In the collaborative effort towards standardisation of out-of-plane permeability measurement, an international benchmarking exercise was carried out whereby 19 participants worldwide were instructed to measure the out-of-plane permeability following a number of strict guidelines, informed by the outcomes of the first international benchmarking exercise completed in 2021. This paper presents the results of the exercise and an assessment of the reproducibility of the data and the suitability of the proposed test method. The data returned were subjected to a number of statistical analysis methods, which showed that adherence to the test guidelines resulted in a high likelihood of a participant not being an outlier and therefore providing evidence that the test method proposed in this paper is a suitable way forward for a standardised test method.@en

A common approach to toughen epoxy matrix systems is to dissolve a thermoplastic phase, in the uncured thermosetting monomers. These can interdiffuse within the thermoplastic, followed by reactioninduced phase separation, leading to intricate graded morphologies with a high fracture toughness. Here, we first architect the thermoplastic phase, made of poly(etherimide) films as a macroscopic layered scaffold, and infiltrate it by an epoxy system, leading to dual-scale morphologies with distinct spatial control of morphological feature at the microscopic and macroscopic scale. The fracture toughness of the modified epoxy system is investigated as a function of varying cure temperature (120–200 °C) for interphase formation and poly(etherimide) film thickness (50–120 μm). Results show that the fracture toughness of the heterogeneous system is mainly controlled by the macroscopic feature, the final PEI layer thickness. Remarkably, as the PEI layer thickness exceeds the plastic zone around the crack tip, around 60 μm, the fracture toughness of the dual scale morphology surpasses the property of bulk PEI. Additionally, decreasing the gradient microscale interphase morphology triggers higher crack tortuosity, which seems to be the dominating mechanism for the synergistic toughening. Ultimately, this knowledge will lead to novel toughening approaches to increase the damage tolerance of fibre reinforced composites by suppressing delamination damage modes.@en

Utilization of sustainable feedstocks to fabricate renewable thermosetting epoxy resins has been of great interest recently; however, their translation into composite structures and benchmark comparisons are poorly understood. Phloroglucinol is a phenolic molecule obtained from brown algae, and its epoxidized form is a high viscosity, high reactivity monomer. In this study, the potential of epoxidized phloroglucinol as a laminating resin was examined in comparison with a bisphenol A diglycidyl ether (BADGE) epoxy monomer employing the Epikure 04908 linear amine hardener system. Utilization of a reactive diluent for PHTE resin was necessary for room temperature laminating applications to reduce viscosity, and the thermomechanical properties of PHTE-based resins and composites are superior to those of BADGE systems.@en

The research focuses on the simultaneous deformation of uncured metal-composite laminates under the press forming process of a double-curved dome part. The study is designed to evaluate the influence of material type, inter-ply friction and clamping force by the use of a finite element modelling method. The result shows that the main failure mode for aluminium-based hybrid materials is metal cracking while prepreg buckling dominates the failure of stainless steel-based hybrid materials. The increase of clamping force contributes to the deformation of fibre reinforced prepreg layer and decrease the risk of prepreg buckling, but the simultaneous increase of plastic deformation tends to induce the failure of metal cracking. The variation of friction coefficient at the metal-prepreg interface affects the inter-ply sliding displacement during the forming process. This work provides a numerical method on the material selection and process parameter optimisation where compatible deformation of the individual layers can be achieved through its own mechanisms.@en

As the aviation industry strives to minimise its environmental footprint, understanding the full life cycle impacts, including maintenance, becomes essential for sustainable development. This paper addresses the critical research gap in the environmental assessment of aircraft maintenance by conducting a comprehensive life cycle assessment based on an Airbus A320 aircraft. By combining a top-down check-level analysis and a detailed examination of the aircraft manufacturer's maintenance planning document, this study provides significant insights into the environmental implications of maintenance activities. The check-level analysis provides a general overview, while the analysis of the maintenance planning document delves into individual tasks, enabling the identification of components with the highest ecological impacts. This research emphasises the importance of including aircraft maintenance activities in life cycle assessment studies and provides valuable guidance for researchers, industry practitioners, and policy makers in prioritising sustainability measures and enhancing the environmental performance of aircraft throughout their life cycle.

