NASA/TM–2011–215869

A Comparison of the Capability of Sensitivity Level 3 and

Sensitivity Level 4 Fluorescent Penetrants to Detect Fatigue

Cracks in Various Metals

Bradford H. Parker

Goddard Space Flight Center, Greenbelt, MD

January 2011
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NASA/TM–2011–215869

A Comparison of the Capability of Sensitivity Level 3 and

Sensitivity Level 4 Fluorescent Penetrants to Detect Fatigue

Cracks in Various Metals

Bradford H. Parker

Goddard Space Flight Center, Greenbelt, MD

National Aeronautics and

Space Administration

Goddard Space Flight Center

Greenbelt, Maryland 20771

January 2011
Trade names and trademarks are used in this report for identification only.

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or implied, by the National Aeronautics and Space Administration.

Available from:

NASA Center for AeroSpace Information National Technical Information Service

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Hanover, MD 21076-1320 Springfield, VA 22161
Introduction

Under the auspices of the NASA Nondestructive Evaluation (NDE) Program, the Goddard

Space Flight Center (GSFC) Materials Engineering Branch initiated a task in October 2008

to investigate the effect of liquid penetrant sensitivity level on probability of crack

detection. The purpose of the task was to address the requirement in NASA-STD-50091,

“Nondestructive Evaluation Requirements for Fracture Critical Metallic Components,”

which was released in April 2008, that states only sensitivity level 4 penetrants could be

used for NASA Standard Level liquid penetrant inspections.

During the development of NASA-STD-5009, there was considerable debate within the

NASA NDE community about this requirement, as it was a change from the Agency’s

previous policy of allowing the use either sensitivity level 3 or level 4 liquid penetrants for

NASA Standard Level liquid penetrant inspections. The rationale for the requirement

change was the fact that the data used to establish the reliably detectable crack sizes for

liquid penetrant inspection was from studies performed in the 1970s using penetrants

deemed to be equivalent only to modern day sensitivity level 4 penetrants. Hence, it was

argued that there was a lack of data to support allowing the use of sensitivity level 3 liquid

penetrants.

In this study, NDE inspectors performed probability of detection (POD) demonstration

tests on aluminum (6061-Al), a high temperature cobalt based alloy (Haynes 188) and

titanium (Ti-6Al-4V) crack panels sets using various sensitivity level 3 and level 4

penetrants. In order to quantify the POD, the hit-miss data from the individual

demonstration tests were analyzed using both the point estimate method and the curve

fitting method of data analysis. In addition, combinations of hit-miss data sets were

analyzed using the curve fitting technique in order to further compare the data not only by

penetrant sensitivity level, but also by inspection method, inspector and metal. The end

goal was to determine if there is a significant difference in the crack detection capability of

sensitivity level 3 and level 4 penetrants.

Penetrant Inspection Materials

The penetrants selected for the study were Method A, Water Washable and Method D, Post

Emulsifiable, Hydrophilic, as these are the most common penetrant inspection methods

used in the aerospace industry. Sensitivity level 3 and level 4 penetrants from two

manufactures, Sherwin and Magnaflux, were selected. Table 1 shows the matrix of eight

penetrant inspection materials (four sensitivity level 3 and four sensitivity level 4) used in

the study. The same nonaqueous developer (Sherwin D-100, which is applied by spray

can) was used for all the Method A inspections and likewise a consistent dry powder

developer (Magnaflux ZP-4B) was used for the Method D inspections.
Table 1: Summary of the penetrant materials used in this study.

Method Sensitivity

Level Penetrant Remover Developer

A 3 Magnaflux ZL-67 N/A Sherwin D-100

A 4 Magnaflux ZL-56 N/A Sherwin D-100

A 3 Sherwin HM-607 N/A Sherwin D-100

A 4 Sherwin HM-704 N/A Sherwin D-100

D 3 Magnaflux ZL-27A Magnaflux ZR-10B Magnaflux ZP-4B

D 4 Magnaflux ZL-37 Magnaflux ZR-10B Magnaflux ZP-4B

D 3 Sherwin RC-65 Sherwin ER-83A Magnaflux ZP-4B

D 4 Sherwin RC-77 Sherwin ER-83A Magnaflux ZP-4B

Crack Panels

As mentioned previously, three different sets of crack panels were used in the study. Table

2 summarizes the attributes of each panel set. In the table, the crack aspect ratio is defined

as the crack depth divided by the crack length. In all three sets, the cracks were produced

in low cycle bending fatigue. The 6061-Al panels are owned by GSFC and their

production is described in NASA Technical Memorandum 2009-215850 2. The Haynes

188 set was borrowed from NASA Marshall Space Flight Center (MSFC) and their

production is described in NASA-CR-170879 . The Ti-6Al-4V panel set was borrowed

from NASA Johnson Space Center (JSC). The production of the Ti-6Al-4V panels is

described in Appendix B.

Table 2: Attributes of the three crack panel sets used in the study.

Crack Length Number Panel Dimensions Total Number Nominal Crack Metal Ranges of Panels (inches) of Cracks (inches) Aspect Ratio

6061-Al 30 8 x 3.5 x 0.25 82 0.023-0.215 0.3

Haynes 188 30 16 x 4 x 0.25 107 0.010-0.350 0.2

Ti-6Al-4V 29 6 x 3.9 x 0.125 54 0.032-0.094 0.4

Figure 1 shows statistical box plots of the crack length distributions for each panel set.

There is a sharp contrast between the distribution in the Ti-6Al-4V set and the other two

metals. The Ti-6Al-4V set was designed for point estimate POD demonstration testing at a

crack length of 0.050 inches. Hence, 45 of the 54 cracks in the set have lengths between

0.045 and 0.055 inches. For the Haynes 188 and 6061-Al sets there is broader distribution

2
of crack lengths and these two sets are better suited for the curve fitting POD data analysis

technique.

Figure 1: Box plots of crack length distribution for each of the crack panel sets.

Procedures

General Procedures

Seven of the nine inspectors that participated in the study were at a vendor local to NASA

GSFC. The remaining two inspectors were members of the GSFC Materials Engineering

Branch. Because of inspector turn over at the NDE vendor and the availability of the crack

panel sets (the Haynes 188 and the Ti-6Al-4V sets were borrowed from other NASA

Centers), not all nine inspectors performed POD demonstration tests on all three metals

using the complete matrix of Method A and Method D penetrants. Table 3 summarizes the

POD demonstration testing performed in the study. The table shows that a total of 92

penetrant POD demonstration tests were performed. Forty-six of the demonstrations were

with sensitivity level 3 penetrants and 46 were performed with sensitivity level 4

penetrants.

3
As a consequence of having to borrow two of the crack panels sets, the demonstration tests

were performed in three phases. First, all the Method A and Method D demonstration tests

were performed on the 6061-Al panel set. Once the 6061-Al testing was complete, the

request was placed to borrow the Haynes 188 panels from MSFC. The testing with the

Haynes 188 panels was then completed prior to receipt of the Ti-6Al-4V panel set from

JSC.

