Search Results (127)

Carbon-fiber-reinforced polymer (CFRP) composites fabricated with bio-based epoxy resins represent a promising approach for sustainable lightweight structures produced by out-of-autoclave (OoA) technologies. The curing schedule influences the state of the epoxy matrix and, consequently, can affect the fiber–matrix interaction and laminate performance. This study investigates the effect of practical curing conditions on 2 × 2 twill woven carbon fiber laminates fabricated by vacuum infusion using a commercially available bio-based epoxy resin IB2. The manufacturer’s recommended room-temperature conditions (25 °C for 24 h) were compared with accelerated mold heating schedules at 40, 50, 60, and 70 °C for 12 h. The laminates were characterized by three-point tensile and flexural tests, heat deflection temperature (HDT) measurements, differential scanning calorimetry (DSC), and SEM fractography. The tensile response showed limited sensitivity to the investigated curing conditions, with mean tensile strengths ranging from 634.01 to 672.95 MPa; T60 exhibited the highest mean numerical tensile strength (672.95 ± 53.60 MPa) and tensile modulus (53.39 ± 11.53 GPa), although the differences were small relative to the experimental spread. In contrast, the flexural response was more sensitive to the processing conditions. T70 exhibited the highest average flexural strength (980.60 ± 129.03 MPa), strain at maximum flexural stress, and strain energy density to maximum stress (8.75 ± 1.89 MJ/m³). The heat deflection temperature (HDT) systematically increased from 65.20 °C for T25 to 85.83 °C for T70. DSC revealed clear differences in the calorimetric response after curing during the first heating cycle, while the glass transition temperatures at the middle of the second heating occupied a relatively narrow range of 80.9–85.6 °C. SEM fractography revealed mixed tensile failure mechanisms related to fibers, matrix, and interface under all curing conditions. Overall, the results demonstrate that accelerated 12 h heated mold programs can reduce cure time while maintaining tensile performance generally comparable to the 24 h room-temperature IB2 reference condition and providing higher average flexural performance and thermal deformation resistance under load. These findings establish processing–property relationships relevant to the development of biomass-based CFRP OoA laminates for lightweight aerospace applications. Full article

Carbon-fiber-reinforced polymer (CFRP) composites are widely used in lightweight aerospace structures because their mechanical performance can be adapted to different structural requirements through appropriate manufacturing conditions. This study investigates the influence of curing temperature applied using temperature-controlled heated molds on the mechanical and thermo-mechanical behavior of vacuum-infused CFRP laminates manufactured with an IN2 epoxy infusion resin. Laminates were cured at room temperature (25 °C) and at 40, 50, 60, and 70 °C using heated molds. Their performance was evaluated by tensile testing, three-point bending, and heat deflection temperature (HDT) measurements. The highest tensile strength (675.06 MPa) was obtained for laminates cured at 40 °C, whereas increasing the curing temperature beyond this value did not provide further improvement in tensile performance. The highest flexural stress at the first peak (983.36 MPa) and flexural modulus (67.17 GPa) were obtained for laminates cured at 70 °C, while the highest energy absorption during bending (0.57 J) was measured for laminates cured at 40 °C. The HDT increased from 59.77 °C for room-temperature curing to 87.60 °C for laminates cured at 70 °C, indicating improved thermo-mechanical stability with increasing curing temperature. The results indicate that no single curing temperature simultaneously maximized the tensile, flexural, and thermo-mechanical properties. Instead, the optimum curing temperature depended on the specific mechanical and thermo-mechanical requirements of the intended application. The results further indicate that controlling the temperature of heated molds during manufacturing provided a practical approach for tailoring the mechanical and thermo-mechanical performance of CFRP laminates without modifying the reinforcement architecture, laminate stacking sequence, or constituent materials. Full article

Hybrid composites combining natural and synthetic fibers have emerged as promising materials for lightweight ballistic protection systems due to their ability to balance mechanical performance, energy absorption, and sustainability. This study investigates the influence of stacking sequence on the ballistic response of epoxy hybrid composites reinforced with raffia and carbon woven fabrics subjected to 9 mm projectile impact. Four stacking-sequence configurations were investigated: alternating laminates (R₂C₂)₃ and (C₂R₂)₃, and block laminates R₆C₆ and C₆R₆, all containing identical reinforcement contents. Ballistic response was evaluated through impact and residual velocities, velocity reduction, absorbed energy, energy absorption efficiency, momentum reduction, and Doppler radar velocity profiles. All laminates were completely perforated but maintained their structural integrity after impact, without catastrophic fragmentation. The results showed that architectures with carbon fiber layers positioned at the impact face, namely (C₂R₂)₃ and C₆R₆, exhibited the lowest residual velocities and the highest absorbed energies, reaching 136.8 J and 135.4 J, respectively. Energy absorption efficiencies ranged from 16.6% to 19.1%, indicating similar ballistic responses among all configurations. Statistical analysis revealed no significant differences among the investigated architectures (p > 0.05). Overall, the results indicate that the impact-face reinforcement exerts a secondary influence on ballistic response, whereas the total reinforcement content governs energy dissipation during projectile penetration. Full article

