Interlaminar crack initiation and growth under modes I and II in a carbon-fibre epoxy composite subjected to fatigue

In this paper, interlaminar crack initiation and propagation under modes I and II dynamic loading of a composite material, with epoxy matrix and unidirectional carbon fibres were evaluated. Delamination commenced in mode I employing the DCB (Double Cantilever Beam) test. In mode II, the ENF (End Notched Flexure) test was used. The fracture toughness in mode I was obtained using the methods of the ASTM D5528 Standard, while in mode II, the methods were applied in accordance with the ESIS (European Structural Integrity Society) Protocol. Employing this experimental program, the fatigue curves (Delta G, N) and growth rate curves (Delta G, da/dN) in both fracture modes were determined for an asymmetry ratio of R=0,2. The influence of the manufacturing process of the material on its behaviour with respect to crack growth onset may be deduced from the experimental results. Moreover, as the crack growth rate decreases for large crack lengths, crack growth may even cease if the critical fracture energy does not increase above the values obtained in the static characterization of the material.