Total Number of Trials Metal Method Inspectors Sensitivity Level 3 Sensitivity Level 4

Vendor 1

Vendor 2

6061-Al A Vendor 3 10 10

Vendor 4

GSFC 1

Vendor 3

Vendor 4

Vendor5 6061-Al D 12 12 Vendor 6

Vendor 7

GSFC 1

Vendor 3

Haynes 188 A Vendor 5 6 6

GSFC 1

Vendor 3

Haynes 188 D Vendor 5 6 6

GSFC 1

Vendor 1

Ti-6Al-4V A Vendor 3 6 6

Vendor 7

Vendor 1

Ti-6Al-4V D GSFC 1 6 6

GSFC 2

Table 3: Summary of the POD demonstration tests performed in the study.

In order to help avoid memorization of the crack panel identification numbers that were

either etched or stamped on the panels (many inspectors inspected the same panel set eight

times), the panels were given random and new numbers for each inspection. The new

numbers were placed on the panel using a “permanent” marker. The inspectors were

instructed to use this temporary number when identifying the panel during a demonstration

test. After each inspection, the numbers were removed during the panel cleaning process

and then new numbers were assigned after cleaning.

For all three panels sets, a grid was used for identification of crack locations. Figure 2 is an

optical photograph of a 6061-Al panel with the corresponding inspection grid. Similar

grids were used for the Haynes 188 and Ti-6Al-4V panel inspections. All inspections

were of the hit-miss type. In addition to hits and misses, the number of false calls was also

recorded.

4
Figure 2: Optical photograph of an aluminum crack panel and the grid used to identify the

detected crack locations.

Method A Procedure

For the Method A inspections, the vendor’s procedure was utilized by all the inspectors

including the GSFC inspector. The minimum penetrant dwell time was 20 minutes and the

developer time was 10 minutes. All the other parameters, i.e., wash pressure, wash

temperature and drying oven temperature were within established limits. Quality assurance

checks for black lights, background lighting, etc. were in place and maintained. As

important, all these parameters were constant throughout the test matrix.

Each inspector progressed through the matrix of four Method A penetrants with no

knowledge of the actual penetrant being used. The order in which a particular inspector

progressed through the penetrants was varied. In addition, the inspectors were given no

feedback about their performance until the study was complete. Inspectors were typically

tested once a week and due to scheduling there were often several week delays between a

specific inspector taking the test.

Method D Procedure

The vendor did not address Method D in their penetrant procedure. As a consequence,

GSFC introduced a procedure for this process. The minimum penetrant dwell time was 30

minutes, the emulsifier concentration for both the Magnaflux ZR-10B and the Sherwin ER83A was 16.7 percent by volume (one gallon of emulsifier mixed with five gallons of

water) and the emulsification time was 2 minutes. Specimens were prewashed prior to

emulsification, however there was no established limit on the prewash or post wash times.

After oven drying, the dry powder developer was applied by a manual dip and drag

technique and there was no minimum developer time. All other parameters (wash pressure,
oven temperature, etc. ) were the same as those used in the vendor procedure for Method A.

Again, the sequence that an inspector progressed through the matrix of penetrants was

varied and no performance feedback was supplied.

Data Analysis

The hit-miss data from each demonstration test was analyzed using two different

approaches to yield crack lengths that have a 90 percent probability of detection with 95

percent confidence (a90/95). The first technique was a point estimate technique, which is

based on binominal distribution statistics. The second method is a curve fitting method

based on the Logit model.

Point Estimate Method

Usually, the point estimate method is applied to a set of cracks with a narrow range of

sizes. For example, NASA uses this technique to qualify inspectors for the NASA Special

Level penetrant inspection crack length of 0.050 inches. During such a demonstration test,

the inspector is given a set of crack panels that ideally contains 29 cracks of exactly 0.050

inches length. Because of the difficulty in controlling the crack length during panel

production, the 29 cracks typically have a range of 0.045 to 0.055 inches (plus or minus 10

percent). Based on the binomial statistics, if the inspector finds all 29 cracks, then an

a90/95 at 0.050 inches has been demonstrated. Volume 11 of the Metals Handbook 8 th

Edition4 gives an excellent explanation of the statistics used by the point estimate method.

As previously noted the Ti-6Al-4V panel set from JSC is ideally suited for the point

estimate method of analysis.

Another outcome of the panel production process, is that in order to get 29 cracks within

the desired length range, one usually ends up with a significant number of cracks both

smaller and larger than the desired range. This is a consequence of the fact that many

panels in a set contain more than one crack. If a panel ends up with any crack within the

desired range it will probably be included in the set even if it has both cracks larger and

smaller than the desired range. This is the reason the Ti-6Al-4V panel set has some crack

lengths outside the 0.045 to 0.055 inch range.

During a NASA Special Level demonstration test with a panel set containing 29 cracks in

the target range of 0.050 inches and with both some larger and smaller cracks, the inspector

successfully demonstrates an a90/95 of 0.050 inches if they find all 29 cracks in the target

range as well as all the larger cracks. There is no penalty for missing cracks smaller than

the target range.

In the case of the demonstration tests in this study, there is not a requirement that an

inspector demonstrates an a90/95 of 0.050 inches. On the contrary, we are interested in

seeing at what length a90/95 is successfully demonstrated and then comparing this

demonstrated length for the different sensitivity Level penetrants. For this reason, the point

6
estimate data analysis method was also applied to the Haynes 188 and the 6061-Al sets,

even though these sets were not specifically design for this analysis technique. The

implementation of the point estimate in this context requires finding a group of 29 cracks

that are found where all the cracks with lengths above the range of these 29 cracks are also

found. The demonstrated size is then considered to be the largest crack length in the set of

29 flaws (not the average).

The hit-miss data set is also evaluated to look for sequences of flaws that meet the other

a90/95 hit-miss combinations identified in the Metals Handbook article 4, i.e., 45 hits for 46

cracks, 59 hits for 61 cracks and 72 hits for 75 cracks. Again there is the consideration that

all of the cracks larger than the identified set must also be correctly hit or found. The

combination that yields the lowest a90/95 is the reported number.

Curve Fit Technique

In this study, the curve fit technique is used only to compare the results between different

penetrant sensitivity levels, inspection methods, inspectors or metals and is not used to

quantify or demonstrate NASA Special Level capability.

Statisticians have proposed numerous models such as the log normal distribution function,

log-logistics (also referred to as the log odds) functions and Probit and Logit models to fit

hit-miss inspection data. The statisticians recognized that most nondestructive inspection

techniques produce a situation where below a certain crack length almost all of the cracks

are missed and above a slightly larger crack length almost all of the cracks are hit. In this

case, a plot of the probability of detection (POD) as a function of the crack length (or the

log of the crack length) yields a characteristic “S” shape curve. This “S” shaped curve can

be well fit by the various models mentioned above to yield an a90/95 crack size. This

approach is explained in detail in Volume 17 of the Metals Handbook 9

th Edition5

 for the

log normal distribution and the log-logistics function. The statistics book by J.A. Cramer6

gives a full explanation and history of the Probit and Logit models.

The hit-miss inspection results generated in this study for the 6061-Al and Haynes 188

panel sets generally produce “S” shaped curves and hence using a curve fit technique to

analyze the data is appropriate. There were eight trials out of 68 with the 6061-Al and

Haynes 188 panels where the data did not conform to the curve fit technique. In seven

trials with the 6061-Al panels, there were no missed flaws and as a consequence no curve

fit. In one additional trial with the 6061-Al panels, the inspector missed only one crack out

of 82, however the crack length was 0.070 inches and hence was near the center of the

distribution of lengths. In this situation the curve fit also failed. As will be seen in the

results section, applying the curve fitting technique to the individual hit-miss data sets from

the Ti-6Al-4V panels was much less successful, which is not surprising given the narrow

distribution of flaw sizes in the set.