Aramid nanofibers (ANFs) inherit the exceptional properties of the bulk counterparts, while introducing a large specific surface area and excellent processability. However, the strong inter-fibrillar interactions and tendency to agglomerate hinder their high-content incorporation into polymer matrices. Here, we report a one-pot reprotonation–compounding strategy for fabricating aramid nanofiber-reinforced poly(vinyl alcohol) (ANF–PVA) composite films, where the ANF dispersion was mixed with polymers during the protonation process to form a continuous 3D network in the ANF-PVA film. The optimized film with a loading of 20 wt% ANFs exhibited a tensile strength of 122.2 MPa and a toughness of 28.36 J m⁻³. Furthermore, the high ANF loading enabled versatile applications. A robust ANF-PVA hydrogel, prepared via salt-induced gelation, delivered 132% enhancement in tear strength and 38.2% increase in cyclic compressive strength compared with the PVA hydrogel. Moreover, inspired by the “brick-and-mortar” architecture of natural nacre, the fully organic ANF-PVA film was incorporated into carbon fiber/epoxy laminates, with a configuration of one nacre-inspired film per five prepreg plies, achieving a 36.5% improvement in impact toughness and minimal loss in flexural strength. This scalable reprotonation–compounding approach provides a general route for producing high-loading ANF-based composites, paving the way for the broader utilization in advanced materials. Full article

The wood-based grid sandwich structure with a high load-to-mass ratio and specific strength was prepared with the core of KH-560-modified jute (Corchoruscapsularis) fabric-reinforced epoxy laminated composite (JFRELC). The compressing behavior and energy absorption characteristics of pure grid cores (GC50#, GC80#) and grid sandwich structures (GS50#, GS80#) were analyzed and compared. The failure mechanism of the fracture surfaces of jute fabrics of grid sandwich cores was clarified by SEM. The results showed that the core made of JFRELC-80# had a good performance for the grid sandwich structure by tenon-and-mortise linking. The load-bearing capacity and energy absorption performance of this wood-based grid sandwich structure can be comparable to that of some glass and carbon fiber reinforced composite sandwich structures, and even show certain advantages. The failure modes of the grid sandwich structure were panel cracking, core buckling and core collapse. The failure mechanisms of jute fabrics in epoxy resin were fiber pull-out and fiber splitting. Full article

The in-plane shear performance of carbon fiber-reinforced polymer (CFRP) composites is critical for structural design but is challenged by significant property scatter. This study aims to achieve individualized prediction of the complete shear stress–strain curve for each composite specimen using only a single early-stage digital image correlation (DIC) strain field. Systematic in-plane shear tests were conducted on 45 laminated carbon fiber/epoxy specimens with synchronized full-field DIC data and macroscopic load–displacement records. A lightweight encoder–decoder convolutional neural network was developed, taking a single DIC strain contour map at 0.2% global strain as input and mapping it directly to the full-range stress–strain curve up to failure for that specific specimen. Data augmentation and Dropout regularization mitigated the small-sample challenge. The proposed model achieved strong predictive performance across the five-fold cross-validation yielded a mean R² of 0.926 ± 0.022 and a mean RMSE of 6.37 ± 1.14 MPa for stress. Individual specimen predictions on the test set yielded an average R² of 0.945, with a minimum of 0.821, confirming robust capability across scattered properties. Residual analysis elucidated error characteristics across deformation stages. This research provides a novel paradigm for non-destructive, early-stage individualized assessment of composite mechanical properties, with applications in structural health monitoring and probabilistic design. Full article

