OM NDT TRAINING & CONSULTANCY

PENETRANT TESTING L-II

Prepared by

MAHESH PANDIT

ASNT NDT L-III

Basic principle of a Liquid Penetrant

• DPI is based upon capillary action, where low surface tension fluid penetrates into clean and dry surface-breaking discontinuities. Penetrant may be applied to the test component by dipping, spraying, or brushing. After adequate penetration time has been allowed, the excess penetrant is removed and a developer is applied. The developer helps to draw penetrant out of the flaw so that an invisible indication becomes visible to the inspector. Inspection is performed under ultraviolet or white light, depending on the type of dye used - fluorescent or non fluorescent (visible).

Capillary action is the ability of a liquid to flow in narrow spaces without the assistance of, and in opposition to, external forces like gravity. The effect can be seen in the drawing up of liquids between the hairs of a paint-brush in a thin tube, in porous materials such as paper. It occurs because of intermolecular forces between the liquid and surrounding solid surfaces

Intermolecular forces are forces of attraction or repulsion which act between neighboring particles.Surface tension is a contractive tendency of the surface of a liquid that allows it to resist an external force. The cohesive forces between liquid molecules are responsible for the phenomenon known as surface tension. Surface tension is typically measured in dynes/cm, the force in dynes required to break a film of length 1 cm. Water at 20°C has a surface tension of 72.8 dynes/cm . The surface tension of water decreases significantly with temperature . Soaps and detergents further lower the surface tension.

Basic principle of a Liquid Penetrant

• When a liquid comes into contact with a surface both cohesive and adhesive forces will act on it. These forces govern the shape which the liquid takes on. Due to the effects of adhesive forces, liquid on a surface can spread out to form a thin, relatively uniform film over the surface, a process known as wetting. Alternatively, in the presence of strong cohesive forces, the liquid can divide into a number of small, roughly spherical beads which stand on the surface, maintaining minimal contact with the surface.

Basic principle of a Liquid Penetrant

• Cohesive forces are the intermolecular forceswhich cause a tendency in liquids to resist separation. These attractive forces exist between molecules of the same substance. For instance, rain falls in droplets, rather than a fine mist, because water has strong cohesion which pulls its molecules tightly together, forming droplets.

Basic principle of a Liquid Penetrant

• Adhesive forces are the attractive forces between unlike molecules. In the case of a liquid wetting agent, adhesion causes the liquid to cling to the surface on which it rests. When water is poured on clean glass, it tends to spread, forming a thin, uniform film over the glasses surface. This is because the adhesive forces between water and glass are strong enough to pull the water molecules out of their spherical formation and hold them against the surface of the glass, thus avoiding the repulsion between like molecules

Basic principle of a Liquid Penetrant

• When the cohesive force of the liquid is stronger than the adhesive force of the liquid to the wall, the liquid concaves down in order to reduce contact with the surface of the wall. When the adhesive force of the liquid to the wall is stronger than the cohesive force of the liquid, the liquid is more attracted to the wall than its neighbors, causing the upward concavity.

Basic principle of a Liquid Penetrant

• The meniscus : is the curve in the upper surface of a liquid close to the surface of the container or another object, caused by surface tension. It can be either convex or concave, depending on the liquid and the surface.

• The viscosity of the liquid is not a factor in the basic equation of capillary rise. Viscosity is related to the rate at which a liquid will flow under some applied unbalanced stress; in itself, viscosity has a negligible effect on penetrating ability.

• In general, however, very viscous liquids are unsuitable as penetrants because they do not flow rapidly enough over the surface of the work piece; consequently, they require excessively long periods of time to migrate into fine flaws.

Basic principle of a Liquid Penetrant

The ability of a given liquid to flow over a surface and enter surface cavities depends principally on the following:• Cleanliness of the surface• Configuration of the cavity• Cleanliness of the cavity• Size of surface opening of the cavity• Surface tension of the liquid• Ability of the liquid to wet the surface• Contact angle of the liquid

Basic principle of a Liquid Penetrant

• If θ is less than 90° (Fig. 1a), the liquid is said to wet the surface, or to have good wetting ability;

• if the angle is equal to or greater than 90° (Fig. 1b and c), the wetting ability is considered poor.

• If θ is greater than 90°, the liquid is depressed in the tube and does not wet the tube wall, and the meniscus is convex (Fig. 2c).

