Mode II interlaminar fracture toughness of interleaved composite materials

Interlaminar mode II fracture toughness, GIIC, of thermoset and thermoplastic interleaved (TSI and TPI) composites were investigated over a wide range of interleaf thickness. TPI specimens had four to about seven times larger GIIC than those without an interleaf. Poor adhesion observed for some TPI specimens were likely to be due to contaminated film materials. Thermoset interleaves were less effective in enhancing the mode II fracture toughness. However, even 0.043 mm thermoset interleaves gave three times larger G$\sb{\rm IIC}$ than those without an interleaf. Estimates of the volume of the yielded material around the crack tip based on a quasi-elastic finite element approach and Irwin's model showed that the yield zone height reaches a peak value for increasing interleaf thickness for both TSI and TPI specimens. Furthermore, fracture toughness data correlates well with yield zone heights.