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.
Pulsed lasers can generate ultrasound from stresses due to rapid thermal expansion. In this low power thermoelastic regime the material is not damaged. This paper concentrates on epoxy resins and aims to relate the observed amplitude of the longitudinal wave to the optical absorption depth of the epoxy. The ultrasound is generated using a high power pulsed laser and the absolute amplitude of the ultrasound is measured with a Michelson interferometer. In the thermoelastic regime, the laser beam is focused onto the sample, causing rapid expansion in times that are comparable to the rise time of the laser pulse. In metals, the laser radiation is absorbed in the thin electromagnetic skin depth but in non-metals the phenomenon is dominated by the optical absorption depth. The latter can vary from a few microns to several millimetres for materials such as epoxy resins. As a consequence, 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”. Two lasers were used in this study: a TEA CO2 and a XeCl excimer laser. The results are compared with optical transmission measurements.
Techniques are being developed worldwide for non-contact ultrasonic inspection of composite materials. These include laser generation and optical detection of ultrasound; both with interferometers and simpler beam deflection techniques, air coupled transducers are also used as generators and/or detectors of ultrasound. This paper compares the generation efficiency and damage thresholds of a range of different laser types: A fundamental Nd:YAG laser (1.06 micrometer), a TEA CO2 laser (10.6 micrometer normally preferred for carbon- fiber reinforced composites) and a Nd:YAG laser with an Optical Parametric Oscillator (OPO) tunable up to 4 micrometer. The laser energy is absorbed with the optical absorption depth, the temperature rise is affected by the wavelength and laser pulse duration. It is essential to remain in the thermoelastic regime in order not to damage the material. A modified Michelson interferometer is used to detect the absolute displacement of the ultrasound. Optical beam deflection techniques and air-coupled transducers are also evaluated as detectors.