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The development of epoxy resin formulations from renewable feedstocks has been thoroughly explored in the chemical literature. A simple one-pot chemical reaction involving sustainable phenolic molecules and epichlorohydrin results in the production of renewable epoxy monomers. These monomers can be cured with amines or anhydrides to yield cross-linked thermosetting resins. Although a wide variety of recipes exist, there is a notable gap in the application of these sustainable resin formulations to engineering contexts. This gap is primarily due to the lack of comprehensive, standardized analyses of these resin recipes, which impede their potential use in advanced composite applications. In this study, we reveal a high-performance resin formulation utilizing epoxidized phloroglucinol derived from brown algae in combination with an aerospace-grade amine hardener. The resin processing and thermomechanical properties are investigated using ASTM standard tests including tensile strength, flexural strength, fracture toughness, and interlaminar shear strength. Given the detailed comparative analysis, the partially renewable resin recipe outperforms petroleum derived analogues.

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Epoxy resins have been used as composite matrix materials for over half a century, enabling lightweight materials for a variety of applications. Their properties including high strength, chemical resistance, and ease of processing feature them as prime candidates for composite applications. Currently, high performance epoxy monomers are derived from non-renewable feedstocks, which presents a problem for future generations and technologies regarding sustainability. Many bio-based alternatives have been investigated, but properties fall short of high-performance industry standards due to a lack of appropriate chemical groups. Herein 3 bio-based epoxy monomers, with potential to replace high-performance standards, are compared against 3 petroleum derived industry standards. Industrial high performance aromatic diamine crystalline hardener is used to synthesise resin systems. Tensile, flexural and fracture toughness properties are investigated following ASTM standards. Beyond a practical investigation of the physical properties of these resins a comparison of chemical structure to mechanical properties is made. Free volume investigations using positron annihilation lifetime spectroscopy (PALS) were performed, and can be used to explain to some degree the observed properties of the resin systems. @en

Thin-ply carbon fiber reinforced polymers (CFRP) have claimed significant attention for their potential to surpass traditional composite materials in terms of performance metrics such as first-ply damage criteria, fatigue life, and ultimate strength. This study focuses on investigating the friction behavior of dry carbon fiber tow during mechanical bar spreading, a crucial process in the manufacturing of thinply CFRP. By systematically examining the interplay of wrap angle, tow pre-tension, and final tension, insights are provided into the frictional forces exerted on the carbon fibers. The study utilizes an experimental framework to analyze single-bar and multi-bar setups, considering both symmetric and asymmetric configurations. Results reveal non-linear friction behavior, with increasing wrap angles leading to decreased dynamic friction coefficients. Additionally, results seem to suggest that higher pretension reduces internal tow movement, thereby decreasing friction losses. Multi-bar setups exhibit distinct friction profiles compared to single-bar setups, especially for larger wrap angles and asymmetric cases, indicating the influence of superimposed wrap angles on friction. Recommendations for future research include further exploration of factors such as non-uniform normal loads and relaxation distances between spreader bars to enhance modeling accuracy and optimize friction performance.@en

Modeling the consolidation of fiber-reinforced thermoplastic composites at the part level presents a formidable computational challenge due to the multi-scale nature of the process. In this article, a method to bypass the multi-scale problem by homogenizing the micro scale and describing the medium with characteristic parameters is described. The model is intended for press molding of hybrid textiles and considers a free-form plate with non-uniform thickness and can describe consolidation in three dimensions with some restrictions. 2D implementation in FEM shows how in-plane matrix pressure gradients can arise in parts and cause fiber disorientation. Experimental verification demonstrates that fiber disorientation arises at the predicted location, and that defect size is proportional to matrix pressure gradient. This novel consolidation model provides new insights, enables part and process optimization, and paves the way for high-quality composite part production. Highlights: A consolidation model for press molding of hybrid textiles is presented. A method to extend consolidation models for complex geometry is presented. The origin of defect formation in complex geometries is explained.

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This study presents the first steps of a benchmarking exercise on image processing of composite materials. Employing three distinct imaging protocols and five different image processing algorithms, the research explains the variability in capturing microstructural features of common micrograph dataset. Results highlight the sensitivity of different methods to factors like illumination inhomogeneities and pixel density, influencing the accuracy and consistency of obtained results. By comparing methodologies from different researchers in a blind format, the study identifies strengths and limitations, laying the groundwork for future benchmarking activities. Moving forward, this research sets the stage for standardized protocols and guidelines, aiming to enhance the reproducibility and reliability of microstructural analysis in composite materials. Such efforts are crucial for advancing material design and development, ultimately creating tailored composite materials with enhanced performance and functionality.@en

