An accurate method for measurement of transverse elastic-wave velocities

We demonstrate a novel analysis that improves the accuracy of angle-dependent elastic-wave velocities in plates for water-immersion measurements. Water-immersion measurements utilize the conversion from longitudinal to transverse mode to determine the velocities of transverse waves. However, beyond the critical angle for longitudinal waves, the phase of the transmitted transverse waves is affected by the presence of evanescent longitudinal waves at the interface. With pulse superposition analysis this effect systematically decreases the apparent velocities at some angles. Previous researchers have corrected the apparent velocities by calculating the theoretical phase shift from known parameters, but this correction requires narrow-band pulses or tone bursts as well as estimations of these parameters. An advantage of our analysis is that it eliminates the need for a priori estimations of parameters. In addition, this analysis allows the use of broad-band pulses that are more applicable to thin plates. To demonstrate these advantages, measurements were conducted on an isotropic steel plate as it was rotated in an immersion tank and insonified with a broad-band ultrasonic transducer. We compared velocities determined by the pulse-echo superposition analysis with velocities determined by our analysis. As expected, there was no significant difference between the longitudinal velocities, but systematic variations in the transverse velocities were lower with this new method.