Fatigue Crack Detection via Load- Differential Guided Wave Methods (preprint)

Detection of fatigue cracks originating from fastener holes is an important application for structural health monitoring (SHM) of civil, mechanical and aerospace structures, but their detection via ultrasonic guided waves can be problematic when cracks are tightly closed in the absence of applied tensile loads. Proposed here are load-differential guided wave methods, which compare signals at one load to those at another load at the same damage state. The main advantage of such methods is that cracks can be detected and localized by analyzing current signals obtained from different loading conditions without using baseline data from the damage-free state. The efficacy of the proposed load-differential methods is examined using fatigue test data where multiple cracks grow from a single through-hole. Data were acquired with a spatially distributed array of piezoelectric discs by recording ultrasonic signals as a function of applied uniaxial load at intervals through the fatigue test. Load-differential guided wave images are generated from residual signals via delay-and-sum imaging methods, and these images are evaluated in terms of their ability to detect and localize fatigue cracks.