Ultrasonic welding of fibre-reinforced thermoplastics is a joining technology with high potential for short welding times and low energy consumption. While the majority of the current studies on continuous ultrasonic welding have so far focused on woven reinforcements, unidirectional materials are preferred for highly loaded aerospace components due to their better mechanical performance. Therefore, this paper investigates the influence and interdependence of the welding speed, amplitude, and energy director thickness on the weld quality of adherends made of unidirectional composites. The quality of the welded joints is assessed by a single-lap shear strength and fracture surface analysis complemented by the microscopic analysis of cross-sections and comparison to a co-consolidated reference. The results showed that the welding process is highly affected by changing welding speeds for a given amplitude. Furthermore, while lower amplitudes lead to significant scatter in the welding quality, higher amplitudes result in increased heating rates and a fully molten energy director even for high welding speeds. Nevertheless, insufficient consolidation at high welding speeds results in porosity in the weld line. Finally, it was observed that thicker, and therefore more compliant, energy directors lead to more uniform melting of the energy director and less deviation in the weld quality for a wider range of welding speeds.@en

This study focuses on the spring-back as a function of the degree of cure on single-curved metal-composite laminates. The manufacturing through a hot-pressing process involves different (curing) stages and can reduce the spring-back with the proper combination of forming and curing. The cure-dependent spring-back is measured and analysed as a function of material constituents, fibre directions, laminate layups, and the process parameters including temperature, holding time and pressure. The results demonstrate that the spring-back ratio after full-cured process is relatively small and mainly depends on the mechanical properties of the metal sheet in laminates. However, temperature and time have a significant effect on the spring-back of partially-cured laminates and the same type of fibre prepreg combined with two different metal sheets have similar trends of spring-back reduction. Moreover, the study found that the hybrid laminates with aluminium sheet delaminate at low pressure after full-cured, while the delamination disappears as the pressure increases. The characterisation on cure-dependency of the spring-back contributes to a better understanding of the deformability of the metal-composite laminates during the hot-pressing process and offers an opportunity to tune the spring-back of these laminates.

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The bias-extension test is one of the test methods to characterise the intra-ply shear behaviour of continuous fibre reinforced composites including fabrics and unidirectional (UD) materials. For the determination of the major mechanical properties of metals, often a uniaxial tensile test is used. Combination of these two methods for the shear deformation of hybrid metal-composite laminates is proposed comparing the method for cross-plied unidirectional prepregs and woven fabric prepregs. The effects of material constituent, shear rate, preheat temperature and normal pressure on the intra-ply shear behaviour are investigated. The results indicate that the material constituents and the frictional responses depending on processing parameters play a critical role in the shear characterisation of the hybrid laminate. The shear angle measurement at four typical strains demonstrates that the support of metal layers improves the shear deformability by delaying the onset of fibre wrinkling. This modified intra-ply shear test contributes to a better understanding of the process design for wet (uncured) hybrid metal-composite laminate manufacturing.

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This paper evaluates the potential of changing the welding force and the compliance of the energy director (ED) to reduce the effects caused by misaligned adherends, which were: increased through-thickness heating, reduced size of welded area and increased heating time. In the methodology that was followed, we welded adherends misaligned by approximately 4.5° in different scenarios: with higher welding force; with increased ED compliance by the use of a thicker ED and; with increased ED compliance by the use of a discontinuous ED. The most significant reduction of the effects caused by misaligned adherends was obtained when combining the use of both increased welding force and discontinuous ED. Such improvement derives from the imposed parallelism caused by the use of a higher welding force and from a more efficient concentration of heat generation at the weld line that occurs when a discontinuous ED is used.

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The production of advanced composites from recycled carbon fibres (rCFs) is critical for the sustainable development of carbon fibre industry. Herein, non-woven mats consisting of commingled rCFs and Polyphenylene-sulfide (PPS) fibres were compression moulded to manufacture rCF/PPS composites, with the fibre/matrix adhesion being tailored by UV-irradiating the non-woven mats. The intralaminar and interlaminar fracture resistance and mechanical performance of the rCF/PPS composites were characterised. The experimental results had demonstrated that improving the PPS/rCF adhesion of the composites significantly increased the intralaminar fracture energies and mechanical properties under tensile and shear loading conditions. However, it also negatively affected the interlaminar fracture resistance. The main fracture mechanism was observed to be fibre evulsion for the intralaminar fracture mode, while crack bridging by the rCFs was the primary fracture mechanism for the interlaminar fracture condition. That led to the contrary influences of the improved fibre/matrix adhesion on the intralaminar and interlaminar fracture resistance of the rCF/PPS composites. In summary, this study had shedded lights on tailoring the crack resistance and mechanical performance of rCFRPs by adjusting the fibre/matrix adhesion using the UV-treatment technique.

