Nonlinear behavior of ultrasonic wave at crack

Abstract

A nonlinear model for contact acoustic nonlinearity (CAN) of closed crack was proposed and analyzed to investigate the nonlinear characteristics of transmitted wave for normal incident wave. Closed crack was modeled as an imperfectly jointed interface that has nonlinear discontinuity in displacement across its boundary. Based on the interfacial stiffness of contact interface, a nonlinear parameter was defined as the ratio of second harmonic component to first harmonic, and derived in terms of discontinuous displacements across the interface. It is found that the amplitude of second order harmonic wave is highly dependent on interfacial stiffness. This was verified by experiments for a real crack. For this, a CT fatigue specimen of Al6061 with V-notch was prepared to initiate the crack, and the amplitude of second order harmonic was measured by scanning along the crack direction. From the results of pre-test to evaluate the contact condition of crack interfaces by monitoring the amplitudes of echo and through transmission, the crack was regarded as gradually closed. It could be observed that the harmonic amplitude had a good correlation with the crack closure and agreed well with the theoretical expectation.