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• 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]

This investigation is aimed to study the dynamic failure behavior of 3-D braided carbon/epoxy composite by capturing the high-speed infrared image and simulating the temperature-rise response under high strain-rate compression. It is found that localized temperature rise is a good indication in damage and failure of the 3-D braided composite. Localized temperature can rise above a hundred degrees Celsius, reaching or even exceeding the T g of the matrix resin and imperiling the polymer along damage regions in specimen. Temperature rise in localized paths of the specimens was caused by adiabatic shear along the boundaries of braided fibers or inside the fiber bundles. Sustaining decrease in stress after the peak strength is related to shear deformation, temperature rise and progressive damage. There is a correlation among localization of temperature rise, adiabatic shear band, resin softening, damages and failures in the 3-D braided carbon/epoxy composite under dynamic compression loading. For the out-of-plane compression, temperature always conducts from the broken interface to the inside of the fiber bundles along the radial, which will lead to the weakening of resin in fiber bundles. And damages, penetrating cracks and openings are verified with 'X'-shape localization of temperature rise. With the increase of strain rate, localization of temperature rise indicates that adiabatic shear bands begin to connect together to form larger and longer paths by the mutual shear effect among adjacent fiber bundles. For the in-plane compression, multi-cracks are found by observing the independent 'zigzag' shear bands converting to dependent 'zigzag' network. The temperature is easily localized at the crimp position of fiber bundle when they forming the 'zigzag' localization along the braiding paths of fiber bundles. However, some regions in 3-D braided carbon/epoxy composite have not experienced plastic shear deformation, which are isolated among 'zigzag' adiabatic shear network and their boundaries are the very paths for localized temperature rise. • Temperature rise in 3-D braided carbon/epoxy composite under dynamic loading is associated with damage and failure. • The localized temperature rise can be observed by high-speed infrared imaging and simulated by mesostructural modeling. • There is a correlation among localization of temperature rise, adiabatic shear band, resin softening, damages and failures in the 3-D braided carbon/epoxy composite. [ABSTRACT FROM AUTHOR]