption coefficient

(g) �=� .cm2=g/

Li 6.941 0.5

Nb 92.906 145

O 15.999 11.5

The molar weight(molar mass) M per 1 mole of LiNbO3 is given in the following:

M D 6:941 C 92:906 C .15:999 � 3/ D 147:844 .g/

The weight ratio wj of three components of Li, Nb, and O is to be obtained.Libr

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wLi D 6:941

147:844D 0:047; wNb D 92:906

147:844D 0:628; wO D 47:997

147:844D 0:325

Then, the mass absorption coefficient of lithium niobate can be obtained as follows:

��

�

�LiNbO3

DX

wj

��

�

�jD 0:047 � 0:5 C 0:628 � 145 C 0:325 � 11:5

D 94:8 .cm2=g/

Question 1.11 A thin plate of pure iron is suitable for a filter for Co-K˛

radiation, but it is also known to easily oxidize in air. For excluding suchdifficulty,we frequently utilize crystalline hematite powder (Fe2O3:density5:24�106 g=m3). Obtain the thickness of a filter consisting of hematite powderwhich reduces the intensity of Co-Kˇ radiation to 1/500 of the K˛ radiationcase. Given condition is as follows. The intensity ratio between Co-K˛ andCo-Kˇ is found to be given by 5:1 without a filter. The packing density ofpowder sample is known usually about 70% of the bulk crystal.

Answer 1.11 The atomic weight of Fe and oxygen (O) and their mass absorptioncoefficients for Co-K˛ and Co-Kˇ radiations are obtained from Appendix A.2, asfollows:

Atomic � /� for Co-K˛ � /� for Co-Kˇ

weight (g) (cm2=g) (cm2=g)

Fe 55.845 57.2 342

O 15.999 18.0 13.3

The weight ratio of Fe and O in hematite crystal is estimated in the following:

MFe2O3 D 55:845 � 2 C 15:999 � 3 D 159:687

wFe D 55:845 � 2

159:687D 0:699; wO D 0:301

The mass absorption coefficients of hematite crystals for Co-K˛ and Co-Kˇ radia-tions are, respectively, to be calculated.

��

�

�˛

Fe2O3

D 0:699 � 57:2 C 0:301 � 18:0 D 45:4 .cm2=g/

��

�

�ˇ

Fe2O3

D 0:699 � 342 C 0:301 � 13:3 D 243:1 .cm2=g/Libr

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It is noteworthy that the density of hematite in the filter presently prepared is equiv-alent to 70% of the value of bulk crystal by considering the packing density, so thatwe have to use the density value of �f D 5:24 � 0:70 D 3:67 g=cm3 Therefore, thevalue of the linear absorption coefficient of hematite powder packed into the filterfor Co-K˛ and Co-Kˇ radiations will be, respectively, as follows:

�˛ D�

�

�

�˛

Fe2O3

� �f D 45:4 � 3:67 D 166:6 .cm�1/

�ˇ D�

�

�

�ˇ

Fe2O3

� �f D 24:1 � 3:67 D 892:2 .cm�1/

The intensity ratio of Co-K˛ and Co-Kˇ radiations before and after passing throughthe filter consisting of hematite powder may be described in the following equation:

ICo�Kˇ

ICo�K˛

D Iˇ0 e��ˇt

I ˛0 e��˛ t

From the given condition, the ratio between I ˛0 and I

ˇ0 is 5:1 without filter, and it

should be 500:1 after passing through the filter. They are expressed as follows:

1

500D 1

5

e��ˇt

e��˛ t! 1

100D e.�˛��ˇ/t

Take the logarithm of both sides and obtain the thickness by using the values of �˛

and �ˇ .

.�˛ � �ˇ /t D � log 100 .* log e D 1; log 1 D 0/

.166:6 � 892:2/t D �4:605

t D 0:0063 .cm�1/ D 63 .m/

Question 1.12 For discussing the influence of X-rays on the human bodyetc., it would be convenient if the effect of a substance consisting of multi-elements, such as water (H2O) and air (N2, O2, others), can be described byinformation of each constituent element (H, O, N, and others) with an appro-priate factor. For this purpose, the value of effective element number NZ isoften used and it is given by the following equation:

NZ D 2:94

qa1Z2:94

1 C a2Z2:942 C � � �

where a1; a2 : : : is the electron component ratio which corresponds to the rateof the number of electrons belonging to each element with the atomic numberLi

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Z1; Z2; : : : to the total number of electrons of a substance. Find the effectiveatomic number of water and air. Here, the air composition is given by 75.5%of nitrogen, 23.2% of oxygen, and 1.3% of argon in weight ratio.

Answer 1.12 Water (H2O) consists of two hydrogen atoms and one oxygen atom,whereas the number of electrons are one for hydrogen and eight for oxygen. Thevalues of atomic weight per mole (molar mass) of hydrogen and oxygen (molarmass) are 1.008 and 15.999 g, respectively. Each electron density per unit mass isgiven as follows:

For hydrogen N He D 0:6022 � 1024

1:008� 1 D 0:597 � 1024 .g�1/

For oxygen N Oe D 0:6022 � 1024

15:999� 8 D 0:301 � 1024 .g�1/

In water (H2O), the weight ratio can be approximated by 2=18 for hydrogen and16=18 for oxygen, respectively. Then, the number of electrons in hydrogenand oxygen contained in 1 g water are 0:597 � 1024 � .2=18/ D 0:0663 � 1024

and 0:301 � 1024 � .16=18/ D 0:2676 � 1024,respectively, so that the numberof electrons contained in 1 g water is estimated to be .0:0663 C 0:2676/ � 1024 D0:3339�1024. Therefore, the electron component ratio of water is found as follows:

aH D 0:0663

0:3339D 0:199

aO D 0:2262

0:3339D 0:801

NZ D 2:94p

0:199 � 12:94 C 0:801 � 82:94

D 2:94p

0:199 C 362:007 D 2:94p

362:206 D 7:42

Here, we use the relationship of NZ D X1y ! ln NZ D 1

ylnX ! NZ D e

1y

lnX

On the other hand, the molar masses of nitrogen, oxygen, and argon are 14.01,15.999, and 39.948 g, respectively. Since 75.5% of nitrogen (7 electrons), 23.2% ofoxygen (8 electrons), and 1.3% of argon (18 electrons) in weight ratio are containedin 1 g of air, each electron numbers is estimated in the following:

For nitrogen N Ne D 0:6022 � 1024

14:01� 0:755 � 7 D 0:2272 � 1024

For oxygen N Oe D 0:6022 � 1024

15:999� 0:232 � 8 D 0:0699 � 1024

For argon N Are D 0:6022 � 1024

39:948� 0:013 � 18 D 0:0035 � 1024Li

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Therefore, the value of .0:2272 C 0:0699 C 0:0035/ � 1024 D 0:3006 � 1024 iscorresponding to the number of electrons in 1 g of air. The rate to the total numberof electrons of each element is as follows:

aN D 0:2272

0:3006D 0:756

aO D 0:0699

0:3006D 0:232

aAr D 0:0035

0:3006D 0:012

Accordingly, the effective atomic number of air is estimated in the following:

NZ D 2:94p

0:756 � 72:94 C 0:232 � 82:94 C 0:012 � 182:94

D 2:94p

230:73 C 104:85 C 58:84 D 2:94p

394:42 D 7:64

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http://www.springer.com/978-3-642-16634-1Li

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Radiographic Testing Page 1 of 47

Radiographic Testing

Radiography is used in a very wide range of aplications including medicine, engineering, forensics, security, etc. In NDT, radiography is one of the most important and widely used methods. Radiographic testing (RT) offers a number of advantages over other NDT methods, however, one of its major disadvantages is the health risk associated with the radiation.

In general, RT is method of inspecting materials for hidden flaws by using the ability of short wavelength electromagnetic radiation (high energy photons) to penetrate various materials. The intensity of the radiation that penetrates and passes through the material is either captured by a radiation sensitive film (Film Radiography) or by a planer array of radiation sensitive sensors (Real-time Radiography). Film radiography is the oldest approach, yet it is still the most widely used in NDT.

Basic Principles

In radiographic testing, the part to be inspected is placed between the radiation source and a piece of radiation sensitive film. The radiation source can either be an X-ray machine or a radioactive source (Ir-192, Co-60, or in rare cases Cs-137). The part will stop some of the radiation where thicker and more dense areas will stop more of the radiation. The radiation that passes through the part will expose the film and forms a shadowgraph of the part. The film darkness (density) will vary with the amount of radiation reaching the film through the test object where darker areas indicate more exposure (higher radiation intensity) and liter areas indicate less exposure (higher radiation intensity).

This variation in the image darkness can be used to determine thickness or composition of material and would also reveal the presence of any flaws or discontinuities inside the material.

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The primary advantages and disadvantages in comparison to other NDT methods are:

Advantages

Both surface and internal discontinuities can be detected.

Significant variations in composition can be detected.

It has a very few material limitations.

Can be used for inspecting hidden areas (direct access to surface is not required)

Very minimal or no part preparation is required.

Permanent test record is obtained.

Good portability especially for gamma-ray sources.

Disadvantages

Hazardous to operators and other nearby personnel.

High degree of skill and experience is required for exposure and interpretation.

The equipment is relatively expensive (especially for x-ray sources).

The process is generally slow.

Highly directional (sensitive to flaw orientation).

Depth of discontinuity is not indicated.

It requires a two-sided access to the component.

PHYSICS OF RADIATION

Nature of Penetrating Radiation

Both X-rays and gamma rays are electromagnetic waves and on the electromagnetic spectrum they ocupy frequency ranges that are higher than ultraviolate radiation. In terms of frequency, gamma rays generaly have higher frequencies than X-rays as seen in the figure . The major distenction between X-rays and gamma rays is the origion where X-rays are usually artificially produced using an X-ray generator and gamma radiation is the product of radioactive materials. Both X-rays and gamma rays are waveforms, as are light rays, microwaves, and radio waves. X-rays and gamma rays cannot been seen, felt, or heard. They possess no charge and no mass and, therefore,

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are not influenced by electrical and magnetic fields and will generally travel in straight lines. However, they can be diffracted (bent) in a manner similar to light.

Electromagentic radiation act somewhat like a particle at times in that they occur as small “packets” of energy and are referred to as “photons”. Each photon contains a certain amount (or bundle) of energy, and all electromagnetic radiation consists of these photons. The only difference between the various types of electromagnetic radiation is the amount of energy found in the photons. Due to the short wavelength of X-rays and gamma rays, they have more energy to pass through matter than do the other forms of energy in the electromagnetic spectrum. As they pass through matter, they are scattered and absorbed and the degree of penetration depends on the kind of matter and the energy of the rays.

Properties of X-Rays and Gamma Rays

They are not detected by human senses (cannot be seen, heard, felt, etc.).

They travel in straight lines at the speed of light.

Their paths cannot be changed by electrical or magnetic fields.

They can be diffracted, refracted to a small degree at interfaces between two different materials, and in some cases be reflected.

They pass through matter until they have a chance to encounter with an atomic particle.

Their degree of penetration depends on their energy and the matter they are traveling through.

They have enough energy to ionize matter and can damage or destroy living cells.

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X-Radiation

X-rays are just like any other kind of electromagnetic radiation. They can be produced in packets of energy called photons, just like light. There are two different atomic processes that can produce X-ray photons. One is called Bremsstrahlung (a German term meaning “braking radiation”) and the other is called K-shell emission. They can both occur in the heavy atoms of tungsten which is often the material chosen for the target or anode of the X-ray tube.

Both ways of making X-rays involve a change in the state of electrons. However, Bremsstrahlung is easier to understand using the classical idea that radiation is emitted when the velocity of the electron shot at the tungsten target changes. The negatively charged electron slows down after swinging around the nucleus of a positively charged tungsten atom and this energy loss produces X-radiation. Electrons are scattered elastically or inelastically by the positively charged nucleus. The inelastically scattered electron loses energy, and thus produces X-ray photon, while the elastically scattered electrons generally change their direction significantly but without loosing much of their energy.

Bremsstrahlung Radiation

X-ray tubes produce X-ray photons by accelerating a stream of electrons to energies of several hundred kiloelectronvolts with velocities of several hundred kilometers per hour and colliding them into a heavy target material. The abrupt acceleration of the charged particles (electrons) produces Bremsstrahlung photons. X-ray radiation with a continuous spectrum of energies is produced with a range from a few keV to a maximum of the energy of the electron beam.

The Bremsstrahlung photons generated within the target material are attenuated as they pass through, typically, 50 microns of target material. The beam is further attenuated by the aluminum or beryllium vacuum window. The results are the elimination of the low energy photons, 1 keV through 15 keV, and a significant reduction in the portion of the spectrum from 15 keV through 50 keV. The spectrum from an X-ray tube is further modified by the filtration caused by the selection of filters used in the setup.

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K-shell Emission Radiation

Remember that atoms have their electrons arranged in closed “shells” of different energies. The K-shell is the lowest energy state of an atom. An incoming electron can give a K-shell electron enough energy to knock it out of its energy state. About 0.1% of the electrons produce K-shell vacancies; most produce heat. Then, a tungsten electron of higher energy (from an outer shell) can fall into the K-shell. The energy lost by the falling electron shows up as an emitted X-ray photon. Meanwhile, higher energy electrons fall into the vacated energy state in the outer shell, and so on. After losing an electron, an atom remains ionized for a very short time (about 10-14 second) and thus an atom can be repeatedly ionized by the incident electrons which arrive about every 10-12 second. Generally, K-shell emission produces higher-intensity X-rays than Bremsstrahlung, and the X-ray photon comes out at a single wavelength.

Gamma Radiation

Gamma radiation is one of the three types of natural radioactivity. Gamma rays are electromagnetic radiation just like X-rays. The other two types of natural radioactivity are alpha and beta radiation, which are in the form of particles. Gamma rays are the most energetic form of electromagnetic radiation.

Gamma radiation is the product of radioactive atoms. Depending upon the ratio of neutrons to protons within its nucleus, an isotope of a particular element may be stable or unstable. When the binding energy is not strong enough to hold the nucleus of an atom together, the atom is said to be unstable. Atoms with unstable nuclei are constantly changing as a result of the imbalance of energy within the nucleus. Over time, the nuclei of unstable isotopes spontaneously disintegrate, or transform, in a process known as “radioactive decay” and such material is called “radioactive material”.

Types of Radiation Produced by Radioactive Decay

When an atom undergoes radioactive decay, it emits one or more forms of high speed subatomic particles ejected from the nucleus or electromagnetic radiation (gamma-rays) emitted by either the nucleus or orbital electrons.

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Alpha Particles

Certain radioactive materials of high atomic mass (such as Ra-226, U-238, Pu-239), decay by the emission of alpha particles. These alpha particles are tightly bound units of two neutrons and two protons each (He-4 nucleus) and have a positive charge. Emission of an alpha particle from the nucleus results in a decrease of two units of atomic number (Z) and four units of mass number (A). Alpha particles are emitted with discrete energies characteristic of the particular transformation from which they originate. All alpha particles from a particular radionuclide transformation will have identical energies.

Beta Particles

A nucleus with an unstable ratio of neutrons to protons may decay through the emission of a high speed electron called a beta particle. In beta decay a neutron will split into a positively charged proton and a negatively charged electron. This results in a net change of one unit of atomic number (Z) and no change in the mass number (A). Beta particles have a negative charge and the beta particles emitted by a specific radioactive material will range in energy from near zero up to a maximum value, which is characteristic of the particular transformation.

Gamma-rays

A nucleus which is in an excited state (unstable nucleus) may emit one or more photons of discrete energies. The emission of gamma rays does not alter the number of protons or neutrons in the nucleus but instead has the effect of moving the nucleus from a higher to a lower energy state (unstable to stable). Gamma ray emission frequently follows beta decay, alpha decay, and other nuclear decay processes.

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Activity (of Radioactive Materials)

The quantity which expresses the radiation producing potential of a given amount of radioactive material is called “Activity”. The Curie (Ci) was originally defined as that amount of any radioactive material that disintegrates at the same rate as one gram of pure radium. The International System (SI) unit for activity is the Becquerel (Bq), which is that quantity of radioactive material in which one atom is transformed per second. The radioactivity of a given amount of radioactive material does not depend upon the mass of material present. For example, two one-curie sources of the same radioactive material might have very different masses depending upon the relative proportion of non-radioactive atoms present in each source.

The concentration of radioactivity, or the relationship between the mass of radioactive material and the activity, is called “specific activity”. Specific activity is expressed as the number of Curies or Becquerels per unit mass or volume. Each gram of Cobalt-60 will contain approximately 50 Ci. Iridium-192 will contain 350 Ci for every gram of material. The higher specific activity of iridium results in physically smaller sources. This allows technicians to place the source in closer proximity to the film while maintaining the sharpness of the radiograph.

Isotope Decay Rate (Half-Life)

Each radioactive material decays at its own unique rate which cannot be altered by any chemical or physical process. A useful measure of this rate is the “half-life” of the radioactivity. Half-life is defined as the time required for the activity of any particular radionuclide to decrease to one-half of its initial value. In other words one-half of the atoms have reverted to a more stable state material. Half-lives of radioactive materials range from microseconds to billions of years. Half-life of two widely used industrial isotopes are; 74 days for Iridium-192, and 5.3 years for Cobalt-60.

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In order to find the remaining activity of a certain material with a known half-life value after a certain period of time, the following formula may be used. The formula calculates the decay fraction (or the remaining fraction of the initial activity) as:

Where; : decay fraction (i.e., remaining fraction of the initial activity) : Half-Life value (hours, days, years, etc.) : Elapsed time (hours, days, years, etc.)

Or alternatively, the equitation can be solved to find the time required for activity to decay to a certain level as:

Radiation Energy, Intensity and Exposure

Different radioactive materials and X-ray generators produce radiation at different energy levels and at different rates. It is important to understand the terms used to describe the energy and intensity of the radiation.

Radiation Energy

The energy of the radiation is responsible for its ability to penetrate matter. Higher energy radiation can penetrate more and higher density matter than low energy radiation. The energy of ionizing radiation is measured in electronvolts (eV). One electronvolt is an extremely small amount of energy so it is common to use kiloelectronvolts (keV) and megaelectronvolt (MeV). An electronvolt is a measure of energy, which is different from a volt which is a measure of the electrical potential between two positions. Specifically, an electronvolt is the kinetic energy gained by an electron passing through a potential difference of one volt. X-ray generators have a control to adjust the radiation energy, keV (or kV).

The energy of a radioisotope is a characteristic of the atomic structure of the material. Consider, for example, Iridium-192 and Cobalt-60, which are two of the more common

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industrial Gamma ray sources. These isotopes emit radiation in two or three discreet wavelengths. Cobalt-60 will emit 1.17 and 1.33 MeV gamma rays, and Iridium-192 will emit 0.31, 0.47, and 0.60 MeV gamma rays. It can be seen from these values that the energy of radiation coming from Co-60 is more than twice the energy of the radiation coming from the Ir-192. From a radiation safety point of view, this difference in energy is important because the Co-60 has more material penetrating power and, therefore, is more dangerous and requires more shielding.

Intensity and Exposure

Radiation intensity is the amount of energy passing through a given area that is perpendicular to the direction of radiation travel in a given unit of time. One way to measure the intensity of X-rays or gamma rays is to measure the amount of ionization they cause in air. The amount of ionization in air produced by the radiation is called the exposure. Exposure is expressed in terms of a scientific unit called a Roentgen (R). The unit roentgen is equal to the amount of radiation that ionizes 1 cm3 of dry air (at 0°C and standard atmospheric pressure) to one electrostatic unit of charge, of either sign. Most portable radiation detection safety devices used by radiographers measure exposure and present the reading in terms of Roentgens or Roentgens/hour, which is known as the “dose rate”.

