Penetrant Testing Level 2 Notes

7/25/2019 Penetrant Testing Level 2 Notes

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Introduction and History ofPenetrant Inspection

Liquid penetrant inspection is a method that is used to reveal surface breaking flaws bybleedout of a colored or fluorescent dye from the flaw. The technique is based on theability of a liquid to be drawn into a "clean" surface breaking flaw by capillary action.

After a period of time called the "dwell," excess surface penetrant is removed and adeveloper applied. This acts as a blotter. It draws the penetrant from the flaw to revealits presence. Colored (contrast) penetrants require good white light while fluorescentpenetrants need to be used in darkened conditions with an ultraviolet "black light".

A very early surface inspection technique involved therubbing of carbon black on glazed pottery, wherebythe carbon black would settle in surface cracksrendering them visible. Later, it became the practicein railway workshops to examine iron and steelcomponents by the "oil and whiting" method. In thismethod, a heavy oil commonly available in railwayworkshops was diluted with kerosene in large tanksso that locomotive parts such as wheels could besubmerged. After removal and careful cleaning, thesurface was then coated with a fine suspension ofchalk in alcohol so that a white surface layer was formed once the alcohol hadevaporated. The object was then vibrated by being struck with a hammer, causing theresidual oil in any surface cracks to seep out and stain the white coating. This methodwas in use from the latter part of the 19th century to approximately 1940, when themagnetic particle method was introduced and found to be more sensitive forferromagnetic iron and steels.

A different (though related) method was introduced in the 1940's. The surface underexamination was coated with a lacquer, and after drying, the sample was caused tovibrate by the tap of a hammer. The vibration causes the brittle lacquer layer to crackgenerally around surface defects. The brittle lacquer (stress coat) has been usedprimarily to show the distribution of stresses in a part and not for finding defects.

Many of these early developments were carried out by Magnaflux in Chicago, IL, USA inassociation with Switzer Bros., Cleveland, OH, USA. More effective penetrating oils

containing highly visible (usually red) dyes were developed by Magnaflux to enhanceflaw detection capability. This method, known as the visible or color contrast dyepenetrant method, is still used quite extensively today. In 1942, Magnaflux introducedthe Zyglo system of penetrant inspection where fluorescentdyes were added to theliquid penetrant. These dyes would then fluoresce when exposed to ultraviolet light(sometimes referred to as "black light") rendering indications from cracks and othersurface flaws more readily visible to inspectors.

Why a Penetrant Inspection Improves the Detectabil ity of Flaws


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The advantage that a liquid penetrant inspection (LPI) offersover an unaided visual inspection is that it makes defects

easier to see for the inspector. There are basically two waysthat a penetrant inspection process makes flaws more easilyseen. First, LPI produces a flaw indication that is much largerand easier for the eye to detect than the flaw itself. Many flawsare so small or narrow that they are undetectable by theunaided eye. Due to the physical features of the eye, there is athreshold below which objects cannot be resolved. Thisthreshold of visual acuity is around 0.003 inch for a person with20/20 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 alsohelping to make the indication more easily seen. When a visible dye penetrantinspection is performed, the penetrant materials are formulated using a bright red dyethat provides for 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 thetrapped 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 awavelength that the eye is most sensitive to under dim lighting conditions.

Basic Processing Steps of a Liquid Penetrant Inspection

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

2. Penetrant Application:Once the surface has beenthoroughly cleaned and dried, the penetrant materialis applied by spraying, brushing, or immersing the

part in a penetrant bath.

3. Penetrant Dwell: The penetrant is left on the surfacefor a sufficient time to allow as much penetrant aspossible to be drawn from or to seep into a defect.Penetrant dwell time is the total time that thepenetrant is in contact with the part surface. Dwelltimes are usually recommended by the penetrant producers or required by thespecification 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 defect being inspected for. Minimum dwell times typically range


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from five to 60 minutes. Generally, there is no harm in using a longer penetrantdwell time as long as the penetrant is not allowed to dry. The ideal dwell time isoften determined by experimentation and may be very specific to a particularapplication.

4. Excess Penetrant Removal:This is the most delicate part of the inspectionprocedure because the excess penetrant must be removed from the surface ofthe sample while removing as little penetrant as possible from defects.Depending on the penetrant system used, this step may involve cleaning with asolvent, direct rinsing with water, or first treating the part with an emulsifier andthen rinsing with water.

5. Developer Application:A thin layer of developer is then applied to the sampleto 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

powdered), dipping, or spraying (wet developers).

6. Indication Development:The developer is allowed to stand on the part surfacefor a period of time sufficient to permit the extraction of the trapped penetrant outof 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 detectindications 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.

Common Uses of Liquid Penetrant Inspection

Liquid penetrant inspection (LPI) is one of the most widely used nondestructiveevaluation (NDE) methods. Its popularity can beattributed to two main factors: its relative ease ofuse and its flexibility. LPI can be used toinspect almost any material provided that itssurface is not extremely rough or porous.Materials that are commonly inspected using LPI

include the following:

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

Glass Many ceramic materials Rubber Plastics

LPI offers flexibility in performing inspections because it can be applied in a large varietyof applications ranging from automotive spark plugs to critical aircraft components.


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Penetrant materials can be applied with a spray can or a cotton swab to inspect forflaws known to occur in a specific area or it can be applied by dipping or spraying toquickly inspect large areas. In the image above, visible dye penetrant is being locallyapplied to a highly loaded connecting point to check for fatigue cracking.

Penetrant inspection systems have been developed to inspect some very largecomponents. In the image shown right, DC-10 banjo fittings are being moved into apenetrant inspection system at what used to be the Douglas Aircraft Company's LongBeach, California facility. These large machined aluminum forgings are used to supportthe number three engine in the tail of a DC-10 aircraft.

Liquid penetrant inspection can only be used to inspect for flaws that break the surfaceof the sample. Some of these flaws are listed below:

Fatigue cracks Quench cracks Grinding cracks Overload and impact fractures Porosity Laps Seams Pin holes in welds Lack of fusion or braising along the edge of the bond line

As mentioned above, one of the major limitations of a penetrant inspection is that flawsmust be open to the surface. To learn more about the advantages and disadvantages of

LPI, proceed to the next page.

Advantages and Disadvantages of Penetrant Test ing

Like all nondestructive inspection methods, liquid penetrant inspection has bothadvantages and disadvantages. The primary advantages and disadvantages whencompared to other NDE methods are summarized below.

Primary Advantages

The method has high sensitivity to small surface discontinuities. The method has few material limitations, i.e. metallic and nonmetallic, magnetic

and nonmagnetic, and conductive and nonconductive materials may beinspected.

Large areas and large volumes of parts/materials can be inspected rapidly and atlow cost.

Parts with complex geometric shapes are routinely inspected. Indications are produced directly on the surface of the part and constitute a visual

representation of the flaw. Aerosol spray cans make penetrant materials very portable. Penetrant materials and associated equipment are relatively inexpensive.


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Primary Disadvantages

Only surface breaking defects can be detected. Only materials with a relatively nonporous surface can be inspected. Precleaning is critical since contaminants can mask defects. Metal smearing from machining, grinding, and grit or vapor blasting must be

removed prior to LPI. 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.

Penetrant Testing Materials

The penetrant materials used today are much more sophisticated than the keroseneand whiting first used by railroad inspectors near the turn of the 20th century. Today'spenetrants are carefully formulated to produce the level of sensitivity desired by theinspector. To perform well, a penetrant must possess a number of importantcharacteristics. A penetrant must:

spread easily over the surface of the material being inspected to providecomplete 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.

