Travel-Time Difference Extracting in Experimental Study of Rayleigh Wave Acoustoelastic Effect

In order to identify the travel-time difference accurately in the experimental study of Rayleigh wave acoustoelastic effect, an experimental system is constructed by the ultrasonic pulser-receiver, digital oscilloscope, Rayleigh wave transmitter and receiver, and a personal computer. And then, the digital correlation method and the Fourier transform frequency analysis method are used to obtain the travel-time difference of the Rayleigh wave corresponding to different subsurface stress conditions. Furthermore, the simulated ultrasonic signals are used to verify the reliability of the two kinds of ultrasonic signal information extracting algorithms, respectively. Finally, the proposed signal processing methods are applied to extract the time delay between different Rayleigh wave signals corresponding to different subsurface stress level. 1. Introduction In recent years, stress measurement has been a hot issue in the mechanical engineering. For instance, by using the strain gauge stress measurement is the traditional detection method, which has been widely used in the principal engineering stress estimate. On the other hand, some new nondestructive techniques have been developed to obtain the stress station of the metal or nonmetal material for engineering structure. After the acoustoelastic effect of the ultrasonic is proposed and proved by the experimental studies [1, 2], this type of physical phenomenon is applied to evaluate the stress level of propagation medium. As we know, the longitudinal and transverse ultrasonic waves are used for estimating the average stress of a structure along the thickness direction because of the changing of the propagating characteristic. However, Rayleigh wave, which propagates along the surface of the material, makes it possible to probe the subsurface stress of the detected object. However, the acoustoelastic effect of longitudinal, transverse ultrasonic wave, and Rayleigh wave is a weak effect caused by the subsurface stress influence. From the previous experimental research, the experimental phenomenon cannot be captured easily. So identifying the difference in time of flight accurately is the key point in the experimental study of acoustoelastic effect. In this paper, two different ultrasonic process methods are discussed to obtain the travel-time difference. The signal process algorithms are verified by digital simulation and experimental approach. 2. The Acoustoelastic Effect of Rayleigh Wave The propagating velocity of the Rayleigh wave in a uniformly deformed solid medium surface is changed due to the subsurface