ABSOLUTE OPTICAL-ABSORPTION SPECTRA IN GRAPHITE-EPOXY BY FOURIER-TRANSFORM INFRARED PHOTOACOUSTIC-SPECTROSCOPY

Optical absorption is obviously of prime interest in the efficiency of laser generation of ultrasound in graphite-epoxy laminates. However, no quantitative spectrum of optical absorption in this composite mater ial has yet been published in the literature. Transmission techniques are inefficient, and other techniques, like attenuated total reflectan ce or diffusive reflectance, do not give absolute values. The Fourier transform photoacoustic spectroscopy technique seems to be a good alte rnative that can analyze adequately and quantitatively a graphite-epox y laminate. We used three different methods to compute the absolute op tical absorption from the photoacoustic signal. The three methods are: the saturation of the real part of the photoacoustic spectrum, the co mparison of the spectra obtained with two different mirror velocities, and the calibration of the photoacoustic cell with a transmission mea surement. The spectra obtained in the IR band of 2.5 to 25 mum are pre sented, and the problems and limitations of each method are discussed. The results permit a better understanding of the absorption process i n the composite laminate, and in this way, will help us enhance the ef ficiency of laser generation of ultrasound in graphite epoxy.