All penetrant materials do not perform the sameand are not designed to perform the same.Penetrant manufactures have developed differentformulations to address a variety of inspectionapplications. Some applications call for thedetection of the smallest defects possible andhave smooth surfaces where the penetrant is

easy to remove. In other applications, therejectable defect size may be larger and apenetrant formulated to find larger flaws can beused. The penetrants that are used to detect thesmallest defect will also produce the largestamount of irrelevant indications.

Penetrant materials are classified in the various industry and government specificationsby their physical characteristics and their performance. Aerospace MaterialSpecification (AMS) 2644, Inspection Material, Penetrant, is now the primary


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specification used in the USA to control penetrant materials. Historically, MilitaryStandard 25135, Inspection Materials, Penetrants, has been the primary document forspecifying penetrants but this document is slowly being phased out and replaced by

AMS 2644. Other specifications such as ASTM 1417, Standard Practice for Liquid

Penetrant Examinations, may also contain information on the classification of penetrantmaterials but they are generally referred back to MIL-I-25135 or AMS 2644.

Penetrant materials come in two basic types. These types are listed below:

Type 1 - Fluorescent Penetrants Type 2 - Visible Penetrants

Fluorescent penetrants contain a dye or severaldyes that fluoresce when exposed to ultravioletradiation. Visible penetrants contain a red dye that

provides high contrast against the white developerbackground. Fluorescent penetrant systems aremore sensitive than visible penetrant systemsbecause the eye is drawn to the glow of thefluorescing indication. However, visible penetrantsdo not require a darkened area and an ultravioletlight in order to make an inspection. Visiblepenetrants are also less vulnerable to contaminationfrom things such as cleaning fluid that can significantly reduce the strength of afluorescent indication.

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

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

Water washable (Method A) penetrants can be removed from the part by rinsing withwater alone. These penetrants contain an emulsifying agent (detergent) that makes itpossible to wash the penetrant from the part surface with water alone. Water washable

penetrants are sometimes referred to as self-emulsifying systems. Post-emulsifiablepenetrants come in two varieties, lipophilic and hydrophilic. In post-emulsifiers,lipophilic 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 excesspenetrant from the surface of the part with a water wash. Solvent removable penetrantsrequire the use of a solvent to remove the penetrant from the part.


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Penetrants are then classified based on the strength or detectability of the indicationthat is produced for a number of very small and tight fatigue cracks. The five sensitivitylevels 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 industryspecifications currently rely on the US Air ForceMaterials Laboratory at Wright-Patterson Air Force

Base to classify penetrants into one of the fivesensitivity levels. This procedure uses titanium and Inconel specimens with smallsurface cracks produced in low cycle fatigue bending to classify penetrant systems.The brightness of the indication produced is measured using a photometer. Thesensitivity 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 originallyplanned. However, when some penetrants were judged to have sensitivitiessignificantly less than most others in the level 1 category, the level was created.

Penetrants

The industry and military specifications that control penetrant materials and their use, allstipulate certain physical properties of the penetrant materials that must be met. Someof these requirements address the safe use of the materials, such as toxicity, flashpoint, and corrosiveness, and other requirements address storage and contaminationissues. Still others delineate properties that are thought to be primarily responsible forthe performance or sensitivity of the penetrants. The properties of penetrant materialsthat are controlled by AMS 2644 and MIL-I-25135E include flash point, surface wettingcapability, viscosity, color, brightness, ultraviolet stability, thermal stability, water

Surface Energy (Surface Wetting Capabili ty)

As previously mentioned, one of the important characteristics of a liquid penetrantmaterial is its ability to freely wet the surface of the object being inspected. At the liquid-


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solid surface interface, if the molecules of the liquid have a stronger attraction to themolecules of the solid surface than to each other (the adhesive forces are stronger thanthe cohesive forces), wetting of the surface occurs. Alternately, if the liquid moleculesare more strongly attracted to each other than the molecules of the solid surface (the

cohesive forces are stronger than the adhesive forces), the liquid beads-up and doesnot wet the surface of the part.

One way to quantify a liquid's surface wetting characteristics is to measure the contactangle of a drop of liquid placed on the surface of an object. The contact angle is theangle formed by the solid/liquid interface and the liquid/vapor interface measured fromthe side of the liquid. (See the figure below.) Liquids wet surfaces when the contactangle is less than 90 degrees. For a penetrant material to be effective, the contact angleshould be as small as possible. In fact, the contact angle for most liquid penetrants isvery close to zero degrees.

Wetting ability of a liquid is a function of the surface energies of the solid-gas interface,the liquid-gas interface, and the solid-liquid interface. The surface energy across aninterface or the surface tension at the interface is a measure of the energy required toform a unit area of new surface at the interface. The intermolecular bonds or cohesiveforces between the molecules of a liquid cause surface tension. When the liquidencounters another substance, there is usually an attraction between the two materials.The adhesive forces between the liquid and the second substance will compete againstthe cohesive forces of the liquid. Liquids with weak cohesive bonds and a strongattraction to another material (or the desire to create adhesive bonds) will tend tospread over the material. Liquids with strong cohesive bonds and weaker adhesiveforces 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 involved, the solid-gas interface, the liquid-gas interface, and the solid-liquid interface. For a liquid tospread over the surface of a part, two conditions must be met. First, the surface energyof the solid-gas interface must be greater than the combined surface energies of theliquid-gas and the solid-liquid interfaces. Second, the surface energy of the solid-gasinterface must exceed the surface energy of the solid-liquid interface.

A penetrant's wetting characteristics are also largely responsible for its ability to fill avoid. Penetrant materials are often pulled into surface breaking defects by capillaryaction. The capillary force driving the penetrant 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.

At times, it is the adhesion tension that is primarily responsible for a penetrant'smovement into a flaw and not the surface energy of the liquid-gas interface. Adhesiontension is the force acting on a unit length of the wetting line from the direction of thesolid. The wetting performance of the penetrant is degraded when adhesion tension isthe primary driving force.

Since the contact angle for penetrants is very close to zero, other methods have beendevised to make relative comparisons of the wetting characteristics of these liquids.


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One method is to measure the height that a liquid reaches in a capillary tube. However,the solid interface in this method is usually glass and may not accurately represent thesurface that the penetrant inspection will be performed on. Another method ofcomparative evaluation is to measure the radius, the diameter, or the area of a spot

formed when a drop of penetrant is placed on the test surface and allowed to standundisturbed for a specific amount of time. However, using this method, other factors arealso acting in the comparison. These methods include the density, viscosity, andvolatility of the liquid, which do not enter into the capillarity equations, but may have aneffect on the inspection as discussed in the related pages.

Density or Specific Gravity

The density or specific gravity of a penetrant material has a slight to negligible effect onthe performance of a penetrant. The gravitational force acting on the penetrant liquidcan be working either with or against the capillary force depending on the orientation of

the flaw during the dwell cycle.

Viscosity

Viscosity describes a fluid's resistance to flow. Liquids such as water that flow easily,have a lower viscosity than do liquids such as ketchup. Viscosity has little effect on theability of a penetrant material to enter a defect but it does have an effect on the speed atwhich the penetrant fills a defect.

Color and Fluorescent Brightness

Penetrant Color and Fluorescence

The color of the penetrant material is of obvious importance in a visible dye penetrantinspection, as the dye must provide good contrast against the developer or part beinginspected. Remember from the earlier discussion of contrast sensitivity that generallythe higher the contrast, the easier objects are to see. The dye used in visible dyepenetrant is usually vibrant red but other colors can be purchased for specialapplications.


