Intralaminar fracture toughness of a cross-ply laminate and its constituent sub-laminates

The intralaminar mode I fracture toughness values for a M55J/M18 carbon/epoxy cross-ply laminate [0°/90°]15 (alternate 0 and 90° layers) and its constituent sub-laminates, namely the [0°]30 and [90°]30 laminates are theoretically evaluated on the basis of a modified crack-closure integral (MCCI) method corresponding to the fracture loads obtained by testing C(T) specimens. A simple relationship between the intralaminar fracture toughness at the specimen level of a cross ply and its constituent sub-laminates is presented. Test data for fracture loads and crack-opening displacements at the opposite end (far end) to the crack tip of the three types of C(T) specimens are obtained. A comparison of fracture toughness values KIc and crack-opening displacements obtained from tests with theoretical predictions by finite-element analysis shows good agreement for the type of laminates considered. Comparison of the analytically determined KIc of the cross-ply C(T) specimen based on the new relationship using the respective values of KIc of its constituent C(T) specimens shows good agreement with test data.