Adhesive joints typically require high safety factors, as their mechanical performance is highly sensitive to environmental and manufacturing variations. Health monitoring can reduce these safety factors by continuously assessing the condition of the joint. While intrinsic and extrinsic sensing approaches exist, they are often based on periodic inspection or manual sensor integration, which limits their suitability for continuous in-service monitoring. This study investigates a novel sensor placement using additively manufactured strain sensors deposited by jet dispensing across the adhesive gap. Tensile lap-shear specimens were fabricated using CFRP (carbon-fiber-reinforced plastic) laminate, an epoxy adhesive, and silver-ink strain sensors placed internally within the joint and externally across the adhesive gap. Mechanical testing revealed that externally printed sensors produced an average resistance change of 65.3% near the failure stress of the adhesive joint, an order of magnitude higher than sensors embedded within the adhesive layer with 6.6% average resistance change. However, the average coefficient of variation increased as well, from 7.6% for internal to 32.6% for external. This sensor response exceeds reported environmentally induced variations in printed sensors and thus represents a promising candidate for condition monitoring. Further work is required to demonstrate actual damage detection capabilities and assess long-term stability under environmental and cyclic loading conditions. Full article

Interlaminar fracture toughness (ILFT) is a key factor governing the damage tolerance and service reliability of carbon fiber-reinforced polymer (CFRP) laminates. This study aims to clarify how the deformation capability of epoxy resin affects the Mode I and Mode II ILFT of carbon fiber/epoxy laminates under comparable fiber, resin-content, and laminate-configuration conditions. Two epoxy systems were compared: a high-strength/high-modulus (HSHM) resin system, designated as Group B, and a high-toughness (HT) resin system, designated as Group T. Neat resin castings were characterized by tensile and flexural tests, and the corresponding CFRP laminates were evaluated using double cantilever beam (DCB) and end-notched flexure (ENF) tests. Although Group T showed slightly lower tensile strength and modulus than Group B, its elongation at break increased from 4.0% to 6.5%, corresponding to an increase of approximately 62.5%. The Mode I ILFT (GIC) increased from approximately 279 J/m² for Group B to 487 J/m² for Group T, while the Mode II ILFT (GIIC) increased from approximately 530 J/m² to 708 J/m², corresponding to improvements of approximately 74.6% and 33.6%, respectively. Scanning electron microscopy (SEM) observations indicated that Group T promoted more resin-covered fibers, resin tearing, crack-tip blunting, crack deflection, shear deformation features, and crack-path reconstruction. These results indicate that, within the present two-system comparison, resin ductility-related deformation capability and local crack-tip deformability should be considered together with strength and modulus when evaluating interlaminar crack resistance. The toughening effect also showed loading-mode dependence, with Mode I improvement mainly related to crack-tip blunting and resin tearing, whereas Mode II improvement was mainly associated with matrix shear deformation, resistance to interfacial sliding, and crack-path deflection. Full article

Carbon fiber-reinforced polymer (CFRP) composites are widely used in engineering applications due to their high strength-to-weight ratio and corrosion resistance. However, their mechanical performance depends strongly on laminate architecture, processing conditions, and environmental exposure. This study investigates the effects of fiber orientation, thermal post-curing, and corrosive SO₂ atmosphere on the mechanical properties of CFRP laminates. Three-layer carbon/epoxy laminates with 90°, 45°, and [90°/45°/90°] fiber orientations were manufactured by vacuum-assisted lamination. Selected specimens were post-cured at 80 °C for 10 h and exposed to sulfur dioxide according to ISO 3231. Tensile and Charpy impact tests showed that the 90° laminate exhibited the highest tensile strength (484 MPa), whereas the 45° laminate showed the lowest value due to shear-dominated load transfer. Post-curing increased tensile strength by approximately 10–30%, while exposure to the corrosive environment reduced both tensile strength and impact toughness. The observed behavior was associated with differences in load-transfer mechanism, possible increased degree of cure and/or residual stress relaxation after post-curing, and degradation of the epoxy–matrix and fiber–matrix interface after SO₂ exposure. The results demonstrate that suitable selection of laminate architecture and thermal treatment can significantly improve the durability of CFRP structures intended for aggressive environments. Full article