Ionization

As penetrating radiation moves from point to point in matter, it loses its energy through various interactions with the atoms it encounters. The rate at which this energy loss occurs depends upon the type and energy of the radiation and the density and atomic composition of the matter through which it is passing.

The various types of penetrating radiation impart their energy to matter primarily through excitation and ionization of orbital electrons. The term “excitation” is used to describe an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom. Excited electrons may subsequently emit energy in the form of X-rays during the process of returning to a lower energy state. The term “ionization” refers to the complete removal of an electron from an atom following the transfer of energy from a passing charged particle. Libr

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Because of their double charge and relatively slow velocity, alpha particles have a relatively short range in matter (a few centimeters in air and only fractions of a millimeter in tissue). Beta particles have, generally, a greater range.

Since they have no charge, gamma-rays and X-rays proceeds through matter until there is a chance of interaction with a particle. If the particle is an electron, it may receive enough energy to be ionized, whereupon it causes further ionization by direct interactions with other electrons. As a result, gamma-rays and X-rays can cause the liberation of electrons deep inside a medium. As a result, a given gamma or X-ray has a definite probability of passing through any medium of any depth.

Newton's Inverse Square Law

Any point source which spreads its influence equally in all directions without a limit to its range will obey the inverse square law. This comes from strictly geometrical considerations. The intensity of the influence at any given distance (d) is the source strength divided by the area of a sphere having a radius equal to the distance (d). Being strictly geometric in its origin, the inverse square law applies to diverse phenomena. Point sources of gravitational force, electric field, light, sound, and radiation obey the inverse square law.

As one of the fields which obey the general inverse square law, the intensity of the radiation received from a point radiation source can be characterized by the diagram above. The relation between intensity and distance according to the inverse square law can be expresses as:

Where are the intensities at distances form the source, respectively. Library

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All measures of exposure or dose rate will drop off by the inverse square law. For example, if the received dose of radiation is 100 mR/hr at 2 cm from a source, it will be 0.01 mR/hr at 2 m.

Interaction between Penetrating Radiation and Matter (Attenuation)

When X-rays or gamma rays are directed into an object, some of the photons interact with the particles of the matter and their energy can be absorbed or scattered. This absorption and scattering is called “Attenuation”. Other photons travel completely through the object without interacting with any of the material's particles. The number of photons transmitted through a material depends on the thickness, density and atomic number of the material, and the energy of the individual photons.

Even when they have the same energy, photons travel different distances within a material simply based on the probability of their encounter with one or more of the particles of the matter and the type of encounter that occurs. Since the probability of an encounter increases with the distance traveled, the number of photons reaching a specific point within the matter decreases exponentially with distance traveled. As shown in the graphic to the right, if 1000 photons are aimed at ten 1 cm layers of a material and there is a 10% chance of a photon being attenuated in this layer, then there will be 100 photons attenuated. This leaves 900 photos to travel into the next layer where 10% of these photos will be attenuated. By continuing this progression, the exponential shape of the curve becomes apparent.

The formula that describes this curve is:

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Where; : initial (unattenuated) intensity : linear attenuation coefficient per unit distance : distance traveled through the matter

Linear and Mass Attenuation Coefficients

The “linear attenuation coefficient” ( ) describes the fraction of a beam of X-rays or gamma rays that is absorbed or scattered per unit thickness of the absorber (10% per cm thickness for the previous example).

Using the transmitted intensity equation above, linear attenuation coefficients can be used to make a number of calculations. These include:

The intensity of the energy transmitted through a material when the incident X-ray intensity, the material and the material thickness are known.

The intensity of the incident X-ray energy when the transmitted X-ray intensity, material, and material thickness are known.

The thickness of the material when the incident and transmitted intensity, and the material are known.

The material can be determined from the value of when the incident and transmitted intensity, and the material thickness are known.

Linear attenuation coefficients can sometimes be found in the literature. However, it is often easier to locate attenuation data in terms of the “mass attenuation coefficient”. Tables and graphs of the mass attenuation coefficients for chemical elements and for several compounds and mixtures as a function of radiation energy (in keV) are available in literature (such information can be found at the National Institute for Standards and Technology website).

Since a linear attenuation coefficient is dependent on the density of a material, the mass attenuation coefficient is often reported for convenience. Consider water for example. The linear attenuation for water vapor is much lower than it is for ice because the molecules are more spread out in vapor so the chance of a photon encounter with a water particle is less. Normalizing by dividing it by the density of the element or compound will produce a value that is constant for a particular element or compound. This constant ( ) is known as the mass attenuation coefficient and has units of cm2/gm. The mass attenuation coefficient can simply be converted to a linear attenuation coefficient by multiplying it by the density ( ) of the material.

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Sometimes instead of specifying the

HVL, the Tenth Value Layer (TVL) is

specified. The TVL is the thickness

that attenuates 90% of the intensity

(only 10% passes through).

In that case, the equation becomes:

Half-Value Layer

The thickness of any given material where 50% of the incident energy has been attenuated is known as the half-value layer (HVL). The HVL is expressed in units of distance (mm or cm). Like the attenuation coefficient, it is photon energy dependant. Increasing the penetrating energy of a stream of photons will result in an increase in a material's HVL.

The HVL is inversely proportional to the attenuation coefficient. If an incident energy of 1 and a transmitted energy of 0.5 are plugged into the intensity attenuation equation introduced earlier, solving for which will correspond to the HVL gives:

The HVL is often used in radiography simply because it is easier to remember values and perform simple calculations. In a shielding calculation, such as illustrated to the right, it can be seen that if the thickness of one HVL is known, it is possible to quickly determine how much material is needed to reduce the intensity to less than 1%.

In order to calculate the ratio of intensity attenuation (or reduction) resulting from passing through a certain thickness of a material for which the HVL is known, the following equation may be used:

Where is intensity reduction ratio.

Or alternatively, the equitation can be solved to find the material thickness required for reducing the intensity to a certain level as:

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Approximate HVL for various materials when radiation is from a gamma-ray source:

Half-Value Layer (mm)

Source

Concrete Steel Lead Tungsten Uranium

Iridium-192 44.5 12.7 4.8 3.3 2.8

Cobalt-60 60.5 21.6 12.5 7.9 6.9

Approximate HVL for some materials when radiation is from an X-ray source:

Half-Value Layer (mm)

X-ray Tube Voltage (kV)

Lead Concrete

50 0.06 4.32

100 0.27 15.10

150 0.30 22.32

200 0.52 25.0

250 0.88 28.0

300 1.47 31.21

400 2.5 33.0

1000 7.9 44.45

Sources of Attenuation

The attenuation that results due to the interaction between penetrating radiation and matter is not a simple process. A single interaction event between a primary X-ray photon and a particle of matter does not usually result in the photon changing to some other form of energy and effectively disappearing. Several interaction events are usually involved and the total attenuation is the sum of the attenuation due to different types of interactions. These interactions include the photoelectric effect, scattering, and pair production.

Photoelectric (PE) absorption of X-rays occurs when the X-ray photon is absorbed, resulting in the ejection of electrons from the outer shell of the atom, and hence the ionization of the atom. Subsequently, the ionized atom returns to the neutral

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state with the emission of an X-ray characteristic of the atom. This subsequent emission of lower energy photons is generally absorbed and does not contribute to (or hinder) the image making process. Photoelectron absorption is the dominant process for X-ray absorption up to energies of about 500 keV. Photoelectric absorption is also dominant for atoms of high atomic numbers.

Compton scattering (C) occurs when the incident X-ray photon is deflected from its original path by an interaction with an electron. The electron gains energy and is ejected from its orbital position. The X-ray photon loses energy due to the interaction but continues to travel through the material along an altered path. Since the scattered X-ray photon has less energy, it, therefore, has a longer wavelength than the incident photon.

Pair production (PP) can occur when the X-ray photon energy is greater than 1.02 MeV, but really only becomes significant at energies around 10 MeV. Pair production occurs when an electron and positron are created with the annihilation of the X-ray photon. Positrons are very short lived and disappear (positron annihilation) with the formation of two photons of 0.51 MeV energy. Pair production is of particular importance when high-energy photons pass through materials of a high atomic number.

EQUIPMENT & MATERIALS

X-ray Generators

The major components of an X-ray generator are the tube, the high voltage generator, the control console, and the cooling system. As discussed earlier in this material, X-rays are generated by directing a stream of high speed electrons at a target material such as tungsten, which has a high atomic number. When the electrons are slowed or stopped

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by the interaction with the atomic particles of the target, X-radiation is produced. This is accomplished in an X-ray tube such as the one shown in the figure.

The tube cathode (filament) is heated with a low-voltage current of a few amps. The filament heats up and the electrons in the wire become loosely held. A large electrical potential is created between the cathode and the anode by the high-voltage generator. Electrons that break free of the cathode are strongly attracted to the anode target. The stream of electrons between the cathode and the anode is the tube current. The tube current is measured in milliamps and is controlled by regulating the low-voltage heating current applied to the cathode. The higher the temperature of the filament, the larger the number of electrons that leave the cathode and travel to the anode. The milliamp or current setting on the control console regulates the filament temperature, which relates to the intensity of the X-ray output.

The high-voltage between the cathode and the anode affects the speed at which the electrons travel and strike the anode. The higher the kilovoltage, the more speed and, therefore, energy the electrons have when they strike the anode. Electrons striking with more energy results in X-rays with more penetrating power. The high-voltage potential is measured in kilovolts, and this is controlled with the voltage or kilovoltage control on the control console. An increase in the kilovoltage will also result in an increase in the intensity of the radiation. The figure shows the spectrum of the radiated X-rays associated with the voltage and current settings. The top figure shows that increasing the kV increases both the energy of X-rays and also increases the intensity of radiation (number of photons). Increasing the current, on the other hand, only increases the intensity without shifting the spectrum.

A focusing cup is used to concentrate the stream of electrons to a small area of the target called the “focal spot”. The focal spot size is an important factor in the system's ability to produce a sharp image. Much of the energy applied to the tube is transformed into heat at the focal spot of the anode. As mentioned above, the anode target is commonly made from tungsten, which has a high melting point in addition to

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a high atomic number. However, cooling of the anode by active or passive means is necessary. Water or oil re-circulating systems are often used to cool tubes. Some low power tubes are cooled simply with the use of thermally conductive materials and heat radiating fins.

In order to prevent the cathode from burning up and to prevent arcing between the anode and the cathode, all of the oxygen is removed from the tube by pulling a vacuum. Some systems have external vacuum pumps to remove any oxygen that may have leaked into the tube. However, most industrial X-ray tubes simply require a warm-up procedure to be followed. This warm-up procedure carefully raises the tube current and voltage to slowly burn any of the available oxygen before the tube is operated at high power.

In addition, X-ray generators usually have a filter along the beam path (placed at or near the x-ray port). Filters consist of a thin sheet of material (often high atomic number materials such as lead, copper, or brass) placed in the useful beam to modify the spatial distribution of the beam. Filtration is required to absorb the lower-energy X-ray photons emitted by the tube before they reach the target in order to produce a cleaner image (since lower energy X-ray photons tend to scatter more).

The other important component of an X-ray generating system is the control console. Consoles typically have a keyed lock to prevent unauthorized use of the system. They will have a button to start the generation of X-rays and a button to manually stop the generation of X-rays. The three main adjustable controls regulate the tube voltage in kilovolts, the tube amperage in milliamps, and the exposure time in minutes and seconds. Some systems also have a switch to change the focal spot size of the tube.

Radio Isotope (Gamma-ray) Sources

Manmade radioactive sources are produced by introducing an extra neutron to atoms of the source material. As the material gets rid of the neutron, energy is released in the form of gamma rays. Two of the most common industrial gamma-ray sources for industrial radiography are Iridium-192 and Cobalt-60. In comparison to an X-ray generator, Cobalt-60 produces energies comparable to a 1.25 MV X-ray system and Iridium-192 to a 460 kV X-ray system. These high energies make it possible to penetrate thick materials with a relatively short exposure time. This and the fact that

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sources are very portable are the main reasons that gamma sources are widely used for field radiography. Of course, the disadvantage of a radioactive source is that it can never be turned off and safely managing the source is a constant responsibility.

Physical size of isotope materials varies between manufacturers, but generally an isotope material is a pellet that measures 1.5 mm x 1.5 mm. Depending on the level of activity desired, a pellet or pellets are loaded into a stainless steel capsule and sealed by welding. The capsule is attached to short flexible cable called

a pigtail.

The source capsule and the pigtail are housed in a shielding device referred to as a exposure device or camera. Depleted uranium is often used as a shielding material for sources. The exposure device for Iridium-192 and Cobalt-60 sources will contain 22 kg and 225 kg of shielding materials, respectively. Cobalt cameras are often fixed to a trailer and transported to and from inspection sites. When the source is not being used to make an exposure, it is locked inside the exposure device.

To make a radiographic exposure, a crank-out mechanism and a guide tube are attached to opposite ends of the exposure device. The guide tube often has a collimator (usually made of tungsten) at the end to shield the radiation except in the direction necessary to make the exposure. The end of the guide tube is secured in the location where the radiation source needs to be to produce the radiograph. The crank-out cable is stretched as far as possible to put as much distance as possible between the exposure device and the radiographer. To make the exposure, the radiographer quickly cranks the source out of the exposure device and into position in the collimator at the end of the guide tube. At the end of the exposure time, the source is cranked back into the exposure device. There is a series of safety procedures, which include several radiation surveys, that must be accomplished when making an exposure with a gamma source.

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Radiographic Film

X-ray films for general radiography basically consist of an emulsion-gelatin containing radiation-sensitive silver halide crystals (such as silver bromide or silver chloride). The emulsion is usually coated on both sides of a flexible, transparent, blue-tinted base in layers about 0.012 mm thick. An adhesive undercoat fastens the emulsion to the film base and a very thin but tough coating covers the emulsion to protect it against minor abrasion. The typical total thickness of the X-ray film is approximately 0.23 mm. Though films are made to be sensitive for X-ray or gamma-ray, yet they are also sensitive to visible light. When X-rays, gamma-rays, or light strike the film, some of the halogen atoms are liberated from the silver halide crystal and thus leaving the silver atoms alone. This change is of such a small nature that it cannot be detected by ordinary physical methods and is called a “latent (hidden) image”. When the film is exposed to a chemical solution (developer) the reaction results in the formation of black, metallic silver.

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Film Selection

Selecting the proper film and developing the optimal radiographic technique for a particular component depends on a number of different factors;

Composition, shape, and size of the part being examined and, in some cases, its weight and location.

Type of radiation used, whether X-rays from an X-ray generator or gamma rays from a radioactive source.

Kilovoltage available with the X-ray equipment or the intensity of the gamma radiation.

Relative importance of high radiographic detail or quick and economical results.

Film Packaging

Radiographic film can be purchased in a number of different packaging options and they are available in a variety of sizes. The most basic form is as individual sheets in a box. In preparation for use, each sheet must be loaded into a cassette or film holder in a darkroom to protect it from exposure to light.

Industrial X-ray films are also available in a form in which each sheet is enclosed in a light-tight envelope. The film can be exposed from either side without removing it from the protective packaging. A rip strip makes it easy to remove the film in the darkroom for processing.

Packaged film is also available in the form of rolls where that allows the radiographer to cut the film to any length. The ends of the packaging are sealed with electrical tape in the darkroom. In applications such as the radiography of circumferential welds and the examination of long joints on an aircraft fuselage, long lengths of film offer great economic advantage.

Film Handling

X-ray film should always be handled carefully to avoid physical strains, such as pressure, creasing, buckling, friction, etc. Whenever films are loaded in semi-flexible holders and external clamping devices are used, care should be taken to be sure pressure is uniform. Marks resulting from contact with fingers that are moist or contaminated with processing chemicals, as well as crimp marks, are avoided if large films are always grasped by the edges and allowed to hang free. Use of envelope-packed films avoids many of these problems until the envelope is opened for processing.

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RADIOGRAPHY CONSIDERATIONS & TECHNIQUES

Radiographic Sensitivity

The usual objective in radiography is to produce an image showing the highest amount of detail possible. This requires careful control of a number of different variables that can affect image quality. Radiographic sensitivity is a measure of the quality of an image in terms of the smallest detail or discontinuity that may be detected. Radiographic sensitivity is dependant on the contrast and the definition of the image.

Radiographic contrast is the degree of density (darkness) difference between two areas on a radiograph. Contrast makes it easier to distinguish features of interest, such as defects, from the surrounding area. The image to the right shows two radiographs of the same stepwedge. The upper radiograph has a high level of contrast and the lower radiograph has a lower level of contrast. While they are both imaging the same change in thickness, the high contrast image uses a larger change in radiographic density to show this change. In each of the two radiographs, there is a small dot, which is of equal density in both radiographs. It is much easier to see in the high contrast radiograph.

Radiographic definition is the abruptness of change in going from one area of a given radiographic density to another. Like contrast, definition also makes it easier to see features of interest, such as defects, but in a totally different way. In the image to the right, the upper radiograph has a high level of definition and the lower radiograph has a lower level of definition. In the high definition radiograph it can be seen that a change in the thickness of the stepwedge translates to an abrupt change in radiographic density. It can be seen that the details, particularly the small dot, are much easier to see in the high definition radiograph. It can be said that a faithful visual reproduction of the stepwedge was produced. In the lower image, the radiographic setup did not produce a faithful visual reproduction. The edge line between the steps is blurred. This is evidenced by the gradual transition between the high and low density areas on the radiograph.

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Radiographic “Image” Density

After taking a radiographic image of a part and processing the film, the resulting darkness of the film will vary according to the amount of radiation that has reached the film through the test object. As mentioned earlier, the darker areas indicate more exposure and liter areas indicate less exposure. The processed film (or image) is usually viewed by placing it in front of a screen providing white light illumination of uniform intensity such that the light is transmitted through the film such that the image can be clearly seen. The term “radiographic density” is a measure of the degree of film darkening (darkness of the image). Technically it should be called “transmitted density” when associated with transparent-base film since it is a measure of the light transmitted through the film. Radiographic density is the logarithm of two measurements: the intensity of light incident on the film ( ) and the intensity of light transmitted through the film ( ). This ratio is the inverse of transmittance.

Similar to the decibel, using the log of the ratio allows ratios of significantly different sizes to be described using easy to work with numbers. The following table shows numeric examples of the relationship between the amount of transmitted light and the calculated film density.

Transmittance (It/I0)

Transmittance (%) Inverse of Transmittance (I0/It)

Density (Log(I0/It))

1.0 100% 1 0 0.1 10% 10 1 0.01 1% 100 2 0.001 0.1% 1000 3 0.0001 0.01% 10000 4

From the table, it can be seen that a density reading of 2.0 is the result of only one percent of the incident light making it through the film. At a density of 4.0 only 0.01% of transmitted light reaches the far side of the film. Industrial codes and standards typically require a radiograph to have a density between 2.0 and 4.0 for acceptable viewing with common film viewers. Above 4.0, extremely bright viewing lights is necessary for evaluation.

Film density is measured with a densitometer which simply measures the amount of light transmitted through a piece of film using a photovoltic sensor.

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Secondary (Scatter) Radiation Control

Secondary or scatter radiation must often be taken into consideration when producing a radiograph. The scattered photons create a loss of contrast and definition. Often, secondary radiation is thought of as radiation striking the film reflected from an object in the immediate area, such as a wall, or from the table or floor where the part is resting.

