Laser-Ultrasonic Characterization of Fiber Reinforced Composites: Effect of the Generation Laser Transverse Modes

Laser-ultrasonics has been used with success for the microstructure evaluation of metals by measuring ultrasonic attenuation and velocity. Extension of this technique to fiber reinforced polymer composites could provide useful information such as fiber contents and porosity. Ultrasonic generation in composite materials is usually performed by using a pulsed TEA-CO2 laser operated on several transverse modes. The intensity profile of this laser could change from shot-to-shot, thus affecting the ultrasonic generation pattern and the accuracy of attenuation measurement. Two solutions are possible to overcome this problem. The first reported approach consists in numerically correcting for the diffraction effect by using a Monte Carlo integration after simultaneous acquisition of the ultrasonic signal and the laser mode pattern. The second reported approach consists in modifying the laser resonator to get a single transverse mode while maintaining high energy per pulse, and to make the diffraction correction much simpler. Both approaches have been tested on a bare carbon epoxy plate with laser generation in the thermoelastic regime. The results obtained from using either approaches show a significant improvement and are compared to those obtained by conventional ultrasonics.Peer reviewed: NoNRC publication: Ye