m 468 Content

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PDHonline Course M468 (6 PDH)

Common Nondestructive Testing NDT Fundamentals Part 1

Instructor: Jurandir Primo, PE

2013

PDH Online | PDH Center

5272 Meadow Estates Drive Fairfax, VA 22030-6658

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www.PDHcenter.com PDHonline Course M468 www.PDHonline.org

2013 Jurandir Primo Page 2 of 67

CONTENTS:

Introduction

1. Visual Inspection (VT)

2. Liquid Penetrant Inspection

Detectability of Flaws Steps of a Liquid Penetrant Inspection Common Uses of Liquid Penetrant Inspection Advantages and Disavantages of Penetrant

Testing Penetrant Testing Conditions Penetrant Testing Materials Surface Wetting Capability Color and Fluorescent Brightness Ultraviolet and Thermal Stability Removability Developers Developers Forms Selection of a Penetrant Technique Penetrant Application Penetrant Dwell Time Inspection of Liquid Pentrant Penetrant Removal Processes Use and Selection f a Developer Control of Temperature Convenience of the Penetrant Application Quality Control of Drying Process Dry Powder Developer Development Time Nature of the Defect Practical Examples

3. Magnetic Particle Inspection

History of Magnetic Particle Inspection Magnetism Diamagnetic, Paramagnetic and Ferromagnetic

Materials Magnetic Properties Magnetic Field Around a Bar Magnet Magnetic Fields Around a Horse Shoe and Ring

Magnets Electromagnetic Fields Magnetic Field of a Coil Flaw Detectability Ferromagnetic Materials Magnetizing Current Longitudinal Magnetic Fields Circular Magnetic Fields Demagnetization Measuring Magnetic Fields Field Indicators Hall-Efect (Gaus/Tesla) Meter Portable MT Equipment Permanent Magnets Electromagnets Prods Portable Coils and Conductive Cables Stationary MT Equipment Portable Power Supplies Lights for MT Inspection Magnetic Field Indicators Dry and Wet Magnetic Particles Suspension Liquids Dry Particle Inspection Wet Suspension Inspection Inspection with Magnetic Rubber Magnetization Techniques Quantitative Quality Indicators (QQI) Shims Gauss Meter Inspection Particle Concentration Water Break Test Practical Examples

References



Introduction: Nondestructive examination (NDE) methods of inspection make it possible to verify compliance to the standards by examining the surface and subsurface of welding and engineered materials for construction purposes. Six basic inspection methods are commonly used: visual, liquid penetrant, magnetic parti-cle, (discussed in Part 1), ultrasonic, eddy current and radiographic, (discussed in Part 2). Visual Inspection (VT): Visual inspection is often the most cost-effective method, but it must take place prior to, during and after welding. Many standards require its use before other methods, because there is no point in submitting an obviously bad weld to sophisticated inspection techniques. The ANSI/AWS D1.1, Structural Welding Code-Steel, states, "Welds subject to nondestructive examination shall have been found acceptable by visual inspection." Visual inspection requires little equipment. Aside from good eye sight and sufficient light, all it takes is a pocket rule, a weld size gauge, a magnifying glass, and possibly a straight edge and square for checking straightness, alignment and perpendicularity.

"Visual inspection is the best buy in NDE, but it must take place prior to, during and after welding."

