Application of laser ultrasonics to graphite/polymer composite materials

The development and application of a laser-based ultrasonic inspection system for noncontact evaluation of graphite/polymer composite materials has been realized. The use of lasers to generate and detect ultrasonic waveforms in materials provides a means to detect material properties remotely. The study consisted of three main aspects:; (1). A confocal Fabry-Perot (CFP) based ultrasonic detection system has been developed which uses light reflected from the CFP interferometer to derive the ultrasonic signal. In a reflection configuration, higher frequency components of the detected waveforms can be discerned when compared to the same system using light transmitted through the CFP interferometer. With improvements in photodetector circuit design, rise times as short as 35 ns have been realized. This also resulted in improved resolution of waveforms created by concurrent but distinct generation processes.; (2). Thermoelastic and ablative laser generation of acoustic pulses in polymer/graphite composite materials have been investigated. Thermoelastic generation of ultrasound occurs when thermal energy deposited by a pulsed laser creates a localized expansion in the material. Ablative generation of ultrasound results from the vaporization of surface material when the laser pulse surpasses a power density threshold. The superposition of the thermoelastic and ablatic waveforms was examined. The evolution of waveforms as a function of laser power density, identification of graphite fibers as the ablation source material, and surface damage on composites created by the laser light impact are discussed.; (3). A novel technique for the detection of low-amplitude ultrasonic waveforms in a gaseous medium by laser beam deflection has been developed. The compression and rarefaction of gas density associated with an acoustic pressure creates corresponding fluctuations in the index of refraction of air. A probe laser beam passing through this region is deflected as a result of the gradients in the index of refraction, as described by the eikonal equation. An electronic signal results when the beam is deflected from its original position on a position-sensitive photodetector, which is proportional to the first time derivative of the acoustic pressure. Airborne acoustic waveforms with acoustic pressures as low as 2 Pascals have been detected. Gas-coupled laser acoustic detection (GCLAD) enables laser ultrasonic inspection of materials that is independent of the optical surface characteristics of the test sample. Applications in both the ultrasonic and audible frequency ranges have been demonstrated.