The curve fit technique adds value to this study because it allows for the evaluation of

combination hit-miss data sets, i.e., all Haynes 188 Method A sensitivity Level 3 hit-miss

results versus all Haynes 188 Method A sensitivity Level 4 results. The use of the point

7
estimate method to evaluate such data sets can produce misleading results. For example, if

one of the inspectors performing the Method A inspections during a particular inspection

missed a very large crack when using one of the sensitivity Level 3 penetrants, and if we

were to combine the hit-miss data from all inspectors into a single data set, the overall point

estimate a90/95 would be determined by this one inspector’s result (a consequence of the

point estimate method is that the a90/95 will never be smaller than the largest missed crack

length). On the other hand, the overall data set would have the characteristic “S” shape and

evaluation of the data set using the curve fit technique would not produce a result

dominated by a single inspection, yet still would include the results from all inspectors.

In support of an effort to update MIL-HDBK-18237, Charles Annis developed a R (an open

source mathematics software program for statistical computing and analysis) package titled

“mh1823 POD.” The menu driven package uses R commands for all data manipulation ,

statistical analysis and graphics. For hit-miss data, the package offers the user eight

different curve fit options all of which are appropriate models for studies were the response

or outcome is binary, i.e., a crack is hit (1) or missed (0). Four of the models are based on

the log of the crack length and four are not. Each is based on a link to an existing R

function, e.g., logit, probit, loglog or cloglog. For each of the eight models, the package

provides a deviance value which is a measure of the goodness of fit. For this study, the

Logit link using the log of the crack length was selected for all the data set analyses, as the

deviance for this choice was generally the lowest. Note that for the data sets in this study

all the model options produced similar a90/95 results. For example, a Method A

inspection of the 6061-Al panel set with 77 hits and 4 misses was fitted using the eight

possible links and the a90/95 values ranged from 0.052 to 0.055 inches (the deviance

values ranged from 13 to 15).

Note again that in this study we are not using the curve fit technique to qualify penetrant

inspectors and we are not attempting to determine which model is the best. Here, a single

curve fit technique (Logit) is universally applied to compare results from the two different

sensitivity levels, and the different inspectors. We state this because the point estimate

method is a more conservative technique for quantifying a90/95 and is the approach

preferred by NASA for demonstrating capability at a single crack length.

False Calls

False calls were not included in the analysis of the data. The tables in Appendix A do

include the number of false calls for each inspection. The grid provided for locating crack

indications had 180, 160 and 96 cells for the 6061-Al, Haynes 188 and Ti-6Al-4V panel

sets, respectively. Hence, for a demonstration test with 30 6061-Al panels and a one-sided

inspection, there are 5400 possible crack locations. For the Haynes 188 and Ti-6AL-4V

panels sets the total number of grid locations was 4800 and 2784, respectively. The

inspectors were instructed to identify only “crack like” indications, however they were

given no feedback about their false call rates between inspections. Based on the available

number of grid locations, the average false call rates were 0. 17, 0.14 and 0.04 percent for

the 6061-Al, Haynes 188 and Ti-6Al-4V demonstration tests, respectively.

8
Results

The results from all the individual inspection trials for each metal and for each penetrant

inspection method are provided in Appendix A tables. These tables include the number of

cracks hit and missed and the number of false calls for each trial. The tables also include

point estimate and curve fitting data analysis results for each individual trial. The tables

included in the various results sections below provide a summary of all the individual trials

found in the Appendix A tables as a function of penetrant sensitivity level. These tables

include the total number of cracks hit and missed, the overall percentage of cracks hit and

curve fit analysis results for combined hit/miss data sets. In addition to the summary

tables, the sections below contain graphical representations of the a90/95 Logit curve fits

for various combined hit/miss sets.

6061-Al Method A

Table 1-A in Appendix A contains all of the individual 6061-Al Method A inspection

results. The first vendor inspector (Vendor 1) to go through the matrix of tests was given

23 to 26 crack panels with 59 to 66 cracks. With this number of cracks, there was not a

point estimate method solution for two of the four trials. As a consequence, additional

crack panels were added to the set to increase the number of larger cracks for all

subsequent inspectors.

Point estimate analysis for the remaining four inspectors with 81 to 82 total cracks

produced a solution in all cases. As mentioned in the Data Analysis section, there was one

trial where the Logit curve fit technique did not produce a solution. This is the case for the

Vendor 3 inspector using Sherwin HM-704 where the one crack missed was 0.070 inches

long.

Table 4 provides a comparison of the sensitivity level 3 versus the level 4 penetrants. The

table shows that the total percentage of cracks found for the 10 trials with each sensitivity

level of penetrant is essentially identical (92.7 versus 93.0 percent). The values in the final

column of the table were obtained by combining all the sensitivity level 3 hit-miss data

(774 cracks) into a single record and all the level 4 hit-miss data (775 cracks) in to a single

record and then performing Logit curve fits to obtain a90/95 values. With this approach,

the a90/95 values for the two sensitivity level penetrants differ by only 0.001 inches.

Table 4: Summary of sensitivity level 3 versus level 4 penetrant performance for 6061-Al

Method A demonstration tests.

Logit

Curve Fit

Sensitivity Number Total Total Total Percent on

Level of Number Number Number of Flaws Combined

Trials of Flaws of Hits of Hit Hit/Miss

Misses Data

(inches)
3 10 774 718 56 92.7 0.052

4 10 775 721 54 93.0 0.051

In order to compare the individual inspectors and the individual penetrants, hit-miss results

from individual trials were combined and analyzed using the Logit curve fit technique.

Figure 3 shows the a90/95 curve fits for the four different Method A penetrants. Each

curve fit was performed on the combined hit-miss data from the five

Figure 3: Logit curve fit a90/95 results as a function of Method A penetrant for 6061-Al

POD demonstration tests.

trials (one trial for each of the five inspectors) performed using the penetrant. The a90/95

values for the penetrants range from 0.051 to 0.055 inches.

In a similar fashion, all of the hit-miss results for each inspector (four trials, two with

sensitivity level 3 and two with sensitivity level 4 penetrants) were combined and analyzed

using the Logit curve fit technique. These results, which are plotted in Figure 4, show that

the demonstrated a90/95 is a stronger function of the inspector than of the penetrant. Here,

the a90/95 values varied between 0.041 and 0.058 inches.

10
Figure 4: Logit curve fit a90/95 results as a function of Method A inspector for the 6060-

Al demonstration tests.

For the Method A inspections, the largest variable in the process was the application of the

nonaqueous Sherwin D-100 developer. One inspector (Vendor 1) applied a heavy coating

of the developer (there was no evidence of the aluminum beneath the developer), whereas

the remaining inspectors applied lighter coatings. One inspector (Vendor 3) inspected the

panels completely before developing and then inspected the panels again during the

development process, i.e., the developer was applied to one crack panel and the panel was

immediately observed under the black light. After recording all crack locations, the

inspector proceeded to the next panel. This inspector had the lowest number of misses.

