Parametric modulation mechanism of surface acoustic wave on a partially closed crack

Low-frequency parametric modulation of a pulsed surface acoustic wave by a partially closed fatigue crack is described. The crack is initiated in the plastic-yielding zone induced by a surface cavity and clamped due to the constraint of the surrounding elastic medium. Small periodic loading, superimposed on a static crack opening load, changes periodically the crack closure state and produces a nonlinear modulation of the reflected ultrasonic pulses. We describe quantitative experimental and modeling results relating the reflected modulation spectrum to crack length, closure, and modulation load. Surface acoustic wave measurements, which were performed in situ during a fatigue test, show that the modulation response varies significantly depending on the ratio of the fatigue crack length to the size of the plastic-yielding zone.