pt05

Charles W. Eick, Dassault Falcon Jet, Little Rock,Arkansas

Art Cedillos, Palomar Plating Company, Escondido,California

Donald J. Hagemaier, Boeing Company, Long Beach,California

J. Thomas Schmidt, J.T. Schmidt Associates, CrystalLake, Illinois

5C H A P T E R

Interpretation of LiquidPenetrant Indications

Interpreting Results ofLiquid Penetrant TestsAll nondestructive testing methods,including liquid penetrant testing,produce indirect indications of conditionsbut do not by themselves tell exactly whatthese material conditions are. The testindications must be correctly interpretedbefore they give any useful information asto the actual conditions that exist in amaterial.

Definitions of LiquidPenetrant Interpretationand EvaluationThere is often a tendency on the part ofan inspector to confuse the termsinterpretation and evaluation and to fail torecognize that there is a significantdistinction between them. Actually, theterms refer to two entirely different stepsin the test process and require entirelydifferent categories of knowledge andexperience. To interpret an indicationmeans to give a decision as to whatmaterial condition is causing theindication. There may be many possibleconditions: cracks, porosity, lack of resultsfrom the assembly of parts, such as aforced fit. Evaluation then followsinterpretation. If it has been decided thata crack of given size and direction exists,the extent of its effect on the usefulnessof the part must be evaluated before thepart can be disposed of: either accepted asis, reworked or scrapped.

Experience Needed forLiquid PenetrantInterpretationTo interpret liquid penetrant indicationscorrectly, the inspector must first of all bethoroughly familiar with the test methodin use. The inspector must know thatliquid penetrant processing has beencorrect. In the case of fluorescent liquidpenetrants, for example, the inspectormust be certain that washing has beenthorough, so that the indication the

inspector sees can be accepted as that of abona fide discontinuity. The inspectormust further be able to derive all possiblesignificant information from theappearance of the indication itself. Fromthis, the inspector may be able to declarethat a crack or porosity or somenonrelevant condition is present.

The task, however, becomes easier andthe declaration more authoritative if theinspector knows the background of thepart being tested. The inspector shouldknow the material, the process by whichit is made and the various processes it hasbeen through. The inspector should knowthe kinds of discontinuities characteristicof the material and should be aware ofwhat discontinuities various processes arelikely to introduce. The inspector shouldknow enough about failure of parts inservice to know where fatigue cracks aremost likely to occur. In addition, ofcourse, the inspector must be familiar,from past experience, with the appearanceof indications of these discontinuities insimilar parts or materials.

Experience Needed forEvaluation of LiquidPenetrant IndicationsOnce it is known that a crack or porosityor other discontinuity of given size ordirection is present, a decision as to thedisposition of the test parts depends onan evaluation of the significance of thediscontinuity to the serviceability of thepart. This is a question of stress and stressdistribution or environmentalrequirements and calls for the knowledgeof the designer and engineer responsiblefor the performance of the part. For agiven service, one direction ofdiscontinuity may be harmless but thesame discontinuity that may be at rightangles to the tensile stress may be highlyobjectionable. In the same way, adiscontinuity in an area of low stress canperhaps be tolerated but a similardiscontinuity in an area of high stresswould cause rejection of the part. Theusual way of expressing this idea is toevaluate a discontinuity as a defect onlywhen it will interfere with the usefulnessof the part in service.

126 Liquid Penetrant Testing

PART 1. Inspector Functions and Terminologyfor Interpretation and Evaluation of LiquidPenetrant Test Indications

Importance of theInspector’s EvaluationBecause correct liquid penetrantevaluation depends on accurateinterpretation, the inspector is the keyperson in the test process. In some massinspection operations, the person whooperates the test equipment merelysegregates the parts that show indications.Others decide their disposition. In mostcases, however, the inspector who firstsees the indication is also expected tointerpret it. Actually, if the inspector hasany of the qualifying background, he orshe is the one best able to do this, becausethe inspector is most familiar with allkinds of indications as they occur on theparts the inspector handles. Also, theinspector is best able, because he or sheperformed the tests, to know that theprocess has been properly carried out orto know and assess any variations fromnormal behavior during the test. Theinspector therefore must be thoroughlyhonest and dependable. The inspectormust be fully alert to the importance ofthe work he or she is doing.

Discontinuity detection with liquidpenetrants is more art than science. Thisis necessarily true unless or until somesuccessful means is evolved that canautomatically see and accurately interpretliquid penetrant indications. In themeantime, the inspector must be reliedon to perform these functions. Skill andproficiency in developing and theninterpreting indications must be acquiredby experience. A skilled and experiencedinspector can add greatly to the inherenteffectiveness of liquid penetrant testing.

Assuming that the inspector knowshow to operate the instruments and carryout the techniques, it is almost selfevident that the inspector cannot do acompetent job unless he or she alsoknows the materials under test. Theinspector must know what they are, howthey are made and how they have beenprocessed. This knowledge must includean understanding of what discontinuitiesmay be introduced during manufactureand processing. If the inspector knowswhat discontinuities to look for andwhere they are likely to occur, theinspector can select the techniquesavailable for their detection much moreeffectively.

General Classification ofDiscontinuities by TheirOriginationDiscontinuities are most commonlyclassified on the basis of the progress of

material from its raw state to finishedform and on into its service life. Thisclassification is as follows.

1. Inherent discontinuities arediscontinuities introduced into thebasic materials as the result of itsinitial production from ore to rawcomponents up to the point where itis ready for initial processing.

2. Processing discontinuities fall readilyinto two groups: (a) discontinuitiesintroduced during primary processessuch as casting, rolling, forging,drawing, welding etc. and (b)discontinuities introduced duringfinishing processing, such asmachining, heat treating, grinding,plating etc.

3. Service discontinuities are thosediscontinuities that develop as theresult of the material or part being inservice. This group includes fatigue,corrosion and damage from impact,overloads and environment.

Testing in Primary MetalsProductionBy far the largest field of application ofnondestructive tests is in the metalmaking industry, including ferrous alloysand the growing list of nonferrous metalsand alloys now being produced and used.Time and space do not permit discussionof all the types of discontinuities that arespecial to many of these materials orconsideration of the many specialprocesses used to work them and the testproblems they present. However, for theusual metals and alloys, there is aconsiderable similarity both of processesand discontinuities that are encountered.Describing the production of basic metalsand pointing out the sources ofdiscontinuities covers, by analogy exceptfor detail, the discontinuities found inmany of the common metallic materialsas well. The problems of nondestructivetesting are still overwhelmingly concernedwith the more common metals.

Methods for Producing Metalsand AlloysIron and the nonferrous metals arerefined from their ores by a variety ofprocesses but most of them are ultimatelyproduced in molten form and the moltenmetal is poured into molds and allowed tosolidify. These molds may be ingot moldsfrom which the solidified ingot is takenthrough a variety of forming processes orthey may be molds for casting the metalinto final shape, with little or no physicaldeformation to be made on the productsubsequently.

127Interpretation of Liquid Penetrant Indications

Sources of Impurities in MoltenMetalAll care is taken and elaborate techniqueshave been devised to ensure that themetal be as free as possible fromunwanted foreign materials when itsolidifies. Inevitably, however, the metaldoes contain amounts of (1) dissolved orentrapped gasses; (2) various nonmetallicmaterials such as oxides and sulfides ofthe metal; and (3) finely dispersedparticles of slag. To some extent thebehavior of the metal during cooling canbe controlled by size and shape of themold, the technique of pouring and theaddition of suitable materials to assist inthe removal of gasses.

Sources of Discontinuities Relatedto Metal SolidificationIn spite of all precautions to produce andsolidify metal sound and free of foreignmaterials and gasses, the solidifiedproduct invariably contains a greater orless quantity of such nonmetallicmaterials. Most metals containnonmetallic inclusions of one sort oranother. Their presence is not in itself anindication of poor quality; this is true ofsteel, copper, brass, aluminum and othermetals. However, if not well distributed orif the segregates are large, they may bevery objectionable. They will appear asstringers or discontinuities in the finalrolled or forged product, which mayseriously affect the suitability of the metalfor many purposes.

Discontinuities in Metal IngotsIn solidifying and cooling, the metalshrinks. This shrinkage may cause shrinkcavities in the center of the ingot, usuallymostly confined to the top orlast-to-freeze portion. This section of theingot is normally cropped and discarded.If it is not completely eliminated, suchinternal discontinuities may show up inthe finished product as pipe in rolled barsor as laminations in plate or strip.

Cracking also may occur on the surfaceof ingots. These cracks are removed ascompletely as possible by chipping orscarfing so that they will not show up asseams on the finished product.

Sources of Discontinuities in MetalCastingsIn the case of castings, where the metalsolidifies in final form, great care is takento place gates and risers by which surplusmetal is provided at favorable locations sothat impurities can float out of the castingproper and gas can have a chance toescape. Porosity and gas pockets alongwith slag inclusions are still a major

source of discontinuities in castings andmuch testing focuses on the location ofthese undesirable conditions (Figs. 1and 2). This is true of nonferrous castingsas well as steel, and special types ofporosity are related to individual metalsand casting processes, as for instancesurface porosity in die cast magnesiumalloys.

Shrinkage on cooling results in thermalcracks in castings, most often on thesurface and related to the shape of thecasting but sometimes internal (see Figs. 3and 4). Other casting discontinuities arehot tears and cold shuts, both beingsurface discontinuities.


FIGURE 1. Fluorescent liquid penetrant indications of porosityand shrinkage cracking in cast light alloy aircraft fittings.