The remaining inspectors developed batches of panels and waited at least 10 minutes

before beginning the crack identification process.

6061-Al Method D

Table 2-A in Appendix A contains all of the inspection results for the Method D

demonstration tests on 6061-Al. Point estimate method solutions were obtained for all 24

cases. There were seven cases where no cracks were missed. The Logit curve technique

for data sets with all hits and no misses does not yield an a90/95 solution. Switching the

smallest crack in the set (0.023 inches) from a hit to a miss yielded an a90/95 solution of

0.026 inches and this is the value reported for the seven cases with no misses.

Table 5 is a summary of the sensitivity level 3 versus level 4 Method D penetrants. The

overall percentage of the cracks found is slightly higher for the level 3 penetrants (97.4)

versus the level 4 penetrants (96.1). The values in the final column of the table were

obtained by combining all the sensitivity level 3 hit-miss data into a single record and all

11
the level 4 hit-miss data in to a single record and then performing Logit curve fits to obtain

a90/95 values. With this approach, the a90/95 values for the two sensitivity level

penetrants differ by only 0.005 inches. In all these modes of comparison, the level 3

penetrants performed better than the level 4 penetrants.

Table 5: Comparison of sensitivity level 3 versus level 4 penetrants for Method D

demonstration tests performed on 6061-Al.

Logit

Curve Fit

Sensitivity Number Total Total Total Percent on

Level of Number Number Number of Flaws Combined

Trials of Flaws of Hits of Hit Hit/Miss

Misses Data

(inches)

3 12 960 935 25 97.4 0.040

4 12 967 929 38 96.1 0.045

In order to compare the different penetrants and inspectors, hit-miss results from individual

trials were combined and analyzed using the Logit curve fit technique. Figure 7 shows the

a90/95 curve fits for the four different Method D penetrants. Each curve fit was performed

on the combined hit-miss data from the six trials (one trial for each of the six inspectors)

performed using the penetrant. The plots show that one penetrant, Magnaflux ZL-27A,

performed better than the remaining three penetrants. Interestingly, this penetrant is

sensitivity level 3. This result is not unprecedented. In a report by John Lively, 8 four of six

inspectors who performed penetrant deomonstrations tests on Inconel 718 panels using

Magnaflux ZL-37 (sensitivity level 4) and Magnaflux ZL-27A (sensitivity level 3)

performed better with ZL-27A.

12
Figure 7: Logit curve fit a90/95 results as a function of Method D penetrant for 6061-Al

demonstration tests.

Table 6 provides summary results for all four Method D penetrants. This data again shows

that the inspectors performed better with ZL-27A and that the remaining three penetrants

performed similarly. There was no characteristic of ZL-27A that surfaced during the

inspections that would lead one to believe that this penetrant would perform better.

Again, all of the hit-miss results for each inspector (four trials, two with sensitivity level 3

and two with sensitivity level 4 penetrants) were combined and analyzed using the Logit

curve fit technique. These results, which are plotted in Figure 8, show that the

demonstrated a90/95 is again a stronger function of the inspector than of the penetrant.

Here, the a90/95 values varied between 0.031 and 0.057 inches, which is similar to the

range for the Method A inspectors (0.041 to 0.058 inches). It should also be noted, that the

inspector with the largest overall demonstrated a90/95 for Method A, also had the largest

a90/95 for Method D.

13
Table 6: Comparison of the four Method D penetrants used in the 6061-Al demonstration

tests.

Logit

Average Average Curve Fit

Penetrant Number Total Total Total Percent Point Logit on

and of Number Number Number of Flaws Estimate Curve Combined

(Sensitivity Trials of Flaws of Hits of Hit a90/95 Fit Hit/Miss

Level) Misses (inches) a90/95 Data

(inches) (inches)

Magnaflux

ZL-27A 6 480 475 5 99.0 0.064 0.035 0.033

(3)

Sherwin

RC-65 6 480 460 20 95.8 0.084 0.053 0.048

(3)

Magnaflux

ZL-37 6 484 465 19 96.1 0.090 0.053 0.046

(4)

Sherwin

RC-77 6 483 464 19 96.1 0.068 0.049 0.046

(4)

Figure 8: Logit curve fit a90/95 results as a function of Method D inspector for 6061-Al

demonstration tests.

14
Haynes 188 Method A

Table 3-A in Appendix A contains the results from all the Method A inspections performed

on the Haynes 188 panel set. Point estimate solutions were found in 11 of the 12 trials and

curve fit solutions were found for all 12 trials. Table 7 summarizes the sensitivity level 3

results versus level 4 results for all the trials. The table shows essentially identical

percentages of cracks detected for the two sensitivity levels (94.7 and 94.5 percent).

Table 7: Comparison of sensitivity level 3 versus level 4 penetrants for Method A

demonstration tests performed on Haynes 188.

Logit

Curve Fit

Sensitivity Number Total Total Total Percent on

Level of Number Number Number of Flaws Combined

Trials of Flaws of Hits of Hit Hit/Miss

Misses Data

(inches)

3 6 642 608 34 94.7 0.046

4 6 638 603 35 94.5 0.049

The values in the final column of Table 7, which were obtained by performing Logit curve

fits on the combination of all 6 sensitivity level 3 hit-miss data sets and all six of the level 4

sets, show the a90/95 values for the two levels of penetrants differ by 0.003 inches. These

results are shown graphically in Figure 9.

All of the hit-miss results for each inspector (four trials, two with sensitivity Level 3 and

two with sensitivity Level 4 penetrants) were also combined and analyzed using the Logit

curve fit technique. These results, which are plotted in Figure 10, show that the

demonstrated a90/95 is again a stronger function of the inspector than of the penetrant.

Here, the a90/95 values varied between 0.026 and 0.075 inches. Note that the range of

a90/95 values as a function of inspector is much broader for the Haynes 188 compared to

the 6061-Al using the same inspection method and materials. This larger range of values

was attributed to variations in the application of the D-100 developer.

15
;o 0-02 fo?om [o'o"e fo'os (o!1^

c^ack,len,6tf l(inchesf

Figure 9: Logit curve fit a90/95 results as a function of Method A penetrant sensitivity

level for Haynes 188 demonstration tests.

,o V0 (oTO4, (o:os (o;oe (0]

grack,en I,I gth (i mches)'

Figure 10: Logit curve fit a90/95 results as a function of Method A inspector for Haynes

188 demonstration tests.

16
During his first trial, the GSFC inspector noticed that for some of the longer cracks, the

indications were intermittent, almost as if the crack was partially clogged with debris.

Upon low magnification (10 to 20 times) microscopic examination, he observed that the

intermittent indications were a result of the non-uniform and sparse nature of his developer

application. As a consequence, this inspector applied an unusually heavy coating of the

developer and the indications became stronger and continuous. This technique was then

adopted for all his inspections. The Vendor 3 inspector to some extent discovered the

same thing. In his case, the discovery occurred during the checking of some flaw

indications. The inspector was witnessed wiping potential flaw indications with a alcohol

dampened swab and then re-developing the area with a concentrated developer application,

i.e., a much heavier application than was initially applied to the entire panel. The Vendor

5 inspector consistently gave the panels a light dusting of developer and never discovered

the strong influence of the development process on the crack detection in Haynes 188. The

strong influence of developer application for detection of cracks in Haynes 188 was

previously noted by Ward Rummel in his 1998 paper9 . In this paper, the POD for Haynes

188 panels before and after developing was compared. Rummel gives the example of an

inspector detecting only 86 of 284 cracks without developer with no a90/95 achieved.