Before the first welding arc is struck, materials should be examined to see if they meet specifications for quality, type, size, cleanliness and freedom from defects. Grease, paint, oil, oxide film or heavy scale should be removed. The pieces to be joined should be checked for flatness, straightness and dimensional accuracy. Likewise, alignment, fit-up and joint preparation should be examined. Finally, process and pro-cedure variables should be verified, including electrode size and type, equipment settings and provisions for preheat or postheat. All of these precautions apply regardless of the inspection method being used. During fabrication, visual examination of a weld bead and the end crater may reveal problems such as cracks, inadequate penetration, and gas or slag inclusions. Among the weld detects that can be recog-nized visually are cracking, surface slag in inclusions, surface porosity and undercut. On simple welds, inspecting at the beginning of each operation and periodically as work progresses may be adequate. Where more than one layer of filler metal is being deposited, however, it may be desirable to inspect each layer before depositing the next. The root pass of a multipass weld is the most critical to weld soundness. It is especially susceptible to cracking, and because it solidifies quickly, it may trap gas and slag. On sub-sequent passes, conditions caused by the shape of the weld bead or changes in the joint configuration can cause further cracking, as well as undercut and slag trapping. Repair costs can be minimized if visual in-spection detects these flaws before welding progresses. VT can only locate defects in the weld surface. Visual inspection at an early stage of production can also prevent underwelding and overwelding. Welds that are smaller than called for in the specifications cannot be tolerated. Beads that are too large increase costs unnecessarily and can cause distortion through added shrinkage stress. After welding, visual inspection can detect a variety of surface flaws, including cracks, porosity and unfilled craters, regardless of subsequent inspection procedures. Dimensional variances, warpage and appear-ance flaws, as well as weld size characteristics, can be evaluated. Before checking for surface flaws, welds must be cleaned of slag. Shot-blasting should not be done before examination, because the peening action may seal fine cracks and make them invisible. The AWS D1.1 Structural Welding Code, for example, does not allow peening "on the root or surface layer of the weld or the base metal at the edges of the weld."

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After removal and careful cleaning, the surface was then coated with a fine suspension of chalk in alcohol so that a white surface layer was formed once the alcohol had evaporated. The object was then vibrated by being struck with a hammer, causing the residual oil in any surface cracks to seep out and stain the white coating. Detectability of Flaws: The advantage that a liquid penetrant inspection (LPI) offers over an unaided visual inspection is that it makes defects easier to see for the inspector. There are basically two ways that a penetrant inspection process makes flaws more easily seen. First, LPI 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. Due to the physical features of the eye, there is a threshold below which objects cannot be resolved. This threshold of visual acuity is around 0.003 inch for a person with 20/20 vision. The second way that LPI improves the detectability of a flaw is that it produces a flaw indication with a high level of contrast between the indication and the background also helping to make the indication more easi-ly seen. When a visible dye penetrant inspection is performed, the penetrant materials are formulated us-ing a bright red dye, providing a high level of contrast between the white developer. In other words, the developer serves as a high contrast background as well as a blotter to pull the trapped penetrant from the flaw. When a fluorescent penetrant inspection is performed, the penetrant materials are formulated to glow brightly and to give off light at a wavelength that the eye is most sensitive to under dim lighting conditions. Steps of a Liquid Penetrant Inspection:

Surface Preparation: One of the most critical steps of a liquid penetrant inspection 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 op-erations can smear metal over the flaw opening and prevent the penetrant from entering.

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material is applied by spraying, brushing, or immersing the part in a penetrant bath.

Penetrant Dwell: The penetrant is left on the surface for a sufficient time to allow as much pene-trant 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. Minimum dwell times typically range from five to 60 minutes. The ideal dwell time is often determined by experimentation and may be very specific to a particular application.

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Water washable (Method A) 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. Post-emulsifiable penetrants come in two varieties, lipophilic and hydrophilic. In post-emulsifiers, lipo-philic systems (Method B), the penetrant is oil soluble and interacts with the oil-based emulsifier to make removal possible. Post-emulsifiable, hydrophilic systems (Method D), use an emulsifier that is a water soluble detergent which lifts the excess penetrant from the surface of the part with a water wash. Solvent removable penetrants require the use of a solvent to remove the penetrant from the part. 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 shown below:

Level - Ultra Low Sensitivity Level 1 - Low Sensitivity Level 2 - Medium Sensitivity Level 3 - High Sensitivity Level 4 - Ultra-High Sensitivity

The major US government and industry specifications currently rely on the US Air Force Materials Labora-tory at Wright-Patterson Air Force Base to classify penetrants into one of the five sensitivity levels. This procedure uses titanium and Inconel specimens with small surface cracks produced in low cycle fatigue bending to classify penetrant systems. The brightness of the indication produced is measured using a photometer. The sensitivity levels and the test procedure used can be found in Military Specification MIL-I-25135 and Aerospace Material Specification 2644, Penetrant Inspection Materials: An interesting note about the sensitivity levels is that only four levels were originally planned. However, when some penetrants were judged to have sensitivities significantly less than most others in the level 1 category, the level was created. An excellent historical summary of the development of test specimens for evaluating the performance of penetrant materials can be found in the following reference. Penetrant: The industry and military specifications that control ma-terials and their use, all stipulate certain physical properties of the penetrant materials that must be met. Some of these requirements address the safe use of the materials, such as toxicity, low flash point, and corrosiveness, and other requirements address storage and contamination issues. Other properties that are thought to be primarily responsible for the performance or sensitivity of the penetrants. The properties of pene-trant materials that are controlled by AMS 2644 and MIL-I-25135E include flash point, surface wetting capability, viscosity, color, brightness, ultraviolet stability, thermal stability, water tolerance, and removability.



