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DTD Handbook

Handbook for Damage Tolerant Design

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    • About
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    • PDF Versions
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    • Sections
      • 1. Introduction
      • 2. Fundamentals of Damage Tolerance
      • 3. Damage Size Characterizations
        • 0. Damage Size Characterizations
        • 1. NDI Demonstration of Crack Detection Capability
          • 0. NDI Demonstration of Crack Detection Capability
          • 2. NDI Capability Evaluation for Cracks
            • 0. NDI Capability Evaluation for Cracks
            • 1. Basic Considerations in Quantifying NDI Capability
            • 2. Design of NDI Capability Demonstrations
            • 3. Sample Size Requirements
            • 4. POD Analysis
          • 3. NDI Capability Evaluation for Corrosion
        • 2. Equivalent Initial Quality
        • 3. Proof Test Determinations
        • 4. References
      • 4. Residual Strength
      • 5. Analysis Of Damage Growth
      • 6. Examples of Damage Tolerant Analyses
      • 7. Damage Tolerance Testing
      • 8. Force Management and Sustainment Engineering
      • 9. Structural Repairs
      • 10. Guidelines for Damage Tolerance Design and Fracture Control Planning
      • 11. Summary of Stress Intensity Factor Information
    • Examples

Section 3.1.2.4. POD Analysis

As noted there are two approaches to quantifying NDI capability – fitting a model that expresses probability of detection as a function of crack size and demonstrating a POD capability for a particular crack size.  Data from the single crack size demonstration approach are analyzed using a straightforward binomial distribution analysis.  Fitting a POD(a) model to the results of an NDI demonstration depends on the nature of the data (hit/miss or â versus a), the function chosen to represent POD(a), and the method for fitting the parameters of the function and determining the confidence bound on the reliably detected crack size.  Experience with â versus a data from eddy current inspections has shown that a cumulative normal equation provides a reasonable model for the POD(a) function when transformations of crack size or inspection signal response are considered.  Further, Berens and Hovey [1981], showed that the lognormal cumulative distribution provided as good or better a model than the eight others that were considered.  Accordingly, the Air Force has generally adopted the cumulative normal distribution function as the model for POD(a) analyses.  Note that the cumulative lognormal model is the cumulative normal model after crack size is transformed.  The log odds equation is also often used to fit NDI data.  The log odds equation and the cumulative lognormal equation are essentially indistinguishable.

A computer program, POD Version 3, is recommended by MIL-HDBK-1823 for the analysis of both â versus a and hit/miss POD(a) analyses (see also Berens [2000]). The program calculates the maximum likelihood estimates of the cumulative normal model as well as confidence bounds on estimates of ap. The program permits transformations of the data. Since the default analysis is based on the natural logarithm transformation, the default analysis is for the cumulative lognormal POD(a) function. In POD Version 3, data are input through an Excel spreadsheet and output is provided as separate tables and graphs in the spreadsheet.

The following paragraphs present a general description of the analysis methods.

â Versus a Analysis

All NDE systems make find/no find decisions by interpreting the response to an inspection excitation.  In some inspections, the response is a recordable metric, â, that is related to the flaw size.  Find/no find decisions are made by comparing the magnitude of â to the decision threshold value, âdec.  The â versus flaw size analysis is a method of estimating the POD(a) function based on the correlation between â and flaws of known size, a.  The general formulation of the â versus a model is expressed as

(3.1)

where f(a) represents the average (or median) response to a crack of size a and d represents the sum of all the random effects that makes the inspection of a particular crack of size a different from the average of all cracks of size a.  In principle, any f(a) and distribution of d that fit the observations can be used. However, if f(a) is linear in a,

(3.2)

and d is normally distributed with constant standard deviation, sd, then the resulting POD(a) function is a cumulative normal distribution function.  Monotonic transformations of â or a can be analyzed in this framework.  In fact, the model has been shown to fit a large number of cases in which a logarithmic transformation of both a and â was applied.

