Carbon composites based on multi-axial multi-ply stitched preforms – Part 6. Fatigue behaviour at low loads: Stiffness degradation and damage development

This article studies the fatigue properties of a carbon-fibre cross-ply non-crimp fabric reinforced epoxy composite. Tensile–tensile fatigue cycling was carried out at load levels corresponding to the onset of damage in a static tensile test, in machine, cross and bias direction. Specimens in machine and cross direction did not fail up to 106 cycles; specimens in bias direction had an average fatigue life Nmax of 3 × 105 cycles. Stiffness degradation in bias direction samples was found to be more severe than in machine or cross direction. Damage development in the samples was studied by means of X-ray photography and appears to show remarkable resemblance to the development under a static tensile test and can be qualitatively compared to the behaviour of non-stitched UD laminates. Post-fatigue tensile tests were done at various stages of the fatigue life. Small differences in damage onset strain level can be found. Failure strain of bias direction tested samples shows significant decrease upon cycling.