Crack Measurements by Laser Ultrasonic at High Temperatures

A noncontact optical system for generation and detection of ultrasound is described. A confocal Fabry-Perot interferometer combined with a multimode optical fiber is used to reconstruct ultrasound images on a sample with a rough surface. The optical fiber is automatically adjusted to optimal positions for effective collection of scattered light from the sample. The confocal Fabry-Perot interferometer provides sensitive detection independent of surface roughness. The system is applied to a fatigue test sample, with which a conventional interferometer has poor sensitivity because of irregularities on the sample surface. The 2D profiles of the fatigue crack are successfully obtained at temperatures up to 1000°C. The crack length estimated from the profile agrees well with that measured using an optical method.