Emulsifiers: When removal of the penetrant from a defect due to over-washing of the part is a concern, a post-emulsifiable penetrant system can be used. Post-emulsifiable penetrants require a separate emulsifi-er to break the penetrant down and make it water-washable. Most penetrant inspection specifications clas-sify penetrant systems into four methods of excess penetrant removal. These are listed below:

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

Method C relies on a solvent cleaner to remove the penetrant from the part being inspected. Method A has emulsifiers built into the penetrant liquid that makes it possible to remove the excess penetrant with a simple water wash. Method B and D penetrants require an additional processing step where a separate emulsification agent is applied to make the excess penetrant more removable with a water wash. Lipo-philic emulsification systems are oil-based materials that are supplied in ready-to-use form. Hydrophilic systems are water-based and supplied as a concentrate that must be diluted with water prior to use . The lipophilic emulsifiers (Method B) were introduced in the late 1950's and work with both a chemical and mechanical action. After the emulsifier has coated the surface of the object, mechanical action starts to remove some of the excess penetrant as the mixture drains from the part. During the emulsification time, the emulsifier diffuses into the remaining penetrant and the resulting mixture is easily removed with a water spray. The hydrophilic emulsifiers (Method D) also remove the excess penetrant with mechanical and chemi-cal action but the action is different because no diffusion takes place. Hydrophilic emulsifiers are basically detergents that contain solvents and surfactants. The hydrophilic emulsifier breaks up the penetrant into small quantities and prevents these pieces from recombining or reattaching to the surface of the part. The mechanical action of the rinse water removes the displaced penetrant from the part and causes fresh re-mover to contact and lift newly exposed penetrant from the surface. The hydrophilic post-emulsifiable method (Method D) was intro-duced in the mid 1970's. Since it is more sensitive than the lipo-philic post emulsifiable method it has made the later method virtu-ally obsolete. The major advantage of hydrophilic emulsifiers is that they are less sensitive to variation in the contact and removal time. While emulsification time should be controlled as closely as possi-ble, a variation of one minute or more in the contact time will have little effect on flaw detectability when a hydrophilic emulsifier is used. However, a variation of as little as 15 to 30 seconds can have a significant effect when a lipophilic system is used. Surface Wetting Capability: As previously mentioned, one of the important characteristics of a liquid penetrant material is its ability to freely wet the surface of the object being inspected. At the liquid-solid surface interface, if the molecules of



the liquid have a stronger attraction to the molecules of the solid surface than to each other (the adhesive forces are stronger than the cohesive forces), wetting of the surface occurs. Alternately, if the liquid mole-cules are more strongly attracted to each other than the molecules of the solid surface (the cohesive forc-es are stronger than the adhesive forces), the liquid beads-up and does not wet the surface of the part. One way to quantify a liquid's surface wetting characteristics is to measure the contact angle of a drop of liquid placed on the surface of an object. The contact angle is the angle formed by the solid/liquid interface and the liquid/vapor interface measured from the side of the liquid. (See the figure below.) Liquids wet sur-faces when the contact angle is less than 90 degrees. For a penetrant material to be effective, the contact angle should be as small as possible. In fact, the contact angle for most liquid penetrants is very close to zero degrees. Wetting ability of a liquid is a function of the surface energies of the solid-gas interface, the liquid-gas inter-face, and the solid-liquid interface. The surface energy across an interface or the surface tension at the interface is a measure of the energy required to form a unit area of new surface at the interface. The in-termolecular bonds or cohesive forces between the molecules of a liquid cause surface tension. When the liquid encounters another substance, there is usually an attrac-tion between the two materials. The adhesive forces between the liquid and the second substance will compete against the cohesive forces of the liquid. Liquids with weak cohesive bonds and a strong attraction to another material (or the desire to create adhesive bonds) will tend to spread over the material. Liquids with strong cohesive bonds and weaker adhesive forces will tend to bead-up or form a droplet when in contact with another material. In liquid penetrant testing, there are usually three surface interfaces in-volved, the solid-gas interface, the liquid-gas interface, and the solid-liquid interface. For a liquid to spread over the surface of a part, two conditions must be met. First, the surface energy of the solid-gas interface must be greater than the combined surface energies of the liquid-gas and the solid-liquid interfaces. Second, the surface energy of the solid-gas interface must exceed the surface energy of the solid-liquid interface. A penetrant's wetting characteristics are also largely responsible for its ability to fill a void. Penetrant mate-rials are often pulled into surface breaking defects by capillary action. The capillary force driving the pene-trant into the crack is a function of the surface tension of the liquid-gas interface, the contact angle, and the size of the defect opening. Color and Fluorescent Brightness: Penetrant Color and Fluorescence: The color of the penetrant material is of obvious importance in a visible dye penetrant inspection, as the dye must provide good contrast against the developer or part being inspected. Remember from the earlier discussion of contrast sensitivity that generally the higher the con-trast, the easier objects are to see. The dye used in visible dye penetrant is usually vibrant red but other colors can be purchased for special applications.