Control of side scatter can be achieved by moving objects in the room away from the film, moving the X-ray tube to the center of the vault, or placing a collimator at the exit port, thus reducing the diverging radiation surrounding the central beam.

When scarered radiation comes from objects behind the film, it is often called “backscatter”. Industry codes and standards often require that a lead letter “B” be placed on the back of the cassette to verify the control of backscatter. If the letter “B” shows as a “ghost” image on the film, a significant amount of backscatter radiation is reaching the film. The image of the “B” is often very nondistinct as shown in the image to the right. The arrow points to the area of backscatter radiation from the lead “B” located on the back side of the film.

The control of backscatter radiation is achieved by backing the film in the cassette with a sheet of lead that is at least 0.25 mm thick such that the sheet will be behind the film when it is exposed. It is a common practice in industry to place thin sheets of lead (called “lead screens”) in front and behind the film (0.125 mm thick in front and 0.25 mm thick behind).

Radiographic Contrast

As mentioned previously, radiographic contrast describes the differences in photographic density in a radiograph. The contrast between different parts of the image is what forms the image and the greater the contrast, the more visible features become. Radiographic contrast has two main contributors; subject contrast and film (or detector) contrast.

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Subject Contrast

Subject contrast is the ratio of radiation intensities transmitted through different areas of the component being evaluated. It is dependant on the absorption differences in the component, the wavelength of the primary radiation, and intensity and distribution of secondary radiation due to scattering.

It should be no surprise that absorption differences within the subject will affect the level of contrast in a radiograph. The larger the difference in thickness or density between two areas of the subject, the larger the difference in radiographic density or contrast. However, it is also possible to radiograph a particular subject and produce two radiographs having entirely different contrast levels. Generating X-rays using a low kilovoltage will generally result in a radiograph with high contrast. This occurs because low energy radiation is more easily attenuated. Therefore, the ratio of photons that are transmitted through a thick and thin area will be greater with low energy radiation.

There is a tradeoff, however. Generally, as contrast sensitivity increases, the latitude of the radiograph decreases. Radiographic latitude refers to the range of material thickness that can be imaged. This means that more areas of different thicknesses will be visible in the image. Therefore, the goal is to balance radiographic contrast and latitude so that there is enough contrast to identify the features of interest but also to make sure the latitude is great enough so that all areas of interest can be inspected with one radiograph. In thick parts with a large range of thicknesses, multiple radiographs will likely be necessary to get the necessary density levels in all areas.

Film Contrast

Film contrast refers to density differences that result due to the type of film being used, how it was exposed, and how it was processed. Since there are other detectors besides film, this could be called detector contrast, but the focus here will be on film. Exposing a film to produce higher film densities will generally increase the contrast in the radiograph.

A typical film characteristic curve, which shows how a film responds to different amounts of radiation exposure, is shown in the figue. From the shape of the curves, it can be seen that when the film has not seen many photon interactions (which will

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result in a low film density) the slope of the curve is low. In this region of the curve, it takes a large change in exposure to produce a small change in film density. Therefore, the sensitivity of the film is relatively low. It can be seen that changing the log of the relative exposure from 0.75 to 1.4 only changes the film density from 0.20 to about 0.30. However, at film densities above 2.0, the slope of the characteristic curve for most films is at its maximum. In this region of the curve, a relatively small change in exposure will result in a relatively large change in film density. For example, changing the log of relative exposure from 2.4 to 2.6 would change the film density from 1.75 to 2.75. Therefore, the sensitivity of the film is high in this region of the curve. In general, the highest overall film density that can be conveniently viewed or digitized will have the highest level of contrast and contain the most useful information.

As mentioned previously, thin lead sheets (called “lead screens”) are typically placed on both sides of the radiographic film during the exposure (the film is placed between the lead screens and inserted inside the cassette). Lead screens in the thickness range of 0.1 to 0.4 mm typically reduce scatter radiation at energy levels below 150 kV. Above this energy level, they will emit electrons to provide more exposure of the film, thus increasing the density and contrast of the radiograph.

Other type of screens called “fluorescent screens” can alternatively be used where they produce visible light when exposed to radiation and this light further exposes the film and increases density and contrast.

Radiographic Definition

As mentioned previously, radiographic definition is the abruptness of change from one density to another. Both geometric factors of the equipment and the radiographic setup, and film and screen factors have an effect on definition.

Geometric Factors

The loss of definition resulting from geometric factors of the radiographic equipment and setup is refered to as “geometric unsharpness”. It occurs because the radiation

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does not originate from a single point but rather over an area. The three factors controlling unsharpness are source size, source to object distance, and object to detector (film) distance. The effects of these three factors on image defenetion is illustrated by the images below (source size effect; compare A & B, source to object distance; compare B & D, and object to detector distance; compare B & C).

The source size is obtained by referencing manufacturers specifications for a given X-ray or gamma ray source. Industrial X-ray tubes often have focal spot sizes of 1.5 mm squared but microfocus systems have spot sizes in the 30 micron range. As the source size decreases, the geometric unsharpness also decreases. For a given size source, the unsharpness can also be decreased by increasing the source to object distance, but this comes with a reduction in radiation intensity. The object to detector distance is usually kept as small as possible to help minimize unsharpness. However, there are situations, such as when using geometric enlargement, when the object is separated from the detector, which will reduce the definition.

In general, in order to produce the highest level of definition, the focal-spot or source size should be as close to a point source as possible, the source-to-object distance

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should be as large as practical, and the object-to-detector distance should be a small as practical.

Codes and standards used in industrial radiography require that geometric unsharpness be limited. In general, the allowable amount is 1/100 of the material thickness up to a maximum of 1 mm. These values refer to the width of penumbra shadow in a radiographic image.

The amount of geometric unsharpness ( ) can be

calculated using the following geometric formula:

Where; : source focal-spot size : distance from the source to the front surface

of the object : distance from the front surface of the object

to the detector (or the thickness of the object if a thick object is placed immediately on top of the detector)

The angle between the radiation and some features will also have an effect on definition. If the radiation is parallel to an edge or linear discontinuity, a sharp distinct boundary will be seen in the image. However, if the radiation is not parallel with the discontinuity, the feature will appear distorted, out of position and less defined in the image.

Abrupt changes in thickness and/or density will appear more defined in a radiograph than will areas of gradual change. For example, consider a circle. Its largest dimension will be a cord that passes through its centerline. As the cord is moved away from the centerline, the thickness gradually decreases. It is sometimes difficult to locate the edge of a void due to this gradual change in thickness.

Lastly, any movement of the specimen, source or detector during the exposure will reduce definition. Similar to photography, any movement will result in blurring of the image. Vibration from nearby equipment may be an issue in some inspection situations.

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Film and Screen Factors

The last set of factors concern the film and the use of fluorescent screens. A fine grain film is capable of producing an image with a higher level of definition than is a coarse grain film. Wavelength of the radiation will influence apparent graininess. As the wavelength shortens and penetration increases, the apparent graininess of the film will increase. Also, increased development of the film will increase the apparent graininess of the radiograph.

The use of fluorescent screens also results in lower definition. This occurs for a couple of different reasons. The reason that fluorescent screens are sometimes used is because incident radiation causes them to give off light that helps to expose the film. However, the light they produce spreads in all directions, exposing the film in adjacent areas, as well as in the areas which are in direct contact with the incident radiation. Fluorescent screens also produce screen mottle on radiographs. Screen mottle is associated with the statistical variation in the numbers of photons that interact with the screen from one area to the next.

Film Characteristic Curves

In film radiography, the number of photons reaching the film determines how dense the film will become when other factors such as the developing time are held constant. The number of photons reaching the film is a function of the intensity of the radiation and the time that the film is exposed to the radiation. The term used to describe the control of the number of photons reaching the film is “exposure”.

Different types of radiographic films respond differently to a given amount of exposure. Film manufacturers commonly characterize their film to determine the relationship between the applied exposure and the resulting film density. This relationship commonly varies over a range of film densities, so the data is presented in the form of a curve such as the one for Kodak AA400 shown to the right. This plot is usually called a film characteristic curve or density curve. A log scale is sometimes used for the x-axis or it is more common that the values are reported in log units on a linear scale as seen in the figure. Also, relative exposure values (unitless) are often

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used. Relative exposure is the ratio of two exposures. For example, if one film is exposed at 100 kV for 6 mA.min and a second film is exposed at the same energy for 3 mA.min, then the relative exposure would be 2.

The location of the characteristic curves of different films along the x-axis relates to the speed of the film. The farther to the right that a curve is on the chart, the slower the film speed (Film A has the highest speed while film C has the lowest speed). The shape of the characteristic curve is largely independent of the wavelength of the X-ray or gamma ray, but the location of the curve along the x-axis, with respect to the curve of another film, does depend on radiation quality.

Film characteristic curves can be used to adjust the exposure used to produce a radiograph with a certain density to an exposure that will produce a second radiograph of higher or lower film density. The curves can also be used to relate the exposure produced with one type of film to exposure needed to produce a radiograph of the same density with a second type of film.

Example 1: Adjusting the Exposure to Produce a Different Film Density

A type B Film was exposed with 140 kV at 1 mA for 10 seconds (i.e., 10 mA.s) and the resulting radiograph had a density of 1.0. If the desired density is 2.5, what should be the exposure?

From the graph, the log of the relative exposure of a density of 1.0 is 1.62 and the log of the relative exposure when the density of the film is 2.5 is 2.12. The difference between the two values is 0.5. 10 0.5 = 3.16 Therefore, the exposure used to produce the initial radiograph with a 1.0 density needs to be multiplied by 3.16 to produce a radiograph with the desired density of 2.5. So the new exposure must be: 10 mA.s x 3.16 = 31.6 mA.s (at 140 kV) Example 2: Adjusting the Exposure to Allow Use of a Different Film Type

Suppose an acceptable radiograph with a density of 2.5 was produced by exposing Film A for 30 seconds at 1mA and 130 kV. What should be the exposure if we want to produce the same density using Film B?

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From the graph, the log of the relative exposure that produced a density of 2.5 on Film A is 1.82 and the log of the relative exposure that produces the same density on Film B is 2.12. The difference between the two values is 0.3. 10 0.3 = 2 So the exposure for Film B must be: 30 mA.s x 2 = 60 mA.s (at 130 kV)

Exposure Calculations

Properly exposing a radiograph is often a trial and error process, as there are many variables that affect the final radiograph. Some of the variables that affect the density of the radiograph include:

The spectrum of radiation produced by the X-ray generator.

The voltage potential used to generate the X-rays (kV).

The amperage used to generate the X-rays (mA).

The exposure time.

The distance between the radiation source and the film.

The material of the component being radiographed.

The thickness of the material that the radiation must travel through.

The amount of scattered radiation reaching the film.

The film being used.

The use of lead screens or fluorescent screens.

The concentration of the film processing chemicals and the contact time.

The current industrial practice is to develop a procedure that produces an acceptable density by trail for each specific X-ray generator. This process may begin using published exposure charts to determine a starting exposure, which usually requires some refinement.

However, it is possible to calculate the density of a radiograph to an acceptable degree of accuracy when the spectrum of an X-ray generator has been characterized. The calculation cannot completely account for scattering but, otherwise, the relationship between many of the variables and their effect on film density is known. Therefore, the change in film density can be estimated for any given variable change. For example, from Newton's Inverse Square Law, it is known that the intensity of the radiation varies inversely with the square of the distance from the source. It is also

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known that the intensity of the radiation transmitted through a material varies exponentially with the linear attenuation coefficient and the thickness of the material. By calculating the intensity from these equations one can directly calculate the required exposure knowing that the exposure is inversely related to the intensity as:

The figure below shows exemplary exposure charts for two materials for a specific X-ray generator for the flowing parameters: film density of 2.0 without screens, 910 mm source-to-film distance, Industrex AA film & 7 minutes development time.

For gamma-ray sources, however, the required exposure can be more easily calculated since the radiation spectrum is well known for each different radiation source. The exposure is usually expressed in Curie-Time units and the data can be represented in the form of chars or in tabulated form. The figure shows a typical exposure chart for Ir-192 at the following parameters: film density of 1.75 without screens, 455 mm source-to-film distance, II-ford film & 6 minutes development time.

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It should be noted that such charts are valid for the specified parameters, but of course using the data in the charts one can calculate the exposure for different set of parameters such as different source-to-film distance, different type of film, or different density.

To make such calculations more easy, radiographic modeling calculators and programs can be used. A number of such programs are available from different sources and some are available online. These programs can provide a fair representation of the radiograph that will be produce with a specific setup and parameters. The figure shows a screen shot of an online calculator available at the (www.ndt-ed.org) website.

Example 1:

A 25 mm thick Aluminum plate is to be radiographed on type C film without screens using X-ray generator at 80 kV and 500 mm distance. What is the minimum required exposure time to get 3.0 density (for same development parameters as used for the chart, and considering the film used for the chart to be type A) knowing that the max current setting for the X-ray machine is 20 mA?

Answer: 190 s

Example 2:

A 12.5 mm thick Steel plate is to be radiographed without screens using Ir-192 source at 455 mm distance. Knowing that the source activity was 100 Ci before 30 days, what is the required exposure time (for same density, film type, and development parameters as used for the chart) if the plate is to be place behind a 50 mm thick concrete wall while it is being exposed?

Answer: 104 s

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http://www.ndt-ed.org/




Controlling Radiographic Quality

One of the methods of controlling the quality of a radiograph is through the use of image quality indicators (IQIs), which are also referred to as penetrameters. IQIs provide means of visually informing the film interpreter of the contrast sensitivity and definition of the radiograph. The IQI indicates that a specified amount of change in material thickness will be detectable in the radiograph, and that the radiograph has a certain level of definition so that the density changes are not lost due to unsharpness. Without such a reference point, consistency and quality could not be maintained and defects could go undetected.

IQIs should be placed on the source side of the part over a section with a material thickness equivalent to the region of interest. If this is not possible, the IQI may be placed on a block of similar material and thickness to the region of interest. When a block is used, the IQI should be the same distance from the film as it would be if placed directly on the part in the region of interest. The IQI should also be placed slightly away from the edge of the part so that at least three of its edges are visible in the radiograph.

Image quality indicators take many shapes and forms due to the various codes or standards that invoke their use. The two most commonly used IQI types are: the hole-type and the wire IQIs. IQIs come in a variety of material types so that one with radiation absorption characteristics similar to the material being radiographed can be used.

Hole-Type IQIs

ASTM Standard E1025 gives detailed requirements for the design and material group classification of hole-type image quality indicators. Hole-type IQIs are classified in eight groups based on their radiation absorption characteristics. A notching system is used to indicate the IQI material. The numbers on the IQI indicate the sample thickness that the IQI would typically be placed on. Also, holes of different sizes are present where these holes should be visible on the radiograph. It should be noted however that the IQI is used to indicate the quality of the radiographic technique and not intended to be used as a measure of the size of a cavity that can be located on the radiograph.

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Wire IQIs

ASTM Standard E747 covers the radiographic examination of materials using wire IQIs to control image quality. Wire IQIs consist of a set of six wires arranged in order of increasing diameter and encapsulated between two sheets of clear plastic. Wire IQIs are grouped in four sets each having different range of wire diameters. The set letter (A, B, C or D) is shown in the lower right corner of the IQI. The number in the lower left corner indicates the material group.

Film Processing

As mentioned previously, radiographic film consists of a transparent, blue-tinted base coated on both sides with an emulsion. The emulsion consists of gelatin containing microscopic, radiation sensitive silver halide crystals, such as silver bromide and silver chloride. When X-rays, gamma rays or light rays strike the crystals or grains, some of the Br- ions are liberated leaving the Ag+ ions. In this condition, the radiograph is said to contain a latent (hidden) image because the change in the grains is virtually undetectable, but the exposed grains are now more sensitive to reaction with the developer.

When the film is processed, it is exposed to several different chemical solutions for controlled periods of time. Film processing basically involves the following five steps:

Development: The developing agent gives up electrons to convert the silver halide grains to metallic silver. Grains that have been exposed to the radiation develop more rapidly, but given enough time the developer will convert all the silver ions into silver metal. Proper temperature control is needed to convert exposed grains to pure silver while keeping unexposed grains as silver halide crystals.

Stopping the development: The stop bath simply stops the development process by diluting and washing the developer away with water.

Fixing: Unexposed silver halide crystals are removed by the fixing bath. The fixer dissolves only silver halide crystals, leaving the silver metal behind.

Washing: The film is washed with water to remove all the processing chemicals.

Drying: The film is dried for viewing.

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Film processing is a strict science governed by rigid rules of chemical concentration, temperature, time, and physical movement. Whether processing is done by hand or automatically by machine, excellent radiographs require a high degree of consistency and quality control.

Viewing Radiographs

After the film processing, radiographs are viewed using a light-box (or they can be digitized and viewed on a high resolution monitor) in order to be interpreted. In addition to providing diffused, adjustable white illumination of uniform intensity, specialized industrial radiography light-boxes include magnifying and masking aids. When handing the radiographs, thin cotton gloves should be worn to prevent fingerprints on the radiographs.

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RADIATION SAFETY

Radiation Health Risks

As mentioned previously, the health risks associated with the radiation is considered to be one the major disadvantages of radiogaphy. The amount of risk depends on the amount of radiation dose received, the time over which the dose is received, and the body parts exposed. The fact that X-ray and gamma-ray radiation are not detectable by the human senses complicates matters further. However, the risks can be minimized and controlled when the radiation is handled and managed properly in accordance to the radiation safety rules. The active laws all over the world require that individuals working in the field of radiography receive training on the safe handling and use of radioactive materials and radiation producing devices.

Today, it can be said that radiation ranks among the most thoroughly investigated (and somehow understood) causes of disease. The primary risk from occupational radiation exposure is an increased risk of cancer. Although scientists assume low-level radiation exposure increases one's risk of cancer, medical studies have not demonstrated adverse health effects in individuals exposed to small chronic radiation doses.

The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including:

Type of radiation involved. All kinds of ionizing radiation can produce health effects. The main difference in the ability of alpha and beta particles and gamma and X-rays to cause health effects is the amount of energy they have. Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues.

Size of dose received. The higher the dose of radiation received, the higher the likelihood of health effects.

Rate at which the dose is received. Tissue can receive larger dosages over a period of time. If the dosage occurs over a number of days or weeks, the results are often not as serious if a similar dose was received in a matter of minutes.

Part of the body exposed. Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the upper body.

The age of the individual. As a person ages, cell division slows and the body is less sensitive to the effects of ionizing radiation. Once cell division has slowed, the

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effects of radiation are somewhat less damaging than when cells were rapidly dividing.

Biological differences. Some individuals are more sensitive to radiation than others. Studies have not been able to conclusively determine the cause of such differences.

Sources of High Energy Radiation

There are many sources of harmful, high energy radiation. Industrial radiographers are mainly concerned with exposure from X-ray generators and radioactive isotopes. However, it is important to understand that eighty percent of human exposure comes from natural sources such as radon gas, outer space, rocks and soil, and the human body. The remaining twenty percent comes from man-made radiation sources, such as those used in medical and dental diagnostic procedures.

One source of natural radiation is cosmic radiation. The earth and all living things on it are constantly being bombarded by radiation from space. The sun and stars emit electromagnetic radiation of all wavelengths. The dose from cosmic radiation varies in different parts of the world due to differences in elevation and the effects of the earth’s magnetic field. Radioactive materials are also found throughout nature where they occur naturally in soil, water, plants and animals. The major isotopes of concern for terrestrial radiation are uranium and the decay products of uranium, such as thorium, radium, and radon. Low levels of uranium, thorium, and their decay products are found everywhere. Some of these materials are ingested with food and water, while others, such as radon, are inhaled. The dose from terrestrial sources varies in different parts of the world. Locations with higher concentrations of uranium and thorium in their soil have higher dose levels. All people also have radioactive isotopes, such as potassium-40 and carbon-14, inside their bodies. The variation in dose from one person to another is not as great as the variation in dose from cosmic and terrestrial sources.