FIGURE 2. Fluorescent liquid penetrant indications of porosityin light alloy aircraft casting.

MOVIE.Fluorescentbleedoutrevealsshrinkage.

Sources of Discontinuities inBlooms and BilletsWhen cast into ingots, the metal is thenfurther worked down into usable forms byforging, rolling or pressing. The steelingot, for example, is reheated to theproper temperature and passed betweenheavy rolls to form blooms or billets. Thesize of ingots varies widely, depending onthe ultimate product to be made fromthem. Ingots of alloy and tool steel maybe quite small, weighing perhaps 100 kg(220 lbm), whereas ingots from whichlarge forgings or large thick plate are to bemade may weigh many tons. Blooms andbillets are formed in an intermediate stepbetween the ingot and the finished size orproduct.

The surface of blooms and billets maycontain seams due to rolled out ingotcracks resulting from cooling stresses orbursts or tears or rolled-in scale or metalparticles. All of these surfaceimperfections should be removed beforethe final finishing operation. Unless theyare completely removed, the end productwill contain seams or other surfacediscontinuities that would beobjectionable. The surfaces of blooms andbillets may be freed of such objectionableconditions by chipping, grinding or flamescarfing. Billets, especially when intendedfor seamless tube piercing, are oftencleaned up by scaling, i.e., by taking a cutoff the surface in a lathe if the billet isround or by planing the surface if thebillet is square.

Sources of Discontinuities in HotRolled Bars, Shapes, Plate or StripThe blooms or billets are often reheated toproper working temperature and rolledinto bars, shapes, plate or strip. As hasbeen pointed out, seams, stringers andlaminations may appear in the finishedproduct as a result of rolling, forging orextrusion. The rolling operation itself mayintroduce certain discontinuities, ofwhich the most common are lapsresulting from too much or too littlemetal to fill the rolls. Too rapid reductionof cross section, especially if the metal isat too low a temperature, may producetears and internal rupture called cupping.

Internal shrinkage cavities from theingot or gas pockets not completelywelded shut in the rolling operation mayresult in laminations in plate or strip or inpipe, bars or other shapes. Unevencooling of some types of steel may causecooling cracks to appear in bars andshapes — a type of damage that may bevery severe.

Sources of Discontinuities inForgingsIf the ingot or billet is used for forging, avariety of discontinuities can be produced,usually because of improper handling ofthe metal under the hammer or toworking at too low a temperature.Common discontinuities are laps andfolds. Forging bursts, both external andinternal, can also occur. Figure 5 shows aforging lap.

In the case of die or drop forging, lapsand surface tears and bursts may beproduced. In die forgings, excess metal isforced out between the two halves of thedie forming the flash and this excessmetal is subsequently trimmed off. If notproperly done, flashline cracks or tearsmay be formed. Another type of forgingdiscontinuity is the internal rupture


FIGURE 3. Fluorescent liquid penetrant indication of crack atattachment lug in cast light alloy aircraft part.

FIGURE 4. Thermal cracks and shrinkage in cast motorhousings.

usually called flake, which occurs in theprocess of cooling relatively large forgings.These may be very severe and extremelyobjectionable.

Sources of Discontinuities inRolled and Pierced ProductsIn the production of seamless tubing, thebillet is pierced. Discontinuities on theinside surface of the pierced billet maytake the form of tears. Seams may occuron the outside surface, especially if thesurface of the billet or blank had not beenproperly cleaned.

Sources of Discontinuities inExtrusionsNonferrous seamless tubing is often madeby extrusion, starting with hollow orpierced billets or from solid ingots.Discontinuities that occur may be checksand tears or imperfections may resultfrom uncovering or bringing to thesurface internal discontinuities of thebillets or ingots. In addition to tubing,many shapes and products are madetoday by various extrusion processes.Because of flow of the metal of theoriginal ingot during extrusion, oxide andsurface skin sometimes becomes foldedinto the interior. A form of pipe alsoresults from this action, which is mostobjectionable when it appears in afinished shape.

Sources of Discontinuities in ColdWorked Metal ProductsCold rolling and cold drawing of strip, rodand wire is a process for improving thesurface finish of steel products. Close

dimensional tolerances can thus bemaintained and the metal becomessomewhat work hardened. Die marks,scratches and surface imperfectionssometimes make their appearance in thisprocess. Of course, the process does notremove any seams or other discontinuitiespresent in the material before the coldfinishing is applied.

Sources of Discontinuities inFusion WeldmentsWelding is a process that has its owngroup of typical discontinuities that mustbe guarded against. In fusion weldingprocesses, where liquid metal solidifies inthe weld, the same sort of discontinuitiesmay occur as in castings — namely, slaginclusions, gas porosity and thermalcracks. In addition, cracking in the parentmetal may occur because of thermalstresses or as a consequence of hydrogenpressure. There are also discontinuitiesdue to lack of penetration of the weld andfailure to get proper fusion of the weldmetal to the parent metal.

Sources of Discontinuities in HeatTreating ProcessesLooking now at the various finishingprocesses, most metal products undergosome form of heat treatment to producedesirable physical properties. In the courseof heat treatment, discontinuities may beproduced, most often as the result ofwarping or cracking. Quenching cracksoccur in steel when the process is notproperly carried out or when the design ofthe part or the steel used is not adapted tothe operation. Quenching cracks are aptto appear in connection with changes incross section of the material that causeexcessive cooling stresses or at locationswhere the contour of the surface permitsstress concentrations to occur. Sometimeshardened parts are cracked when anattempt is made to straighten those thathave warped slightly out of shape.

Special nondestructive test techniqueshave been devised for checking hardnessagainst specification tolerances and forsuch unwanted conditions as abnormalgrain size, segregations etc.

Sources of Discontinuities inMachined and Ground SurfacesSometimes machining operations, as forinstance too heavy a cut with impropertools, may cause surface damage, leavingdiscontinuities in the form of surfacetears. Grinding of surfaces for accuratedimensions or for finish is a prolificsource of surface cracking, especially onhardened steel surfaces. Parts that havebeen hardened may have residual internal


FIGURE 5. Fluorescent liquid penetrant indication of forginglap in aircraft turbine bucket (shown on as-forged surfacewhich has not been machined).

MOVIE.Quenchingcracks.

stresses, produced in the quenchingprocess and not subsequently removed bydrawing or stress relieving. Such parts areparticularly sensitive to the formation ofgrinding checks.

Sources of Cracks during Platingor Chemical TreatmentParts containing residual stresses fromheat treatment or perhaps simply fromcold working may crack during platingprocesses or during the pickling thatprecedes plating. This is because thepickling etch removes some of the metalsurface containing compressive stresses,permitting the internal tensile stress to berelieved through cracking. Embrittlementby absorbed hydrogen during picklingmay sometimes be a factor in suchcracking.

Sources of Fatigue Cracks Incurredduring Service OperationThe last group of discontinuities are thosethat occur after the part or material hasbeen placed in service. Of this group,fatigue cracking and failure resulting fromfatigue is by far the most importantgroup. Fatigue occurs during repeated orcyclic loading or stressing of materials (seeFigs. 6 and 7).

A fatigue failure starts with a crackinitiated by a variety of causes. Undercyclic loads, this crack progresses throughthe cross section of the metal until only asmall portion of the original sectionremains sound. Finally failures occur evenwhen calculated or average stresses are farbelow the elastic limit and do not dependdirectly on whether the tensile strength ofthe metal, as determined by static tests, iseither high or low.

Significance of SurfaceDiscontinuities under RepeatedLoading or VibrationFatigue cracks almost invariably start atthe surface and are initiated by conditionsthat bring about locally high stresses.These stress concentrations involvestresses above average for the part. Thestresses may, in their local area, be abovethe fatigue strength of the metal. Surfacescratches, cracks or other discontinuitiesmay cause these local areas of high stress(Fig. 7). Localized high stresses may alsobe caused by design features of the part —for example, high stresses occur aroundholes, fillets and stiffening members.

Prevention of Fatigue Failures inServiceOne of the main purposes of liquidpenetrant nondestructive testing is tolocate and eliminate discontinuities likelyto lead to fatigue failures. However,fatigue cracking still occurs and is aserious indication sought bynondestructive techniques when the


FIGURE 6. Fluorescent liquid penetrant indications of fatiguecracks in aircraft wheel.

FIGURE 7. Example of working standard and interpretationguide for 356-T6 aluminum alloy casting with line ofporosity that will propagate into a fatigue crack and causefailure. This condition is in stressed area, so fan castingshould be rejected.

machine or structure is inspected atintervals during its service life.Fortunately, fatigue cracks do not oftenpropagate so rapidly that they cannot belocated early in their existence and thepart replaced before failure.

Sources of Failures Related toCorrosionIf a part that is subject to frequent stressreversals or fluctuations is at the sametime subject to corrosion, fatigue cracksmay appear very rapidly and may progressto failure in a very short period of time.

Corrosion leads to another type ofservice failure through cracking of metalsubject to corrosion at the same time it isunder tensile stress. Stress corrosioncracking of steel beams around bolt holes,leading to bridge collapse, is an example.Such cracking is analogous to cracks inthe surface of heat treated or cold workedparts produced in pickling before plating,where residual tensile stresses are left fromsome previous operation. The corrosionallows the tension stress at the surface tobe relieved by cracking (Fig. 8).

Other Causes of Service DamageOther sources of service damage causingcracking that can be located bynondestructive techniques are abnormallyhigh stresses caused by vibration, as insteam turbine blades and jet enginebuckets; impact due to wrecks and othersudden stoppages; and overloads due to

abnormal service conditions or simpleabuse of the engine or machine.

