This study presents an investigation into the energy absorbing characteristics and failure mechanisms in composite nR multicellular tubes inspired by the macro- and micro-structure of a lotus stem. A 1/n three-dimensional progressive damage model is developed using a user-defined VUMAT subroutine within the Abaqus/Explicit platform, with material parameters being obtained through mechanical property tests. Numerical simulations demonstrate that the 1/n finite element model provides significant computational efficiency improvements over full-scale models, while maintaining a high degree of accuracy. Furthermore, the three-dimensional progressive meshing strategy outperforms conventional global meshing in balancing computational precision and efficiency. A damage degree parameter is introduced to quantitatively assess material degradation. The results reveal a strong correlation between the damage severity and the load-bearing curve, with the degree of fibre damage exhibiting a significant influence on the energy absorbing characteristics. Subsequently, the effects of geometric parameters on the energy absorption characteristics of composite multicellular tubes are analysed, including rib number (n) and inner diameter (d). Parametric studies highlight a coupled interaction of both parameters on energy absorption. The findings highlight excellent energy absorption characteristics, in cases when the inner diameter and the number of ribs are small, and when they are large.

In situ consolidation automated fiber placement of thermoplastic composites (ICAT) is one of the key engineering technologies for the widespread application of environmentally friendly materials in important fields such as aerospace. Nevertheless, the high melting point and low viscosity characteristics of high-performance thermoplastic composites such as PEEK are key challenges limiting the large-scale industrialization of ICAT technology. As a consequence, researching the defect formation mechanism and suppression methods of the interlayer fusion process of ICAT technology in new high-performance low-melting-point thermoplastic matrix composites is an important direction for promoting the large-scale application of thermoplastic composites. Hence, this paper investigated the ICAT in situ forming process of low-melting-point polyaryletherketone matrix (LM/PAEK) composites. The temperature history was analyzed using a high-frequency temperature acquisition system. Simultaneously, the Taguchi algorithm was used to analyze the coupled effects of multiple consolidation parameters. The results indicate that consolidation temperature is the most significant factor affecting void and interlayer performance. The consolidation parameters primarily influence the extrusion and permeation behavior of the matrix. Furthermore, the optimized consolidation parameter combination is as follows: consolidation temperature of 400 degrees C, consolidation speed of 100 mm/s, and consolidation force of 300 N. Additionally, under the optimized consolidation parameter combination conditions, the porosity and interlayer shear strength were 1.42% and 39.53 MPa, respectively. The porosity was reduced by 78.39%, and the interlayer shear strength was increased by 50.59%.

The fabrication of complex hollow structures with variable cross-sections and curvatures remains challenging due to the limited deformability and poor demolding performance of conventional rigid molds. This study proposes a novel strategy that integrates 3D printing of continuous fiber-reinforced smart mold with a dimension-reduction wire-drawing demolding method. A photothermal dual-crosslinked polymer network was developed to enable reversible stiffness modulation, ensuring high rigidity during molding and flexibility during demolding. To optimize fabrication performance, a machine learning framework based on gradient boosting regression was employed to model and analyze the influence of key printing parameters on both mechanical strength and dimensional accuracy. Using this approach, smart mold with a bending strength of 620.71 MPa and a printing error of 2.46 % were successfully fabricated. The method was further validated through the forming and demolding of representative geometries, including dumbbell-shaped and variable-section components. Results confirm the feasibility and robustness of the approach under extreme forming conditions. This digitally driven, material-process-structure integrated solution offers broad application potential for the precision manufacturing of complex hollow composite structures in solid rocket motors, aerospace engineering, and architectural fabrication.

Aiming at reducing the warpage that occurs during in-situ consolidation automated fiber placement (ISC-AFP) of thermoplastic composites, this article explores, for the first time, the through-thickness crystallinity distribution characteristics of laminate and its effect on warpage. A tool-temperature-compensation isothermal consolidation (TTC-IC) technology is proposed to achieve warpage suppression. The research results indicate that laminate crystallinity decreases gradually from the bottom to the top layer. At a fixed initial consolidation temperature, higher tool temperatures (Ttool) reduce the through-thickness temperature gradient during consolidation. However, as Ttool increases, the through-thickness crystallinity gradient of the laminate also increases, resulting in greater warpage. This indicates that crystallinity gradient dominates the warpage. Via the proposed TTC-IC technology, the through-thickness crystallization gradient of the laminate was reduced, achieving a reduction of 33.33% in warpage and 46.1% in porosity. This study addresses the knowledge gap in warpage formation mechanisms for laminates fabricated via ISC-AFP and introduces a novel in-situ technological approach for warpage control.

