A new mode I fracture test for composites with translaminar reinforcements

A new test method is presented for Mode I delamination fracture toughness testing of laminated composites containing a high density of stitches or translaminar reinforcements. The test set up, which is similar to the standard Double Cantilever Beam test, induces an axial tension in the specimen in addition to the transverse forces responsible for propagation of delamination. The tensile stresses reduce the compressive stresses in the vicinity of the crack tip caused by the large bending moments required for crack propagation. The nonlinear differential equations of equilibrium of the new specimen are solved using an iterative procedure to obtain the strain energy release rate as a function of load and crack length. Experiments were conducted using carbon/epoxy specimens containing 6.2 stitches per square centimeter (40 stitches per square inch). Results include Mode I fracture toughness, crack tip bending moment, transverse deflection and slope as a function of crack length. It is found that the apparent fracture toughness of the specimens tested remains constant as the stitches break and crack propagates, and is about sixty times that of unstitched specimens.