On the subject of sources ofdiscontinuities in all kinds of materials onwhich nondestructive testing techniquesare commonly used, books have beenwritten that give useful references forthose in the field of nondestructivetesting.1-3 The material in the presentchapter gives merely a general idea ofhow many typical and often encountereddiscontinuities are produced and toprovide background for later discussionsof nondestructive testing techniquesdesigned to locate such discontinuities.


FIGURE 8. Visible dye liquid penetrant indications (withoutdeveloper) of stress corrosion cracking of metallic sheet.


PART 2. General Interpretation of LiquidPenetrant Indications

Mechanism of Formationof Liquid PenetrantIndicationsAny liquid penetrant indication marks thelocation of a discontinuity on the surfaceof the test object. There must be a surfaceopening; liquid penetrants cannot detectinclusions, chemical segregation, thepresence of foreign material or any otherabnormality unless an opening is present.Because of the nature of liquid penetranttesting, even a crack or void will remainundetected unless open to the partsurface. Liquid penetrants work equallywell on any nonporous metal, regardlessof magnetic properties, size, shape orchemical composition.

Even ferromagnetic metals, which areusually checked by magnetic particletesting, are sometimes more easilyinspected for surface cracks by liquidpenetrants. For example, the suddenchange in section in sharp fillets or in theroots of threads may cause a nonrelevantmagnetic indication. Because liquidpenetrant testing deals only with thesurface, it can be used with equal accuracyin sharp corners or on wide, flat areas.

Evaluation of Liquid PenetrantIndicationsThe presence of an indication poses threequestions.

1. What type of discontinuity causes thisindication?

2. What is the extent of thisdiscontinuity?

3. What effect will this discontinuityhave on the anticipated service of thepart?

On the answers to these three questionsdepends the fate of the part — acceptanceor rejection.

Quantitative information on the typeand size of discontinuity is not alwaysobtainable from surface inspection alone.Liquid penetrant indications do, however,provide the experienced inspector withqualitative data on which to base adecision in all obvious cases. Theinspector must know the kind ofdiscontinuity and its approximatemagnitude before attempting to solve thethird problem, that of estimating probabledamage to the part.

Appearance of LiquidPenetrant IndicationsIf fluorescent liquid penetrant is used, theexamination is made under ultravioletradiation, sound areas appear as deepviolet-blue, whereas discontinuities glowwith a brilliant, generally yellow-greenlight. The intensity of the fluorescence isassociated with the volume andconcentration of liquid penetrant retainedin the discontinuity. If visible dye liquidpenetrant is used, the examination ismade in ordinary white light. Thedeveloper forms a white background anddiscontinuities are made visible by a redcolor indication, whose richness is closelyrelated to the volume of entrapped liquidpenetrant.

Several factors influence the exactappearance of individual liquid penetrantindications. However, there are certaingeneral trends that hold true for all sortsand forms of materials.

Interpretation of Continuous LineLiquid Penetrant IndicationsA crack usually shows up as a continuouslinear liquid penetrant indication, asshown in Fig. 9. The width and brightness

FIGURE 9. Typical liquid penetrantindications: (a) large crack or opening;(b) tight crack or cold shut; (c) partiallywelded lap; (d) pits or porosity.

(c)

(a) (b)

(d)

MOVIE.Open andpartially opencracks.

MOVIE.Pitting andporosity.

MOVIE.Lineardiscontinuity.


of fluorescence or color depend on thevolume of the crack. The line may befairly straight or jagged, because it followsthe intersection of the crack with thesurface. A cold shut on the surface of acasting also appears as a continuous line,generally a relatively narrow one. Becausecold shuts are caused by imperfect fusionwhere two streams of metal meet but donot merge, the liquid penetrant indicationis likely to be smooth in outline ratherthan jagged. A forging lap may also causea continuous line liquid penetrantindication.

Interpretation of Intermittent LineLiquid Penetrant IndicationsMany forging laps are partially weldedduring subsequent blows of the forginghammer. The liquid penetrant indicationcaused by such forging laps is therefore anintermittent linear indication. Asubsurface crack that does not reach thesurface for its entire length or a seam thatis partially filled also produces anintermittent line or liquid penetrantindication, as shown in Fig. 9c.

Interpretation of Rounded Areasof Liquid Penetrant IndicationsRounded areas of liquid penetrantindication signify gas holes or pin holesin castings or relatively large areas ofunsoundness in any form of metal. Theindications appear rounded because thevolume of liquid penetrant entrapped; theactual discontinuities may be irregular inoutline. Deep crater cracks in weldsfrequently show up as roundedindications also, because a large amountof liquid penetrant is entrapped and theindividual crack indications merge.

Interpretation of Small Dot LiquidPenetrant IndicationsLiquid penetrant indications in the formof small dots, as in Fig. 9d, result from aporous condition. Such indications maydenote small pin holes or excessivelycoarse grains in castings or may be causedby a shrinkage cavity. In the latter case,an overall indication pattern of fernlike ordendritic outline is usually noted.

Interpretation of Diffuse LiquidPenetrant IndicationsSometimes a large area presents a diffusedappearance. If fluorescent liquidpenetrants are used, the whole surfacemay glow feebly; if visible dye liquidpenetrants are used, the background maybe pink instead of white. This diffusedcondition may result from very fine,widespread porosity, such asmicroshrinkage in magnesium. Or it may

be caused by insufficient cleaning beforetesting, by incomplete removal of excessliquid penetrant, by too thick a coat ofdeveloper. Weak indications extendingover a wide area should be regarded withsuspicion. It is usually wise to repeat theliquid penetrant test and to eliminate anyfalse indications due to faulty technique,rather than to attempt immediateevaluation of a diffused indication.

Edges of IndicationsThe sharpness of liquid penetrantindications is affected by the volume ofliquid retained in the discontinuity, thetest conditions such as temperature andtime allowed for indications to developand the type of liquid penetrant used.Generally, clear cut indications come fromnarrow linear discontinuities.

Brilliance and Extent of LiquidPenetrant IndicationsThe color or fluorescent brightness ofliquid penetrant indications can be usefulin estimating the seriousness of thediscontinuity. Brightness is directly relatedto the amount of liquid penetrant presentand therefore to the size of thediscontinuity. It is difficult for the humaneye to detect slight differences in color ofdye or brilliance of fluorescence. Testsshow that although instruments canrecord four percent difference inbrightness, the eye cannot see less than10 percent difference. It is fortunate thatlarger discontinuities nearly alwaysproduce larger indications in addition tothe increased brightness.

FIGURE 10. Visible dye liquid penetrantindication of crack in heat resistant alloydiesel valve.

Time for Liquid PenetrantIndication to DevelopOther things being equal, the timerequired for an indication to develop isinversely proportional to the volume ofthe discontinuity. The larger thediscontinuity, the more quickly will liquidpenetrant entrapped therein be pulled outby the developer. The crack in the railroaddiesel engine valve in Fig. 10 appearedimmediately, showing that it was adiscontinuity of significant size. It isimportant to allow sufficient time for theappearance of minute indications fromfine discontinuities, such as tight fatiguecracks. To use the time required for anindication to develop as a measure of theextent of the discontinuity, othervariables such as type of liquid penetrant,sensitivity of technique, temperature ofpart, dwell time and condition ofexamination must be controlled.

Persistence of LiquidPenetrant IndicationsOne good way to estimate the size ofdiscontinuity is by the persistence of theindication. If it reappears after thedeveloper has been removed andreapplied, there must be a reservoir ofliquid penetrant present. In case of faintor weak indications where there is somedoubt as to the type or even the existenceof a discontinuity, it is good practice torepeat the entire liquid penetrant test. Ifthe indication reappears, it is probablydue to a small discontinuity rather thanto incomplete cleaning.



Influence of LiquidPenetrant SystemSelection on InterpretationVarious commercial materials are availablefor liquid penetrant testing, each of whichhas its special field of optimumapplication. To a large extent, thesensitivity of the liquid penetrant testprocess can be varied at will by the properselection of liquid penetrant anddeveloper. It is important for best resultsto discuss with the manufacturers thetype of discontinuity that should befound by liquid penetrant testing. Veryhigh performance liquid penetrantsystems may cause unnecessary rejectionsdue to indications from discontinuitiesthat do not adversely affect that particularpart.

Effects of MetalsManufacturing Processeson Liquid PenetrantIndicationsLiquid penetrant indications can beinfluenced seriously by previousprocessing during manufacturing,inspection or surface treatments of testparts. Although the chemical compositionor form of material does not affect theindications obtained from liquidpenetrant testing, it is true that the sametechnique will produce varyingindications on rough castings, finish

machined parts or die forgings becausethe various manufacturing processes resultin characteristic surface conditions thatfrequently have individual types ofdiscontinuities. Furthermore, someoperations may interfere with accurateliquid penetrant testing. Table 1 gives theeffects of metals manufacturing processeson liquid penetrant indications.

Therefore, any interpretation of liquidpenetrant indications should includeconsideration of the test part and itshistory. Is the surface porous? Is there apossibility of embedded material thatwould clog openings, say, in lapped parts?Can the production techniques obscure adiscontinuity, such as a seam or forginglap, by further working that tends to closethe discontinuity? Or does the shape ofthe surface tend to produce an irrelevantindication, perhaps by trapping liquidpenetrant in the undercut area of a weld?Obviously, paint or plating films interferewith the entrance of liquid penetrantsinto discontinuities. Anodic or chromatetreatments reduce the detectability ofmany fluorescent liquid penetrantindications, primarily because the porousoxide absorbs penetrants and creates abright fluorescent background. However,special fluorescent liquid penetrants canbe used successfully after anodic orchromate treatments. It is essential thatthe fabrication methods be consideredwhen interpreting liquid penetrantindications.

