Journal of Composite Materials

The mechanical properties of polymer composites under the influence of external agents (heat, temperature, contact fluids) have been investigated in several industrial areas, such as aerospace, automotive, and petroleum. This study aimed to explore the relationship between environmental exposure and mechanical properties, given its impact on the final mechanical performance, service life, and industrial applications of reinforced composites. In the present study, three laminate plates with a polymeric matrix were manufactured, and reinforced with bidirectional E-glass fiber and Kevlar-49 fabric. The laminates were industrially produced using a hand lay-up process and immersed in fluids (crude oil and seawater) to investigate the relationship between fluid immersion and mechanical properties, highlighting the effects observed under the imposed conditions, apparent morphology, the microstructure of post-immersed samples, and final mechanical performance. Comparative results (dry vs wet) demonstrated that fluid contact did not significantly reduce strength, except for the IGK laminate, which showed a 19% reduction when immersed in petroleum and tested under bending, all other values remained below 10%. However, the losses in elastic modulus were more significant, reaching up to 50% rigidity loss in some cases. Morphological analysis showed resin degradation and petroleum impregnation due to chloride ions from seawater and crude oil exposure.

This paper deals with quasi-static and transient-dynamic crush testing of carbon composite material at close-to-cryogenic temperatures. Self-supported crush specimens of trapezoidal shape were fabricated from Cycom 6k HTA 5 HS carbon fabric material infiltrated with HexFlow RTM6 resin. Crush tests were performed at the temperatures T 1 ≈ 20°C, T 2 = −55°C, and T 3 = −170°C, as well as at loading rates of 20 mm/min and 2 m/s. A new test setup was used that enabled precise temperature control at close-to-cryogenic conditions in particular for high strain-rate testing. The steady-state crush stress (SSCS) and mass-specific energy absorption (SEA) are determined and compared for varying loading rates and temperatures. Main objective of this study is to obtain a first insight in trends for transient-dynamic crush performance at low temperatures which is why only two tests per configuration were performed. The test results indicate SSCS and SEA reduction for increasing loading rate and decreasing temperature. Strain-rate dependency showed twice the SSCS and SEA reduction at low temperatures compared to room temperature. Concerning the crash-relevant dynamic loading, the SSCS and SEA values are reduced by −15% from T1 ≈ 20°C to T2 = −55°C while the reduction is −19% from T1 ≈ 20°C to T3 = −170°C. Summarized, compared to quasi-static test data at room temperature, the total reduction for crush loading at −170°C and 2 m/s is −28% for SSCS and SEA. As a novel aspect, this result was experimentally determined under precisely controlled close-to-cryogenic temperatures during high-speed testing.

This paper presents an experimental investigation into the moisture absorption behavior and mechanical properties of glass fiber-reinforced polymer (GFRP) laminates used in stay vane extensions for hydraulic turbines. The research addresses critical challenges posed by prolonged water exposure, focusing on two types of resins—Araldite RenInfusion 8601/Ren 8602 (epoxy) and Derakane 411-350 (vinylester)—and two types of fiberglass weaves—Non-Crimp Fabrics (NCF) and Satin. Laminates were subjected to accelerated aging at 40°C and natural aging at room temperature to simulate long-term water immersion. The study reveals that epoxy-based laminates exhibit higher moisture absorption rates and more pronounced mechanical degradation compared to vinylester-based laminates, primarily due to differences in resin susceptibility to hydrolysis and matrix plasticization. These findings underscore the importance of resin choice and moisture management in optimizing the durability and mechanical performance of GFRP laminates, providing valuable insights for improving composite materials in underwater applications within hydropower systems.

