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.

Advanced materials for use in the aerospace industry are presently being developed and applied at an astonishing rate. This pace is driven by the need for materials that can withstand higher operating temperatures and loads, yet remain cost competitive. The future in NDE is influenced by the increased performance demands on materials, while on the other hand, a new need has developed for inspection of old materials in aging aircraft. As the performance demands of aerospace materials push nearer and nearer the theoretical limit for strength, the allowed flaw size in traditional materials is driven smaller, making quality control more stringent. The promise of improved performance characteristics is also generating strong interest in other materials such as: exotic alloys, ceramics and reinforced composites. The last two issues involve increasing the performance of materials, but the aerospace industry is also going through a critical period in that many of the original commercial airliners are reaching their design-life limits. With natural resources becoming more and more limited, the cost of replacement is often prohibitive. The next decade will bring about many changes in the aerospace industry in that cost effective airworthiness inspection programs will be developed to extend the life of older aircraft.

Ultrasonic nondestructive evaluation can be used to quantitatively interpret received electrical signals from ultrasonic measurements. In order to preform this interpretation, several techniques have been used. Two of these techniques are the standard procedure and the measurement model. The standard procedure compares the waves reflected from the flawed specimen with the waves reflected from many specimens of the same host material with different known flaws in the interior. By matching reflections, the approximate size of the flaw can be estimated by comparison with the response from the known standard flaw model; however, this method requires a large number of known, standard flaw models, which makes it a difficult procedure to use. In addition, different flaw shapes of different sizes can result in the same overall peak-to-peak response. Therefore, precise flaw sizing can not be obtained with this technique.