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To explore the enhancement mechanism of CFRP overwrap in the aluminum-CFRP square hybrid, hybrid tubes with different braided angles were investigated both experimentally and numerically. Hybrid tube of 30 degrees had severe CFRP layers splitting damage in the tube corner. As the braided angle up to 60 degrees, CFRP layers with high transverse resistance force compressed the aluminum tube and caused it to fold inward; This led to the hybrid tube failing in a distinct inward eccentric fold deformation pattern and had an obvious energy absorption improvement in its inner aluminum tube due to its increase in the corner deformation. [ABSTRACT FROM AUTHOR]

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]

Composite tubes have an asymmetry deformation and loading distribution under transverse bending. This work investigated the effect of the stacking sequence on the critical damage to a hybrid tube with 45° and 60° braided layers. In-situ infrared thermal imaging was adopted to observe the initiation and development of internal damage. As the outermost reinforcing material in the stacking structure, the 60° braided layer with high contact strength inhibited damage to the contact area at the top and improved the bending strength of the specimen at the initial stage of loading. As the inner reinforcing material in the stacking structure, the 45° braided layer with high bending toughness maintained the high stability of the load-carrying capacity during bending and restrained the tensile failure at the bottom of the tube. However, the interlaminar interface of braided layers with different braiding angles reduced the interlaminar fracture toughness. The interlaminar delamination failure developed rapidly as the bending at the bottom of the tube increased. This delamination damage determines the peak load-carrying capacity of the laminated structure. • The mechanical behavior of the stacking structure braided tube with the outer 60° braid and the inner 45° braid be studied. • Infrared thermography is applied to detect and analyze damage distribution degree in the loading process. • The load and damage distribution of stacking structure braided tubes are analyzed. [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 delamination damage distribution induced by multi-impact was identified. • The sensitivity of axial compression performance to impact damage was studied. • Proposed FE model provided accurate predication for CAI scenario of braided tube. In this study, damage behaviors and residual crashworthiness characteristics of braided composite tube with transverse pre-impact damages subjected to axial compression were investigated by experimental and numerical methods. The compression tests were conducted on intact, single impact and repeated impact tubes at identical, adjacent and opposite positions to analyze the effects of impact damages. A multi-step finite element model considering the progressive damage characteristics was developed in ABAQUS/Explicit to predict the failure behaviors of braided tube. It was found that the damage behavior and residual performance of braided tube subjected to axial compression were dominated by impact induced delamination distribution with respect to area and position. A relatively small delamination area induced by single impact resulted in similar progressive folding damage mode and energy absorption capacity as in intact tube. For repeated impact, delamination with increased area at identical position leaded to local buckling damage mode, while small delamination areas distributed at adjacent and opposite positions caused U-shape and W-shape fracturing damage modes, respectively. Consequently, the transformation of damage mode resulted in significantly reduced residual crashworthiness characteristics in repeated impact tubes compared to intact and single impact tubes. [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 effect of the number of carbon-fiber-reinforced plastic (CRFP) layers on the axial crushing capacity of aluminum–CFRP square hybrid tubes was investigated both experimentally and numerically. Increasing the number of CRFP layers led to more collapsed lobes and decreased collapsed lobe widths, resulting in an improvement of the energy absorption capacity. The simulation results revealed a distinct tube wall collapse mode in the 4-layer hybrid tubes. The higher bending stiffness of the composite layers in the 4-layer hybrid tube led to debonding of the interface between the aluminum and composite layers. After interface failure, the composite layers did not collapse following the aluminum tube wall and instead rebounded in the approximate longitudinal direction, which directly suffering the longitudinal crushing loading. This behavior led to the shortened plastic collapse width and improvement of the single-layer CFRP energy dissipation due to CFRP delamination. • The effect of CFRP layer number on single-layer CFRP energy absorption was explored. • Different wall collapse mode appeared as CFRP layer number increases. • The increase of CFRP layer number led to decrease of collapse lobe width. • The decrease of collapse lobe width would improve the single-layer CFRP energy absorption. [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]

• 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]