With developer, the same inspector found 277 of 311 cracks with an a90/95 of 0.077

inches.

As previously noted, for the Method A 6061-Al demonstration tests, inspectors also

displayed extreme variations in the amount of D-100 developer applied, but the resultant

variation in POD was much less. Observations in this study showed that the increase in

indication brightness after developer application was much greater for the Haynes 188

panels as compared to the 6061-Al and Ti-6Al-4V panels. It was also noted that the

majority of the cracks in the 6061-Al and Ti-6Al-4V panels could be detected prior to

developer application, but this was not the case for the Haynes 188 panels.

Haynes 188 Method D

Table 4-A in Appendix A contains the results from all the Method D inspections performed

on the Haynes 188 panel set. Point estimate and curve fit solutions were found for all 12

trials. Table 8 summarizes the sensitivity level 3 results versus level 4 results for all the

trials. This table shows that the Method D sensitivity level 4 penetrants performed better

on the Hayes 188 panels. The overall percentages of cracks detected were 95.6 and 96.7

percent for the sensitivity level 3 and level 4 penetrants, respectively. The final column of

Table 8, which was obtained by performing Logit curve fits on the combination of all 6

sensitivity level 3 hit-miss data sets and all six of the level 4 sets, show the a90/95 values

for the two levels of penetrants differ by 0.004 inches. These results are shown graphically

in Figure 11.

17
Table 8: Comparison of sensitivity level 3 versus level 4 penetrants for Method D

demonstration tests performed on Haynes 188.

Logit

Curve Fit

Sensitivity Number Total Total Total Percent on

Level of Number Number Number of Flaws Combined

Trials of Flaws of Hits of Hit Hit/Miss

Misses Data

(inches)

3 6 637 609 28 95.6 0.043

4 6 641 618 23 96.7 0.039

Figure 11: Logit curve fit a90/95 results as a function of Method D penetrant sensitivity

level for Haynes 188 demonstration tests.

Again, all of the hit-miss results for each inspector (four trials, two with sensitivity level 3

and two with sensitivity level 4 penetrants) were combined and analyzed using the Logit

curve fit technique. These results, which are plotted in Figure 12, show that the

demonstrated a90/95 is again a stronger function of the inspector than of the penetrant.

The range of variation for the three inspectors, which were the same three that performed

the Method A inspections on this panels set, is smaller (0.026 to 0.075 inches for Method A

and 0.023 to 0.059 inches for Method D). The biggest change was with Vendor 5

inspector. This may be due to the fact that the dry powder developer, by its nature of

18
application (dip and drag), will completely coat the surface, whereas this may not be the

case for a light dusting with the D-100 developer. This appears more critical for the

Haynes 188, where crack detection is strongly influenced by the development process.

Figure 12: Logit curve fit a90/95 results as a function of Method D inspector for Haynes

188 demonstration tests.

Ti-6Al-4V Method A

All of the individual inspector demonstration test results for the inspections of the Ti-6Al4V panels set using the Method A penetrants is shown in Table 5-A in Appendix A. The

table shows that in 5 of the 12 trials there was no point estimate solution. This is a

consequence of the small number of cracks in this panel set; as few as two missed cracks

can result in no point estimate solution. In addition, there were two trials with no Logit

curve fit solution. Table 9 shows that the overall average number of cracks detected for the

sensitivity level 3 trials (97.8 percent) was higher than for the level 4 trials (96.2 percent).

The Logit curve fit a90/95 values obtained by combining all six level 3 trials (0.043 inches)

is with in 0.001 inches of the a90/95 obtained by combining the six level 4 trials (0.044

inches). These results are shown graphically in Figure 13.

19
Table 9: Comparison of sensitivity level 3 versus level 4 penetrants for Method A

demonstration tests performed on Ti-6Al-4V.

Logit

Curve Fit

Sensitivity Number Total Total Total Percent on

Level of Number Number Number of Flaws Combined

Trials of Flaws of Hits of Hit Hit/Miss

Misses Data

(inches)

3 6 324 317 7 97.8 0.043

4 6 315 303 12 96.2 0.044

Figure 13: Logit curve fit a90/95 results as a function of Method A penetrant sensitivity

level for Ti-6Al-4V demonstration tests.

Again, all of the hit-miss results for each inspector (four trials, two with sensitivity level 3

and two with sensitivity level 4 penetrants) were combined and analyzed using the Logit

curve fit technique. These results, which are plotted in Figure 14, show that the

demonstrated a90/95 varies little between the three inspectors (0.041-0.048 inches).

Having observed all the inspections for the three panel sets, this may be attributed to the

very low background found on the Ti-6Al-4V panels. In addition, the cracks in the Ti-6Al20
4V set produced relatively bright indications before developing, which minimizes the

variability created by variations in development technique.

Water Washable

Ti-6Al-4V

Vendor 1 a90/95 = 0.044 inches

Vendor 3 a90/95 = 0.041 inches

Vendor 7 a90/95 = 0.048 inches

0 0.02 0.04 0.06 0.08 0.1

Crack Length (inches)

Figure 14: Logit curve fit a90/95 results as a function of Method A inspector for Ti-6Al-4V

demonstration tests.

Ti-6Al-4V Method D

All of the individual inspector demonstration test results for the inspections of the Ti-6Al4V panels set using the Method D penetrants is shown in Table 6-A in Appendix A. The

table shows that there was no point estimate solution in eight of the 12 trials. This is again

a consequence of the small number of cracks in this panel set. In addition, there were four

trials with no Logit curve fit solution. Table 10 shows that the overall average number of

cracks detected for the sensitivity level 4 trials (97.2 percent) was higher than for the level

3 trials (96.4 percent). The Logit curve fit a90/95 values obtained by combining all six

level 3 trials (0.041 inches) is with in 0.001 inches of the a90/95 obtained by combining the

six level 4 trials (0.042 inches). These results are shown graphically in Figure 15.

0.8

0.6

On

0.4

0.2

0

21
Table 10: Comparison of sensitivity level 3 versus level 4 penetrants for Method D

demonstration tests performed on Ti-6Al-4V.

Logit

Curve Fit

Sensitivity Number Total Total Total Percent on

Level of Number Number Number of Flaws Combined

Trials of Flaws of Hits of Hit Hit/Miss

Misses Data

(inches)

3 6 306 295 11 96.4 0.041

4 6 319 310 9 97.2 0.042

Post Emulsifiable

Ti-6Al-4V

0.8

0.6

On

0.4

0.2

0

Sensitivity Level 3 a90/95 = 0.041 inches

Sensitivity Level 4 a90/95 = 0.042 inches

0 0.02 0.04 0.06 0.08 0.1

Crack Length (inches)

Figure 15: Logit curve fit a90/95 results as a function of Method D penetrant sensitivity

level for Ti-6Al-4V demonstration tests.

Again, all of the hit-miss results for each inspector (four trials, two with sensitivity level 3

and two with sensitivity level 4 penetrants) were combined and analyzed using the Logit

curve fit technique. These results, which are plotted in Figure 16, show that the

demonstrated a90/95 varies little between the three inspectors (0.043-0.047 inches).