PART 3. Processing Effects Influencing LiquidPenetrant Indications

TABLE 1. Effects of metals manufacturing processes on liquid penetrant indications.

Process Obscured Discontinuities Irrelevant Indications

Casting ——— rough surface retains liquid penetrant Honing, lapping compound clogs openings oils and greases may fluoresce Forging partially seals laps scale holds liquid penetrantShot blasting seals openings ———Polishing metal flows over discontinuities ———Heat treating ——— scale holds liquid penetrant Painting or plating fills openings ———Anodizing reduces fluorescence porous oxide absorbs liquid penetrant Chromate treatment reduces fluorescence ———Rough machining in blind holes ——— hard to remove excess liquid penetrantWelding ——— rough surface retains liquid penetrant


Effects of Previous Testingon Liquid PenetrantIndicationsTest processes, like fabrication, can changethe surface condition. It is not possible todetermine the relative efficiency ofvarious test processes by subsequentchecks of the same part because many ofthe materials used in magnetic particle,fluorescent liquid penetrant and visibledye liquid penetrant techniques areincompatible.

Discontinuities may remain undetectedby liquid penetrant testing. If magneticparticle testing has been previously used,the residual iron oxide may fill or bridgethe discontinuity. Similarly, fluorescentliquid penetrant will often fail to showdiscontinuities previously found by visibledye liquid penetrant because the dyeabsorbs incident ultraviolet radiation orperhaps reduces or even kills fluorescence.Subsequent visible dye liquid penetranttesting may miss discontinuities indicatedby fluorescent liquid penetrant, becausethe later affects the visible color dye.Therefore, in judging the presence orabsence of liquid penetrant indications,the inspector must know whether theparts have been subjected to other testprocesses. If such is the case, extreme caremust be taken to clean parts thoroughlybefore the additional liquid penetranttesting.

Chromic Acid Anodic Treatmentto Provide Acid Stain IndicationsAnodic treatment interferes withfluorescent liquid penetrant testing.However, chromic acid anodization servesboth manufacturing and inspectionneeds. Although its primary purpose doesnot include the detection of cracks, lapsor other surface discontinuities,experience has shown that chromic acidanodization does render suchdiscontinuities more visible. Any surfaceconnected void retains some of the acid,which then produces a directionalchromic acid stain on the surface.Although not so sensitive as liquidpenetrant testing, chromic acidanodization is accepted by some as analternate method for detection of surfacediscontinuities in aluminum alloyforgings.

Other Variables AffectingIndication AppearanceThe following variables have a markedeffect on the size, brilliance andappearance of liquid penetrant

indications: condition of surface,temperature of the part or the liquidpenetrant, dwell times, amount ofwashing, developer and conditions ofexamination.

Effects of Test Object SurfaceCondition on Liquid PenetrantIndicationsSurface conditions that influence liquidpenetrant indications are given in Table 2.

There are four general ways in whichthe condition of the surface can interferewith the proper results from liquidpenetrant testing.

1. Surface openings may be closed. Thiscan occur when lubricants, polishingcompounds, dirt, scale or othercontaminants are forced into cracks orholes. Or metal can be peened oversurface discontinuities by forging,polishing or shot blasting.

2. Rough or porous areas may retainliquid penetrant, producing irrelevantindications. Or naturally fluorescentmaterials (such as many oils andgreases) may cause confusingindications.

3. Deposits on the surface or in openingsmay dilute the liquid penetrant andthus reduce its effectiveness, or suchmaterials may react with the liquidpenetrant, destroying fluorescence ordye color.

4. Water or moisture within crackdiscontinuities can prevent the liquidpenetrant from entering the crack.

Proper cleaning before liquid penetranttesting will remove any dirt or othercontaminants, leaving surface openingsfree to receive the liquid penetrant. Forcorrect evaluation of the discontinuitiesfound by liquid penetrant testing, it isessential that the surface be adequately

TABLE 2. Effects of surface conditions on liquid penetrantindications.

Surface Condition Possible Result

Oily or greasy no indications (openings closed) or falsefluorescence

Dirty no indications (openings closed)Shot blasted no indications (openings closed)Acid weak indications (reduced fluorescence)Caustic weak indications (reduced fluorescence)Wet weak indications (diluted penetrant)Scale or rust no indications (openings closed) or false

indicationsExcessively rough weld false indicationsRough castings surface false indicationsClean excellent


clean and dry. If it is impossible to attainsatisfactory cleanliness, the inspectormust try to take this fact into accountwhen weighing the importance of liquidpenetrant indications.

Effects of Temperature of Part orLiquid Penetrant on IndicationsThe viscosity of most liquids increases atlow temperature and liquid penetrants areno exception. If a part is quite cold, sayunder 16 °C (60 °F), the liquid penetrantmay be chilled and thickened so that itdoes not enter very fine discontinuities inthe same dwell time as for highertemperatures. If the part or liquidpenetrant is too hot, the volatilecomponents of the liquid penetrant mayevaporate, changing the capability forrevealing small discontinuities. Materialtemperatures from 10 to 52 °C (50 to125 °F) produce optimum results withconventional liquid penetrants.4 However,special liquid penetrants developed foraerospace and nuclear power programscan be used in extremely coldtemperatures and others at relatively hightemperatures.

Effects of Dwell Time andWashing on Liquid PenetrantIndicationsFine liquid penetrant indications usuallydenote fine discontinuities. However, fineindications may be caused by insufficientliquid penetrant dwell time. A diffusedindication may mean a generally porouscondition but may also imply incompletewashing. If excess liquid penetrant is notremoved, false indications are evident. Onthe other hand, washing too long and toohard can remove liquid penetrant fromlarge or shallow discontinuities, resultingin less intense indications. Problemsresulting from poor washing are discussedelsewhere in this volume.

Effect of Developer on LiquidPenetrant IndicationsThe developer renders indications moreeasily visible by (1) providing acontrasting background, (2) pulling liquidpenetrant out of openings by a blotting

action and (3) reducing glare fromreflected light during observations offluorescent indications. A thick film ofdeveloper may absorb liquid penetrant,resulting in faint indications or evenmasking very fine discontinuities. The netresult of most faulty liquid penetrantprocessing techniques is loss ofindications, as indicated in Table 3.

Effect of Examination Conditionson Visibility of Liquid PenetrantIndicationsThe correct interpretation of indicationsfrom liquid penetrant testing is up to theinspector. In addition to a knowledge ofthe meaning of indications and the

TABLE 3. Results of faulty liquid penetrant processing techniques.

Operation Too Low or Too Little Too High or Too Much

Temperature of part or fine defects missed fine defects missedliquid penetrant

Dwell time fine defects missed removal difficultWashing false indications liquid penetrant removed from shallow defectsDeveloper poor contrast hide fine defects

FIGURE 11. Heat treatment cracks in forging:(a) fluorescent indications; (b) cross section.

(b)

(a)

variables affecting them, a good inspectormust have good eyesight and properlighting. The latter is especially importantfor fluorescent liquid penetrants, whichshould be viewed in a darkened area withultraviolet illumination.

An inspector whose eyesight was belowaverage because of a vitamin A deficiencyor whose eyes had not properly adjustedto the darkened examination booth couldnot have detected the fine heat treatmentcracks shown in Fig. 11a. As shown inFig. 11b, these cracks could be veryserious in this highly stressed aircraftforging.


Types of Specifications forLiquid Penetrant TestingSpecifications and standards applicable toliquid penetrant testing may be dividedinto two broad classes: (1) those dealingwith techniques and (b) those dealingwith materials. Each of these groups canbe broken down further. Some of thesubgroups are below.

TechniquesSpecifications and standards may apply tothe following types of techniques:

1. broad procedural guidelines;2. company procedural guidelines;3. procedural guides for specific types of

products or for industries;4. procedures for testing specific articles,

specified by a purchaser or by processspecifications internal to thecompany;

5. procedures specified for overallinspection of a company’s products;

6. specifications for certification ofoperators;

7. standards for acceptance or rejection,set up by buyer or by company forquality control;

8. repair station requirements;9. equipment specifications; and

10. instructions for operating specifictypes of equipment or individualspecial units.

MaterialsSpecifications that apply to materials maybe of the following types:

1. specifications designed for use inpurchasing liquid penetrants andother materials and

2. specifications designed for testing andevaluating performance of liquidpenetrants and other materials.

Lack of “Standard” LiquidPenetrant IndicationsThere are as yet no “standard” liquidpenetrant indications for comparison.Radiographers can judge the seriousnessof a discontinuity revealed on X-ray film

by comparing it with standard film butthe many factors affecting liquidpenetrant testing have retarded theestablishment of standards for thisprocess. Differences in indications may bedue to personnel or technique, as well asto variations in discontinuities. Theinspector must know the technique usedand its effect on liquid penetrantindications. Possible causes of inadequateliquid penetrant indications are shown inTable 4.


PART 4. Establishing Acceptance Standards forLiquid Penetrant Indications

FIGURE 12. Crack in stainless steel tube: (a) fluorescent liquidpenetrant indication; (b) cross section.

