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 effects 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 IM7/8551-7A graphite/rubber-toughened epoxy system. Delamination starter films of two different thicknesses were used: 25.4 μm and 12.7 μm. High-temperature degradation was accomplished by placing the specimens on a wire rack in a forced-air convection oven held at 177 °C. A continuous flow of fresh laboratory air existed at all times. G_IC decreased with increased exposure, showing a 40% reduction after 8000 h. The specimens with thicker inserts showed a higher G_IC. Weight loss of the material occurred throughout the aging cycle, indicating that thermo-oxidation was occurring. G_IC was not affected by the wedging open of the precrack during exposure. Crack propagation in the unexposed specimens occurred in the matrix material, whereas for the exposed specimens, it occurred along the fiber-matrix interface. This indicated possible exposure-induced interfacial degradation.