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At an elevated temperature, penetrants can experience heat degradation or "heat fade." Excessive heat: 1. Evaporates the more volatile constituents which increases viscosity and adversely affects the rate of penetration. 2. Alters wash characteristics. 3."Boils off" chemicals that prevent separation and gelling of water soluble penetrants. 4. Kills the fluorescence of tracer dyes. This fourth degradation mechanism involves the molecules of the penetrant materials. The phenomenon of fluorescence involves electrons that are delocalized in a molecule. These electrons are not specifically associated with a given bond between two atoms. When a molecule takes up sufficient energy for the exci-tation source, the delocalized bonding electrons rise to a higher electronic state. After excitation, the elec-trons will normally lose energy and return to the lowest energy state. This loss of energy can involve a "radiative" process such as fluorescence or "non-radiative" processes. Non-radiative processes include relaxation by molecular collisions, thermal relaxation, and chemical reac-tion. Heat causes the number of molecular collisions to increase, which results in more collision relaxation and less fluorescence. This explanation is only valid when the part and the penetrant are at an elevated temperature. When the materials cool, the fluorescence will return. However, while exposed to elevated temperatures, penetrant solutions dry faster. As the molecules become more closely packed in the dehydrated solution, collision relaxation increases and fluorescence decreases. This effect has been called "concentration quenching" and experimental data shows that as the dye con-centration is increased, fluorescent brightness initially increases but reaches a peak and then begins to decrease. Airflow over the surface on the part will also speed evaporation of the liquid carrier, so it should be kept to a minimum to prevent a loss of brightness. Generally, thermal damage occurs when fluorescent penetrant materials are heated above 71oC (160oF). It should be noted that the sensitivity of an FPI inspection can be improved if a part is heated prior to apply-ing the penetrant material, but the temperature should be kept below 71oC (160oF). Some high tempera-ture penetrants in use today are formulated with dyes with high melting points, greatly reducing heat relat-ed problems. The penetrants also have high boiling points and the heat related problems are greatly reduced. However, a loss of brightness can still take place when the penetrant is exposed to elevated temperatures over an extended period of time. When one heat resistant formulation was tested, a 20 % reduction was measured after the material was subjected to 163oC (325oF) for 273 hours. The various types of fluorescent dyes commonly employed in today's penetrant materials begin decomposition at 71oC (160oF). When the tem-perature approaches 94oC (200oF), there is almost total attenuation of fluorescent brightness of the com-position and sublimation of the fluorescent dyestuffs. Removability: Removing the penetrant from the surface of the sample, without removing it from the flaw, is one of the most critical operations of the penetrant inspection process. The penetrant must be removed from the sample surface as completely as possible to limit background fluorescence. In order for this to happen, the adhesive forces of the penetrant must be weak enough that they can be broken by the removal methods used.