(b)

(a)

Liquid Penetrant ReferenceStandards for RepetitiveTestingCompanies performing repetitive testingof the same or similar parts make theirown standards by photographing theindications and the sectioned parts. Aseries of such indications produced byexactly the same technique, together withthe discontinuities that were thus located,help the inspector estimate both the typeof discontinuity present and itsseriousness. For example, the indicationon the stainless steel tube shown inFig. 12a was caused by the crack picturedin Fig. 12b. Where it is possible to displayactual sectioned parts instead ofphotographs, this has proved even better.As in any other nondestructive test, liquidpenetrant test standards must becorroborated by destroying the part andconfirming the existence of thediscontinuity. The time and money spentin selecting, photographing andsectioning typical parts is a goodinvestment in quality control.

Indications ofMiscellaneous PartsMany test laboratories handling a widevariety of work do not have the advantageof inspecting large quantities of any oneitem. Most of these commerciallaboratories are performing tests togovernment specifications andconsequently, the liquid penetrantinspectors are trained and tested. There isno substitute for experience, which isgained only by seeing whichdiscontinuities caused which indications.

Liquid PenetrantIndications of LeaksIn any part such as a gas tank, tubing,fitting, flange or cylinder that mustmaintain pressure, a liquid penetrantindication through the wall is consideredto be cause for rejection. It makes nodifference whether the discontinuity is acrack, a hole, lack of penetration in a weldor a porous area. The fact that a liquidpenetrant can pass through the wallproves that the vessel cannot holdpressurized fluids without leakage.


Table 4. Possible causes of inadequate liquid penetrant indications compared to validindications from defects.

Cause Indications__________________________________________________________________________Weak Faint Line Diffused Many, Small Heavy

Defect fine crack or cold shut general porosity porosity large flawsFabrication prior anodize or chromate embedded grease rough surface ———Cleaning wet, acid, or caustic oily or greasy scale or rust ———Temperature too low ——— ——— ———Washing inadequate inadequate ——— ———Lighting inadequate ——— ——— ———Inspector poor sighta poor sighta ——— ———

a. Inadequate vision of inspector may be due to (1) inattention, (2) maladaption to darkness or (3) poor vision acuity,which may be due to vitamin A deficiency.

Cracks Occurring onSolidificationCracks may occur in castings during thesolidification and cooling of the metal,because of contraction. Indications ofcracks are considered a basis for rejectionin almost all cases. Cracks not only reducethe strength of a part but they also maypropagate, especially under alternating orfatigue loads and they afford entrance forcorrosive media. Shrinkage cracks incastings are usually found in areas wherethere is an abrupt change in the thicknessof metal. They are caused by internalstresses that develop because of theunequal cooling of the area between theheavy metal and the thin wall. Vibrationand external stress would cause thesecracks to become more extensive,resulting in failure of the part.

Cold shuts are often confused withcracks but the indications from cold shutsare usually narrow lines of smooth,curving outline.

Cracks in Fusion WeldsFusion welds may have cracks in the weldmetal like that shown in Fig. 13. Cratercracks may produce rounded indicationsbecause so much liquid penetrant isretained. These cracks are usuallyrejectable. Liquid penetrant testing has

made possible the successful repair ofmany fusion welds because discontinuitiesare located immediately and accuratelyand small cracks can be ground out andthe area rewelded. Cracks in the parentmetal adjacent to the weld are nearlyalways cause for rejection. Welds may givestraight line indications that look likecracks but which are actually caused bylack of penetration or lack of fusion.Whether or not this is a harmfuldiscontinuity depends on the design andthe service expected of the part.

Cracks in Brazed BondsBrazed assemblies, like welds, may havecracks or lack of adhesion. Cracks arerejectable, mainly because of theirtendency to spread. A certain amount oflack of adhesion is tolerated in brazedjoints; only in very critical assemblieswould 100 percent braze be required. Theinspector must observe liquid penetrantindications closely to differentiatebetween the straight line from lack ofadhesion and the uneven meandering of acrack.

Fluorescent liquid penetrantindications on tipped cutting tools areshown in Fig. 14. These indications showa lack of bond between the shank and thehard metal insert. Lack of bond in thisarea would shorten the life of the toolsbecause there would not be sufficientsupport for the tip. The broad, smudgyindication in the illustration results from


PART 5. Interpretation of Liquid PenetrantIndications of Cracks

FIGURE 13. Fluorescent liquid penetrant indication of crack instainless steel weldment.

FIGURE 14. Lack of bond on tools tipped with hard metal.


the large amount of liquid penetranttrapped in the void between the twopieces of metal.

Cracks Occurring duringProcessing of MetalsHot and cold working of metals and alloyscan induce cracking. “Hot short”materials can crack when hot workingstresses exceed loads that can besupported at grain boundaries. Coldworking to excess leads to cracking as wellas work hardening. Heating and cooling(as when hardening by quenching parts inwater after removal from hot furnaces)can lead to thermal stress and quenchcracking.

Forging CracksForgings may have indications of cracks,often at the juncture of light and heavysections or in thin fins. As in castings,cracks are regarded as rejectable.Indications similar to those from cracksare produced by forging laps, discussedelsewhere in this chapter.

Cracks in Sheets or TubingFormed parts such as sheet or tubing orcastings that have been straightened maycrack at the point of maximum stress.Such cracks may be very small; there areusually several in close proximity andthey are generally considered rejectablediscontinuities.

Heat Treatment CracksHeat treatment cracks should be lookedfor at the meeting of light and heavysections. They are nearly always deemeddetrimental and therefore a cause forrejection.

Machining MarksMachining practice may produce cracklikeindications from dull tools, chatter marksand similar tool marks. Such indicationsare ordinarily not intense and are notconsidered sufficient cause for rejection.

Grinding CracksGrinding cracks are minute, ofteninvisible to the unaided eye and are causefor rejection. The type of precision partthat must have a ground surface cannottolerate any surface cracks, however small.While grinding cracks are extremely fineand may be shallow, they usually occur ina definite network that is easily identified.Extensive grinding cracks are shown inthe two carbide tipped cutting toolsillustrated in Fig. 15. The photographs in

Fig. 15a were taken under normal light;the photographs in Fig. 15b show thesame tools, processed with water washableliquid penetrant, under ultravioletradiation. These cracks would soonprogress to failure if the tools were used.Notice that the indications are bright andwell defined because the cracks are deepand rather tight lipped.

FIGURE 15. Carbide-tipped tools:(a) photographed with white light;(b) fluorescent indications of grinding cracks.

(b)

(a)

Cracks Occurring duringServiceCracks can develop in service as aconsequence of fatigue under repeatedloading, thermal cycling, heat checking orstress corrosion.

Fatigue Cracks in Turbine BladesOne very serious type of crack occursduring service as the result of fatigue orintermittent loading. Fatigue cracks oftenstart at a stress raiser such as a sharp toolmark, a nonmetallic inclusion or ascratch. However, even in carefullyprepared parts such as turbine blades,interrupted loading produces fatigue.Figure 16a shows a faint liquid penetrant

indication on a cobalt base superalloyturbine blade. The original indication hadbeen removed with a pencil grinder in thehope that it would be a shallow surfacediscontinuity. When developer wasreapplied, the indication again appeared,showing that all the crack had not beenground out. Figure 16b shows a crosssection through that crack and Fig. 16cillustrates the path of the crack (about100× magnification). A fatigue crack is arejectable discontinuity.

Two of the rotor buckets shown inFig. 17 have fatigue cracks that wouldgrow in size if the rotor were left inservice. The Christmas tree dovetails wherethe blades are fitted into the wheel aresharply visible because liquid penetranthas lodged in space between the dovetailsand the wheel. This latter indication isnormal in such a part, because the bladesand wheel, although a press fit, are notintended to be bonded to each other.

Fatigue Cracks in AluminumPistonsThe aluminum piston in Fig. 18 clearlyshows the difference between tight andbroad cracks. The crack between the finsin the top of the piston is broad; theliquid penetrant has bled out of thiscrack. This is a condition that usuallyoccurs in wide, deep discontinuities. Incontrast, the cracks at the base of eachboss are much finer, indicating a crack ofsmaller dimensions.

Fatigue Cracks in Light AlloyCastingsSix of the eight spokes in the magnesiumwheel in Fig. 19 have fatigue cracks, fourof them almost to the web. Notice alsothe start of cracks in the spokes near therim. There are also several generallyfluorescent areas indicating some porositybut because no cracks seem to have grown


FIGURE 16. Crack in superalloy turbineblade: (a) visible dye liquid penetrantindication; (b) cross section throughindication; (c) photomicrograph of crack.

(b)

(a)

(c)

FIGURE 17. Fatigue cracks in assembled rotor buckets.


from these areas, this porosity is probablyharmless.

Cracks in Valve HeadsSome valves are faced with special alloysto increase their resistance to heat andwear. Because this facing is nonmagnetic,cracks can be detected only by liquidpenetrant testing (Fig. 20). Such cracks are

cause for rejection unless they can belapped out within the acceptabledimensional tolerance.

Stress Corrosion CracksAnother crack that is caused by operatingconditions is stress corrosion cracking.Deep drawn parts used to containchemicals and items for maritime serviceare especially likely to crack from stresscorrosion. Stress corrosion cracking doesnot necessarily start from an edge or stressraiser. Indications of these cracks oftenappear in a pattern at the area ofmaximum stress and are easily identified.Stress corrosion cracks are consideredcause for rejection.

Heat ChecksThe third type of crack that may show upon maintenance overhaul is the heatcheck, occurring when surfaces areoverheated. Like grinding cracks (whichare initiated in the same way by localoverheating), heat checks are usually quiteshallow but occur in definite, recognizablenetwork. These indications may or maynot, be cause for rejection, depending onthe use of the part.