As an example consider the formulation of the â versus a analysis that has been used exclusively in the evaluation of the RFC/ENSIP automated eddy current inspection system.  The relation between â and a is expressed in terms of the natural logarithms of â and a.

(3.3)

where d is Normal (0, sd). For a decision threshold of âdec,

(3.4)

where F(z) is the cumulative standard normal distribution function and

(3.5)

(3.6)

The calculation is illustrated in Figure 3.1.5.  The parameters of the â versus a model (B0, B1, and sd) are estimated from the data of the demonstration specimens.  The probability density function of the ln â values for a 13 mil crack depth is illustrated in the figure.  The decision threshold in the example is set at âdec = 165.  The POD for a randomly selected 13 mil crack would be the proportion of all 13 mil cracks that would have an â value greater than 165, i.e. the area under the curve above 165.  In this example, the decision threshold was selected so that POD(13) = 0.90.  The estimate of the POD(a) function and its 95 percent confidence bound for the decision threshold of 165 counts is presented in Figure 3.1.6.  It might be noted that when all cracks have a recorded response between the signal minimum and maximum, the maximum likelihood estimates are identical with those obtained from a standard regression (least squares) analysis.  However, when crack response is below the signal minimum or above the maximum (saturation level of the recorder), more sophisticated calculations are required to obtain parameter estimates and the confidence bound.  For complete details of the maximum likelihood calculations and more discussion of the â versus a analysis, see MIL-HDBK-1823, Berens [1988], and Berens [2000].

Figure 3.1.5.  Example POD(a) Calculation from â versus a Data

 

Figure 3.1.6.  POD(a) Function with 95 Percent Confidence Bound for an Example â versus a Analysis


The preceding formulation of the â versus a model is based on three assumptions:

a)      the mean of the log responses, ln â, is linearly related to log crack size, ln a;

b)      the differences of individual ln  â values from the mean response have a normal distribution; and,

c)      the standard deviation of the residuals, sd, is constant for all a.

These assumptions can be tested using the results of the data from the demonstration. When the assumptions are not acceptable, current practice is to restrict the analysis to a range of crack sizes for which the assumptions are acceptable.

These assumptions can be easily checked and statistical tests for all three assumptions are built into the standard analysis of the POD Version 3 computer program of MIL-HDBK-1823.

If the ln â versus ln a relation is not linear, it may be possible to use other transformations of either the signal response or the crack size.  If the three assumptions are reasonably valid for other transformations of the data, the above analysis can be applied using the different transformation.  The inverse transformation of the results provides the answers in the correct units.  Data sets have been observed in which no transformation was required and the fit was made directly to â versus a data (i.e. without the logarithmic transform).  Other data sets have been analyzed in which the three assumptions were acceptable when the analysis was performed in terms of ln â versus 1/a. It should be noted that extreme caution must be exercised when extrapolating the results beyond the range of crack sizes in the data.  The POD Version 3 computer program has been designed to perform the POD analyses using transformations other than the logarithmic.  The logarithmic transform of both crack size and inspection response is the default transform.

Hit/Miss Analysis

The results of an inspection system are often recorded only as a decision as to the presence (hit, find, or pass) or absence (miss, no find or fail) of a crack.  The available data from the capability demonstration of such inspections comprise data pairs of crack size and the inspection result.  The parameters of a POD(a) model for such data can be estimated using maximum likelihood as follows:

Let ai represent the size of the ith crack and Zi  represent the result of the inspection: Zi = 1 if the flaw was found (hit) and Zi = 0 if the flaw was not found (miss).  Assume POD(ai) is the equation relating probability of detection to flaw size for the inspection.  The likelihood of obtaining a specific set of (ai, Zi) results when inspecting the specimens is

(3.7)

where q = (q1, q2, , qk) is a vector of the parameters of the POD(a) function. Values of q1, q2, , qk are determined to maximize L(q). For typical POD(a) models, it is more convenient to perform the analyses in terms of logarithms.

(3.8)

The maximum likelihood estimates are given by the solution of the k simultaneous equations:

(3.9)

In general, an iterative solution will be required to solve Equations 3.9.