This paper examines the damage and failure behavior of woven ceramic matrix composites (CMCs) under thermomechanical loading. The study focuses on S200H (Hi-Nicalon™ SiC fiber with a boron nitride coating in a SiNC matrix) and S400N (carbon fiber with a pyrolytic carbon coating in a SiNC matrix) CMCs. Quasi-static and creep-fatigue tests were conducted at various temperatures and stress levels to investigate failure mechanisms at intermediate and high temperatures. Creep-fatigue tests for S200H were performed at 800°C in an oxidative environment at stress levels corresponding to fractions of the ultimate tensile strength (UTS). S400N samples were tested at 600–900°C and 1200°C under low and high stress levels. Residual strength tests were conducted after cooldown to assess mechanical degradation in samples that did not fail. Fracture surface characterization using confocal microscopy, scanning electron microscopy, and energy dispersive spectroscopy provided insights into failure mechanisms. Results indicate that chemical reactions of non-stoichiometric phases in SiC/SiNC, thermal property mismatch, and pyrolytic carbon coating volatilization in C/SiNC govern quasi-static failure mechanisms. Both CMC systems exhibited a 30% reduction in UTS at elevated temperatures, with strain-to-failure decreasing by 17% in SiC/SiNC and 31% in C/SiNC. The SiC/SiNC samples exhibited an increase in yield strength with higher applied stress levels after 100 hours of creep-fatigue testing due to matrix residual compressive stresses, but a 37% reduction in UTS due to oxidation. In contrast, C/SiNC samples experienced significant diffusion-limited oxidation, leading to rapid fracture within 20 hours under creep-fatigue loading.

Materials that are lightweight, durable, and require low maintenance in various applications despite exposure to harsh environmental conditions are greatly desired. Polymer matrix composites have been regarded as a material with a true balance between cost-effectiveness and deliverable performances. Due to their appealing properties, polymer composites have found extensive applications within various industries, such as aerospace, automotive, and even construction. The escalating demand for bio-friendly sustainable materials has caused natural fibre-reinforced polymer (FRP) composite to attract considerable interest due to its potential to reduce environmental impacts. Nevertheless, these composites face challenges in high-performance engineering applications, where synthetic fibre composites such as carbon and glass FRP are generally favoured. Striking a balance between the performance of the composite, its biodegradability, and its costs is essential for developing sustainable composite materials. To overcome these challenges, surface modifications, such as alkali, enzyme and silane treatment have been reported to affect the fibre’s physical and chemical properties, leading to the mechanical performance enhancement of the composite. This review also explores the inclusion of natural fibre and nanomaterials in the different formations of natural fibre, namely 2D and 3D braids, knit and woven architectures to enhance the properties in both static and dynamic loadings. This review comprehensively examines the hybridisation effect of various combinations of natural fibre and nanomaterials that further enhance the properties of these composites. Additionally, the fabrication methods involving natural fibre, such the hand (wet)layup, vacuum-assisted resin transfer molding (VARTM), compression molding, injection molding and additive manufacturing (AM), are also included in this review. This review presents an overview that highlights the recent advancements in hybrid natural fibre composites and the deeper understanding of the synergistic effects between natural fibres and nanofillers, improved overall properties, and enhanced applications in multiple industries. The potential of hybrid natural composite as a sustainable and high-performance material features the need for continued research and innovation to address current challenges and explore new opportunities in composite engineering.

The leading edges of helicopter rotor blades are susceptible to high-velocity impact from small particles of a wide variety of materials. In this study, a glass fiber-reinforced epoxy composite was subjected to single impacts from spherical particles at velocities up to Mach 1. Target samples were cut from a 15-layer composite with 3070-style woven E-glass reinforcement. Erosive simulant particles were 1.6 mm in diameter spheres of silicon nitride, 2017 aluminum, hardened 52,100 steel, 260 brass, and 102 copper. In situ images were captured of each impact event, and samples were examined post-mortem with a scanning electron microscope. The crater formation in the target material was observed to follow a two-stage process. The Young’s Modulus of the projectile has a distinct effect on the depth of damage during both stages of material removal, and the yield stress of the projectile places a limit on the extent of damage during the second stage.

This paper introduces new rule-of-thumb equations for estimating the effective elastic modulus of randomly oriented short fiber reinforced composites (SFRCs), developed using the Tandon and Weng method combined with the simplifications from the Tsai-Pagano model. These types of rule-of-thumb equations are useful for a quick first approximation when designing SFRCs. The analysis reveals a linear relationship between the effective Young's modulus of randomly oriented SFRCs and the effective axial and transverse elastic moduli of unidirectional SFRCs, highlighting that the slope of this relationship is sensitive to the axial-to-transverse modulus ratio. Equations are provided to estimate this slope. Additionally, the study utilizes a two-step homogenization analysis, applied theoretically in this work, which proved to be a practical method for predicting the elastic modulus of randomly oriented SFRCs. Finally, a comprehensive comparative analysis with similar theoretical models demonstrates that the proposed approach delivers comparable prediction performance in two-dimensional random fiber distribution cases and slightly superior performance in three-dimensional random fiber distribution cases. Notably, the rule-of-thumb model developed in this work outperformed other two-step homogenization rule-of-thumb models, achieving better predictions of Young's modulus.

