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Triaxial braided composites employed as structural components need to be drilled for mechanical connections. This study is to investigate the thrust force, temperature elevation, and drilling-induced damages with varying machining parameters in the drilling of triaxial braided CFRP. A high-resolution infrared thermography was carried out to capture the temperature of the panel side surface and the drill-exit surface of CFRP. The damage characteristic of drilled hole was evaluated, and a relationship between force, temperature, and hole damage was established. The obtained results indicate that the temperature of triaxial braided CFRP was mainly transferred along the fiber direction. The temperature isothermal at the drill-exit surface developed from polygonal to roughly circular with the increase of drilling depth. The braided yarns tended to deflect, and induced poorly cut fibers due to the more heat generation with the low feed, making the matrix behaved from brittle to ductile and therefore resulting in more burrs. The increase of spindle speed could restrain the burrs. A larger thrust force with high feed will result in fiber-resin debonding and more tearing defects. The delamination occurred at the interface of fiber bundles at the outermost layer with high thrust force and cracks propagated along the yarn direction. [ABSTRACT FROM AUTHOR]

Failure mechanism of complex profile component is always different from that of conventional plate counterpart due to the coupling effect of material and structure. In this work, the low-velocity impact (LVI) and compression after impact (CAI) behaviors of Ω-shape hybrid carbon/Kevlar 3 D orthogonal woven (3DOW) composite made for vehicle B-pillar were comprehensively studied by mechanical tests and mesoscale finite element (FE) analysis at component level, high-speed infrared (IR) thermal imaging, acoustic emission (AE) detection, and microscopic damage morphology characterization. It is found that a through-thickness stress concentration ring leads to high stress state and damage zone penetrating from the impact side to non-impact side along the ring path instead of at the lowest impactor position. The slope effect can not only help the stress conduction downward, but also inhibit the damage propagation from the impact side to the slope. Impact-induced cracks are concentrated around the R corners and extended along the axial direction of the specimen, forming the strip-shaped damage concentration zone along the upper eave of the slope. The Progressive Top-Down Crushing (PTDC) mode of compression after impact is due to the complex deformation process of each yarn such as squeezing, folding and eversion in the crushing process from the top of specimen. And the Middle Indentation Fracture (MIF) mode is the result of bending instability and abrupt fracture. This work presents a reference significance for the further development of composite strengthening components in vehicle bodywork. [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]

Previous investigation on the crashworthiness of braided composite tubes did not take the relationship between adiabatic shear failure and temperature rise into account during dynamic loading. In this study, transient temperature rise caused by adiabatic effect was detected and captured for the three-dimensional braided carbon/epoxy composite specimens during axial impact compression under 600–800/s. A mesostructure model was established based on three-dimensional braided tube architecture to numerically characterize the mechanical and thermal response in material. Based on the results, non-uniform temperature distribution shows good correlation with adiabatic shear failure in the material. Key scientific issues are discovered including the position, morphology, time sequence, and response process of the temperature rise. The catastrophic shear behavior with accelerated temperature rise occurs after the peak force and accompanies the progressive failure process. Nodes having room temperature in the adiabatic shear zone indicates that some positions in plastic zone may still behave as elastic state. There exists different plastic slip distances due to shear instability in the path along or crossing the adiabatic shear band. Through this investigation, the model considering the adiabatic effect was able to show the dynamic shear mechanism involving the fracture position, morphology, and progressive thermo-mechanical response of the temperature rise, which cannot be revealed by experimental testing. [ABSTRACT FROM AUTHOR]

