Mode I Interlaminar Fracture of Interleaved Graphite/Epoxy

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Abstract
Mode I interlaminar fracture of interleaved graphite/epoxy has been stud ied experimentally using double cantilever beam (DCB) specimens and optical micros copy. The interleaves were either thermoset or thermoplastic films placed at the midplane along the anticipated fracture plane. To examine the possible influence of constraints on the crack tip plastic zone due to the rigid composite plies, a large range of interleaf thick nesses was investigated. At each crack increment, the crack extension initiated close to the maximum load and progressed in a stable manner during the displacement controlled test ing. Fiber bridging was not observed on the fracture specimens, or indicated by the shape of the crack growth resistance curves. The thermoplastic interleaves enhanced the fracture toughness, G IC , to a much larger extent than the thermoset interleaves, due to a larger en ergy absorption capability in the fracture process. For the thermoplastic interleaves, the maximum fracture toughness was achieved at very small film thicknesses. At larger film thicknesses GIC remained large. For the thermoset film material, the enhancement of GIC was only nominal and the thicker thermoset interleaves failed in a film delamination mode, resulting in low GIC values.