Flexural fatigue performance of 3D woven composites

Under flexural deformation, one of the dominant fatigue fracture mechanisms of laminated composites is delamination. For 3D textile composites, the introduction of thickness reinforcement will increase interlaminar strength, enabling them to better resist initial crack formation, and provide bridging for interlaminar cracks once they have formed. In this study three-point bending tests were conducted to investigate the fatigue behavior of a 3D woven composite. By recording the loss of flexural stiffness of specimens as a function of testing cycles, the fatigue behavior of the material was observed and the progressive damage evolution illustrated. For comparison, specimens of a UD-laminate were also fabricated and tested under the same conditions as their 3D counterparts. The results show a rapid loss of flexural stiffness in the early stage of fatigue testing for both the 3D woven composite and the UD-laminate. Then the stiffness reduction moderated during the rest of the testing cycles for the 3D woven composite, and no delamination was observed when the tests approached the proposed failure criterion. For the UD-laminate, however, a sudden loss of flexural stiffness was observed at the final stage of fatigue testing, leading to delamination, and the tests ended with catastrophic failure. The study demonstrates that the fatigue behavior of composites relies very much upon the architecture of fibrous reinforcement and that thickness reinforcement helps avoid unpredictable fatigue failure of composites.