| The Mode-I and Mode-II fracture toughnesses for both unidirectional and angle-ply laminates for three different composite material systems were investigated both experimentally and numerically. The propensity of certain laminate orientations to delaminate under mixed-mode conditions was predicted, employing a singular finite element analysis. The angle-ply specimens had cracks both at the mid-plane of the laminate, between two laminae of similar orientation, and at an interface, between two laminae of opposing orientation. The Mode-I and Mode-II fracture toughnesses for the materials tested were found to be independent of the fiber orientation in the laminate. This suggests that the fracture toughness of a unidirectional composite material can be used as a design value even in structural components that employ multidirectional laminate orientations. The strain energy release rate was found to have a distinct distribution across the width of the specimens under both Mode-I and Mode-II loading conditions. This was attributed to the ratio of the in-plane properties of the composite materials and to the existence of a plane stress/plane strain behavior in the laminates. It was found that a compliance calibration technique gave values of fracture toughness that were independent of specimen geometry, suggesting that such a method be used. Similar values were also obtained using a finite element analysis, suggesting that the compliance calibration technique gave accurate values for the fracture toughness. |
