Improvements in instrumentation for spatially resolved acoustic spectroscopy (SRAS)

Characterizing microstructure of manufactured materials is a necessary process for determining material properties. Spatially resolved acoustic spectroscopy (SRAS) is a method for non-destructively characterizing microstructure using laser induced surface acoustic waves (SAWs). It provides grain orientation maps similar to those acquired from electron backscatter diffraction (EBSD) over a significantly greater measurement area and in a shorter time period. In this work, we introduce the fundamentals of the technique and improvements to the SRAS measurement systems discussed in the literature. The following developments in instrumentation are focused on improving the speed, resolution, and versatility of SRAS for characterizing polycrystalline materials. A photon Doppler velocimeter (PDV), often used in short duration high-velocity experiments, was adopted for measuring SAWs with a higher sampling rate than instruments previously used. The multi-channel design of the PDV is advantageous for performing measurements at multiple locations and orientations simultaneously. A high-powered pulsed laser provided increased signal-to-noise ratio and high precision of excitation timing. Lastly, a spatial light modulator (SLM) was used to structure the laser light from the excitation source enabling the ability to quickly change diffraction patterns, increasing the speed of measurements. It also has highly customizable patterns that are only limited by pixel size.