Estimation the mean correlation length of metals by using mode-converted diffuse ultrasonic backscatter model

Abstract

Diffuse ultrasonic backscatter measurements can explain the phenomenon of the scattering at interfaces in heterogeneous materials. The theoretical models of diffuse ultrasonic backscatter for longitudinal (L-L) and shear (T-T) wave scattering within polycrystalline materials have recently been developed. A mean correlation length of metals is successfully calculated from the L-L model at normal incidence in a pulse-echo inspection. Above the first critical angle, mode-conversion scattering occurs when the longitudinal waves are converted to shear waves (L-T) at material grain boundaries. With a similar formalism, a mode-conversion scattering (L-T) model is presented to describe this process. The model is then to fit the experimental response for a pitch-catch transducer configuration and the correlation length is extracted by modifying the beam function. The mean correlation length from the L-T model is in agreement with both the L-L model and the results obtained from optical micrographs. This presentation outlines the theoretical framework and the method to extract the mean correlation length. Mode-converted backscatter removes the influence of the front-wall reflection and may lead to improvements in microstructural characterization and material property evaluation. [Research supported by FRA.]