Development of a Scan System for Rayleigh Wave Velocity Mapping

A velocity map contains more quantitative information than an amplitude image because Rayleigh wave velocity is related to the elastic constants of the material. A high-precision scanning acoustic microscope system has been developed by the University of Dayton Research Institute with the capability to generate Rayleigh wave velocity map images of aerospace materials. The velocity map is presented in a C-scan format in order to visualize the velocity distribution in a material or around a defect. Rayleigh wave velocity is measured using a time-of-flight (TOF) technique. This system utilizes impulse excitation in order to separate the direct reflected signal and the Rayleigh wave signal in the time domain. Time differences between these signals at two defocus depths are used to calculate Rayleigh wave velocity in real-time and display a 2D x/y velocity map image during the scan. Velocity measurement accuracy is demonstrated to be better than 1%. Software techniques that were developed to improve time measurement accuracy will be quantitatively compared with the velocity in standard materials. Both measurement accuracy and standard deviation of experimental data are used as the basis of comparison for each investigated technique. Techniques discussed include improved peak detection, signal averaging requirements, digitization rate, and software gate placement. C- scan images of Rayleigh wave velocity maps of several materials will be presented