Low-Velocity Impact And Post-Impact Bending Performance Of Stitched Braided Composites

Stitched braided composites are a new structural type of composites, which are manufactured by stitching layers of braided sheets together. Because they combine the stitching technology and braided structure, stitched braided composites show lots of outstanding properties. Up to now, however, studies on the low-velocity impact and post-impact bending properties of stitched braided composites still remain few, which are focused primarily on the tensile and bending performance of braided composites with stitching joint. And it is of critical importance to understand the impacting damage characteristics of stitched braided composites for their further application.In this project, four classes of stitched braided composites with different structural parameters were chosen as research objects. To study their low-velocity impact and post-impact bending damage properties, low-velocity impact tests and bending tests were conducted, and then damage distribution and deformation characteristics were observed. All of these researches were expected to lay a firmer foundation of application of stitched braided composites.By observing and analyzing the results, following conclusions have been drawn.1. Under the low-velocity impact of3.2J/mm, the bending properties of stitched braided composites degrade and the primary impact-damage mode is resin-matrix cracks. And, bending modulus shows a higher sensitivity to the impact damage than bending strength. Within stitched braided composites, stitching threads hinder the evolution of damage, resulting in a smaller damage area, a less serious property degradation and a higher impact damage tolerance.2. As for stitched braided composites with the same structural parameters, they show the similar load-time curves and energy-time curves. Under the same impact energy, four-directional stitched braided composites show better energy-absorption abilities than five-directional ones, which suggests the worse impact-resistance in four-directional stitched braided composites. 3. The bending load-displacement curves show bilinear relation and failures present brittleness damage. Under bending tests, the resin-matrix cracks remain within each braided sheets rather than in the resin between adjacent layers. And, even inside the neutral planes of specimens where in-plane shear stress reach its maximum, there is few obvious damage occurring.4. The mode and propagation of impact damages change with the structural parameters. For the existence of axial yarns, the resin-matrix cracks within five-directional stitched braided composites intend to propagate more along the longitudinal direction, and their bending modulus, bending strength and impact damage tolerance are better than the four-directional specimens. For the higher volume content of inner cells, the stitched braided composites with thick braided sheets show better bending toughness than with thin braided sheets.