Interferometric Measurement of Ultrasonic Velocities with Line-Focusing P(VDF-TrFE) Transducer: V(f) Curves
Acoustic microscopy  is used to measure the V(Z) curves and thereby determine the ultrasonic velocities on the surfaces of solid materials. The acoustic transducer, point- or line-focusing, is defocused below the sample surface through the coupling water. The focal depth Z is changed to extract the interference between the specular reflection signal and the leaky surface wave signal(s). Their phase difference produces the periodic patterns of the combined amplitude as a function of Z, V(Z). The periodicity, ΔZ, together with the velocity in water and the operating frequency, gives the surface wave velocity, which is related to residual stress, preferred grain orientation (texture), or other material attribute of interest in the surface region. The V(Z) curve is an unique technique of ultrasonic NDE, because it provides a direct and localized measurement of surface wave velocities.