The role of epoxy resin in the mechanism of laser generated ultrasound in carbon fiber reinforced composites

Epoxy resins are essential to the fabrication of carbon fiber reinforced composites (CFRCs). This paper investigates laser generated ultrasound in epoxy resins using three pulsed lasers: A TEA CO₂, a fundamental Nd:YAG and a XeCl excimer. In the low power thermoelastic regime, the laser beam causes the surface of the sample to expand rapidly, in times that are comparable to the rise time of the laser pulse. In non-metals the phenomenon is dominated by the optical absorption depth, which is a function both of the properties of the material and the laser wavelength, and for epoxy resins, varies from a few microns to several millimeters. Compared to the thermoelastic source in metals, a bigger volume of the material is affected, the temperature rise is less and the amplitude of the longitudinal wave is greater. This condition is referred to as "a buried thermoelastic source". In CFRCs, the superficial layer of epoxy resin (typically 50 - 100 microns thick) plays an important role to the generation mechanism. At the Nd:YAG wavelength the epoxy is transparent and acts as a constrained layer. At the TEA CO₂ and the XeCl excimer wavelengths both the epoxy and the underlying fibers absorb strongly. Experiments were carried out on CFRC and pure epoxy resin samples, comparative results and efficiency graphs are presented.