This study investigates the flexural behavior and failure mechanisms of 3D-printed continuous carbon fibre-reinforced polylactic acid (CCF/PLA) composites with varying thicknesses under three-point bending. Experimental results show that the peak load increases exponentially with increasing thickness, while the displacement at failure decreases, indicating a transition from ductile to brittle behavior. Macroscopic observations and X-ray tomography reveal a progressive change in failure modes, from matrix deformation and localized stress concentration in thin specimens to extensive matrix cracking, fibre pull-out, and fibre fracture in thicker specimens. The proportion of the damaged region through the thickness increases from approximately 27 % in 4-layer specimens to nearly 86 % in 20-layer specimens. A finite element model incorporating anisotropic material properties and interlayer interfaces is developed to simulate the bending response. Numerical results agree well with experiments and provide insight into stress redistribution and shear-dominated deformation in thick specimens. These findings highlight the critical role of thickness in flexural performance and failure evolution, offering guidance for the design and optimization of additively manufactured fibre-reinforced composites.

To address the problem of insufficient toughness of carbon fiber (CF) reinforced thermosetting epoxy (EP) composites, a discontinuous fiber/thermoplastic PA12 particle co-toughened CF/EP composite laminate was designed in this paper to maximize its toughness while considering its high strength advantages. Firstly, inspired by the strong and tough biological structures in nature, a discontinuous fiber/interlayer toughened CF/EP laminate structure and its forming process were proposed. Secondly, a three-point bending (3 PB) finite element simulation of the discontinuous fiber laminate was conducted to investigate the influence of the discontinuous fiber structural parameters on the mechanical properties. The optimized design of the discontinuous fiber toughened laminate structure was hence carried out based on machine learning. Then, the interlaminar fracture toughness, interlaminar shear strength, and 3 PB tests of interlaminar toughened laminates were carried out to investigate the influence of the type of interlaminar toughened material and its density on the mechanical properties of the laminates. Finally, it was experimentally demonstrated that the discontinuous structure and interlaminar toughening method yielded a joint toughening effect.

Uncured Unidirectional (UD) thermoset prepregs are extensively used in advanced automated composite manufacturing processes, such as the Automated Fiber Placement (AFP) technique. Pre-consolidation is a common method for preparing uncured specimens, which is crucial for testing in-plane properties needed in numerical simulations of AFP. We aim to identify a reliable process window for vacuum pre-consolidation by examining its impact on the in-plane performance of prepregs, using uniaxial tensile and in-plane shear tests. Two key process parameters, temperature and duration, were investigated. The results show that heating during pre-consolidation significantly improves testing consistency, while excessive treatment can lead to overestimated data. Ultimately, the recommended pre-consolidation process windows are established based on the test data.

This study investigates the effect of bi-axial preloading (tension and compression) on the low velocity impact behaviour of GLARE (GLAss REinforced laminate) through both experimental testing and numerical simulations. In this study, a bi-axial preloading apparatus has been integrated into a conventional drop-weight impact system, coupled with high-speed three-dimensional digital image correlation, to quantify the full-field deformation profile of the plate. The experimental results demonstrate that tensile preloading enhances the stiffness of the laminate as well as the maximum impact load, but reduces the out-of-plane displacement, the impact duration and the overall level of energy absorption. In contrast, compressive preloading results in effects that run counter to those mentioned above. A finite element model involving a user-defined subroutine VUMAT has been developed, which successfully reproduced the failure modes in the preloaded panels. Discrepancies between the experimental and numerical predictions were within 13 %. The numerical analysis revealed that preloading modifies the damage modes within the laminates, wherein tensile pre-loading reduces delamination, but increases the level of fibre and matrix damage. In contrast, under 7.5 J impact energy, compressive preloading induces a more complex response, i.e. Al-GF debonding is reduced, whereas GF-GF delamination is enhanced. The net effect is dominated by the debonding reduction, resulting in an overall decrease in total delamination. Further, preloading leads to a redistribution of the in-plane stresses, thereby influencing the ability of the FMLs to absorb and dissipate impact energy, it also changes the impact response and damage characteristics of the GLARE laminates. It is believed that the current study provides an insight into the impact response of pre-stressed hybrid materials.