Any monotone increasing function between zero and one can be used for POD(a).  However, an early study of data with multiple inspections per crack [Berens & Hovey, 1981] indicated that the log odds or, equivalently, the cumulative lognormal models were more generally applicable than the others investigated.  Further, the assumptions leading to a cumulative log normal model for the POD(a) function for â versus a data have often been verified for eddy current data.  The log odds and cumulative lognormal models are equivalent in a practical sense in that the maximum difference in POD(a) between the two for fixed location and scale parameters is about 0.02 which is well within the scatter from repeated determinations of a POD(a) capability.

POD Version 3, the computer program recommended by MIL-HDBK-1823, is based on a cumulative normal equation but allows transformations of the crack size. The default transform of POD Version 3 is the natural logarithm transform so that the program will fit the cumulative lognormal equation by default. However, the program also provides a solution based on the log odds equation. Other models for the POD(a) function may be appropriate but, if preferred, would require a different computer implementation.

Repeating Equation 3.4, the cumulative log normal equation for the POD(a) functions is:

(3.10)

where F(z) is the standard normal cumulative distribution function. The log odds model for the POD(a) function is:

(3.11)

Equation 3.10 or 3.11 is substituted in Equations 3.7 through 3.9 for POD(a). and are determined so as to maximize L(m,s), the likelihood of obtaining the observed inspection results.  Note that POD(m) = 0.5 for both models. s is a scale parameter that determines the degree of steepness of the POD(a) function.  A negative value of s is not contradictory but, for a negative s, the POD(a) function will decrease with increasing a.

There are occasions when Equations 3.9 do not converge.  No solution will be obtained if the sizes of found cracks do not overlap with the sizes of missed cracks.  Little information is obtained from cracks that are so large they are always found or so small they are always missed.  More overlap is needed for the cumulative lognormal model than for the log odds model.  It is also possible to obtain negative estimates of s from erratic data sets.  Results of this nature are due to the wrong range of crack sizes in the demonstration or to an inspection process that is not under proper control.  When the crack sizes in the specimens are not in the range of increase of the POD(a) function, the effective sample size is smaller and the effect is reflected in larger standard deviations of the sampling distributions of the parameter estimates and, thus, wider confidence bounds.

Damage tolerance analyses are driven by the single crack size characterization of inspection capability for which there is a high probability of detection.  Typically, the one number characterization of the capability of the NDE system is expressed in terms of the crack length for which there is 90 percent probability of detection, a90.  But a90 can only be estimated from a demonstration experiment and there is there is sampling uncertainty in the estimate.  To cover this variability, an upper confidence bound can be placed on the best estimate of a90.  The use of an upper 95 percent confidence bound, the a90/95 crack size has become the de facto standard for this characterization of NDE capability.  The use of a90/95 is intended to be conservative from the viewpoint of damage tolerance analyses.

In the hit/miss analysis of POD Version 3 a single value of POD(a), say 0.90, is selected and an upper confidence bound, say 95 percent is calculated for the POD value.  This procedure is known as a point by point confidence bound.  These are valid confidence bounds for any one POD value but not for the entire POD(a) curve.

The confidence bounds for the estimates of a90 are calculated using the asymptotic normality properties of the maximum likelihood estimates [Berens, 2000].  Figure 3.1.7 presents an example of a fit to hit/miss data from a semi-automated, directed eddy current inspection.

 

 

Figure 3.1.7.  Example POD(a) for a Semi-Automated, Directed Eddy Current Inspection

 

Binomial Analysis for Cracks of Fixed Size

Because of the individual physical differences between cracks, cracks of the same size will have different detection probabilities for a given NDI system.  However, a single POD for all cracks of that size can be postulated in terms of the probability of detecting a randomly selected crack from the population of all cracks of the given size.  In this formalism, the proportion detected in a random sample of the cracks is an estimate of POD for that size and binomial distribution theory can be used to calculate a lower confidence bound on the estimate.  Given a sample of inspection results from cracks of a target size, say aNDI, the inspection system is considered adequate if the lower confidence bound on the proportion of detected cracks exceeds the desired POD value.

