Fractography and failure mechanisms in static mode I and mode II delamination testing of unidirectional carbon reinforced composites

The mode I and mode II interlaminar fracture surfaces of a unidirectional AS4/8552 carbon/epoxy laminate were studied by means of SEM microscopy and numerical analysis in order to correlate the surface appearance with the stress state. SEM micrographs of fractured surfaces showed that the most predominant fractographic features in modes I and II were 'river markings' and 'hackle markings' respectively. Finite Element Analysis (FEA) showed that the principal tensile stress direction near the crack tip was perpendicular to the 'hackle' or 'stacked lamellae' microstructure and at 45° to the fracture plane. As was observed by other authors, these results suggest that matrix microcracking perpendicular to the principal stress direction is the driving mechanism forming the 'hackle marking' microstructure.