Abstract
With a laser Doppler vibrometer the propagation of ultrasonic waves at the free surface of a solid can be measured contactless. While a fixed transducer transmits ultrasound pulses into the sample with high repetition rate, its surface is scanned point by point with high spatial resolution and the out-ofplane displacement is recorded. The recorded data of all points can be visualized as a slow motion video sequence and show the propagating waves. The wave front of a surface skimming longitudinal wave (SSLW) at the scanned surface is locally influenced by the material's near-surface grain structure. The grain structure can be extracted directly from the recorded data by appropriate signal processing. This method is called "Grazing Incidence Ultrasound Microscopy" (GIUM). Due to the fine laser focus and high scanning resolution, the grain structure images show details smaller than the acoustic wavelength. This method was applied on austenitic weld specimens as an alternat ive to classical metallography. A comparison between GIUM image and electron backscatter diffraction (EBSD) image of the same specimen is shown.