Figure 2

History of PT

• A very early surface inspection technique involved the rubbing of carbon black on glazed pottery, whereby the carbon black would settle in surface cracks rendering them visible. Later, it became the practice in railway workshops to examine iron and steel components by the "oil and whiting" method by Magna flux in (Chicago)

History of PT

• In this method, a heavy oil was diluted with kerosene in large tanks so that locomotive parts such as wheels could be submerged. 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

Why a Penetrant Inspection Improves the Detectability of Flaws?

• 1) It produces a flaw indication that is much larger and easier for the eye to detect than the flaw itself.

• 2) it produces a flaw indication with a high level of contrast between the indication and the background

• 3) The developer serves as a high contrast background as well as a blotter to pull the trapped penetrant from the flaw.

Visual Acuity of the Human 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 (0.076mm) inch for a person with 20/20 vision.

• 20/20 vision, it means that when you stand 20 feet away from the chart you can see what the "normal" human being can see.

• The human eye is more sensitive to a light indication on a dark background and the eye is naturally drawn to a fluorescent indication.

• With a light indication on a dark background, indications down to 0.003 mm (0.0001 inch) may be seen when the contrast between the flaw and the background was high.

• But dark indication on a lighter background can’t.

Visual Acuity of the Human Eye

The eye has a visual acuity threshold below which an object will go undetected. This threshold varies from person to person, but as an example, the case of a person with normal 20/20 vision can be considered. As light enters the eye through the pupil, it passes through the lens and is projected on the retina at the back of the eye. Muscles called extra ocular muscles, move the eyeball in the orbits and allow the image to be focused on the central retinal or fovea.

The retina is a mosaic of two basic types of photoreceptors: rods, and cones. Rods are sensitive to blue-green light with peak sensitivity at a wavelength of 498 nm, and are used for vision under dark or dim conditions. There are three types of cones that give us our basic color vision: L-cones (red) with a peak sensitivity of 564 nm, M-cones (green) with a peak sensitivity of 533 nm, and S-cones (blue) with a peak sensitivity of 437 nm.

Visual Acuity of the Human Eye

• The standard definition of normal visual acuity (20/20 vision) is the ability to resolve a spatial pattern separated by a visual angle of one minute of arc. Since one degree contains sixty minutes, a visual angle of one minute of arc is 1/60 of a degree.

• For the case of normal visual acuity the angle Theta is 1/60 of a degree. By bisecting this angle we have a right triangle with angle Theta/2 that is 1/120 of a degree. Using this right triangle it is easy to calculate the distance X/2 for a given distance d.

• X/2 = d (tan Theta/2) • under normal lighting conditions, the eye is most sensitive

to a yellowish-green color.

Visual Acuity of the Human Eye

• When the light levels drop to near total darkness, the response of the eye changes significantly by the scotopicresponse curve .

• At this level of light, the rods are most active and the human eye is more sensitive to the light present, and less sensitive to the range of color.

• At this very low light level, sensitivity to blue, violet, and ultraviolet is increased, but sensitivity to yellow and red is reduced.

• Fluorescent penetrant inspection materials are designed to fluoresce at around 550 nanometers to produce optimal sensitivity under dim lighting conditions.

Basic Processing Steps of a Liquid Penetrant Inspection

• 1) Surface preparation: The surface must be free of oil, grease, water, or other contaminants that may prevent penetrant from entering flaws.

• 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.

System performance checks

• System performance checks involve processing a test specimen with known defects to determine if the process will reveal discontinuities of the size required.

• The most commonly used test specimen is the TAM or PSM panel. These panel are usually made of stainless steel that has been chrome plated on one half and surfaced finished on the other half to produced 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 to produce range of crack sizes. Each panel has a characteristic “signature” and variances in that signature are indications of process variance.

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. 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. 4) Excess Penetrant Removal: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.

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.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.

Contaminants • Coatings, such as paint, are much more elastic than

metal and will not fracture even though a large defect may be present just below the coating.

• The part must be thoroughly cleaned as surface contaminates can prevent the penetrant from entering a defect.

• Surface contaminants can also lead to a higher level of background noise since the excess penetrant may be more difficult to remove.

