Nondestructive evaluation with laser ultrasound
This article describes how laser-based systems generate and detect ultrasonic waves from a distance for on-line process control and rapid NDE inspection. Not long after the laser was invented in the 1960s, scientists proposed using the powerful new optical device for a novel nondestructive examination (NDE) technique that could probe the interior of an object from a distance. A short, intense laser pulse, they suggested, could non-destructively ''whack'' the surface of a test article by rapidly heating a small target area. The spot would thermally expand nearly instantaneously and send ultrasonic waves through the material. When these acoustic waves hit the boundaries of the test object or any internal flaws or discontinuities, they would rebound elastically and produce minute surface vibrations at the target area. This target would then be illuminated by a second detection laser, whose light is scattered by the surface. Ultrasonic motion at the surface causes a small phase or frequency shift (Doppler effect) in the scattered laster light, which is detected by an interferometer--a precision instrument that uses interference fringe phenomena to measure small displacements and their velocities. When the resulting interference patterns are analyzed to determine the frequency content of the tiny surface perturbations, the test item's dimensions, defect content, and even its physical state of matter can be determined.
- OSTI ID:
- 6817283
- Journal Information:
- Mechanical Engineering; (United States), Vol. 116:10; ISSN 0025-6501
- Country of Publication:
- United States
- Language:
- English
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