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When fluorescent materials are involved, the effect of color and fluorescence is not sostraightforward. LPI materials fluoresce because they contain one or more dyes thatabsorb electromagnetic radiation over a particular wavelength and the absorption ofphotons leads to changes in the electronic configuration of the molecules. Since the

molecules are not stable at this higher energy state, they almost immediately re-emit theenergy. There is some energy loss in the process and this causes photons to be re-emitted at a slightly longer wavelength that is in the visible range. The radiationabsorption and emission could take place a number of times until the desired color andbrightness is achieved. Two different fluorescent colors can be mixed to interact by amechanism called cascading. The emission of visible light by this process involves onedye absorbing ultraviolet radiation to emit a band of radiation that makes a second dyeglow. Since the human eye is the most commonly used sensing device, mostpenetrants are designed to fluoresce as close as possible to the eyes' peak response..

Penetrant Brightness

Fluorescent brightness was erroneously once thought to be the controlling factor withrespect to flaw detection sensitivity. Measurements have been made to evaluate theintrinsic brightness of virtually all commercially available penetrants and they all haveabout the same brightness. Intrinsic brightness values are determined for thick liquidfilms but the dimensional threshold of fluorescence (discussed on the next page) is amore important property. The measurement of fluorescent brightness is detailed in

ASTM E-1135, "Standard Test Method for Comparing the Brightness of FluorescentPenetrants."

Ultraviolet and Thermal Stability of Penetrant Indications

Exposure to intense ultraviolet light and elevated temperatures can have a negativeeffect on fluorescent penetrant indications. Fluorescent materials can lose theirbrightness after a period of exposure to high intensity UV light. One study measured theintensity of fluorescent penetrant indications on a sample that was subjected to multipleUV exposure cycles. Each cycle consisted of 15 minutes of 800 microwatt/cm UV lightand 2.5 minutes of 1500 microwatt/cm UV light. Two penetrants were tested in thestudy, water washable, level 3 and a post emulsifiable, level 4. The results from thestudy showed that the indications from both penetrants faded with increased UVexposure. After eight exposure cycles, the brightnesses of the indications were lessthan one half their original values.

At an elevated temperature, penetrants can experience heat degradation or "heat fade."Excessive heat:

1. evaporates the more volatile constituents which increases viscosity and adverselyaffects 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.


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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 excitation source, the delocalized

bonding electrons rise to a higher electronic state. After excitation, the electrons willnormally lose energy and return to the lowest energy state. This loss of energy caninvolve a "radiative" process such as fluorescence or "non-radiative" processes. Non-radiative processes include relaxation by molecular collisions, thermal relaxation, andchemical reaction. Heat causes the number of molecular collisions to increase, whichresults in more collision relaxation and less fluorescence.

This explanation is only valid when the part and the penetrant are at an elevatedtemperature. When the materials cool, the fluorescence will return. However, whileexposed to elevated temperatures, penetrant solutions dry faster. As the moleculesbecome more closely packed in the dehydrated solution, collision relaxation increases

and fluorescence decreases. This effect has been called "concentration quenching" andexperimental data shows that as the dye concentration is increased, fluorescentbrightness initially increases but reaches a peak and then begins to decrease. Airflowover the surface on the part will also speed evaporation of the liquid carrier, so it shouldbe kept to a minimum to prevent a loss of brightness.

Generally, thermal damage occurs when fluorescent penetrant materials are heatedabove 71oC (160oF). It should be noted that the sensitivity of an FPI inspection can beimproved if a part is heated prior to applying the penetrant material, but the temperatureshould be kept below 71oC (160oF). Some high temperature penetrants in use today areformulated with dyes with high melting points, greatly reducing heat related problems.

The penetrants also have high boiling points and the heat related problems are greatlyreduced. However, a loss of brightness can still take place when the penetrant isexposed to elevated temperatures over an extended period of time. When one heatresistant formulation was tested, a 20 % reduction was measured after the material wassubjected to 163oC (325oF) for 273 hours. The various types of fluorescent dyescommonly employed in today's penetrant materials begin decomposition at 71oC(160oF). When the temperature approaches 94oC (200oF), there is almost totalattenuation of fluorescent brightness of the composition and sublimation of thefluorescent dyestuffs.

Dimensional Threshold of Fluorescence

The dimensional threshold of fluorescence is a property that is not currently controlledby the specifications but appears to largely determine the sensitivity of a fluorescentpenetrant. A. L. Walters and R. C. McMaster conducted an experiment that led to theunderstanding of this condition. Two optically flat plates of glass were clamped tightlytogether. A drop of fluorescent penetrant was placed at the interface of the plates. The


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penetrant could be seen migrating in between the plates but when exposed to blacklight, no fluorescence was seen. The phenomenon was not fully understood until 1960when Alburger introduced the concept of thin-film transition of fluorescent response.

The dimensional magnitudes of typical crack defects correspond to the dimensionalthresholds of fluorescence response which are characteristic of the available penetrant.

Alternately stated, the degree of fluorescence response, under a given intensity ofultraviolet radiation, is dependent on the absorption of ultraviolet radiation, which in turndepends on dye concentration and film thickness. Therefore, the ability of a penetrant toyield an indication depends primarily on its ability to fluoresce as a very thin film. Theperformance of penetrants based on the physical constraints of the dyes can bepredicted using Beer's Law equation. This law states that the absorption of light by asolution changes exponentially with the concentration of the solution. This equationdoes not hold true when very thin layers are involved but works well to establish generalrelationships between variables

Removability

Removing the penetrant from the surface of the sample, without removing it from theflaw, is one of the most critical operations of the penetrant inspection process. Thepenetrant must be removed from the sample surface as completely as possible to limitbackground fluorescence. In order for this to happen, the adhesive forces of thepenetrant 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, theadhesive forces (forces of attraction between the penetrant and the solid surface beinginspected) must be stronger than the cohesive forces (forces holding the liquid

together). Proper formulation of the penetrant materials provides the correct balancingof these forces.

Another consideration in the formulation of the penetrant liquid is that it should noteasily commingle and become diluted by the cleaning solution. Dilution of the penetrantliquid will affect the concentration of the dye and reduce the dimensional threshold offluorescence

Emulsifiers

When removal of the penetrant from a defect due to over-washing of the part is aconcern, a post-emulsifiable penetrant system can be used. Post-emulsifiablepenetrants require a separate emulsifier to break the penetrant down and make it water-washable. Most penetrant inspection specifications classify penetrant systems into fourmethods of excess penetrant removal. These are listed below:

1. Method A: Water-Washable2. Method B: Post-Emulsifiable, Lipophilic3. Method C: Solvent Removable4. Method D: Post-Emulsifiable, Hydrophilic


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Method C relies on a solvent cleaner to remove the penetrant from the part beinginspected. Method A has emulsifiers built into the penetrant liquid that makes it possibleto remove the excess penetrant with a simple water wash. Method B and D penetrantsrequire an additional processing step where a separate emulsification agent is applied

to make the excess penetrant more removable with a water wash. Lipophilicemulsification 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 bediluted with water prior to use .

Lipophilic emulsifiers (Method B) were introduced in the late 1950's and work with botha chemical and mechanical action. After the emulsifier has coated the surface of theobject, mechanical action starts to remove some of the excess penetrant as the mixturedrains from the part. During the emulsification time, the emulsifier diffuses into theremaining penetrant and the resulting mixture is easily removed with a water spray.

