Nondestructive Material Evaluation in a Small Area by Surface Waves (Quantitative Evaluation of Surface-Wave Velocities in Heat-Treated Steels by Ultrasonic Reflectivity Measurement)

A newly developed ultrasonic reflectivity measurement technique is applied for the evaluation of surface-weve velocity in a small surface area in polycrystalline materials. The surface-wave velocity is determined by observing either the phase change or the amplitude change of the ultrasonic reflection coefficient on the specimen surface. The measurement system, with a sensor unit consisting of a transmitter with a planar lens and a receiver with a spherical lens, provides high spatial resolution for estimation of frequency dependence of the velocity using impulse waves. Measurements of steel with various elastic properties and structures have been carried out in a frequency range from 40 to 120 MHz. The measurement resolution and accuracy were dependent on the frequency. The velocities on quenched and tempered steels could be estimated accurately by measuring the phase change, while the velocity on annealed steel was estimated by measuring the amplitude change, because the phase change of the annealed steel was anomalous due to significant attenuation in the solid in the high-frequency region.