The need to create lightweight materials with better mechanical properties has led to the use of Fiber Reinforced Composites (FRCs)s in the aerospace and automotive industries. The mechanical behavior of FRCs is heterogeneous, especially in conditions of low-velocity impact (LVI). The impact events cause structural damage, where most of the available literature deals with mono- or bi-composites in controlled situations. This work will present the results of studying the behavior of mono, bi- and tri-hybrids with carbon, glass and Kevlar fiber-reinforced epoxy. The sequences of the laminate stacks, number of plies and laminate thickness in the drop weight testing were across velocities of 1.91 to 3.91 m/s at drop heights of 19 to 79 cm. The dominant pillars of LVI, such as peak load, energy absorption and the modes of damage, were analyzed. The glass-dominated laminates peaked at 5.67 kN, while the Kevlar-dominated laminates reached peak flow in ductile collapse with greater quantities of absorbed energy. The leaders in strength and energy were the hybrids of Kevlar–glass (KG) cross-ply at 8.08 kN and 47.28 J and quasi-isotropic Kevlar–carbon–glass (KCG) at 9.12 kN and 47.25 J, showcasing a balance of strength and toughness. The rest, holding a greater quantity of Kevlar, ranging in thickness and cross-plies, were shaped with a load center. The experimental conclusion is that hybridization improved impact resistance and ductility, which is best supported by the glass/carbon rigidity-layered laminates. Such understanding directs the design work of future composite materials for better impact control. Full article

This study investigates the residual stresses developed during the curing process of polymer fiber-reinforced composites and their influence on fracture behavior, particularly the initiation and propagation of interlaminar cracks. The main objective is to quantify how different curing histories, including incomplete cure, alter the spatial distribution of residual stresses and, in turn, affect the mode-I fracture response of carbon-fiber/epoxy laminates. A transient thermal–structural finite element framework incorporating an autocatalytic cure kinetics model was used to simulate the curing process and predict residual stress development in a unidirectional carbon-fiber/epoxy laminate with an edge crack, considering thermal, chemical, and geometric effects. The cure model was calibrated using isothermal differential scanning calorimetry data to determine the degree of cure under different thermal conditions. The key novelty of this work is the integration of a validated cure-kinetics-based curing simulation with fracture analysis, enabling direct correlation of thermal history and degree of cure with spatially varying residual stresses at the crack front and their effect on fracture toughness. Numerical load–displacement predictions were compared with double cantilever beam experimental results and showed good agreement for the curing profiles examined. The results demonstrate that residual stresses generated by different cure cycles, including hold conditions and incomplete curing, significantly influence fracture toughness. In particular, the incomplete-cure profile produced an approximately 40% reduction in toughness compared with profiles that achieved complete cure, highlighting the importance of cure history in determining final structural performance. Full article

High-cycle fatigue (HCF) behavior of multi-scale hybrid composites remains a critical area of investigation for advanced applications in aerospace and automotive industries. This study aims to experimentally investigate and optimize the HCF performance of carbon–Kevlar/epoxy hybrid composites through synergistic incorporation of nano-silica (nSiO₂) and nano-graphene (nGr). Laminates were fabricated using a hand lay-up process followed by press molding, with a [2 carbon fiber/4 Kevlar fiber/2 carbon fiber] stacking sequence. Sixteen material configurations were investigated based on a Taguchi design of experiment (DOE), with two input parameters (nanoparticle percentages) at four different levels each. Following tensile screening tests, three optimal formulations were selected for fatigue evaluation alongside a non-reinforced baseline. Axial fatigue tests were conducted under load-controlled conditions with a stress ratio of R = 0.01 at a constant frequency of 5 Hz. Stress levels were set at 65%, 70%, and 75% of the ultimate tensile strength (UTS), which ranged from 211 MPa for the baseline composite to 390 MPa for the optimal hybrid formulation (1.2 wt.% nSiO₂ and 0.75 wt.% nGr). Scanning electron microscopy (SEM) analysis of fracture surfaces was performed to correlate microstructural features with fatigue performance. The results demonstrate a remarkable synergistic effect. The optimal hybrid nanocomposite exhibited superior fatigue life, sustaining significantly higher maximum stress (253 MPa vs. 137 MPa at 65% UTS) and achieving a life increase of several-fold compared to the non-modified baseline. SEM observations revealed that this enhancement stems from complementary microstructural mechanisms: nSiO₂ particles are uniformly dispersed without agglomeration, providing matrix toughening through crack deflection, while nGr sheets enhance interfacial adhesion, as evidenced by complete matrix coverage on fiber surfaces. The optimal formulation uniquely displays both mechanisms operating simultaneously, creating a true multi-scale reinforcement architecture. In contrast, sub-optimal formulations showed nanoparticle agglomerations that acted as stress concentrators under cyclic loading, explaining their intermediate fatigue performance despite high static strength. Full article

