December 2009

NASA/TM-2009-215850

A Comparison of the Capability of Sensitivity Level 3 and 

Sensitivity Level 4 Fluorescent Penetrants to 

Detect Fatigue Cracks in Aluminum

Bradford H. Parker

Goddard Space Flight Center, Greenbelt, MD
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December 2009

NASA/TM-2009-215850

A Comparison of the Capability of Sensitivity Level 3 and 

Sensitivity Level 4 Fluorescent Penetrants to 

Detect Fatigue Cracks in Aluminum

Bradford H. Parker

Goddard Space Flight Center, Greenbelt, MD
Available from:

NASA Center for AeroSpace Information National Technical Information Service

7115 Standard Drive 5285 Port Royal Road

Hanover, MD 21076-1320 Springfield, VA 22161
1

Introduction

The release of NASA‐STD‐5009,

1 “Nondestructive Evaluation Requirements for 

Fracture Critical Metallic Components,” in April 2008 overturned the long standing 

requirement that either sensitivity Level 3 or Level 4 liquid penetrants could be used 

for NASA Standard Level liquid penetrant inspections.  The new requirement imposed 

by NASA‐STD‐5009 states that:  “The penetrant system used shall be a fluorescent 

penetrant of Level 4 sensitivity.”  The ostensible rationale for this change was that the 

liquid penetrant capability studies performed in the 1970s, which are the basis for the 

NASA Standard Level crack sizes in NASA‐STD‐5009, were based on tests conducted 

with penetrants that were deemed to be equivalent to the modern day sensitivity 

Level 4 designation.  

The requirement that only sensitivity Level 4 penetrants can be used for NASA 

Standard Level NDE created an uproar from many NASA contractors who complained 

about the cost of switching penetrant materials and developing new procedures, or 

having to perform demonstration tests with existing  sensitivity Level 3 penetrants in 

order to qualify them for use.  To address the issue, the NASA Nondestructive 

Evaluation Program funded a task in October 2008 to compare the performance of 

sensitivity Level 3 and Level 4 penetrants.  This NASA Technical Memorandum

documents the results from the study.

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 penetrant inspection materials used in the study.  Note that the same 

nonaqueous developer (Sherwin D‐100) 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.

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

Table 1:  Summary of the penetrant materials used in this study.
2

Procedures

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

NASA Goddard Space Flight Center (GSFC).  The remaining inspector was a member of 

the GSFC Materials Engineering Branch.  First, four vendor inspectors and the GSFC 

inspector completed the Method A test matrix. Two of the vendor inspectors that 

completed the Method A matrix then also completed the Method D matrix.  The GSFC 

inspector also completed both matrices.  Three vendor inspectors that did not 

complete the Method A matrix completed the Method D matrix.  Hence, six inspectors 

completed demonstration tests using the Method D matrix.

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 feed back 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 single inspector taking the test.  A total of five 

inspectors completed demonstration tests using the matrix of four Method A 

penetrants.

Method D Procedure

The vendor did not address Method D in their penetrant procedure, as this is not a 

technique they typically use because of the quality assurance checks on the emulsifier.  

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 ER‐83A 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 feed back was supplied.
3

Crack Panels

The crack panels used in the study were 6061 aluminum and have dimensions of 

approximate 8 by 3.5 by 0.25 inches thick.  The panels contain low cycle fatigue cracks 

with aspect ratios (crack depth divided by crack length) of approximately 0.3.  The 

cracks were grown using a Fatigue Dynamics Inc. Model LFE‐500 Life, Fatigue and 

Endurance Testing Machine (see Figure 1).  This testing machine was setup to induce 

tension‐tension fatigue in bending with maximum tensile stresses of approximately 

35,000 pounds per square inch.  The starter notch for the crack growth consisted of  

three closely spaced and aligned 0.025 inch diameter holes that were 0.020 inch deep.  

Figure 2 shows an optical photograph of a fatigue crack broken open showing the 

aspect ratio achieved by this configuration.  The starter holes were removed using a 

fly cut and the panels were etched in a sodium hydroxide solution to remove metal 

smear.