However, in order for the penetrant to have good surface wetting characteristics, the adhesive forces (forces of attraction between the penetrant and the solid surface being inspected) must be stronger than the cohesive forces (forces holding the liquid together). Proper formulation of the penetrant materials pro-vides the correct balancing of these forces. Another consideration in the formulation of the penetrant liquid is that it should not easily commingle and become diluted by the cleaning solution. Dilution of the penetrant liquid will affect the concentration of the dye and reduce the dimensional threshold of fluorescence. 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. The fine developer particles both reflect and refract the incident ultraviolet light, allowing more of it to interact with the penetrant, causing more efficient fluo-rescence. The developer also allows more light to be emitted through the same mechanism. This is why indications are brighter than the penetrant itself under UV light. Another function that some developers perform is to create a white background so there is a greater degree of contrast between the indication and the surrounding background. Developer Forms: The AMS 2644 and Mil-I-25135 classify developers into six standard forms. These forms are listed below: 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 The developer classifications are based on the method that the developer is applied. The developer can be applied as a dry powder, or dissolved or suspended in a liquid carrier. Each of the developer forms has advantages and disadvantages.

a) Form a - Dry Powder: Dry powder developer is generally considered to be the least sensitive but it is 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. The developer can be applied by dipping parts in a container of developer, or by using a puffer to dust parts with the de-veloper. Parts can also be placed in a dust cabinet where the developer is blown around and allowed to settle on the part. Electrostatic powder spray guns are also available to apply the developer. The goal is to allow the developer to come in contact with the whole inspection area.



Unless the part is electrostatically charged, the powder will only adhere to areas where trapped penetrant has wet the surface of the part. The penetrant will try to wet the surface of the penetrant particle and fill the voids between the particles, which brings more penetrant to the surface of the part where it can be seen. Since dry powder developers only stick to the area where penetrant is present, the dry developer does not provide a uniform white background as the other forms of developers do. Having a uniform light background is very important for a visible inspection to be effective and since dry developers do not provide one, they are seldom used for visible inspections. When a dry developer is used, indications tend to stay bright and sharp since the penetrant has a limited amount of room to spread.

b) Form b - Water Soluble: As the name implies, water soluble developers consist of a group of chemicals that are dissolved in water and form a developer lay-er when the water is evaporated away. The best method for apply-ing 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. Aqueous developers contain wetting agents that cause the solution to func-tion much like dilute hydrophilic emulsifier and can lead to additional removal of entrapped penetrant. Drying is achieved by placing the wet but well drained part in a recirculating, warm air dryer with the tem-perature held between 70 and 75F. If the parts are not dried quickly, the indications will will be blurred and indistinct. Properly developed parts will have an even, pale white coating over the entire surface.

c) Form c - 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 sus-pension. Water suspendable developers are applied to parts in the same manner as water soluble devel-opers. Parts coated with a water suspendable developer must be forced dried just as parts coated with a water soluble developer are forced dried. The surface of a part coated with a water suspendable developer will have a slightly translucent white coating.

d) Form d/e - Nonaqueous: Nonaqueous developers suspend the developer in a volatile sol-vent and are typically applied with a spray gun. Nonaqueous de-velopers are commonly distributed in aerosol spray cans for port-ability. The solvent tends to pull penetrant from the indications by solvent action. Since the solvent is highly volatile, forced drying is not required. A nonaqueous developer should be applied to a thoroughly dried part to form a slightly translucent white coating.



