Interpreting MIL-HDBK-1823 for Application to Probability of Detection Demonstration Tests on a Fluorescent Penetrant Inspection System: Balancing Practical and Statistical Considerations when Designing a Specimen Standard

摘要

Probability of Detection (POD) demonstration tests are required by the National Aeronautics and Space Administration (NASA) for programs under Fracture Control. To be qualified to perform Fluorescent Penetrant Inspection (FPI) on production hardware that is classified as fracture critical, an inspector must be able to demonstrate the ability to reliably detect fatigue flaws equal to or greater than the critical initial flaw size. To determine the smallest size indication that an inspector can distinguish as linear with a specified level of reliability, Pratt & Whitney Rocketdyne (PWR) periodically conducts POD demonstrations tests on both in-house inspectors and on approved external vendor's inspectors. Each inspector is tested on each production FPI system they perform work on using a known specimen standard. The results are analyzed using established statistical methods, the outcome of which is used to determine an inspector's capability. However, the validity of the statistical analysis and the associated uncertainty is highly dependent on the design of the specimen standard. The guidelines stated in MIL-HDBK-1823 provide only minimum guidelines for designing a specimen standard for POD demonstration tests on an FPI system. This paper provides a practical and statistical review of the guidelines stated in MIL-HDBK-1823 regarding the number of flaws, the distribution of flaw sizes, and how the fl aw sizes are distributed across the specimen standard. A general overview of the accepted POD analysis method for FPI systems and interpretation of results will also be discussed since the results of the statistical analysis can be subject to misinterpretation even with a well-designed specimen standard.