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Ward D. Rummel, D&W Enterprises, Limited, Littleton,Colorado

9C H A P T E R

Liquid Penetrant Testing

Crack Detection

Capabilities and Reliability

Copyright 1999 by D&W Enterprises, Limited. Reprinted with permission.

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IntroductionLiquid penetrant testing is one of themost widely applied methods ofnondestructive testing because of the lowcost of application, the perceivedsimplicity of the process and its ability tobe applied to complex shapes with littleprocess adjustment. Liquid penetrant

testing is not, however, absolute and isnot capable of finding all cracks. Theend-to-end liquid penetrant process isbounded by a lower limit for thedetection of small cracks (detectionthreshold). In addition, the detectionthreshold is not a constant value butdepends on multiple parameters inherentto the test object and liquid penetrantprocedure applied or are a function ofrigid process control in application.Although some confidence may beprovided by the ability of a liquidpenetrant procedure to reveal a knowncrack in a test coupon, the brightness orcontrast intensity of the liquid penetrantindication may be such that it would notbe detected under field conditions if itspresence and location were not known. Itis foolhardy to assume crack detection ifthe indication is small or dim or both.The inspection result of importance is notthe smallest discontinuity detectedbut thelargest discontinuity missedby applicationof a given liquid penetrant procedure.

In many liquid penetrant testingapplications where requirements are forthe detection of large cracks, exceeding25 mm (1 in.) long, demonstration of

liquid penetrant detection capabilitiesmay not be required. Inspectionsrequiring detection of small cracks andcritical inspection applications mayrequire demonstration of detectioncapabilities and rigid process control thatincludes periodic revalidation of detectioncapabilities.

Probability of Detection(POD) As Measure ofLiquid PenetrantPerformanceThe recognized metric for ascertaining thedetection capability for a liquid penetrantprocedure is the probability of detection(POD). A characteristic POD curve isgenerated by passing a large number ofcracks of varying size through a liquidpenetrant procedure and recording thecracks that are detected. A standardizeddata analysis procedure is then used to

produce a plot of probability of detectionas a function of crack size (typically cracklength). Figure 1 is an example of a PODcurve for a given liquid penetrant testprocedure.

By convention, the threshold detectionpoint is that point where the POD curvecrosses the 90 percent threshold and isreferred to as the 90/95 probability ofdetection value (assuming the number anddistribution of crack sizes meet the criteriafor standardized analysis). In the exampleshown, the single valued detectioncapability is at the 3.5 mm (0.14 in.) crack

276 Liquid Penetrant Testing

FIGURE 1. General form of probability of detection curve for liquid penetrant testingprocedure. Accepted 90 percent threshold detection point is noted.

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Actual crack length, mm (in.)

Probabilityofdetection

(percent)

Legend

= predicted probability of detectionX = hit datum (along top edge) or miss datum (along bottom edge)

Threshold

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length. Although some cracks smallerthan the 3.5 mm (0.14 in.) crack lengthwere detected by the applied liquidpenetrant procedure (Xs plotted at the100 percent detection level) , theprocedure did not reliably detect crackssmaller than the 3.5 mm (0.14 in.) cracksize.

The capabilityof the procedure is statedas 3.5 mm (0.14 in.) crack length.

The reliabilityfor detection of cracks atthe 3.5 mm (0.14 in.) level is 90 percentand is a measure of the repeatability andreproducibility for finding a crack at thatsize.

The confidence level for detection at the3.5 mm (0.14 in.) level is derived from thenumber and distribution of cracks used todetermine the detection capability.

Multiparameter ProcessLiquid penetrant testing is amultiparameter process. Changes in one

or more of the process parameters cansignificantly change the liquid penetrantcrack detection performance capability.Liquid penetrant process performancedepends on the following: (1) test object,(2) discontinuity type, (3) discontinuitysize, (4) discontinuity form and state,(5) test materials, (6) test equipment,(7) test process, (8) test environment,(9) test procedure applied and (10) humanfactors.

The test objectmaterial, shape(configuration), surface texture (porosityand surface finish), stress state, surfacecondition and object size are parameters

that may affect liquid penetrant processcapability. For example, titanium alloyswet differently than aluminum alloysused in aircraft.

The discontinuity type may be asignificant factor; for example, acondition for detection is that thediscontinuity must be open to the surfaceand must have an opening such thatliquid penetrant can be retained using theapplied liquid penetrant procedure. It isobvious that a crack that is covered by apaint layer or smeared metal from amachining or grinding process will not beconsistently detectable.

