Damage initiation and evolution in glass/epoxy tubes subjected to combined tension-torsion fatigue loading

The matrix-dominated behaviour of unidirectional composite laminae subjected to multiaxial fatigue loading was extensively investigated. Tubular specimens with three different lay-up ([90n], [0F/90U,3] and [0F/90U,3/0F]) were tested under combined tension/torsion loadings. Attention was focused on the initiation and propagation of cracks in the 90 layers, and fatigue data are presented in terms of S–N curves (for crack initiation) and Paris-like curves (for crack propagation) under several biaxiality ratios. Damage onset and evolution have been found to be strongly affected by the multiaxial loading condition, showing a particularly detrimental effect of the shear stress component. SEM investigations on the fracture surfaces confirmed a strong dependence also of the microscopic damage modes on the local multiaxial stress state.