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The structural properties of braided composite laminates are significantly affected by the low‐velocity impact (LVI). In this paper, the impact resistance and damage tolerance of triaxial braid structure laminates at different positions were mainly studied. Three hybrid biaxial (B)/triaxial (T) braided composite structures of BBTT, BTTB, and TTBB were designed, and LVI test and post‐impact compression tests were carried out. The results show that the triaxial braided fabric with quasi‐isotropic on the impact side caused more significant matrix cracks along the axis yarn direction, and the matrix damage area and delamination area were larger. For triaxial braided ply with high curl levels, severe fiber fracture and relatively concentrated damage occurred first on the non‐impact side. Interestingly, triaxial braided fabrics exhibited better impact resistance in terms of mechanical response when distributed across the specimen surface. However, a global damage pattern appears at high energy levels, which seriously reduced the residual strength of the structure and showed the worst damage tolerance. The damage quantification analysis of cracks, dent depths, delamination and residual compressive strength caused by the LVI of hybrid shaft number braided laminates was carried out in this work, which provided a valuable reference for engineering failure analysis and rational structure optimization of composite laminates. [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]

• Advantages of asymmetric hybridization on impact resistance and resilience. • Rules of external damage area and internal crack volume distribution. • Damage mechanism of carbon/Kevlar hybridization laminates. The ply structure has a significant effect on the impact resistance and failure behavior of composite laminates. In this paper, carbon/Kevlar hybrid fabric reinforcements were prepared with different Kevlar ratio (0%, 25%, 50%, 75%, 100%) by replacing carbon fabric plies on the non-impact side by Kevlar plies. The low-velocity impact tests for all configurations were carried out under 15 J and 25 J energy respectively. The results show that the maximum deflection, impact duration, impact absorption energy and absorption energy efficiency of hybrid laminates increased with the Kevlar hybrid ratio, whereas the peak force presented a decreasing trend, which indicates that the adding of Kevlar layer makes the laminates more ductile than non-hybrid carbon structure. The damage mechanism of the carbon fiber ply was brittle fiber fracture, resin cracking and resin shedding, while the fibers pull-out and tows splitting was found in Kevlar ply. The critical structural damages start from the non-impact side and evolve to the impact side. And the rules of internal crack volume distribution with Kevlar plies are consistent with the trend of external damage area on the non-impact side. It is suggested that in order to take into account the peak strength and excellent structural resilience, Kevlar asymmetric-hybrid ratio of 25% is the most appropriate. The reason is that an appropriate small ratio of Kevlar cushion ply can not only effectively avoid the tensile fracture at non-impact side happened in the non-hybrid carbon structure, but also withstand enough impact load by maintaining a high carbon fiber content. [ABSTRACT FROM AUTHOR]

• The structural hybridization effect on low-velocity impact response of braided/unidirectional (UD) laminates was investigates. • Continuum shell finite element models based on the anisotropic constitutive relationship of braided and UD plies were established to reveal the damage onset and propagation. • The extensive delamination formed rapidly when impacted on the fabric side. • The damage characterized by severe fiber breakage occurs at the impact site when impacted on UD ply side. This work investigated the structural hybridization effect on the low-velocity impact response of braided/unidirectional(UD) laminate. Two groups of hybridization laminates, namely interleaving and clustering groups, were manufactured. The impact performance of the specimens was characterized by combining dropping testbench, optical camera and ultra-sound detector. A continuum shell finite element model was established to reveal the progressive damage behavior of hybridization laminate. The specimen in the interleaving group presented overwhelming performance than that in the clustering group. The impact side effect on failure modes was discussed in detail. When impacted on the braided fabric side, extensive delamination formed on the edge of the specimen due to bending deformation. It further resulted in-plane crushing failure of UD sub-laminate. In contrast, severe fiber breakage occurs at the impact site due to combination stress of tensile and shear when impacted on UD side. Therefore, the fabric-impacted specimen exhibits better impact resistance than that impacted on UD side with respect to higher peak load and lower absorbed energy. [Display omitted] [ABSTRACT FROM AUTHOR]