An automated laser-scattering system for nondestructive evaluation (NDE) was developed to scan silicon-nitride engine valves for distributed subsurface defects/damage. The technique is based on a cross-polarization detection method that measures only the optical scattering from the subsurface. The system utilizes two-rotation and two-translation stages to align and focus the laser beam on the valve surface during the entire scan, and the resulting two-dimensional scattering image is used to identify the location, size, and relative severity of subsurface defects. Laser-scattering images were obtained for two silicon-nitride valves made from NT551 (Norton-Saint Gobain) that were subjected to ambient-temperature cyclic-impact loads for 100 hours. These data were analyzed and compared with surface photomicroscopy results.
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