FATIGUE AND FRACTURE.

Graphite composite materials have demonstrated superior fatigue properties at room temperature and to a limited extent at elevated and cryogenic temperatures. But the environment of the body-flap presented potential temperature extremes that have not been studied in a deliberate sense. In this study, the effects of the load/temperature environment on the fatigue life of the body-flap was investigated by conducting real-time and accelerated flight-by-flight tests up to 500 flights on coupons containing holes. On the fracture side, much is known about the sensitivity of the tensile strengths of composites to holes and other flaws, predominantly at room temperature. In this study, the effects of notches were assessed for graphite polyimides at the expected temperature extremes of the body-flap: 117 K ( minus 250 degree F) to 589 K (600 degree F). The two composite materials to be used for the majority of tests are Celion graphite fiber in mattrices on PMR-15 and NR 150-B2. The preliminary results are presented with a description of specially-developed testing equipment.