CRACK TIP YIELD ZONE ESTIMATES IN MODE II INTERLAMINAR FRACTURE OF INTERLEAVED COMPOSITES

An elastic stress analysis of the interleaved end notched flexure specimen was conducted by two-dimensional finite element modeling. The purpose of the study was to identify mechanisms that influence the enhancement of mode II interlaminar fracture toughness of interleaved composites. The study focused on a graphite/epoxy composite interleaved with thermoplastic and thermoset polymeric film materials. Crack tip yield zones were estimated from the elastic stress distribution beyond the crack tip and von Mises yield criterion. The dependence of yield zone height on interleaf film thickness agrees qualitatively with the trends observed experimentally for mode II interlaminar fracture toughness, G(IIC), over a range of interleaf thickness. The plastic zone height and G(IIC) increase rapidly with adhesive thickness and attain a region of less variations at larger film thicknesses. The high yield strength thermoplastic interleaf material attained a constant plastic zone height and a relatively constant fracture toughness at much smaller film thicknesses than the lower yield strength thermoset interleaf material. Irwin's plastic zone model was examined and was found to be a promising candidate for initial design of interleaved composites.