Evaluation of crack closure stress by analyses of ultrasonic phased array images during global preheating and local cooling

Crack closure stress (CCS) is an important parameter that affects crack propagation rate. However, the method for measuring CCS in practical fields has yet to be developed. In this study, we propose a practical method of estimating CCS. In our experiment, a closed fatigue crack was imaged by a linear phased array (PA) method during crack opening by global preheating and local cooling (GPLC). Here, we assumed that the crack appears in PA images after the thermal stress induced by GPLC exceeds CCS. Therefore, we calculated the thermal stress induced by GPLC using an analytical solution and thereby estimated CCS. Then, to validate the CCS estimation method, we simulated the PA images by a finite difference time domain (FDTD) method with a damped double node (DDN) model, where CCS was relieved by the calculated thermal stress. Consequently, the temporal variation in crack depth observed in the PA image was successfully reproduced in the simulation. Thus, the CCS estimation method was verified by comparing experimental results with analysis results.