The overall set of panels used in the study contains 82 cracks with lengths ranging 

from 0.023 to 0.215 inches.  The majority of the crack lengths are close to 0.050 

inches, as a portion of the panel set is used to qualify inspectors for the NASA Special 

Level penetrant inspection crack length of 0.050 inches.  The distribution of crack 

lengths is shown in Figure 3.  The cracks are all on one side only of the panels and the 

inspectors are asked only to inspect this one side.  This approach was adopted to 

minimize the amount of scratching and other handling damage that occurs during 

demonstration testing, as the non crack side of the panel can be placed on any surface 

with out concern for the panel condition.  This policy has helped extend the useful life 

of the panels.

Figure 1:  Photograph of the Fatigue Dynamics Inc. Model LFE‐500 Life, Fatigue and 

Endurance Testing Machine with a 0.25 thick aluminum plate in bending.
4

Figure 2:  Optical photograph of a fatigue crack which was broken open to reveal the 

crack aspect ratio.  The two halves of the specimen have been placed back‐to‐back.   

The surface crack length for this crack was approximately 0.32 inches.

The inspectors were typically given a set of 30 panels with a total of 79 to 82 cracks. 

One or two of the panels in the set were blank, i.e., contained no cracks.  The 

inspectors are provided with an inspection grid with 3/8 inch squares and asked to 

record all crack locations as a function of grid location (see Figure 4).  The actual hits 

(correct crack finds), misses (missed cracks) and false calls (erroneous crack finds) 

were then tabulated based on the inspector’s list of crack locations.

Figure 3:  Distribution of crack lengths binned by 0.005 inches (5 mils) for the entire 

82 flaws in the aluminum panel set.
5

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

the detected crack locations.

Data Analysis

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

approaches to yield a crack length that corresponds to detection with 90 percent 

reliability and 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 fit 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 8th Edition2 gives an excellent explanation of the statistics 

used by the point estimate method.

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 
6

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 set of 

GSFC aluminum panels has the wide range of crack lengths seen in Figure 3.  

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.  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 article2, 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 or inspectors and is not used to quantify or 

demonstrate NASA Special Level capability.  

If using the point estimate method to assess the capability of a nondestructive 

evaluation technique (instead of simply demonstrating a90/95 at a single crack 

length), then one would ideally have a set of panels with 29 cracks in each of a range 

of crack lengths.   Well‐trained inspectors using sound techniques pass point estimate 

penetrant inspection demonstration tests at crack lengths of 0.050 inches (the NASA 

Special Level crack length).  However, not all inspectors pass the 0.050 inch 

demonstration test while on the other hand some inspectors not only find the 29 

cracks in the 0.050 inch set but they also find the majority of the smaller cracks in the 

set.  If it is desired to know exactly how well an inspector could perform, then we 

would potentially have a crack panel set with 29 cracks at each length of 0.075, 0.070, 

0.065, 0.60, 0.055, 0.050, 0.045, 0.040, 0.035, 0.030, and 0.025 inches (or some similar 

distribution).  This set would have over 300 cracks and the time and cost to produce 

as well as inspect such a set of panels is prohibitive.  
7

In order to overcome this problem, 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 Metal Handbook 9th Edition3 for 

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

Cramer gives a full explanation of the Probit and Logit models.4

The hit‐miss inspection results generated in this study for the most part (43 out of 44 

cases) produce “S” shaped curves and hence using a curve fit technique to analyze the 

data is appropriate.  In the one case where data did not conform to the curve fit 

technique, 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. 

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

combination hit‐miss data sets, i.e., all Method A sensitivity Level 3 hit‐miss results 

versus all Method A sensitivity Level 4 results.  The use of the point estimate method 

to evaluate such data sets can produce misleading results.  For example, if one of the 

five 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 five 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‐18235, 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 with 77 hits and 4 misses was 
8

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 quantitatively 

measure the capability of penetrant inspections and we are not attempting to 

determine which model should be used.  Here, a single curve fit  technique (Logit) is 

universally applied to compare results from different penetrants or 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 the results 

section do include the number of false calls for each inspection.  The grid provided for 

locating crack indications has 90 cells, hence for a demonstration test with 30 panels 

and a one sided inspection there are 2700 possible crack locations.   The inspectors 

were instructed to identify only “crack like” indications, however they were given no 

feedback about their false call rates between inspections.

