Search Results (116)

Spherical caps exploit their intrinsic curvature to achieve efficient stress distribution, delivering exceptional strength-to-weight ratios. This advantage renders them indispensable for aerospace systems, pressurized containers, architectural domes, and structures operating in extreme environments, such as deep-sea or nuclear containment. Their superior load-bearing capacity enables diverse applications, including satellite casings and high-pressure vessels. Meticulous optimization of geometric parameters and material selection ensures robustness in demanding scenarios. Given their significance, this study examines the natural frequency and static response of bio-inspired helicoidally laminated carbon fiber–reinforced polymer matrix composite spherical panels surrounded by Winkler elastic foundation support. Utilizing a 3D elasticity approach and the finite element method (FEM), the governing equations of motion are derived via Hamilton’s Principle. The study compares five helicoidal stacking configurations—recursive, exponential, linear, semicircular, and Fibonacci—with traditional laminate designs, including cross-ply, quasi-isotropic, and unidirectional arrangements. Parametric analyses explore the influence of lamination patterns, number of plies, panel thickness, support rigidity, polar angles, and edge constraints on natural frequencies, static deflections, and stress distributions. The analysis reveals that the quasi-isotropic (QI) laminate configuration yields optimal vibrational performance, attaining the highest fundamental frequency. In contrast, the cross-ply (CP) laminate demonstrates marginally best static performance, exhibiting minimal deflection. The unidirectional (UD) laminate consistently shows the poorest performance across both static and dynamic metrics. These investigations reveal stress transfer mechanisms across layers and elucidate vibration and bending behaviors in laminated spherical shells. Crucially, the results underscore the ability of helicoidal arrangements in augmenting mechanical and structural performance in engineering applications. Full article

Closed-section composite structures with corners present significant challenges during forming and molding for achieving the desired thickness distribution over the profile. The experimental investigation in the present work was designed to compare laminates constructed entirely from twill-weave carbon fabric prepregs with different hybrid laminates constructed by combining unidirectional (UD) carbon fiber prepregs around the flat and twill-weave fabric prepregs around the curved section. Although the UD fiber prepregs were found to be more compressible than the twill-weave prepregs, the desired thickness distribution (to within 2% of design geometry), along with the proper level of consolidation, was obtained only with the hybrid construction that had an equal number of UD plies around the flat and twill-weave plies around the curved section. In contrast, the thickness distribution obtained with the all-twill prepreg laminate deviated from the design geometry by 5.4%. Forming simulations incorporating experimentally derived compaction behavior of different plies were used to predict the local compaction, tool–ply contact pressures, and thickness profile of the molded part. The simulation results for thickness profiles showed similar trends to those observed in experiments. Full article

Additive Manufacturing (AM) using Fused Layer Modelling (FLM) often results in polymer components with limited and highly anisotropic mechanical properties, exhibiting structural weaknesses in the layer direction (Z-direction) due to low interlaminar adhesion. The main objective of this work was to investigate and quantify these mechanical limitations and to develop strategies for their mitigation. Specifically, this study aimed to (1) characterize the anisotropic behavior of unreinforced Polycarbonate (PC) components, (2) evaluate the effect of continuous, unidirectional (UD) carbon fiber tape reinforcement on mechanical performance, and (3) validate experimental findings through Finite Element Method (FEM) simulations to support predictive modeling of reinforced FLM structures. Methods involved experimental tensile and 3-point bending tests on specimens printed in all three spatial directions (X, Y, Z), validated against FEM simulations in ANSYS Composite PrepPost (ACP) using an orthotropic material model and the Hashin failure criterion. Results showed unreinforced samples had a pronounced anisotropy, with tensile strength reduced by over 70% in the Z direction. UD tape integration nearly eliminated this orthotropic behavior and led to strength gains of over 400% in tensile and flexural strength in the Z-direction. The FEM simulations showed very good agreement regarding initial stiffness and failure load. Targeted UD tape reinforcement effectively compensates for the weaknesses of FLM structures, although the quality of the tape–matrix bond and process reproducibility remain decisive factors for the reliability of the composite system, underscoring the necessity for targeted process optimization. Full article

