Influence of the load type on cross-ply composites under Very High Cycle Fatigue

The Very High Cycle Fatigue (VHCF) regime of composites is not yet fully understood due to difficulties in testing, like long testing times, specimen heating and self-fatigue of the test equipment. As most fatigue research is conducted using tensile load, knowledge about the fatigue life under other load types is scarce. In this study, the influence of the load type is investigated for four-point bending compared to tensile fatigue. Therefore, experimental results under four-point bending and tensile fatigue are compared for two different cross-ply lay-ups. Furthermore, the influence is determined by parametric crack and delamination simulations. The evaluations of the experimental data show earlier transverse crack initiation, a lower fatigue limit load and higher damage contents in terms of crack density and delamination area ratio. Numerical simulations show a slightly slower crack growth but significantly higher delamination growth under tensile loading. For very low load levels this results in a difference in the fatigue damage ratio with fewer cracks and critical quantities of delamination under tensile fatigue. These very large delamination areas, can lead to the total loss of the contribution of the outer 90°-plys to the material parameters of the layup. • Lower fatigue limit and earlier crack initiation under tensile VHCF. • Higher crack densities and higher delamination under tensile VHCF. • Low loads can lead to lower crack density and larger delamination under tensile VHCF. • Critical quantities of delamination under tensile VHCF.