FIGURE 18. Fatigue cracks in aluminum piston head.

FIGURE 19. Fatigue cracks in magnesium wheel.

FIGURE 20. Comparison of cracks in diesel valves: (a) waterwashable liquid penetrant indications; (b) postemulsificationliquid penetrant indications.

(b)

(a)

3PT05 (125-160) 8/13/99 12:43 PM Page 145

Cracks in Plastics and GlassLiquid penetrant testing can be used onany nonporous material. It should not beused on materials that might be adverselyaffected by the liquid penetrant. Someplastics are attacked by certain oil baseliquid penetrants but many plastics arenot affected. A water base liquid penetrantshould be used where it is found that theusual oil base liquid penetrants react withthe plastic. Before inspecting plastics withliquid penetrant techniques, it is wise totry out the reaction of the test materialson material similar to that of which thepart is made. Fatigue cracks may be foundin plastics as well as in metals. Liquidpenetrant testing can also be used onglass, because it is nonporous. Electrifiedparticle testing in preferred for inspectionof glass and glazed ceramics; however,liquid penetrants are more suitable fordetermining leaks and lack of bondbetween glass and metal.



Cold Shut or FoldAs mentioned in the discussion of cracks,cold shuts or folds give liquid penetrantindications that are definitely lines,usually curving in a smooth outline. Inhighly stressed castings, a cold shut is arejectable discontinuity because, like acrack, it lessens the strength of the partand is a point of high stressconcentration. If the surface is to bemachined and the cold shut can beremoved, it may be accepted on conditionthat an additional liquid penetrant testingbe performed after machining.

Die CastingsIn die castings, however, slight cold shutsor skin folds, less than 0.1 mm (0.004 in.)in depth, are common. Because diecastings are seldom used for applicationsrequiring very high strength, faintindications of cold shuts or skin folds arenot causes for rejection. The usual practiceis to remove some of the worst lookingdiscontinuities and estimate how deepthey were, then make the decision ofacceptance or rejection.

Forging LapsForging laps, like cracks, produce linearliquid penetrant indications and areconsidered equally detrimental. It isespecially hard to estimate thesignificance of a lap from the liquidpenetrant indication because (1) forgingtends partially to seal the lap on thesurface and (2) embedded scale and/or dielubricant may partially fill a lap. In eitherof these cases, the resultant indicationwould not denote the true degree ofseverity of the discontinuity. Forging lapsare serious discontinuities and are causefor rejection unless it is definitely knownthat they will be removed duringsubsequent machining. Figure 21 showssuch laps in an aluminum forging. It isdoubtful that the machining operationwould remove these laps, so the partwould probably be rejected. Thebrightness of the indication and thebleeding out of liquid penetrant showthat these laps have considerable depthbecause they harbor so much liquidpenetrant.

SeamsSeams in bar stock, laminations in sheetor plate and seam conditions in forgingsgive liquid penetrant indications similarto those from extrusion discontinuities.Seams are often extremely narrow, havingbeen compressed and elongated inmanufacture. Indication lines are usuallyquite straight, easy to recognize and inthe longitudinal direction. They may beonly short lines when examinedtransverse to the direction of rolling.Seams are usually bases for rejection(Fig. 22).

PART 6. Interpretation of Liquid PenetrantIndications of Laminar Discontinuities

FIGURE 21. Forging laps in aluminum forging.

FIGURE 22. Fluorescent liquid penetrant indications ofprominent seam in bar stock.

Extrusion DiscontinuitiesDepending on the orientation of thesurface being examined, an extrusiondiscontinuity may cause continuous ordotted liquid penetrant indications. Atypical extrusion discontinuity is an internalunsound area, longitudinally squeezedduring extrusion and sometimesbreaching the surface (Fig. 23). If theextrusion is sectioned and a longitudinalsurface examined, the extrusiondiscontinuity will show up as a line, like acrack. If a cross section is inspected, liquidpenetrant indications will appear as shortlines or dots. An extrusion discontinuityis usually considered a basis for rejectionin aircraft applications but may beacceptable for other service where onlylongitudinal strength is required.


FIGURE 23. Liquid penetrant indications of discontinuities inextruded, milled, aircraft wing spar.

Gas HolesLiquid penetrant indications of gas holesare round blobs of rich color orfluorescence. Large gas holes are bases forrejection, not only because they reducestrength but also because they produce arough surface. Small gas holes may not berejectable discontinuities, because they donot have the serious effect on strengththat a crack or a lap does. If pressuretightness or smooth surface is required,then even tiny pinholes are considered abasis for rejection. Figure 24 showsfluorescent liquid penetrant indications ofextreme porosity in a casting.

Shrinkage CavitiesPorosity resulting from shrinkage cavitiesmay produce rounded liquid penetrantindications or may give the appearance ofcracks. Usually, shrinkage cavities have adendritic pattern that helps the inspectoridentify them. For high stressapplications, shrinkage cavities arerejectable discontinuities. These are caseswhere the amount of shrinkage and itslocation on the part do not detrimentallyaffect service; in such cases the part isacceptable.


PART 7. Interpretation of Liquid PenetrantIndications of Porosity and Leaks

FIGURE 24. Fluorescent liquid penetrant reveals extremeporosity in casting.

FIGURE 25. Leak testing of welded seam with fluorescentliquid penetrant: (a) spray application of liquid penetrant tointerior wall at weld seam; (b) spray application of developerto exterior wall at weld seam; (c) leak indications in weldedseam shown by fluorescence.

(b)

(a)

(c)

MOVIE.Porosity incasting.

MicroshrinkageSome materials, especially magnesiumcastings, have shrinkage cavities so finethat they are almost invisible. Theseopenings are so tiny that instead ofgetting a speckled condition with anumber of small spots, the liquidpenetrant indication is more likely to be agenerally diffused condition. To confirmthe actual presence of microshrinkage, itis best to clean and reprocess the part.Microshrinkage is not always a rejectablediscontinuity; rejection depends on sizeand severity of microshrinkage, itslocation and the end use of the part.

Porosity in CeramicsLiquid penetrant testing is a good way tolocate porosity in ceramics. Theappearance and interpretation ofindications are the same as in metals,with the one extra caution that specialattention must be paid to cleaningbecause most ceramics are somewhatporous.

Leak Testing of WeldedSeamsLiquid penetrants are often effectivetracers for detecting leaks that wouldallow gases or liquids to pass through thewalls of welded containers such as pipes,cans, boxes or other containment systems.Figure 25 shows typical procedures andindications produced by fluorescent liquidpenetrants on a corner welded sheet metalbox. Liquid penetrant is applied to seamsor other suspect areas on the outside (orinside) surface of the containment. Afterallowing dwell time sufficient to permitliquid penetrant to seep through leaks,examination for liquid penetrantindications is carried out on the oppositesurface of the containment wall (with orwithout developers, as appropriate). Eithervisible dye or fluorescent liquidpenetrants can be used as leak tracers, asconvenient for visual examination.


Common Causes ofNonrelevant LiquidPenetrant and FalseIndicationsNonrelevant indications may be due tomisapplied or improper testing proceduresor to actual discontinuities that do notaffect the usability of a part. Falseindications, a subcategory of nonrelevantindications, are best defined as indicationsthat distract from the quality of theinspection but do not indicate adiscontinuity in a part. False indicationsare due to several causes but the primarycause is usually a deficiency in processcontrol.

Poor Process ControlRegardless of the liquid penetranttechnique used, the level of sensitivitycan be adversely affected by poor processcontrol, such as improper application ofliquid penetrant, improper removal ofliquid penetrant, lack of cleanliness in theinspection area and poor handling ofparts being processed. Any of these cancause false indications that will interferewith or cause a distraction during thevisual detection and interpretation ofindications.

Improper application of liquidpenetrant can cause pooling of the liquidpenetrant. This produces an unevencoating of liquid penetrant on the surfaceof a part and, on certain types ofmaterials, results in areas of highbackground bleedout.

Improper removal of liquid penetrantcan cause either an overremoval orunderremoval of excess surface liquidpenetrant. Overremoval is indicated by acomplete lack of background orindications; a slight background isnormal. Overremoval will not result in afalse indication but increases the chancesof removing a valid discontinuityindication. However, underremoval ofexcess liquid penetrant can cause a highbackground intensity and decrease thecontrast needed to reveal smallindications. Another problem withunderremoval is that the remaining liquidpenetrant background may mask arelevant discontinuity indication. Ineither case, a part that has been subject to

improper removal of excess surface liquidpenetrant must be cleaned andreprocessed.

Lack of cleanliness in the inspectionarea is often a cause of false indications.Except for the liquid penetrant containers,an inspection area should be free of allliquid penetrant. Liquid penetrant onroller assemblies, dryer shelves orinspection tables should be cleaned upbefore parts are processed. Otherwise thisliquid penetrant can be absorbed by thedeveloper and cause nonrelevantindications.

Proper handling of parts during allphases of the liquid penetrant process isimportant. Rubber gloves should becleaned between processing steps. Smallparts processed together should behandled in a manner to prevent theirrubbing against each other. Towels or ragsthat are not lint free will leave lint on apart that will fluoresce under nearultraviolet radiation and give theappearance of a crack. False indicationsdue to improper handling of parts can beeasily prevented.

