Practical considerations for the rapid inspection of composite materials using laser-based ultrasound

Non destructive inspection of aerospace composites by a fiber-coupled laser ultrasonics system

“…Literature shows that the efficiency of generating longitudinal waves in graphite/epoxy depends on the optical penetration depth, itself function of the wavelength [11] . Furthermore, short pulses (in the nanosecond or tens of nanosecond range) are required.…”

A Linear Systems Approach to Laser Generation of Ultrasound in Composites

“…This indicates that the thermal conductivity in the illuminated region is quite low. It has been shown that, when generating with a Nd: Y AG Iaser in a graphite-epoxy specimen, a significant portion ofthe Iaser energy is absorbed by the graphite fibers [3]. The present results suggest that either the one-dimensional heat flow in the fibers is was not sufficient to remove heat from the illuminated region during the nanosecond time-scale of the incident Iaser pulse or that enough Iaser energy is absorbed by the matrix material ( extremely low thermal conductivity) to cause vaporization independent of the fiber temperature.…”

“…Laser ultrasonic generation and detection systems have been shown to be effective in the inspection and evaluation ofboth metals and composite materials [1][2][3]. Advantages of these noncontact systems include rapid scanning capability, the inspection of parts with complex geometries, and the ability for use in hostile environments.…”

Laser Ultrasonic Thermoelastic/Ablation Generation with Laser Interferometric Detection in Graphite/Polymer Composites

“…[2] The relatively sparse literature on laser generation in other materials includes a paper by Taylor et ai., [3] describing experimental work on generation by a CO2 laser in pure polymer, and a paper by McKie and Addison, [4] comparing the effectiveness of Nd:YAG and CO2 lasers for generation in graphite/polymer composites. Taylor et al demonstrated that ultrasound waveforms at intermediate laser power levels can be interpreted as a superposition of waves generated by thermoelastic and ablation processes, with an additional component due to plasma breakdown at higher powers.…”