Multi-mode ultrasonic waves focusing in a variable focus technique for simultaneous sound-velocity and thickness measurement

Abstract

A double-focus technique, using a spherically focused immersion transducer, has been developed for simultaneous sound-velocity and thickness measurement. This is an effective method that does not require pre-experiments or predictive parameters. In previous studies, the single-mode wave model, usually used, cannot explain the additional reflection peaks on the V(z,t) curve, which denote two-dimensional amplitudes as a function of the movement along distance z and the time t. In this research, a multi-mode wave model, interpreting the results of depth scanning of thin solid plates, using a spherically focused ultrasound transducer in pulse-echo mode, is presented. The existence of additional reflection peaks in the solid plate due to mode conversion, which are usually neglected, are described in the new model. A comprehensive interpretation of the measurements is proposed, instead of ignoring the transverse waves and mode conversion. The distribution of energy among multiple reflected focuses and the vertical displacements of the transducer are numerically analysed. The experiments on a 0.5 mm thick stainless steel and a 0.5 mm thick aluminium alloy have been carried out to verify the model. This study contributes to the identification of longitudinal-wave focuses, especially in cases of simultaneous measurement on multi-layered materials.