The study relates to the filament winding of a flexible conical aeroshell that belongs to the deployable thermal protection system for the controlled re-entry and safe recovery of CubeSat class satellites. A review of the flexible thermal protection systems made it possible to allocate materials for deployable mechanisms into a separate group by highlighting their principal features. Following the review, the filament winding of conical structures does not pose significant difficulties, except for the limitations in the available fiber paths and lack of data on the tow tension fluctuations during the manufacturing process. In the study, the available fiber paths for two configurations of the conical aeroshell were compared with the principal stress distributions expected during the operation of the flexible thermal protection system. The stress distributions have been evaluated by several theories related to the bending of thin plates or membranes under transverse load. It was shown that the stress distributions did not agree with the stiffness distribution of the filament wound structure, which was computed by the methods adopted in the classic lamination theory. The effect of various machine paths on the tow tension fluctuations was studied experimentally for the conical structures with the help of a laboratory 4-axis filament winding machine. The analytically computed machine path, which does not require a mechanism to rewind the tow over the packages during the winding process, was used to manufacture a small-scale flexible aeroshell. The features of the wound flexible aeroshell were discussed in conclusion.

This study evaluates the impact response and damage tolerance of carbon fiber-reinforced polymer (CFRP) composites with varying Aramid pulp reinforcement ratios subjected to low-velocity impact (LVI) at an energy level of 5 J. Pristine (unreinforced) and reinforced samples with 1X, 2X, and 4X Aramid pulp ratios were analyzed through drop-weight impact tests, flexural tests, and micro-CT scans. Results indicate that increasing reinforcement enhances energy absorption and maximum displacement but decreases peak force due to improved load distribution via delamination and matrix cracking. While 4X reinforced samples exhibit the highest energy absorption, they also show the most significant internal damage, including extensive delamination, void formation, and intralaminar failure. Flexural behavior reveals a decreased in stiffness and strength with higher reinforcement levels, which is attributed to weak fiber-matrix adhesion and resin-rich zones. Optimal reinforcement was observed at 2X, where flexural strain peaked due to effective crack bridging. Excessive reinforcement led to agglomeration effects, exacerbating void formation and structural weakening. The findings underscore the importance of optimizing reinforcement ratios and ensuring uniform dispersion to balance between energy dissipation and mechanical performance in CFRP laminates.

This study investigates the feasibility of enhancing the damage tolerance of carbon fiber-reinforced polymer (CFRP) composites using Aramid pulp as an interlaminar reinforcement under low-velocity impact (LVI) conditions at Arctic temperatures (AT). Woven carbon fiber and vinyl-ester resin laminates were fabricated with and without Aramid pulp reinforcement and subjected to drop-weight impact tests at various energy levels (5, 10, 15, and 20 J). The study compares the LVI performance of Aramid pulp- reinforced and unreinforced (pristine) CFRPs at room temperature (RT, 25°C) and AT (−60°C), focusing on key parameters such as contact force, displacement, and energy absorption. Results show that AT positively affected the CFRP laminates, increasing peak impact forces and bending stiffness while resulting in less damage compared to samples impacted at RT. However, AT also reduced displacement, as the increased rigidity and stiffness limited laminate deflection. Aramid pulp-reinforced laminates demonstrated higher peak forces and bending stiffness than pristine samples across all impact energies and temperatures. Additionally, reinforced samples exhibited reduced deflection and damage compared to pristine samples under all tested conditions. Fractographic analysis revealed matrix cracking as the primary failure mode, with less damage in Aramid pulp-reinforced and AT samples. These findings demonstrate that Aramid pulp reinforcement improves the impact resistance and stiffness of CFRP laminates, particularly at AT, making them more suitable for harsh environmental applications.

