In-situ monitoring of fatigue crack growth at fastener holes using Rayleigh-like waves

The development of fatigue cracks at fastener holes is a common problem in aircraft maintenance. This contribution investigates the use of Rayleigh-like waves for the in-situ monitoring of fatigue crack growth in tensile, aluminum specimens. Experimentally the Rayleigh-like wave is excited using standard angle beam transducers and measured using either standard pulse-echo equipment or laser interferometry. The combined scattered field of the Rayleigh-like wave at the fastener hole and crack is measured and compared to finite difference (FD) simulations. The measured changes in the pulse-echo ultrasonic signal due to fatigue crack growth are evaluated using an energy ratio parameter and correlated to the optically measured crack length.