This study presents a comparative analysis of two industrially relevant technologies for manufacturing of prepreg composite materials based on polyimide (PI) resin: hot-melt and solvent-based technology. More specifically, the study focuses on evaluating the relationship between key processing parameters and the final properties of the composite material manufactured with unidirectional (UD) C-fibers and woven fabrics used as reinforcement for both technologies. The impregnation process was carried out using a custom-designed coating equipment developed by Mikrosam D.O.O. Manufactured prepregs were characterized in terms of their resin content, volatile content, weight, width, and quality of the applied resin film. The hot-melt method that involves applying the resin in a semi-molten state with minimal solvent content provided a stable resin content (34–35%) and low volatiles (~1.2–1.5%) in the final product. The solvent-based method, using a resin/solvent ratio of 50:50, enabled deeper resin penetration into the fibers, particularly in woven fabrics (resin content: 34–37%) and lower residual volatiles (~0.3–0.5%). These results showed that the hot-melt technology consistently produced prepregs with very stable resin content, which is critical for structural applications requiring increased mechanical performance. In contrast, the solvent-based method demonstrated better adaptability to different reinforcement forms, improved impregnation depth, and excellent film uniformity, particularly suitable for woven fabrics. Representative SEM micrographs confirmed uniform resin distribution, full fiber wetting, and absence of voids, validating the impregnation quality obtained by both techniques. These findings highlight the technological relevance of selecting the appropriate impregnation route for each reinforcement architecture, offering direct guidance for industrial-scale composite manufacturing, where the hot-melt method is preferred for UD prepregs requiring precise resin control, while solvent-based impregnation ensures deeper and uniform resin distribution in woven fabric structures. Full article

The increasing demand for sustainable engineering solutions has driven extensive research into natural fibre-reinforced composites (NFCs), notably flax fibre-reinforced polymers (FFRPs), which offer promising eco-friendly alternatives to synthetic composites. This study presents a comprehensive application of continuum damage mechanics (CDM) methodology to unidirectional (UD) FFRPs, addressing critical gaps in predictive modelling of progressive intralaminar damage for sustainable structural applications. A systematic experimental characterisation protocol was developed to identify material parameters that account for the inherent variability and complex nonlinear behaviour of natural fibres. The CDM model was calibrated using comprehensive quasi-static testing on multiple laminate configurations and validated through finite element analysis (FEA) in Siemens Simcenter Samcef. The model accurately captures the nonlinear behaviour and damage mechanisms of notched flax–epoxy laminates, achieving prediction accuracies of 97.61% and 88.98% for the force-displacement response in open-hole tensile (OHT) validation tests. Integrating experimental characterisation with FEA enables robust virtual prototyping of sustainable composite structures, supporting design optimisation and lifecycle assessment. This research establishes validated damage modelling methodologies for eco-friendly NFC, facilitating accelerated adoption in automotive, marine, and construction sectors. Full article

The potential for structural health monitoring (SHM) in fibre-reinforced polymers (FRPs) using electrical resistance measurements (ERMs) has gained increasing attention, particularly in carbon fibre-reinforced polymers (CFRPs). Most existing studies are limited to single-axis measurements on coupon-scale specimens, whereas industrial applications demand scalable solutions capable of monitoring large areas, with more complex sensing configurations. Structural health monitoring (SHM) of carbon fibre-reinforced polymers (CFRPs) using electrical resistance measurements offers a low-cost, scalable sensing approach. However, predicting surface resistance between arbitrarily placed electrodes on unidirectional (UD) CFRP laminates remains challenging due to anisotropic conductivity and geometric variability. This study introduces a practical analytical model based on two geometry-dependent parameters, effective width and effective distance, to estimate resistance between any two electrodes arbitrarily placed on UD CFRP laminates with 0° or 90° fibre orientations. Validation through finite element (FE) simulations and experimental testing demonstrates good matching, confirming the model’s accuracy across various configurations. Results show that the dominant electrical current path aligns with the fibre direction due to the material’s anisotropic conductivity, allowing simplification to a single-axis resistance model. The proposed model offers a reliable estimation of surface resistance and provides a valuable tool for electrode array configuration design in CFRP-based SHM. This work contributes to enabling low-cost and scalable electrical sensing solutions for the real-time monitoring of composite structures in aerospace, automotive, and other high-performance applications. Full article