Glass fiber-reinforced epoxy composites were modified with carbon nanotubes (CNTs), Al₂O₃, and TiO₂ nanoparticles to comparatively evaluate their influence on tensile, flexural, and low-velocity impact performance within an integrated experimental–numerical framework. Nanoparticles were incorporated at controlled weight fractions to identify dispersion-controlled reinforcement regimes and the onset of heterogeneity-driven mechanical transitions. Among all formulations, 0.5 wt% CNTs provided the most pronounced static mechanical enhancement, increasing tensile strength to 419.50 MPa (≈21% improvement over the reference GF laminate) and flexural strength to 230.23 MPa (≈26% increase). In contrast, impact performance exhibited a non-monotonic evolution; the highest absorbed energy (9.64 J) was observed at 2 wt% CNTs, indicating that dynamic energy dissipation mechanisms do not necessarily scale proportionally with static strength gains. Oxide-filled systems demonstrated stiffness-dominated behavior, where increasing filler content amplified elastic mismatch and progressively reduced strength despite modulus enhancement. Finite element simulations conducted in ANSYS LS-DYNA (MAT_022) reproduced global stiffness trends within the dispersion-controlled regime. Tensile strength predictions agreed within 0–9% at optimal CNT loading, whereas larger deviations (up to ~33%) emerged under bending-dominated loading in oxide-rich systems, reflecting amplified sensitivity to microstructural heterogeneity. The coupled evolution of stiffness–strength decoupling (SSDI) and FEM deviation (η) enabled identification of a Composite Heterogeneity Threshold (CHT), defined as the nanoparticle concentration beyond which stiffness enhancement no longer translates into proportional strength or toughness improvement. Beyond this threshold, dispersion-induced heterogeneity not only reduces mechanical efficiency but also marks the boundary of homogenized continuum model adequacy across static and dynamic loading conditions. Full article

The recycling of carbon fiber-reinforced polymers (CFRPs), particularly carbon fiber-reinforced epoxy polymers (CFREPs), is a challenging problem because of their broad application spectrum, the amount of laminates produced per year, and the cost per kg of the carbon fiber fabric. Recently, several papers were published on the recycling of CFREPs through solvothermal methods that allow the recovery of the carbon fiber fabrics with a relatively low environmental impact. In the present paper, for the first time, the effect of the presence of flame retardants is discussed. A carbon fiber-reinforced epoxy polymer (CFREP) charged with P-, Zn-, B- and Al-based flame retardants, supplied by the aerospace industry, was subjected to a double-step solvothermal treatment. The epoxy matrix was successfully dissolved in monoethanolammine after a preswelling step in acetic acid. The experimental results show that the proposed process allows the full recovery of the carbon fabric with its original sizing layer without injury to the fiber. As confirmation, CFREP laminates produced with the recycled carbon fiber fabrics exhibited mechanical properties close to that of laminates obtained from the virgin epoxy/carbon prepreg. Contrary to what is reported in the literature, the present paper also shows that, in the studied case, whilst acetic acid treatment promotes swelling, it also causes the formation of a degraded surface layer that would impede complete removal of the polymeric matrix and full recovery of the carbon fabric if only acetic acid was used. On the basis of the known mechanism of flame retardancy of phosphates and borates, the degraded layer formation is attributed to the acidic character of the acetic acid. It is worth pointing out that the paper suggests, therefore, that the presence of flame retardants may strongly affect the solvothermal processes. Full article

Polymer nanocomposites offer significant potential for improving the strength-to-weight ratio and dynamic behavior of drone blades. This study examines the vibration characteristics of tapered aramid (Kevlar)/epoxy composite blades reinforced with nanocarbon fillers—graphene (2D), multi-walled carbon nanotubes (MWCNTs, 1D), and fullerene (0D)—to determine the most effective filler for enhancing stiffness and operational stability. The laminated blades (300 mm length, 200 mm width, root thickness 13 mm, tip thickness 8 mm) incorporate ply drop-offs and a central honeycomb core. Modeling was performed using classical laminate plate theory integrated with the finite element method (FEM) in MATLAB (R2016a). Under clamped–free–free–free boundary conditions, the study considered rotational speeds of 750–2250 rpm, setting angles of 30–60°, various fiber orientations, and nanofiller contents of 0–10 wt.%. The results indicate that while the setting angle minimally affects natural frequency, it significantly influences damping in modes (1,2) and (2,1). Increasing nanofiller content improves stiffness, with optimal performance observed near 5 wt.%. At 1500 rpm in mode (1,1), MWCNTs provided the greatest enhancement. Overall, MWCNTs exhibited superior stiffness improvement and rotational stability compared to other fillers. Full article