The discontinuity size i.e., length,

depth and opening greatly affectdetectability. The brightness of a liquidpenetrant indication for a small crack (lessthan 0.75 mm [0.030 in.]) will besignificantly less than that for a largercrack because of the reservoir size. Theability of a human reader to discriminatesmall indications is typically greater than0.75 mm (0.030 in.) in length. Becausediscontinuity size (length and depth) is abasic material parameter used in staticdesign and life cycle analyses, it is the

variable that is most often measured,reported or claimed.

The discontinuity form and state aremore difficult to quantify but must beconsidered in each liquid penetrantapplication. It is obvious that adiscontinuity must be clean and dry tosupport reliable liquid penetrantperformance. Less obvious is the closureand stress state of a discontinuity (crack).

It is known that the brightness of a liquidpenetrant indication is decreased for acrack under compressive load, thus smallcracks will not be detectable under someloading conditions. In addition, the pastload history (in particular, a highoverload) may change the configurationof a crack such that a liquid penetrantindication is segmented, with the crackends visible and the connecting ligamentproducing a very dim indication.

The test materials (liquid penetrantmaterials) are known to affect thesensitivityof a liquid penetrant and liquidpenetrant materials are classified by their

ability to produce an indication understandardized test conditions. A variety ofliquid penetrants of differing sensitivityarecommercially available and are intendedfor use in different applications. Forexample, a high sensitivityliquidpenetrant would not be suitable for theinspection of a porous material surfacebecause of the background indicationsproduced.

The inspection equipmentintroducesvariables in the test process because of theinherent limits and variations forprocesscontrol. For example, a temperaturenonuniformity in a drying oven will addto the end-to-end process variance.Ultraviolet radiation intensity, white lightbackground, tank contamination etc. arevariables added by the type of equipmentor equipment maintenance.

The inspection process may introducemajor variance in end-to-endperformance. For example, dry developerversus wet developer; hydrophilicemulsifier versus a lipophilic emulsifier;or inspection without a developer.

The inspection environmentcan producewide variances in process performance.Consider variance between in-lineprocessing in a factory environmentduring component overhaul versus field

inspection in the arctic.The inspection procedure appliedmay

cause wide variance in processperformance in the form of processingtimes. The most critical process parameteris the remover time or emulsification timebut wash times, dwell times, drying timeetc. can add significant performancevariance.

Human factors variables are listed lastbecause the human operator has littlechance for discontinuity detection if other

277Liquid Penetrant Testing Crack Detection Capabilities and Reliability

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end-to-end parameters are not controlled.Human factors include the traditionalhealth, attitude, skill level and attentionto detail that are related to training,experience and recent experience with thesame or similar parts, equipment andprocesses.

Process Control Is Criticalto Liquid PenetrantPerformanceThe multivariate nature of a liquidpenetrant process demands processcontrol to effect repeatable and

reproducible results. Indeed, liquidpenetrant testing without attention toprocess control is often an exercise inparts washing. Process control aids andmeasurement tools such as ultravioletradiation intensity measurement; artifactpanels such as the testing and monitoring(TAM) panels; and periodic measurementof liquid penetrant material properties areessential to reliable liquid penetrant

process performance. In addition,attention to detail in equipment andprocessing materials unique to a particulartest object or process line is required forconsistent performance.

Periodic proficiency demonstration andvalidation for human operators is also


FIGURE 2. Effect of ultraviolet radiation level for liquid penetrant testing procedure (water washprocess without developer, on tightly closed fatigue cracks in cobalt alloy; with 55 to 107 lx[5 to 10 ftc] white light illumination): (a) 4 Wm2 (400 Wcm2) ultraviolet radiation;(b) 12 Wm2 (1200 Wcm2) ultraviolet radiation.1

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(percent)

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(a)

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essential to end-to-end process control.

Additional training may be beneficial butadditional training without proficiencydemonstration is often a rerun of previousinformation and may have little benefit inmaintaining skill levels.

Process PerformanceAssessment andProficiency DemonstrationVariation in end-to-end liquid penetrantprocess performance is readily observedby comparison of probability of detection(POD) data as a function of varying liquidpenetrant process parameters. Table 1 andFigs. 2 to 5 are examples of the assessmentof liquid penetrant process parameters


FIGURE 3. effect of developer for liquid penetrant testing procedure (water wash process ontightly closed fatigue cracks in cobalt alloy): (a) without developer, (b) with developer.1

(b)

(a)

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Prob

abilityofdetection

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Legend


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FIGURE 4. Effects of etching and proof test for liquid penetrant testing procedure (solventremover process; on tightly closed fatigue cracks in 6Al-4V titanium material): (a) asmachined condition; (b) after etch; (c) after proof load.1

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FIGURE 5. Effects of etching and proof test for liquid penetrant testing procedure (solventremover process; on tightly closed fatigue cracks in AISI 4340 steel material): (a) as machinedcondition; (b) after etch; (c) after etch and proof load.1

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using the POD metric. Figures 2 and 3show results with a heat resistant cobaltalloy resembling SAE AMS 5608D.2

Figure 2 shows the effect of ultravioletradiation level on liquid penetrantperformance. Figure 3 shows the benefitof using developer on liquid penetrantperformance. Figures 4 and 5 show theeffects of etching and proof loading ontitanium and steel flat plate specimen.