Hydrophilic emulsifiers (Method D) also remove theexcess penetrant with mechanical and chemicalaction but the action is different because no diffusiontakes place. Hydrophilic emulsifiers are basicallydetergents that contain solvents and surfactants.The hydrophilic emulsifier breaks up the penetrantinto small quantities and prevents these pieces fromrecombining or reattaching to the surface of the part.The mechanical action of the rinse water removesthe displaced penetrant from the part and causesfresh remover to contact and lift newly exposed

penetrant from the surface.

The hydrophilic post-emulsifiable method (Method D) was introduced in the mid 1970's.Since it is more sensitive than the lipophilic post emulsifiable method it has made thelater method virtually obsolete. The major advantage of hydrophilic emulsifiers is thatthey are less sensitive to variation in the contact and removal time. While emulsificationtime should be controlled as closely as possible, a variation of one minute or more inthe contact time will have little effect on flaw detectability when a hydrophilic emulsifieris used. However, a variation of as little as 15 to 30 seconds can have a significanteffect when a lipophilic system is used.

Developers

The role of the developer is to pull the trapped penetrant material out of defects andspread it out on the surface of the part so it can be seen by an inspector. The finedeveloper particles both reflect and refract the incident ultraviolet light, allowing more ofit to interact with the penetrant, causing more efficient fluorescence. The developer alsoallows more light to be emitted through the same mechanism. This is why indicationsare brighter than the penetrant itself under UV light. Another function that somedevelopers perform is to create a white background so there is a greater degree ofcontrast between the indication and the surrounding background.


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Developer Forms

The AMS 2644 and Mil-I-25135 classify developers into six standard forms. Theseforms are listed below:

1. Form a - Dry Powder2. Form b - Water Soluble3. Form c - Water Suspendable4. Form d - Nonaqueous Type 1 Fluorescent (Solvent Based)5. Form e - Nonaqueous Type 2 Visible Dye (Solvent Based)6. 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 liquidcarrier. Each of the developer forms has advantages and disadvantages.

Dry Powder

Dry powder developer is generallyconsidered to be the least sensitive but itis inexpensive to use and easy to apply.Dry developers are white, fluffy powdersthat can be applied to a thoroughly drysurface in a number of ways. Thedeveloper can be applied by dipping partsin a container of developer, or by using a

puffer to dust parts with the developer.Parts can also be placed in a dust cabinetwhere the developer is blown around andallowed to settle on the part. Electrostaticpowder spray guns are also available toapply the developer. The goal is to allowthe developer to come in contact with thewhole inspection area.

Unless the part is electrostatically charged, the powder will only adhere to areas wheretrapped 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 bringsmore penetrant to the surface of the part where it can be seen. Since dry powderdevelopers only stick to the area where penetrant is present, the dry developer does notprovide a uniform white background as the other forms of developers do. Having auniform light background is very important for a visible inspection to be effective andsince 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 thepenetrant has a limited amount of room to spread.

Water Soluble


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As the name implies, water soluble developersconsist of a group of chemicals that are dissolved inwater and form a developer layer when the water isevaporated away. The best method for applying

water soluble developers is by spraying it on thepart. The part can be wet or dry. Dipping, pouring, orbrushing the solution on to the surface is sometimesused but these methods are less desirable. Aqueousdevelopers contain wetting agents that cause thesolution to function much like dilute hydrophilicemulsifier and can lead to additional removal ofentrapped penetrant. Drying is achieved by placing the wet but well drained part in arecirculating, warm air dryer with the temperature held between 70 and 75F. If the partsare not dried quickly, the indications will will be blurred and indistinct. Properlydeveloped parts will have an even, pale white coating over the entire surface.

Water Suspendable

Water suspendable developers consist of insolubledeveloper particles suspended in water. Watersuspendable developers require frequent stirring oragitation to keep the particles from settling out of

suspension. Water suspendable developers are applied toparts in the same manner as water soluble developers.Parts coated with a water suspendable developer must beforced dried just as parts coated with a water solubledeveloper are forced dried. The surface of a part coatedwith a water suspendable developer will have a slightlytranslucent white coating.

Nonaqueous

Nonaqueous developers suspend the developer in avolatile solvent and are typically applied with a spraygun. Nonaqueous developers are commonlydistributed in aerosol spray cans for portability. Thesolvent tends to pull penetrant from the indicationsby solvent action. Since the solvent is highly volatile,forced drying is not required. A nonaqueousdeveloper should be applied to a thoroughly driedpart to form a slightly translucent white coating.


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Special Applications

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

Preparation of Part

One of the most critical steps in the penetrant inspection process is preparing the partfor inspection. All coatings, such as paints, varnishes, plating, and heavy oxides mustbe removed to ensure that defects are open to the surface of the part. If the parts havebeen machined, sanded, or blasted prior to the penetrant inspection, it is possible that athin layer of metal may have smeared across the surface and closed off defects. It iseven possible for metal smearing to occur as a result of cleaning operations such as gritor vapor blasting. This layer of metal smearing must be removed before inspection.

Contaminants

Coatings, such as paint, are much more elastic thanmetal and will not fracture even though a large defectmay be present just below the coating. The part must bethoroughly cleaned as surface contaminates can preventthe penetrant from entering a defect. Surface

contaminants can also lead to a higher level ofbackground noise since the excess penetrant may bemore difficult to remove.

Common coatings and contaminates that must beremoved include: paint, dirt, flux, scale, varnish, oil,etchant, smut, plating, grease, oxide, wax, decals,machining fluid, rust, and residue from previous penetrantinspections.

Some of these contaminants would obviously prevent

penetrant from entering defects, so it is clear they mustbe removed. However, the impact of other contaminants such as the residue fromprevious penetrant inspections is less clear, but they can have a disastrous effect on theinspection. Take the link below to review some of the research that has been done toevaluate the effects of contaminants on LPI sensitivity.

A good cleaning procedure will remove all contamination from the part and not leaveany residue that may interfere with the inspection process. It has been found that somealkaline cleaners can be detrimental to the penetrant inspection process if they havesilicates in concentrations above 0.5 percent. Sodium metasilicate, sodium silicate, and


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related compounds can adhere to the surface of parts and form a coating that preventspenetrant entry into cracks. Researchers in Russia have also found that some domesticsoaps and commercial detergents can clog flaw cavities and reduce the wettability ofthe metal surface, thus reducing the sensitivity of the penetrant. Conrad and Caudill

found that media from plastic media blasting was partially responsible for loss of LPIindication strength. Microphotographs of cracks after plastic media blasting showedmedia entrapment in addition to metal smearing.

It is very important that the material being inspected has not been smeared across itsown surface during machining or cleaning operations. It is well recognized thatmachining, honing, lapping, hand sanding, hand scraping, grit blasting, tumbledeburring, and peening operations can cause some materials to smear. It is perhapsless recognized that some cleaning operations, such as steam cleaning, can also causemetal smearing in the softer materials.

Selection of a Penetrant Technique

The selection of a liquid penetrant system is not a straightforward task. There are avariety of penetrant systems and developer types that are available for use, and one setof penetrant materials will not work for all applications. Many factors must be consideredwhen selecting the penetrant materials for a particular application. These factors includethe sensitivity required, materials cost, number of parts, size of area requiringinspection, and portability.

When sensitivity is the primaryconsideration for choosing a penetrant

system, the first decision that must bemade is whether to use fluorescentpenetrant or visible dye penetrant.Fluorescent penetrants are generallymore capable of producing a detectableindication from a small defect. Also, thehuman eye is more sensitive to a lightindication on a dark background and theeye is naturally drawn to a fluorescentindication.

