The effect of ply thickness on the notch sensitivity and bearing properties on carbon fibre reinforced polymer composites and their hybrid laminates with steel foils were studied. Laminates with ply thicknesses of 0.3 mm and 0.03 mm comprising of CFRP and hybrid laminates were manufactured and characterized using tension, open hole tension and double lap bearing tests. A 25% ply substitution was found to double the bearing load with extensive plastic deformation in the joint while maintaining high stress and maintaining constant cross-sectional thickness in the laminate. With a good agreement between the finite element predicted values and failure behaviour, the damage initiation and progression behaviour could be observed experimentally. We numerically captured (i) rapid failure of 0° plies in the thin ply CFRP hybrid and (ii) continuous delamination with significant plastic deformation for the thick ply CFRP hybrid. The numerical results significantly reduce future experimental work when designing hybrid laminates and could allow the laminate lay-up to be tailored for load cases. Both the experiments and numerical models underline the distinct size effects occurring with respect to the ply thicknesses when hybridising a very ductile metal with a brittle yet strong composite material.

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The increasing use of carbon fibre reinforced polymer (CFRP) composites in aviation and automotive industries has led to the adoption of automated production methods such as pultrusion or resin transfer moulding (RTM) for cost reduction in the production lightweight structures. These processes however, offer limited freedom to locally reinforce structures. This paper describes an approach to utilise a basic geometry for several similar parts and add local reinforcement patches only in regions of load introduction or high local stress. The approach offers the benefit of being able to combine automated production methods with unprecedented design freedom. The specific bearing performance for three different local reinforcement using (1) add-on CFRP patches, (2) surface mounted steel foils and (3) steel foil interleaving in replacement of 90° plies with foils of the same thickness as the CFRP plies (0.125 mm) is compared by double lap bearing tests. The bearing strength improves with the addition of patches, for surface mounted steel foils, more so as CFRP co-cured patches, and most as an interleaved configuration. Quasi ductile failure of the bearing joints was maintained due to additional plasticity of the steel foils, producing a joint that fails safely while enhancing the bearing strength. When examining the hybrid laminates, all samples buckled and failed in bearing compression/shear. Brooming was evident on the compressive side of the hole where the bolt indented the laminate. Indentation led to shear kink bands along the washer supported region and appear as large compression/shear damage above the washer confined region of the laminate. When normalised by weight, the three approaches show similar bearing performance. However, each approach has specific advantages with regards to processing, electrolytic potential, or absolute bearing strength, depending on the design of the load introduction.

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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.@en

The rheological properties of epoxy matrices and adhesives during cure are modelled in this work. Three epoxy-amine systems, designed for different applications with distinct curing behaviours, were investigated. Curing times ranged from a few seconds to upwards of 220 min depending on the cure temperature and formulation of the epoxy. The presented time-dependent model adds an exponential fit to Kiuna-Fontana's master curve of dimensionless viscosity, as well as a weighting function for the overall fit, aiming at creating a more accurate model. A major benefit of the approach is that time consuming determination of the cure kinetics using differential scanning calorimetry (DSC) is not required. The developed model was also used to predict the viscosity of a typical, non-isothermal curing cycle for composite manufacturing. It proved to correlate well with both isothermal and non-isothermal processes, demonstrating that it is particularly useful model to describe viscosity for flow modelling or process optimisation.

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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.

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Fast-cure resins combined with high pressure liquid composite moulding processes (LCM) such as compression resin transfer moulding (CRTM) may be used to produce composite parts with cycle times of only a few minutes. Processing of such fast-cure resins adds difficulties to manufacturing due to the high exothermic reaction during cure, where gelation may occur before fibre impregnation. Combined modelling of rheo-kinetics, flow and heat transfer can help optimising the process parameters and ensure complete impregnation without sacrificing cycle time. The numerical model developed in this study was used to model the CRTM process for a 4 mm plate with a mould temperature of 100°C. It was shown that cure times ofa few minutes are possible with the studied epoxy but large variations of the degree of cure at different locations over the thickness can occur.@en

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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Fast-curing epoxy polymers allow composite 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 manufactured parts. The kinetic properties were measured as a function of part thickness and it was noted that the exothermic heat of reaction can be significantly reduced with the addition of silica nanoparticles, which were well-dispersed in the epoxy. 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. No significant viscosity increase was found with the addition of up to 20 wt% of silica nanoparticles. The predictions were compared to the experimental data, and the agreement was found to be very good.@en