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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]

• The transverse impact response of hybrid biaxial/uniaxial braided tube was studied. • The effects of architecture hybridization and stacking sequence were identified. • Proposed FE model provided accurate predication of the impact behaviors. Tubular composites in automobile engineering are susceptible to transverse loading events. This paper deals with transverse low-velocity impacts on interply hybrid tubes made of biaxial/uniaxial braided fabric layers. For comparison, drop weight tests were performed on hybrid tubes with biaxial braided surface layers and uniaxial braided inner layer (BUB), with contrary stacking sequence (UBU), pure biaxial and uniaxial braided tubes (BBB and UUU). Numerical models were established to predict the impact behaviors and evaluate the hybrid effects. X-ray micro-computed tomography (Micro-CT) was also employed to identity the cracking location and characterize the damage mechanism. The results showed that the impact response of braided tube was related to its structural deformation resistance, which was determined by properties of reinforced layers varying with different stacking sequence. The hybrid effect became more obvious at higher impact energy. At impact energy of 15.4 J, hybrid tube (BUB) yielded the highest impact resistance with small structural deformation. The biaxial braided layer protected the specimen surface by constraining intralaminar crack propagation through the interlacing patterns. Meanwhile, it promoted crack tip propagating to the subsequent uniaxial braided layer and transferred the impact load. Thus, the material involvement of the inner layer was improved, which was responsible for providing a higher in-plane tensile property due to its low yarn crimp. [ABSTRACT FROM AUTHOR]

• The impact response of carbon/glass intralaminar hybrid braided pipes was studied. • Impact behaviors of hybrid pipes with and without internal pressure were compared. • The damage extension mechanisms were analyzed by micro-CT 3D damage reconstruction. • Ductile and brittle hybrid fibers restrict each other and promote impact resistance. • The hybrid fiber layout and the internal pressure had a synergistic effect. This study investigated the low-velocity impact behaviors of intralaminar hybrid carbon/glass braided composite pipes under internal pressure. The effects of intralaminar fiber layout and internal pressure on the mechanical responses, damage extension, and material cost-effectiveness were analyzed. The internal damage distribution of composite pipes with different hybrid fiber alternating numbers but identical mix ratios was evaluated through Micro-CT three-dimensional damage reconstructions. The yarn splitting of brittle carbon fiber materials was mutually prevented by the induction of intra-yarn debonding and delamination damages by hybridization with ductile glass fiber materials, which were prone to produce out-of-plane displacement and shear stresses under impact loading. A smaller hybrid fiber alternating unit was conducive to exploiting the hybrid impact toughening effect by avoiding pure reinforced fiber materials aggregated under the impactor. Internal pressure had a supporting effect on the composite pipe and was effectively synergistic with the hybridization effect. The change in the dominant damage mode resulted in significant reductions in impact deformation and damage severity compared to the unpressurized case. [ABSTRACT FROM AUTHOR]

This work investigated off-axial angle effect on in-plane compression behavior of 3D orthogonal woven(3DOW) composite under both quasi-static and dynamic loading. Mechanical responses of 3DOW specimens with different off-axial angles were evaluated using a universal material tester and a split Hopkinson pressure bar(SHPB) testbench. An infrared thermographic camera was used to record temperature rising of the specimens in order to characterize the damage development in quasi-static compression. A yarn-level finite element model considering progressive damage behavior of impregnated yarn, matrix resin and their interface were also established to reveal the deformation behavior and failure mechanism. It is found that the off-axial angle has significant influence on compressive response and failure mode. The modulus and compressive strength of specimens exhibited a decreasing tendency with increase of off-axial angle, while failure strain had a rising trend. In addition, the role of z-yarn was identified in different off-axial angle cases. For on-axial specimen, the z-yarn suppressed out-of-plane deformation of superficial weft yarns failing in kinking mode. It in turn caused intensive z-yarn breakage, which accompanied by high temperature rising. For off-axial specimens, rotation deformation of weft and warp yarns was observed, which was restrained by z-yarn. It leaded to extensive debonding between matrix and yarns and delamination characterized by relative low temperature rising. • The effect of off-axial effect on the compressive impact response of 3D orthogonal woven composite was identified. • The FEA model predicted accurately the compressive impact behavior of specimens. • The effect of z-yarn was identified by FEA analysis model. [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]

