Fatigue modeling for carbon/epoxy unidirectional composites under various stress ratios considering size effects

•The size effects on the fatigue properties of CFRP UD composites with various fiber orientations are studied experimentally.•The underlying mechanisms of size effects of the fatigue of CFRP composites are investigated through SEM observations.•A fatigue life prediction method for CFRP UD composites considering size effects is presented.•Constant fatigue life diagrams for 0°, 10°, and 90° UD composites are established. Fatigue tests for unidirectional (UD) CFRP composite specimens of various sizes are conducted under different stress ratios. The results show that the specimen size has considerable influence on the transverse tension-tension fatigue performance of UD laminates. The underlying fatigue mechanisms are also investigated through SEM observations. Moreover, a fatigue life prediction method considering size effects is presented, and a good correlation is observed between the prediction and the test data. Finally, to allow for an evaluation of the effects of mean stress, constant fatigue life diagrams for 0°, 10°, and 90° UD composites are established based on four-segment anisomorphic model.