Influence of stress ratio and stress concentration on the fatigue behaviour of hygrothermal aged multidirectional CFRP composite laminate

•Study of fatigue behavior of hygrothermal aged multidirectional CFRP laminates.•Focus on synergetic effect involving stress ratio, ageing and stress concentration.•Dry fatigue behavior is worst at low stress ratio due to stress concentration.•After ageing, the laminate fatigue behavior at high stress ratio also deteriorated. The fatigue behaviour of carbon fibre reinforced polymer (CFRP) under the dry and hygrothermal aged conditions was investigated at two stress ratios, 0.1 and 0.5. Two types of layup, named as the unconstrained [06], and constrained [±45/03]S laminates were investigated to understand the ‘constraining effect’ on the zero-degree plies, imposed by the ±45 plies of the constrained laminate. The experiments revealed that the constraining effect significantly deteriorated the fatigue behaviour of the constrained laminate at low stress ratio due to stress concentration. In contrast, the laminate had better fatigue behaviour at the high stress ratio, which deteriorated after hygrothermal ageing.