Warping remains a critical challenge in fused deposition modeling (FDM) of thermoplastic composites, leading to dimensional inaccuracies and compromised mechanical properties in printed parts. Existing studies have predominantly examined individual factors that affect warpage, such as infill specifications and printing orientation, without a comprehensive assessment of their combined effects. This study addresses this gap by employing a transient thermomechanical simulation approach in Digimat-AM, which integrates material crystallinity evolution, fiber orientation effects, and toolpath-dependent thermal history to enhance warpage prediction accuracy. A short carbon fiber-reinforced polyamide 12 composite was selected as the material system due to its complex thermal behavior, including the negative thermal expansion coefficient of carbon fibers. An unmanned aerial vehicle (UAV) camera holder was used as a case study, where the effects of the infill density, the infill pattern, the printing orientation, and the raster angle on warpage and mechanical performance were systematically analyzed. The high-fidelity simulation method in Digimat-AM was utilized to predict warpage, while experimental validation was performed through tensile and bending tests. Analysis of variance was applied to optimize the process parameters. The results demonstrated that a 90% infill density, a triangular infill pattern, a flat printing orientation, and a 90° raster angle significantly reduced warpage, achieving a deviation angle of as low as 0.04°. Furthermore, the combination of 90% infill density, concentric infill pattern and 90° raster angle improved tensile strength (45.58 MPa) and stiffness (2.57 GPa), confirming improved mechanical integrity.

The hygrothermal-oxidative aging of a phenolic/short glass fiber composite was studied in a humid environment (100% relative humidity) for over a year at three temperatures: 50°C, 70°C, and 90°C. Wet aging of this composite is rarely studied in the literature due to the nature of its matrix. Indeed, phenolic resins are usually used to resist thermal stress at very high temperatures. Infrared spectroscopy by ATR mode, dynamic thermomechanical analyses, water sorption measurements and optical microscopy were used in a complementary way to monitor the influence of humidity, oxygen and temperature on the composite structure. From the first aging times, resin plasticizing and post-curing of resin are associated to cracks occurring from the resin/fiber interface, due to the hydration of the fibers which would be treated by silane-type coupling agent. Sub-crosslinking would be related to the presence of intermediate products such as amides, imides, or phenol groups in excess, strongly hydrophilic. Consequently, along with time, hydrolysis is accelerated. Moreover, the oxidation products revealed by FTIR analyses will favor interactions with water. The cracks are then deviated inwards the resin matrix.

In this study, novel tetra-chiral spline missing-rib structure is designed and experimentally tested to enhance energy-absorbing in crash conditions. Out-of-plane quasi-static compression tests are carried out for both classical and novel missing-rib structures made with additive manufacturing. The numerical models are created and required details are used to predict the energy absorption (EA) of structures. Two criteria namely, energy absorption efficiency and deformation efficiency are examined to determine the EA of the structures. By using these criteria, the numerical finite element analysis is validated and calibrated with the test experiments. Then based on Response Surface Methodology (RSM), design of experiments (DOE) is carried out compression analysis are modeled using FEA. For optimization, four regression models are examined and 2FI model is chosen. Then by maximizing energy absorption, optimum geometrical parameters are determined. Finally, the optimal model is constructed and FEM results are verified again. Great enhancement in energy absorption capacity of the novel missing-rib is substantial which can be served by designers as basis for future lightweight structures.

Microstructure and mechanical properties of polymer matrix composites (PMCs) using as-received biochar derived from kitchen bio-waste and Hexion LR160 epoxy as the matrix material were investigated. The PMCs were produced via glass moulding with higher biochar contents of 10 wt.%, 20 wt.%, and 30 wt.%. Scanning electron microscopy (SEM), Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR) were used to analyze the microstructure, chemical composition, and interfacial bonding. SEM analysis revealed biochar agglomeration at lower concentrations and increased homogeneity at higher biochar contents. Raman spectroscopy confirmed biochar’s disordered carbon structure (ID/IG = 1.10), while FTIR analysis identified characteristic functional groups suggesting mechanical adhesion between biochar and the epoxy matrix. Mechanical testing showed a modulus increase from 3.61 GPa (pure epoxy) to 4.98 GPa (PMC-C30). Tensile strength, initially lower than pure epoxy (62.00 MPa), increased from 21.00 MPa (PMC-C10) to 32.2 MPa (PMC-C30) with higher biochar content, indicating its reinforcing potential. Hardness increased from 175 HB (pure epoxy) to 237 HB (PMC-C30), further confirming biochar’s strengthening effect. The results of this study contribute to demonstrating biochar’s potential as a sustainable filler, supporting eco-friendly composite development while reducing the environmental footprint of polymer-based materials.

