Thickness effect on Mode I trans-laminar fracture toughness of quasi-isotropic carbon/epoxy laminates

The thickness dependency of trans-laminar fracture toughness was studied in centre-notched quasi-isotropic IM7/8552 carbon/epoxy laminates with central double 0° plies with thicknesses between 1 and 8 mm. A reduction in trans-laminar fracture toughness with thickness was measured experimentally in the specimens with a 25.4 mm notch. For specimens with a shorter 12.7 mm notch, no significant dependency on specimen thickness was found. The thickness dependency was captured in detailed Finite Element (FE) models with cohesive interface elements for sub-critical damage and a Weibull criterion for fibre breakage. The reason for the thickness dependency is explained through the damage states of the individual plies which determine whether or not premature fracture occurs before the damage process zone is fully developed.