Mixed-Mode Strain-Energy-Release Rate Effects on Edge Delamination of Composites

Unnotched [±θ/0/90]_s graphite/epoxy laminates, designed to delaminate at the edges under static and cyclic tensile loads, were tested and analyzed. The specimen stacking sequences were chosen so that the total strain-energy-release rate, G, for edge delamination was identical for all three layups. However, each layup had different percentages of crack-opening and shear-mode strain-energy-release rates, G_I and G_II, respectively. Results with composites made from T300 graphite fibers and 5208 epoxy, a brittle resin, indicated that only G_I contributed to delamination onset under static loading. However, results with composites made from C6000 fibers and H205 epoxy, a tougher resin, indicated that the total G governed the onset of edge delaminations under cyclic loads. In addition, for both materials, the threshold level of G for delamination onset in fatigue was significantly less than the critical G_c measured in static tests. Furthermore, although the C6000/H205 material had a much higher static G_c than T300/5208, its fatigue resistance was only slightly better. A series of mixed-mode tests, like the ones in this study, may be needed to evaluate toughened-resin composites developed for highly strained composite structures subjected to cyclic loads.