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.