Local phase measurements with focused acoustic transducer

A focused acoustic transducer consisting of three separated elements has been designed to perform local phase measurements. The outer elements have been used to generate and receive Rayleigh Surface Acoustic Waves (RSAW) generated at the interface between water and the sample. The central element is utilized to generate and receive direct reflected signal from the surface of the sample. A tone burst signal is split into two parts; one part is fed to the central element and other to one of the outer elements. A phase comparison of the two received signals at a defocus is related to the relative changes in the RSAW velocity. Measurements across an electron beam weld in a sample of titanium alloy using the new lens is compared with absolute RSAW measurements performed using an impulse excitation and two-point defocus method, as well as with residual stress measured with x-ray diffraction. The results of relative phase measurements indicate significant improvement over the absolute RSAW measurements. The changes in the relative phase measurements are in agreement with the x-ray diffraction residual stress measurements.