Is there a ply thickness effect on the mode I intralaminar fracture toughness of composite laminates?

The apparent crack resistance curves associated with longitudinal tensile failure of carbon fibre reinforced epoxy composite systems with ply thicknesses of 0.075 mm, 0.134 mm and 0.268 mm are determined experimentally from the size effect law of geometrically similar double edge notch tension cross-ply specimens. An increase in notched strength as a function of the ply thickness and a corresponding increase of the measured intralaminar fracture toughness is observed. This increase is shown to be a consequence of the appearance of split cracks in the thicker 0 degrees plies in the vicinity of the notches. The numerical models developed demonstrate that if the notch blunting mechanisms are properly represented, the laminate strength is well predicted for a constant value of the ply intralaminar fracture toughness. This supports the hypothesis that these mechanisms are responsible for the higher strength of the thicker plies, and that the intralaminar fracture toughness of the 0 degrees plies should not be scaled with the ply thickness.