Human factors effects are reflected in alldata presented. Variations in humanfactors were reduced by using the sameoperators for each test sequencepresented.

The usefulness of the POD method ofliquid penetrant performance assessmentis self-evident for purposes of proceduredevelopment, procedure improvementand the qualification and validation ofprocess and procedure. For criticalapplications, POD qualification andvalidation is often used as a requirementfor both facility and personnelperformance demonstration.

Human Performance inLiquid Penetrant TestingThe individual performance of the humanoperator is a significant element inend-to-end process performance capabilityand reliability. Inspection processparameters are readily measured andcontrolled by application of appropriatetools when applied by knowledgeablesupervision. The readout or interpretationof liquid penetrant indications by humanoperators is more difficult and

encompasses the multiple factors of theworkplace environment and developmentof skill levels applicable to the procedurebeing performed.

Training and skill development arecomplimentary but not interchangeable.Trainingdenotes transfer of knowledgewhereas skill developmentdenotesapplication of knowledge, procedure andexperience in performing a specific task.When an operator views a liquidpenetrant indication, interpretation is notayes/no decision task as commonlyviewed and desired in productionapplication but is instead a problem in

conditional probability. The operator ispresented with a signal characterizedprimarily by brightness, size and pattern.The signal is superimposed on abackground (nonrelevant indicationsinherent to the test object and liquidpenetrant process also termed clutter)characterized by the same parameters butof a lesser degree or level. For purposes ofdiscussion, the signal characteristics willbe termed signal and the backgroundcharacteristics will be termed noise. The

interpretation problem is to identify thesignal in a field that consists ofsignal plusnoise.

Varying cases of relative levels of signaland noise are shown in Fig. 6. Figure 6a isa case where the signal level issignificantly greater than the noise andclear discrimination is possible. Figure 6bis a case where the signal level overlapsthe noise and noise may be interpreted as

signal (false call) or a signal may be


Probabilitydensity

distribution

(relativeunits)

FIGURE 6. Three conditions of signal and noise, representingdiffering levels of defect detection: (a) clear signal-to-noisediscrimination; (b) small overlap of signal and noise, somefalse calls and misses; (c) overlapping signal and noise, poordiscrimination.

Noise ThresholdDecision

Level

Signal

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Noise

False calls

Signal

(b)

Probabilitydensitydist

ribution

(relativeunits)

Noise Signal

(c)

Probabilitydensitydistribution

(relativeunits)



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interpreted as noise (miss). Figure 6c is acase where the signal and noise aresuperimposed and discrimination is notpossible. The greater the separation ofsignal and noise, the greater the margin forcorrect interpretation. Because signal is adirect function of the size and brightnessof a liquid penetrant indication, large andbright indications should be readilydetectable and have a high probability ofdetection.

The varying cases of signal-to-noisediscrimination are consistent with thePOD results shown in Figs. 2 to 5. Larger

and brighter liquid penetrant indicationsproduce discrimination at a low level i.e., developer versus no developer, higherultraviolet radiation levels and removal ofsmeared surface material by etching.

As a result of this observation of theliquid penetrant indications, the decisionof a human operator has four possibleoutcomes.

1. A discontinuity may be called when adiscontinuity is present (correct call).

2. A discontinuity may be called whenno discontinuity is present (Type IIerror).

3. A no discontinuity condition may becalled when no discontinuity ispresent (correct call).

4. A no discontinuity condition may becalled when a discontinuity is present(Type I error).

The conditional probabilitydiscrimination decision process is shownschematically in Fig. 7 and Table 1.

Liquid Penetrant ProcessPerformanceDemonstrationThe structural integrity of modernengineering materials, components,structures and systems are increasinglydependent on the ability ofnondestructive testing processes to findsmall discontinuities. Componentsrequiring test capabilities below thegeneral detection level are often termedfracture critical. Although well intentioned,the assumedcapabilities for a given liquidpenetrant test operation are generallyincorrect and, in some cases, the criteria

noted on engineering drawings are belowthe capabilities of the best performing testfacilities. Qualification and validation oftest facilities, procedures and personnelare therefore required for fracture criticalcomponents.

