Modulation Enhanced Detectability of Cracks Using Surface Acoustic Waves

The parametric effect in surface acoustic wave interaction with a partially closed fatigue crack initiated in the cavity‐induced plastic yielding zone is investigated experimentally and theoretically. In‐situ ultrasonic measurements were made for different crack lengths and variable static closure load during a fatigue test. Small periodic loading, superimposed on the static closure load, changes the crack interfacial condition and/or the open crack segment length resulting in nonlinear modulation of the surface wave reflected signals. The modulation spectrum is related to the crack initiation and evolution and to the crack opening‐closure behavior. It is shown that the modulation even at a low mean load could enhance considerably crack detectability. The increase of the second harmonic in the modulation spectrum is pronounced when the modulation is applied at two critical mean static loads: when the crack is nearly closed and when it is nearly open. It is also observed that the maximum modulation occurs ...