There are also a number of manmade radiation sources that present some exposure to the public. Some of these sources include tobacco, television sets, smoke detectors, combustible fuels, certain building materials, nuclear fuel for energy production, nuclear weapons, medical and dental X-rays, nuclear medicine, X-ray security systems

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and industrial radiography. By far, the most significant source of man-made radiation exposure to the average person is from medical procedures, such as diagnostic X-rays, nuclear medicine, and radiation therapy.

Measures Relative to the Biological Effects of Radiation Exposure

There are four measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or gamma-rays. These measures are: Exposure, Dose, Dose Equivalent, and Dose Rate. A short description of these measures and their units is given below

Exposure: Exposure is a measure of the strength of a radiation field at some point in air (the amount of charge produced in a unit mass of air when the interacting photons are completely absorbed in that mass). This is the measure made by radiation survey meters since it can be easily and directly measured. The most commonly used unit of exposure is the “roentgen” (R).

Dose or Absorbed Dose: While exposure is defined for air, the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter. In other words, the dose is the amount of radiation absorbed by and object. The SI unit for absorbed dose is the “gray” (Gy), but the “rad” (Radiation Absorbed Dose) is commonly used (1 Gy = 100 rad). Different materials that receive the same exposure may not absorb the same amount of radiation. In human tissue, one Roentgen of X-ray or gamma radiation exposure results in about one rad of absorbed dose. The size of the absorbed dose is dependent upon the intensity (or activity) of the radiation source, the distance from the source, and the time of exposure to radiation.

Dose Equivalent: The dose equivalent relates the absorbed dose to the biological effect of that dose. The absorbed dose of specific types of radiation is multiplied by a “quality factor” to arrive at the dose equivalent. The SI unit is the “Sievert” (Sv), but the “rem” (Roentgen Equivalent in Man) is commonly used (1 Sv = 100 rem). The table below presents the “Q factors” for several types of radiation.

Type of Radiation Rad Q Factor Rem

X-Ray 1 1 1

Gamma Ray 1 1 1

Beta Particles 1 1 1

Thermal Neutrons 1 5 5

Fast Neutrons 1 10 10

Alpha Particles 1 20 20

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Dose Rate: The dose rate is a measure of how fast a radiation dose is being received. Dose rate is usually presented in terms of mR/hr, mrem/hr, rad/min, mGy/sec, etc. Knowing the dose rate, allows the dose to be calculated for a period of time.

Controlling Radiation Exposure

When working with radiation, there is a concern for two types of exposure: acute and chronic. An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time. An acute exposure has the potential for producing both non-stochastic and stochastic effects. Chronic exposure, which is also sometimes called “continuous exposure”, is long-term, low level overexposure. Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures.

The three basic ways of controlling exposure to harmful radiation are: 1) limiting the time spent near a source of radiation, 2) increasing the distance away from the source, 3) and using shielding to stop or reduce the level of radiation.

Time

The radiation dose is directly proportional to the time spent in the radiation. Therefore, a person should not stay near a source of radiation any longer than necessary. If a survey meter reads 4 mR/h at a particular location, a total dose of 4 mR will be received if a person remains at that location for one hour. The received dose can be simply calculated as: Dose = Dose Rate x Time

When using a gamma camera, it is important to get the source from the shielded camera to the collimator (a device that shields radiation in some directions but allow it pass in one or more other directions) as quickly as possible to limit the time of exposure to the unshielded source.

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Distance

Increasing distance from the source of radiation will reduce the amount of radiation received. As radiation travels from the source, it spreads out becoming less intense. This phenomenon can be expressed by the Newton inverse square law, which states that as the radiation travels out from the source, the dosage decreases inversely with the square of the distance: I1 / I2 = D2

2/ D12

Shielding

The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation. In general, the more dense the material the more shielding it will provide. Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials. Concrete is commonly used in the construction of radiation vaults. Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside.

Exposure Limits

Over the years, numerous recommendations regarding occupational exposure limits have been developed by international radiation safety commissions. In general, the guidelines established for radiation exposure have had two principal objectives: 1) to prevent acute exposure; and 2) to limit chronic exposure to “acceptable” levels.

Current guidelines are based on the conservative assumption that there is no safe level of exposure. This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure “as low as reasonably achievable” (ALARA). ALARA is a basic requirement of current radiation safety practices. It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible.

In general, most international radiation safety codes specify that the dose rate must not exceed 2mR/hour in any unrestricted area. The specifications for the accumulated dose per year differ between radiation workers and non-workers. The limits are as follows:

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Regulatory Limits for Occupational Exposure

Most international codes set the annual limit of exposure for industrial radiographers who generally are not concerned with an intake of radioactive material as follows:

1) the more limiting of:

A total effective dose equivalent of 5 rem (0.05 Sv) or

The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rem (0.5 Sv).

2) The annual limits to the lens of the eye, to the skin, and to the extremities, which are:

A lens dose equivalent of 15 rem (0.15 Sv)

A shallow-dose equivalent of 50 rem (0.50 Sv) to the skin or to any extremity.

The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation. This value is the dose equivalent at a tissue depth of 0.007 cm averaged over an area of 10 cm2.

The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation. This value is the dose equivalent at a tissue depth of 0.3 cm.

The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation. This value is the dose equivalent at a tissue depth of 1 cm.

The total effective dose equivalent is the dose equivalent to the whole-body.

Declared Pregnant Workers and Minors

Because of the increased health risks to the rapidly developing embryo and fetus, pregnant women can receive no more than 0.5 rem during the entire gestation period

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(this is 10% of the dose limit that normally applies to radiation workers). The same limit also applies to persons under the age of 18 years.

Non-radiation Workers and the General Public

The dose limit to non-radiation workers and members of the public is only 2% of the annual occupational dose limit. Therefore, a non-radiation worker can receive a whole body dose of no more that 0.1 rem/year from industrial ionizing radiation. This exposure would be in addition to the 0.3 rem/year from natural background radiation and the 0.05 rem/year from man-made sources such as medical X-rays.

Over-Dose Health Symptoms

Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period.

0-25 rem No injury evident. First detectable blood change at 5 rem.

25-50 rem Definite blood change at 25 rem. No serious injury.

50-100 rem Some injury possible.

100-200 rem Injury and possible disability.

200-400 rem Injury and disability likely, death possible.

400-500 rem Median Lethal Dose (MLD) 50% of exposures are fatal.

500-1,000 rem Up to 100% of exposures are fatal.

Over 1,000 rem 100% likely fatal.

The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure.

Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period.

100 - 200 rem

First Day No definite symptoms

First Week No definite symptoms

Second Week No definite symptoms

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Third Week Loss of appetite, malaise, sore throat and diarrhea

Fourth Week Recovery is likely in a few months unless complications develop because of poor health

400 - 500 rem

First Day Nausea, vomiting and diarrhea, usually in the first few hours

First Week Symptoms may continue

Second Week Epilation, loss of appetite

Third Week Hemorrhage, nosebleeds, inflammation of mouth and throat, diarrhea, emaciation

Fourth Week Rapid emaciation and mortality rate around 50%

Radiation Detectors

Instruments used for radiation measurement fall into two broad categories:

Rate measuring instruments.

Personal dose measuring instruments.

Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity). Survey meters, audible alarms and area monitors fall into this category. These instruments present a radiation intensity reading relative to time, such as R/hr or mR/hr. An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time.

Dose measuring instruments are those that measure the total amount of exposure received during a measuring period. The dose measuring instruments, or dosimeters, that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual. An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units.

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Survey Meters

The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation. There are many different models of survey meters available to measure radiation in the field. They all basically consist of a detector and a readout display. Analog and digital displays are available. Most of the survey meters used for industrial radiography use a gas filled detector.

Gas filled detectors consists of a gas filled cylinder with two electrodes having a voltage applied to them. Whenever the device is brought near radioactive substances, the gas becomes ionized. The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode. This results in an electrical signal that is amplified, correlated to exposure and displayed as a value.

Audible Alarm Rate Meters

Audible alarms are devices that emit a short "beep" or "chirp" when a predetermined exposure has been received. It is required that these electronic devices be worn by an individual working with gamma emitters. These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to exposure levels or dosages of radiation above a preset amount. It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters. Modern survey meters have this alarm feature already built in.

Pocket Dosimeter

Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to X-rays or gamma rays. As the name implies, they are commonly worn in the pocket. The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure. It also has the advantage of being reusable. The limited range, inability to provide a permanent record, and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter.

The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter.

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Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen. The accumulated dose value can be read by pointing the instrument at a light source and observing the internal fiber through a system of built-in lenses. The fiber is viewed on a translucent scale which is graduated in units of exposure. Typical industrial radiography pocket dosimeters have a full scale reading of 200 mR but there are designs that will record higher amounts. During the shift, the dosimeter reading should be checked frequently. The measured exposure should be recorded at the end of each shift.

Digital Electronic Dosimeter These dosimeters measure both dose information and dose rate and display them in digital form. Also, some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure. Consequently, the frequency or chirp rate of the alarm is proportional to the radiation intensity. Some models can also be set to provide a continuous audible signal when a preset exposure has been reached.

Film Badges

Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays, X-rays and beta particles. The detector is, as the name implies, a piece of radiation sensitive film. The film is packaged in a light proof, vapor proof envelope preventing light, moisture or chemical vapors from affecting the film. Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives. Whole body badges are worn on the body between the neck and the waist, often on the belt or a shirt pocket.

The film is contained inside a film holder or badge. The badge incorporates a series of filters to determine the quality of the radiation. Radiation of a given energy is attenuated to a different extent by various types of absorbers. Therefore, the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter. By comparing these results, the energy of the radiation

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can be determined and the dose can be calculated knowing the film response for that energy. The badge holder also contains an open window to determine radiation exposure due to beta particles (since beta particles are shielded by a thin amount of material).

The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record, it is able to distinguish between different energies of photons, and it can measure doses due to different types of radiation. It is quite accurate for exposures greater than 100 mR. The major disadvantages are that it must be developed and read by a processor (which is time consuming) and prolonged heat exposure can affect the film.

Thermoluminescent Dosimeter (TLD)

Thermoluminescent dosimeters (TLD) are often used instead of the film badge. Like a film badge, it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received, if any. TLDs can measure doses as low as 1 mR and they have a precision of approximately 15% for low doses which improves to approximately 3% for high doses. TLDs are reusable, which is an advantage over film badges. However, no permanent record or re-readability is provided and an immediate, on the job readout is not possible.

A TLD has a phosphor, such as lithium fluoride (LiF) or calcium fluoride (CaF), in a solid crystal structure. When a TLD it is exposed to ionizing radiation at ambient temperatures, the radiation interacts with the phosphor crystal causing some of the atoms in the material to produce free electrons and become ionized. The free electrons are trapped and locked into place in the imperfections in the crystal lattice structure.

Heating the crystal causes the crystal lattice to vibrate, releasing the trapped electrons in the process. Released electrons return to the original ground state, releasing the captured energy from ionization as light, hence the name thermoluminescent. Instead of reading the optical density (blackness) of a film, as is done with film badges, the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured. The “glow curve” produced by this process is then related to the radiation exposure. The process can be repeated many times.

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Safety Controls

Since X-ray and gamma radiation are not detectable by the human senses and the

resulting damage to the body is not immediately apparent, a variety of safety controls

are used to limit exposure. The two basic types of radiation safety controls used to

provide a safe working environment are engineered and administrative controls.

Engineered controls include shielding, interlocks, alarms, warning signals, and material

containment. Administrative controls include postings, procedures, dosimetry, and

training.

Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a “radiation vault”. Fixed shielding materials are commonly high density concrete and/or lead. Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced. Warning lights are used to alert workers and the public that radiation is being used. Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present. Safety controls should never be tampered with or bypassed.

When portable radiography is performed, most often it is not practical to place alarms or warning lights in the exposure area. Ropes (or cordon off tape) and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation. Occasionally, radiographers will use battery operated flashing lights to alert the public to the presence of radiation.

Safety regulations classify the areas surrounding the location where ionizing radiation is present into restricted areas and controlled areas according to the radiation intensity level:

Restricted areas: Areas with a dose rate higher than 300 mR/h must be secure so that nobody can enter this area. If anybody accidently enters this area, radiation must be terminated and the person must be checked. Access is only permitted under specific conditions and if there is an absolute need for it, the body dose should be calculated and the personal dose measured.

Control areas: These are areas with dose rates which are equivalent to or higher than 0.75 mR/h. Control areas must be cordoned off and provided with a radiation warning signs.

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Dr SP Tyagi

X-RAY FILM AND ACCESSORIES

Dr. S. P. Tyagi

X-ray film:

The X-ray film is the medium that record the image of part exposed with X-rays. The x-ray film

is somewhat similar to photographic film in its basic composition. However unlike photographic

film, the light (or radiation) sensitive emulsion is usually coated on both sides of the base of X-

ray film so that it can be used with intensifying screens.

The x-ray film is composed of following –

1: Film base: The central portion of the x-ray film is the base which supports the fragile

photographic emulsion on both of its surface. Ideally the base must be flexible as well as quite

strong so that the films can be repeatedly snapped into x-ray illuminators (Viewing boxes).

Secondly, it must withstand any geometric distortion due to the heat of the developing process

and finally, the base must provide a uniform, highly transparent, optical background.

Historically, photographic glass plates were used as the X-ray film base followed by

cellulose nitrate in early 1920’s. Later cellulose triacetate base was developed in 1924 to avoid

the highly flammable nature of cellulose nitrate. Finally, a stronger, thinner, more

dimensionally stable film base made of polyester was developed in 1960 and that has replaced

all above materials for making of film base.

2: Film Emulsion: The X-ray film emulsion is composed of a mixture of gelatin (derived from

cadaver bones) and small silver halide crystals (grains). The gelatin serves as a matrix which

keeps the silver halide grains well dispersed and prevents their clumping. The developing and Libr

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http://www.amershamhealth.com/medcyclopaedia/medical/article.asp?vol=VOLUME%20I&article=SILVER_HALIDE


Dr SP Tyagi

fixing solutions can penetrate the gelatin very rapidly without changing the strength or

permanence of the gelatin. Small crystal grains of silver halide (1.0 to 1.5 microns in diameter)

comprise the light sensitive substance in the emulsion. These grains, known as silver-iodo-

bromide, are typically between 90 and 99% silver bromide and between 1 and 10% silver

iodide.

The atoms in the silver-iodo-bromide crystal are arranged in a cubic lattice and each crystal contains many point defects, where a silver ion is displaced and is free to move through the crystal. It is the mobility of these silver ions that contributes to the formation of the latent image. In its pure form the silver halide crystal has low photographic sensitivity. The emulsion is sensitized by heating it under controlled conditions with a reducing agent containing sulphur. This result in the production of silver sulphide at a site on the surface of the crystal referred to as a sensitivity speck. It is the sensitivity speck that traps electrons to begin formation of the latent image centres.

In the process of film exposure, the energy from absorbing a photon of light is sufficient to liberate an electron from a bromide ion in the crystal. The electron travels freely through the crystal until it is trapped at a site of crystal imperfection such as a dislocation defect or a sensitivity speck composed of an AgS molecule. A free silver ion is attracted to the negative charge and combines with the charge (is reduced) to form an atom of metallic silver (which is optically black). The single atom of silver acts as an electron trap for another electron and then attracts another atom of silver which is then reduced to metallic silver. This process continues while the exposure to light continues.

3: Adhesive layer: In general, the emulsion and the base do not adhere to each other. For this

reason, the emulsion must be attached to the film base using a thin layer of suitable adhesive

which is generally a clear thin layer of gelatin only.

4: Protective layer: To protect the emulsion, which would be easily scratched and damaged by

normal handling, a very thin outer protective layer is applied (again usually made of gelatin).

Types of X-ray films:

1. On the basis of photosensitive emulsion layers:

Single coated: In such type of x-ray films the photosensitive emulsion is coated only on

one surface of film base. These films are used with single intensifying screen cassette with the

film placed in front of the screen, i.e. on the side facing the X-ray tube. These are specific

purpose films used when higher spatial resolution of image is desired.

Double coated: These are routine purpose x-ray films having photosensitive coatings on

both sides of base and used with double screen cassette with the film sandwiched between the Libr

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http://www.amershamhealth.com/medcyclopaedia/medical/article.asp?vol=VOLUME%20I&article=FIXING

http://www.amershamhealth.com/medcyclopaedia/medical/article.asp?vol=VOLUME%20I&article=LATENT_IMAGE



http://www.amershamhealth.com/medcyclopaedia/medical/article.asp?vol=VOLUME%20I&article=FILM_EXPOSURE

http://www.amershamhealth.com/medcyclopaedia/medical/article.asp?vol=VOLUME%20I&article=SENSITIVITY_SPECK


Dr SP Tyagi

screens. Such films require lesser exposure factors and lesser processing times. For example

the image can be produced in 1/2 the time required to produce an image on the single sided

film.

2. On the basis of use with intensifying screens:

Screen films: These films are used along with intensifying screens and are therefore

ultimately exposed by light and not the X-rays. These films require lesser exposure factors and

processing time for development of radiographic image. The emulsion coating of such films is

also thinner. Such films are versatile and used for most general purpose diagnostic

radiography.

Non-screen films: These films are used without intensifying screens and require more

exposure factors and prolonged processing time for production of comparable radiographic

density to that of non-screen films. They have relatively thicker emulsion and therefore

radiographic image formed on such films have excellent details. Such films are used for specific

purposes such as detection of hail-line fracture or any subtle tissue change that remains

unrecognized in traditional routine radiograph.

3. On the basis of types of light sensitive emulsion coating:

Blue light sensitive films:

Green light sensitive Orthochromatic films:

Red light sensitive Panchromatic films:

The spectral sensitivity of the film must be matched to the emission spectrum of the

intensifying screen in order to increase the sensitivity of the system. The principle emission

from traditionally used calcium tungstate intensifying screens is blue light. Therefore, it is

imperative that the films to be used with such intensifying screens must be sensitive more

towards blue light. The photographic emulsion containing silver bromide is coincidently cream Libr

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Dr SP Tyagi

coloured that absorbs ultraviolet and blue light, but reflects green and red light and therefore

such films have been used without any problem with calcium tungustate intensifying screens.

However many rare earth intensifying screens principally emit greener lights and

therefore, x-ray films to be used with such screens should be made sensitive to greener

spectrum of light as well. For this, suitable dyes are added in their photosensitive emulsion of

the films. (Such green light sensitive orthochromatic films also require suitable change in x-ray

darkroom safe light colour and intensity). Now a day blue light emitting rare earth intensifying

screens are also available.

“High lite” films from 3M company were more or less not sensitive to room light

(particularly yellow lights) and therefore, allowed all the procedures of dark room in a

yellow lighted room.

4. On the basis of film speed:

Film speed refers to the relative sensitivity of X-ray film to a given amount of radiation.

Faster films require lesser exposure but produce grainy images that lack definition. They

also have narrow film latitude. Speed wise x-ray films may be categorized as following-

Standard or par speed films

Fast speed films

Ultrafast films

Standard speed films are versatile as they have wide film latitude but require greater exposure.

Film Latitude: It refers to the range of exposure factors that produce diagnostically useful range

of radiographic densities.