Unidirectional carbon fiber-reinforced polymer (UD-CFRP) composites demonstrate superior tensile creep strain and stress relaxation behavior along fiber orientation. However, prolonged transverse compressive loading in structural connection zones induces significant interfacial stress relaxation and creep deformation, primarily driven by resin matrix degradation and interfacial slippage under thermal-mechanical interactions, and remains poorly understood. This study systematically investigates the transverse stress relaxation characteristics of UD-CFRP through controlled experiments under varying thermal conditions (20–80 °C) and compressive stress levels (30–80% ultimate strength). Post-relaxation mechanical properties were quantitatively evaluated, followed by the development of a temperature-stress-time-dependent predictive model aligned with industry standards. The experimental results reveal bi-stage relaxation behavior under elevated temperatures and compressive stresses, characterized by a rapid primary phase and stabilized secondary phase progression. Notably, residual transverse compressive strength remained almost unchanged, while post-relaxation elastic modulus increased by around 10% compared to baseline specimens. Predictive modeling indicates that million-hour relaxation rates escalate with temperature elevation, reaching 51% at 60 °C/60% stress level—about 1.8 times higher than equivalent 20 °C conditions. These findings provide crucial design insights and predictive tools for ensuring the long-term integrity of CFRP-based structures subjected to transverse compression in various thermal environments. Full article

As the importance of sustainability and performance increases, new developments in the manufacturing of fiber-reinforced polymer composites (FRPC) are requested. Ultraviolet (UV) curing offers a faster, more economical, and eco-friendlier alternative to conventionally used thermal curing methods, e.g., autoclave curing, but according to extant research, also presents some shortcomings, such as limitations to thin FRPCs and transparent glass fibers (GFs). This study analyses the UV light transmission in different thermoset FRPCs by irradiating various fiber samples on one side, while a sensor on the opposite side measures the transmitted irradiance. The materials investigated include unidirectional (UD) carbon fibers (CF), UD flax fibers (FF), and six GF fabrics with different ply structures. The fiber samples are tested in a dry, non-impregnated state and a resin-impregnated state using a UV-curable vinyl-ester-based resin. The results show that up to 16 plies of five GF fabrics are fully cured within the 20 s irradiation time and still exhibit a relatively high light transmission, revealing the potential of curing thick FRPCs with UV light. Furthermore, up to three plies of non-transparent FFs are cured, which is promising for the UV curing of natural fibers. Full article

Spherical shells exhibit superior strength-to-geometry efficiency, making them ideal for industrial applications such as fluid storage tanks, architectural domes, naval vehicles, nuclear containment systems, and aeronautical and aerospace components. Given their critical role, careful attention to the design parameters and engineering constraints is essential. The present paper investigates the buckling responses of bio-inspired helicoidal laminated composite spherical shells under normal and torsional loading, including the effects of a Winkler elastic medium. The pre-buckling equilibrium equations are derived using linear three-dimensional (3D) elasticity theory and the principle of virtual work, solved via the classical finite element method (FEM). The buckling load is computed using a nonlinear Green strain formulation and a generalized geometric stiffness approach. The shell material employed in this study is a T300/5208 graphite/epoxy carbon fiber-reinforced polymer (CFRP) composite. Multiple helicoidal stacking sequences—linear, Fibonacci, recursive, exponential, and semicircular—are analyzed and benchmarked against traditional unidirectional, cross-ply, and quasi-isotropic layups. Parametric studies assess the effects of the normal/torsional loads, lamination schemes, ply counts, polar angles, shell thickness, elastic support, and boundary constraints on the buckling performance. The results indicate that quasi-isotropic (QI) laminate configurations exhibit superior buckling resistance compared to all the other layup arrangements, whereas unidirectional (UD) and cross-ply (CP) laminates show the least structural efficiency under normal- and torsional-loading conditions, respectively. Furthermore, this study underscores the efficacy of bio-inspired helicoidal stacking sequences in improving the mechanical performance of thin-walled composite spherical shells, exhibiting significant advantages over conventional laminate configurations. These benefits make helicoidal architectures particularly well-suited for weight-critical, high-performance applications in aerospace, marine, and biomedical engineering, where structural efficiency, damage tolerance, and reliability are paramount. Full article