e) Form f - Special Applications: Plastic or lacquer developers are special developers that are primarily used when a permanent record of the inspection is required. Selection of a Penetrant Technique: The selection of a liquid penetrant system is not a straightforward task. There are a variety of penetrant systems and developer types that are available for use, and one set of penetrant materials will not work for all applications. Many factors must be considered when selecting the penetrant materials for a particular application. These factors include the sensitivity required, materials cost, number of parts, size of area requiring inspection, and portability. When sensitivity is the primary consideration for choosing a penetrant system, the first decision that must be made is whether to use fluorescent penetrant or visible dye penetrant. Fluorescent penetrants are gen-erally more capable of producing a detectable indication from a small defect. Also, the human eye is more sensitive to a light indication on a dark background and the eye is naturally drawn to a fluorescent indica-tion. The graph below presents a series of curves that show the contrast ratio required for a spot of a certain diameter to be seen. The ordinate is the spot diameter, which was viewed from one foot. The abscissa is the contrast ratio between the spot brightness and the background brightness. To the left of the contrast ratio of one, the spot is darker than the background (representative of visible dye penetrant testing); and to the right of one, the spot is brighter than the background (representative of fluo-rescent penetrant inspection). Each of the three curves right or left of the contrast ratio of one are for different background brightness (in foot-Lamberts), but simply consider the general trend of each group of curves right or left of the contrast ratio of one. The curves show that for indication larger than 0.076 mm (0.003 inch) in diameter, it does not really matter if it is a dark spot on a light background or a light spot on a dark background. However, when a dark indication on a light background is further reduced in size, it is no longer detectable even though contrast is increased. Furthermore, with a light indication on a dark background, indications down to 0.003 mm (0.0001 inch) were detectable when the contrast between the flaw and the background was high. From this data, it can be seen why a fluorescent penetrant offers an advantage over a visible penetrant for finding very small defects. Data presented by De Graaf and De Rijk supports this statement. They inspect-ed "identical" fatigue cracked specimens using a red dye penetrant and a fluorescent dye penetrant. The fluorescent penetrant found 60 defects while the visible dye was only able to find 39 of the defects. Under certain conditions, the visible penetrant may be a better choice. When fairly large defects are the subject of the inspection, a high sensitivity system may not be warranted and may result in a large number



of irrelevant indications. Visible dye penetrants have also been found to give better results when surface roughness is high or when flaws are located in areas such as weldments. Since visible dye penetrants do not require a darkened area for the use of an ultraviolet light, visible sys-tems are more easy to use in the field. Solvent removable penetrants, when properly applied, can have the highest sensitivity and are very convenient to use. However, they are usually not practical for large area inspection or in high-volume production settings. Another consideration in the selection of a penetrant system is whether water washable, post-emulsifiable or solvent removable penetrants will be used. Post-emulsifiable systems are designed to reduce the pos-sibility of over-washing, which is one of the factors known to reduce sensitivity. However, these systems add another step, and thus cost, to the inspection process. Penetrants are evaluated by the US Air Force according to the requirements in MIL-I-25135 and each penetrant system is classified into one of five sensitivity levels. This procedure uses titanium and Inconel specimens with small surface cracks produced in low cycle fatigue bending to classify penetrant systems. The brightness of the indications produced after processing a set of specimens with a particular penetrant system is measured using a photometer. A procedure for producing and evaluating the penetrant qualifica-tion specimens was reported on by Moore and Larson at the 1997 ASNT Fall Conference. Most commercially available penetrant materials are listed in the Qualified Products List of MIL-I-25135 according to their type, method and sensitivity level. Visible dye and dual-purpose penetrants are not clas-sified into sensitivity levels as fluorescent penetrants are. The sensitivity of a visible dye penetrant is re-garded as level 1 and largely dependent on obtaining good contrast between the indication and the back-ground. Penetrant Application: The penetrant material can be applied in a number of different ways, including spraying, brushing, or immersing the parts in a penetrant bath. The method of penetrant application has little ef-fect on the inspection sensitivity but an electrostatic spraying method is reported to produce slightly better results than other methods. Once the part is covered in penetrant it must be allowed to dwell so the penetrant has time to enter any defect present. There are basically two dwell mode options, immersion-dwell (keeping the part immersed in the penetrant during the dwell period) and drain-dwell (letting the part drain during the dwell period). Prior to a study by Sherwin, the immersion-dwell mode was generally considered to be more sensitive but recognized to be less economical because more penetrant was washed away and emulsifiers were con-taminated more rapidly. The reasoning for thinking this method was more sensitive was that the penetrant was more migratory and more likely to fill flaws when kept completely fluid and not allowed to lose volatile constituents by evapora-tion.



However, Sherwin showed that if the specimens are allowed to drain-dwell, the sensitivity is higher be-cause the evaporation increases the dyestuff concentration of the penetrant on the specimen. As pointed-out in the section on penetrant materials, sensitivity increases as the dyestuff concentration increases. Sherwin also cautions that the samples being inspected should be placed outside the penetrant tank wall so that vapors from the tank do not accumulate and dilute the dyestuff concentration of the penetrant on the specimen. 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 fol-lowed. The time required to fill a flaw depends on a number of variables which include the following:

The surface tension of the penetrant. The contact angle of the penetrant.

The dynamic shear viscosity of the penetrant, which can vary with the diameter of the capillary. The vis-cosity of a penetrant in microcapillary flaws is higher than its viscosity in bulk, which slows the infiltration of the tight flaws.