22
Post Emulsifiable

Ti-6Al-4V

0.8

0.6

On

0.4

0.2

0

GSFC 1 a90/95 = 0.046 inches

GSFC 2 a90/95 = 0.043 inches

Vendor 1 a90/95 = 0.047 inches

0 0.02 0.04 0.06 0.08 0.1

Crack Length (inches)

Figure 16: Logit curve fit a90/95 results as a function of Method D inspector for Ti-6Al4V demonstration tests.

23
Conclusions

Table 11 provides a summary for all 92 demonstration tests performed in this study. The

table presents the overall percentage of cracks found and the Logit curve fit a90/95 value

for the combined hit/miss data sets as a function of metal, penetrant inspection method and

penetrant sensitivity level. These results show that for all three metals and both inspection

methods, the crack detection capability of the sensitivity level 3 penetrants is essentially

identical to the sensitivity level 4 penetrants.

Metal Method Number

of Trials

Sensitivity

Level

Percent of

Cracks Hit

Logit Curve Fit

a90/95 on

Combined Data

Set

(inches)

6061 Al A 10 3 92.7 0.052

6061 Al A 10 4 93.0 0.051

Haynes 188 A 6 3 94.7 0.046

Haynes 188 A 6 4 94.5 0.049

Ti-6Al-4V A 6 3 97.8 0.043

Ti-6Al-4V A 6 4 96.2 0.044

6061 Al D 12 3 97.4 0.040

6061 Al D 12 4 96.1 0.045

Haynes 188 D 6 3 95.6 0.043

Haynes 188 D 6 4 96.7 0.039

Ti-6Al-4V D 6 3 96.4 0.041

Ti-6Al-4V D 6 4 97.2 0.042

Table 11: Summary of all demonstration tests performed in the study.

The largest difference in the overall percentage of cracks detected was 1.6 percent for the

Ti-6Al-4V Method A case. In this case the sensitivity level 3 penetrants performed better

(97.8 percent) than the sensitivity level 4 penetrants (96.2 percent). There were two

additional cases (Haynes 188 Method A and 6061-Al Method D) where based on the

24
overall percentage of cracks hit, the sensitivity level 3 penetrants performed slightly better

than the level 4 penetrants. For the other three cases (6061-Al Method A, Haynes 188

Method D and Ti-6Al-4V Method D) the sensitivity level 4 penetrants performed slightly

better.

The table also shows that the Logit curve fit a90/95 values for the combined hit/miss data

sets for the two penetrant sensitivity levels vary no more than 0.005 inches. Also the

results show that in four of the six cases, the a90/95 is lower for the sensitivity level 3

penetrants.

The results from this study show that there is no significant difference in crack detection

capability between sensitivity level 3 and sensitivity level 4 penetrants. As a consequence,

sensitivity level 3 penetrants should be acceptable for NASA Standard Level penetrant

inspections.

25
References:

1NASA-STD-5009, “Nondestructive Evaluation Requirements for Fracture-Critical

Metallic Components,” Baseline Release, April, 2008.

2NASA/TM-2009-215850, “A Comparison of the Capability of Sensitivity Level 3 and

Sensitivity Level 4 Fluorescent Penetrants to Detect Fatigue Cracks in Aluminum,”

3NASA-CR-170879, “NDE Detectability of Fatigue Type Cracks in High-Strength

Alloys,” Martin Marietta Corporation, July 1983.

4P.F. Packman et al., “Reliability of Flaw Detection by Nondestructive Inspection,” Metals

Handbook, 8

th Edition, Volume 11, 1976.

5A.P. Berens, “NDE Reliability Data Analysis,” Metals Handbook, 9

th Edition, Volume 17,

1989.

6

J. S. Cramer, Logit Models from Economics and Other Fields, Cambridge University

Press, 2003.

7MIL-HDBK-1823A, “Nondestructive Evaluation System Reliability Assessment,” April

2009.

8J.A. Lively and T.L. Aljundi, “Fluorescent Penetrant Inspection Probability of Detection

Demonstration Performed for Space Propulsion,” AIP Conf. Proc., Vol. 657, Issue 1,

2003.

9W. D. Rummel, “Probability of Detection as a Quantitative Measure of Nondestructive

Testing End-To-End Process Capabilities”, Materials Evaluation, January 1998.

26
Appendix A

27
Date Inspector Penetrant Sensitivity Flaws Hits Misses False

Calls

a90/95

Logit

Curve Fit

a90/95

Point

Estimate

12/10/08 Vendor 1 ZL-67 3 66 59 7 1 0.073 N/A

11/12/08 Vendor 1 ZL-56 4 61 56 5 1 0.052 0.063

11/5/08 Vendor 1 HM607 3 59 51 8 0 0.081 N/A

12/3/08 Vendor 1 HM704 4 63 57 6 0 0.056 0.085

1/28/09 Vendor 2 ZL-67 3 81 70 11 15 0.071 0.130

1/21/09 Vendor 2 ZL-56 4 82 75 7 22 0.061 0.085

2/11/09 Vendor 2 HM607 3 81 75 6 21 0.057 0.080

2/4/09 Vendor 2 HM704 4 81 74 7 19 0.059 0.076

1/28/09 GSFC 1 ZL-67 3 81 78 3 5 0.050 0.062

1/21/09 GSFC 1 ZL-56 4 82 79 3 3 0.056 0.082

1/14/09 GSFC 1 HM607 3 82 77 5 5 0.056 0.070

1/7/09 GSFC 1 HM704 4 82 77 5 8 0.055 0.065

3/4/09 Vendor 3 ZL-67 3 81 80 1 9 0.042 0.059

3/11/09 Vendor 3 ZL-56 4 81 80 1 7 0.042 0.059

2/25/09 Vendor 3 HM607 3 81 77 4 5 0.054 0.065

2/18/09 Vendor 3 HM704 4 81 80 1 7 N/A 0.070

4/15/09 Vendor 4 ZL-67 3 81 73 8 2 0.071 0.090

4/22/09 Vendor 4 ZL-56 4 81 73 8 5 0.061 0.080

4/1/09 Vendor 4 HM607 3 81 78 3 9 0.048 0.062

4/29/09 Vendor 4 HM704 4 81 70 11 1 0.077 0.130

Table 1-A: Individual results for each inspector performing Method A demonstration tests

on the 6061-Al panel set.