Traditional sandwich composite panels consist of stiff facesheets that support bending loads and a core that bears shearing load. In this study, the core and facesheet were assembled in a single process to eliminate the possibility of delamination between the layers, and make the structure unitary in its construction. The drop tower impact test was conducted in accordance to ASTM standard D7136 to evaluate the impact strength of the panels while taking into consideration the impact velocity and the coefficient of restitution. The impact velocities used were 1 m/s and 4.6 m/s denoted as V 1 m s and V 4.6 m s respectively. A Phased Array Ultrasonic Testing (PAUT) inspection was conducted to evaluate damage extent in the high velocity samples. At V 4.6 m s , the CoR ranged between 0.05 and 0.363, suggesting some amount of elastic collision. The displayed Energy Loss Percentage (ELP) was more than 85%, suggesting that the specimen experienced total perforation. There were mainly dents on the front side and bulges on the opposite sides of the specimens tested at V 1 m s while at V 4.6 m s , the specimens experienced considerable damage including perforation. Ultrasound (UT) inspection of the damaged specimens did not show delamination. Avoiding dislocation of structural members when manufacturing composite panels for intermodal containers and truck beds was achieved.

The combination of metals and carbon fiber reinforced polymer (CFRP) composites to fabricate thin-walled tubes has excellent energy absorption potential. Introducing origami patterns and filling with polyurethane foam can affect the deformation process of the composite tube, significantly improving its energy absorption performance. This study investigates the energy absorption characteristics of CFRP/metal composite origami thin-walled filled components under quasi-static axial loads through experiments and numerical simulations. First, the energy absorption performance of metal thin-walled tubes and composite thin-walled tubes is analyzed, and the effect of foam filling on their energy absorption performance is examined. It is found that CFRP/metal composite tubes exhibit significant improvement in energy absorption compared to individual metal or CFRP tubes. Additionally, the incorporation of origami patterns and polyurethane foam enhances the deformation stability of the composite tube and improves its energy absorption characteristics. Subsequently, the effects of design parameters, such as the fiber winding angle of the CFRP tube, the ratio of axial (10°) to circumferential (90°) winding angles, the dihedral angle of the metal tube pattern, and the number of pattern layers, on the energy absorption characteristics of the composite tube are studied in detail. The results show that with an increase in fiber winding angle and the ratio of axial (10°) to circumferential (90°) winding angles, the load fluctuations of the composite tube decrease, and stable progressive fracture deformation occurs, improving the energy absorption characteristics. When the dihedral angle of the metal tube origami pattern is 150° and the number of layers is 5, the composite energy absorption effect is optimal.

A bespoke test apparatus has been developed to investigate the buckling and post-buckling behaviours of a stiffened composite panel under combined shear and compression. A pair of four-bar linkage shear frames coupled with actuators have been introduced to produce the shear force along the lateral edges of the panel, enabling adaptive adjustment to the direction of shear force as the panel undergoes in-plane shear deformation. Experimental results have verified the capability of the apparatus for producing a uniform strain distribution over the area of interest for the panel and the effectiveness in independently controlling the shear and compressive loads without interference during the combined loading. The failure loads of the panel are 50% greater than the initial buckling loads in this combined loading case, showing a sufficient load carrying capability in the post-buckling regime. Results show that local buckling in skin bays is the primary buckling mode for the panel studied. The stiffeners are capable of sustaining axial compression while the skin has buckled locally. The buckling-induced waviness in skin bays produces a twist effect on the stiffeners, contributing to skin-stiffener debonding and stiffener breakage, and therefore ultimately leading to the collapse of the stiffened panel.

Because of the notch stress singularity, the research on notch stress field and fatigue evaluation of anisotropic V-notch plate has always been the focus of safety evaluation. For anisotropic materials, the notch stress field is not only related to the geometric size and opening angle, but also closely related to the material parameters in all directions. The accurate characterization of notch stress field is the key to evaluate stress concentration and fatigue life of notched structures. By introducing singular strength factor, the semi-analytical formula of stress field in vertical crack direction is deduced theoretically. On the basis of theoretical formula, combined with a large number of numerical model results, a simplified formula of notch stress field under bending load or tensile load is fitted and recommended. Finally, through a series of fatigue tests of V-notched laminates, the notch stress field and fatigue life are obtained, and the formula of notch stress field and fatigue assessment method proposed in this paper are verified. The verification results show that the proposed notch stress field and N-SIF formulas have high prediction accuracy, which provides a reference for the rapid evaluation of notch stress field and fatigue life of anisotropic materials.

