Load-differential imaging for detection and localization of fatigue cracks using Lamb waves

Fatigue cracks are common and potentially critical defects in metallic plate-like structures, and ultrasonic guided wave methods provide an efficient and relatively low-cost means of crack detection and monitoring. However, widely used baseline subtraction methods may fail under mismatched environmental and operational conditions. In particular, varying applied loads change not only the contact state of a crack but also specimen dimensions and wave speeds, which affect the ultrasonic signal response. The load dependence of crack opening provides a possibility for enhanced crack detection, which is well-known for higher frequency bulk waves. A load-differential method is proposed in this paper whereby guided wave signals obtained at different loads under the same damage state are compared without utilizing previously recorded damage-free data. To demonstrate this method, a fatigue test was performed on an aluminum plate specimen instrumented with a sparse array of piezoelectric transducers. Signal changes due to crack opening effects caused by increasing tensile loads are visualized using delay-and-sum imaging. The results show that the load-differential method is capable of detecting cracks and visualizing their locations.