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. 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 appli-cation. For example, AMS 2647A requires that the dwell time for all aircraft and engine parts be at least 20 minutes, while ASTM E1209 only requires a five minute dwell time for parts made of titanium and other heat resistant alloys. Generally, there is no harm in using a longer penetrant dwell time as long as the penetrant is not allowed to dry. Inspection of Liquid Penetrant: Unlike magnetic particle inspection, which can reveal subsurface defects, liquid penetrant inspection reveals only those defects that are open to the surface. Four groups of liquid penetrants are pres-ently in use:

Group I is a dye penetrant that is nonwater washable. Group II is a water washable dye penetrant. Group III and Group IV are fluorescent penetrants.

Carefully follow the instructions given for each type of penetrant since there are some differences in the procedures and safety precautions required for the various penetrants. Before using a liquid penetrant to inspect a weld, remove all slag, rust, paint, and moisture from the surface. Except where a specific finish is required, it is not necessary to grind the weld surface as long as the weld surface meets applicable specifications. Ensure the weld contour blends into the base metal without un-



der-cutting. When a specific finish is required, perform the liquid penetrant inspection before the finish is made. This enables you to detect defects that extend beyondt he final dimensions, but you must make a final liquid penetrant inspection after the specified finish has beengiven. Before using a liquid penetrant, clean the surface oft he material very carefully, including the areas next tothe inspection area. You can clean the surface by swabbing it with a clean, free cloth saturated in a non-volatile solvent or by dipping the entire piece into a solvent. After the surface has been cleaned, remove all traces of the cleaning material. It is extremely importantto remove all dirt, grease, scale, lint, salts, or other materials that would interfere with the inspection. After the surface has dried, apply another substance, called a developer. Allow the developer (powder or liquid) to stay on the surface for a minimum of 7 minutes before starting the inspection. Leave it on no longer than 30 minutes, thus allowing a total of 23 minutes toevaluate the results. The indications during a liquid penetrant inspection must be carefully interpreted and evaluated. In almost every inspection, some insignificant indications are present. Most of these are the result of the failure to remove the excess penetrant from the surface. At least 10 percent of all indications must be re-moved from the surface to determine whether defects are actually present or whether the indications are the result of excess penetrant. Penetrant Removal Processes: The penetrant removal procedure must effectively remove the penetrant from the surface of the part without removing an ap-preciable amount of entrapped penetrant from the defect. If the removal process extracts penetrant from the flaw, the flaw indi-cation will be reduced by a proportional amount. If the pene-trant is not effectively removed from the part surface, the con-trast between the indication and the background will be re-duced. Removal Method: Penetrant systems are classified into four methods of excess penetrant removal. These include the follow-ing: Method A: Water-Washable Method B: Post-Emulsifiable, Lipophilic Method C: Solvent Removable Method D: Post-Emulsifiable, Hydrophilic Of the three production penetrant inspection methods, Method A, Water-Washable, is the most economi-cal to apply. Water-washable or self-emulsifiable penetrants contain an emulsifier as an integral part of the formulation. 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, there-fore, too labor intensive for most production situations. The excess penetrant may be removed from the object surface with a simple water rinse. These materials have the property of forming relatively viscous gels upon contact with water, which results in the formation of gel-like plugs in surface openings. While they are completely soluble in water, given enough contact time, the plugs offer a brief period of protection against rapid wash removal. Thus, water-washable pene-trant systems provide ease of use and a high level of sensitivity.



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. 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. Spray application is not recommended for lipo-philic emulsifiers because it can result in non-uniform emulsification if not properly applied. 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 al-lowed 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. The penetrant that has reacted with the emulsifier is easily cleaned away. Controlling the reaction time is of essential importance when using a post-emulsifiable system. If the emul-sification 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 en-trapped 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). Since these methods are generally only used when very high sensitivity is needed, the hydrophilic method renders the lipophilic method virtually obsolete. The major advantage of hydrophilic emulsifiers is that they are less sensitive to variation in the contact and removal time. While emulsification time should be controlled as closely as possible, a variation of one mi-nute or more in the contact time will have little effect on flaw detectability when a hydrophilic emulsifier is used. On the contrary, a variation of as little as 15 to 30 seconds can have a significant effect when a lipophilic system is used. Using an emulsifier involves adding a couple of steps to the penetrant process, slightly increases the cost of an inspection. When using an emulsifier, the penetrant process includes the following steps (extra steps in bold): 1. pre-clean part, 2. apply penetrant and allow to dwell, 3. pre-rinse to remove first layer of penetrant, 4. ap-ply 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: The method used to rinse the excess from the object surface and the time of the rinse should be controlled so as to prevent over-washing. It is generally 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 immer-sion time should be kept to a minimum through frequent inspections of the remaining background level. 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 pro-cedure 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 neces-