Handling and storage care of unexposed and exposed x-ray films:

1. Films should be stored in a cool (10-200c) and low humidity (40-60%) environment.

2. Film boxes should be kept vertically without any pressure on them.

3. Films should never be stored near a source of heat, irradiation or water.

4. Films should be loaded and unloaded from a cassette on a dry and clean bench inside

the dark room under a proper safe light.

5. Films should be handled delicately and any accidental splashing of processing solutions

should be avoided.

6. Films should not be used after their expiry period. Libr

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Dr SP Tyagi

7. If an x-ray film has been exposed, the cassette should immediately be transferred to the

dark room or in a lead shielded box to avoid inadvertent subsequent exposures

particularly in cases where serial radiography is being done.

8. The wet processed film should be kept upright in a film drier for its drying.

9. The wet films should never be touched with fingers to avoid finger marks over films.

Intensifying screen:

These screens are fitted in x-ray cassettes and interact with x-rays to convert most of

their radiant energy (>95%) in to visible light thereby, exposing the x-ray film finally with light

(and not the x-rays). The amount of light emitted by the intensifying screen is proportional to

the amount of x-radiation passing through it.

Generally the x-ray films are more sensitive to light rays than the x-rays and therefore

the use of intensifying screens allow reduction in the exposure factors without affecting the

general quality of radiograph.

The intensifying screen typically has following components-

1. Base: Provides a strong, smooth, but flexible support for the fluorescent layer. This is

constructed usually from paper, cardboard or polyester with total thickness not exceeding

approximately 0.18 mm.

Ideal properties of an intensifying screen include: Chemically inert, moisture resistant,

no discolouring with age

2. Substratum: It is the bonding layer between the base & the phosphor layer. It may be

reflective, absorptive or transparent in nature.

3. Phosphor (Fluorescent) Layer: This is the “active” layer of the intensifying screen that

consists of fluorescent crystals, which emit light when struck by x-radiation. Examples of Libr

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Dr SP Tyagi

typical phosphor materials include calcium tungstate & rare earth phosphors. Earlier

barium lead sulphate and zinc cadmium sulphide were also used as phosphor materials.

The rare earth screens may have any of the following types of phosphor material-

Terbium activated gadolinium oxysulphide

Terbium activated lanthanum oxysulphide

Terbium activated yttrium oxysulphide

Thulium activated lanthanum oxybromide

X-ray absorption efficiency and their light conversion ratio of rare earth screens are far

superior to calcium tungustate type films. For example rare earth screen film combination

has 12 times faster speed than par speed tungustate screen film combination and

exposure is reduced by 15-50%.

4. Super-coat: This is a transparent external protective layer which helps in resisting surface

abrasion. It is constructed from cellulose acetate and has anti-static and waterproofing

qualities.

Fluorescence: It is a kind of luminescence where a cold (nonglowing) substance releases electromagnetic radiation in the form of visible light while absorbing another form of energy, but ceases to emit the radiation immediately upon the cessation of the input energy. The emission of light from an intensifying screen during absorption of X-rays is one example of fluorescence.

However, if the emission is delayed somewhat, it is called phosphorescence (after glow).

Film holders (cassettes):

The material in the cassette box must be as little absorbing as possible. Presently, the

best material for this is carbon fibre, giving a very rigid structure combined with low density

and a low atomic number.

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http://www.amershamhealth.com/medcyclopaedia/medical/article.asp?vol=VOLUME%20I&article=INTENSIFYING_SCREEN


Radiography

Ref: http://www.ge-mcs.com/download/x-ray/GEIT-30158EN_industrial-radiography-image-forming-techniques.pdf

http://onlineshowcase.tafensw.edu.au/ndt/content/radiographic/task8/accessible.htm

http://radiopaedia.org/articles/pair-production

http://en.wikipedia.org/wiki/Latent_image

EME-062

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http://www.ge-mcs.com/download/x-ray/GEIT-30158EN_industrial-radiography-image-forming-techniques.pdf















http://onlineshowcase.tafensw.edu.au/ndt/content/radiographic/task8/accessible.htm







Structure of the X-ray film

• An X-ray film, total thickness approx. 0.5 mm, is made up of seven layers,(see figure)

• a transparent cellulose triacetate or polyester base (d). On both sides of this base are applied:

• a layer of hardened gelatine (a) to protect the emulsion

• emulsion layer (b) which is suspended in gelatine, sensitive to radiation

• a very thin layer called the substratum (c) which bonds the emulsion layer to the base Librar

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Radiographic image

Latent image (A latent image is an invisible image produced by the exposure to light of a photosensitive

material such as photographic film.)(Ref: http://en.wikipedia.org/wiki/Latent_image)

When light or X-radiation strikes a sensitive emulsion, the portions receiving a sufficient quantity of radiation undergo a change; extremely small particles of silver halide crystals are converted into metallic silver.

These traces of silver are so minute that the sensitive layer remains to all appearances unchanged. The number of silver particles produced is higher in the portions struck by a greater quantity of radiation and less high where struck by a lesser quantity.

Developing the latent image

Development is the process by which a latent image is converted into a visible image. This result is obtained by selective reduction into black metallic silver of the silver halide crystals in the emulsion. Librar

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http://en.wikipedia.org/wiki/Light

http://en.wikipedia.org/wiki/Photographic_film



Characteristics of the X-ray film

This is the science which studies the Photographic properties of a film, and the methods enabling these to be measured.

Density (optical)

• When a photographic film is placed on an illuminated screen for viewing, it will be observed that the image is made up of areas of differing brightness, dependent on the local optical densities (amount of silver particles) of the developed emulsion.

• Density (D) is defined as the logarithm to base 10 of the ratio of the incident light Io and the transmitted light through the film It, therefore:

D = log (Io/ It) .

Density is measured by a densitometer.

Industrial radiography on conventional film covers a density range from 0 to 4, a difference corresponding with a factor 10,000. Librar

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Contrast

• The contrast of an image is defined as the

relative brightness between an image and the

adjacent background.

• The contrast between two densities D1 and

D2 on an X-ray film is the density difference

between them and is usually termed the

radiographi o trast .

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Characteristic curve (density curve)

• The characteristic or density curve indicates the relationship between increasing exposures and resulting density. By exposure (E) is meant the radiation dose on the film emulsion. It is the product of radiation intensity (Io) and exposure time (t), therefore:

E = Io.t

The ratio between different exposures and related densities is not usually plotted on a linear scale but on a logarithmic scale; i.e. density D versus log E.

• Density (D) of a photographic emulsion does not increase linearly with exposure (E) over the entire density range, but has a shape as in figure. The lower part of the curve (a- is alled the toe , the middle part (b- is alled the straight li e li ear portio , a d the upper part -d) is called the shoulder .

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Film speed (sensitivity)

In radiography the relationship between exposure (in C/kg) and resulting density is commonly referred to as film speed.

Graininess

When a developed X-ray film is viewed in detail on an illuminated screen, minute density variations are visible in a grainy sort of structure. This isual i pressio is alled grai i ess a d a measurement of this phenomenon establishes a degree of gra ularity .

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Film interpretation

and reference radiographs The common term for film interpretation is film viewing. Film

viewing in fact means the evaluation of the image quality of a radiograph for compliance with the code requirements and the interpretation of details of any possible defect visible on the film. For this purpose, the film is placed in front of an illuminated screen of appropriate brightness/luminance. The edges of the film and areas of low density need to be masked to avoid glare.

The following conditions are important for good film interpretation: • right ess of the illu i ated s ree lu i a e

• de sity of the radiograph

• diffusio a d e e ess of the illu i ated s ree

• a ie t light i the ie i g roo

• fil ie er’s eye-sight

Poor viewing conditions may cause important defect information on a radiograph to go unseen. Librar

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• The lassi fil a e ie ed after photochemical treatment (wet process) on a film viewing screen. Defects or irregularities in the object cause variations in film density (brightness or transparency). The parts of the films which have received more radiation during exposure – the regions under cavities, for example – appear darker, that is, the film density is higher.

• Digital radiography gives the same shades of black and white images, but viewing and interpretation is done on a computer screen (VDU).

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X-ray Videos

x-ray inspection of CAN:

https://www.youtube.com/watch?v=acljU7PoDS4

case study of x-ray

https://www.youtube.com/watch?v=ksv1JEOWlvw&index=2&list=PLML1Ygt3lPBV0f9qL3sHn4qyAb9x5RAAY

what is x-ray machine:

https://www.youtube.com/watch?v=eLYLzBpprYM

having an x-ray

https://www.youtube.com/watch?v=dybncisweZE Library Study M

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https://www.youtube.com/watch?v=acljU7PoDS4



https://www.youtube.com/watch?v=eLYLzBpprYM

https://www.youtube.com/watch?v=dybncisweZE



Ultrasonic Testing Page 1 of 36

Ultrasonic Testing

Ultrasonic Testing (UT) uses high frequency sound waves (typically in the range between 0.5 and 15 MHz) to conduct examinations and make measurements. Besides its wide use in engineering applications (such as flaw detection/evaluation, dimensional measurements, material characterization, etc.), ultrasonics are also used in the medical field (such as sonography, therapeutic ultrasound, etc.).

In general, ultrasonic testing is based on the capture and quantification of either the reflected waves (pulse-echo) or the transmitted waves (through-transmission). Each of the two types is used in certain applications, but generally, pulse echo systems are more useful since they require one-sided access to the object being inspected.

Basic Principles

A typical pulse-echo UT inspection system consists of several functional units, such as the pulser/receiver, transducer, and a display device. A pulser/receiver is an electronic device that can produce high voltage electrical pulses. Driven by the pulser, the transducer generates high frequency ultrasonic energy. The sound energy is introduced and propagates through the materials in the form of waves. When there is a discontinuity (such as a crack) in the wave path, part of the energy will be reflected back from the flaw surface. The reflected wave signal is transformed into an electrical signal by the transducer and is displayed on a screen. Knowing the velocity of the waves, travel time can be directly related to the distance that the signal traveled. From the signal, information about the reflector location, size, orientation and other features can sometimes be gained.

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Advantages

It is sensitive to both surface and subsurface discontinuities.

The depth of penetration for flaw detection or measurement is superior to other NDT methods.

Only single-sided access is needed when the pulse-echo technique is used.

It is highly accurate in determining reflector position and estimating size and shape.

Minimal part preparation is required.

It provides instantaneous results.

Detailed images can be produced with automated systems.

It is nonhazardous to operators or nearby personnel and does not affect the material being tested.

It has other uses, such as thickness measurement, in addition to flaw detection.

Its equipment can be highly portable or highly automated.

Disadvantages

Surface must be accessible to transmit ultrasound.

Skill and training is more extensive than with some other methods.

It normally requires a coupling medium to promote the transfer of sound energy into the test specimen.

Materials that are rough, irregular in shape, very small, exceptionally thin or not homogeneous are difficult to inspect.

Cast iron and other coarse grained materials are difficult to inspect due to low sound transmission and high signal noise.

Linear defects oriented parallel to the sound beam may go undetected.

Reference standards are required for both equipment calibration and the characterization of flaws.

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PHYSICS OF ULTRASOUND

Wave Propagation

Ultrasonic testing is based on the vibration in materials which is generally referred to as acoustics. All material substances are comprised of atoms, which may be forced into vibrational motion about their equilibrium positions. Many different patterns of vibrational motion exist at the atomic level; however, most are irrelevant to acoustics and ultrasonic testing. Acoustics is focused on particles that contain many atoms that move in harmony to produce a mechanical wave. When a material is not stressed in tension or compression beyond its elastic limit, its individual particles perform elastic oscillations. When the particles of a medium are displaced from their equilibrium positions, internal restoration forces arise. These elastic restoring forces between particles, combined with inertia of the particles, lead to the oscillatory motions of the medium.

In solids, sound waves can propagate in four principal modes that are based on the way the particles oscillate. Sound can propagate as longitudinal waves, shear waves, surface waves, and in thin materials as plate waves. Longitudinal and shear waves are the two modes of propagation most widely used in ultrasonic testing. The particle movement responsible for the propagation of longitudinal and shear waves is illustrated in the figure.

In longitudinal waves, the oscillations occur in the longitudinal direction or the direction of wave propagation. Since compression and expansion forces are active in these waves, they are also called pressure or compression waves. They are also sometimes called density waves because material density fluctuates as the wave moves. Compression waves can be generated in gases, liquids, as well as solids because the energy travels through the atomic structure by a series of compressions and expansion movements.

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In the transverse or shear waves, particles oscillate at a right angle or transverse to the direction of propagation. Shear waves require an acoustically solid material for effective propagation, and therefore, are not effectively propagated in materials such as liquids or gasses. Shear waves are relatively weak when compared to longitudinal waves. In fact, shear waves are usually generated in materials using some of the energy from longitudinal waves.

Modes of Sound Wave Propagation

In air, sound travels by the compression and rarefaction of air molecules in the direction of travel. However, in solids, molecules can support vibrations in other directions. Hence, a number of different types of sound waves are possible. Waves can be characterized by oscillatory patterns that are capable of maintaining their shape and propagating in a stable manner. The propagation of waves is often described in terms of what are called “wave modes”.

As mentioned previously, longitudinal and transverse (shear) waves are most often used in ultrasonic inspection. However, at surfaces and interfaces, various types of elliptical or complex vibrations of the particles make other waves possible. Some of these wave modes such as Rayleigh and Lamb waves are also useful for ultrasonic inspection.

Though there are many different modes of wave propagation, the table summarizes the four types of waves that are commonly used in NDT.

Wave Type Particle Vibration

Longitudinal (Compression) Parallel to wave direction Transverse (Shear) Perpendicular to wave direction Surface - Rayleigh Elliptical orbit - symmetrical mode Plate Wave - Lamb Component perpendicular to surface

Since longitudinal and transverse waves were discussed previously, surface and plate waves are introduced here.

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Surface (or Rayleigh) waves travel at the surface of a relatively thick solid material penetrating to a depth of one wavelength. A surface wave is a combination of both a longitudinal and transverse motion which results in an elliptical motion as shown in the image. The major axis of the ellipse is perpendicular to the surface of the solid. As the depth of an individual atom from the surface increases, the width of its elliptical motion decreases. Surface waves are generated when a longitudinal wave intersects a surface slightly larger than the second critical angle and they travel at a velocity between .87 and .95 of a shear wave. Rayleigh waves are useful because they are very sensitive to surface defects (and other surface features) and they follow the surface around curves. Because of this, Rayleigh waves can be used to inspect areas that other waves might have difficulty reaching.

Plate (or Lamb) waves are similar to surface waves except they can only be generated in materials a few wavelengths thick (thin plates). Lamb waves are complex vibrational waves that propagate parallel to the test surface throughout the thickness of the material. They are influenced a great deal by the test wave frequency and material thickness. Lamb waves are generated when a wave hits a surface at an incident angle such that the parallel component of the velocity of the wave (in the source) is equal to the velocity of the wave in the test material. Lamb waves will travel several meters in steel and so are useful to scan plate, wire, and tubes. o With Lamb waves, a number of modes of particle vibration are possible, but

the two most common are symmetrical and asymmetrical. The complex motion of the particles is similar to the elliptical orbits for surface waves.

Symmetrical Lamb waves move in a symmetrical fashion about the median plane of the plate. This is sometimes called the “extensional mode” because the wave is stretching and compressing the plate in the wave motion direction. The asymmetrical Lamb wave mode is often called the “flexural mode” because a large portion of the motion is in a normal direction to the plate, and a little motion occurs in the direction parallel to the plate. In this mode, the body of the plate bends as the two surfaces move in the same direction.

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Properties of Acoustic Waves

Among the properties of waves propagating in isotropic solid materials are wavelength, frequency, and velocity. The wavelength is directly proportional to the velocity of the wave and inversely proportional to the frequency of the wave. This relationship is shown by the following equation:

Where; : wavelength (m) : velocity (m/s) : frequency (Hz)

The velocity of sound waves in a certain medium is fixed where it is a characteristic of that medium. As can be noted from the equation, an increase in frequency will result in a decrease in wavelength. For instance, the velocity of longitudinal waves in steel is 5850 m/s and that results in a wavelength of 5.85 mm when the frequency is 1 MHz.

Wavelength and Defect Detection

In ultrasonic testing, the inspector must make a decision about the frequency of the transducer that will be used in order to control the wavelength. The wavelength of the ultrasound used has a significant effect on the probability of detecting a discontinuity. A general rule of thumb is that a discontinuity must be larger than one-half the wavelength to stand a reasonable chance of being detected.

Sensitivity and resolution are two terms that are often used in ultrasonic inspection to describe a technique's ability to locate flaws. Sensitivity is the ability to locate small discontinuities. Sensitivity generally increases with higher frequency (shorter wavelengths). Resolution is the ability of the system to locate discontinuities that are close together within the material or located near the part surface. Resolution also generally increases as the frequency increases.

The wave frequency can also affect the capability of an inspection in adverse ways. Therefore, selecting the optimal inspection frequency often involves maintaining a balance between the favorable and unfavorable results of the selection. Before selecting an inspection frequency, the material's grain structure and thickness, and the discontinuity's type, size, and probable location should be considered. As frequency

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increases, sound tends to scatter from large or course grain structure and from small imperfections within a material. Cast materials often have coarse grains and thus require lower frequencies to be used for evaluations of these products. Wrought and forged products with directional and refined grain structure can usually be inspected with higher frequency transducers.

Since more things in a material are likely to scatter a portion of the sound energy at higher frequencies, the penetration depth (the maximum depth in a material that flaws can be located) is also reduced. Frequency also has an effect on the shape of the ultrasonic beam. Beam spread, or the divergence of the beam from the center axis of the transducer, and how it is affected by frequency will be discussed later.

It should be mentioned, so as not to be misleading, that a number of other variables will also affect the ability of ultrasound to locate defects. These include the pulse length, type and voltage applied to the crystal, properties of the crystal, backing material, transducer diameter, and the receiver circuitry of the instrument. These are discussed in more detail in a later section.

Sound Propagation in Elastic Materials

It was mentioned previously that sound waves propagate due to the vibrations or oscillatory motions of particles within a material. An ultrasonic wave may be visualized as an infinite number of oscillating masses or particles connected by means of elastic springs. Each individual particle is influenced by the motion of its nearest neighbor and both inertial and elastic restoring forces act upon each particle.

A mass on a spring has a single resonant frequency (natural frequency) determined by its spring constant k and its mass m. Within the elastic limit of any material, there is a linear relationship between the displacement of a particle and the force attempting to restore the particle to its equilibrium position. This linear dependency is described by Hooke's Law. In terms of the spring model, the relation between force and displacement is written as F = k x.

The Speed of Sound

Hooke's Law, when used along with Newton's Second Law, can explain a few things about the speed of sound. The speed of sound within a material is a function of the

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properties of the material and is independent of the amplitude of the sound wave. Newton's Second Law says that the force applied to a particle will be balanced by the particle's mass and the acceleration of the particle. Mathematically, Newton's Second Law is written as F = m a. Hooke's Law then says that this force will be balanced by a force in the opposite direction that is dependent on the amount of displacement and the spring constant. Therefore, since the applied force and the restoring force are equal, m a = k x can be written.

Since the mass m and the spring constant k are constants for any given material, it can be seen that the acceleration a and the displacement x are the only variables. It can also be seen that they are directly proportional. For instance, if the displacement of the particle increases, so does its acceleration. It turns out that the time that it takes a particle to move and return to its equilibrium position is independent of the force applied. So, within a given material, sound always travels at the same speed no matter how much force is applied when other variables, such as temperature, are held constant.

