Interlaminar fracture is one of the primary damage modes in laminated composites and it is generally caused by mixed mode deformations. To assess the resistance to interlaminar fracture under mixed mode deformations, it is necessary to perform the suitable evaluation for mixed mode interlaminar fracture toughness in laminated composite with respect to damage tolerance.
In this study, the antisymmeric test fixture with the fracture specimen and the modified ENF specimen are used to investigate the interlaminar fracture characteristics of laminated composite under mixed mode deformations. For the antisymmetric test fixture with the fracture specimen, accurate finite correction factors for the fracture specimen are determined with several crack lengths by finite element analysis. Interlaminar fracture toughness under mode-I, mode-II and mixed mode deformations is evaluated by experiments on the specimens with [0/0] fiber orientation on the crack surface and a mixed mode interlaminar fracture criterion is determined to predict interlaminar fracture behavior under mixed mode deformations. The effect of different fiber orientations on the crack surface on interlaminar fracture toughness is also investigated. For the modified ENF specimen, total energy release rate is determined by the linear elastic fracture mechanics and the linear beam theory including the effect of shear deformation. The partitioning of total energy release rate into the mode-I and mode-II component is performed and the condition for stability in crack growth is investigated. Mixed mode interlaminar fracture toughness is also evaluated on the modified ENF specimens with various thickness ratios experimentally. Finally, fracture surfaces are examined to obtain informations on interlaminar fracture mechanisms by means of a scanning electron microscope.
For [0/0] fiber orientation on the crack surface, energy required for crack growth in the mode-II loading is 5 times that required for crack growth in the mode-I loading. This is a result of increased fracture energy necessary for crack growth due to dominantly severe matrix deformations. For the effect of different fiber orientations on interlaminar fracture toughness, interlaminar fracture toughness for [0/0] case is greater than those for [0/30], [0/45] and [0/60] cases due to fiber bridging. Mixed mode test results show good agreement with the mixed mode fracture criterion for material characteristic parameters m=2 and n=3, which predicts mixed mode interlaminar fracture behavior of graphite/epoxy composite. The modified ENF specimen is controlled different mixed mode ratios by changing thickness ratios and is applicable to investigate mixed mode interlaminar fracture behavior from the mode-I through the mode-II deformation. Morphology of fracture surfaces is sensitive to applied loading condition and provides an insight to estimate the state of stress at fracture.
