Determination of the mode I, mode II, and mixed-mode I–II delamination toughness of a graphite/polyimide composite at room and elevated temperatures

A study is described in which the mode I, mode II, and mixed-mode delamination toughnesses of a woven fabric graphite/polyimide composite are determined at room temperature and at 300℃. It is shown that, with some modifications, commonly used delamination toughness test methods can be extended for use into this relatively high-temperature regime. At room temperature, the mode II toughness was found to be significantly higher than that obtained in mode I. Relative to their fracture toughnesses at room temperature, the mode I and mixed-mode toughnesses increased at elevated temperature, while the mode II toughness decreased. Thus, at 300℃, the mode I toughness was larger than that obtained for mode II. The temperature dependence in all three modes was found to be consistent with what is predicted based on micromechanical considerations for the effects of temperature on the resin. In addition, the local tow orientations influenced the fracture path and produced toughening mechanisms that were both temperature and mode mix dependent.