Abstract

Varying loading conditions of aircraft structures result in stress concentration at fastener holes, where multi-layered components are connected, possibly leading to the development of fatigue cracks. The potential of guided ultrasonic waves, propagating along large plate-like structures, for the Structural Health Monitoring (SHM) of aerospace structures has been identified. However, the sensitivity for the detection of small, potentially hidden, defects has to be ascertained. This contribution presents a study of the application of guided ultrasonic waves in multi-layered tensile specimens for the monitoring of fatigue crack growth at fastener holes in the 2 nd (bottom) layer of such structures. Fatigue crack growth was monitored optically and the changes in the ultrasonic signal caused by the crack development were quantified. It was shown that hidden fatigue crack detection and monitoring using the low frequency guided waves is possible. The sensitivity and repeatability of the measurements were ascertained, having the potential for fatigue crack growth monitoring at critical and difficult to access fastener locations from a stand-off distance. The robustness of the methodology for practical in-situ ultrasonic monitoring of fatigue crack growth was discussed.

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