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Toughness of epoxies is commonly improved by adding thermoplastic phases, which is achieved through dissolution and phase separation at the microscale. However, little is known about the synergistic effects of toughening phases on multiple scales. Therefore, here, we study the toughening of epoxies with layered poly(ether imide) (PEI) structures at the meso- to macroscale combined with gradient morphologies at the microscale originating from reaction-induced phase separation. Characteristic features of the gradient morphology were controlled by the curing temperature (120–200 °C), while the layered macro structure originates from facile scaffold manufacturing techniques with varying poly(ether imide) layer thicknesses (50–120 μm). The fracture toughness of the modified epoxy system is investigated as a function of varying cure temperature (120–200 °C) and PEI film thickness (50–120 μm). Interestingly, the result shows that the fracture toughness of modified epoxy was mainly controlled by the macroscopic feature, being the final PEI layer thickness, i.e., film thickness remaining after partial dissolution and curing. Remarkably, as the PEI layer thickness exceeds the plastic zone around the crack tip, around 62 μm, the fracture toughness of the dual scale morphology exceeds the property of bulk PEI in addition to a 3 times increase in the property of pure epoxy. On the other hand, when the final PEI thickness was smaller than 62 μm, the fracture toughness of the modified epoxy was lower than pure PEI but still higher than pure epoxy (1.5–2 times) and “bulk toughened” system with the same volume percentage, which indicates the governing mechanism relating to microscale interphase morphology. Interestingly, decreasing the gradient microscale interphase morphology can be used to trigger an alternative failure mode with a higher crack tortuosity. By combining facile scaffold assemblies with reaction-induced phase separation, dual-scale morphologies can be tailored over a wide range, leading to intricate control of fracture mechanisms with a hybrid material exceeding the toughness of the tougher phase.@en

Autoclave curing is one of the most energy consuming processes in manufacturing carbon fibre reinforced polymers. In order to improve the energy efficiency, one needs to understand energy usage in an autoclave and factors that influence it. This work presents two thermodynamic based models for estimating energy consumption in an autoclave. The first model is an analytical approach based on simplified heat capacity equation. The second model combines the Multi-Relaxation-Time Lattice Boltzmann method (MRT LBM) with Fourier heat equation to simulate autoclave temperature flow and energy consumption. The output from the two models were compared to energy consumption data collected using a power meter. The estimated values from the MRT LBM method showed a better match with only 1% difference from the experimental value. Since the two models are parametric and scalable, a what-if analysis was carried out to investigate the influence of varying process parameters on autoclave energy consumption. Parameters including cure cycle, autoclave size and loading capacity.

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A new model is proposed for the consolidation of hybrid textiles, in which air entrapment and dissolution are considered. One of the key parameters is tow permeability, which is described by the analytical model of Gebart and validated at very high fibre volume fractions by direct tow permeability measurement. The model also takes into account the presence of fibres limiting gas diffusion in the molten polymer. Experimental validation of the proposed model is then conducted with quasi-unidirectional glass textile and either polypropylene or polyethylene by measuring the impregnation degree as a function of the consolidation time. Good agreement is found between predictions and measurements for the two matrix systems at different pressures. It is shown that entrapped air significantly influences impregnation. The model offers new and comprehensive insights about the phenomena taking place during consolidation and enables future process optimization.

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Chopped Tape Thermoplastic Composites (CTTCs) offer high formability and performance for complex-shaped components in the aerospace and automotive industries. However, the mesoscopic discontinuity leads to spatial variabilities and correspondingly high scatter in the elastic properties of CTTCs due to the random orientations of chopped tapes and chopped tape-cavity edge interactions. Here we propose a new approach to investigate the effect of mould cavity edges on chopped tape orientation and hence the mechanical properties of CTTCs. Based on this approach, a Set Voronoi tessellation was implemented to represent the variability of local Young's Modulus and chopped tape-cavity edge interactions occurring during the manufacturing process. It was confirmed that the chopped tapes align along the edges, and progressively transition to a random orientation towards the middle of the specimen. The results were validated on moulded specimens and demonstrated the ability to deconvolute the edge interaction.

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