Fiber-reinforced thermoplastic composites (FRTP), characterized by their lightweight nature, high specific strength, and recyclability, are increasingly adopted as core materials for structural components in aircraft and spacecraft. They are driving its leapfrog development from structural weight reduction to functional integration. Ultrasonic welding, as an efficient, clean, and automation-friendly technique for joining method, has become a focal point attracting significant attention from researchers. This review presents a comprehensive analysis of the ultrasonic welding of FRTP from four interrelated dimensions: interfacial bonding mechanisms, welding processes, defect influencing factors, and performance regulation strategies. Although a complete ultrasonic welding process involves both frictional heating and viscoelastic heating, it can be meticulously divided into five distinct stages. Quantitative findings from recent studies are summarized: under ultrasonic spot welding process parameters (amplitude 50-85 mu m, welding energy 500-1500 J, and welding time 0.3-1.5 s), lap shear strengths of 20-35 MPa have been achieved for FRTP, while innovative energy directors (EDs) can further enhance joint strength by 20-40 %. Continuous ultrasonic welding holds significant development potential. Although it can achieve a maximum welding speed of 3.6 m/min, it still faces challenges such as localized overheating and porosity, resulting in joint strength lower than that of spot welding. The review identifies that controlling interfacial melting behavior and resin flow is crucial for defect suppression and performance optimization. Strategies such as welding parameters optimization, innovative ED design, and the application of auxiliary processes are discussed as promising avenues to enhance weld integrity and reproducibility. Finally, key research directions are proposed, including continuous ultrasonic welding for curved thick plates, multiscale modeling of ultrasonic energy transmission, and intelligent process monitoring.

Lightweight energy-absorbing structures are crucial for enhancing crashworthiness in transportation vehicles, where managing impact loads and mitigating occupant injury risk are paramount. Inspired by the fibre-reinforced composite architecture and unique segmented nodes of natural bamboo, this study proposes a Bamboo Bio-inspired Composite Column (BBCC) fabricated from carbon fibre/epoxy composite via vacuum bag moulding. A comprehensive experimental and statistical investigation is conducted to evaluate the effects of key reinforcement parameters-internode lengthl, reinforcement axial length d, and ply number n-on crashworthiness performance of BBCC. Quasi-static compression tests reveal that these parameters critically control failure modes, ranging from stable progressive crushing, local buckling to interactive bilateral failure. Main-effect and ANOVA analyses quantify that n is the dominant factor for energy absorption capacity (specific energy absorption (SEA) and mean crushing force (MCF)) and crushing force efficiency (CFE), while d and its interaction with l govern the initial peak crushing force (IPCF). Entropy-based TOPSIS multi-objective optimisation resolves the inherent trade-off between maximising SEA and minimising IPCF, yielding the optimal design achieving an exceptional balance: a high SEA of 53.04 kJ/kg, a controlled IPCF of 28.84 kN, and an outstanding CFE of 84.4%. This study provides valuable reference for the design, analyse, and optimisation on lightweight, high-performance energy-absorbing components.

This study investigates the multi-factor coupling effects on the deposition quality of 3D-printed continuous carbon fiber reinforced thermosetting epoxy composites. Utilizing Response Surface Methodology (RSM) combined with a Box-Behnken Design (BBD), the synergistic impacts of printing temperature, speed, spacing, and layer height on flexural properties were systematically evaluated. The established high-precision quadratic regression models (R-2 > 0.98) revealed that internal porosity and fiber damage are predominantly governed by strong "saddle-shaped" coupling effects: the geometric matching between printing spacing and layer height, and the rheological compensation between temperature and speed. By correlating 3D response surfaces with scanning electron microscopy (SEM) and micro-computed tomography (CT) analyses, the physical mechanisms of defect formation under parameter mismatches were systematically elucidated. Multi-objective optimization determined a global optimal process window (printing temperature (T) = 120 degrees C, printing speed (V) = 500 mm/min, printing spacing (S) = 1.3 mm, and layer height (H) = 0.4 mm). Validation experiments demonstrated excellent predictive accuracy for flexural strength (604.97 MPa) with a relative error of only 2.1%. Notably, this optimal configuration minimized the internal void content to an ultra-low level of 2.3% and effectively suppressed premature delamination, resulting in an actual flexural strength increase of 27.7% compared to the unoptimized baseline. Meanwhile, the observed deviation in flexural modulus was theoretically linked to deposition-induced micro-level fiber waviness.