• Hybridization of glass fiber into carbon composite presents toughening effect. • Infrared thermography accurately evaluates the progressive damage. • Off-axial tensile properties are more sensitive than on-axial for hybrid composites. • The quick crack propagation in hybrid composites is prevented. • Damage of interior carbon fiber layers can be restrained through hybridization. Hybrid composites are increasingly attractive for advanced engineering applications. In this study, the open-hole tensile (OHT) properties and failure mechanism of plain woven carbon fiber, glass fiber, and carbon/glass hybrid fiber composites under on-axis (0°) and off-axis (45°) loads were investigated through experimental and numerical approaches. The progressive damage was characterized by high-speed infrared thermography. The numerical simulation presented a good explanation on the damage development. It indicates that the carbon/glass hybrid composites exhibited significant improvement in failure strain and toughness compared to pure carbon fiber composite. With the insertion of glass fiber layers, the failure modes of hybrid structures were modified and quick propagation of cracks was prevented. Once the cracking on their carbon layers had initiated there was an increase in damage to glass fiber layers showing the transfer of stress from carbon layers to glass layers. Similar mechanical properties were reached among sandwich-like hybrid samples in on-axis tension, but much greater properties can be achieved when the carbon fiber at the interior layers of the hybrid specimens under off-axis load. Since glass fiber with great ductility was laminated as the surface layers in hybridization, resulting in less delamination and higher ultimate strength in 45○ off-axis tension. [Display omitted] [ABSTRACT FROM AUTHOR]

It is difficult to establish a numerical model for a certain structure of electrospun nanofibrous mats, due to their high porosity and non-linear characteristics, that can fully consider these characteristics and describe their mechanical behaviors. In this paper, an analytical method of meso-mechanics was adopted to establish the tensile constitutive relationship between a single fiber and mats from fiber-web microstructures. Meanwhile, a macroscopic finite element model was developed and verified through uniaxial tensile stress-strain experimental data of silk fibroin (SF)/polycaprolactone (PCL) nanofibrous mats. The compared results show that the constitutive relation and finite element model could satisfactorily express elastic-plastic tensile mechanical behaviors of the polymer. This model helps regulate the microstructure of nanofibrous mats to meet the mechanical requirements in engineering applications. [ABSTRACT FROM AUTHOR]

This investigation examines the transient deformation and heat generation of a solid polyurethane subjected to dynamic compression. A special method is presented to prepare the solid polyurethane from raw materials which are commonly used to make polyurethane foams. Testing methods including infrared spectrum, differential scanning calorimetry, quasi-static and dynamic compression were applied to study the basic physical properties of the solid polyurethane. High-speed optical and infrared imaging systems are used to obtain visual and thermo-graphic images during impact tests. Under quasi-static compression, the solid polyurethane presents a good performance in toughness. This is confirmed by its Poisson's ratio. Under impact compression, the adiabatic heat generation are identified statistically. Temperature distribution confirms the fact of transient heat generation in specimens. Adiabatic self-heating mechanism provides a consideration to understand the negative strain-rate effect and post-yield softening effect found in the solid polyurethane. Mechanical properties including quasi-static and dynamic responses are related with the composition of molecular and structure of polymer. [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]

3D angle interlock woven fabric(3DAWF) has great potential for impact protection. This paper investigates the ballistic mechanism of 3DAWF(5 layers of angle interlock – through the thickness) under normal and oblique impact. The full-size mesoscale model of 3DAWF under different impact directions and angles was established and systematically studied to reveal the 3DAWFs' ballistic mechanism. The numerical studies of 3DAWF subjected to 0°, 15°, 30°, 45°, and 60° oblique impacts from two impact directions along 3DAWF structure configurations were carried out. We found that 3DAWFs' ballistic performance increases non-linearly with impact obliquity. The ballistic mechanisms change with impact directions because of 3DAWFs' anisotropic structure. This work also demonstrates the impact damage mechanism, energy absorption evolution, and stress wave distribution of the 3DAWF under oblique high-velocity impact. The findings are constructive for the 3DAWF applicated in ballistic protection. [ABSTRACT FROM AUTHOR]

Progress of dynamic failure of 3-D textile structural composites have not been sufficiently studied. This work is aimed to investigate the progressive failure of 3-D braided, biaxial warp-knitted and angle-interlock woven composites by high-speed imaging system during impact loadings. Real-time images are listed frame by frame and located in stress-strain curves. There is a sharp decrease stage and a steady decrease stage in stress under catastrophic failures. These two stages reveal different damage morphologies in progressive failures. For the out-of-plane impact compression, if there is no catastrophic failure, composites get limited rebound because of structural recovery of their reinforcements. If catastrophic failure occurs, composite structures would be split. For the in-plane impact compression, the 3-D braided, biaxial warp-knitted and angle-interlock woven structures protect themselves from being thoroughly split under explosive cracks/shear/delamination failures. [ABSTRACT FROM AUTHOR]

