Using requirements on merit ratios for assessing reliability of NDE flaw detection

Abstract

Current approaches to qualification or reliability assessment of NDE procedures by traditional probability of detection (POD) testing per MIL-HDBK-1823A require significant number of specimens with known size real or simulated flaws. For many practical applications in aerospace industry, either it is difficult to produce or it is economically not feasible to fabricate the necessary flaw specimens in required quantity. Thus, there is a strong need for alternative methodologies to provide assessments of reliability of NDE procedures using fewer flaw specimens. The paper provides an alternate NDE reliability assessment approach using dependence of the probability of detection (POD) and probability of false positive (POF), on the contrast-to-noise ratio (CNR) and decision threshold-to-noise ratio (TNR) for selected POD and POF models. The NDE reliability assessment approach is termed as limited sample (LS) POD assessment. Current work assumes a single-hit flaw detection methodology i.e. the flaw detection calls are not mapped to form an indication with cluster of pixels in a 2D image. In the LS POD approach, the signal response from a fixed size flaw of interest is assumed to follow a normal distribution. The approach uses statistical tolerances computed using signal response sample data and 𝑘₁-factor. Similar to traditional POD analysis, a flaw size with chosen POD and confidence is determined. Alternately, POD/Conf. can be determined for a chosen decision threshold for the flaw size used in the analysis. The POF is also estimated in the analysis. Noise is assumed to follow either a normal or lognormal distribution. For reliable detection of the target size flaw, minimum POD/Conf. of 90/95% and maximum POF of 1% are assumed. The sample of signal responses is assumed to be representative of the assumed population of signal responses. Similarly, the noise measurements are assumed to be representative of those expected in the inspection of real hardware. If representative flaw sample and noise measurements are used, LS POD results pose no risk in the POD and POF estimation. Risk to meeting NDE flaw size detection requirements can be assessed based on how representative the flaw sample and noise measurements are, and based on positive margin between LS POD results and procedure reliability requirements.