Damage Development and Shear Modulus Degradation in Graphite/Epoxy Laminates

A method developed recently for nondestructive monitoring of in-plane shear modulus of composite specimens was used to detect longitudinal fatigue cracks in cross-ply composite laminates. Cross-ply graphite/epoxy specimens were subjected to tension-tension fatigue (R = 0.1) at five different cyclic stress levels: 85. 66, 53, 35, and 28% of the static strength of the specimen. In-plane shear modulus was measured periodically, and its reduction due to transverse and longitudinal cracks was compared with the reduction of the longitudinal modulus. It was found that the sensitivity of the longitudinal modulus to longitudinal cracks is very low, whereas the sensitivity of the in-plane shear modulus is approximately four times higher. The onset of longitudinal cracking can be detected clearly. The results were compared with X-radiographs of the tested specimens. Good correlation was found between the initiation and accumulation of longitudinal cracks sensed by the in-plane shear modulus and the actual cracks observed in the corresponding X-radiographs. The relation between the residual life of the cross-ply specimen and the shear modulus reduction is also discussed.