Three-point Bending Behavior and Energy Absorption Capacity of Composite Tube Reinforced by Gradient Braided Structure in Radial Direction

This study aims to investigate the radial gradient structural effect on energy absorption and failure behavior of braided composite tubes. Two-layer tubular braided fabrics were fabricated using over-braiding technology with three types of radial gradient configurations in terms of uninform, descending and ascending pattern. The structure of surface layer touching with indenter roll was found to have a significant role on the bending behavior and failure mode. The composite tube possessing the small angle surface layer tends to fail in top-surface mode characterized by compression damage in contact area between indenter roll and tube. With the increasing of the braiding angle in surface layer, the bottom-surface failure mode characterized by penetrating crack in circumferential direction was observed, which accompanies abruptly loading drop and loss of energy absorption capacity. In addition, the tube in descending gradient pattern contributes to higher flexural modulus and peak load due to high fiber volume fraction, while keeps top-surface failure mode because of load spreading ability provided by surface layer with small braiding angle. The results show that a proper selection of stacking sequence and braiding angle in multi-layer braided tube is capable of effectively enhancing the energy absorption of tubal structures under bending load.