Hybrid structures are increasingly important in joints of composite components. For carbon, glass, and bilateral carbon/glass hybrid composites, a thermal-mechanical characterization experimental study was implemented to explore the hybrid effect on the drilling temperature evolutions and damage mechanism. Infrared thermography was applied to analyze the temperature distribution from the side and exit surface of the composites. The results indicate that the hybrid structure generated an abrupt change of thrust force during drilling the carbon/glass interfacial region and the maximum peak force was induced in 5C5G specimen. The temperature isothermal on the side surface of hybrid composites gradually evolved into a trapezoidal shape. The variations of hole positions on the plain fabric structure can affect the temperature generation and heat transfer. The carbon layer can prevent bending deformation and act as a support in drilling the last layer of 5G5C and showed better performance. Severe damage was induced for the less mechanical strength in the glass layer and low heat dissipation of exit material leading to concentrated heat. Accumulated high temperature reduced the bonding strength between the fiber and resin, and thus the debonding, matrix smearing and resin strips occurred. [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]

• Advantages of symmetric intercalation hybridization on impact strength and resilience. • Correlation of apparent damage area index and internal damage volume data. • Damage mechanism of symmetric and asymmetric carbon/Kevlar structures. Hybrid fiber composite laminates with the same materials, manufacturing methods and molding processes may present different mechanical behaviors. In this paper, the effect of intercalation sequence on the mechanical performance of carbon/Kevlar plain laminates were studied. According to different intercalation methods, the hybrid layout was divided into symmetric and asymmetric configurations. It is found that the symmetric intercalation structures (KCCK and CKKC) possess better load-bearing and resilience capacity compared with asymmetric structures (CCKK and CKCK), even better than pure carbon structure (CCCC) under high energy impact. The impact damage volume of symmetric specimens was significantly smaller than that of asymmetric specimens by quantitative micro-CT analysis. The apparent damage index (d f and d r) can be used to estimate the internal damage data (d CT , H and V). KCCK, a typical symmetric sandwich-like structure, presented the most slight damage among all symmetric and asymmetric configurations, with shear cracks observed at the brittle carbon fiber core layers, while the delamination cracks between Kevlar-carbon interlayers were away from the central damaged region. In CKCK, interlaminar cracks were formed at each carbon-Kevlar interlayers, which shared and relieved the primary interlaminar crack at the core layers compared with CCKK. [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]

This paper presents an approach to simulate the high strain-rate compression of 3-D braided basalt/epoxy composite materials under room and low temperatures. A microstructure model of 3-D braided composite was established to characterize a fully coupled thermo-mechanical response during the fast deformation. High stress state and low heat generation have been found in braided reinforcement. For out-of-plane compression, adiabatic heat concentration is along single diagonal direction at −100 °C, while a cross-shape heat concentration region is found along two crossed diagonal directions at 26 °C. There are fewer cracks among fiber tows at low temperature. Once the single diagonal shear failure occurs at low temperature, the 3-D braided reinforcement can not keep structure integrity and will be separated into two parts. For in-plane compression, the damage morphology at −100 °C is similar to that at 26 °C. Under the influence from the fiber tows, the adiabatic heating develops the zigzag-shape damage with veins and stripes along the braiding angle in composite, while the 3-D braided reinforcement still keeps its structural integrity. [ABSTRACT FROM AUTHOR]

This article reports the compressive behaviors of 3-D braided basalt fiber tows/epoxy composite materials under the temperature range of 23–210°C with the strain-rate range of 1300–2300 s−1. A split Hopkinson pressure bar apparatus with a heating device was designed to conduct the out-of-plane compression tests. It was found that compression modulus, specific energy absorption, and peak stress decreased with the elevated temperatures, while failure strain gradually increased with the elevated temperatures. Compression modulus and peak stress were more sensitive to the temperature effect, whereas failure strain and specific energy absorption were more easily affected by the strain rate effect. The plasticity can be divided into two types: (a) the platform-shape plasticity; or (b) the slope-shape plasticity. The experimental condition of 150°C with 1827 s–1was a dividing threshold to differentiate the compression-failure mode and the shear-failure mode. The authentic microstructural finite element analysis results revealed that the distribution and accumulation of the inelastic heat led to the development of shear bands. Braided reinforcement had an important influence on the damage characteristics. When the temperature was belowTg, the material underwent a significant temperature rise during failure. But aboveTg, the temperature rise was relatively steady. [ABSTRACT FROM PUBLISHER]

