Influence of fiber direction and mixed-mode ratio on delamination fracture toughness of carbon/epoxy laminates

Surface free energy has been often treated as a scalar constant without considering its dependence on propagation direction. It is desirable, however, to investigate how surface free energy or fracture toughness of delamination in a single interface varies with both the local mismatch angle of fiber directions and the direction of crack propagation in polymeric laminate composites. As a materials constant, fracture toughness is effectively used for various mechanical analyses of fiber-reinforced composites as well as conventional materials. This study investigates quantitatively and qualitatively the dependence of delamination fracture toughness on mismatch angle and crack propagation direction in laminated structures. AS4-Carbon/Epoxy prepregs were used for fabricating test specimens, and 50 different mismatch angles of fiber direction were applied on the delaminated interface of laminates. Fracture toughness was measured using the Mixed-mode Bending (MMB) test. This test method is composed of fracture mode-I and mode-II, and the mixed-mode ratio (GII/G) can be controlled. The mixed-mode ratios used here were 20, 35, 50, 65, and 80%. The crack path and the delamination fracture toughness were observed and calculated for specimens, and the dependence of the toughness was shown to be related to the mismatch angle of ply fibers at the delaminated interface. The relationship between mismatch angle and delamination fracture toughness was newly revealed and discussed for various angles. These results can be usefully applied to various fracture mechanics analyses in fiber-reinforced laminated composites.