The theory of the binomial analysis is as follows.  Given independent inspection results from specimens containing n cracks of size aNDI, the target reliably detected crack size.  Assume that r of the cracks are detected.  If POD is the true (but unknown) probability of detection for the population of cracks, the number of detections is modeled by the binomial distribution.  The probability of r detections in n independent inspections of cracks of size aNDI is:

(3.12)

The unbiased, maximum likelihood estimate of POD is

(3.13)

The 100(1-g) percent lower confidence bound, PODCL, on the estimate of POD is obtained as the solution to the equation:

(3.14)

The interpretation of PODCL as a lower confidence bound is as follows.  If the demonstration was completely and independently repeated a large number of times, 100(1-g) percent of the calculated lower bounds would be less than the true value of POD.  There is 100(1-g) percent confidence that PODCL from a single demonstration will be less than the true value.

Solutions to Equation 3.14 are tabulated in Natrella [1963] for 90, 95, and 99 percent confidence limits and selected sample sizes. General solutions expressed in terms of the incomplete beta function and the normal approximation to the binomial distribution can be found in many statistical references, for example, Mood [1950]. Minimum values of n and r which yield predefined values of PODCL and confidence level, 100(1-g), are often quoted. Selected values can be found in Packman, et al. [1976].

For example, consider a demonstration that there is 95 percent confidence that at least 90 percent of all cracks of size aNDI will be detected by a given inspection system.  To achieve the desired level of confidence and POD would require results as given in Table 3.1.2.

 

Table 3.1.2.  Minimum Number of Detections Require to Conclude that
POD > 0.90 with 95 Percent Confidence

Number of Cracks
of Size aNDI

Number of Cracks
Detected

29

29

46

45

61

59

75

72

89

85

103

98

 

If there were 28 cracks in the demonstration and all 28 were detected, the lower 95 percent confidence bound on the estimate of POD would be 0.899.  If less than 28 were detected, the lower confidence bound would be even lower.  Since the minimum number of specimens that can yield a 90 percent POD at 95 percent confidence is 29, this approach to capability demonstration has been referred to as the “29 out of 29” method.

There are several objections to the use of this approach to quantifying inspection capability:

1)      This demonstration approach to capability provides only minimal and reasonably gross POD information for the single crack size used for the inspections.  Steep POD(a) functions are generally considered superior to flat POD(a) functions and a single crack size capability demonstration provides no information regarding POD(a) steepness.

2)      Passing or failing the demonstration provides no discrimination of degree of detectability at the high POD levels.  For example, consider the 29 finds out of 29 cracks criterion for demonstrating the 90/95 capability.  If the true POD is less than 0.9, there is up to a 5 percent chance that the demonstration will conclude that the true POD is 0.9 or greater.  Conversely, if the true POD is 0.995, there is a 15 percent chance that at least one crack out of 29 will be missed and the demonstration will fail to conclude that there is 95 percent confidence that the POD is greater than 0.9.  At POD = 0.976, there is about a fifty-fifty chance of concluding the POD is greater than 0.9. POD(a) tends to be relatively flat above 0.9 and there could easily be a very large crack size difference between, say, a 0.9 capability and 0.995 capability.  Even when crack detection is absolutely certain for the given size, only a 90/95 capability can be claimed after the demonstration.

3)      When attempting to demonstrate a 90/95 capability and one crack out of 29 is missed, the demonstration must be repeated with at least additional 17 cracks.  Since demonstrations are planned with the expectation of meeting the criteria, the need for additional specimens can create significant problems.

For these reasons, quantifying inspection capability in terms of the entire POD(a) function has evolved as the preferred method [MIL-HDBK-1823].

It might be noted that attempts have been made to use a binomial approach to the analysis of demonstration data comprising a range of crack sizes [Yee, et al., 1976].  These approaches have been generally abandoned but a Bayesian approach to such analyses is being considered [Bruce, 1998].