THE EFFECT OF HIGH-TEMPERATURE DEGRADATION ON THE MODE-I CRITICAL STRAIN-ENERGY RELEASE RATE OF A GRAPHITE EPOXY COMPOSITE/

The double cantilever beam (DCB) test was used to determine the effect s of high-temperature degradation on the Mode I critical strain energy release rate (G(IC)) of a continuous fiber-reinforced polymer-matrix composite material. The composite material investigated was IM7/8551-7 A graphite/rubber-toughened epoxy system. Delamination starter films o f two different thicknesses were used: 25.4 mu m and 12.7 mu m. High-t emperature degradation was accomplished by placing the specimens on a wire rack in a forced-air convection oven held at 177 degrees C. A con tinuous flow of fresh laboratory air existed at all times. G(IC) decre ased with increased exposure, showing a 40% reduction after 8000 h. Th e specimens with thicker inserts showed a higher G(IC). Weight loss of the material occurred throughout the aging cycle, indicating that the rmo-oxidation was occurring. G(IC) was not affected by the wedging ope n of the precrack during exposure. Crack propagation in the unexposed specimens occurred in the matrix material, whereas for the exposed spe cimens, it occurred along the fiber-matrix interface. This indicated p ossible exposure-induced interfacial degradation.