The graph below presents a series of curves that show the contrast ratio required for aspot of a certain diameter to be seen. The ordinate is the spot diameter, which wasviewed from one foot. The abscissa is the contrast ratio between the spot brightnessand the background brightness. To the left of the contrast ratio of one, the spot is darkerthan the background (representative of visible dye penetrant testing); and to the right ofone, the spot is brighter than the background (representative of fluorescent penetrantinspection). Each of the three curves right or left of the contrast ratio of one are fordifferent background brightness (in foot-Lamberts), but simply consider the generaltrend of each group of curves right or left of the contrast ratio of one. The curves showthat for indication larger than 0.076 mm (0.003 inch) in diameter, it does not really


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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 nolonger detectable even though contrast is increased. Furthermore, with a light indicationon 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 avisible penetrant for finding very small defects. Data presented by De Graaf and De Rijksupports this statement. They inspected "identical" fatigue cracked specimens using ared dye penetrant and a fluorescent dye penetrant. The fluorescent penetrant found 60defects while the visible dye was only able to find 39 of the defects.

Ref: De Graaf, E. and De Rijk, P., Comparison Between Reliability, Sensitivity, andAccuracy of Nondestructive Inspection Methods, 13th Symposium on NondestructiveEvaluation Proceedings, San Antonio, TX, published by NTIAC, Southwest Research

Institute, San Antonio, TX, April 1981, pp. 311-322.

Ref: Thomas, W.E., An Analytic Approach to Penetrant Performance, 1963 LesterHonor Lecture, Nondestructive Testing, Vol. 21, No. 6, Nov.-Dec. 1963, pp. 354-368.

Under certain conditions, the visible penetrant may be a better choice. When fairly largedefects are the subject of the inspection, a high sensitivity system may not be warrantedand may result in a large number of irrelevant indications. Visible dye penetrants havealso been found to give better results when surface roughness is high or when flaws arelocated in areas such as weldments.

Since visible dye penetrants do not require a darkened area for the use of an ultravioletlight, visible systems 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.


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However, they are usually not practical for large area inspection or in high-volumeproduction settings.

Another consideration in the selection of a penetrant system is whether water washable,

post-emulsifiable or solvent removable penetrants will be used. Post-emulsifiablesystems are designed to reduce the possibility of over-washing, which is one of thefactors known to reduce sensitivity. However, these systems add another step, and thuscost, 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 inlow cycle fatigue bending to classify penetrant systems. The brightness of the

indications produced after processing a set of specimens with a particular penetrantsystem is measured using a photometer. A procedure for producing and evaluating the

penetrant qualification specimens was reported on by Moore and Larson at the 1997ASNT Fall Conference. Most commercially available penetrant materials are listed in the

Qualified Products List of MIL-I-25135 according to their type, method and sensitivitylevel. Visible dye and dual-purpose penetrants are not classified into sensitivity levels asfluorescent penetrants are. The sensitivity of a visible dye penetrant is regarded as level

1 and largely dependent on obtaining good contrast between the indication and thebackground.

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. The method of penetrantapplication has little effect on the inspection sensitivity but an electrostatic sprayingmethod is reported to produce slightly better results than other methods. Once the partis covered in penetrant it must be allowed to dwell so the penetrant has time to enterany defect present.

There are basically two dwell mode options,immersion-dwell(keeping the part

immersed in thepenetrant during thedwell period) anddrain-dwell (letting thepart drain during thedwell period). Prior toa study by Sherwin,the immersion-dwellmode was generallyconsidered to be


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more sensitive but recognized to be less economical because more penetrant waswashed away and emulsifiers were contaminated more rapidly. The reasoning forthinking this method was more sensitive was that the penetrant was more migratory andmore likely to fill flaws when kept completely fluid and not allowed to lose volatile

constituents by evaporation. However, Sherwin showed that if the specimens areallowed to drain-dwell, the sensitivity is higher because the evaporation increases thedyestuff concentration of the penetrant on the specimen. As pointed-out in the sectionon penetrant materials, sensitivity increases as the dyestuff concentration increases.Sherwin also cautions that the samples being inspected should be placed outside thepenetrant tank wall so that vapors from the tank do not accumulate and dilute thedyestuff 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 necessaryto seep or be drawn into a defect. Dwell times are usually recommended by thepenetrant producers or required by the specification being followed. The time requiredto 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 viscosity of a penetrant in microcapillary flaws is higher thanits 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 toa particular application. For example, AMS 2647A requires that the dwell time for allaircraft and engine parts be at least 20 minutes, while ASTM E1209 only requires a fiveminute 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.

The following tables summarize the dwell time requirements of several commonly usedspecifications. The information provided below is intended for general reference and noguarantee is made about its correctness. Please consult the specifications for the actualdwell time requirements.


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Researchers found that the optimal dwell time varied with the type of defect anddeveloper used. The following table summarizes some of the findings.

Penetrant Removal Process

The penetrant removal procedure must effectivelyremove the penetrant from the surface of the partwithout removing an appreciable amount ofentrapped penetrant from the defect. If theremoval process extracts penetrant from the flaw,

the flaw indication will be reduced by aproportional amount. If the penetrant is noteffectively removed from the part surface, thecontrast between the indication and thebackground will be reduced. As discussed in theContrast Sensitivity Section, as the contrastincreases, so does visibility of the indication.

Removal Method

Penetrant systems are classified into four methods of excess penetrant removal. These

include the following:

1. Method A: Water-Washable2. Method B: Post-Emulsifiable, Lipophilic3. Method C: Solvent Removable4. 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 solventremover, and is, therefore, too labor intensive for most production situations. Of thethree production penetrant inspection methods, Method A, Water-Washable, is the most


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economical to apply. Water-washable or self-emulsifiable penetrants contain anemulsifier as an integral part of the formulation. The excess penetrant may be removedfrom the object surface with a simple water rinse. These materials have the property offorming 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, givenenough contact time, the plugs offer a brief period of protection against rapid washremoval. Thus, water-washable penetrant systems provide ease of use and a high levelof sensitivity.

When removal of the penetrant from the defect due to over-washing of the part is aconcern, a post-emulsifiable penetrant system can be used. Post-emulsifiablepenetrants require a separate emulsifier to breakdown the penetrant and make it waterwashable. The part is usually immersed in the emulsifier but hydrophilic emulsifiers mayalso be sprayed on the object. Spray application is not recommended for lipophilicemulsifiers 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 ofthe brush may force emulsifier into discontinuities, causing the entrapped penetrant tobe removed. The emulsifier is allowed sufficient time to react with the penetrant on thesurface of the part but not given time to make its way into defects to react with thetrapped penetrant. The penetrant that has reacted with the emulsifier is easily cleanedaway. Controlling the reaction time is of essential importance when using a post-emulsifiable system. If the emulsification time is too short, an excessive amount ofpenetrant will be left on the surface, leading to high background levels. If theemulsification time is too long, the emulsifier will react with the penetrant entrapped indiscontinuities, making it possible to deplete the amount needed to form an indication.

The hydrophilic post-emulsifiable method (Method D) is more sensitive than thelipophilic post-emulsifiable method (Method B). Since these methods are generally onlyused when very high sensitivity is needed, the hydrophilic method renders the lipophilicmethod virtually obsolete. The major advantage of hydrophilic emulsifiers is that theyare less sensitive to variation in the contact and removal time. While emulsification timeshould be controlled as closely as possible, a variation of one minute or more in thecontact time will have little effect on flaw detectability when a hydrophilic emulsifier isused. On the contrary, a variation of as little as 15 to 30 seconds can have a significanteffect when a lipophilic system is used. Using an emulsifier involves adding a couple ofsteps to the penetrant process, slightly increases the cost of an inspection. When usingan 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 layerof 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 todevelop, and 8. inspect.


