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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 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]

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]

Abstract Temporary structures, which are designed to be quickly erected and in service for short periods of time, are widely used in performances and sports competitions. This temporary characteristic requires special construction to be implemented. This paper presents a new modular support structure that can be used as temporary grandstands, stages, ski runs and disaster relief rooms. This modular support structure is composed of the foldable plane frame with corresponding joints. The assembly property is achieved through joint connection among these frames. The local folding characteristic of these plane frames improves both erection and transportation efficiency. As a core component of this modular support structure, the aluminum alloy assembly column was particularly concerned and the axial stability of the column was investigated experimentally. The results indicated that the use of the connection ring could improve the stability of the assembly column along with the connections between the foldable plane frames. Numerical analysis of the tested specimens under axial compression was developed. The ultimate strength, failure modes and load-displacement curves obtained from the numerical results agreed well with the test results. Equivalent stability factors of the assembly columns were calculated to further illustrate axial stability of the column. Graphical abstract fx1 Highlights • A new modular support structure used to build the temporary structure is presented. • The stability of the aluminum alloy assembly column is studied experimentally. • Numerical models of the aluminum alloy assembly column are developed. • Equivalent stability factors of the aluminum alloy assembly columns are calculated. [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]

• The virtual fabric model can be used for the simulation of actual weaving process. • Virtual fabric composed of truss elements predicts the microstructure of 3D woven fabric. • Friction and yarn tension affect the trajectory and cross-sectional shape of the yarn. • The intersection-induced tension between warp and weft yarn causes fabric deformation. • The corners of the mold induce the interlayer slippage of fabric and alter the trajectory of the warp yarns. This article introduces a modeling method aimed at predicting the internal geometric structure of 3D warp interlock fabric. Inspired by the digital element method, the method employs truss elements to model the yarns as fiber bundles, and considers their material properties. This technique allows for predicting the microstructure of 3D woven fabric based on simple input characteristics. By varying the elastic modulus of virtual fibers, the friction coefficient between fibers, and the tension load at the ends of yarns, and designing compression molding molds to compress the fabric into shape, the virtual fiber fabric predicted the influence of variables on the fabric. The study found that the structure of the samples obtained through CT scanning is generally consistent with the structure of the virtual fiber fabric. The elastic modulus has a relatively minor impact on the fabric structure, whereas the friction coefficient and tension load significantly affect the cross-sectional shape of the yarns. During compression molding, deformation within the mold leads to interlayer fabric slippage and changes in the cross-sectional shape of the fabric. This suggests the feasibility and applicability of virtual simulation testing methods for analyzing the behavior of various types of 3D woven fabrics. [ABSTRACT FROM AUTHOR]

In this work, the 3D and 2D fabric reinforcements were made of glass/polypropylene (GF/PP) commingle filaments. The effect of heat treatment on the multiple impact mechanical responses and damage behaviors of GF/PP/epoxy composites were investigated. It is found that the pre-impact heat treatment (P) could maintain the best performance in the mechanical responses at the 3rd impact; The post-impact heat treatment (VH) could play the role of filler or healing agent after each impact load, but polypropylene fibers existing as reinforcement were melted into fusant and flowed out of the material along cracks, which results in reduction of mechanical properties. Under low-velocity impact, 3D woven composites were more sensitive to heat treatment because of the stress concentration of heated Z-binder yarns as the anti-delamination role; Perforation failure at the 3rd impact was found in the 3DOW structure with post-impact heat treatment, while the 2D woven laminated structure could still withstand more impacts. At high strain rates, heat treatment made polypropylene fibers be melted and recrystallized, and changed their volume and morphology, resulting in global micro voids in the material under impact load, which inhibits the formation of localized macro shear cracks. • Heat treatment makes PP fibers be melted, recrystallized and changed in morphology. • Pre-impact heat treatment restrains crack propagation and maintains mechanical responses. • Post-impact heat treatment fills cracks but results in reduction of mechanical properties. • 2D woven laminates present better perforation resistance than 3DOW structures. • Heat treatment results in global micro-voids but inhibits localized macro-shear cracks. [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]

To meet the demand of lightweight truss, thin-walled single-/double-/four-cell carbon fiber reinforced polymethacrylimide foam sandwich composite beams were manufactured by tubular braiding. Transverse impact damage and axial compression failure of the braided sandwich beams were investigated by finite element modeling, micro-CT detection, acoustic emission and infrared imaging characterization. It is found that impact damage concentrated at edges of the indentation region, internal CFRP walls and surrounding foams. Different from the continuous vertical wall in double-cell beam, cracks on the internal vertical wall of four-cell beam were suppressed at half height due to the discontinuous horizontal wall. During the axial compression, six failure modes were observed for non-impacted(0J) and impacted(40J, 60J, 80J) specimens, which were determined by the combined effect of impacted damages and inner configurations. And the inner configurations have an advantage over the transverse impact effect on the axial failure mechanism. Unlike the single-cell beams, the double- and four-cell counterparts did not present buckling or bending failures owing to the effective support from internal CFRP walls, however, more significant fracture was found at their outer CFRP layers due to the progressive mechanism of shear, tearing, splaying, and separation. • Integral braiding and net forming of sandwich beams with complex cross sections. • Mutual verification by micro-CT, FE analysis, and morphology characterization. • Real-time failure analysis by joint application of AE and IR method. • Effect of pre-impact and inner configurations on CAI properties of sandwich beams. [ABSTRACT FROM AUTHOR]