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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]

• 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]

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]

• 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]

Spatial variability of tensile strength, widely existing in concrete gravity dams due to the influence of long construction period and heterogeneous construction quality, has a profound impact on the nonlinear behaviour of structures. In this paper, a method to simulate the random field of the tensile strength of concrete gravity dams based on the random field theory is proposed. A typical non-overflow dam section of the Guandi gravity dam is taken as an example, artificial seismic waves with similar response spectra and natural seismic waves with different magnitudes are selected for the comparative analysis. Based on the concrete damage plasticity (CDP) model and incremental dynamic analysis (IDA) method, the damage characteristics, failure modes, probability density evolution processes, and fragility curves obtained from gravity dams with homogenous and random fields of tensile strength are compared with each other. The differences among waves and realisations of random fields are also investigated. It is seen that the spatial variability of tensile strength aggravates the damage degree, decreases the continuity of local damaged areas, and increased the failure probability of global dams. It reveals that the random characteristics affect the dams' nonlinear behaviour directly. [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]

• Air flow heating without tank is reliable and effective for drastic impact loads. • Mechanics of woven CF/PEEK is more sensitive to temperature than strain rate. • Ultimate service temperature significantly influences impact dynamic behavior. • Brittle-Loose transition of damage modes were found from glassy to high-elastic state. • Failure modes at high temperature were verified by micro-scale characterization. A reliable hot air-flow heating approach is creatively carried out for high strain rate tests without environment tank. The out-of-plane and in-plane compression of carbon fiber reinforced polyetheretherketone (CF/PEEK) woven composites are investigated from 400 to 4000/s at the glassy state and high-elastic state. The real-time and in-situ deformation and failure process of the composite specimens under the extreme thermo-mechanical loads are successfully captured, which can reflect the impact dynamic behavior of the material near the ultimate service temperature. Empirical laws for mechanical index of the woven composites are given. The results reveal that the thermal softening effect outperforms the strain-rate strengthening effect. For out-of-plane impact at low strain rates, damage mode transitions from the interface and matrix cracking at glassy state to the "parallel" shear and fiber extrusion at high-elastic state. Under high strain rates, it reveals the "fragmented" shear mode at glassy state. For in-plane impact, the temperature effect leads to the failure mode changing from the delamination and local shear to the mixed feature of kinking bands, delamination, and multiple-band shear with "bulge" behavior. And the multiple-band shear and delamination are more likely to occur along the in-plane direction as the strain rate increased. [Display omitted] [ABSTRACT FROM AUTHOR]

Two-dimensional triaxial braided composite (2DTBC) often fails by external force from different directions in service. In this work, the in-plane compression failure mechanism of 2DTBC was studied along the axial (0°), off-axial (±60°) and perpendicular (90°) directions at quasi-static and high strain rate. The progressive failure process was recorded by infrared thermography with the final morphology using computer tomography technology. A three-dimensional mesoscale finite element model was established to simulate the initiation, accumulation and propagation of various damages. The results show that the in-plane compression strength, strain, energy absorption and failure mode of 2DTBC depends on whether there are main load-bearing (axial or braided) yarns directly transferring stress along load directions. The occurrence and development of localized damages can be captured by high-speed IR thermography and verified by final morphology. The in-plane failure mechanism of 2DTBC is induced by different dominating factors under different load directions, such as the out-of-plane movement of braided yarns, kinking effect at crimp positions, structure antisymmetry effect, buckling and shear instability. • Effect of main load-bearing (axial or braided) yarns during the in-plane compression. • Occurrence and development of damages captured by high-speed IR thermography. • Effect of yarns out-of-plane movement, kinking, buckling and shear instability. [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]

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]