• contaminates that must be removed include: paint, dirt, flux, scale, varnish, oil, etchant, smut, plating, grease, oxide, wax, decals, machining fluid, rust, and residue from previous penetrant inspections

Pre-cleaning

• Regardless of the penetrant chosen, adequate pre-cleaning of work pieces prior to penetrant inspection is absolutely necessary for accurate results. Without adequate removal of surface contamination, relevant indications may be missed because:

• The penetrant does not enter the flaw

• The penetrant loses its ability to identify the flaw because it reacts with something already in it

• The surface immediately surrounding the flaw retains enough penetrant to mask the true appearance of the flaw

Cleaning

• Alkaline cleaners can be detrimental to the penetrant inspection process if they have silicates in concentrations above 0.5 percent.

• Sodium metasilicate, sodium silicate, and related compounds can adhere to the surface of parts and form a coating that prevents penetrant entry into cracks.

• some domestic soaps and commercial detergents can clog flaw cavities and reduce the wettability of the metal surface, thus reducing the sensitivity of the penetrant.

Cleaning methodsSelection of a cleaning method depends upon the type of contaminant to be removed and the type of alloy being cleaned.

This cleaning methods are generally classified as

• Chemical,

• Mechanical,

• Solvent,

• or any combination of these.

Cleaning methods• Chemical cleaning methods include alkaline or acid

cleaning, pickling or chemical etching.• Mechanical cleaning methods include tumbling, wet

blasting, dry abrasive blasting, wire brushing, and high pressure water or steam cleaning.

• Mechanical cleaning methods should be used with care because they often mask flaws by smearing adjacent metal over them.

• Solvent cleaning methods include vapor degreasing, solvent spraying, solvent wiping, and ultrasonic immersion using solvents.

• Probably the most common method is vapor degreasing. However, ultrasonic immersion is by far the most effective means of ensuring clean parts, but it can be a very expensive capital equipment investment.

Mechanical methods• Abrasive tumbling : Removing light scale, burrs,

welding flux, braze stop-off, rust, casting mold, and core material;

• Wire brushing removing light deposits of scale, flux, and stopoff.

• Stop-Off is a brazing aid commonly used in silver and aluminum brazing. It is used to prevent the flow of flux and metal to unwanted areas during brazing.

• High-pressure water and steam used with an alkaline cleaner or detergent; removing typical machine shop soils such as cutting oils, polishing compounds, grease, chips etc.

• Ultrasonic cleaning used with detergent and water or with a solvent; removing adherent shop soil from large quantities of small parts

Chemical methods

• Alkaline cleaning Removing braze stopoff, rust, scale, oils, greases, polishing material, and carbon deposits; ordinarily used on large articles where hand methods are too laborious;

• Acid cleaning Strong solutions for removing heavy scale; mild solutions for light scale; weak (etching) solutions for removing lightly smeared metal

Solvent methods

• Vapor degreasing removing typical shop soil, oil, and grease; usually employs chlorinated solvents; not suitable for titanium

• Solvent wiping Same as for vapor degreasing except a hand operation; may employ non-chlorinated solvents; used for localized low-volume cleaning

Common Uses of Liquid Penetrant Inspection

• LPI can be used to inspect almost any material provided that its surface is not extremely rough or porous. It include the following:

• Metals (aluminum, copper, steel, titanium, etc.)

• Glass

• Many ceramic materials

• Rubber

• Plastics

It can only be used to inspect for flaws that break the surface of the sample. Some of these flaws are listed below: 1. Fatigue cracks 2. Quench cracks 3. Grinding cracks 4. Overload and impact fractures 5. Porosity 6. Laps 7. Seams 8. Pin holes in welds 9. Lack of fusion along the edge of the bond line

Advantages of Penetrant Testing

• High sensitivity to small surface discontinuities.

• Large areas and large volumes of parts/materials can be inspected rapidly and at low cost.

• Parts with complex geometric shapes are routinely inspected

• Aerosol spray cans make penetrant materials very portable.

Disadvantages of Penetrant Testing

• 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 prior to LPI.

Disadvantages of Penetrant Testing

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

• Surface finish and roughness can affect inspection sensitivity.

• Post cleaning of acceptable parts or materials is required.

• Chemical handling and proper disposal is required.

TYPES OF PENETRANT MATERIALS

Type 1 - Fluorescent Penetrants: High sensitive, comes usually green in color and fluoresce brilliantly under ultraviolet light.