The representative volume element (RVE) is frequently used to forecast the mechanical properties of composites, where the distribution of fibers plays a significant role. This paper proposes a new RVE modeling method for unidirectional fiber-reinforced polymer (UD-FRP) composites, which takes into account the random distribution of fiber positions and inclinations. The fiber inclination in the RVE is normally or uniformly distributed. The suggested RVE model was validated using static tests and the fiber structure observed by micro-computed tomography (CT). The effects of fiber volume fraction and maximum fiber inclination on the elastic properties were investigated based on the proposed RVE model. The results indicate that the prediction of transverse properties is considerably impacted by fiber inclination in RVE, with uniformly distributed inclination having a more significant influence than normally distributed inclination. For the transverse Young’s modulus of UD-FRP, the predicted results of the proposed model and the models in the literature differed from the experimental results by 0.30% and 11.45%, respectively. For the in-plane shear modulus of UD-FRP, the predicted results of the proposed model and the models in the literature differed from the experimental results by 1.65% and 8.44%, respectively. Moreover, the fiber volume fraction has a significant effect on the elastic properties, and the maximum inclination of the fibers has a significant effect on the elastic properties except for the longitudinal Poisson’s ratio. The proposed RVE model in this paper can predict the elastic properties of composites more accurately. Full article

Polyetherimide (PEI)/molecular semiconductor-based all-organic dielectric composites have garnered significant attention due to their exceptional energy storage performance at elevated temperatures. In this work, the high-electron-affinity semiconductor 5,6,12,13-tetrachloro-2,9-bis(2-ethylhexyl)anthra[2,1,9-def:6,5,10-d′e′f]diisoquinoline-1,3,8,10(2H,9H)-tetraone (TCEHAQ) is employed as a filler to enhance the dielectric energy storage performance of PEI. It is believed that TCEHAQ can immobilize electrons and reduce charge transport in dielectric composites. The results demonstrate that the breakdown strength of PEI with only 0.5 wt% of TCEHAQ increased from 450 MV/m to 600 MV/m at room temperature, while the maximum discharge energy density (U_d) reached 5.99 J/cm³, and the discharge efficiency (η) was 96.5%. Meanwhile, the breakdown strength of the 0.5 wt% TCEHAQ/PEI blend at 150 °C was 500 MV/m, and the maximum U_d and η were 3.68 J/cm³ and 81.0%, respectively. This is a straightforward and effective method for fabricating large-area, high-quality dielectric energy storage films suitable for use in harsh environments. Full article

The thermal, thermomechanical, and viscoelastic properties of continuous unidirectional (UD) glass fiber/high-density polyethylene (GF/HDPE) and ultra-high-molecular-weight polyethylene/high-density polyethylene (UHMWPE/HDPE) tapes are characterized in this paper in order to support their use in extreme environments. Unlike prior studies that focus on short-fiber composites or limited thermal conditions, this work examines continuous fiber architectures under five operational environments derived from Army Regulation 70-38, reflecting realistic defense-relevant extremes. Differential scanning calorimetry (DSC) was used to identify melting transitions for GF/HDPE and UHMWPE/HDPE, which guided the selection of test conditions for thermomechanical analysis (TMA) and dynamic mechanical analysis (DMA). TMA revealed anisotropic thermal expansion consistent with fiber orientation, while DMA, via strain sweep, temperature ramp, frequency sweep, and stress relaxation, quantified their temperature- and time-dependent viscoelastic behavior. The frequency-dependent storage modulus highlighted multiple resonant modes, and stress relaxation data were fitted with high accuracy (R² > 0.99) to viscoelastic models, yielding model parameters that can be used for predictive simulations of time-dependent material behavior. A comparative analysis between the two material systems showed that UHMWPE/HDPE offers enhanced unidirectional stiffness and better low-temperature performance. At the same time, GF/HDPE exhibits lower thermal expansion, better transverse stiffness, and greater stability at elevated temperatures. These differences highlight the impact of fiber type on thermal and mechanical responses, informing material selection for applications that require directional load-bearing or dimensional control under thermal cycling. By integrating thermal and viscoelastic characterization across realistic operational profiles, this study provides a foundational dataset for the application of continuous fiber thermoplastic tapes in structural components exposed to harsh thermal and mechanical conditions. Full article

