Physical manifestation of a < inf > 90/95 < /inf > in remnant life revision studies of aero-engine components

Abstract

Damage Tolerance (DT) lifing methodology for aero-engines require the reliability of Non-Destructive Testing (NDT) techniques used. Probability of Detection (POD) for measuring NDT reliability yields the a90/95 (flaw detection with 90% probability and 95% confidence) value. This a90/95 or the largest crack size missed by an NDT technique is in general, incorporated into the DT calculations for estimating the remaining fatigue cycles the component can withstand before failure. Hence, it is essential to estimate the a90/95 value to the closest accuracy. However, the NDT inspection data at a site containing multiple cracks results in ambiguity of HIT/MISS approaches to be adopted for the estimation of POD or a90/95 values. Several approaches were attempted by the researchers to minimize the ambiguity but with limited success due to the restrictions in implementing them. Moreover, to the best of the author's knowledge, the physical significance of the a90/95 value obtained from different HIT/MISS approaches on the remnant life calculations of aero-engine components was not available in the literature. Therefore, in the current study, physical manifestation of a90/95 in remnant life calculations obtained from the maximum flaw size and sum of flaw sizes approaches for inspection of natural fatigue cracks in a nickel based superalloy using fluorescent penetrant (FPI) and eddy current inspection (ECI) techniques was attempted. It was observed that ECI technique provides the higher number of remnant cycles than the FPI technique due to its higher sensitivity. In addition, it was also observed that regardless of the NDT techniques used, maximum flaw size approach results in higher number of fatigue cycles. However, the actual number of remnant cycles of the component can be exactly known provided the capability of the current NDT techniques in resolving a group of flaws in a particular location is enhanced.