Thin-ply composites are rapidly gaining interest in the composite industry, not only because of the larger design possibilities that they offer, but also because of positive size effects that have ...been shown to improve performance in various loading conditions 1. In this work, carbon fiber–epoxy composites of different ply thicknesses (30–300g/m2 fiber areal weight) were produced from the same batch of Toray M40JB fiber and NorthTPT TP80ep matrix to study the influence of ply thickness on the ultimate strength and on the onset of damage in lamina, laminates and components. Uniaxial tension, open-hole compression and open-hole tensile fatigue tests on quasi isotropic 45°/90°/−45°/0°ns laminates showed very significant improvements regarding the on-set of damage, and in some cases ultimate strength, when decreasing the ply thickness. These performance improvements are the result of major changes in the damage progression and failure modes of the laminates caused by a systematic delay or near suppression of transverse cracking and delamination growth in thin-ply composites. On the component level, thin-ply laminates enabled a marked improvement for bolted-joint bearing, especially in hot–wet conditions. Under impact, the 30μm thin ply laminate exhibited a quasi-brittle failure with extensive translaminar cracking while a ply thickness of 100μm was found as optimum to minimize the projected damage area. Ply thickness scaling of transverse and in-plane shear strength was identified based on classical laminate theory and unnotched tensile tests on quasi-isotropic specimens. The empirical scaling was found to follow a linear trend over a range of ply thicknesses from 30 to 250μm. Due to the near suppression of delamination, the strength of thin-ply composites could then be modeled more effectively than thick ply composites using classical laminate theory or standard multilayer shell modeling.
Thin-ply composites represent a promising approach to further improve the performance of carbon fibre composite structures thanks to their ability to delay the onset of matrix cracking and ...delamination up to the point of fibre dominated failure. However, this increased strength comes with a more brittle failure response which raises concerns on damage tolerance. Thus a careful material optimization is needed to address this trade-off. In this work, eight different formulations of thin-ply composites ranging from low modulus to high modulus carbon fibres are evaluated to understand the effects of the fibre and matrix constituents on the onset of damage and strength in unnotched tensile (UNT) tests of quasi isotropic laminates for ply thicknesses between 300 and 30 microns. The obtained experimental data are combined in master curve diagrams for simplified material selection process. It is observed that certain thin-ply composites with a ply thickness t < 134 μm can reach UNT strength corresponding to or approaching the ultimate strain of the fibres as well as UNT stress at onset of damage as high as 92% of the latter. Based on this knowledge, a novel aerospace grade toughened thin-ply composite system is developed which can reach a quasi-isotropic UNT strength above 1 GPa (>95% of the fibre strain). The newly developed composite is further optimized to improve damage tolerance by toughening the resin and selected interfaces. The effect of those modifications on damage tolerance are evaluated through compression strength after impact (CAI) tests and open hole tensile tests (OHT). It is found that an optimized interlayer toughened thin-ply composite based on 68 microns plies of intermediate modulus fibre can reach both outstanding strength properties with comparable or better CAI and OHT strength compared to current aerospace grade composites.
Fast-cure epoxy systems are used for the mass production of composite parts with cycle times in the minute range. One of the major difficulties observed when processing fast-cure resins is their ...strong exothermic reaction during cure. This may result in a significant temperature overshoot and large temperature gradients over the thickness, and therefore gradients, in glass transition temperature, shrinkage and residual stress. Hence, the cure reaction during the injection, impregnation, compaction and curing stages need to be understood and optimised in order to reduce cycle time without sacrificing mould filling and part quality. A possibility to process such highly reactive epoxies is the compression resin transfer moulding process (CRTM), where the preform is impregnated in through-thickness direction, leading to reduced cycle times. The aim of this work is to identify processing constraints that the different CRTM variations present when using fast-curing resins. We have developed a multi-physics model to compare three variations of the CRTM process: gap injection, direct dosing and wet pressing, by studying their fluid flow and the exothermic reaction. The models show the importance of the injection strategy to avoid temperature overshoot, which can occur before the part is fully impregnated. Our results show that when impregnation time is a limiting factor, wet pressing appears to be a favourable approach for fast composite processing, doubling the available impregnation time before gel.
Understanding the three-dimensional variability of unidirectional composites is relevant to the material performance and the development of advanced material modelling strategies. This work proposes ...a new methodology for the characterization of unidirectional composites, showcased on carbon fibre/poly(ether-ether-ketone) tapes. Three microstructural descriptors were here introduced, each representing an increasing level of complexity in the fibre architecture: from a tortuosity-based single fibre trajectory analysis to fibre groups’ behaviour, to fibre network interconnectivity. The methodology was developed and validated on real material datasets acquired via X-ray computed tomography. A facile method for image analysis was used to reconstruct the three-dimensional fibrous architecture at a single fibre path resolution. The approach bridges a gap in the traditional approach and nomenclature typical of the composite field to describe and quantify complex fibre organization in unidirectional composites, highlighting micro- and mesoscopic features, such as edge-core effects in the fibre arrangement, possibly occurring in tow spreading. The study of the parameter interdependence showed relationships, which will provide further insight for future research in the study of microstructure formation of unidirectional composites, its evolution during processing or loading, and input for advanced modelling techniques based on Representative Volume Elements.