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Rinse Method and Time for Water-Washable Penetrants

The method used to rinse the excess penetrant from the object surface and the time ofthe 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 beused. When a spray is being used, it should be directed at a 45 angle to the partsurface 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 inspectionsof the remaining background level.

Hand Wiping of Solvent Removable Penetrants

When a solvent removable penetrant is used, care must also be taken to carefullyremove the penetrant from the part surface while removing as little as possible from theflaw. 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 thatshould be used to remove as much penetrant as possible. Next, the surface should bewiped with one pass in one direction with a rag moistened with cleaner. One dry passfollowed by one damp pass is all that is recommended. Additional wiping maysometimes be necessary; but keep in mind that with every additional wipe, some of theentrapped penetrant will be removed and inspection sensitivity will be reduced.

To study the effects of the wiping process, Japanese researchers manufactured a testspecimen out of acrylic plates that allowed them to view the movement of the penetrantin a narrow cavity. The sample consisted of two pieces of acrylic with two thin sheets ofvinyl 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 betweenthe 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 clampingpoints was believed to be 30 mm (1.18 inch).

Although the size of the flaw represented by this specimen is large, an interestingobservation was made. They found that when the surface of the specimen was wipedwith a dry cloth, penetrant was blotted and removed from the flaw at the corner areaswhere the clearance between the plate was the least. When the penetrant at the sideareas was removed, penetrant moved horizontally from the center area to the ends ofthe 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 thesurface in the corners but depressed in the center. This shows that each time thecleaning cloth touches the edge of a crack, penetrant is lost from the defect. This alsoexplains why the bleedout of an indication is often largest at the corners of cracks.


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Use and Selection of a Developer

The use of developer is almost always recommended. One study reported that theoutput from a fluorescent penetrant could be multiplied by up to seven times when asuitable powder developer was used. Another study showed that the use of developercan have a dramatic effect on the probability of detection (POD) of an inspection. Whena Haynes Alloy 188, flat panel specimen with a low-cycle fatigue crack was inspectedwithout a developer, a 90 % POD was never reached with crack lengths as long as 19mm (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 theinspector identifying 277 of 311 cracks with no false-calls. However, some authors havereported that in special situations, the use of a developer may actually reducesensitivity. These situations primarily occur when large, well defined defects are beinginspected on a surface that contains many nonrelevant indications that cause excessivebleedout.

Type of Developer Used and Method of Application

Nonaqueous developers are generally recognized as the most sensitive when properlyapplied. There is less agreement on the performance of dry and aqueous wetdevelopers, but the aqueous developers are usually considered more sensitive.

Aqueous wet developers form a finer matrix of particles that is more in contact with thepart surface. However, if the thickness of the coating becomes too great, defects can bemasked. Also, aqueous wet developers can cause leaching and blurring of indicationswhen used with water-washable penetrants. The relative sensitivities of developers andapplication 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, butsome industry experts feel that water suspendable developers are more sensitive thanwater-soluble developers.

Sensitivity Ranking (highest to lowest) Developer Form Application Technique.

Ranking12345678910

Developer FormNonaqueous, Wet

SolventPlastic Film

Water-SolubleWater-Suspendable

Water-SolubleWater-Suspendable

DryDryDryDry

Method ofAppl ication

SpraySpraySpraySpray

ImmersionImmersionDust Cloud

(Electrostatic)Fluidized Bed

Dust Cloud (AirAgitation)

Immersion (Dip)


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

Developer Advantages Disadvantages

Dry

Indications tend to remainbrighter and more distinctover time

Easily to apply

Does not form contrastbackground so cannot beused with visible systems

Difficult to assure entirepart surface has beencoated

Soluble

Ease of coating entire part

White coating for goodcontrast can be producedwhich work well for bothvisible and fluorescentsystems

Coating is translucent andprovides poor contrast (notrecommended for visual

systems)

Indications for waterwashable systems are dimand blurred

Suspendable

Ease of coating entire part

Indications are bright andsharp

White coating for goodcontrast can be producedwhich work well for bothvisible and fluorescentsystems

Indications weaken andbecome diffused after time

Nonaqueous

Very portable

Easy to apply to readilyaccessible surfaces

White coating for good

contrast can be producedwhich work well for bothvisible and fluorescentsystems

Indications show-up rapidlyand are well defined

Provides highest sensitivity

Difficult to apply evenly to

all surfaces

More difficult to clean partafter inspection


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Process Control of Temperature

The temperature of the penetrant materials and the part beinginspected can have an effect on the results. Temperatures from27 to 49oC (80 to 120oF) are reported in the literature to produceoptimal results. Many specifications allow testing in the range of 4to 52oC (40 to 125oF). A tip to remember is that surfaces that canbe touched for an extended period of time without burning theskin are generally below 52oC (125oF).

Since the surface tension of most materials decrease as thetemperature increases, raising the temperature of the penetrantwill increase the wetting of the surface and the capillary forces. Of course, the converseis also true, so lowering the temperature will have a negative effect on the flowcharacteristics. Raising the temperature will also raise the speed of evaporation of

penetrants, which can have a positive or negative effect on sensitivity. The impact willbe positive if the evaporation serves to increase the dye concentration of the penetranttrapped in a flaw up to the concentration quenching point and not beyond. Highertemperatures and more rapid evaporation will have a negative effect if the dyeconcentration exceeds the concentration quenching point, or the flow characteristics arechanged to the point where the penetrant does not readily flow.

The method of processing a hot part was once commonly employed. Parts were eitherheated or processed hot off the production line. In its day, this served to increaseinspection sensitivity by increasing the viscosity of the penetrant. However, thepenetrant materials used today have 1/2 to 1/3 the viscosity of the penetrants on the

market in the 1960's and 1970's. Heating the part prior to inspection is no longernecessary and no longer recommended.

Quality Control of Penetrant

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

freezing and exposure to high heat. Freezing can cause separation to occur andexposure to high temperature for a long period of time can affect the brightness of thedyes.

Contamination can occur during storage and use. Of course, open tank systems aremuch more susceptible to contamination than are spray systems. Contamination byanother liquid will change the surface tension and contact angle of the solution. Wateris the most common contaminant. Water-washable penetrants have a definite tolerancelimit for water, and above this limit they do not function properly. Cloudiness andviscosity both increase with increasing water content. In self-emulsifiable penetrants,


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water contamination can produce a gel break or emulsion inversion when the waterconcentration becomes high enough. The formation of the gel is an important featureduring the washing processes, but must be avoided until that stage in the process. Dataindicates that the water contamination must be significant (greater than 10%) for gel

formation to occur. Most specifications limit water contamination to around 5% to beconservative. Water does not readily mix with the oily solution of lipophilic post-emulsifiable systems and it generally settles to the bottom of the tank. However, theinspection of parts that travel to the bottom of the tank and encounter the water could beadversely affected.

Most other common contaminates, such as cleaning solvents, oils, acids, caustics andchromates, must be present in significant quantities to affect the performance of thepenetrant. Organic contaminants can dilute the dye and absorb the ultraviolet radiationbefore it reaches the dye, and also change the viscosity. Acids, caustics, andchromates cause the loss of fluorescence in water-soluble penetrants.

