Transverse ply cracking in cross-ply CFRP laminates - Initiation or propagation controlled?

The initiation and propagation stages of transverse ply cracking in crossply CFRP laminates under quasi-static loading have been studied using laminates with a range of transverse ply thicknesses. Specimens of two types have been tested-coupons with polished edges and coupons with defects (notches) introduced into the transverse ply prior to testing. Detailed observations of crack propagation across the transverse ply thickness and width have been made, along with measurements of crack density and residual properties. In laminates with thin transverse plies (less than 0.25 mm for the materials tested here), fully formed transverse cracks are observed at about the same strain in undamaged and notched laminates. In laminates with thicker transverse plies (greater than 0.25 mm), the notched laminates show fully formed transverse cracks at lower strains than the undamaged laminates. The experimental results have illustrated the situations (laminate properties, presence of flaws) in which fracture mechanics models for transverse cracking available in the literature can be applied. In particular, it is shown that the fracture mechanics models which assume implicitly the presence of initial defects spanning the thickness of the ply may underestimate the failure strain of laminates with thick transverse plies. The effect of composite intralaminar toughness is also discussed.