Multi-mode nonlinear ultrasonic phased array for closed crack imaging

Crack closure is one of the serious issues in ultrasonic testing that may cause underestimation or misdetection. Nonlinear ultrasonics is a primary means to solve this problem. Recently, several techniques using ultrasonic phased arrays to measure elastic nonlinearity, referred to commonly as nonlinear ultrasonic phased array imaging, have been proposed and have had their efficacy demonstrated in closed-crack specimens. Note that most of them use nonlinearly scattered longitudinal waves. On the other hand, nonlinear scattering at closed cracks can generate not only longitudinal waves but also mode-converted shear waves, which may have different characteristics from longitudinal one. Here we propose to use both longitudinal and mode-converted shear waves in nonlinear ultrasonic phased array imaging, which is called multi-mode nonlinear ultrasonic phased array imaging. We examined the nonlinear scattering in numerical simulation. Subsequently, it was also experimentally demonstrated.