In this study, damage behavior and failure mode of braided composite tube subjected to repeated transverse low-velocity impact were investigated by combining experimental and numerical methods. The effects of impact energy (2.0 J, 5.6 J and 10.5 J) and impact position (identical, adjacent and opposite positions) on impact responses were evaluated. A finite element (FE) model with high computational efficiency was also established to predict the effect of single impact on the repeated impact response by varying boundary conditions. When subjected to single impact, the braided tube presented delamination-dominated damage behaviors caused by obvious structural deformation. The top part sunken and both lateral sides bulged, were found to have correlation with the mechanical response of tube with respect to initial scope and peak load, respectively. The delamination area increased at higher impact energies due to increasing shear stress caused by adjacent braided layers. Delamination induced by the first impact had different role on repeated impact responses. At identical position, it caused larger sinking deformation at top part of the tube and thus a lower initial scope. At adjacent position with impact energy of 5.6/10.5 J, the delamination damage caused a reduced resistance to bulging deformation at one lateral side of the tube, leading to a lower peak load. At opposite position, it had no obvious influence on repeated impact response. • The damage behavior of braided tube subjected to single transverse impact with different impact energy was investigated. • Mechanical response of repeated impact depended on delamination position caused by single impact. • The proposed numerical model can provide accuracy results for repeated impact scenario. [ABSTRACT FROM AUTHOR]

The study aims to understand the effect of stacking sequence on the impact response and residual flexure characteristics of braided composite tube by combination of low-velocity impact, fracture observation and acoustic emission monitoring. Two stacking sequence configurations of braided tube in terms of [I60/O40] and [I40/O60] were designed and their low-velocity impact response and damage behavior were examined. Uniform structures namely [I40/O40] and [I60/O60] were also employed as reference specimens. The quasi-static bending performances of intact and impacted tubes were further compared to evaluate the influence of impact damage on residual performance. The results show that failure mode of impacted tubes strongly depends on braided structure of outer ply, of which large braiding angle(60°) leads to the formation of bottom delamination and degradation of structural integrity. The comparative experiments indicate that [I60/O40] tube improves flexure after impact performance with respect to specific energy absorption by 45.98% over [I40/O60] counterpart because of absence of bottom delamination. Thus, structure design of braided composite consisting of intra-ply structure and stacking sequence would be an economically feasible method to improve the transverse mechanical performance. • The braided structure of outer ply has a decisive role on the failure mode of composite tube under transverse impact. • There was a sharp load dropping when a bottom delamination occurred in intact braided tube under quasi-static bending. • Bottom delamination induced by impact significantly reduced residual bending performance of braided tube. [ABSTRACT FROM AUTHOR]

In this work, the synergistic effect of 3D orthogonal woven structure and asymmetric carbon/glass hybridization on impact response was studied by experimental and numerical methods. The hybrid 3D orthogonal woven (H3DOW) composite plates were manufactured on a home-made 3D weaving machine. Impacts with various energies 42.9, 54.9, 66.9 and 78.9 J were applied to carbon (CG) and glass (GC) sides. The impact response was examined using fracture morphology observation and mechanical curves. Yarn-level finite element models including failure criteria and progressive damage law were established in ABAQUS/Explicit to compare the structural deformation and damage behavior. The impact side significantly influences the failure modes of H3DOW materials. GC specimen failed through delamination between carbon layers at non-impact side and subsequent fiber tensile breakage, whereas the crushing damage in the carbon yarn located in the impact-side and large area debonding between yarn and matrix resin in superficial layers was the dominant failure mode in CG specimen. This caused the CG configuration to exhibit better impact resistance with respect to the perforation threshold energy. [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]