28
Date Inspector Penetrant Sensitivity Flaws Hits Misses False

Calls

a90/95

Logit

Curve Fit

a90/95

Point

Estimate

5/8/09 GSFC 1 ZL-27A 3 81 81 0 5 0.026 0.058

5/6/09 GSFC 1 ZL-37 4 81 80 1 8 0.044 0.059

7/9/09 GSFC 1 RC65 3 81 81 0 2 0.026 0.058

7/10/09 GSFC 1 RC77 4 81 81 0 3 0.026 0.058

5/20/09 Vendor 4 ZL-27A 3 81 79 2 6 0.05 0.065

5/13/09 Vendor 4 ZL-37 4 81 75 6 1 0.083 0.165

8/6/09 Vendor 4 RC65 3 80 72 8 3 0.078 0.165

8/14/09 Vendor 4 RC77 4 80 73 7 11 0.061 0.07

6/2/09 Vendor 3 ZL-27A 3 81 81 0 1 0.026 0.058

6/10/09 Vendor 3 ZL-37 4 80 80 0 0 0.026 0.058

8/25/09 Vendor 3 RC65 3 79 77 2 2 0.052 0.065

9/1/09 Vendor 3 RC77 4 80 79 1 3 0.043 0.059

6/17/09 Vendor 5 ZL-27A 3 79 79 0 16 0.026 0.058

6/30/09 Vendor 5 ZL-37 4 81 75 6 47 0.062 0.09

8/18/09 Vendor 5 RC65 3 80 77 3 3 0.056 0.076

8/20/09 Vendor 5 RC77 4 80 78 2 1 0.05 0.075

6/24/09 Vendor 6 ZL-27A 3 78 78 0 8 0.026 0.058

7/8/09 Vendor 6 ZL-37 4 81 80 1 6 0.026 0.058

7/20/09 Vendor 6 RC65 3 80 77 3 2 0.048 0.062

7/22/09 Vendor 6 RC77 4 81 77 4 2 0.052 0.075

8/13/09 Vendor 7 ZL-27A 3 80 77 3 16 0.058 0.09

8/11/09 Vendor 7 ZL-37 4 80 75 5 30 0.077 0.112

7/25/09 Vendor 7 RC65 3 80 76 4 45 0.059 0.076

7/27/09 Vendor 7 RC77 4 81 76 5 35 0.059 0.07

Table 2-A: Individual results for each inspector performing Method D demonstration tests

on the 6061-Al panel set.

29
Date Inspector Penetrant Sensitivity Flaws Hits Misses False

a90/95

Curvee

Fit

a90/95

Point

Estimate

1/5/10 GSFC 1 ZL-67 3 107 106 1 7 0.030 0.055

11/30/09 GSFC 1 ZL-56 4 103 101 2 12 0.012 0.055

12/14/09 GSFC 1 HM-607 3 107 106 1 6 0.030 0.055

12/7/09 GSFC 1 HM-704 4 107 105 2 7 0.037 0.055

12/21/09 Vendor 3 ZL-67 3 107 102 5 7 0.048 0.085

1/12/10 Vendor 3 ZL-56 4 107 103 4 13 0.048 0.087

12/16/09 Vendor 3 HM-607 3 107 104 3 14 0.020 0.055

12/2/09 Vendor 3 HM-704 4 107 102 5 10 0.055 0.105

1/19/10 Vendor 5 ZL-67 3 107 96 11 6 0.087 0.229

3/2/10 Vendor 5 ZL-56 4 107 91 16 2 0.189 n/a

3/9/10 Vendor 5 HM-607 3 107 94 13 1 0.085 0.136

1/26/10 Vendor 5 HM-704 4 107 101 6 2 0.051 0.087

Table 3-A: Individual results for each inspector performing Method A demonstration tests

on the Haynes 188 panel set.

30
Date Inspector Penetrant Sensitivity Flaws Hits Misses False

Calls

a90/95

Logit

Curve

Fit

a90/95

Point

Estimate

5/4/10 GSFC 1 ZL-27A 3 102 101 1 8 0.031 0.055

5/6/10 GSFC 1 ZL-37 4 107 106 1 10 0.030 0.055

5/3/10 GSFC 1 RC-65 3 107 106 1 15 0.030 0.055

5/5/10 GSFC 1 RC-77 4 107 106 1 9 0.030 0.055

3/31/10 Vendor 3 ZL-27A 3 107 104 3 6 0.047 0.095

4/6/10 Vendor 3 ZL-37 4 106 102 4 6 0.048 0.085

2/18/10 Vendor 3 RC-65 3 107 104 3 7 0.020 0.055

2/24/10 Vendor 3 RC-77 4 107 105 2 9 0.040 0.082

4/27/10 Vendor 5 ZL-27A 3 107 96 11 1 0.087 0.232

4/20/10 Vendor 5 ZL-37 4 107 95 12 1 0.087 0.137

3/23/10 Vendor 5 RC-65 3 107 98 9 1 0.066 0.133

3/17/10 Vendor 5 RC-77 4 107 104 3 4 0.020 0.055

Table 4-A: Individual results for each inspector performing Method D demonstration tests

on the Haynes 188 panel set.

31
Date Inspector Penetrant Sensitivity Flaws Hits Misses False

Calls

a90/95

Logit

Fit

a90/95

Point

Estimate

8/26/10 Vendor 1 ZL-67 3 54 53 1 0 0.047 0.058

8/31/10 Vendor 1 ZL-56 4 54 54 0 0 0.032 0.052

8/18/10 Vendor 1 HM-607 3 54 53 1 1 0.048 0.058

8/24/10 Vendor 1 HM-704 4 54 52 2 2 0.063 n/a

7/27/10 Vendor 3 ZL-67 3 54 51 3 0 0.051 0.059

7/29/10 Vendor 3 ZL-56 4 54 54 0 0 0.032 0.052

8/20/10 Vendor 3 HM-607 3 54 54 0 1 0.032 0.052

8/13/10 Vendor 3 HM-704 4 52 50 2 0 n/a n/a

8/2/10 Vendor 7 ZL-67 3 54 52 2 2 0.059 n/a

7/15/10 Vendor 7 ZL-56 4 47 43 4 7 0.055 n/a

8/9/10 Vendor 7 HM-607 3 54 54 0 2 0.032 0.052

8/11/10 Vendor 7 HM-704 4 54 48 4 2 n/a n/a

Table 5-A: Individual results for each inspector performing Method A demonstration tests

on the Ti-6Al-4V panel set.

32
Date Inspector Penetrant Sensitivity Flaws Hits Misses False

a90/95

Curvee

Fit

a90/95

Point

Estimate

10/6/10 GSFC 1 ZL-27A 3 50 50 0 0 0.032 0.052

9/27/10 GSFC 1 ZL-37 4 54 52 2 4 n/a n/a

10/4/10 GSFC 1 RC-65 3 48 45 3 2 n/a n/a

10/5/10 GSFC 1 RC-77 4 54 54 0 2 0.032 0.052

9/13/10 GSFC 2 ZL-27A 3 54 53 1 0 0.047 0.058

9/21/10 GSFC 2 ZL-37 4 54 51 3 0 n/a n/a

9/7/10 GSFC 2 RC-65 3 54 52 2 0 0.063 n/a

9/16/10 GSFC 2 RC-77 4 53 53 0 1 0.032 0.059

11/16/10 Vendor 1 ZL-27A 3 50 47 3 1 n/a n/a

11/8/10 Vendor 1 ZL-37 4 50 48 2 0 0.196 n/a

11/18/10 Vendor 1 RC-65 3 50 48 2 0 0.196 n/a

11/22/10 Vendor 1 RC-77 4 54 52 2 0 0.056 n/a

Table 6-A: Individual results for each inspector performing Method D demonstration tests

on the Ti-6Al-4V panel set.

33
34
Appendix B

35
Production of Ti-6Al-4V Crack Panels

The JSC set of specimens was made from the solution treated and aged titanium alloy, 6

aluminum, 4 vanadium (Ti-6Al-4V STA). The intent was to produce 29 primary fatigue

cracks with surface lengths of 0.050 +/- 0.005 inches and with aspect ratios of 0.5. A

number of secondary cracks were also produced, some with longer and shorter lengths

than the primary set of cracks. Two such sets of specimens were produced.

