Laser/Materials Interaction Studies for Enhanced Sensitivity of Laser Ultrasonic Systems

摘要

Under this program, laser ultrasonic wave generation and propagation in composite materials was studied to improve the sensitivity of laser ultrasonic inspection of composite materials. The obvious advantages of laser- based systems aside, compared to conventional counterparts, the sensitivity of these systems must be improved to yield comparable inspection capabilities. To address sensitivity issues, those factors that are critical to improving the generation efficiency and propagation of laser ultrasound in composite materials were investigated to provide directions for advancing the performance of laser ultrasonics systems. The directions that were identified and pursued included the following: optimization of laser pulse duration for maximum ultrasonic energy transmission, determination of the effects of composite anisotropy and homogeneity on ultrasonic transmission, modification of the laser source to improve detectability using signal processing and investigation of laser ablation damage mechanisms. Owing to inhomogeneity and anisotropy, composite materials behave differently than do traditional aircraft materials such as aluminum. This behavior was assessed to identify the frequency dependent effects on various ultrasonic modes in composites. The effects of pulse duration on ultrasonic generation deficiency were determined; the ability to deliver more energy at the frequencies of interest, that are supported by the composite, greatly improves the transmission of ultrasound through composites. These accomplishments of the program were broadly applicable to laser ultrasonic inspection of composites and directly indicated methods to improve laser ultrasonic system sensitivity.