Part Geometry and SurfaceConditionThe geometry of a part or assembly cancause nonrelevant indications. Theindication of an interface between aturbine blade and a disk can not onlyproduce a nonrelevant indication but alsomask possible small indications if thebleedout from the interface is excessive.Figure 17 shows an example of assembledrotor buckets being liquid penetrantinspected. Note the liquid penetrantindications of the Christmas tree junctionsof the rotor (disk) and turbine blades. Oneaid to the inspector is that the regularpattern of the indications make them easyto identify. However, in many casesinspection procedures will require that anassembly be completely disassembledbefore inspection.

Press fittings, like the example ofturbine blades and disks above, are asource of nonrelevant indications causedby part geometry. It is sometimesimpossible to completely disassemble apart that must be inspected. The bleedoutof the liquid penetrant around pressfittings can be considerable and can maskrelevant indications. In these situations


PART 8. Nonrelevant and False Liquid PenetrantIndications

MOVIE.Process controlcan maskdiscontinuities.

MOVIE.Nonrelevantindications canmask relevantones.

MOVIE.Nonrelevantindication frompart geometry.

MOVIE.Falseindications.

inspectors must monitor the part veryclosely during the developer dwell time toproperly evaluate the indications as theyare forming.

Another cause of false and nonrelevantindications is a rough part surface. Weldsare very difficult to inspect, especiallywith solvent removable method, becauseof rough weld bead surfaces and weldspatter. The major concern on a weldbead is masking of relevant indications.As-cast and as-forged surfaces may alsolead to nonrelevant indications due tosurface roughness or other irregularities.Machined parts may have machininggrooves or marks or may have scratchesdue to rough handling.

Evaluation of NonrelevantIndicationsOne technique that an inspector can useto help identify a nonrelevant indicationis wiping off the indication with a solventdampened lint free swab or cloth andreapplying the developer. The originaldeveloper dwell time must be repeated;however, the inspector should monitorthe bleedout to help evaluate theindication. If there is no bleedout, theoriginal indication can be considerednonrelevant or false. This technique mayalso reveal multiple relevant indicationsthat had previously merged into one orhad been hidden by excessive bleedoutfrom an adjacent nonrelevant indicationduring the developer dwell.


Sample Anomalous PartsThe most effective means for trainingliquid penetrant inspectors to recognizeand identify discontinuities is frequentreference to a collection of parts withtypical discontinuities. Parts that havebeen rejected because of discontinuitiesshould be clearly marked or partiallydamaged so that they will not beconfused with acceptable parts.

The parts with known discontinuitiescould be processed with regularproduction parts. This would serve twopurposes: the inspector in training couldbe judged for discontinuity recognitionand at the same time, the inspector couldbe familiarizing himself with the type ofdiscontinuity indication considered to because for rejection. However, withcontinued processing, the knowndiscontinuities will clog with liquidpenetrant and developer residues. Specialcleaning of these parts with cloggeddiscontinuities will be necessary tocontinue their use in recognition training.

Visible liquid penetrants tend to killthe fluorescent qualities in fluorescentliquid penetrants. After a part has beeninspected by using visible liquidpenetrant, it is not desirable to attempt toreprocess with fluorescent liquidpenetrant. The results will not be reliable.In all cases, parts should be cleanedthoroughly and degreased beforereprocessing.

Evaluating Indications toDetermine Causes ofDiscontinuitiesIt is possible to examine an indication ofa discontinuity and to determine its causeas well as its extent. Such an appraisal canbe made if something is known about themanufacturing processes to which thepart has been subjected. The extent of theindication or accumulation of liquidpenetrant, will show the extent ofdiscontinuity and the breadth andbrilliance will be a measure of its depth.Deep cracks will hold more liquidpenetrant and therefore will be broaderand more brilliant. Very fine openings canhold only small amounts of liquid

penetrant and therefore will appear as finelines.

Although many factors influence theexact size and shape of indications fromliquid penetrant testing, most typicaldiscontinuities are easy to recognize. Aline of liquid penetrant signifies a crack,lap, cold shut, seam or other longdiscontinuity. A spot or blob denotes ahole, large or small.

By photographing typical indicationsand pairing them with sectioned partsshowing the discontinuities causing thoseindications, standards for acceptance orrejection can be established in terms ofthe photographic appearance of liquidpenetrant indications.


PART 9. Recognition Training of LiquidPenetrant Inspectors

Specifications andReferences Applicable toLiquid PenetrantInterpretationSome industries have prepared standardsfor evaluation and acceptance/rejection ofhardware on the basis of liquid penetrantindications. These can be anything fromquite general to very detailed. Liquidpenetrant users may find some of thesehelpful or may prefer to prepare theirown. If liquid penetrant inspectors areworking under a contract, it is mandatorythat they determine and conform to thespecifications to which the contractingagency intends to hold them.

Commonly, a general statement maybe encountered, such as “The inspectiondepartment shall pass only those partsthat are free of liquid penetrantindications. Parts showing liquidpenetrant indications should be referredto the metallurgy or design departmentsfor disposition. The metallurgy or designdepartment shall decide which parts shallbe accepted, reworked or rejected”.

ASTM E 433, Standard ReferencePhotographs for Liquid Penetrant Inspection,contains reference photographs to be usedas a means of establishing and classifyingtypes and characteristics of surfacediscontinuities detectable by liquidpenetrant testing.5 They may be used as areference for acceptance standards,specifications and drawings. However, noattempt has been made to establish limitsof acceptability or the extent of themetallurgical discontinuity.

Blueprint Notations ControllingInterpretation of Liquid PenetrantIndicationsIdeally, the manufacturer’s drawing orprint for the test part or surface underexamination will specify thenondestructive test method or methodsrequired for acceptance. Moreover, it willeither specify the acceptance or rejectioncriteria or refer the inspector tosupplemental documents such asacceptance/rejection specifications. Ifcritical parts are involved (such as nuclearhardware or jet engine components), an

expert in evaluation of indications mayhave to be called on for a judgment.

Establishing Criteria forAcceptance or Rejection inLiquid Penetrant TestingTo establish acceptance/rejection criteria,it may be necessary to conduct anextensive correlation study betweennondestructive test indications anddestructive test results. This is theultimate procedure but even it may leavesome doubt because discontinuities orindications do not always occur in exactlythe same place, with the same frequencyor to the same extent.

It should be obvious that a number offactors enter into the final judgment ofacceptability of test parts during liquidpenetrant testing, including: (1) the metalor metal alloy involved; (2) if anonmetallic surface, the composition ofthe nonmetal; (3) locations of the liquidpenetrant indications, for example, incritical radii, on edges that will be groundoff, in parts designed for high strengthapplications or in thick sections that mayallow for removal without sacrifice offunction; (4) whether or not the surface orsurfaces are repairable by welding or othermeans; and (5) the cost of the part. It maybe that the cost of a new part is so lowthat the expense of repair or rework of ananomalous part is not warranted.Conversely, of course, one would notwant to discard an expensive piece ofhardware that could be reworked at aconsiderable saving over the cost of a newpart.

In summary, it can be seen that liquidpenetrant evaluation is dependent onseveral factors that are not easilystandardized. Further work in detecting,defining, describing and evaluatingindications could be very helpful to thescience of liquid penetrant testing.


PART 10. Specifications and Guides forEvaluation of Liquid Penetrant Indications

Examples of InterpretationGuides Based onAppearance of LiquidPenetrant IndicationsIn some cases, specifications provide aguide to parts evaluation based primarilyon the size, shape or location of liquidpenetrant indications. For such purposes,a linear liquid penetrant indication isdefined as having a length greater thanthree times the width. Rounded liquidpenetrant indications are thoseindications that are circular or ellipticalwith the length less than three times thewidth. In some code applications,unacceptable discontinuities are thendefined in terms such as (1) any crack orlinear indication; (2) rounded indicationsgreater than 5 mm (or 0.2 in.) indimension; (3) four or more roundedindications in a line separated by 1.5 mm(or 0.06 in.) or less, edge to edge; and(4) ten or more rounded indications inany 37.5 cm2 (6.0 in.2) of surface with themajor dimension of this area not toexceed 150 mm (6.0 in.). The area mustbe taken in the most unfavorable locationrelative to the indications beingevaluated.

Representative AerospaceManufacturer’s LiquidPenetrant TestInterpretation GuideA typical aerospace manufacturer’s processspecification requires that visualinspection areas shall be illuminated withessentially white light. The intensity ofwhite light at the visual test level shall beequivalent to at least 750 lx (70 ftc).Fluorescent liquid penetrant testing shallbe conducted in a suitable darkened area(with an ultraviolet radiation intensity ofat least 10 W·m–2 (1000 µW·cm–2) andbackground illumination preferably notexceeding 20 lx (2.0 ftc). The followinglists the criteria for parts acceptance orrejection in accordance with the aerospacecompany’s quality requirements.

1. Propagating discontinuities, regardlessof location, are cause for rejectionunless completely removed withindrawing tolerances. Propagatingdiscontinuities are thosediscontinuities that, because of theirnature or geometry, may enlarge inany way during service life. Includedare linear porosity, laps, seams andcracks.

2. Any indication discernible as a crackwhen observed with 10×magnification shall be rejectable.

3. Nonpropagating imperfections areacceptable and do not require removalif the dimensional and smoothnessrequirements established on theengineering drawing can be met.

Table 5 lists one company’s acceptanceand rejection criteria for commonly usedaerospace materials and conditions.Questionable conditions are required tobe referred to quality control engineeringfor resolution. Rejected parts shall bedisposed of in accordance with theapplicable procedures.

