Finite difference method analysis of ultrasonic nonlinearity in partially degraded material

In recent years, the fact that the nonlinear acoustic effect has a strong c orrelation to the microdeformation of materials has been considered to be u seful for nondestructive evaluation (NDE) of material degradation. However, in the past, most research considered only the case in which the material degradation was distributed evenly along the path of the probing wave. In c ontrast, in this study, the nonlinear behavior of ultrasonic waves in a par tially degraded material is considered. For this purpose, the finite differ ence method (FDM) model for the nonlinear wave equation is developed consid ering only I-D longitudinal wave motion and the contribution of the partial degradation in the material to the measurable nonlinear parameter is analy zed quantitatively. In the simulation, three types of distribution of degra dation are considered; single point, discrete distribution and continuous d istribution. Simulation results reveal that the degradation level and the r ange of degradation have a strong correlation with the nonlinear parameter beta which is estimated by measuring the harmonic generation. In order to v erify the validity of this numerical analysis method, the simulation result s in the case of continuous distribution of degradation are compared with t he previous experimental results, and they are found to show a similar tend ency.