This study investigates the tensile properties of composite scarf repairs reinforced by a stitching technique using a thick carbon fibre thread, through both experimental and numerical approaches. Stitched scarf joints were designed and manufactured using carbon fiber prepreg sheets and tapes. Tensile tests were subsequently conducted to evaluate the stiffness, strength characteristics, and failure mechanisms of the stitched repairs. The effects of scarf angle and stitching thread diameter on joint performance were also examined. In addition, finite element models of the stitched joints were developed to predict their mechanical responses under tension, and to investigate the corresponding damage mechanisms. The results demonstrate that the stitching technique significantly enhances both the load-carrying capacity and stiffness of the repairs, with the diameter of stitching thread playing a critical role in determining joint performance and failure modes. The numerical simulations show strong agreement with experimental measurements, offering deeper insight into failure mechanisms that are challenging to observe during testing. Thus, a thick thread stitching is an effective method for enhancing the performance of scarf repairs, though the stitching thread diameter must be carefully selected to optimize outcomes.

Combining topology optimization with continuous fiber 3D printing offers a promising route to lightweight design of continuous fiber-reinforced polymer (CFRP) composites. However, conventional methods aligning fibers with principal stresses often cause fiber discontinuities and stress mismatches at biaxial junctions, limiting overall structural integrity. In biological junctions such as wood branches and bamboo nodes, fibers provide superior strength and toughness through interwoven and interlocking arrangements. This study proposes a synergistic framework for the design and fabrication of CFRP structures. The framework integrates dual-stressdriven Solid Orthotropic Material with Penalization (SOMP) topology optimization, stress-guided structural decomposition and path reorganization, and a bio-inspired woven junction printing strategy. By matching biaxial stresses, orthogonal fiber orientations are assigned and converted into manufacturable continuous toolpaths. At junctions, the bio-inspired sinusoidal woven fibers create macroscale interweaving with microscale interlocks. These features enhance stress transfer and shear resistance. The effectiveness of the framework was validated using the Messerschmitt-Bo & uml;lkow-Blohm (MBB) beam. Compared with the non-crossing SOMP-optimized MBB baseline, the laminated orthogonal junctions increased the specific load and specific stiffness by 46.18% and 37.14%, respectively. Bio-inspired woven junctions achieved superior enhancements of 96.04% and 68.27%. Strain and failure analyses revealed that the woven bundle interlocks establish stress-transfer pathways. This mechanism reduces stress concentrations and prevents junction delamination and fracture. Overall, this framework provides a practical design-to-manufacture route for high-performance 3D printed CFRP composite structures.

To address the tedious assembly of traditional beam-rib UAV wings and meet the urgent demand for integrated and efficient manufacturing of complex structures in fixed-wing UAVs, this study, inspired by the longitudinal and transverse vein distribution of dragonfly wings, proposes a novel variable-stiffness integrated UAV wing skeleton structure. Combined with the nonlinear load distribution along the wing span, a three-stage gradient variable-stiffness configuration from wing root to wing tip is designed. The main load-bearing wing beams and cross ribs form an integrated network load-bearing architecture, simulating the multi-path collaborative force transmission mechanism of dragonfly wings to optimize load transfer pathways. Continuous fiber-reinforced composite (CFRP) 3D printing technology is employed to achieve the integrated manufacturing of the bionic wing skeleton with ±45° cross-fiber trajectories. Performance verification is conducted through quasi-static tests and field flight tests. The results show that the measured wingtip deflection is 20.07% lower than the simulated value, and the structural stiffness meets the requirements of reliability design. The study confirms that the bionic variable-stiffness design inspired by dragonfly wing veins, combined with continuous fiber 3D printing technology, can efficiently achieve lightweight, high-performance, and integrated manufacturing of UAV wings, providing important reference and technical support for the engineering application of bionic structural design and additive manufacturing in the aerospace field.

Lightweight composite energy-absorbing structures are increasingly required in aerospace vehicles, where in-service low-velocity impacts may compromise their residual crashworthiness and damage tolerance. This study investigates the damage behaviour and residual crashworthiness of lotus stem-inspired multi-cell composite tubes containing transverse impact damage using a combined experimental and numerical approach. An efficient residual-performance prediction framework is developed by employing an impact damage-mapping technique, which enables the accurate transfer of intralaminar damage, interlaminar delamination, and permanent deformation between different analysis models. To further elucidate the energy-absorption mechanisms during impact and subsequent axial crushing, energy absorption and dissipation calculations are integrated into a VUMAT subroutine. The finite element model demonstrates a high level of predictive accuracy for both the transverse impact response and the post-impact compressive behaviour. Using this framework, the effects of circumferential and height impact locations on residual performance are systematically examined. The results show that transverse impact loading markedly affects the energy-absorption characteristics of composite tubes, while the multi-cell configuration reduces the degradation caused by prior impact damage. The residual energy-absorption capacity of the circular tube decreases by approximately 61 %, whereas the multi-cell tube exhibits a more moderate reduction of approximately 23 %. During the impact phase, interlaminar damage dominates energy dissipation, whereas during axial crushing intralaminar failure represents the primary energy-absorbing mechanism. Although circumferential impact locations produce different local responses, their effect on global residual performance is limited; in contrast, impact height has a significant influence on the remaining load-bearing capacity and ensuing failure modes. The proposed framework provides a methodological basis for the damage-tolerant design of lightweight aerospace composite energy-absorbing structures.

