An interface-dependent model of plateau fracture toughness in multidirectional CFRP laminates under mode I loading

Obvious R-curve behavior exists in multidirectional carbon fiber reinforced polymers (CFRP) laminates under mode I loading, in which the plateau fracture toughness represents the steady-state delamination growth behavior. By decomposing the fracture toughness into the fracture work for pure delamination at the interface and that for the intra-ply damage of adjacent layers, a mechanism-based correlation between the plateau value of interface mode I fracture toughness in multidirectional laminates and the interface angle and/or the two angles of the fiber orientations of the two layers surrounding the crack tip was developed, which is an explicitly interface-dependent formula. Static tests were performed on six types of DCB specimen with different interfaces. The analytical calculations agreed well with the experimental data, verifying the proposed correlation. The intra-ply damage and fiber bridging during delamination along different interfaces were evaluated, which provides a justification for the proposed model.