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The failure mode is a dominant factor of mechanical performance of braided composite tube subjected to axial compression. The failure mode was modified by tailoring stack-up sequence of composite tubes hybridized by bi/tri-axial braided laminate. The thermal photographing and micro-CT scanning methods were employed to monitor the in-situ damage progression and post-mortem specimen, respectively. The results show that the delamination was easy to be induced at the interface nearby tri-axial braided laminate. It attributes to the combination between the crack caused by local buckling of the axial yarns and the crack caused by shear deformation of braiding yarns. By the illumination of this behavior, delamination is controlled to occur at the different position along thickness direction of braided tube by adjusting the stacking sequence of braided laminates. When delamination position transfer from the inner/outer sides to the middle of tube wall, the failure mode transfer from the progressive folding to splaying, which increase the specific energy absorption by 45.77%. [ABSTRACT FROM AUTHOR]

In this study, the performance of braided composite tubes under low-velocity transverse impact loading at mid-span was investigated using both numerical and experimental methods. Three types of braided composite tubes with different braiding angles (30°, 45°, and 60°) were manufactured. The transverse punch behavior of the tubes was examined on a low-velocity imspact test bench. A meso-level finite element model of the composite tube was also established for identifying the damage initiation and development. The numerical results showed a good correlation with the experimental data. The mechanical response including force–time histories, force–displacement histories, and fracture morphologies was compared between three types of composite tubes for analyzing the influence of braiding angle on the impact response and failure mode. Although suffering from the low bending stiffness depends on fiber volume fraction at initial impact stage, the braided tube with 30° angle engaged more portion to resist impact loading in subsequent process and thus presented higher peak loading than the one with large angle. In addition, there are distinct different failure modes between composite tubes with various braiding angles. Shear yarn breakage underneath the punch was prone to occur in 30° sample because the braiding yarn was closer to the axial direction of tube. In contrast, the resin was deboned severely from the braiding yarn and then the braiding yarn exhibits plastic deformation in 60° sample due to the stress concentration caused by the large braiding angle. [ABSTRACT FROM AUTHOR]

• Difference of damage modes caused by the boundary conditions of primary load-bearing yarns. • Straightening effect of braided yarns with undulations under low-velocity impact. • Variation of off-axial angles leading to the transition of failure modes. The sensitivity of off-axial angle on impact response was investigated by experimental and numerical methods in this work. Three types of braided laminates with different on/off-axial angles in terms of OA00, OA225 and OA45 were prepared and low-velocity impact tests with various energy levels were performed. Meso-level finite element model (FEM) was also developed to reveal impact damage initiation and development. We discovered that the boundary condition of primary load-bearing yarns depending on the off-axial angles caused the difference of structural deformation and stress distribution of braiding yarns during impact process. Due to the restriction of the pressurized tips, the straightening effect of primary load-bearing yarns with heavy waviness caused its out-of-plane movement and related main cracks in OA00 specimen. In addition, the matrix resin cracks were found at the edge of the specimen because of in-plane shear movement of braiding yarns. As for OA45 specimen, more evident bending deformation was observed, resulting in elevated tensile stress of braiding yarns on non-impact side. The variation of off-axial angle from 0 to 45 degree led to the failure mode transforming from the extensive matrix damage on specimen edge to the severe fiber breakage on non-impact side. As a medium off-axial angle, OA225 specimen exhibited a mixture failure mode to resist impact and achieved excellent performance with respect to higher peak force and delayed load dropping. [Display omitted] [ABSTRACT FROM AUTHOR]

Braided composite laminates allow a reinforcement architecture design method of varying ply-to-ply braided structure instead of rotating fiber orientation in traditional unidirectional laminates. The stacking sequence sensitivities of braided laminates subjected to low-velocity impact were experimentally studied. Uniform ([±45° 6 ]) and non-uniform ([±30° 2 /±45° 2 /±60° 2 ]) architectures were designed to evaluate the effect of braided structure on impact performance. In addition, non-uniform laminate was impacted on both sides to identify the role of impact side on the failure mode. Quasi-static indentation was conducted to understand the damage initiation and propagation of braided laminates. The experimental results show that braided ply with high crimp level in impact side induced the main cracks along the load-carrying yarn, causing that the laminate failed in a kink-band dominated mode. For braided ply with low crimp level, a micro-buckling damage was found in the impact side, leading to the delamination dominated failure mode. It was also found that uniform braided laminate presents inferior impact resistance with respect to lower peak load and more concentrated damage because the kink-band can propagate into the specimen along braiding yarn easily. [ABSTRACT FROM AUTHOR]

• The effect of ply number on the impact response was identified. • The effect of impact damage on the residual axial compression behavior was analyzed. • The proposed two-step FE model predicted accurately the damage behaviors of braided tube. In this study, transverse low-velocity impact response and residual axial compression behavior of braided composite tube with different ply number was investigated by experimental and numerical methods. The transverse low-velocity impact tests with 5.6 J energy were conducted on the composite tubes. The quasi-static axial compression performance of intact and pre-impacted tubes was compared to evaluate the effect of impact damage. A two-step finite element (FE) model was also established to reveal damage mechanisms of braided tube under impact loading and following axial compression. It was found that the wall thickness had a significant influence on the impact response. Obvious structural deformation occurred in 2-ply tube when subjected to impact loading, resulting in a large projected delamination area. In following axial compression process, the delamination failure significantly reduced the local compression stiffness and caused a structural instability, which led to a buckling failure mode. In contrast, increasing bending stiffness of 3 or 4-ply tube suppressed its structural deformation during impact, leading to a confined projected delamination area and therefore the buckling of tube wall was prevented effectively when subjected to axial compression. For all impacted tubes, the transition of failure mode from progressive folding mode in intact tube led to a lower energy absorption capacity per composite ply, especially for 2-ply tube. [ABSTRACT FROM AUTHOR]