Regular checks must be performed to ensure that the material performance has notdegraded. When the penetrant is first received from the manufacturer, a sample of thefresh solution should be collected and stored as a standard for future comparison. Thestandard 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 todetect changes in color, odor and consistency. When using fluorescent penetrants, abrightness comparison per the requirements of ASTM E 1417 is also often required.This check involves placing a drop of the standard and the in-use penetrants on a pieceof Whatman #4 filter paper and making a side by side comparison of the brightness ofthe two spots under UV light.

Additionally, the water content of water washable penetrants must be checkedregularly. Water-based, water washable penetrants are checked with a refractometer.The rejection criteria is different for different penetrants, so the requirements of thequalifying specification or the manufacturer's instructions must be consulted. Non-water-based, water washable penetrants are checked using the procedure specified in

ASTM D95 or ASTM E 1417.

Appl ication of the Penetrant

The application of the penetrant is the step of the process that requires the least amount

of control. As long as the surface being inspected receives a generous coating ofpenetrant, it really doesn't matter how the penetrant is applied. Generally, theapplication method is an economic or convenience decision.

It is important that the part be thoroughly cleaned and dried. Any contaminates ormoisture on the surface of the part or within a flaw can prevent the penetrant materialfrom entering the defect. The part should also be cool to the touch. The recommendedrange of temperature is 4 to 52oC (39 to 125oF).


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Quality Control of Wash Temperature and Pressure

Thewash temperature, pressure and time are three parameters that are typicallycontrolled in penetrant inspection process specification. A coarse spray or an immersionwash tank with air agitation is often used. When the spray method is used, the waterpressure is usually limited to 276 kN/m2 (40 psi). The temperature range of the water isusually specified as a wide range (e.g.. 10 to 38oC (50 to 100oF) in AMS 2647A.) A low-pressure, coarse water spray will force less water into flaws to dilute and/or removetrapped penetrant and weaken the indication. The temperature will have an effect on thesurface tension of the water and warmer water will have more wetting action than coldwater. Warmer water temperatures may also make emulsifiers and detergent moreeffective. The wash time should only be as long as necessary to decrease thebackground to an acceptable level. Frequent visual checks of the part should be madeto determine when the part has be adequately rinsed.

Quality Control of Drying Process

The temperature used to dry parts after the application of an aqueous wet developer orprior to the application of a dry powder or a nonaqueous wet developer, must becontrolled to prevent "cooking" of the penetrant in the defect. High drying temperaturecan affect penetrants in a couple of ways. First, some penetrants can fade at hightemperatures due to dye vaporization or sublimation. Second, high temperatures cancause the penetrant to dry in the the flaw, preventing it from migrating to the surface toproduce an indication. To prevent harming the penetrant material, drying temperatureshould be kept to under 71oC.

The drying should be limited to the minimum length of time necessary to thoroughly drythe component being inspected.

Quality Control of Developer

The function of the developer is very important in a penetrant inspection. It must drawout of the discontinuity a sufficient amount of penetrant to form an indication, and it mustspread the penetrant out on the surface to produce a visible indication. In a fluorescent

penetrant inspection, the amount of penetrant brought to the surface must exceed thedye's thin film threshold of fluorescence, or the indication will not fluoresce. Additionally,the developer makes fluorescent indications appear brighter than indications producedwith the same amount of dye but without the developer.

In order to accomplish these functions, a developer must adhere to the part surface andresult in a uniform, highly porous layer with many paths for the penetrant to be moveddue to capillary action. Developers are either applied wet or dry, but the desired endresult is always a uniform, highly porous, surface layer. Since the quality controlrequirements for each of the developer types is slightly different, they will be coveredindividually.


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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 fromprevious inspection. This check is performed by spreading a sample of the developerout and examining it under UV light. If there are ten or more fluorescent specks in a 10cm diameter area, the batch should be discarded.

Apply a light coat of the developer by immersing the test component or dusting thesurface. After the development time, excessive powder can be removed by gentlyblowing on the surface with air not exceeding 35 kPa or 5 psi.

Wet Soluble/Suspendable Developer

Wet soluble developer must be completely dissolved in the water and wet suspendabledeveloper must be thoroughly mixed prior to application. The concentration of powder inthe carrier solution must be controlled in these developers. The concentration should bechecked at least weekly using a hydrometer to make sure it meets the manufacturer'sspecification. To check for contamination, the solution should be examined weekly usingboth white light and UV light. If a scum is present or the solution fluoresces, it should bereplaced. Some specifications require that a clean aluminum panel be dipped in thedeveloper, dried, and examined for indications of contamination by fluorescentpenetrant materials.

These developers are applied immediately after the final wash. A uniform coating

should be applied by spraying, flowing or immersing the component. They should neverbe applied with a brush. Care should be taken to avoid a heavy accumulation of thedeveloper solution in crevices and recesses. Prolonged contact of the component withthe developer solution should be avoided in order to minimize dilution or removal of thepenetrant from discontinuities.

Solvent Suspendable (AKA Nonaqueous Wet)

Solvent suspendable developers are typically supplied in an sealed aerosol spray can.Since the developer solution is in a sealed vessel, direct check of the solution is notpossible. 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. Makesure the can is well shaken and apply a thin coating to a test article. If the sprayproduces spatters or an uneven coating, the can should be discarded.

When applying a solvent suspendable developer, it is up to the inspector to control thethickness of the coating. with a visible penetrant system, the developer coating must bethick enough to provide a white contrasting background but not heavy enough to maskindications. When using a fluorescent penetrant system, a very light coating should beused. The developer should be applied under white light and should appear evenlytransparent.


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Development Time

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

Quality Control of Lighting

After a component has been properly processed, itis ready for inspection. While automated visioninspection systems are sometimes used, the focushere will be on inspections performed visually by ahuman inspector, as this is the dominant method.Proper lighting is of great importance when visually

inspecting a surface for a penetrant indication.Obviously, the lighting requirements are different foran inspection conducted using a visible dyepenetrant than they are for an inspection conductedusing a fluorescent dye penetrant. The lightingrequirements for each of these techniques, as wellas how light measurements are made, are discussed below.

Light ing 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. When usingnatural lighting, it is important to keep in mind that daylight varies from hour to hour, soinspectors must stay constantly aware of the lighting conditions and make adjustmentswhen needed. To improve uniformity in lighting from one inspection to the next, the useof artificial lighting is recommended. Artificial lighting should be white whenever possibleand white flood or halogen lamps are most commonly used. The light intensity isrequired to be 100 foot-candles at the surface being inspected. It is advisable to choosea white light wattage that will provide sufficient light, but avoid excessive reflected lightthat could distract from the inspection.

Light ing for Fluorescent Penetrant Inspections

When a fluorescent penetrant is being employed, the ultraviolet (UV) illumination andthe visible light inside the inspection booth is important. Penetrant dyes are excited byUV light of 365nm wavelength and emit visible light somewhere in the green-yellowrange between 520 and 580nm. The source of ultraviolet light is often a mercury arclamp with a filter. The lamps emit many wavelengths and a filter is used to remove allbut the UV and a small amount of visible light between 310 and 410nm. Visible light ofwavelengths above 410nm interferes with contrast, and UV emissions below 310nminclude some hazardous wavelengths.


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Standards and procedures require verification of lens condition and light intensity. Blacklights should never be used with a cracked filter as output of white light and harmfulblack light will be increased. The cleanliness of the filter should also be checked as acoating of solvent carrier, oils, or other foreign materials can reduce the intensity by up

to as much as 50%. The filter should be checked visually and cleaned as necessarybefore warm-up of the light.

