Grain size evaluation with time-frequency ultrasonic backscatter

The ultrasonic backscatter signal can be used to evaluate the grain size of polycrystalline media nondestructively. In this article, the joint time-frequency analysis is introduced into the ultrasonic backscatter model, with the independent scattering approximation. The time-frequency domain model is applied to predict the spatial variance surface (SVS) and fit the experimental results to evaluate the grain size with the approach giving a relative error of 2.26% in comparison with metallography. Moreover, it shows that the present method sup-presses electrical noise better than either the time domain or the frequency domain models alone. Based on the global sensitivity analysis of Sobol's method, the sensitivities of transducer parameters and material parameters in the ultrasonic backscatter model are estimated. The sensitivity analysis shows that not all the transducer parameters in the model must be calibrated carefully. In addition, it is found that the grain size sensitivity index of the time-frequency backscatter model is higher than the time domain or the frequency-domain backscatter models. Overall, the backscatter model in the time-frequency domain can evaluate the microstructure more accurately.