This study investigates the performance of repaired composite panels under low-velocity impact, as well as their compression-after-impact (CAI) behavior. Damaged composite laminates were repaired by single-patch and double-patch configurations. Low-velocity impact tests were then conducted, followed by damage assessments. Subsequently, CAI tests were performed to evaluate the influences of impact energy, patch size and repair configuration on the load bearing capacity and failure modes of the repaired panels. A finite element model, incorporating continuum damage mechanics, was established by considering three-dimensional composite failure and interlaminar damage. A user-defined subroutine VUMAT and a two-step analysis were implemented in Abaqus/Explicit to simulate the impact and the CAI responses. Experimental results reveal that single-sided repairs have a higher out-of-plane displacement resistance, with only limited damage being observed. However, double-side repairs exhibit a significant damage after impact. Additionally, debonding occurs on the rear side of double-sided repair. CAI results indicate that the failure loads of most double-sided repairs are higher than those of single-sided repairs. The primary failure mode in the repaired structures is featured with laminate cracking propagated from hole edge. The numerical results show that the current model can efficiently predict the dynamic responses of the repaired structures subjected to impact, as well as damage modes. In addition, the CAI results obtained from the FE model also correlate well with the experimental ones.

This study investigates the dynamic shear behavior of carbon fiber-reinforced composite single-lap joints assembled with protruding-head hi-lock bolts by comparing single-bolt single-lap (SBSL) and double-bolt single-lap (DBSL) configurations under quasi-static loading and dynamic loading speeds of 0.02, 0.2 and 2 m/s. Based on load-displacement responses, high-speed imaging/DIC, X-ray CT and SEM observations, a tangent-stiffness-based six-phase failure framework was developed for SBSL joints, and four combined dynamic failure modes were identified: bearing-pull-out, bearing-shear-out, bearing-tear-out and slash-tear-out. The results show that the initial stiffness and peak load of SBSL joints varied by only 6% over the tested speed range, whereas the failure displacement increased from 15.05 to 27.68 mm and the energy dissipation increased by 37%; compared with SBSL joints, DBSL joints increased the mean peak load from 12.9 to 25.4 kN, corresponding to a 1.97-fold increase, but led to more abrupt tear-out or sequential bearing-pull-out failures, indicating that bolt configuration mainly governs the dynamic failure path and energy-dissipation mechanism, while the initial stiffness and peak strength are less sensitive to loading speed.

This study investigates the damage behaviour and residual crashworthiness of multi-cell composite tubes containing low-velocity impact damage, using a combined experimental and numerical approach. An efficient prediction framework for residual-performance is developed by employing an impact damage–mapping technique, which enables the accurate mapping of intralaminar and interlaminar damage as well as permanent deformation between different analysis models. To further elucidate the energy-absorption mechanisms during impact and subsequent axial crushing, energy absorption and dissipation calculations are integrated to a VUMAT subroutine. The finite element model demonstrates a high level of predictive accuracy when predicting both the transverse impact response and the post-impact compressive behaviour. Using this framework, the effects of transverse impacts on circumferential positions and height locations on the residual performance are systematically examined. The results show that transverse impact loading affects the energy-absorption characteristics of the composite tubes, with the multi-cell configuration exhibiting potential for reducing the effect of prior impact damage. The residual energy-absorption capacity of the circular tube decreased by approximately 61 %, whereas the multi-cell tubes suffer a more moderate reduction of approximately 23 %. During the impact phase, interlaminar damage dominates energy dissipation, whereas during axial crushing intralaminar failure represents the primary energy-absorbing mechanism. Although impact at circumferential locations results in different local responses, its effect on the global residual performance is limited. In contrast, impact at different height locations has a significant influence on both the remaining load-bearing capacity and the ensuing failure modes. The proposed framework provides a robust methodological basis for both the damage-tolerant assessment and the crashworthiness design of composite multi-cell thin-walled structures.