Pulse-echo ultrasonic measurements of compressibility coefficient of thick epoxy resin during the curing process

A pulse-echo apparatus was developed to maximize the amplitude of the reflected ultrasonic pulse in monitoring the curing process of epoxy resin. The aim was to increase the ultrasonic wave path in the resin, keeping its amplitude detectable while making it possible to study thick samples. Samples 5.0 x 10(-3) m thick were used in the measurements. During the curing process, the longitudinal wave speed ranged from 1475 to 2300 m/s and the attenuation ranged from 2.5 x 10(3) to 5.5 x 10(3) dB/m. Using a theoretical model for an ultrasonic wave traveling in a viscoelastic medium, and assuming that the density of epoxy resin does not change during the curing process, the compressibility coefficient was obtained as a function of the curing time. The values obtained for the compressibility coefficient ranged from 1.8 x 10(-10) to 4.3 x 10(-10) Pa-1, in good agreement with literature values.