Acceptance/RejectionCriteria for LiquidPenetrant Testing of CastTurbine BladesAn example of how one manufacturerhandles the liquid penetrant testrequirements for gas turbine engineturbine blades and vanes is to issue aspecification that covers theacceptance/rejection criteria for visual,radiographic and liquid penetrantindications. The specification indicatesthat drawings shall designate zones on thecasting identified by letters A, B, C etc.Each of these letters (or grades) as definedin this specification establishes differentdegrees of allowable discontinuities forvisual, fluorescent liquid penetrant andradiographic testing. The zones for eachcasting are established by themanufacturer’s materials engineering andstress analysis group.

The specification emphasizes theimportance of carefully controlled liquidpenetrant testing and the necessity torecord on the technique card or otherapplicable document the exact techniquesused to process the parts. In a number ofspecifications, porosity bleedout diametersare specified as accept/reject criteria.These maximum acceptable size limits forliquid penetrant indications apply to thebleedout indication, viewed immediatelyafter wiping the indication only one timewith a swab or cloth dampened with asuitable solvent. The recurrence of thefluorescent liquid penetrant indicationafter once wiping clean is referred to asbleedback.

Positive surface discontinuities (excessmetal), such as mold ridge, fins, bumpsetc., generally are not consideredpotential stress raisers when notassociated with severe undercutting attheir bases. These discontinuities arepermissible on turbine blade and vanecastings provided they can be removed



TABLE 5. Typical aerospace manufacturer’s liquid penetrant testing acceptance and rejection criteria.

HighMaterial and Stainless Iron TemperatureDiscontinuity Aluminum Copper Magnesium Nickel Steel Alloy Titanium Alloy

Bar Stockcracks N N N N N N N Nseams N N N N N N N N

Castingscold shuts Q Q Q Q Q Q Q Qcracks N N N N N N N Nporosity A A Q A A A A Asand blisters A A A A A A A Ashrinkage Q Q Q Q Q Q Q Q

Extrusionsblisters N N N N N N N Nbroken surface N — N — N N N N deep scratches Q Q Q Q Q Q Q Qdie drag Q Q Q Q Q Q Q Qdie weld cracks N N N — — — — —cracks N N N N N N N Ninclusions Q Q N Q Q Q N Qmetal pick up A A A A A A A Apitting A Q N Q Q A N Q

Forgingscracks N N N N N N N Ninclusions Q Q N Q Q Q N Qlaps N N N N N N N N

Formed Partcracks N N N N N N N Ninclusions Q Q N Q Q Q N Qmetal pick up A A A A A A A Aorange peel Q Q Q Q Q Q Q Q

Heat Treatedcracks N N N N N N N Nscale A A A — — — — —

Machined partscracks N N N N N N N Ngrinding cracks N N N N N N N Ntool marks Q Q N Q Q Q N Q

Platecracks N N N N N N N Ninclusions Q N N Q Q Q N Qlaminations N N N N N N N Npitting A Q N Q Q A N Qscratches A A N A A A N A

Sheetcracks N N N N N N N N inclusions Q Q N Q Q Q N Qlaminations N N N N N N N Npitting A Q N Q Q A N Qscratches A A N A A A N A

A = Acceptable.N = Not acceptable.Q = Questionable.

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Grade B Negative flaw of diameter≤ 0.4 mm (0.016 in.) andestimated depth ≤ 0.2 mm(0.008 in.) or 20 percent oflocal drawing minimumthickness, whichever is less

Any number allowed if clearlyspaced ≥ 3 mm (0.12 in.)apart

Bleedback of diameter≤ 0.4 mm (0.016 in.)

Any number allowed if clearlyseparated a distance ≥ 3 mm(0.12 in.) apart

Diameter ≤ 0.4 mm (0.016 in.)Any number allowed if clearly

separated a distance ≥ 3 mm(0.12 in.) apart, and eachdoes not exceed 20 percentof the local drawing specifiedthickness

Grade C Allows same flaws as Grade B,plus four negative flaws perside of 0.4 to 0.8 mm (0.016to 0.032 in.) diameter andestimated depth ≤ 0.2 mm(0.008 in.) or 20 percent oflocal drawing minimumthickness, whichever is less, ifclearly separated a distance≥ 3 mm (0.12 in.) apart

Bleedback ≤ 0.4 mm (0.016 in.)diameter

Allows any number, plus four0.4 to 0.8 mm (0.016 to0.032 in.) diameterindications per side if allindications clearly separated adistance ≥ 3 mm (0.12 in.)

Allows same indications as inGrade B, plus 0.4 to 0.8 mm(0.016 to 0.032 in.) diameterlimited to four places per areadesignated, if clearlyseparated a distance ≥ 3 mm(0.12 in.), none exceeding20 percent of local drawingspecified thickness

Grade D This grade is usually assigned toblade root serrations andallows essentially same flaws asunder Grade C except thatthere shall be no 0.4 to0.8 mm (0.016 to 0.032 in.)indications in serration rootradii; not more than two 0.2to 0.8 mm (0.008 to 0.032 in.)indications per serration; andnot more than four 0.2 to0.8 mm (0.008 to 0.032 in.)indications in all serrations oneach side of blade

Bleedback allowed same as ingrade C except that thereshall be no 0.4 to 0.8 mm(0.016 to 0.032 in.)indications in serration rootradii; not more than two 0.4to 0.8 mm (0.008 to0.032 in.) indications perserration; and not more thanfour 0.4 to 0.8 mm (0.008 to0.032 in.) indications in allserrations on each side ofblade

Same as Grade C

Grade E Allows same flaws as Grade B,plus four negative flaws perside of 0.4 to 1.5 mm (0.016to 0.06 in.) diameter and anestimated depth ≤ 0.5 mm(0.02 in.) or 25 percent of thelocal drawing thickness,whichever is least, if clearlyseparated a distance ≥ 6 mm(0.24 in.) apart

Negative flaws of diameter≤ 0.8 mm (0.032 in.) allowedif spaced ≥ 3 mm (0.12 in.)

Allows same bleedback as inGrade B, plus four bleedbackindications per side of 0.4 to1.5 mm (0.016 to 0.06 in.)diameter if clearly separated aminimum of 6 mm (0.24 in.)apart

Allows indications ≤ 0.8 mm(0.032 in.) diameter if spaced3 mm (0.12 in.) apart

Allows same indications as inGrade B, plus diameter ≥ 0.4to 1.5 mm (0.016 to 0.06 in.)limited to four places per areadesignated if clearly separateda distance ≥ 3 mm (0.12 in.)apart and each does notexceed 20 percent of thelocal drawing specifiedthickness

Allows indications of diameter≤ 0.8 mm (0.032 in.) ifspaced ≥ 3 mm (0.12 in.)apart

Stock surfaces,all grades

Negative defects allowed todepth of machining stock

Negative flaws of ≤ 0.1 mm(0.004 in.) diameter shall beconsidered not interpretableand shall be acceptableregardless of location if clearlyseparated by ≥ 2.5 mm(0.1 in.)

Cracks, folds, cold shuts, orlinear flaws (width 1/3 itslength) are not allowed

For any grade, bleedbackallowed to depth ofmachining stock


Cracks, folds, cold shuts orlinear flaws (width 1/3 itslength) are not allowed

Unlimited flaws allowed todepth of machining stock


Cracks, folds, cold shuts, orlinear flaws (width 1/3 itslength) are not allowed

TABLE 6. Example of acceptance standards for nondestructive testing of cast turbine blades and vanes.

Grade Visual Testing Liquid Penetrant Testing Radiographic Testing

Grade A No defects allowed No bleedback allowed No defects allowed

without exceeding the minimumdimensions on the drawings and areremoved by approved techniques ofgrinding and polishing.

Negative discontinuities may be eitherpropagating (cracks, cold shuts, folds) ornonpropagating (oxide pits, small gasholes, shallow smooth bottomeddepressions). Propagating discontinuitiesare not acceptable regardless of location.Negative discontinuities of thenonpropagating type are acceptable to thelimits set forth in Table 6. Any of theabove listed discontinuities that occur inareas with stock to be removed in lateroperations shall not be immediate causefor rejection. Such discontinuities may beremoved within the stock allowance limitsto ascertain that the requirements of thiscompany’s standards are met. Figure 26shows examples of working standards andinterpretation guides provided for shoppersonnel for the case of a 356 aluminumfan casting.


FIGURE 26. Example of working standardand interpretation guide for shop personnelinspecting 356-T6 aluminum fan casting forporosity bleedout diameter: (a) indicationsin fan blade fillet area (cause for rejectionbecause area is highly stressed so indicationspropagate through part thickness);(b) indications in rim of fan, which hasheavy wall thickness and is subject to lowerstress levels. Probing indicates depths ofabout 0.3 mm (0.01 in.). Experience hasindicated that these have never extendedthrough fan rim and therefore will not bedetrimental to use of fan.

(b)

(a)

1. Polushkin, E.P. Defects and Failures ofMetals: Their Origin and Elimination.New York, NY: Elsevier PublishingCompany (1956).

2. Barer, R.D. and B.F. Peters. Why MetalsFail. New York, NY: Gordon andBreach Science Publishers (1970).

3. Colangelo, V.J. and F.A. Heiser.Analysis of Metallurgical Failure, secondedition. New York, NY: John Wiley &Sons (1987).

4. ASTM E 1417, Standard Practice forLiquid Penetrant Examination. WestConshohocken, PA: American Societyfor Testing and Materials (1995).

5. ASTM E 433-71, Standard ReferencePhotographs for Liquid PenetrantInspection. West Conshohocken, PA:American Society for Testing andMaterials (1993).


References