Bending fatigue stiffness and strength degradation in carbon–glass/epoxy hybrid laminates: Cross-ply vs. angle-ply specimens

A hybrid glass–carbon non-woven fabric reinforced epoxy matrix composite, constituted by layers of biaxial carbon fabrics, biaxial glass fabrics and hybrid carbon–glass fabrics, was considered for its bending fatigue behaviour. Tensile and flexural static tests as well as displacement-controlled bending fatigue tests (ratio of 0.10) were conducted on two sets of standard specimens, having fibre orientation parallel to the loading direction (cross-ply specimens) and at 45° to the loading direction (angle-ply specimens). Specimens were subjected to different fatigue loading, with the initial maximum load level up to 85% of the laminate ultimate flexural strength, and damage in the laminate was continuously monitored through the loss of bending moment during cycling. After 106 cycles the fatigue test was stopped and residual properties were measured on tested specimens. Stiffness-based stress-number of cycle (SN) curves were drawn for the two sets of specimens. The amount of stiffness loss for cross-ply and angle-ply specimens was observed to depend on the fatigue load level. In particular, cross-ply specimens were observed to damage more significantly than angle-ply specimens only at high fatigue loading. This was attributed to different damage mechanisms for the two sets of specimens. Reduction in material strength and elastic modulus as measured after 106 cycles was also found to depend on the level of fatigue loading and to follow different trends for the two sets of specimens.