A microscale energy-based fatigue damage model for unidirectional composites under multiaxial loading at different stress ratios

•A new non-dimensional effective local energy is proposed and generalized for fiber and matrix based on continuum damage mechanics.•Fatigue behavior of unidirectional composites is studied at the level of fiber and matrix.•Effect of stress ratio on fatigue damage and fatigue life of unidirectional composites is considered.•Fatigue life of unidirectional composite materials can accurately be predicted for different stress levels and stress ratios. The off-axis fatigue behavior of unidirectional composites has been studied using an energy based model. The fatigue model is based on average stresses in fiber and matrix derived from a micromechanical approach. Different fatigue damage modes are identified on the basis of physical interpretation of the driving forces defined from Continuum Damage Mechanics. The model is able to distinguish between damage in fiber and matrix and predict the residual stiffness in transverse and longitudinal direction. The proposed model is verified and validated by studying the fatigue behavior of unidirectional carbon/epoxy and glass/epoxy laminates under non-negative mean stresses. The predictions are in good agreement with the experimental data regardless of stress ratio, stress level and fiber orientation.