On-axis fatigue behaviors and failure characterization of 3D5D braided composites with yarn-reduction using X-ray computed tomography

Abstract This paper mainly provides a thorough experimental investigation of tension-tension fatigue (R=0.1) failure mechanisms in 3D5D braided carbon/epoxy composites without/with yarn-reduction (No-YR/In-YR) subjected to different stress levels of ultimate tensile stress (40%-75%UTS). X-ray computed tomography (Micro-CT) was conducted to visualize and quantify the damage generated in the fatigued samples. More significantly, a novel two-steps segmentation approach was proposed to classify and locate the cracks. The results indicated that the two types of specimens showed similar fatigue behavior, i.e. the extensive debonding of the braided yarns might give rise to the premature significant stiffness degradation of the fatigued samples. However, the tensile strength and the fatigue limit of In-YR 3D5D braided composites were reduced by 28.3% and 22.8% relative to No-YR 3D5D braided composites, respectively, which might be attributed to the variation of failure modes. Particularly, the failure modes of the axial yarns surround the yarn-reduction point evolved from mode I to mode II due to the deformation generated during the fabrication. It is noteworthy that the cracks presented a typical wedge distribution near the yarn reduction point due to the in-plane stress migration.