Journal of Composite Materials

Composite aircraft structures have been thoroughly investigated with regard to various loading combinations, which are capable of inducing different failure modes. The failure modes, particularly the fracture energies associated with modes I and II, are influenced by geometric and material variations, contributing to both epistemic and intrinsic uncertainties. The present work introduces a numerical-experimental methodology to simulate the Mixed Mode Bending (MMB) test conducted on specimens manufactured with carbon fibre and epoxy resin adherents and a ductile-based epoxy adhesive. The methodology employs a two-dimensional finite element model developed in Abaqus^® software aided by a Python^® routine. The Design of Experiments (DoE) technique, based on the Plackett-Burman Design (PBD) method, is applied to reduce the number of simulations needed. To assess the impact of mechanical and geometric variables on fracture strength responses in various mixed-mode ratios, the Compliance-Based Beam Method (CBBM) adapted for the MMB test is used, following the ASTM D6671 standard. The numerical model uses 4-node cohesive elements (COH2D4) for the adhesive layer and 4-node plane strain elements (CPE4R) for the composite adherents, with the loading system modelled through two-node linear rigid links (R2D2). A triangular traction-separation law (TSL) is adopted to simulate adhesive behaviour, allowing validation against experimental results from the literature. The methodology, considering the investigated materials, was applied to mixed-mode ratios of 25%, 50%, 60%, and 75%, and results showed that at 25%, the influence of mode I fracture energy (G _Ic ) was only 8% lower than that of mode II (G _IIc ), whereas at higher ratios the difference exceeded 270%, highlighting the increasing dominance of mode II mechanisms. The numerical model consistently predicted fracture loads conservatively compared to experimental values, and the statistical analysis identified that mode II fracture energy and adhesive modulus are key parameters influencing bonded joint strength. The total mixed-mode fracture energy (G_Tc,MMB) resulting from CBBM was used to determine the Main Effect (ME), clearly ranking the parameters that most affect the mechanical behaviour of the bonded joint. These results demonstrate the effectiveness and robustness of the proposed methodology for assessing bonded composite joints under complex mixed-mode conditions.

Interleaving of thermoplastic veils within carbon fibre composites is a method commonly utilised to increase the fracture toughness of composite materials. Previous studies have presented a range of fracture toughness values attributed to the inclusion of polyamide based thermoplastic veils; however, a large portion of these have remained focused on the use of veils that retain their fibre structure during the laminate cure cycle. This study sets out to explore the Mode-I fracture toughness and resultant failure mechanisms of a carbon/epoxy composite interleaved with a low-melt copolyamide thermoplastic veil at different areal densities. The results reveal that the thermoplastic veil polymer successfully precipitated from the initial veil fibre structure during the cure cycle resulting in a phase transformation from veil fibres to bead-like particles across the interlayer. As the veil density is increased, there is a decrease in the Mode-I fracture toughness with a reduction of 23%, 33% and 52% for 8, 14 and 20 g/m² veil configurations. Fractography of the fracture surface revealed the weak interface between the phase separated Co-PA particle and epoxy matrix as the primary failure mechanism across all interleaved laminates. Microscopic analysis demonstrated that the use of low-melt copolyamide veils resulted in a semi-uniform distribution of precipitated thermoplastic particles in low veil densities (8 g/m²), however led to a slightly larger particle size and onset of agglomerations as the density was increased (14 g/m²) before forming a bi-continuous phase at high veil densities (20 g/m²) leading to larger reductions in the calculated fracture toughness.

Polypropylene (PP) composites are prepared using calcined shell waste (CS) at 8% by weight, with incorporation of Polyethylene grafted with maleic anhydride (PE-g-MA). For comparison purposes, the composites are mixed using either a single-screw extruder (SSE) or a twin-screw extruder (TSE), followed by injection molding. The composites (SSE) with the addition of 10 wt% PE-g-MA showed a reduction in fluidity of approximately 40.78 %, while those produced using TSE showed a 40% decrease. The impact strength of PPCSPE-g-MA10 (TSE) decreased by around 37.92%, while the composites with fine CS showed improved performance. The elongation at yield was reduced by 12.30% for SSE samples, while an increase of 21.26% was observed for composite with 2 wt% PE-g-MA processed by TSE. The yield stress and tensile strength were slightly reduced for TSE samples, while SSE composites showed a similar performance to pure PP. A slight increase in tensile modulus was observed in all composites. The flexural modulus of the composites with 2 wt% PE-g-MA showed better results, with an increase of 21.6% (TSE) and 18.4% (SSE). Flexural strength was higher in the SSE composites, attributed to better dispersion and adhesion of fine CS particles. The analysis of variance (ANOVA) revealed statistically significant differences in the mechanical properties of the composites. Both the Nicolais-Narkis and Pukanszky models demonstrated that the addition of compatibilizer positively influences interfacial adhesion. The initial thermal stability of the PPCSPE-g-MA composites decreased by no more than 13%. A promising solution for the industrial recycling of calcined residue is evaluated.