This study investigates the high-velocity impact mechanics in correlation with piezo-resistance damage sensing characteristics of glass/carbon hybrid composites under projectile impact loading. Inter-ply and Intra-ply hybrid composites consisting of different ply orientations, stacking sequences, and liquid metal (LM) compositions (1 and 2 wt%) are considered for this study. An in-house one-stage gas gun setup is used to conduct projectile impact loading experiments. A novel circumferential four probes electrical resistivity method is employed to investigate the damage-sensing capability of hybrid composites. Two different projectile shapes (cone end projectile and stepped cone end projectile) are considered and investigated their effect on the composites’ ballistic limit, impact energy absorption, damage area, and piezo-resistance response. Projectile shape significantly influences ballistic limit and energy absorption, whereas a stepped cone end projectile demonstrates higher amount of energy absorption of about 42% and peak piezo-resistance change of around 60% compared to cone end projectile. The addition of LM improved the ballistic limit by about 20% and the amount of energy absorption by around 50% but reduced damage-sensing sensitivity due to improved electrical conductivity with its presence. Moreover, the intra-ply hybrid composites exhibited lower ballistic limits owing to weaker fiber strength, while inter-ply hybrids showed better energy absorption capabilities, resulting in higher ballistic limits. Thermal imaging technique is adopted in post-mortem analysis of the damaged area, and it revealed delamination inside the intra-ply hybrid composites.

This study investigates the mechanical, thermal, and morphological properties of polylactic acid (PLA) composites reinforced with 10 wt% flax, hemp, and pineapple leaf fibres (PALF), produced via extrusion and Fused Deposition Modelling (FDM) 3D printing. Alkali treatment and PLA pre-coating were applied to enhance fibre-matrix interactions. Tensile testing revealed that the hemp composites exhibited the highest strength, with values reaching 29.12 MPa for surface-treated fibres, when compared to 23.84 MPa and 18.91 MPa for flax and PALF, respectively. Thermogravimetric analysis (TGA) showed that alkali-treated hemp composites had the greatest thermal stability, with an onset degradation temperature of 306°C, compared to 291°C for flax and 295°C for PALF. Scanning Electron Microscopy (SEM) confirmed improved fibre dispersion and interfacial bonding in treated hemp composites, contributing to enhanced mechanical performance. Fourier Transform Infrared (FTIR) spectroscopy revealed reduced O-H peaks, indicating successful fibre surface modifications. These findings highlight the potential of hemp fibres for automotive applications, offering superior mechanical properties and thermal stability over other natural fibres when incorporated into PLA composites.

This study examined the mechanical, morphological, and electrical insulation properties of alkali-treated palm kernel shell particles reinforced polypropylene (PKSPRPP) developed for industrial applications. The palm kernel shell particles (PKSP) were subjected to alkali treatment, washed, dried, crushed, and sieved to a fine particle size. Compression molding was utilized to create untreated and treated composite samples with a PKSP size of 53 μm and filler contents of 10, 20, and 30 wt%. Polypropylene was the matrix utilized. While resistivity and dielectric strength tests were utilized to ascertain the electrical qualities, tensile and hardness tests were conducted in accordance with ASTM standards. Fourier transform infrared spectroscopy and X-ray diffraction were used to examine the functional and the crystallinity characteristics of PKSP, respectively. Scanning electron microscopy was utilized to investigate the morphology of the composite samples. The results showed that alkali treatment removed the impurities in the filler and enhanced the adhesion between PKSP and PP. The chemically treated PKSPRPP had better morphological, mechanical, and electrical insulation properties than the untreated PKSRPP composites. This study concludes that alkali-treated PKSPRPP is a valuable material for polymer composite applications, as can be seen in its good mechanical and electrical properties, and can be considered for electrical insulation coating and piping applications.

