Characterization of pre-impregnated graphite-epoxy lamina with gas-coupled ultrasonics

In this paper, we demonstrate the feasibility of employing gas-coupled ultrasonics in the nondestructive evaluation of pre-impregnated graphite-epoxy sheets (“pre-preg”). Furthermore, we show that the gas-coupled approach may be useful for online inspection of such lamina. To determine the quality of the prepreg, a 0.5 MHz focused ultrasonic beam is transmitted through a 0.15-mm thick sample of the material and the amplitude is measured as a function of position. The “quality” of the material is then evaluated in terms of pixel histogram statistics. In particular, measurements are made on three samples that were qualitatively characterized as: “good”, “fair”, and “bad”. The two-dimensional scanned images are analyzed quantitatively by calculating pixel histograms of the transmitted sound amplitude for each of the samples. Unambiguous differences, amounting to more than 30 dB when averaged over a 0.0225 m² area, are noted between samples independently judged as acceptable and those judged to be rejectable