Two modes of qualification andvalidation are in general use; these are the

full POD demonstration and the subsetdemonstration (widely known as the29-out-of-29 method). A full PODdemonstration is always the most rigorousmode and is used when dealing with newmaterials, new fabrication processes ornew liquid penetrant processing facilities.A full POD demonstration is desirable forqualification of less experiencedinspectors. Although the full PODdemonstration is desirable, the cost of testcomponents and the time involvedwarrant consideration of alternativemethods. A large amount of liquidpenetrant process characterization data

has been generated and is available.1

When requirements are such that asignificant margin is realized by the use ofstandard, generic liquid penetrantprocessing techniques, no demonstrationmay be necessary. When standard, genericprocess techniques are applied to thedetection of small discontinuities withinthe envelope of previously demonstratedcapabilities, a subset demonstration maybe considered.


FIGURE 7. Four possible outcomes of accept/reject decision.(Probability P is discussed with the published POD data.1)

Nondestructiveevaluation

signal

(Response

to

discontinuity

P(A,a)True positive (hit)

No error

Positive (a)

Stimuli (discontinuity presence)

Positive (A)

Negative (N)

P(A,n)False positiveType II error

Negative (n)

P(N,a)False negative (miss)

Type I error

P(N,n)True negative

No error

TABLE 1. Accept/reject decision process.

Call Error Condition Decision

True positive none (discontinuity found when discontinuity is present) reject (correct reject)

False positive type II (discontinuity found when no discontinuity is present) reject (false call)

False negative type I (no discontinuity found when discontinuity is present) accept (miss)

True negative none (no discontinuity found when no discontinuity is present) accept (correct accept)

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A subset demonstration is conductedby generating at least 29 representativediscontinuities of nominally equal size(but within the previously demonstratedsize envelope and using the samematerials) and near or below theacceptance limit for the test component.The additional specimens are thenconsidered to be a subset of thosepreviously used for full POD

demonstration. The 29 discontinuities arethen subjected to the candidate testprocedure (operator) and the inspection iscompleted. Success requires that all 29 ofthe discontinuities are detected. If adiscontinuity is not detected, thedetection failure must be resolved. If thereis a legitimate cause for missing thediscontinuity, the discontinuity may bereplaced and the process repeated. Failureto resolve and rectify a misseddiscontinuity requires full demonstrationby the POD method. When the subsetmethod is used for operator skilldemonstration and qualification, a failure

to detect may be resolved by additionalskill development (training alone is notsufficient) and retesting at the end of thedefined skill development period. Thesubset, 29-out-of-29 method provides therequired number of test opportunities fora 90 percent confidence level for a singlediscontinuity size and is consistent with asingle point on a demonstrated full PODcurve.

The rationale, complexity andanalytical methods for full PODdemonstration are beyond the scope ofthis publication. The reader is referred toMIL-STD-1823 for requirements andmethodologies for a full PODdemonstration.3

SummaryLiquid penetrant testing is an effectiveand economical method of discontinuitydetection and is widely used in theprocess of ensuring the safety andstructural integrity of engineeringmaterials, components, structures andsystems. Its wide use and superficiallysimple application result in a wide rangeof results that vary from consistentdetection of critical discontinuities toparts washing exercises that do not addvalue to the parts. Fortunately, it costs nomore to perform a valid inspection than itdoes to conduct a parts washing exercise.It is logical that a multiparameter testingprocess requires attention to detail andprocess control for successful application.The tools and techniques for materialsand process control are readily available.The end to end process performance mayalso be quantified using the informationand techniques discussed herein. If the

principles and technique described areused, capable and reliable performancemay be expected and demonstrated toensure continuing excellence in liquidpenetrant processing and continuingconfidence in the safety and structuralintegrity of engineering systems.

The smallest discontinuity found is ofacademic interest. The largest discontinuitymissed is of critical importance to reliable

liquid penetrant testing.


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1. Rummel, W.D. and G.A. Matzkanin.Nondestructive Evaluation (NDE)Capabilities Data Book, third edition.NTIAC DB-97-02. Austin, TX:Nondestructive Testing Informationand Analysis Center (1997).

2. SAE AMS 5608D, Cobalt Alloy,Corrosion and Heat Resistant,Sheet Strip and Plate40Co-22Cr-22Ni-14.5W-0.07LaSolution Heat Treated. Warrendale,PA: Society of Automotive Engineers(1995).

3. MIL-STD-1823, Non-DestructiveEvaluation System Reliability Assessment.Washington, DC: United States

Department of Defense.

285Liquid Penetrant Testing Capabilities and Reliability

References

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