Type 2 - Visible Penetrants : Less sensitive, usually red in color, viewed under adequate white light. less vulnerable to

Type 3 – Dual mode penetrants : Viewed under black light or white light.

The Type- I , Penetrant have five sensitivity levels:-

Level ½ - Ultra Low Sensitivity Level 1 - Low Sensitivity Level 2 - Medium Sensitivity Level 3 - High Sensitivity Level 4 - Ultra-High Sensitivity

Before selection of a type of penetrant method, we must have a knowledge of

• Surface condition of the work piece being inspected

• Characteristics of the flaws to be detected

• Time and place of inspection

• Size of the work piece

• Sensitivity required

• Materials cost, number of parts, size of area requiring inspection, and portability.

Penetrants are classified on the basis of penetrant type

• Type I: Fluorescent

• Type II: Visible

Method A: Water washable

Method B: Post emulsifiable-lipophilic

Method C: Solvent removable

Method D: Post emulsifiable-hydrophilic

Water-washable penetrant (method A)• Designed so that the penetrant is directly

water washable from the surface of the work piece.

• It is a self emulsifying penetrant.

• It is susceptible to over washing.

Post-emulsifiable penetrants

• Emulsifiers are liquids used to render excess penetrant on the surface of a work piece water washable.

• Method B, lipophilic emulsifiers: oil based, are used as supplied in ready-to-use form, and function by diffusion.

• work with both a chemical and mechanical action.• mechanical action remove excess penetrant as the mixture

drains from the part• In Chemical action, the emulsifier diffuses into the

remaining penetrant and the resulting mixture is easily removed with a water spray.

• Water content (method B, lipophilic) Monthly Not to exceed 5%

Emulsifiers• Method D, hydrophilic emulsifiers are water

based and are usually supplied as concentrates that are diluted in water to concentrations of 5 to 30% for dip applications and 0.05 to 5% for spray applications.

• Hydrophilic emulsifiers function by displacing excess penetrant from the surface of the part by detergent action.

• Hydrophilic emulsifier is slower acting than the lipophilic emulsifier Concentration (method D, hydrophilic) Weekly Not greater than 3% above initial concentration

Hydrophilic emulsifiers

• The major advantage of hydrophilic emulsifiers is that they are less sensitive to variation in the contact and removal time.

• It is more sensitive than the lipophilic post emulsifiable.

• No diffusion takes place

• Work with both a chemical and mechanical action.

• Emulsification Time: ranges from approximately 30 s to 3 min.

Pre-rinse• When using method D (hydrophilic), a coarse water

spray pre-rinse is needed to assist in penetrant removal and to reduce contamination of the emulsifier.

• A coarse water spray is recommended, using a pressure of 275 to 345 kPa (40 to 50 psi).

• The pre-rinse water temperature should be 10 to 40 °C (50 to 100 °F).

• The pre-rinse time should be kept to a minimum (that is, 30 to 90 s) because the purpose is to remove excess penetrant so that the emulsifier does not become contaminated quickly.

• Rinse time should be determined experimentally for specific workpieces; it usually varies from 10 s to 2 min.

Drying• Drying is best done in a recirculating hot-air drier

that is thermostatically controlled. • The temperature in the drier is normally between

65 and 95 °C (150 and 200 °F). • The temperature of the work pieces should not

be permitted to exceed 70 °C (160 °F).• Excessive drying at high temperatures can impair

the sensitivity of the inspection. • Because drying time will vary, the exact time

should be determined experimentally for each type of work piece.

Post-emulsifiable penetrants (methods B and D)

• Designed to ensure the detection of minute flaws in some materials.

• Separate emulsification is required to remove the penetrant.

• The danger of over washing the penetrant out of the flaws is reduced.

• These methods are the most reliable for detecting minute flaws.

Methods B and D1. pre-clean part, 2. apply penetrant and allow to dwell, 3. pre-rinse to remove first layer of penetrant, 4. apply 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

Processing steps

Solvent-removable penetrant (method C)

• Used to inspect only a localized area of a work piece

• Inspect a work piece at the site rather than on a production inspection basis.

• Normally, the same type of solvent is used for pre cleaning and for removing excess penetrant.

• This method is labor intensive.• When properly conducted and when used in the

appropriate applications, the solvent-removable method can be one of the most sensitive penetrant methods available.

Solvent-removable penetrant (method C)

• The use of excessive amounts of solvent must be avoided.

