PLASTIC ZONE ESTIMATES IN MODE-I INTERLAMINAR FRACTURE OF INTERLEAVED COMPOSITES

The toughening of interleaved composites that arises from interleaving has been investigated by finite element elastic stress analysis. Interleaving refers to a thin layer of ductile polymeric film placed between the composite plies. The study focused on the hypothesis that interleaving allows formation of a larger crack tip plastic zone and hence increases the plastic energy dissipation during interlaminar crack extension. Calculations of crack tip plastic zones in composites with thermoplastic and thermoset interleaves allowed comparison with previously reported experimental measurements of mode I fracture toughness, G(IC). The trends of G(IC) versus interleaf thickness were consistent with predicted plastic zone heights and yield strengths for the thermoplastic and thermoset interleaves although the maximum fracture toughnesses were observed at thinner interleaves than predicted. Optimum interleaf thicknesses were computed for the thermoplastic and thermoset interleaves based on the condition that the plastic zone height equals the interleaf thickness. The computed values were in close agreement with the experimentally observed thicknesses at maximum fracture toughness, although inadequate interleaf/composite adhesion rendered full quantitative comparison difficult.