MIXED-MODE DELAMINATION OF MULTIDIRECTIONAL CARBON FIBER EPOXY LAMINATES/

Scanning electron microscopy,uas used to identify fractographic featur es that are characteristic of different modes of interlaminar fracture . The cusp angle and the amount of fiber pull-out on the fracture surf ace can he used to characterize the different loading modes. A large a mount 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 charact eristics. The amount of fiber pull-out, and subsequently broken fibers , per unit area was investigated and found to vary in proportion to th e degree of mode I loading. Such methods can he used to analyze failur e and the propagation of delamination in structural components. The en ergy associated with cusp formation constitutes a large proportion of the mode II fracture toughness component, while the amount of fiber pu ll-out and fracture has a considerable influence on. the mode I fractu re component. The cusp angle was seen to provide a quantitative measur e of the fracture surface roughness. A failure criterion that takes th e fracture surface appearance into account was evaluated. The cusp ang le was subsequently used to modify this failure criterion. As a conseq uence, this provided improved agreement with the experimental data.