Laser-based ultrasonic NDE using gas-coupled laser acoustic detection

Abstract

An optical beam-deflection technique has been applied to the ultrasonic inspection of materials. The technique is similar to air-coupled detection, except the airborne wave is detected through beam deflection instead of with an electro-acoustic transducer. It is a noncontact, laser-based, detection technique that does not depend on reflection of the detection laser beam from the surface. It is thus independent of the surface optical properties of the material under test, and is particularly suitable for testing polymer/graphite composites, which have rough surfaces of poor optical reflectivity. The beam is directed parallel to the surface of a solid material under investigation and is incident on a custom-built position-sensitive photodetector. A wave is radiated from the surface into the surrounding gas during reflection of elastic waves in the solid. The transverse acoustic density gradients cause a beam deflection which is clearly observable with the photodetector. Applications of gas-coupled laser acoustic detection (GCLAD) with laser ultrasound generation will be illustrated with (1) waveforms showing numerous, well-resolved echoes, (2) C-scans showing subsurface defects, and (3) surface acoustic waves. The samples tested include several types of polymer/graphite composites. [Work supported by the Army Research Office.]