Since fluorescent brightness is linear with respect to ultraviolet excitation, a change inthe intensity of the light (from age or damage) and a change in the distance of the lightsource from the surface being inspected will have a direct impact on the inspection. ForUV lights used in component evaluations, the normally accepted intensity is 1000microwatt per square centimeter when measured at 15 inches from the filter face(requirements can vary from 800 to 1200 W/cm2). The required check should beperformed when a new bulb is installed, at startup of the inspection cycle, if a change inintensity is noticed, or every eight hours of continuous use. Regularly checking the

intensity of UV lights is very important because bulbs lose intensity over time. In fact, abulb that is near the end of its operating life will often have an intensity of only 25% of itsoriginal output.

Black light intensity will also be affected by voltage variations. A bulb that producesacceptable intensity at 120 volts will produce significantly less at 110 volts. For thisreason it is important to provide constant voltage to the light. Also, most UV light mustbe warmed up prior to use and should be on for at least 15 minutes before beginning aninspection.

When performing a fluorescent penetrant inspection, it is important to keep white light to

a minimum as it will significantly reduce the inspectors ability to detect fluorescentindications. Light levels of less than 2 fc are required by most procedures with someprocedures requiring less than 0.5 fc at the inspection surface. Procedures require acheck and documentation of ambient white light in the inspection area. When checkingblack light intensity at 15 inches a reading of the white light produced by the black lightmay be required to verify white light is being removed by the filter.

Light Measurement

Light intensity measurements are made using a radiometer. A radiometer is aninstrument that translate light energy into an electrical current. Light striking a silicon

photodiode detector causes a charge to build up between internal layers. When anexternal circuit isconnected to the cell, an electrical current is produced. This current is linear withrespect to incident light. Some radiometers have the ability to measure both black andwhite light, while others require a separate sensor for each measurement. Whichevertype is used, the sensing area should be clean and free of any materials that couldreduce or obstruct light reaching the sensor. Radiometers are relatively unstableinstruments and readings often change considerable over time. Therefore, they shouldbe calibrated at least every six months.


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Ultraviolet light measurements should be taken using a fixture to maintain a minimumdistance of 15 inches from the filter face to the sensor. The sensor should be centeredin the light field to obtain and record the highest reading. UV spot lights are oftenfocused, so intensity readings will vary considerable over a small area. White lights are

seldom focused and depending on the wattage, will often produce in excess of the 100fc at 15 inches. Many specifications do not require the white light intensity check to beconducted at a specific distance.

System Performance Check

System performance checks involve processing a test specimen with known defects todetermine if the process will reveal discontinuities of the size required. The specimenmust be processed following the same procedure usedto process production parts. A system performancecheck is typically required daily, at the reactivation of a

system after maintenance or repairs, or any time thesystem is suspected of being out of control. As withpenetrant inspections in general, results are directlydependent on the skill of the operator and, therefore,each operator should process a panel.

The ideal specimen is a production item that has naturaldefects of the minimum acceptable size. Somespecification delineate the type and size of the defectsthat must be present in the specimen and detected.Surface finish is will affect washability so the check

specimen should have the same surface finish as theproduction parts being processed. If penetrant systemswith different sensitivity levels are being used, thereshould be a separate specimen for each system.

There are some universal test specimens that can beused if a standard part is not available. The most commonly used test specimen is theTAM or PSM panel. These panels are usually made of stainless steel that has beenchrome plated on one half and surfaced finished on the other half to produced thedesired roughness. The chrome plated section is impacted from the back side toproduce 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 variancesin that signature are indications of process variance. Panel patterns as well asbrightness are indicators of process consistency or variance.

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


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Nature of the Defect

The nature of the defect can have a large affect on sensitivity of a liquid penetrantinspection. Sensitivity is defined as the smallest defect that can be detected with a highdegree of reliability. Typically, the crack length at the sample surface is used to definesize of the defect. A survey of any probability-of-detection curve for penetrant inspectionwill quickly lead one to the conclusion that crack length has a definite affect onsensitivity. However, the crack length alone does not determine whether a flaw will beseen or go undetected. The volume of the defect is likely to be the more importantfeature. The flaw must be of sufficient volume so that enough penetrant will bleed backout to a size that is detectable by the eye or that will satisfy the dimensional thresholdsof fluorescence.

Above is an example of fluorescent penetrant inspection probability of detection (POD)curve from the Nondestructive Evaluation (NDE) Capabilities Data Book. Please notethat this curve is specific to one set of inspection conditions and should not beinterpreted to apply to other inspection situations.

In general, penetrant inspections are more effective at finding

small round defects than small linear defects.Small round defects aregenerally easier to detect for several reasons. First, they are typically volumetricdefects that can trap significant amounts of penetrant. Second, round defects fillwith penetrant faster than linear defects. One research effort found that elliptical


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flaw with length to width ratio of 100, will take the penetrant nearly 10 timeslonger to fill than a cylindrical flaw with the same volume.

deeper flaws than shallow flaws.Deeper flaws will trap more penetrant thanshallow flaws, and they are less prone to over washing.

flaws with a narrow opening at the surface than wide open flaws.Flaws withnarrow surface openings are less prone to over washing.

flaws on smooth surfaces than on rough surfaces.The surface roughness ofthe part primarily affects the removability of a penetrant. Rough surfaces tend totrap more penetrant in the various tool marks, scratches, and pits that make upthe surface. Removing the penetrant from the surface of the part is more difficultand a higher level of background fluorescence or over washing may occur.

flaws with rough fracture surfaces than smooth fracture surfaces.Thesurface roughness that the fracture faces is a factor in the speed at which apenetrant enters a defect. In general, the penetrant spreads faster over a surfaceas the surface roughness increases. It should be noted that a particular penetrant

may spread slower than others on a smooth surface but faster than the rest on arougher surface.

flaws under tensile or no loading than flaws under compression loading.Ina 1987 study at the University College London, the effect of crack closure ondetectability was evaluated. Researchers used a four-point bend fixture to placetension and compression loads on specimens that were fabricated to containfatigue cracks. All cracks were detected with no load and with tensile loadsplaced on the parts. However, as compressive loads were placed on the parts,the crack length steadily decreased as load increased until a load was reachedwhen the crack was no longer detectable.

Health and Safety Precautions in Liquid Penetrant Inspection

When proper health and safety precautions are followed, liquid penetrant inspectionoperations can be completed without harm to inspection personnel. However, there area number of health and safety related issues that must be addressed. Since eachinspection operation will have its own unique set of health and safety concerns thatmust be addressed, only a few of the most common concerns will be discussed here.

Chemical Safety

Whenever chemicals must be handled, certain precautions must be taken as directedby the material safety data sheets (MSDS) for the chemicals. Before working with achemical of any kind, it is highly recommended that the MSDS be reviewed so thatproper chemical safety and hygiene practices can be followed. Some of the penetrantmaterials 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. Eyeprotection should always be worn to prevent contact of the chemicals with the eyes.Many of the chemicals used contain detergents and solvents that can dermatitis. Glovesand other protective clothing should be worn to limit contact with the chemicals.


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Ultraviolet Light Safety

Ultraviolet (UV) light or "black light" as it is sometimes called, has wavelengths rangingfrom 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 sourceof UV radiation is the the sun and is necessary in small doses for certain chemicalprocesses to occur in the body. However, too much exposure can be harmful to the skinand eyes. Excessive UV light exposure can cause painful sunburn, accelerate wrinklingand increase the risk of skin cancer. UV

Penetrant Testing Level 2 Notes

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