Probability of Detection for Magnetic Particle in NDT
In the damage-tolerance approach to airworthiness, fatigue-critical aircraft structure is subject to regular nondestructive inspection (NDI) to prevent catastrophic structural failure. Periodic inspection intervals are determined using knowledge of the minimum crack size that can be reliably detected, together with information on the structural loads, critical defect size and crack growth rates. The minimum reliably detectable crack size, aNDI, is determined through analysis of the probability of detection (POD) of defects as a function of defect size. Quantitative measurement of aNDI for a given NDI procedure is arguably as challenging to obtain as the other key inputs to damage-tolerance analyses.
The Defence Science and Technology Organisation (DSTO) is conducting a series of critical literature reviews to examine the reliability of standard non-destructive inspection methods used for Australian Defence Force (ADF) aircraft. The first review (DSTO-TR-2623) examined the reliability of liquid penetrant testing (LPT). The present report is the second in the series and is concerned with magnetic particle testing (MPT). MPT is a technically mature inspection method used to detect surface-breaking cracks in high-strength steel components.
A survey of the available literature on the reliability of MPT found some twenty references relevant to POD over the period 1968 to 2011. After critical review, four published studies were considered applicable to detection of fatigue cracks in high-strength steel aerospace components. It was found that the original statistical analysis methods used in these four studies were either outdated or deficient in other aspects. Thus, the original POD data were reanalysed using the currently accepted approach in which maximum likelihood estimation was used to fit a log-normal cumulative distribution function to the POD hit/miss data as a function of crack size and statistical confidence levels were determined using the Q2 likelihood ratio statistic.
Analysis of the MPT POD studies showed a spread in performance between the organisations involved in the various trials. Following a meta-analysis, it was concluded that the aNDI = 2.0 mm currently assumed by the Royal Australian Air Force is consistent with the average performance of MPT derived from the relevant available literature. The smallest aNDI consistent with most implementations of MPT is 2.6 mm. On this basis, the results do not support a reduction in the standard limitation for MPT.