INTERLAMINAR SHEAR FRACTURE OF INTERLEAVED GRAPHITE EPOXY COMPOSITES

Interlaminar shear fracture (Mode II) of interleaved composites has been experimentally examined by means of interleaved end-notched flexure specimens and optical and scanning electron microscopy. Thermoplastic and thermoset interleaves sandwiched between graphite/epoxy composite plies were investigated over a wide range of film thicknesses. The fracture process was initiated by the nucleation of microcracks ahead of the crack tip and continued with coalescence of microcracks that finally resulted in unstable fracture. Both types of interleaves enhanced the interlaminar fracture toughness to a great extent, although thermoplastic interleaves were more effective than the thermoset interleaves because of the larger energy-absorbing capability of the thermoplastic polymer. The fracture toughness increased steeply with film thickness at small interleaf thicknesses and leveled off to a relatively constant value at larger film thicknesses. Cohesive failures, microcracking, and plastic deformation in the thermoset interleaf were observed in an optical microscope. The thermoplastic interleaves displayed adhesive failures, a limited degree of microcracking, and material yield.