Solvent Cleaner/Removers

• Remove excess surface penetrant through direct solvent action.

• There are two basic types of solvent removers: • flammable and nonflammable. • Flammable cleaners are essentially free of

halogens but are potential fire hazards.• Nonflammable cleaners are widely used.

However, they do contain halogenated solvents, which may render them unsuitable for some applications.

Solvent Cleaner/Removers

• Excess surface penetrant is removed by wiping, using lint-free cloths slightly moistened with solvent cleaner/remover.

• It is not recommended that excess surface penetrant be removed by flooding the surface with solvent cleaner/remover,

• Because the solvent will dissolve the penetrant within the defect and indications will not be produced

Penetrant Application

• Penetrants can be applied by :-

• Immersing

• Spraying

• Brushing

• The emulsifier on to the part is not recommended by brushing either because the bristles of the brush may force emulsifier into discontinuities, causing the entrapped penetrant to be removed

Penetrant Application and Dwell Time

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 contaminated 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 evaporation.

Penetrant Application and Dwell Time

• However, Sherwin showed that if the specimens are allowed to drain-dwell, the sensitivity is higher because the evaporation increases the dyestuff concentration of the penetrant on the specimen.

• 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.

Dwell TimeThe 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• The atmospheric pressure at the flaw opening.• 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.

Dwell Time• 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. • Deutsch makes about dwell time is that if the

elliptical flaw has a length to width ratio of 100, it will take the penetrant nearly ten times longer to fill than it will a cylindrical flaw with the same volume

Physical and Chemical Characteristics

• Chemical stability and uniform physical consistency

• A flash point not lower than 95 °C (200 °F);

• penetrants that have lower flash points constitute a potential fire hazard.

• A high degree of wettability

• Low viscosity to permit better coverage and minimum drag out

• Ability to penetrate discontinuities quickly and completely

• Sufficient brightness and permanence of color

Physical and Chemical Characteristics

• Chemical inertness with materials being inspected and with containers

• Low toxicity to protect personnel

• Slow drying characteristics

• Ease of removal

• Inoffensive odor

• Low cost

• Resistance to ultraviolet light and heat fade

Chemical stability• Tendency of a material to resist change or

decomposition due to internal reaction, or due to the action of air, heat, light, pressure, etc.

• The properties of penetrant 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.

ultraviolet & Thermal stability

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.

2. Generally, thermal damage occurs when fluorescent penetrant materials are heated above 71oC

Temperature• The temperature of the penetrant materials and the part

being inspected should be from 10 to 49oC (80 to 120oF) .• 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.

• Raising the temperature will also raise the speed of evaporation of penetrants, which can have a positive or negative effect on sensitivity.

• The impact will be positive if the evaporation serves to increase the dye concentration of the penetrant trapped in a flaw up to the concentration quenching point and not beyond.

Flash point

• The evaporation of the volatile constituents of penetrants can alter their chemical and performance characteristics,

• Resulting in changes in inherent brightness, removability, and sensitivity.

• Liquid penetrant materials qualified to MIL-I-25135D (and subsequent revisions) have a flash point requirement of a minimum of 95 °C

Properties of a penetrant

• The properties of penetrant 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.

• Dilution of the penetrant liquid will affect the concentration of the dye and reduce the dimensional threshold of fluorescence.

A penetrant must:

• 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.

Function of developers• Increase the brightness intensity of

fluorescent indications and the visible contrast of visible-penetrant indications.

• The developer also provides a blotting action, which serves to draw penetrant from within the flaw to the surface, spreading the penetrant and enlarging the appearance of the flaw.

• Decreases inspection time by hastening the appearance of indications.

Developer properties• The developer must be adsorptive to maximize

blotting• It must have fine grain size and a particle shape

that will disperse and expose the penetrant at a flaw to produce strong and sharply defined indications of flaws

• It must be capable of providing a contrast background for indications when color-contrast penetrants are used

• It must be easy to apply• It must form a thin, uniform coating over a

surface

Developer properties• It must be non fluorescent if used with

fluorescent penetrants

• It must be easy to remove after inspection

• It must not contain ingredients harmful to parts being inspected or to equipment used in the inspection

• It must not contain ingredients harmful or toxic to the operator

Developer Forms

• Form A, dry powder

• Form B, water soluble

• Form C, water sus-pendible

• Form d , Non-aqueous Type 1 Fluorescent (Solvent Based)

• Form e ,Non-aqueous Type 2 Visible Dye (Solvent Based)

Form A, dry powder

• Dry developer does not provide a uniform white background as the other forms of developers do

• Least sensitive but it is inexpensive to use and easy to apply.

