Ultrasonic nondestructive evaluation of impact-damaged thermoset and thermoplastic composite laminates
The aerospace industry, driven by a demand to reduce weight, has relied increasingly on composite materials to increase performance of advanced systems. However, impacts by foreign objects such as runway debris can damage composite components without leaving indentations or other visually identifiable marks. Microcracking of the matrix, fiber pullout, and delaminations are among the types of resultant internal damage that can weaken the structure and affect the ultimate load strength as well as fatigue life. The field of nondestructive evaluation (NDE) provides a means for the detection and evaluation of barely visible impact damage. In this study glass/epoxy, carbon/epoxy, and carbon/thermoplastic material systems were evaluated ultrasonically after being struck at low impact energy levels. Laminates were impacted following NASA specifications; two different clamped impact boundaries were evaluated. Impact energies were varied by adjusting the drop height or the mass of the impactor. Impact events were recorded with a video camera and the energetics were obtained from the video tape; these included the incident and rebound energy and velocity. The energy dissipated in the laminate as a consequence of the impact can therefore be determined. The total delamination area, as summed over all the ply interfaces through the thickness of the laminate, was determined from ultrasonic scans and then quantitatively correlated to the energy dissipated in the laminate. A destructive deplying technique was applied to woven laminates to obtain the size and morphology of the impact delaminations. These results were compared to NDE results. The ability of delaminations to block ultrasound and the resulting shadowing effects were investigated. Finally, the impact resistance of a thermoset system (carbon/epoxy) and a thermoplastic system (carbon/PPS) were compared and no significant difference was found.