This work analyzes the aero-elastic oscillations of cantilever beams reinforced by carbon nanotubes (CNTs). Four different distributions of single-walled CNTs are assumed as the reinforcing phase, in the thickness direction of the polymeric matrix. A modified third-order piston theory is used as an accurate tool to model the supersonic air flow, rather than a first-order piston theory. The nonlinear dynamic equation governing the problem accounts for Von Kármán-type nonlinearities, and it is derived from Hamilton’s principle. Then, the Galerkin decomposition technique is adopted to discretize the nonlinear partial differential equation into a nonlinear ordinary differential equation. This is solved analytically according to a multiple time scale method. A comprehensive parametric analysis was conducted to assess the influence of CNT volume fraction, beam slenderness, Mach number, and thickness ratio on the fundamental frequency and lateral dynamic deflection. Results indicate that FG-X reinforcement yields the highest frequency response and lateral deflection, followed by UD and FG-A patterns, whereas FG-O consistently exhibits the lowest performance metrics. An increase in CNT volume fraction and a reduction in slenderness ratio enhance the system’s stiffness and frequency response up to a critical threshold, beyond which a damped beating phenomenon emerges. Moreover, higher Mach numbers and greater thickness ratios significantly amplify both frequency response and lateral deflections, although damping rates tend to decrease. These findings provide valuable insights into the optimization of CNTR composite structures for advanced aeroelastic applications under supersonic conditions, as useful for many engineering applications. Full article

The LaRC02 criterion is a classical criterion for determining fiber kinking failure of UD laminates under longitudinal compression (LC), but its basis for determining matrix cracking in a fiber kinking coordinate system is based on stress-invariant theory rather than on a physical mechanism. Herein, three Puck-like physical-mechanism-based inter-fiber failure criteria are extended to LC failure of UD composites, and thus three failure criteria (denoted as LC-Guo, LC-Li, and LC-Puck failure criteria) are constructed for fiber kinking failure determination. The stresses in the global coordinate system are transformed to the fiber kinking coordinate system by a three-level coordinate system transformation, and then the failure determination is performed using the three Puck-like criteria. The results show that the overall accuracy of the three proposed criteria is higher than that of the LaRC02 criterion, especially the LC-Guo criterion. Additionally, an analysis of the influence of material properties shows that the failure envelope curves tend to be conservative, and the predicted off-axial compression strength decreases as the transverse compression strength and in-plane shear strength increase and the transverse tensile strength decreases. This work proposes a more reasonable assessment methodology for the determination of LC failure of UD composites, which has important theoretical significance and engineering value. Full article

Polyphenols are valuable contributors to skin health, offering potent antioxidant and anti-inflammatory effects that help counteract the process of inflammaging. According to the literature, Urtica dioica L. is a rich source of polyphenolic compounds, suggesting its potential for applications in cosmetology and dermatology. This study aimed to evaluate the antioxidant and anti-inflammatory activity of polyphenol-rich fractions isolated from U. dioica leaves (UdLs) and flowers (UdFs) using human skin cells subjected to oxidative stress and lipopolysaccharide (LPS) stimulation, respectively. Extracts were obtained via an accelerated solvent extraction and further purified by a solid-phase extraction to concentrate their polyphenolic content. Their chemical composition was analyzed using UPLC-DAD-MS. Biological activity was assessed through cytotoxicity assays (NR and MTT), chemical and cellular antioxidant assays (DPPH, ABTS, FRAP, CUPRAC, TPC, and H₂DCFDA), an evaluation of antioxidant enzyme activity (SOD, CAT), lipid peroxidation (MDA), and cytokine production (IL-1β, IL-6, IL-10). Our study showed that both fractions were abundant in phenolic compounds, with chlorogenic acid identified as the predominant constituent. UdLs contained higher levels of phenolic acids, whereas the UdF was richer in flavonoids, particularly derivatives of isorhamnetin. Both the UdL and UdF were non-cytotoxic and exhibited strong radical scavenging potential, with the UdL being slightly more effective. They significantly reduced intracellular ROS levels, enhanced the activity of antioxidant enzymes, and attenuated lipid peroxidation in cells exposed to oxidative stress. Moreover, both fractions reduced the secretion of pro-inflammatory cytokines in LPS and H₂O₂-stimulated fibroblasts. These results highlight the potential of polyphenolic fractions derived from U. dioica leaves and flowers as multifunctional ingredients for anti-aging and skin-protective cosmetics. Full article