Enhancement of fatigue crack monitoring by surface acoustic wave reflection and modulation in a space-cycle-load domain: an imaging approach

The surface wave acoustic method is applied to the in situ ultrasonic monitoring (during fatigue) of fatigue crack initiation and evolution. A small low-frequency periodic loading is also applied, resulting in a nonlinear modulation of reflected pulses. The acoustic wave reflections are collected for: each experimental cycle; a range of applied fatigue and modulation load levels; and a range of spatial propagation positions, and are presented in image form to aid pattern identification. Salient features of the image are then extracted and processed to evaluate the initiation time of the crack, its opening/closing behavior and its subsequent size evolution until sample failure. This image representation improves the visibility of the small-crack reflectivity pattern and thus enhances the detectability of small cracks.