Results

Method A Results

Table 2 contains all of the individual 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 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 was the 

case for the Vendor 3 inspector using Sherwin HM‐704 where the one crack missed 

was 0.070 inches long.  

Table 3 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).  

Likewise the average of the point estimate method a90/95 values for the two 

sensitivity Levels differ by only 0.003 inches and the average of the Logit curve fit 

a90/95 values for the two sensitivity Levels differ by only 0.002 inches.  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 
9

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

differ by only 0.001 inches.

Date Inspector Penetrant Sensitivity Flaws Hits Misses False 

Calls

a90/95 

Logit 

Curve 

Fit

(inches)

a90/95 

Point 

Estimate

(inches)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

tests.

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 5 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 

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 0.054 inches.
10

Sensitivity

Level

Number

of

Trials

Total 

Number 

of Flaws

Total 

Number 

of Hits

Total

Number 

of 

Misses

Percent 

of Flaws 

Hit

Average 

Point 

Estimate 

a90/95 

(inches)

Average 

Logit 

Curve 

Fit 

a90/95 

(inches)

Logit 

Curve Fit 

on 

Combined 

Hit/Miss 

Data 

(inches)

3 10 774 718 56 92.7 0.077* 0.060 0.052

4 10 775 721 54 93.0 0.080 0.058+ 0.051

*8 trials +

9 trials

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

demonstration tests.

Figure 5:  Logit curve fit a90/95 results as a function of Method A penetrant.

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 
11

Figure 6, 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.

Figure 6:  Logit curve fit a90/95 results as a function of Method A inspector.

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

minutes before beginning the crack identification process.

Method D Results

Table 4 contains all of the inspection results for the Method D process.  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.

0 0.02 0.04 0.06 0.08 0.1

0

0.2

0.4

0.6

0.8

1

Water Washable

Vendor 1 a90/95 = 0.056 inches

Vendor 2 a90/95 = 0.056 inches

GSFC 1 a90/96 = 0.049 inches

Vendor 3 a90/95 = 0.041 inches

Vendor 4 a90/95 = 0.058 inches

Crack Length (inches)

POD
12

Table 5 provides a summary of the sensitivity Level 3 versus Level 4 Method D 

penetrants.  The percentage of the total cracks found is slightly higher for the Level 3 

penetrants (97.4) versus the Level 4 penetrants (96.1).   The average of the point 

estimate method a90/95 values for the two sensitivity Levels differ by only 0.005 

inches and the average of the Logit curve fit a90/95 values for the two sensitivity 

Levels differ by only 0.007 inches.  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 

(960 cracks) and all 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 

cases, the Level 3 penetrants performed better than the Level 4 penetrants.

Date Inspector Penetrant Sensitivity Flaws Hits Misses False 

Calls

a90/95 

Logit 

Curve 

Fit

a90/95 

Point 

Estimate

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

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

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

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

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

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 4: Individual results for each inspector performing Method D demonstration 

tests.
13

Sensitivity

Level

Number

of

Trials

Total 

Number 

of Flaws

Total 

Number 

of Hits

Total 

Number 

of 

Misses

Percent 

of Flaws 

Hit

Average 

Point 

Estimate 

a90/95 

(inches)

Average 

Logit 

Curve 

Fit 

a90/95 

(inches)

Logit

Curve Fit 

on 

Combined 

Hit/Miss 

Data 

(inches)

3 12 960 935 25 97.4 0.074 0.044 0.040

4 12 967 929 38 96.1 0.079 0.051 0.045

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

demonstration tests.

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 

Figure 7: Logit curve fit a90/95 results as a function of Method D penetrant.
14

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,6 four of six inspectors who performed 

penetrant deomonstrations tests using Magnaflux ZL‐37 (sensitivity Level 4) and 

Magnaflux ZL‐27A (sensitivity Level 3) performed better with ZL‐27A.

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.  

Figure 8: Logit curve fit a90/95 results as a function of Method D inspector.

