A quantitative study of the low-frequency parametric modulation of a pulsed surface acoustic wave (SAW) by a partially closed fatigue crack is described. In situ ultrasonic measurements were performed during a fatigue test for different crack lengths and static opening loads. 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 the open crack segment length and/or the crack interfacial condition producing nonlinear modulation of the reflected ultrasonic pulses. The modulation spectrum is related quantitatively to the crack length and to the crack opening–closure behavior. It is demonstrated that the application of a small static crack-opening load with the modulation load could considerably enhance crack detectability. The increase of the second modulation harmonic is pronounced when the crack is nearly closed and when it is nearly open. Also, it is observed that the maximum modulation occurs at different static opening loads depending on the crack length relative to the plastic-yielding zone size. A low-frequency scattering model is presented based on the mechanism of modulation of the open/close segment length of the partially opened crack. The modeling results compare favorably with experiment.
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Kim et al. (2004) studied this question.
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