• Excessive powder can be removed by gently blowing on the surface with air not exceeding 35 kPa or 5 psi.

Form A, dry powder• Widely used with fluorescent penetrants, but should not be

used with visible dye penetrants because they do not produce a satisfactory contrast coating on the surface of the work piece.

• It should be light and fluffy to allow for ease of application and should cling to dry surfaces in a fine film.

• powders should not be hygroscopic, and they should remain dry.

• If they pick up moisture when stored in areas of high humidity, they will lose their ability to flow and dust easily, and they may agglomerate, pack, or lump up in containers or in developer chambers.

• Dry-developer form inspected daily Must be fluffy, not caked.

Safety requirement

• Handled with care because it can dry the skin and irritate the lining of the air passages, causing irritation.

• Rubber gloves and respirators may be desirable if an operator works continuously with this.

Water-soluble developers (form B)• It can be used for both type I or type II

penetrants.• It is not recommended for use with water-

washable penetrants, because of the potential to wash the penetrant from within the flaw if the developer is not very carefully controlled.

• Supplied as a dry powder concentrate• Dispersed in water from 0.12 to 0.24 kg/L • The bath concentration is monitored for specific

gravity with hydrometer.• They should never be applied with a brush.

Water-suspendible developers (form C)

• It can be used with either fluorescent (type I) or visible (type II) penetrants.

• With fluorescent penetrant, the dried coating of developer must not fluoresce, nor may it absorb or filter out the black light used for inspection.

• supplied as a dry powder concentrate, which is then dispersed in water in recommended proportions, usually from 0.04 to 0.12 .

• Specific gravity checks should be conducted routinely, using a hydrometer to check the bath concentration.

Water-suspendible developers (form C)

• It contains dispersing agents to help retard settling and caking as well as inhibitors to prevent or retard corrosion of work pieces

• It contains biocides to extend the working life of the aqueous solutions.

• It contains wetting agents to ensure even coverage of surfaces and ease of removal after inspection.

• They should never be applied with a brush.

Drying

• Drying is achieved by placing the wet but well drained part in a recirculating, warm air dryer with the temperature held between 70 and 75°F.

• If the parts are not dried quickly, the indications will be blurred and indistinct.

• Properly developed parts in water soluble developer will have an even, pale white coating over the entire surface.

• The surface of a part coated with a water suspendable developer will have a slightly translucent white coating.

Advantages

• Not require any agitation in water soluble but water suspendable developers require frequent stirring or agitation to keep the particles from settling out of suspension.

• Applied prior to drying, thus decreasing the development time

• The dried developer film on the work piece is completely water soluble and is thus easily and completely removed by simple water rinsing.

Non-aqueous solvent-suspendible developers (form D)

• used for both the fluorescent and the visible penetrant process.

• This coating yields the maximum white color contrast with the red visible penetrant indication and extremely brilliant fluorescent indication.

• Supplied in the ready-to-use condition and contain particles of developer suspended in a mixture of volatile solvents.

• It also contain surfactants in a dispersant whose functions are to coat the particles and reduce their tendency to clump or agglomerate.

Non-aqueous solvent-suspendible developers

• Most sensitive form of developer used with type I because the solvent action contributes to the absorption and adsorption mechanisms.

• It enters the flaw and dissolves into the penetrant. This action increases the volume and reduces the viscosity of the penetrant.

• There are two types of solvent-base developers:

• nonflammable (chlorinated solvents) and flammable (non-chlorinated solvents). Both types are widely used.

Non-aqueous solvent-suspendibledevelopers

• Since the solvent is highly volatile, forced drying is not required.

• A non-aqueous developer should be applied to a thoroughly dried part to form a slightly translucent white coating.

• If the spray produces spatters or an uneven coating, the can should be discarded.

Stationary Inspection Equipment

The type of equipment most frequently used in fixed installations consists of a series of modular subunits. • Drain and/or dwell stations• Penetrant and emulsifier stations• Pre- and post-wash stations• Drying station• Developer station• Inspection station• Cleaning stations

Developer

• Developer Station. The type and location of the developer station depend on whether dry or wet developer is to be used.

