Mixed Mode delamination of multidirectional carbon fibre/epoxy laminates

Scanning electron microscopy was used to identify fractographic features that are characteristic of different modes of interlaminar fracture. The cusp angle and the amount of fiber pull-out on the fracture surface can be used to characterize the different loading modes. A large amount of fiber pull-out is the dominant feature of a mode I fracture, while in mode II large cusp angles and many cusps are the main characteristics. The amount of fiber pull-out, and subsequently brokenfibers, per unit area was investigated and found to vary in proportion to the degree of mode I loading. Such methods can be used to analyze failure and the propagation ofdelamination in structural components. The energy associated with cusp formation constitutes a large proportion of the mode IIfracture toughness component, while the amount of fiber pull-out and fracture has a considerable influence on the mode I fracture component. The cusp angle was seen to provide a quantitative measure of the fracture surface roughness. A failure criterion that takes the fracture surface appearance into account was evaluated. The cusp angle was subsequently used to modify this failure criterion. As a consequence, this provided improved agreement with the experimental data.