Ultrasonic Nonlinear Responses of a Closed Crack in Plate-like Structure

Lamb wave method is one of the most effective methods to detect plate-like structures. Ultrasonic nonlinear response has high sensitivity to micro-damage. The nonlinear theories of single frequency classical nonlinear theory and contact acoustic nonlinearity are described in this paper. The distortion of the waveform is caused by the asymmetric stiffness near the crack. In this paper, the plate structure, closed crack, excitation, mesh and step size are modeled in detail by finite element method. The results show that the contact of the closed crack surface leads to the nonlinear phenomenon. The length, stress and friction coefficient of the crack are simulated, and it is found that the crack parameters have obvious correlation with the nonlinear response of Lamb wave. Finally, in order to study the effect of the crack direction on the nonlinear response, a horizontal crack is introduced. With the growth of the horizontal crack, growths of harmonics appear periodic changes.