Fatigue and post-fatigue tensile behaviour of non-crimp stitched and unstitched carbon/epoxy composites

An experimental study is described in this paper dealing with the tensile–tensile fatigue and the quasistatic post-fatigue tensile behaviour of a structurally stitched multi-ply carbon composite and the unstitched counterpart. The influence of the stitching on the fatigue life and on the residual post-fatigue quasi-static properties in two principal direction is investigated. The fatigue behaviour of both composites is represented by Wöhler-like diagrams. The damage imparted during fatigue is studied by X-ray analyses. The residual mechanical properties of the fatigued composites after different number of cycles are compared in term of stiffness and strength. The post-fatigue quasi-static tensile tests include acoustic emission (AE) registration and full-field surface strain mapping (SM) to investigate the damage onset and development. The main conclusions of the experimental work are: the fatigue life is improved in the direction of the structural stitching and is reduced in the orthogonal direction; for the considered cyclic stress level the post-fatigue reduction of the mechanical properties is limited by the structural stitching.