The transverse impact deformation and damage of 3-D circular braided composite tubes were studied both in experimental and numerical approaches. A meso-structure geometrical model based on the braided architecture was established to analyze the impact damage and morphology. The transverse impact tests were conducted on a modified split Hopkinson pressure bar (SHPB) apparatus driven with high pressure Nitrogen gas under different pressures. The load-time histories and deformations of the 3-D circular braided composite tubes under transverse impact were obtained from experimental and compared with those in finite element analysis (FEA). The FEA results show satisfactory agreements with experimental data and demonstrate the validity of the model. Based on the finite element results, the deformation process and stress propagation were obtained to analyze the failure mechanism. The stress distribution on the composite tubes showed that the 3-D circular braided reinforcement was the main load-carrying component and absorbed most of the impact energy. The impact region was dented along the impact direction and a clear shear band was found around the impacted region. [ABSTRACT FROM AUTHOR]

This investigation aims to study the impact behaviors of biaxial warp-knitted composite materials under the following conditions—strain rates of 1000–3400 s−1; temperatures of 23℃–210℃. Impact compression tests (in the through-thickness direction of specimens) were conducted using the split Hopkinson pressure bar apparatus. A heating device was applied to achieve required temperatures. It was found that high temperatures of 120℃–210℃ led to a softened initial response on the composite’s stress–strain curves. In addition, pre-peak hardening was observed in the specimen when exposed to temperature between 23℃ and 90℃. The latter could be attributed to further compaction with densification effect during through-thickness compression. Finally, two different failure modes were noticed—(1) the shear-oriented failure and (2) the delamination-oriented failure. This was due to the degradation of the tricot, the brittle–ductile transition of the resin, and the thermal stress in the biaxial warp-knitted composite at different temperatures. [ABSTRACT FROM AUTHOR]

This paper reports the high-strain rate properties of 3-D braided basalt/epoxy composite materials at 26 °C, −50 °C, −100 °C and −140 °C with strain-rate range from 1300 s −1 to 2100 s −1 by experimental study. A simple and effective cryogenic device was applied to the SHPB system to create the low-temperature field of the samples. It was found that the compression modulus, peak stress, failure strain and specific energy absorption of the 3-D braided basalt/epoxy composite materials had different sensitivity to temperatures and strain rates. In the out-of-plane impact, there were two failure modes, namely, compression-failure mode and shear-failure mode. Fracture of fiber tows was irregular with abundant pull-out of fiber and much finely-divided fragmentation of resin among fibers at room temperature. In cryogenic field, the fracture of fiber tows was neat and tidy with few pull-out of fiber and few finely-divided fragmentation of resin. However, in the in-plane impact, there was only compression failure mode. And there was no fracture of fiber tows and no big difference among samples tested under different gas pressures. Because of the function of squeezing and buckling, split-off separation of the composite could be blocked by the tangled fiber tows. As a whole, the reinforcement could still keep its structural integrity. [ABSTRACT FROM AUTHOR]

The strength of filled bolt holes of composites is an important parameter in developing the bearing bypass envelope curves for bolted joint structures in engineering design. This paper presents the procedures of tensile failure strengths and damage for a laminate with a filled bolt hole. Tests of two different cases, open‐hole tension (OHT) and filled‐hole tension (FHT), were performed and repeated seven times, respectively. A new type of virtual testing based on the LaRC05 criteria coupling with XFEM crack technique was developed to predict and analyze the crack initiation and early propagation around the bolt hole in the laminates. Two analytical methods were also adopted to predict the residual strength for a laminate after drilling. The results show that the thinner sample would present a higher strength with the same material system. It is also shown that the virtual test can well capture the failure modes, revealing that the 0° fiber has a natural function of crack arrest, the 90° layers that are the fracture pioneers with matrix tensile failure trigger the fiber breakage of 0° layers, and the failure of 45° layers occurs at the last. Additionally, an interesting phenomenon is observed that the initial failure around the filled hole edge does not affect the structural stiffness. Finally, a good agreement is achieved among the experimental, analytical and numerical results. It suggests that the results could provide a valuable guidance for the design of multi‐bolted joint structure. [ABSTRACT FROM AUTHOR]