A novel buckling design strategy for double-double (DD) laminates under in-plane shear loading has been developed, employing an off-the-shelf approach to enhance the applicability of DD configurations in aeronautical main structures. This strategy relies on four key components: Tsai’s modulus, the Master-Ply concept, the D 22 * maximization criterion, and a homogenization procedure tailored for commercially available unidirectional (0°, 90°) and angle-ply (±30°, ±45°, ±60°) carbon/epoxy prepregs. Finite element analysis (FEA) was used to validate analytical predictions for long/infinite composite plates under two primary boundary conditions: simply supported and clamped edges. From a design standpoint, increasing the number of sub-laminate repetitions [±ψ/±ϕ] rT effectively reduces bend-twist coupling effects ( D 16 * , D 26 * ) in DD laminates. Simultaneously, the bending stiffness components ( D 11 * , D 22 * , D 12 * , D 66 * ) increase by nearly an order of magnitude when the laminate thickness is doubled. Among the 13 DD configurations analyzed, those with smaller angles, such as [±0/±30] 4 and [±0/±45] 4 , exhibit the lowest critical buckling loads under in-plane shear loading. However, configurations like [±0/±45] 4 and [±30/±30] 4 outperform the benchmark case [03/±452/90] s by 45.56% and 44.27%, respectively. A clear trend emerges: increasing the angle orientation reduces the half-wavelength of buckling, leading to higher critical loads. The top-performing DD configuration, [±60/±60] 4 , achieves a buckling load nearly double that of the benchmark case—an improvement of approximately 185%.

3D printing has been increasingly used in aeronautical and automotive industry for the production of integral metallic or fibre-reinforced plastic (FRP) parts. Various additive manufacturing methods have been developed to respond to the growing industrial needs. The Screw Extrusion Additive Manufacturing (SEAM) is a pellet-based production technique relying on a heated screw extruder. This heated single screw extruder facilitates the plasticisation of injection-moulding granules and direct extrusion of high temperature thermoplastics. The mechanical properties of the 3D-printed part depend directly on adjustable process parameters, making process optimisation a strongly empirical task. To gain a solid comprehension of the process, the present article focuses on a first extensive characterization of a carbon fibre reinforced PA6 thermoplastic material with a fixed set of manufacturing parameters. The material anisotropy and fibre characteristics are first analysed on polished micrograph samples. In-situ specimens are extracted from 3D-printed parts and mechanically tested under tension, compression and bending in different material directions. In particular, the work focuses on the investigation of failure patterns through the use of the Digital Image Correlation (DIC) technique. In light of the experimental results, the article discusses potential improvements of the mechanical properties through the optimisation of manufacturing parameters.

This study focused on the effects of three different sets of curing conditions on the creep properties of epoxy resin composites reinforced with unidirectional sisal fibers: (i) curing at room temperature, followed by postcuring at 120°C in an oven; (ii) curing in an oven at 100°C for 4 h; and (iii) curing with an accelerator at a ratio of 100:38:1 at room temperature without postcuring. The sisal/epoxy [0°] composite cured at room temperature presented the highest tensile strength, whereas the sisal/epoxy composite cured with an accelerator presented superior tensile creep resistance and long-term performance. The scanning electron microscopy images confirmed good fiber/matrix adhesion. Finally, the Findley and Burger models effectively predicted the creep behavior of the sisal/epoxy composites under long-term service use.

Yarns from recycled carbon fibre (rCF) and thermoplastic fibres have shown potential in achieving high mechanical properties in thermoplastic composites. As the thermoplastic fibre component of the yarn melted during the composite manufacturing to form the matrix of the composite, the rCF content of the yarn is equivalent to the rCF content of thermoplastic composites (typically 45–55% by volume). However, in order to use such yarns for thermoset composites from rCF, the yarn must be impregnated with a liquid thermoset resin. To ensure good mechanical properties in thermoset composites, the rCF content in the yarns must be as high as possible. Due to the smooth surface and lack of fibre-to-fibre cohesion, producing yarn or slivers from rCF alone through carding and drawing is highly challenging. As a result, the use of rCF yarns in thermoset composites remains underexplored. This research addresses the knowledge gap by examining the influence of rCF yarn structures on tensile and impact properties of composites. Two yarns were developed using friction and wrap spinning techniques, where a core of rCF is wrapped with thermoplastic filaments or fibres (<10 wt%) to achieve an rCF content of >90 wt%. The developed yarns were impregnated with epoxy resin using the resin transfer moulding technique to produce unidirectional composites. The investigations show that while the compactness, hairiness and processability of the yarn are significantly affected by differences in yarn structure, the tensile and impact properties of the composites remain comparable.