Material Properties Affecting the Speed of Sound

Of course, sound does travel at different speeds in different materials. This is because the mass of the atomic particles and the spring constants are different for different materials. The mass of the particles is related to the density of the material, and the spring constant is related to the elastic constants of a material. The general relationship between the speed of sound in a solid and its density and elastic constants is given by the following equation:

Where; : speed of sound (m/s) : elastic constant “in a given direction” (N/m2)

: density (kg/m3)

This equation may take a number of different forms depending on the type of wave (longitudinal or shear) and which of the elastic constants that are used. It must also be mentioned that the subscript “ ” attached to “ ” in the above equation is used to indicate the directionality of the elastic constants with respect to the wave type and

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direction of wave travel. In isotropic materials, the elastic constants are the same for all directions within the material. However, most materials are anisotropic and the elastic constants differ with each direction. For example, in a piece of rolled aluminum plate, the grains are elongated in one direction and compressed in the others and the elastic constants for the longitudinal direction differs slightly from those for the transverse or short transverse directions.

For longitudinal waves, the speed of sound in a solid material can be found as:

Where; : speed of sound for longitudinal waves (m/s) : Young’s modulus (N/m2) : Poisson’s ratio

While for shear (transverse) waves, the speed of sound is found as:

Where; : speed of sound for shear waves (m/s) : Shear modulus of elasticity (N/m2);

From the above equations, it can be found that longitudinal waves travel faster than shear waves (longitudinal waves are approximately twice as fast as shear waves). The table below gives examples of the compressional and shear sound velocities in some metals.

Material Compressional velocity

Shear velocity

Aluminum 6320 3130 Steel (1020) 5890 3240 Cast iron 4800 2400 Copper 4660 2330 Titanium 6070 3310

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Attenuation of Sound Waves

When sound travels through a medium, its intensity diminishes with distance. In idealized materials, sound pressure (signal amplitude) is reduced due to the spreading of the wave. In natural materials, however, the sound amplitude is further weakened due to the scattering and absorption. Scattering is the reflection of the sound in directions other than its original direction of propagation. Absorption is the conversion of the sound energy to other forms of energy. The combined effect of scattering and absorption is called attenuation. Attenuation is generally proportional to the square of sound frequency.

The amplitude change of a decaying plane wave can be expressed as:

Where; : initial (unattenuated) amplitude : attenuation coefficient (Np/m) : traveled distance (m)

The Decibel (dB) is a logarithmic unit that describes a ratio of two measurements. The difference

between two measurements X1 and X2 is described in decibels as:

The intensity of sound waves (I) is quantified by measuring the variation in sound pressure using a

transducer, and then the pressure is transferred to a voltage signal. Since the intensity of sound

waves is proportional to the square of the pressure amplitude, the ratio of sound intensity in

decibels can be expressed as:

where;

: the change in sound intensity between two measurements

: are the two different transducer output voltages (or readings)

Use of dB units allows ratios of various sizes to be described using easy to work with numbers.

Np (Neper) is a logarithmic dimensionless

quantity and it can be converted to Decibels

by dividing it by 0.1151.

Decibel is a more common unit when

relating the amplitudes of two signals.

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Attenuation can be determined by evaluating the multiple back-wall reflections seen in a typical A-scan display (like the one shown in the image in the previous page). The number of decibels between two adjacent signals is measured and this value is divided by the time interval between them. This calculation produces an attenuation coefficient in decibels per unit time. Then knowing the velocity of sound it can be converted to decibels per unit length.

Acoustic Impedance

Sound travels through materials under the influence of sound pressure. Because molecules or atoms of a solid are bound elastically to one another, the excess pressure results in a wave propagating through the solid.

The acoustic impedance ( ) of a material is defined as the product of its density ( ) and the velocity of sound in that material ( ).

Where; : acoustic impedance (kg/m2s) or (N s/m3) : density (kg/m3) : sound velocity (m/s)

The table gives examples of the acoustic impedances for some materials:

Aluminum Copper Steel Titanium

Water (20°C)

Air (20°C)

Acou. Imp.

(kg/m2s) 17.1 x 106 41.6 x 106 46.1 x 106 28 x 106 1.48 x 106 413

Acoustic impedance is important in:

the determination of acoustic transmission and reflection at the boundary of two materials having different acoustic impedances.

the design of ultrasonic transducers.

assessing absorption of sound in a medium.

Reflection and Transmission Coefficients

Ultrasonic waves are reflected at boundaries where there is a difference in acoustic impedances ( ) of the materials on each side of the boundary. This difference in is

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commonly referred to as the impedance mismatch. The greater the impedance mismatch, the greater the percentage of energy that will be reflected at the interface or boundary between one medium and another.

The fraction of the incident wave intensity that is reflected can be derived based on the fact that particle velocity and local particle pressures must be continuous across the boundary. When the acoustic impedances of the materials on both sides of the boundary are known, the fraction of the incident wave intensity that is reflected (the reflection coefficient) can be calculated as:

Where are the acoustic impedances of the two materials at the interface.

Since the amount of reflected energy plus the transmitted energy must equal the total amount of incident energy, the “transmission coefficient” is calculated by simply subtracting the reflection coefficient from one ( ).

Taking for example a water steel interface and calculating the reflection and transmission coefficients (using the acoustic impedance information given in the previous table), we get = 0.88 and = 0.12. This means that the amount of energy transmitted into the second material is only 12% while 88% is reflected back at the interface. If we convert the amounts of reflection and transmission to decibels, we find that to be -1.1 dB and -18.4 dB respectively. The negative sign indicates that individually, the amount of reflected and transmitted energy is smaller than the incident energy.

If reflection and transmission at interfaces is followed through the component, only a small percentage of the original energy makes it back to the transducer, even when loss by attenuation is ignored. For example, consider an immersion inspection of a steel block. The sound energy leaves the transducer, travels through the water, encounters the front surface of the steel, encounters the back surface of the steel and reflects back through the front surface on its way back to the transducer. At the water steel interface (front surface), 12% of the energy is transmitted. At the back surface, 88% of the 12% that made it through the front surface is reflected. This is 10.6% of the intensity of the initial incident wave. As the wave exits the part back through the front surface, only 12% of

Multiplying the reflection coefficient by

100 yields the amount of energy reflected

as a percentage of the original energy.

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10.6 or 1.3% of the original energy is transmitted back to the transducer.

Note that in such calculation the attenuation of the signal as it travels through the material is not considered. Should it be considered, the amount of signal received back by the transducer would be even smaller.

Q: What portion of the signal will be reflected at an Air-Steel interface?

A: 99.996%

Refraction and Snell's Law

When an ultrasonic wave passes through an interface between two materials at an oblique angle, and the materials have different indices of refraction, both reflected and refracted waves are produced. This also occurs with light, which is why objects seen across an interface appear to be shifted relative to where they really are. For example, if you look straight down at an object at the bottom of a glass of water, it looks closer than it really is.

Refraction takes place at an interface of two materials due to the difference in acoustic velocities between the two materials. The figure shows the case where plane sound waves traveling in one material enters a second material that has a higher acoustic velocity. When the wave encounters the interface between these two materials, the portion of the wave in the second material is moving faster than the portion of the wave that is still in the first material. As a result, this causes the wave to bend and change its direction (this is referred to as “refraction”).

Snell's Law describes the relationship between the angles and

the velocities of the waves. Snell's law equates the ratio of

material velocities to the ratio of the sine's of incident and

refracted angles, as shown in the following equation:

Where;

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: the longitudinal wave velocities in the first and second materials

respectively : the angles of incident and refracted waves respectively

Note that in the diagram, there is a reflected longitudinal wave ( ) shown. This wave

is reflected at the same angle as the incident wave because the two waves are traveling in the same material, and hence have the same velocities. This reflected wave is unimportant in our explanation of Snell's Law, but it should be remembered that some of the wave energy is reflected at the interface.

Mode Conversion

When sound travels in a solid material, one form of wave energy can be transformed

into another form. For example, when a longitudinal wave hits an interface at an angle,

some of the energy can cause particle movement in

the transverse direction to start a shear wave. Mode

conversion occurs when a wave encounters an

interface between materials of different acoustic

impedances and the incident angle is not normal to

the interface. It should be noted that mode

conversion occurs “every time” a wave encounters

an interface at an angle. This mode conversion

occurs for both the portion of the wave that passes

through the interface and the portion that reflects

off the interface.

In the previous section, it was pointed out that when sound waves pass through an interface between materials having different acoustic velocities, refraction takes place at the interface. The larger the difference in acoustic velocities between the two materials, the more the sound is refracted. However, the converted shear wave is not refracted as much as the longitudinal wave because shear waves travel slower than longitudinal waves. Therefore, the velocity difference between the incident longitudinal wave and the shear wave is not as great as it is between the incident and refracted longitudinal waves. Also note that when a longitudinal wave is reflected inside the material, the reflected shear wave is reflected at a smaller angle than the

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reflected longitudinal wave. This is also due to the fact that the shear velocity is less than the longitudinal velocity within a given material.

Snell's Law holds true for shear waves as well as longitudinal waves and can be written as follows:

Where; : the longitudinal wave velocities in the

first and second materials respectively

: the shear wave velocities in the first

and second materials respectively

: the angles of incident and refracted longitudinal waves respectively

: the angles of the converted reflected and refracted shear waves respectively

Critical Angles

When a longitudinal wave moves from a slower to a faster material (and thus the wave is refracted), there is an incident angle that makes the angle of refraction for the “longitudinal wave” to become 90°. This is angle is known as “the first critical angle”. The first critical angle can be found from Snell's law by putting in an angle of 90° for the angle of the refracted ray. At the critical angle of incidence, much of the acoustic energy is in the form of an inhomogeneous compression wave, which travels along the interface and decays exponentially with depth from the interface. This wave is sometimes referred to as a "creep wave". Because of their inhomogeneous nature and the fact that they decay rapidly, creep waves are not used as extensively as Rayleigh surface waves in NDT.

When the incident angle is equal or greater than the first critical angle, only the mode converted shear wave propagates into the material. For this reason, most angle beam transducers use a shear wave so that the signal is not complicated by having two waves present.

In many cases there is also an incident angle that makes the angle of refraction for the “shear wave” to become 90°. This is known as the “second critical angle” and at this

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point, all of the wave energy is reflected or refracted into a surface following shear wave or shear creep wave. Slightly beyond the second critical angle, surface (Rayleigh) waves will be generated.

The incident angle for angle-beam transducers is somewhere between the first and second critical angles such that a shear wave, at a desired angle, is introduced into the material being inspected.

The figure shows the mode of waves introduced into a steel surface as a function of the incident angle of the wave generated by the transducer. It can be seen from the figure that the incident angle for angle beam (shear) transducers ranges between 30° to 55°. But it is important to remember that, due to refraction, the angle of the shear wave inside the material is completely different than the incident angle.

Wave Interaction or Interference

The understanding of the interaction or interference of waves is important for understanding the performance of an ultrasonic transducer. When sound emanates from an ultrasonic transducer, it does not originate from a single point, but instead originates from many points along the surface of the piezoelectric element. This results in a sound field with many waves interacting or interfering with each other.

When waves interact, they superimpose on each other, and the amplitude of the sound pressure at any point of interaction is the sum of the amplitudes of the two individual waves. First, let's consider two identical waves that originate from the same point. When they are in phase (so that the peaks and valleys of one are exactly aligned with those of the other), they combine to double the pressure of either wave acting alone. When they are completely out of phase (so that the peaks of one wave are exactly aligned with the valleys of the other wave), they combine to cancel each other out. When the two waves are not completely in phase or out of phase, the resulting wave

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is the sum of the wave amplitudes for all points along the wave.

When the origins of the two interacting waves are not the same, it is a little harder to picture the wave interaction, but the principles are the same. Up until now, we have primarily looked at waves in the form of a 2D plot of wave amplitude versus wave position. However, anyone that has dropped something in a pool of water can picture the waves radiating out from the source with a circular wave front. If two objects are dropped a short distance apart into the pool of water, their waves will radiate out from their sources and interact with each other. At every point where the waves interact, the amplitude of the particle displacement is the combined sum of the amplitudes of the particle displacement of the individual waves.

As stated previously, sound waves originate from multiple points along the face of the transducer. The image shows what the sound field would look like if the waves originated from just three points (of course there are more than three points of origin along the face of a transducer). It can be seen that where the waves interact near the face of the transducer and as a result there are extensive fluctuations and the sound field is very uneven. In ultrasonic testing, this is known as the “near field” or Fresnel zone. The sound field is more uniform away from the transducer in the “far field” or Fraunhofer zone. At some distance from the face of the transducer and central to the face of the transducer, a uniform and intense wave field develops.

Wave Diffraction

Diffraction involves a change in direction of waves as they pass through an opening or around a barrier in their path. Diffraction of sound waves is commonly observed; we notice sound diffracting around corners or through door openings, allowing us to hear others who are speaking to us from adjacent rooms.

In ultrasonic testing of solids, diffraction patterns are usually generated at the edges of sharp reflectors (or discontinuities) such as cracks. Usually the tip of a crack behaves as point source spreading waves in all directions due to the diffraction of the incident wave.

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EQUIPMENT & TRANSDUCERS

Piezoelectric Transducers

The conversion of electrical pulses to mechanical vibrations and the conversion of returned mechanical vibrations back into electrical energy is the basis for ultrasonic testing. This conversion is done by the transducer using a piece of piezoelectric material (a polarized material having some parts of the molecule positively charged, while other parts of the molecule are negatively charged) with electrodes attached to two of its opposite faces. When an electric field is applied across the material, the polarized molecules will align themselves with the electric field causing the material to change dimensions. In addition, a permanently-polarized material such as quartz (SiO2) or barium titanate (BaTiO3) will produce an electric field when the material changes dimensions as a result of an imposed mechanical force. This phenomenon is known as the piezoelectric effect.

The active element of most acoustic transducers used today is a piezoelectric ceramic, which can be cut in various ways to produce different wave modes. A large piezoelectric ceramic element can be seen in the image of a sectioned low frequency transducer. The most commonly employed ceramic for making transducers is lead zirconate titanate.

The thickness of the active element is determined by the desired frequency of the transducer. A thin wafer element vibrates with a wavelength that is twice its thickness. Therefore, piezoelectric crystals are cut to a thickness that is 1/2 the desired radiated wavelength. The higher the frequency of the transducer, the thinner the active element.

Characteristics of Piezoelectric Transducers

The function of the transducer is to convert electrical signals into mechanical vibrations (transmit mode) and mechanical vibrations into electrical signals (receive mode). Many

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factors, including material, mechanical and electrical construction, and the external mechanical and electrical load conditions, influence the behavior of a transducer.

A cut away of a typical contact transducer is shown in the figure. To get as much energy out of the transducer as possible, an impedance matching layer is placed between the active element and the face of the transducer. Optimal impedance matching is achieved by sizing the matching layer so that its thickness is 1/4 of the desired wavelength. This keeps waves that are reflected within the matching layer in phase when they exit the layer. For contact transducers, the matching layer is made from a material that has an acoustical impedance between the active element and steel. Immersion transducers have a matching layer with an acoustical impedance between the active element and water. Contact transducers also incorporate a wear plate to protect the matching layer and active element from scratching.

The backing material supporting the crystal has a great influence on the damping characteristics of a transducer. Using a backing material with an impedance similar to that of the active element will produce the most effective damping. Such a transducer will have a wider bandwidth resulting in higher sensitivity and higher resolution (i.e., the ability to locate defects near the surface or in close proximity in the material). As the mismatch in impedance between the active element and the backing material increases, material penetration increases but transducer sensitivity is reduced.

The bandwidth refers to the range of frequencies associated with a transducer. The frequency noted on a transducer is the central frequency and depends primarily on the backing material. Highly damped transducers will respond to frequencies above and below the central frequency. The broad frequency range provides a transducer with high resolving power. Less damped transducers will exhibit a narrower frequency range and poorer resolving power, but greater penetration.

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The central frequency will also define the capabilities of a transducer. Lower frequencies (0.5MHz-2.25MHz) provide greater energy and penetration in a material, while high frequency crystals (15.0MHz-25.0MHz) provide reduced penetration but greater sensitivity to small discontinuities.

Radiated Fields of Ultrasonic Transducers

The sound that emanates from a piezoelectric transducer does not originate from a point, but instead originates from most of the surface of the piezoelectric element. The sound field from a typical piezoelectric transducer is shown in the figure where lighter colors indicating higher intensity. Since the ultrasound originates from a number of points along the transducer face, the ultrasound intensity along the beam is affected by constructive and destructive wave interference as discussed previously. This wave interference leads to extensive fluctuations in the sound intensity near the source and is known as the “near field”. Because of acoustic variations within a near field, it can be extremely difficult to accurately evaluate flaws in materials when they are positioned within this area.

The pressure waves combine to form a relatively uniform front at the end of the near field. The area beyond the near field where the ultrasonic beam is more uniform is called the “far field”. The transition between the near field and the far field occurs at a distance, , and is sometimes referred to as the "natural focus" of a flat (or unfocused) transducer. Spherical or cylindrical focusing changes the structure of a transducer field by "pulling" the point nearer the transducer. The area just beyond the near field is where the sound wave is well behaved and at its maximum strength. Therefore, optimal detection results will be obtained when flaws occur in this area.

For a round transducer (often referred to as piston source transducer), the near field distance can be found as:

Where;

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: transducer diameter, : transducer frequency, and : sound longitudinal velocity in the medium through which waves are transmitted.

Transducer Beam Spread

As the sound waves exits the near field and propegate through the material, the sound beam continiously spreads out. This phenomenon is usually referred to as beam spread but sometimes it is also referred to as beam divergence or ultrasonic diffraction. It should be noted that there is actually a difference between beam spread and beam divergence. Beam spread is a measure of the whole angle from side to side of the beam in the far field. Beam divergence is a measure of the angle from one side of the sound beam to the central axis of the beam in the far field. Therefore, beam spread is twice the beam divergence.

Although beam spread must be considered when performing an ultrasonic inspection, it is important to note that in the far field, or Fraunhofer zone, the maximum sound pressure is always found along the acoustic axis (centerline) of the transducer. Therefore, the strongest reflections are likely to come from the area directly in front of the transducer.

Beam spread occurs because the vibrating particle of the material (through which the wave is traveling) do not always transfer all of their energy in the direction of wave propagation. If the particles are not directly aligned in the direction of wave propagation, some of the energy will get transferred off at an angle. In the near field, constructive and destructive wave interference fill the sound field with fluctuation. At the start of the far field, however, the beam strength is always greatest at the center of the beam and diminishes as it spreads outward.

The beam spread is largely influenced by the frequency and diameter of the transducer. For a flat piston source transducer, an approximation of the beam divergence angle at which the sound pressure has decreased by one half (-6 dB) as compared to its value at the centerline axis can be caculated as:

Where;

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: the beam divergence angle from centerline to point where signal is at half strength : sound velocity in the material : diameter of the transducer : frequency of the transducer

Transducer Types

Ultrasonic transducers are manufactured for a variety of applications and can be custom fabricated when necessary. Careful attention must be paid to selecting the proper transducer for the application. It is important to choose transducers that have the desired frequency, bandwidth, and focusing to optimize inspection capability. Most often the transducer is chosen either to enhance the sensitivity or resolution of the system.

Transducers are classified into two major groups according to the application.

Contact transducers are used for direct contact inspections, and are generally hand manipulated. They have elements protected in a rugged casing to withstand sliding contact with a variety of materials. These transducers have an ergonomic design so that they are easy to grip and move along a surface. They often have replaceable wear plates to lengthen their useful life. Coupling materials of water, grease, oils, or commercial materials are used to remove the air gap between the transducer and the component being inspected.