• For dry developer, the developer station is downstream from the drier, but for wet developer it immediately precedes the drier, following the rinse station.

• For wet, there should also be a rack or conveyor on which parts can rest after dipping. This will permit excess developer to run back into the tank.

Developer

• Suspendible developer baths settle out when not in use; therefore, a paddle for stirring should be provided. Continuous agitation is essential because the settling rate is rapid.

• Pumps are sometimes incorporated into the developer station for flowing the developer over large work pieces through a hose and nozzle and for keeping the developer agitated.

• In automatic units, special methods of applying developer are required. Flow-on methods are frequently used.

• This technique requires a nozzle arrangement that permits the work pieces to be covered thoroughly and quickly.

Inspection Station

• Inspection station is simply a worktable on which work pieces can be handled under proper lighting.

• For fluorescent methods, the table is usually surrounded by a curtain or hood to exclude most of the white light from the area.

• For visible-dry penetrants, a hood is not necessary.

• Generally, black (ultraviolet) lights (100 W or greater) are mounted on brackets from which they can be lifted and moved about by hand.

• Because of the heat given off by black lights, good air circulation is essential in black light booths.

Black light Intensity• UV ranging from 180 to 400 nanometers. • Recommended black light intensity is 1000 to 1600

W/cm2.• The intensity of the black light should be verified at

regular intervals by the use of a suitable black light meter such as a digital radiometer.

• Warm up prior to use--generally for about 10 min. • UV light must be warmed up prior to use and should be

on for at least 15 minutes before beginning an inspection.

• The inspector should allow time for adapting to darkness; a 1-min period is usually adequate.

• White light intensity should not exceed 20 lx (2 ftc) to ensure the best inspection environment.

Black light Intensity

• 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.

• UV emissions below 310nm include some hazardous wavelengths.

• Bulbs lose intensity over time. In fact, a bulb that is near the end of its operating life will often have an intensity of only 25% of its original output.

Effect of UV light• Excessive UV light exposure can cause painful

sunburn, accelerate wrinkling and increase the risk of skin cancer.

• UV light can cause eye inflammation, cataracts, and retinal damage

• 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.

• UV lamps sold for use in LPI application are almost always filtered to remove the harmful UV wavelengths.

visible light intensity

• visible light intensity should be adequate to ensure proper inspection; 320 to 540 lx (30 to 50 ftc) is recommended.

• Lighting intensity should be verified at regular intervals by the use of a suitable white light meter such as a digital radiometer & it should be calibrated at least every six months.

• Ultraviolet light measurements should be taken using a fixture to maintain a minimum distance of 15 inches from the filter face to the sensor

Dimensional Threshold of Fluorescence

• The performance of penetrants based on the physical constraints of the dyes can be predicted using Beer's Law equation. This law states that the absorption of light by a solution changes exponentially with the concentration of the solution.

• This equation does not hold true when very thin layers are involved but works well to establish general relationships between variables.

• I = Io x e-lCt

Dimensional Threshold of Fluorescence

Where:

I = Transmitted light intensityIo = Incident light intensitye = Base of natural log (2.71828)l = Absorption coefficient per unit of concentrationC = Dye concentrationt = Thickness of the absorbing layer trolled to a certain degree by the concentration of the fluorescent tracer dye in the penetrant

Post cleaning

• Some residue will remain on work pieces after penetrant inspection is completed.

• Residues can result in the formation of voids during subsequent welding or unwanted stopoff in brazing,

• In the contamination of surfaces (which can cause trouble in heat treating), or in unfavorable reactions in chemical processing operations.

Post cleaning

• ultrasonic cleaning may be the only satisfactory way of cleaning deep crevices or small holes. However, solvents or detergent-aided steam or water is almost always sufficient.

• The use of steam with detergent is probably the most effective of all methods.

• It has a scrubbing action that removes developers, the heat and detergent remove penetrants, it leaves a work piece hot enough to promote rapid, even drying, and it is harmless to nearly all materials.

Post cleaning

• Vapor degreasing is very effective for removing penetrants, but it is practically worthless for removing developers.

• It is frequently used in combination with steam cleaning.

• If this combination is used, the steam cleaning should always be done first because vapor degreasing bakes on developer films.

Probability of detection

In general, penetrant inspections are 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

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THE END