This paper investigates the tensile strength of glass fiber-reinforced polymer (GFRP) bars embedded in concrete under harsh environmental conditions and sustained loads. A total of 135 specimens were constructed for this investigation, each consisting of a 1.3 m length of GFRP bar centrally embedded in a concrete prism. After casting and curing, the specimens were exposed to a tensile stress that is 25% of their maximum tensile strength. The specimens were then exposed to six accelerated laboratory conditions for 6, 12, and 18 months, along with a controlled environment in the laboratory. The test conditions involved exposure to both seawater and tap water at temperatures of 23°C and 50°C, alternating wet/dry cycles in seawater at 50°C, and immersion in an alkaline solution maintained at 50°C. Moreover, two harsh field conditions (hot dry and hot humid) were also considered. GFRP bars’ performance was assessed through tensile strength tests. The results indicated that after the exposure of 18 months, the maximum tensile strength loss occurred in bars exposed to environments of tap water and alkaline solution at the temperatures of 50°C with 18.2 and 16.8% strength losses, respectively. Conversely, the two field conditions did not significantly affect the residual strength of the GFRP bars. The test results revealed that the sustained load alone increased the strength losses by an average of 6.4% with a minimum of 1.3% in the environment of seawater at room temperature and a maximum of 11.4% in the seawater (wet/dry cycles) at 50°C after 18 months.

Graphical Abstract

Sugarcane bagasse, a renewable and abundant natural fiber, shows great potential as a reinforcement in composite materials. While fiber length and weight ratio are key factors influencing the mechanical properties of composites, their impact on sugarcane bagasse-based composites remains unexplored. This gap highlights the need for research to determine how these factors affect the performance of such composites. Thus, the present work investigates the effect of fiber length and weight ratio on the mechanical properties of sugarcane bagasse-glass fiber reinforced polyester matrix composites. The fiber length and weight ratio of sugarcane bagasse are varied to assess their influence on the mechanical properties of the composites. The fiber length of sugarcane bagasse is varied in three different levels: short (0.5-1.7 mm), medium (1.7-4 mm), and long (greater than 4 mm). Additionally, the weight ratio of sugarcane bagasse fibers is adjusted at three levels (10, 20, and 30 wt%) with 5 wt% glass fibers mixed in unsaturated polyester resin. The composites are prepared by a hand lay-up method, and ASTM standards are followed in preparing the samples. Experimental results indicate that longer sugarcane bagasse fibers and 30 wt% contribute to improved tensile strength. Based on the results, increasing the SCB fiber length and weight percentage consistently enhanced the mechanical properties of the composite. Tensile strength increased from 27 MPa (short fibers) to 35.8 MPa (long fibers) and from 22.2 MPa (0% fiber) to 39 MPa (30% fiber); flexural strength improved from 73.2 MPa (short fibers) to 84.75 MPa (long fibers) and from 63.3 MPa (0% fiber) to 84.75 MPa (30% fiber); while compression strength rose from 110.44 MPa (short fibers) to 115.152 MPa (long fibers) and from 108.5 MPa (0% fiber) to 118.2 MPa (30% fiber). Moreover, the incorporation of sugarcane bagasse improves compression strength, flexural strength, and impact strength of the composites. The findings give valuable insights for optimizing sugarcane bagasse-glass fiber reinforced polyester composites for different application areas.

This study offers an in-depth examination of the bending behaviour of functionally graded carbon nanotube (FG-CNT) reinforced composite beams, emphasising the effects of diverse porosity distributions and their interaction with sophisticated foundation models, such as Winkler, Pasternak, and Kerr foundations. The inquiry examines various porosity patterns, including uniform and non-uniform distributions (X-CNT, O-CNT, and V-CNT), to clarify their impact on the mechanical properties of nanocomposite beams. This research is novel due to the integration of porosity effects with advanced elastic foundation models, providing a detailed understanding of how these factors collectively influence the bending performance of composite beams. A comprehensive mathematical framework is established to elucidate the relationship between porosity distributions and foundation factors, with findings corroborated by benchmark investigations in existing literature. The study performs a comprehensive parametric analysis, investigating the impacts of geometric configurations, volume fractions, reinforcing schemes, and porosity changes on essential metrics including displacement and stress profiles. The integration of Winkler, Pasternak, and Kerr foundation models represents a significant enhancement in the discipline, since it increases the accuracy of modelling beam-foundation interactions in real-world applications. This study provides critical insights for the optimised design and engineering of advanced structural components across various industries, including aerospace, civil, and mechanical engineering, where the integration of porosity features and realistic foundational conditions is vital for achieving improved performance and reliability.