This paper reports compression properties of three-dimensional carbon fiber/epoxy braided composites at temperatures from 23℃ to 210℃ under strain rate from 1200 s−1 to 2400 s−1. It was found that the elevated temperature has a negative effect on the compression properties, whereas the strain rate effect is positive. The compression modulus has a rapid degradation at the temperature of 120℃ which is close to the glass transition temperature of epoxy resin obtained from the dynamic mechanical analysis. The results also showed that the shear deformation is the main failure mode of three-dimensional braided composites at high strain rates of compression load and high temperatures. [ABSTRACT FROM PUBLISHER]

This paper reports the longitudinal compressive behaviour of 3D braided basalt fibre tows/epoxy composite materials under strain-rate range of 1,200-2,400 s and temperature range of 23-210 °C both in experimental and finite element analyses (FEA). A split Hopkinson pressure bar system with a heating device was designed to test the longitudinal compressive behaviour of 3D braided composite materials. Testing results indicate that longitudinal compression modulus, specific energy absorption and peak stress decreased with elevated temperatures, whereas the failure strain increased with elevated temperatures. At some temperatures above the T of epoxy resin, such as at 120 and 150 °C, strain distributions and deformations in fibre tows and epoxy resin tended to be the same. It results in relatively slighter damage status of the 3D braided composite material. The FEA results reveal that heating of the material due to the dissipative energy of the inelastic deformation and damage processes generated in resin is more than that in fibre tows. The braiding structure has a significant influence on thermomechanical failure via two aspects: distribution and accumulation of the heating leads to the development of the shear band paths along braiding angle; the buckling inflection segment rather than the straight segment generates the maximum of the heating in each fibre tows. The damage occurs at the early stage when the temperature is below T, while at the temperature above T, damage stage occurs at the rear of plastic deformation. [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]

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]

This paper focuses on the in-plane compression mechanism of two-dimensional biaxial braided composite laminates under quasi-static compression and dynamic compression. Three types of specimens with different off-axial angles in respect of OA00, OA60, and OA90 were employed for compression experiments. The surface damage process of the specimen was recorded using optical camera and infrared thermal imaging technology. The internal damage morphology of the tested specimens was analyzed by X-ray computed tomography. The progressive damage behavior of braiding yarns and matrix resin was simulated by a three-dimensional meso-level finite element model. Experimental and simulation results revealed that the off-axial angle significantly affects mechanical properties and damage mechanisms. The stress distribution and compression deformation of braiding yarns are affected by the boundary conditions of load-bearing yarn, which relies on the off-axial angle. The stress of OA00 specimen is concentrated at the interface of yarn and matrix resin. The failure mode of OA00 specimen is dominated by the shear band, which is induced by the interface microcracks between yarn and matrix resin. The stress of OA60 and OA90 specimens are concentrated at the undulation position of the braiding yarn. The critical failure mode of OA60 and OA90 specimens is delamination induced by interlaminar microcracks. The OA60 specimen achieves a better compression response with respect to bearing the load and absorbing the fracture energy because the sub-laminate that is formed after delamination can continue to bear the load until its intralaminar fracture occurs. [ABSTRACT FROM AUTHOR]