Immersion transducers do not contact the component. These transducers are designed to operate in a liquid environment and all connections are watertight. Immersion transducers usually have an impedance matching layer that helps to get more sound energy into the water and, in turn, into the component being inspected. Immersion transducers can be purchased with a planer, cylindrically focused or spherically focused lens. A focused transducer can improve the sensitivity and axial resolution by concentrating the sound energy to a smaller area. Immersion transducers are typically used inside a water tank or as part of a squirter or bubbler system in scanning applications.

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Other Types of Contact Transducers

Contact transducers are available in a variety of configurations to improve their usefulness for a variety of applications. The flat contact transducer shown above is used in normal beam inspections of relatively flat surfaces, and where near surface resolution is not critical. If the surface is curved, a shoe that matches the curvature of the part may need to be added to the face of the transducer. If near surface resolution is important or if an angle beam inspection is needed, one of the special contact transducers described below might be used.

Dual element transducers contain two independently operated elements in a single housing. One of the elements transmits and the other receives the ultrasonic signal. Dual element transducers are especially well suited for making measurements in applications where reflectors are very near the transducer since this design eliminates the ring down effect that single-element transducers experience (when single-element transducers are operating in pulse echo mode,

the element cannot start receiving reflected signals until the element has stopped ringing from its transmit function). Dual element transducers are very useful when making thickness measurements of thin materials and when inspecting for near surface defects. The two elements are angled towards each other to create a crossed-beam sound path in the test material.

Delay line transducers provide versatility with a variety of replaceable options. Removable delay line, surface conforming membrane, and protective wear cap options can make a single transducer effective for a wide range of applications. As the name implies, the primary function of a delay line transducer is to

introduce a time delay between the generation of the sound wave and the arrival of any reflected waves. This allows the transducer to complete its "sending" function before it starts its "receiving" function so that near surface resolution is improved. They are designed for use in applications such as high precision thickness gauging of thin materials and delamination checks in composite materials. They are also useful in high-temperature measurement applications since the delay line provides some insulation to the piezoelectric element from the heat.

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Angle beam transducers and wedges are typically used to introduce a refracted shear wave into the test material. Transducers can be purchased in a variety of fixed angles or in adjustable versions where the user determines the angles of incidence and refraction. In the fixed angle versions, the angle of refraction that is marked on the transducer is only accurate for a particular material, which

is usually steel. The most commonly used refraction angles for fixed angle transducers are 45°, 60° and 70°. The angled sound path allows the sound beam to be reflected from the backwall to improve detectability of flaws in and around welded areas. They are also used to generate surface waves for use in detecting defects on the surface of a component.

Normal incidence shear wave transducers are unique because they allow the introduction of shear waves directly into a test piece without the use of an angle beam wedge. Careful design has enabled manufacturing of transducers with minimal longitudinal wave contamination.

Paint brush transducers are used to scan wide areas. These long and narrow transducers are made up of an array of small crystals and that make it possible to scan a larger area more rapidly for discontinuities. Smaller and more sensitive transducers are often then required to further define the details of a discontinuity.

Couplant

A couplant is a material (usually liquid) that facilitates the transmission of ultrasonic energy from the transducer into the test specimen. Couplant is generally necessary because the acoustic impedance mismatch between air and solids is large. Therefore, nearly all of the energy is reflected and very little is transmitted into the test material. The couplant displaces the air and makes it possible to get more sound energy into the test specimen so that a usable ultrasonic signal can be obtained. In contact ultrasonic testing a thin film of oil, glycerin or water is typically used between the transducer and the test surface. When shear waves are to be transmitted, the fluid is generally selected to have a significant viscosity.

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When scanning over the part, an immersion technique is often used. In immersion ultrasonic testing both the transducer and the part are immersed in the couplant, which is typically water. This method of coupling makes it easier to maintain consistent coupling while moving and manipulating the transducer and/or the part.

Electromagnetic Acoustic Transducers (EMATs)

Electromagnetic-acoustic transducers (EMAT) are a modern type of ultrasonic transducers that work based on a totally different physical principle than piezoelectric transducers and, most importantly, they do not need couplant. When a wire is placed near the surface of an electrically conducting object and is driven by a current at the desired ultrasonic frequency, eddy currents will be induced in a near surface region of the object. If a static magnetic field is also present, these eddy currents will experience forces called “Lorentz forces” which will cause pressure waves to be generated at the surface and propagate through the material.

Different types of sound waves (longitudinal, shear, lamb) can be generated using EMATs by varying the configuration of the transducer such that the orientation of the static magnetic field is changed.

EMATs can be used for thickness measurement, flaw detection, and material property characterization. The EMATs offer many advantages based on its non-contact couplant-free operation. These advantages include the ability to operate in remote environments at elevated speeds and temperatures.

Pulser-Receivers

Ultrasonic pulser-receivers are well suited to general purpose ultrasonic testing. Along with appropriate transducers and an oscilloscope, they can be used for flaw detection and thickness gauging in a wide variety of metals, plastics, ceramics, and composites. Ultrasonic pulser-receivers provide a unique, low-cost ultrasonic measurement

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capability. Specialized portable equipment that are dedicated for ultrasonic inspection merge the pulser-receiver with the scope display in one small size battery operated unit.

The pulser section of the instrument generates short, large amplitude electric pulses of controlled energy, which are converted into short ultrasonic pulses when applied to an ultrasonic transducer. Control functions associated with the pulser circuit include:

Pulse length or damping: The amount of time the pulse is applied to the transducer.

Pulse energy: The voltage applied to the transducer. Typical pulser circuits will apply from 100 volts to 800 volts to a transducer.

In the receiver section the voltage signals produced by the transducer, which represent the received ultrasonic pulses, are amplified. The amplified signal is available as an output for display or capture for signal processing. Control functions associated with the receiver circuit include:

Signal rectification: The signal can be viewed as positive half wave, negative half wave or full wave.

Filtering to shape and smoothing

Gain, or signal amplification

Reject control

Data Presentation

Ultrasonic data can be collected and displayed in a number of different formats. The three most common formats are known in the NDT world as A-scan, B-scan and C-scan presentations. Each presentation mode provides a different way of looking at and evaluating the region of material being inspected. Modern computerized ultrasonic scanning systems can display data in all three presentation forms simultaneously.

A-Scan Presentation

The A-scan presentation displays the amount of received ultrasonic energy as a function of time. The relative amount of received energy is plotted along the vertical axis and the elapsed time (which may be related to the traveled distance within the material) is displayed along the horizontal axis. Most instruments with an A-scan

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display allow the signal to be displayed in its natural radio frequency form (RF), as a fully rectified RF signal, or as either the positive or negative half of the RF signal. In the A-scan presentation, relative discontinuity size can be estimated by comparing the signal amplitude obtained from an unknown reflector to that from a known reflector. Reflector depth can be determined by the position of the signal on the horizontal time axis.

In the illustration of the A-scan presentation shown in the figure, the initial pulse generated by the transducer is represented by the signal IP, which is near time zero. As the transducer is scanned along the surface of the part, four other signals are likely to appear at different times on the screen. When the transducer is in its far left position, only the IP signal and signal A, the sound energy reflecting from surface A, will be seen on the trace. As the transducer is scanned to the right, a signal from the backwall BW will appear later in time, showing that the sound has traveled farther to reach this surface. When the transducer is over flaw B, signal B will appear at a point on the time scale that is approximately halfway between the IP signal and the BW signal. Since the IP signal corresponds to the front surface of the material, this indicates that flaw B is about halfway between the front and back surfaces of the sample. When the transducer is moved over flaw C, signal C will appear earlier in time since the sound travel path is shorter and signal B will disappear since sound will no longer be reflecting from it.

B-Scan Presentation

The B-scan presentation is a type of presentation that is possible for automated linear scanning systems where it shows a profile (cross-sectional) view of the test specimen. In the B-scan, the time-of-flight (travel time) of the sound waves is displayed along the vertical axis and the linear position of the transducer is displayed along the horizontal axis. From the B-scan, the depth of the reflector and its approximate linear dimensions in the scan direction can be determined. The B-scan is typically produced by establishing a trigger gate on the A-scan. Whenever the signal intensity is great enough to trigger the gate, a point is produced on the B-scan. The gate is triggered by the

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sound reflected from the backwall of the specimen and by smaller reflectors within the material. In the B-scan image shown previously, line A is produced as the transducer is scanned over the reduced thickness portion of the specimen. When the transducer moves to the right of this section, the backwall line BW is produced. When the transducer is over flaws B and C, lines that are similar to the length of the flaws and at similar depths within the material are drawn on the B-scan. It should be noted that a limitation to this display technique is that reflectors may be masked by larger reflectors near the surface.

C-Scan Presentation

The C-scan presentation is a type of presentation that is possible for automated two-dimensional scanning systems that provides a plan-type view of the location and size of test specimen features. The plane of the image is parallel to the scan pattern of the transducer. C-scan presentations are typically produced with an automated data acquisition system, such as a computer controlled immersion scanning system. Typically, a data collection gate is established on the A-scan and the amplitude or the time-of-flight of the signal is recorded at regular intervals as the transducer is scanned over the test piece. The relative signal amplitude or the time-of-flight is displayed as a shade of gray or a color for each of the positions where data was recorded. The C-scan presentation provides an image of the features that reflect and scatter the sound within and on the surfaces of the test piece.

High resolution scans can produce very detailed images. The figure shows two ultrasonic C-scan images of a US quarter. Both images were produced using a pulse-echo technique with the transducer scanned over the head side in an immersion scanning system. For the C-scan image on the top, the gate was set to capture the amplitude of the sound reflecting from the front surface of the quarter. Light areas in the image indicate areas that reflected a greater amount of energy back to the transducer. In the C-scan image on the bottom, the gate was moved to record the intensity of the sound reflecting from the back surface of the coin. The details on the back surface are clearly visible but front surface features are also still visible since the sound energy is affected by these features as it travels through the front surface of the coin.

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MEASUREMENT AND CALIBRATION TECHNIQUES

Normal Beam Inspection

Pulse-echo ultrasonic measurements can determine the location of a discontinuity in a part or structure by accurately measuring the time required for a short ultrasonic pulse generated by a transducer to travel through a thickness of material, reflect from the back or the surface of a discontinuity, and be returned to the transducer. In most applications, this time interval is a few microseconds or less. The two-way transit time measured is divided by two to account for the down-and-back travel path and multiplied by the velocity of sound in the test material. The result is expressed in the well-known relationship:

Where is the distance from the surface to the discontinuity in the test piece, is the velocity of sound waves in the material, and is the measured round-trip transit time.

Precision ultrasonic thickness gages usually operate at frequencies between 500 kHz and 100 MHz, by means of piezoelectric transducers that generate bursts of sound waves when excited by electrical pulses. Typically, lower frequencies are used to optimize penetration when measuring thick, highly attenuating or highly scattering materials, while higher frequencies will be recommended to optimize resolution in thinner, non-attenuating, non-scattering materials. It is possible to measure most engineering materials ultrasonically, including metals, plastic, ceramics, composites, epoxies, and glass as well as liquid levels and the thickness of certain biological specimens. On-line or in-process measurement of extruded plastics or rolled metal often is possible, as is measurements of single layers or coatings in multilayer materials.

Angle Beam Inspection

Angle beam transducers and wedges are typically used to introduce a refracted shear

wave into the test material. An angled sound path allows the sound beam to come in

from the side, thereby improving detectability of flaws in and around welded areas.

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Angle beam inspection is somehow different than normal beam inspection. In normal

beam inspection, the backwall echo is always present on the scope display and when

the transducer basses over a discontinuity a new echo will appear between the initial

pulse and the backwall echo. However, when scanning a surface using an angle beam

transducer there will be no reflected echo on the scope display unless a properly

oriented discontinuity or reflector comes into the beam path.

If a reflection occurs before the sound waves reach the backwall, the reflection is

usually referred to as “first leg reflection”. The angular distance (Sound Path) to the

reflector can be calculated using the same formula used for normal beam transducers

(but of course using the shear velocity instead of the longitudinal velocity) as:

where is the shear sound velocity in the material.

Knowing the angle of refraction for the transducer,

the surface distance to the reflector and its depth

can be calculated as:

where is the angle of refraction.

If a reflector came across the sound beam after it has reached and reflected off the

backwall, the reflection is usually referred to as “second leg reflection”. In this case, the

“Sound Path” (the total sound path

for the two legs) and the “Surface

Distance” can be calculated using

the same equations given above,

however, the “Depth” of the

reflector will be calculated as:

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Inspection of Welded Joints

The most commonly occurring defects in welded joints are porosity, slag inclusions, lack of side-wall fusion, lack of intermediate-pass fusion, lack of root penetration, undercutting, and longitudinal or transverse cracks. With the exception of single gas pores all the listed defects are usually well detectable using ultrasonics.

Ultrasonic weld inspections are typically performed using straight beam transducer in conjunction with angle beam transducers.

A straight beam transducer, producing a longitudinal wave at normal incidence into the test piece, is first used to locate any laminations in or near the heat-affected zone. This is important because an angle beam transducer may not be able to provide a return signal from a laminar flaw.

The second step in the inspection involves using an angle beam transducer to

inspect the actual weld. This inspection may include the root, sidewall, crown, and

heat-affected zones of a weld. The process involves scanning the surface of the

material around the weldment with the transducer. This refracted sound wave will

bounce off a reflector (discontinuity) in the path of the sound beam.

To determine the proper scanning

area for both sides of the weld, the

inspector must calculate the skip

distance of the sound beam using

the refracted angle and material

thickness as:

where is the material thickness.

Based on such calculations, the inspector can identify the transducer locations on the

surface of the material corresponding to the face, sidewall, and root of the weld.

The angle of refraction for the angle beam transducer used for inspection is usually

chosen such that ( ). Doing so, the second leg of the

beam will be normal to the side wall of the weldment such that lack of fusion can be

easily detected (the first leg will also be normal to the other wall). However, for

improving the detectability of the different types of weld discontinuities, it is

recommended to repeat the scanning using several transducers having different angles

of refraction.

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Crack Tip Diffraction

When the geometry of the part is relatively uncomplicated and the orientation of a flaw is well known, the length of a crack can be determined by a technique known as “crack tip diffraction”.

One common application of the tip diffraction technique is to determine the length of a crack originating from on the backside of a flat plate as shown below. In this case, when an angle beam transducer is scanned over the area of the flaw, an echo appears on the scope display because of the reflection of the sound beam from the base of the crack (top image). As the transducer moves, a second, but much weaker, echo appears due to the diffraction of the sound waves at the tip of the crack (bottom image). However, since the distance traveled by the diffracted sound wave is less, the second signal appears earlier in time on the scope.

Crack height ( ) is a function of the ultrasound shear velocity in the material ( ), the incident angle ( ) and the difference in arrival times between the two signal ( ). Since the beam angle and the thickness of the material is the same in both measurements, two similar right triangles are formed such that one can be overlayed on the other. A third similar right triangle is made, which is comprised on the crack, the length and the angle . The variable is really the difference in time but can easily be converted to a distance by dividing the time in half (to get the one-way travel time) and multiplying this value by the velocity of the sound in the material. Using trigonometry, we can write:

Therefore, the crack height is found to be:

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If the material is relatively thick or the crack is relatively short, the crack base echo and the crack tip diffraction echo could appear on the scope display simultaneously (as seen in the figure). This can be attributed to the divergence of the sound beam where it becomes wide enough to cover the entire crack length. In such case, though the angle of the beam striking the base of the crack is slightly different than the angle of the beam striking the tip of the crack, the previous equation still holds reasonably accurate and it can be used for estimating the crack length.

Calibration Methods

Calibration refers to the act of evaluating and adjusting the precision and accuracy of measurement equipment. In ultrasonic testing, several forms of calibrations must occur. First, the electronics of the equipment must be calibrated to ensure that they are performing as designed. This operation is usually performed by the equipment manufacturer and will not be discussed further in this material. It is also usually necessary for the operator to perform a "user calibration" of the equipment. This user calibration is necessary because most ultrasonic equipment can be reconfigured for use in a large variety of applications. The user must "calibrate" the system, which includes the equipment settings, the transducer, and the test setup, to validate that the desired level of precision and accuracy are achieved.

In ultrasonic testing, reference standards are used to establish a general level of consistency in measurements and to help interpret and quantify the information contained in the received signal. The figure shows some of the most commonly used reference standards for the calibration of ultrasonic equipment. Reference standards are used to validate that the equipment and the setup provide similar results from one day to the next and that similar results are produced by different systems. Reference standards also help the inspector to estimate the size of flaws. In a pulse-echo type setup, signal strength depends on both the size of the flaw and the distance between the flaw and the transducer. The inspector can use a reference standard with an artificially induced flaw of known size and at approximately the same distance away for the transducer to produce a

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signal. By comparing the signal from the reference standard to that received from the actual flaw, the inspector can estimate the flaw size.

The material of the reference standard should be the same as the material being inspected and the artificially induced flaw should closely resemble that of the actual flaw. This second requirement is a major limitation of most standard reference samples. Most use drilled holes and notches that do not closely represent real flaws. In most cases the artificially induced defects in reference standards are better reflectors of sound energy (due to their flatter and smoother surfaces) and produce indications that are larger than those that a similar sized flaw would produce. Producing more "realistic" defects is cost prohibitive in most cases and, therefore, the inspector can only make an estimate of the flaw size.

Reference standards are mainly used to calibrate instruments prior to performing the inspection and, in general, they are also useful for:

Checking the performance of both angle-beam and normal-beam transducers (sensitivity, resolution, beam spread, etc.)

Determining the sound beam exit point of angle-beam transducers

Determining the refracted angle produced

Calibrating sound path distance

Evaluating instrument performance (time base, linearity, etc.)

Introduction to Some of the Common Standards

A wide variety of standard calibration blocks of different designs, sizes and systems of units (mm or inch) are available. The type of standard calibration block used is dependent on the NDT application and the form and shape of the object being evaluated. The most commonly used standard calibration blocks are those of the; International Institute of Welding (IIW), American Welding Society (AWS) and American Society of Testing and Materials (ASTM). Only two of the most commonly used standard calibration blocks are introduced here.

IIW Type US-1 Calibration Block

This block is a general purpose calibration block that can be used for calibrating angle-beam transducers as well as normal beam transducers. The material from which IIW blocks are prepared is specified as killed, open hearth or electric furnace, low-carbon steel in the normalized condition and with a grain size of McQuaid-Ehn No. 8. Official IIW blocks are dimensioned in the metric system of units.

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The block has several features that facilitate checking and calibrating many of the parameters and functions of the transducer as well as the instrument where that includes; angle-beam exit (index) point, beam angle, beam spared, time base, linearity, resolution, dead zone, sensitivity and range setting. The figure below shows some of the uses of the block.

ASTM - Miniature Angle-Beam Calibration Block (V2)

The miniature angle-beam block is used in a somewhat similar manner as the as the IIW block, but is smaller and lighter. The miniature angle-beam block is primarily used in the field for checking the characteristics of angle-beam transducers.

With the miniature block, beam angle and exit point can be checked for an angle-beam transducer. Both the 25 and 50 mm radius surfaces provide ways for checking the location of the exit point of the transducer and for calibrating the time base of the

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instrument in terms of metal distance. The small hole provides a reflector for checking beam angle and for setting the instrument gain.

