Characterization of composite prepreg with gas-coupled ultrasonics

Abstract The feasibility of employing gas-coupled ultrasonics in the non-destructive evaluation of pre-impregnated graphite-epoxy lamina in a realistic fashion for on-line inspection has been demonstrated. The gas-coupled wave amplitude of a 0.5 MHz focussed ultrasonic beam transmitted through a 0.15 mm thick sample of the prepreg has been measured as a function of position and material quality. Measurements are reported on three qualitatively characterized samples. Scanned images are compared quantitatively by calculating pixel histograms of the transmitted sound amplitude for the samples. Clear, unambiguous differences, amounting to more than 30 dB when averaged over a 225 cm 2 area, have been observed in gas-coupled tests between material classified as acceptable and that determined to be rejectable in independent assessments. These quantitative results imply a high potential for reliable discrimination among material quality levels.