0 0.02 0.04 0.06 0.08 0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Post Emulsifiable

GSFC 1 a90/95 = 0.031 inches

Vendor 3 a90/95 = 0.035 inches

Vendor 4 a90/95 = 0.057 inches

Vendor 5 a90/95 = 0.042 inches

Vendor 6 a90/95 = 0.046 inches

Vendor 7 a90/95 = 0.053 inches

crack length (inches)

POD
15

Penetrant 

and 

(Sensitivity

Level)

Number

of

Trials

Total 

Number 

of Flaws

Total 

Number 

of Hits

Total 

Number 

of 

Misses

Percent 

of 

Flaws 

Hit

Average 

Point 

Estimate 

a90/95 

(inches)

Average 

Logit 

Curve 

Fit 

a90/95 

(inches)

Logit

Curve Fit 

on 

Combined 

Hit/Miss 

Data 

(inches)

Magnaflux 

ZL‐27A

(3)

6 480 475 5 99.0 0.064 0.035 0.033

Sherwin

RC‐65

(3)

6 480 460 20 95.8 0.084 0.053 0.048

Magnaflux

ZL‐37

(4)

6 484 465 19 96.1 0.090 0.053 0.046

Sherwin

RC‐77

(4)

6 483 464 19 96.1 0.068 0.049 0.046

Table 6:  Comparison of the four Method D penetrants.

Conclusions

For Method A demonstration tests, sensitivity Level 3 and Level 4 penetrants on 

average produced nearly identical demonstrated a90/95 crack lengths for five 

different inspectors.  For Method D demonstration tests from six inspectors, one 

sensitivity Level 3 penetrant produced an a90/95 crack length smaller than the 

remaining Level 3 penetrant and the two Level 4 penetrants.  The remaining Level 3 

penetrant and the two Level 4 Method D penetrants on average produced nearly 

identical demonstrated a90/95 crack lengths.  The a90/95 demonstrated crack 

lengths for individual inspectors varied more than for individual penetrants, i.e., the 

dominant factor in the demonstrated a90/95 is the inspector, not the penetrant.

Only two of the 44 demonstration tests in the study produced point estimate a90/95 

crack lengths greater than 0.150 inches long (the NASA Standard Level penetrant 

crack length).  These two results were from the same inspector, one with a Method D 

sensitivity Level 3  penetrant and one with a Method D sensitivity Level 4  penetrant.

The results from both the Method A and Method D demonstration tests strongly 

support the conclusion that sensitivity Level 3 penetrants are acceptable for NASA 

Standard Level penetrant inspections.
16

References:

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

Metallic Components,” Baseline Release, April, 2008.

2P.F. Packman et al., “Reliability of Flaw Detection by Nondestructive Inspection,”  

Metals Handbook, 8th Edition, Volume 11, 1976. 

3A.P. Berens, “NDE Reliability Data Analysis,” Metals Handbook, 9th Edition, Volume 

17, 1989.

4J.S. Cramer, Logit Models from Economics and Other Fields, Cambridge University 

Press, 2003.

5MIL‐HDBK‐1823A, “Nondestructive Evaluation System Reliability Assessment,” April 

7, 2009.

6J.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.
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Prescribed by ANSI Std. Z39-18 

26-01-2010 Technical Memorandum

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

4 Fluorescent Penetrants to Detect Fatigue Cracks in Aluminum

Code 541

Bradford H. Parker

Goddard Space Flight Center

Greenbelt, MD 20771 2010-00980

National Aeronautics and Space Administration

Washington, DC 20546-0001

NASA GSFC

NASA/TM-2009-215850

Unclassified-Unlimited, Subject Catagory: 38

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

Historically both sensitivity level 3 and sensitivity level 4 fluorescent penetrants have been used to perform NASA Standard Level

inspections of aerospace hardware. In April 2008, NASA-STD-5009 established a requirement that only sensitivity level 4 penetrants

were acceptable for inspections of NASA hardware. 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 sensitivity level 3 and level 4 penetrants using both Method A and Method D inspection

processes. The study results strongly support the conclusion that sensitivity level 3 penetrants are acceptable for NASA Standard

Level inspections.

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

Unclassified Unclassified Unclassified Unclassified 16

Bradford Parker

(301) 286-8548