Distance Amplitude Correction (DAC)

Acoustic signals from the same reflecting surface will have different amplitudes at different distances from the transducer. A distance amplitude correction (DAC) curve provides a means of establishing a graphic “reference level sensitivity” as a function of the distance to the reflector (i.e., time on the A-scan display). The use of DAC allows signals reflected from similar discontinuities to be evaluated where signal attenuation as a function of depth has been correlated. DAC will allow for loss in amplitude over material depth (time) to be represented graphically on the A-scan display. Because near field length and beam spread vary according to transducer size and frequency, and materials vary in attenuation and velocity, a DAC curve must be established for each different situation. DAC may be employed in both longitudinal and shear modes of operation as well as either contact or immersion inspection techniques.

A DAC curve is constructed from the peak amplitude responses from reflectors of equal area at different distances in the same material. Reference standards which incorporate side drilled holes (SDH), flat bottom holes (FBH), or notches whereby the reflectors are located at varying depths are commonly used. A-scan echoes are displayed at their non-electronically compensated height and the peak amplitude of each signal is marked to construct the DAC curve as shown in the figure. It is important to recognize that regardless of the type of reflector used, the size and shape of the reflector must be constant.

The same method is used for constructing DAC curves for angle beam transducers, however in that case both the first and second leg reflections can be used for constructing the DAC curve.

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Fundamentals of Resonant Acoustic Method NDT

Gail R Stultz [email protected]

Richard W Bono [email protected]

Mark I Schiefer [email protected]

The Modal Shop, Inc. 3149 East Kemper Rd. Cincinnati, OH 45241

Abstract Rapid conversion of machined parts to powdered metal and cast is driving industries, especially automotive. Due to the high expectations of both primary manufacturers and end consumers, defects cannot be tolerated even in million piece quantities. There is, in effect, a growing requirement for zero defect supply chain commitments. To achieve zero defect output, manufacturers are making the commitment to move to online NDT. This type of online inspection requires accuracy, reliability, and high throughput. Resonant Acoustic Method NDT (RAM NDT) provides a proven technique exhibiting these pivotal performance requirements and automates economically. RAM NDT tests, reports and screens for most common part flaws in a manner similar to the way NASA tests flight hardware and automotive manufacturers validate their new car designs. Utilizing structural dynamics and statistical variation, RAM NDT provides mature, laboratory proven technology in a robust, economical, process-friendly manner. 1. Motivational Example As with most powdered metal component suppliers, Company ABC is already doing spot magnetic particle testing on batches of parts from a given production run. The problem starts when a customer, say an automotive manufacturer, experiences field failures. The result is that Company ABC is put on parts-hold and has to pay for both containment and 100% field inspection on the customer site. At the risk of permanently damaging the company’s reputation and losing both existing and new business, significantly larger part batches are subjected to magnetic particle inspection, with 100% of the production lots inspected via a 300% visual part sort – where each part is visually inspected by three separate technicians. Everyone who can be pulled from another job is pulled in to help out during this time of crisis. To ensure necessary quality, 100% end-of-line part inspection must be implemented; traditional NDT techniques such Libra

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as magnetic particle, liquid penetrant, eddy current and X-ray, or purely visual inspection, are painstaking, subjective manual processes. As a result, rarely does the 100% inspection continue, and the cycle of “flawed-parts roulette” continues. Providing relief and security for the high volume manufacturer, RAM NDT offers reliable inspection, with quantitative, objective results. This technique is easily automated to eliminate human error with fast throughput for cost effective 100% inspection, simple and straightforward with minimal disruption to production. RAM NDT is a volumetric, resonant inspection technique that measures the structural integrity of each part to detect defects on a component level. With a large number of successes on the production lines of powdered metal, cast and forged parts, RAM NDT is the simple, effective solution to this common problem. 2. History The history of NDT techniques used for quality control testing in part manufacturing dates back to the beginning of the industrial manufacturing era. Initially, the basic visual inspection of the operators themselves served as the primary means of monitoring part acceptability. More sophisticated NDT techniques evolved, and magnetic particle inspection eventually became the de facto standard for testing ferrous metallic components such as castings, forgings and, more recently, powdered metals. This subjective and visual technology has remained essentially unchanged for the past 50+ years, yet continues to be the most common inspection tool for such parts. Traditional NDT techniques focus on detecting and diagnosing defects. They use visual techniques or imaging to scan for any indication of defects. For the case presented in our motivational example, identifying the type of defect itself is secondary to identifying the defective parts. While diagnosing specific defects is applicable when evaluating and inspecting some systems, such as gas pipelines or similar, it is not appropriate for high volume 100% manufactured part inspection. For these components it is of primary importance to detect if a part is non-conforming rather than why. Therefore, an end-of-line “go/no go” objective inspection, such as by RAM NDT, is preferred here to a subjective diagnosis. Scanning methods include magnetic particle testing (MT), ultrasonic testing (UT), eddy current/electromagnetic testing (ET), dye penetrant testing (PT), X-ray/radiographic testing (RT) and visual testing (VT). The fundamental difference between these traditional NDT techniques and resonant inspection (RI) is this scanning methodology. Scanning methods are manual and require subjective interpretation by an operator. As a result, the operator requires a certain level of technical training and/or certification to properly diagnose such indications of defect and infer the effects on the functionality of a part. Additionally, whenever such a technique requires the judgment of an operator, overall reliability suffers. In Juran’s Quality Handbook, Juran states that operators average only 80% reliability – this statistic is a reflection of the human interpretation factor, not the accuracy of the techniques themselves, see ref 1. None of these scanning techniques allow for efficient, cost effective or reliable quality control testing of 100% of manufactured parts of any appreciable volume. It should be noted that in some cases eddy current techniques can be implemented as a “whole part” test by using an encircling coil, easily automated with high throughput. However, in these cases the effectiveness of ET’s flaw detection is reduced, limited to detecting on certain types or configurations of surface flaws. Libra

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Resonant inspection, conversely, measures the structural response of a part and evaluates it against the statistical variation from a control set of good parts to screen defects. Its volumetric approach tests the whole part, both for external and internal structural flaws or deviations, providing objective and quantitative results. This structural response is a unique and measurable signature, defined by a component’s mechanical resonances. These resonances are a function of part geometry and material properties and are the basis for RI techniques. By measuring the resonances of a part, one determines the structural characteristics of that part in a single test. Typical flaws and defects adversely affecting the structural characteristics of a part are given in Table 1 for powdered metal, cast and forged applications. Many of the traditional NDT techniques previously discussed can detect these flaws as well, but often only RI can detect all in a single test, throughout the entire part (including deep sub-surface defects), in an automated and objective fashion. Table 1. Typical structural defects detectable by resonant inspection.

Cast Forged Powdered Metal Cracks Cracks Cracks

Cold shuts Missed or double strikes Chips Porosity Porosity Voids

Hardness/density Hardness Hardness/density Inclusions Inclusions Inclusions Heat treat Heat treat Heat treat

Compressive & residual stress Quenching problems Decarb Nodularity Laps Oxides

Gross dimensions Gross dimensions Gross dimensions Raw material contaminants Raw material

contaminants Raw material contaminants

Missed processes/operations Missed processes/operations

Missed processes/operations

After defective parts have been sorted with RI, complimentary traditional NDT techniques may provide a means for subjective diagnosis on the smaller subset of parts. This is useful for determining a defect’s root cause and ultimately improving the production processes. Table 2 provides a generic NDT selection table stating the capabilities of the various methods. The ASME has published standards that detail each of the traditional NDT methodologies mentioned here, see ref 2-8. Table 2. General overview of common NDT techniques. ET MT/PT UT RT RAM Defect Type Cracks/chips/porosity/voids Yes Yes Yes Yes/No Yes Missed processes/operations

Yes/No No Yes/No Yes/No Yes

Material property Yes/No No No No Yes Structurally significant Yes Yes Yes Yes Yes Production lot variations Yes/No Yes Yes Yes Yes/No Defect Location Libra

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Surface (external) Yes Yes Yes No Yes Internal No No Yes Yes Yes Brazing/bonding/welding No No Yes/No Yes/No Yes Speed/Training/Cost Part throughput Medium Low High Low High Training requirements High High Medium High Low Overall inspection costs Medium Medium High High Low Automation Capacity Quantitative results Yes/No No Yes/No No Yes Automation requirements Medium N/A Complex Complex Easy Automation cost Medium N/A High High Low/Medium 3. Theoretical Background Modal analysis is defined as the study of the dynamic characteristics of a mechanical structure or system. All structures, even structures such as metal gears or similar parts that are apparently rigid to the human eye, undergo deformation. These deformations can be described using modal analysis. Specifically, all structures have mechanical resonances, where the structure itself amplifies any energy imparted to it at certain frequencies. For example, tuning forks or bells will vibrate at very specific frequencies, their natural frequencies, for long periods of time with just a small tap. The sound that is made is directly due to these natural frequencies. In fact, any noise generated by a structure is done so by vibration, which is simply a pattern of summed sinusoidal deformations. RAM NDT utilizes this structural dynamic behavior to evaluate the integrity and consistency of parts. For illustrative purposes, consider the single degree-of-freedom (SDOF) mass, spring, damper system in Figure 1. It has one DOF because its state can be be determined b one quantity (x), the displacement of the mass. The elements of this simplified model are the mass (m), stiffness (k) and damping (c). The energy imparted into the system by the excitation force (f) is stored in the system as kinetic energy of the mass and potential energy of the spring and is dissipated by the damping. The mathematical representation of the SDOF system, which is called its equations of motion, is given in Equation (1) below. mx(t) + cx.(t) + kx(t) = f(t) (1) The solution to the equation of motion produces an eigenvalue problem which yields the undamped natural frequency as

n = km (2)

Equation (2) reveals the natural frequencies, or resonances, of a system that are determined by its mass (i.e., volume and density) and stiffness (i.e., Young's modulus and cross-sectional geometry). While in Equation (2) holds only for an SDOF system, the underlying relationship of mass and stiffness can be generalized for more complex systems. That is, an increase in stiffness will increase the natural frequency and an increase in mass will decrease the natural frequency. For example, consider the strings on a guitar. The larger diameter strings (more mass) produce lower tones than the smaller strings (less mass). Also, a string has a higher pitch when tightened Libra

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(increased stiffness) than when loosened (decreased stiffness). It is these fundamental properties of the resonances of a system that RAM NDT utilizes to evaluate the integrity and consistency of parts. Stiffness Damping k c Displacement Mass Excitation Force x m f Figure 1. Single Degree of Freedom (SDOF) discrete parameter model The natural frequencies are global properties of a given structure and the presence of structural defects causes shifts in these resonances. For example, a crack will change the stiffness in the region near the crack and a variation in density or the presence of porosity will change the mass. A crack defect typically reduces the stiffness in the material, thus decreasing the natural frequency. Similarly, porosity in a cast part reduces mass, thus increasing the natural frequency. These shifts are measurable if the defect is structurally significant with respect to the either the size or location of the flaw within a specific resonance mode shape. With some defects, a shift in resonant frequency can also be noticed audibly, such as a cracked bell that does not ring true. 4. Resonant Acoustic Method (RAM NDT) An introductory overview of the resonant inspection technique and theoretical background has previously been presented in Sections 2 and 3, respectively. This portion of the paper discusses in more detail the specific implementation of resonant inspection and the associated advantages of the Resonant Acoustic Method. RI is basically experimental modal analysis simplified for application to high volume production manufacturing and quality control testing. The generic, step-by-step procedure is as follows:

1. Excite the part with a known and repeatable force input. This force is typically generated by a controlled impact or actuator providing broadband or sinusoidal energy over the appropriate frequency range of analysis.

2. Measure the structural response of the part to the applied input force using a dynamic sensor such as a microphone or accelerometer (vibration pickup) and a high-speed analog to digital converter (ADC) with appropriate anti-aliasing filters.

3. Process the acquired time data with a Fast Fourier Transform (FFT) for analysis in the frequency domain.

4. Analyze the consistency of the frequency spectrum from part to part by comparing to a spectral template created from known good parts. Mechanical resonances are indicated as peaks in the frequency spectrum of the response. “Good” parts (structurally sound) have consistent spectral signatures (i.e. the mechanical resonances are the same among Libra

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parts) while “bad” parts are different. Generally these templates are setup to evaluate the consistency of the frequency and amplitude of ten or fewer peaks. Any deviation in (a range of) peak frequency or amplitude constitutes a structurally significant difference that provides a quantitative and objective part rejection.

The Resonant Acoustic Method technique performs resonant inspection by impacting a part and “listening” to its acoustic spectral signature with a microphone. The controlled impact provides broadband input energy to excite the part and the microphone allows for a non-contact measurement of the structural response. The part’s mechanical resonances amplify the broadband input energy at its specific natural frequencies, measured by the microphone above the background noise in the test environment. An example of such a spectrum from 0 to 40 kHz is given in Figure 2.

Figure 2. Typical acoustic signature for powdered metal part. Gross defects can often be distinguished directly by the human ear, but human hearing is subjective and limited to approximately 20 kHz maximum. By analyzing data beyond 20 kHz, to upwards of 50 kHz, much smaller defects can be detected, even across production lots given reasonable process control. Typically, these defects cause frequency shifts as shown in Figure 3. These shifts are a function of how the specific defect affects the mechanical resonance, which is dependent upon the specific defect location with respect to the deformation pattern of the resonance. Fortunately, mechanical resonances are global properties of a structure, and generally a defect will alter at least one resonant frequency. For this reason, it is good practice to set up multiple criteria ranges for analysis.

Figure 3. Data showing frequency shift due to structural defect in part. Libra

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An additional signal processing tool for improving analysis and sorting of good parts versus defective parts is implemented with a time delay function. Often times a defect may not cause a substantial shift in resonant frequency, but instead reduces the structure’s capability to “hold its tone” over time. By delaying the structural response measurement (many times just milliseconds) the resonant peak is not measurable from defective parts because the energy decays too rapidly. The peak in the frequency spectrum disappears, shown in Figure 4. A practical example of this is a cracked bell – when struck, it does not ring for an extended period of time as a “good” bell would.

Figure 4. Data showing resonance of good part against defective part, processed using time delay technique causing the peak to “disappear”. RAM NDT’s basic measurement procedure allows for easy automation and very high part testing throughput. There is no part preparation required – no magnetizing, cleaning, immersion, etc. Expendable costs associated with such preparation, such as chemicals and waste removal, are eliminated. The single impact and non-contact response measurement (via microphone) can be made as a part is moving down a conveyor, often as fast as a part per second. The parts do not need to be physically stopped; nor are they required to be precisely located with expensive robotics on contact actuators and vibration pickups. Simple guides are typically adequate to rotate/position the part for impact and allow flexibility to test many different types of parts or geometries. Given this capacity for automation and throughput, and its quantitative analysis with objective results, RAM NDT is ideal for plant floor, high volume quality control test applications. The core system components are shown in Figures 5 and 6. From the rugged microphone and industrial electric impactor to the NEMA 4 smart digital controller, the packaging is ideally suited for dirty, plant environments such as ductile iron foundries. A typical conveyor system for fully automated testing is shown in Figure 7.

Figure 5. Industrial electric impact hammer designed for millions of impacts and rugged microphone for non-contact response measurement, shown mounted on conveyor section. Libra

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Figure 6. Smart digital controller measures signals and processes data, independent of PC, with internal digital relay outputs.

Figure 7. Fully automated system on 4 ft conveyor section, shown with acoustic chamber for testing parts. Successful implementation of RAM NDT depends upon proper setup of the accept/reject criteria ranges in the part template. Each type and/or geometry of part requires a separate template. Parts need to be tested in the same manufacturing state. Typically, templates can be setup quickly with just a few dozen parts in less than a ½ hour. This sample set should include both good parts (ideally with at least several from different production batches) and parts with the expected variety of flaws. It is recommended to validate the template and resulting part sort with a larger statistical data set of a few hundred pieces. Often other NDT techniques, for example magnetic particle inspection, are complimentary in this regard, or destructive evaluations are commonly used for correlation as well. Once the specific part’s template is verified for accuracy, large volumes of parts can be 100% tested quickly and reliably. System validation can be performed using a controlled set of known parts. Parts of a given type, both good and defective, are kept as “standards” and run through the automated system for validation on a regular basis. Across batches over time, signatures often show trends where mechanical resonances shift due to acceptable variations in material properties (density, etc.) or process variations (heat treatment, etc.) By investing time upfront with this type of system validation procedure, process engineers and technicians have a better understanding of their parts and manufacturing processes and ensure the reliability of their inspection system. 5. Case Study: Powdered Metal Sprocket The manufacturer of the powdered metal sprocket shown in Figure 8 below needed to automate inspection, primarily for cracks and flawed teeth. The initial part template was set up using 70 Libra

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samples. 30 of these were visually inspected as good parts, while the remaining 40 were determined to have a variety of flaws such as broken, chipped and cracked teeth as shown in Figure 9. Typical data from several parts is shown in Figure 10, where frequency shifts down from cracks (two blue traces left of acceptance box) and up from broken teeth (red, pink and olive traces right of box) are clearly displayed against the two good samples (black/gray traces peak within box). These physical flaws correlate nicely with the theory presented in Section 2. A crack is simply a weaker spring (lower stiffness, k, in Eq. 1) and a broken tooth reduces mass (lower mass, m, in Eq. 1) which affects the resonant frequency accordingly.

Figure 8. Powdered metal sprocket.

Figure 9. View of powdered metal sprocket with chipped, broken and cracked teeth, as indicated, clockwise from top.

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Figure 10. Data at 6700 Hz resonance from 7 samples. Results of this evaluation showed that the flawed samples could be reliably sorted from the good parts. Of note, one of the “good” samples had significant shifts in resonances, indicating the presence of structural defects. (This is common, often related to the inability of visual scanning techniques such as magnetic particle testing to detect internal flaws.) Given the volumetric, whole-part testing by RAM, this part was successfully sorted and contained where subjective, visual inspection failed. The resulting template configured for this sprocket was implemented successfully in production, with millions of parts reliably tested per year. Prior to RAM NDT, the facility was scrapping 6-8% of production parts and still had field failures returned by their customers, all while trying to keep up with 100% inspection via magnetic particle testing. RAM NDT reduced this scrap rate to under 2% by eliminating false rejects (for example, a part that has a flaw indication on its surface yet is structurally sound.) Additionally, and more importantly, RAM NDT has prevented any defective parts from shipping to customers. 6. Conclusion The RAM NDT technique serves quality minded manufacturers who are dissatisfied with visual detection techniques such as magnetic particle, liquid penetrant, or X-ray, which are time consuming, costly and subjective. RAM NDT allows for simple integration of a turnkey system that is a reliable, fully automated method for quality control and process improvement. This rapidly growing technique creates an economical, on-line inspection system that provides for zero defect product supply. Unlike previous implementation of resonant inspection which are excessively complicated and costly to automate, RAM NDT is fast, simple and reliable, and easily re-configurable. As a result, powdered metal and casting manufacturers around the world have proven the benefits of RAM NDT resonant inspection over their traditional inspection techniques. 7. References [1] Juran, Joseph M. and Godfrey, A. Blanton, Fifth Edition, Juran’s Quality Handbook, McGraw-Hill. [2] ASTM E1444-01 Standard Practice for Magnetic Particle Examination. Libra

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[3] ASTM E309-95 Standard Practice for Eddy-Current Examination. [4] ASTM B594-02 Standard Practice for Ultrasonic Inspection Examination. [5] ASTM F1467-99 Standard Guide for Use of an X-Ray Tester. [6] ASTM E2001-98 Standard Guide for Resonant Ultrasound Spectroscopy for Defect Detection. [7] ASTM D4086-92a (1997) e1 Standard Practice for Visual Evaluation. [8] ASTM E165-02 Standard Test Method for Liquid Penetrant Examination.

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