Non-crimp fabric reinforced polymer (NCFRP) composite is a suitable choice as structure material in aerospace use. One of the most important issues related to adiabatic failure is the correlation among temperature elevation, adiabatic shear band and failures of the material. In this study, impulsive damage and failure modes with temperature rise in biaxial NCFRP composite specimens were investigated under increased strain rates from 200/s to 2500/s. Path and distribution of temperature rise were experimentally characterized when the composite was subjected to in-plane and out-plane impulsive compression. It is found that (i) localized temperature can rise above a hundred degrees Celsius, reaching or even exceeding the T g of the resin matrix and polyester tricot in specimen, which causes stress concentration and subsequent failure in warp/weft fiber tows in localized positions. (ii) For the out-of-plane compression, the localized temperature rise indicates dispersed inter-layer shear deformation in the biaxial NCFRP composite by stress wave mismatch effect. With the increase of strain rate, intensive temperature rise is a good announcement for an adiabatic shear band quickly and thoroughly splitting the specimen. (iii) For the in-plane compression, slight impact loading triggers small debonding and cracking along warp/weft fiber tows or at the fiber-matrix interface by fiber tows microbuckling effect. With the increase of strain rate, the localized temperature rise reveals that single shear band is developed and further evolved into zigzag shear bands, while catastrophic delamination failure is the dominant mode at higher strain rate. Higher temperature values at kinking positions are caused by fiber tows buckling-fold effect. Thermal degradation effect imperils the polymeric matrix and tricot around the kinking positions where bridging cracking, obstructing the spread of adiabatic shear band, and leading to the pull-out/breakage/splitting damages of warp/weft fiber tows. (iv) This work indicates that high-speed thermographic technology is an important method for investigating the dynamic behavior of composite materials. Localized temperature rise is an effective indicator in complex correlation with the multiple physical interaction including the adiabatic shear band, matrix/tricot softening, fiber tows kinking, localized damages and failures. [ABSTRACT FROM AUTHOR]

This investigation is aimed to study the dynamic failure behavior of 3-D braided carbon/epoxy composite by capturing the high-speed infrared image and simulating the temperature-rise response under high strain-rate compression. It is found that localized temperature rise is a good indication in damage and failure of the 3-D braided composite. Localized temperature can rise above a hundred degrees Celsius, reaching or even exceeding the T g of the matrix resin and imperiling the polymer along damage regions in specimen. Temperature rise in localized paths of the specimens was caused by adiabatic shear along the boundaries of braided fibers or inside the fiber bundles. Sustaining decrease in stress after the peak strength is related to shear deformation, temperature rise and progressive damage. There is a correlation among localization of temperature rise, adiabatic shear band, resin softening, damages and failures in the 3-D braided carbon/epoxy composite under dynamic compression loading. For the out-of-plane compression, temperature always conducts from the broken interface to the inside of the fiber bundles along the radial, which will lead to the weakening of resin in fiber bundles. And damages, penetrating cracks and openings are verified with 'X'-shape localization of temperature rise. With the increase of strain rate, localization of temperature rise indicates that adiabatic shear bands begin to connect together to form larger and longer paths by the mutual shear effect among adjacent fiber bundles. For the in-plane compression, multi-cracks are found by observing the independent 'zigzag' shear bands converting to dependent 'zigzag' network. The temperature is easily localized at the crimp position of fiber bundle when they forming the 'zigzag' localization along the braiding paths of fiber bundles. However, some regions in 3-D braided carbon/epoxy composite have not experienced plastic shear deformation, which are isolated among 'zigzag' adiabatic shear network and their boundaries are the very paths for localized temperature rise. • Temperature rise in 3-D braided carbon/epoxy composite under dynamic loading is associated with damage and failure. • The localized temperature rise can be observed by high-speed infrared imaging and simulated by mesostructural modeling. • There is a correlation among localization of temperature rise, adiabatic shear band, resin softening, damages and failures in the 3-D braided carbon/epoxy composite. [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]

Accurately identifying various meso-morphological features and sub-phases within the damaged braided composite fabric is crucial for assessing the mechanical properties of composites under complex loading conditions and different processing parameters. However, obtaining 3D reconstructions that can be quantitatively analyzed is still a challenge, primarily due to the low contrast of the meso-morphological features (yarn tow and matrix crack networks). In this work, a new data enhancement algorithm is proposed to generate a realistic-looking artificial training dataset for enriching the information of the meso-morphological features. Then, the effect of various training networks and training parameters (the size of real and hybrid training dataset, number of epochs) on segmentation performance were examined. Finally, the meso-morphological features were statistically analyzed to accurately evaluate the mechanical properties of the composite material. The work presented here provides an effective tool that enables the quantitative analysis of the dynamic crack evolution process under the axial load of the composite materials. [Display omitted] [ABSTRACT FROM AUTHOR]

