Location

La Jolla, CA

Start Date

1-1-1983 12:00 AM

Description

A program is described which employs lasers for ultrasonic NDE. A high-power laser is used to generate a brief sound pulse in the test specimen. A second low-power laser then measures the response of the specimen to that sound pulse.

The response of the specimen is measured by a “Laser Vibrometer.” This is a novel type of heterodyne interferometer which focuses a Helium-Neon laser beam onto the surface of the specimen and measures its displacement. Displacements as small as 2×10-12 meters on a 0.15 sec averaging time can be detected and also displacements of 1.5×l0-9 meters on a 10-MHz bandwidth. The Laser Vibrometer has a well defined frequency response and does not introduce distortion.

The sound generating laser is either a pulsed carbon dioxide TEA laser or a YAG laser. The peak power exceeds 10 M watt. Two mechanisms for generating the sound are discussed. The thermoelastic mechanism relies on the thermal expansion of the surface, causing it to move. The reaction to this causes a pressure pulse in the specimen. Another mechanism allows a small amount of the surface to be ablated and the reaction to this causes a substantial pressure pulse in the specimen.

Both laser beams can be scanned over the surface of the specimen by a microprocessor controlled mirror. The microprocessor generates a raster scan of arbitrary size, number of lines, step size and speed.

Eventually this technique will allow the inspection of complex specimens without direct contact. This will eliminate the tedium and contact reliability problems associated with conventional piezo-ceramic NDE.

Book Title

Review of Progress in Quantitative Nondestructive Evaluation

Volume

2B

Chapter

Section 25: Ultrasonic Transducers and Standards

Pages

1763-1782

DOI

10.1007/978-1-4613-3706-5_118

Language

en

File Format

application/pdf

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Jan 1st, 12:00 AM

Ultrasonic Nondestructive Evaluation Using Laser Transducers

La Jolla, CA

A program is described which employs lasers for ultrasonic NDE. A high-power laser is used to generate a brief sound pulse in the test specimen. A second low-power laser then measures the response of the specimen to that sound pulse.

The response of the specimen is measured by a “Laser Vibrometer.” This is a novel type of heterodyne interferometer which focuses a Helium-Neon laser beam onto the surface of the specimen and measures its displacement. Displacements as small as 2×10-12 meters on a 0.15 sec averaging time can be detected and also displacements of 1.5×l0-9 meters on a 10-MHz bandwidth. The Laser Vibrometer has a well defined frequency response and does not introduce distortion.

The sound generating laser is either a pulsed carbon dioxide TEA laser or a YAG laser. The peak power exceeds 10 M watt. Two mechanisms for generating the sound are discussed. The thermoelastic mechanism relies on the thermal expansion of the surface, causing it to move. The reaction to this causes a pressure pulse in the specimen. Another mechanism allows a small amount of the surface to be ablated and the reaction to this causes a substantial pressure pulse in the specimen.

Both laser beams can be scanned over the surface of the specimen by a microprocessor controlled mirror. The microprocessor generates a raster scan of arbitrary size, number of lines, step size and speed.

Eventually this technique will allow the inspection of complex specimens without direct contact. This will eliminate the tedium and contact reliability problems associated with conventional piezo-ceramic NDE.