Off-axis fatigue behaviour of unidirectional laminates based on a microscale fatigue damage model under different stress ratios

- A new non-dimensional effective local stress is generalized for fibre and matrix.
- Fatigue behaviour of UD plies is investigated at the level of fibre and matrix.
- Effect of stress ratio on fatigue damage of UD composite plies is investigated.
- Fatigue life of UD plies can accurately be predicted for different stress ratios.

A fatigue damage micro-model is developed for two-phase composite materials. Constitutive law is derived at the level of fibre and matrix and extended for a damaged lamina using the reformulation of Mori-Tanaka model. A new non-dimensional effective local stress based on a failure criterion which is rewritten in term of average stress of fibre and matrix is coupled with a fatigue damage model for prediction of fatigue damage and fatigue life. The proposed model is able to distinguish between damage in fibre and matrix and also stiffness reduction of the matrix and transverse direction during fatigue loading using three different damage variables. The fatigue behaviour of unidirectional carbon/epoxy and glass/epoxy laminates are studied under off-axis loading conditions. Fatigue life of unidirectional composites is predicted at different stress levels, stress ratios and fibre orientations under tension-tension and tension-compression fatigue. The obtained numerical results from the proposed model are in good agreement with the experimental data.