The effect of axial yarns on progressive bending damage of braided composite tubes is studied in this paper. Four specimens with different number and distribution of axial yarns were experimentally tested by quasi-static three-point bending. At the same time, infrared thermography was used to observe the damage in-situ. Combined with the cross-section morphology of Ultra-Depth 3D Microscope, the damage evolution and distribution characteristics of braided tubes were identified. It is found that axial yarns enhance the mechanical properties of braided composite tube. The detected changes of infrared maximum temperature have a good correlation with the damage initiation and failure mechanism of the specimen. The abrupt changes of the maximum temperature indicate the occurrence of local fiber breakage, especially the axial yarn breakage. If there are axial yarns on the compression side of the tube, the maximum temperature increases by 0.5 °C in the linear elastic stage, and two initial damages of debonding and fiber breakage will occur. The debonding started at the interface of the axial yarn. The axial yarns increased fiber crimp, resulting in obvious wavy delamination between layers of the specimen, and the axial damage range was expanded. In addition, the mixed structure with axial yarns only on the compression side can inhibit the damage from spreading down the braided tube wall. • The progressive bending damage of triaxial braided tube was studied. • The abrupt change of infrared maximum temperature indicates local fiber breakage. • The mixed structure with axial yarns only in the upper half can inhibit damage spreading downward. [ABSTRACT FROM AUTHOR]

Localized deformation and damage mechanism of triaxial braided composite tubes, with different quantities and positions of axial yarns, were experimentally studied under transverse low-velocity impact. A high-speed infrared thermography was applied to analyze the transient thermo-mechanical failure process and afterwards verified by the external and internal damage characterization. It is found that the distribution of axial yarns has a significant effect on the mechanical behavior of braided composite tubes. The bending stiffness and peak force of the tubes were enhanced with the increased quantity of axial yarns, while the impact lasting time and displacement presented an opposite trend. And the more axial yarns assembled at the impact side, the greater force oscillatory of the tubes under low-velocity impact, accompanied by the decreased heat generation in small-angle braided tubes and the alleviated fiber tows fracture, delamination and in-plane shear cracks inside the wall of large-angle braided tubes. On the premise of the same quantity of axial yarns, the position effect plays a key role. Axial yarns concentrated at the impact side could restrain the damages spreading to the non-impact side by changing the damage mode of large-angle braided tubes. If a larger force is needed at the early stage, axial yarns could be inserted at the local position most probably to be impacted; Otherwise, it is highly suggested to braid at least one axial yarn in each layer at the impact side and non-impact side respectively to reinforce localized strength and prevent the impact-side indentation and non-impact-side delamination. • High-speed IR thermography verified by the external and internal damage characterization. • Effect of axial yarn quantity and location on mechanical behavior and damage mechanism. • Force oscillation response caused by dense distribution of axial yarns at the impact side. • Necessity of axial yarns in alleviating the severity of localized damages at the impact side. • Importance to install at least one axial yarn at the non-impact side of large-angle braided tube. [ABSTRACT FROM AUTHOR]

[Display omitted] • Integral braiding and near-net forming of CFRP and CFRP/Al hybrid composite tubes. • In-situ and real-time failure mode analysis of CFRP and CFRP/Al composite tube under transverse impact. • Comprehensive effect of braiding angle, locailzed temperature rise, and ductile aluminum on the damage mechanism. High-speed infrared thermography, micro-CT characterization and full-scale mesostructural simulation were jointly applied to reveal the impact deformation and damage mechanism of composite tubes. Two different failure modes, namely the Impact Side Dominated (ISD) and Impact-/Non-impact Side (INIS), were found to have relation with the braiding angle and localized temperature rise. The hybrid effect can not only prevent resin fragmentation in the triangle region at the lateral side and the diamond region at the non-impact side, but also inhibit the CFRP damage development at the lateral and non-impact side. In small-angle braided tube, the impact-side rebound trend of CFRP layer leads to mismatch at the CFRP-Al interface, which results in opening delamination. In large-angle braided tube, the non-impact-side damage plays an important role in further load-bearing capacity. Apart from the structure effect, temperature rise has an effect on composite damage mode by degrading the localized matrix stiffness. [ABSTRACT FROM AUTHOR]