This paper presents experimental and numerical investigations into transient heat and thermal–mechanical responses of electrically conductive composite under the operating environment of solar-powered aircraft in range of ±120 °C. Four kinds of the flexible electrically conductive composites were manufactured and tested under quasi-static tensile and temperature environmental conditions, respectively. The Fiber Bragg Grating (FBG) technology was used to monitor the in-situ principal strain in composite materials at thermal steady-state condition. Based on its realistic geometry and periodic arrays, a representative volume elements (RVE) of the composite was constructed to investigate the thermal effect on the adhesive layer. The results show that the temperature effect mainly affects the interface of materials with different thermal expansion coefficients, and the more serious risk occurs at the initial process of transition from the moment of expansion to contraction. In addition, the plastic deformation range is one order of magnitude larger than the elastic deformation range and two orders of magnitude larger than the temperature deformation range in the case of 124° rotating thermal coupling analysis. It suggests that the results could provide a reliable reference for the design and lifetime evaluation of the flexible electrically conductive composites. [ABSTRACT FROM AUTHOR]

The helical spring is an essential mechanical component of automobile and machinery and has been widely used in industrial application. The weight reduction is the requirement of current material design and also the research trend for material development. In this study, a novel composite helical spring with "skin‐core" structure was designed and fabricated with an integrated formation process based on the vacuum‐assisted resin infusion technology. Based on the different function of skin and core, the skin structure was weft knitting tube made of aramid and ultra‐high molecular weight polyethylene fiber, and the core structure was unidirectional carbon fiber. By selecting material for knitting tube and adjusting the volume content of carbon fiber core, the static and fatigue compression properties of composite helical spring were investigated. It has been found that the spring torsion and bending performance can be improved by wrapping the core with the knitting tube as the spring wire. Moreover, the finite element method was applied to further analyze the stress evolution of the "skin‐core" composite helical spring during compression and fatigue. The results of compression test and simulation show that the spring stiffness can be effectively improved by using knitting tube and increasing the content of carbon fiber. The stress of the spring cross section is concentrated in the internal radial region of 180°, and the distribution of the corresponding cross sections is relatively uniform. The "skin‐core" structure with aramid knitting tube as the outer skin and carbon fiber as the spring core significantly improves the load‐carrying capacity and stability of the composite spring. The proposed lightweight helical composite could provide a replacement strategy of existing metal or unidirectional filament reinforced springs used in the engineering applications. [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]

Accurate 3D representations of multiphase layered composite fabric, including distinguish and label the pores, matrices, warp and weft yarns, can assist in understanding and ultimately improving composite material design performance. In this work, a new data enhancement algorithm is proposed to generate realistic-looking artificial learning datasets for expanding the datasets. Then, we demonstrate how supervised, the Swin Transformer approach can help to realize accurate segmentation of low-resolution and poor-contrast fabric datasets by populating databases with real and artificial learning datasets. Most of the pixels were correctly identified and the boundaries of the yarn could be identified clearly, except for some spots and a few merged boundaries. The work presented here provides an effective tool that can be widely used in semantic segmentation for various composite materials (woven composites, fiber-reinforced composites, short-fiber materials, etc.), which enables higher-precision reconstruction of fiber-composites and significant reduction in dataset processing time, and provides a route to generating artificial learning dataset. [ABSTRACT FROM AUTHOR]

• The usage of correlation between maximum temperature and peak load. • The reconstruction of CT images identifies the debonding distribution and degree. • A novel inspection way of combing infrared imaging and micro CT reconstruction methods. • The role of axial yarns in axial crushing. In this work, the effect of axial yarns on damage behavior of two-dimensional triaxially braided composite(2DTBC) tubes subjected to axial compression was investigated. A novel damage detection method combining infrared thermography and CT scanning was presented. In the area without axial yarns, yarn crossovers were separated from each other due to interlacing patterns. Axial compression caused the braiding yarns to shear deformation like scissors. Inter-yarn de-bonding appeared in crossovers and could not connect. Consequently, it caused a progressively folding failure mode. In contrast, local buckling occurred on axial yarns because of their primary loading behavior. Cracks along the axial yarns formed, which would combine with debonding crossovers caused by the sheared braiding yarn. It resulted in significant inter-layer delamination and consequently splaying failure mode. The energy absorption of composites tube with 18 axial yarns increased by 79.9% at the cost of 27.8% fiber usage more than 3 axial yarns. [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 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]