Elucidation of subharmonic generation mechanism and challenge to new ultrasonic nondestructive evaluation

In recent years, a new ultrasonic nondestructive evaluation method that utilizes nonlinear ultrasonic behavior generated in cracks or at a bi-material interface defect has emerged. However, the mechanism of generating subharmonics is theoretically still not well understood. Therefore, in this research, dynamic contact problems of a bi-material interface defect and a crack with contact boundary conditions are investigated by using numerical methods such as the convolution quadrature time-domain boundary element method (CQBEM), moving particle semi-implicit simulation, and elastodynamic finite integration technique. In addition, some crack shape reconstruction methods are developed based on the inverse scattering theory, and the applicability is considered. Numerical results show that the subharmonic generation is depend on the angle of incident ultrasonic waves, quasi-natural resonance frequency, and contact boundary conditions.