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Natural fibre (NF) reinforced composites offer high specific mechanical properties and are an ecological alternative to synthetic fibre-reinforced composites. While having great potential, their use ...today is limited to non-structural applications, mostly with epoxy or polypropylene matrices. This work studies suitable high-performance thermoplastic matrices and characterises their bulk properties, fibre-wetting and composite mechanical behaviour. Thermoplastic polymers such as poly-l-lactide (PLLA) and polyoxymethylene (coPOM) are matrices with bulk properties similar to epoxy. The results show that PLLA matrix NF-composites have a longitudinal modulus and strength of 27 GPa and 308 MPa. The tougher coPOM matrix NF-composites show both high transverse stiffness and strength of 2.6 GPa and 41.5 MPa and show that even the drawback of creep can be overcome by the use of hierarchically structured coPOM. The developed NF-composites demonstrate in-plane properties comparable to those with epoxy matrices and can outperform them by up to 26% in the transverse direction.
Ultrasonic welding is a promising technology to join fibre-reinforced thermoplastic composites. While current studies are mostly limited to fabric materials the applicability to unidirectional ...materials, as found in aerospace structures, would offer opportunities for joining primary aircraft structures. However, due to the highly anisotropic flow of a molten unidirectional ply undesired squeeze flow phenomena can occur at the edges of the weld overlap. This paper investigates how the fibre orientation in the plies adjacent to the weld line influences the welding process and the appearance of edge defects. Ultrasonic welding experiments with different layups and energy director configurations were carried out while monitoring temperatures at different locations inside and outside the weld overlap. The joints were characterized by single lap shear tests, analysis of corresponding fracture surfaces and microscopic cross-sections. Results showed that the anisotropic flow and the anisotropic thermal conductivity of the plies adjacent to the weld line have a distinct effect on the appearance and location of edge defects. By using energy directors that cover only part of the weld overlap area a new approach was developed to mitigate edge defects caused by the highly directional properties of the unidirectional plies.
The objective of this paper was to develop cure kinetic models to describe the B-stage curing and co-curing assembly of carbon fibre reinforced thermosetting polymer (CFRP) composites. Starting from ...the analytical model, temperature cycles and experimental procedures are developed to join a B-stage CFRP part to a reinforcing B-stage CFRP patch for local reinforcement. Our results show that by using the analytical model, one may precisely describe the cure reaction and join the composites without any additional adhesive. The co-cured composites were successfully manufactured with stable fibre volume fractions and glass transition temperatures between the two sub-components. Additionally, merits of the process, such as modifying reinforcing areas locally, or formation of net shape detail are discussed.
To simultaneously address the lower toughness and the build-up of internal heat for fast-curing epoxy matrices, the influence of nominal 100 nm and 300 nm core-shell rubber (CSR) particles on the ...properties and rheo-kinetics were studied. The fracture energy was enhanced by a factor of 14.5, up to 2572 ± 84 J m−2 with 14.5 wt% of the nominal 300 nm diameter CSR particles, with evidence of cavitation and plastic void growth of the rubber core combined with shear band yielding of the epoxy matrix. These toughening mechanisms were modelled with an approximately linear increase up to 10 wt% for both particle types. At higher concentrations, deviation between the measured and modelled data was observed due to insufficient epoxy to dissipate additional energy. The CSR particles were not filtered out or damaged during the manufacturing of composites and reduced the total heat of reaction with a linear correlation, demonstrating a multi-functionality of simultaneous toughening and reduction of the exothermic peak.
Fast-curing epoxy polymers allow thermoset parts to be manufactured in minutes, but the curing reaction is highly exothermic with heat flows up to 20 times higher than conventional epoxies. The low ...thermal conductivity of the polymer causes the mechanical and kinetic properties of parts to vary through their thickness. In the present work, silica nanoparticles were used to reduce the exotherm, and hence improve the consistency of the parts. The mechanical and kinetic properties were measured as a function of part thickness. The exothermic heat of reaction was significantly reduced with the addition of silica nanoparticles, which were well dispersed in the epoxy. The silica nanoparticles increased the Young’s modulus linearly from 3.6 to 4.6 GPa with 20 wt% of silica, but the fracture energy was found to increase less than for many slow-curing epoxy resins, with values of 176–211 J m
−2
being measured. Although there was no additional toughening, shear band yielding was observed. Further, the addition of silica nanoparticles increased the molecular weight between crosslinks, indicating the relevance of detailed cure kinetics when studying fast-curing epoxy resins. A model was developed to describe the increase in viscosity and degree of cure of the unmodified and the silica-modified epoxies. A heat transfer equation was used to predict the temperature and resulting properties through the thickness of a plate, as well as the effect of the addition of silica nanoparticles. The predictions were compared to the experimental data, and the agreement was found to be very good.
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