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 fract ure of continuous carbon fiber/epoxy multidirectional-laminates under Mode I, Mode II and Mixed-Mode I/II loading conditions. The present study consid ers the variation of the interlaminar failure energy, G(C), with the extent of crack jumping, and ensuing fiber bridging, which arises during the grow th of the delamination in the multidirectional-laminates under the various modes of loading. The main type of laminate which was studied was a multidi rectional fiber composite prepared from 24 ply lay-ups of (-45 degrees/0 de grees/+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 s pecimen. It has been found that when the values of the interlaminar fractur e energy, G(C), are ascertained as a function of the length of the propagat ing crack, a, then very complex relationships are observed. This was the ca se for all the different modes of loading, and these observations reflected the complex failure paths which occurred as the delamination propagated th rough the multidirectional fiber composites. It was, however, possible to d efine clearly the onset of crack initiation. These results also revealed th at the values for the interlaminar fracture energy, G(C) (initiation), at c rack initiation for the (-45 degrees/0 degrees-45 degrees)(2S) (+45 degrees /0 degrees/-45 degrees)(2S) multidirectional laminates were always signific antly greater than that for the corresponding unidirectional (i.e., 0 degre es/0 degrees) laminates.