Thickness dependence of mode I interlaminar fracture toughness in a carbon fiber thermosetting composite

The relation of the non-singular opening stress component with apparent fracture toughness was investigated for unidirectional carbon-fiber laminates by means of double cantilever beam experiments in which laminate thickness was varied. It was found that a sample configuration with smaller thickness related to a higher apparent fracture toughness measurement. This result was explained by the presence of a negative non-singular opening stress component, which was found to decrease with thickness. The experimental procedure utilized first crack propagation from the initial polytetrafluoroethylene insert to avoid ambiguity of defining crack location due to crack curving, resulting in a wider crack front and fiber bridging. The two dimensional finite element analyses were used to calculate the corresponding singular and non-singular stress components and the energy release rates. Based on the experimental results a two parameter linear elastic fracture mechanics model was calibrated to incorporate the thickness dependence of the apparent fracture toughness.