sary; but keep in mind that with every additional wipe, some of the entrapped penetrant will be removed and inspection sensitivity will be reduced. To study the effects of the wiping process, Japanese researchers manufactured a test specimen out of acrylic plates that allowed them to view the movement of the penetrant in a narrow cavity. The sample consisted of two pieces of acrylic with two thin sheets of vinyl clamped between as spaces. The plates were clamped in the corners and all but one of the edges sealed. The unsealed edge acted as the flaw. The clearance between the plates varied from 15 microns (0.059 inch) at the clamping points to 30 microns (0.118 inch) at the midpoint between the clamps. The distance between the clamping points was believed to be 30 mm (1.18 inch). Although the size of the flaw represented by this specimen is large, an interesting observation was made. They found that when the surface of the specimen was wiped with a dry cloth, penetrant was blotted and removed from the flaw at the corner areas where the clearance between the plate was the least. When the penetrant at the side areas was removed, penetrant moved horizontally from the center area to the ends of the simulated crack where capillary forces are stronger. Therefore, across the crack length, the penetrant surface has a parabola-like shape where the liquid is at the surface in the corners but depressed in the center. This shows that each time the cleaning cloth touch-es the edge of a crack, penetrant is lost from the defect. This also explains why the bleedout of an indica-tion is often largest at the corners of cracks. Use and Selection of a Developer: The use of developer is almost always recommended. One study reported that the output from a fluores-cent penetrant could be multiplied by up to seven times when a suitable powder developer was used. Another study showed that the use of developer can have a dramatic effect on the probability of detection (POD) of an inspection. When a Haynes Alloy 188, flat panel specimen with a low-cycle fatigue crack was inspected without a developer, a 90 % POD was never reached with crack lengths as long as 19 mm (0.75 inch). The operator detected only 86 of 284 cracks and had 70 false-calls. When a developer was used, a 90 % POD was reached at 2 mm (0.077 inch), with the inspector identify-ing 277 of 311 cracks with no false-calls. However, some authors have reported that in special situations, the use of a developer may actually reduce sensitivity. These situations primarily occur when large, well defined defects are being inspected on a surface that contains many nonrelevant indications that cause excessive bleedout. Developer Application Method: Nonaqueous developers are generally recognized as the most sensitive when properly applied. There is less agreement on the performance of dry and aqueous wet developers, but the aqueous developers are usually considered more sensitive. Aqueous wet developers form a finer matrix of particles that is more in contact with the part surface. However, if the thickness of the coating becomes too great, defects can be masked. Aqueous wet developers can cause leaching and blurring of indications when used with water-washable penetrants. The relative sensitivities of developers and application techniques as ranked in Volume II of the Nondestructive Testing Handbook are shown in the table below. There is general industry agreement with this table, but some industry experts feel that water suspendable developers are more sensitive than water-soluble developers. Sensitivity ranking of developers per the Nondestructive Testing Handbook.



Ranking 1 2 3 4 5 6 7 8 9 10

Developer Form Nonaqueous, Wet SolventPlastic 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)

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 back-ground so cannot be used with visible systems Difficult to assure entire part sur-face has been coated

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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 pro-vides poor contrast (not recom-mended for visual systems). Indications for water washable systems are dim and blurred.

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Ease of coating entire part Indications are bright and sharp White coating for good contrast can be produced which work well for both visible and fluorescent systems

Indications weaken and become diffused after time.

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Very portable Easy to apply to readily accessible surfaces White coating for good contrast can be produced which work well for both visible and fluorescent systems Indications show-up rapidly and are well defined Provides highest sensitivity

Difficult to apply evenly to all sur-faces. More difficult to clean part after inspection.

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Page 29

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