The cracks were introduced into panels that were initially 18 inches long by 3.9 inches

wide by 0.250 inches thick. Seven electric-discharge machine (EDM) fatigue crack

starter notches were randomly introduced along the 18-inch length on one side of the

panel. The intended crack sites were first prepared with a pair of slotting cutters that were

used to scoop metal out at each site. The scooped out area was approximately one inch

square and was cut to a depth of 0.050 inches. The closely spaced slotting cutters, shown

in Figure B-1, left a thin (0.010 inches) upstanding, lengthwise rib centered in the cut.

The rib was subsequently EDM-notched to a depth of 0.020 inches. This scoop-rib

method of initiating cracking was performed to facilitate producing short fatigue cracks

with aspect ratios near 0.5.

Figure B-1. Slotting Cutters and resulting Scoop-Rib Areas on a Test Panel

Fatigue cracking was accomplished in cantilever bending using a Vishay fatigue

machine. Growing fatigue cracks in bending with conventional EDM notches tended

toward low aspect ratio cracks. The scoop-rib method allowed short, 0.5 aspect ratio

cracks to be produced relatively easily where the crack initiated in the notched rib and

36
grew into the base metal at a high aspect ratio. Each crack was grown separately by

clamping the scooped out site such that the notch in the rib was at the point of maximum

stress in bending (i.e., just beyond the end of the clamp). A moment arm of four inches

was used during cycling and the displacement was controlled so as not to exceed 50

percent of the material’s yield strength. A typical test set up is shown in Figure B-2. As

each crack was grown to the desired length, the panel was re-clamped at the next scooprib site and the completed crack was moved under the clamps so that no additional crack

growth occurred. This process of fatigue cycling a scoop-rib site to the desired crack

length and moving the finished crack under the clamps was continued until all seven

cracks were completed.

Figure B-2. Typical Fatigue Test Set Up Showing Loading Arrangement

Crack growth was monitored with a long focal length traveling microscope at a

magnification of 50x. As the crack grew in depth and length, the surface length was

measured with the traveling microscope. By interrupting the fatigue cycling, applying the

maximum stress to the panel, and reading the crosshair locations of the crack tips, the

37
crack length was determined to the nearest 0.001 inches. At 50x with a static maximum

stress applied, the crack tips were readily visible.

After the cracking was completed, the individual panels were surface milled on both

surfaces to near final thickness and then surface ground on both surfaces to the final

thickness of 0.125 inches. The final surface grinding of the crack was performed to

remove any trace of the scoop machined surfaces at the crack sites. After final grinding to

the finished thickness, the 18 inch panels were cut into three panels 6 inches long. The

final surface preparation was completed by chemically-etching to remove smeared metal

from the milling and grinding operations. The etchant used was a 20 percent muriatic

acid that removed approximately 0.0003 to 0.0004 inches of metal per side. After

etching, the final recorded crack length measurement was made on an optical comparator

to the nearest 0.0001 inches. The detectability of each crack was verified with a spot

solvent removable penetrant test.

To validate the cracking process, a number of the cracks were broken open. Breaking

open the cracks allowed the crack depths to be measured and the crack aspect ratios to be

determined. Four cracks were broken open before any machining occurred and six cracks

were opened after the machining and etching were completed. The pre-machined cracks

did have high aspect ratios and ranged from 0.48 to 0.57. The post-machined and etched

cracks had aspect ratios ranging from 0.36 to 0.46. The lower than intended aspect ratios

of the finished cracks was due to the shallowness of the crack depths and the need to

insure the complete removal of any evidence of the scoop machined surfaces by

removing a few ten-thousandths of an inch past the scooped surface. The end result was

that the cracks in the final specimen set had aspect ratios closer to 0.4 than to 0.5.

Several broken open crack surfaces from a similarly manufactured set of 0.025 inch long

cracks are shown in Figures B-3 and B-4.

As a result of the etching process, the surface crack opening for the cracks in the set is on

the order of 0.0003 to 0.0004 inches. The crack opening gets significantly tighter just

below the surface where the etching process had little effect. This fact became evident

when one of the cracks in the set was sacrificed for a metallurgical examination of crack

tightness as a function of depth. The metallurgical cross section of the sacrificed crack,

which is also from the set of 0.025 inch long cracks, is shown in Figure B-5. The crack

opening at the surface measures about 0.0004 inches and is V-shaped to a depth of about

0.002 inches after which the opening is very tight and measures less than 0.0001 inches.

The V-shape is due to the removal of metal from the crack faces by the etchant.

38
Figure B-3. Pre-machined Crack Surface with Rib. Mag: 70x

39
Figure B-4. Post-machined and Etched Crack Surface. Mag: 70x

40
Figure B-5. Metallographic Cross Section of a Crack. Mag: 200x

41
REPORT DOCUMENTATION PAGE

Form Approved

OMB No. 0704-0188

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PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

1. REPORT DATE (DD-MM-YYYY) 2. REPORT TYPE 3. DATES COVERED (From - To)

19-01-2011 Technical Memorandum

4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER

A Comparison of the Capability of Sensitivity Level 3 and Sensitivity Level

4 Fluorescent Penetrants to Detect Fatigue Cracks in Various Metals 5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S) 5d. PROJECT NUMBER

Bradford H. Parker

5e. TASK NUMBER

5f. WORK UNIT NUMBER

Code 541

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION

REPORT NUMBER

Goddard Space Flight Center

Greenbelt, MD 20771 201100999

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING/MONITOR'S ACRONYM(S)

National Aeronautics and Space Administration NASA Washington, DC 20546-0001

11. SPONSORING/MONITORING

REPORT NUMBER

NASA/ TM-2011-215869

12. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified-Unlimited, Subject Category: 38

Report available from NASA Centerfor Aerospace Information, 7115 Standard Drive, Hanover, MD 21076. (443) 757-5802

13. SUPPLEMENTARY NOTES

14. ABSTRACT

In April 2008, NASA-STD-5009 established a requirement that only sensitivity level 4 penetrants

are acceptable for NASA Standard Level liquid penetrant inspections. Having NASA contractors change existing processes or perform

demonstration tests to certify sensitivity level 3 penetrants posed a potentially huge cost to the Agency. This study was conducted to

directly compare the probability of detection (POD) of sensitivity level 3 and level 4 penetrants using both Method A and Method D

i nspection processes. POD demonstration tests were performed on 6061-Al, Haynes 188 and Ti-6Al-4V crack panel sets. The study

results strongly support the conclusion that sensitivity level 3 penetrants are acceptable for NASA Standard

Level inspections.

15. SUBJECT TERMS

Quality assurance and Reliability, Nondestructive Evaluation, Probability of Detection, Penetrant Inspection, Fatigue Cracks

16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF RESPONSIBLE PERSON

ABSTRACT OF

PAGES Bradford Parker a. REPORT b. ABSTRACT c. THIS PAGE

19b. TELEPHONE NUMBER (Include area code) Unclassified Unclassified Unclassified Unclassified 41 (301) 286-8548

Standard Form 298 (Rev. 8-98)

Prescribed by ANSI Std. Z39-18