Delamination fracture of multidirectional carbon-fiber/epoxy composites under Mode I, Mode II and Mixed-Mode I/II loading

The purpose of the present study was to characterize the delamination fracture of continuous carbon fiber/epoxy multidirectional-laminates under Mode I, Mode II and Mixed-Mode I/II loading conditions. The present study considers the variation of the interlaminar failure energy, G(C), with the extent of crack jumping, and ensuing fiber bridging, which arises during the growth of the delamination in the multidirectional-laminates under the various modes of loading. The main type of laminate which was studied was a multidirectional fiber composite prepared from 24 ply lay-ups of (-45 degrees/0 degrees/+45 degrees)(2S) (+45 degrees/0 degrees/-45 degrees)(2S). The initial delamination was located at the +45 degrees/-45 degrees mid-plane of the specimen. It has been found that when the values of the interlaminar fracture energy, G(C), are ascertained as a function of the length of the propagating crack, a, then very complex relationships are observed. This was the case for all the different modes of loading, and these observations reflected the complex failure paths which occurred as the delamination propagated through the multidirectional fiber composites. It was, however, possible to define clearly the onset of crack initiation. These results also revealed that the values for the interlaminar fracture energy, G(C) (initiation), at crack initiation for the (-45 degrees/0 degrees-45 degrees)(2S) (+45 degrees/0 degrees/-45 degrees)(2S) multidirectional laminates were always significantly greater than that for the corresponding unidirectional (i.e., 0 degrees/0 degrees) laminates.