POD of NDT Techniques Using High Temperature Oxidized Fatigue Cracks in an Aero Engine Alloy

Abstract

Establishing probability of detection (POD) or reliability of various nondestructive testing (NDT) techniques is essential for implementing damage tolerant (DT) methodology for aero-engines. This POD is usually established with the help of a large number of service expired aero-engine components containing several fatigue cracks. In the absence of such components, artificial defects such as electrical discharge machining (EDM) notches or starter cracks were explored. However, such artificial defects would not meet the key features such as tightness of the fatigue cracks and the possible oxidation in the crack opening and thus, limiting their usage. Therefore, in the current study, an innovative approach of generating fatigue cracks at 650 °C (~ typical aero-engine service temperatures) with key high temperature service degradation aspects of oxidation and fatigue cracking is demonstrated for the first time using Gleeble® test system. Further, POD is estimated by inspecting these laboratory generated fatigue cracks using fluorescent liquid penetrant technique (FLPT) and eddy current technique (ECT) under HIT (defect detected) vs. MISS (defect not detected) and â (signal response) vs. a (crack size) methodologies. The current study also discusses a statistical approach of random generation of crack sizes for use in NDT reliability analysis. In addition, an attempt has been made to understand the effect of a90/95 values on remnant life calculations. It is concluded that the eddy current response of oxidized fatigue cracks results in better (high sensitive) a90/95 values compared to the eddy current response obtained from non-oxidized fatigue cracks.