A macroscale finite element approach for simulating the bending behaviour of biaxial fabrics

A macroscale finite element (FE) model was developed to simulate the forming behaviour of biaxial fabrics, incorporating the effects of bending stiffness to predict fabric wrinkling. The dependency of the bending stiffness on the fibre orientation was addressed by extending a non-orthogonal constitutive framework previously developed for biaxial fabric materials. The nonlinear bending behaviour of a biaxial non-crimp fabric (NCF) with pillar stitches was characterised by a revised cantilever test using structured light scanning to measure specimen curvature, providing input data for the material model. Simulations were performed to replicate the bias-extension behaviour of the NCF material, showing good agreement with experimental data. Wrinkles were observed within the central area of the specimen at low extension, which consequently affect the uniformity of the shear angle distribution in the region where pure shear is expected.EPRSC Doctoral Training Partnership awar

Frictional behaviour of non-crimp fabrics (NCFs) in contact with a forming tool

Microscopic observation and analysis are used to examine the role that contact conditions play in determining the frictional behaviour of non-crimp fabrics (NCFs). The true fibre contact length is measured over a range of normal pressures. For the NCF considered, the contact length is 67% lower than for a corresponding unidirectional tow-on-tool contact at a pressure of 240 kPa. The difference in contact behaviour is associated with the fabric architecture, specifically stitching and gaps between tows. These microscopic observations are used to predict friction using a constant interface shear strength model. These predictions are found to compare well with macroscopic friction measurements taken using a sliding sled arrangement, once the roughness of the sled tool is taken into account

Long discontinuous carbon fibre/polypropylene composites for high volume structural applications

A processing route is presented to manufacture discontinuous carbon fibre reinforced polypropylene (CF.PP) composites, using much longer fibre lengths (25mm) and higher volume fractions (up to 45%) than previously reported in the literature. Carbon fibre tows are coated with different ratios of polypropylene, blended with a maleic anhydride coupling agent, to investigate the influence of the interfacial shear strength at the microscale on the macroscale composite properties. Improvements in the tensile performance at the macroscale (70% increase) are not as high as those reported for the interfacial shear strength at the microscale (300%), following the addition of the coupling agent. Consequently, the tensile strength of the CF.PP material is only 45% of values reported for carbon fibre/epoxy systems, however, the tensile stiffness is comparable. This demonstrates the potential for using CF.PP for structural applications, following further process optimisation to overcome the current high levels of porosity (3.3% at 0.45Vf) to improve the tensile strength

3D geometric modelling of discontinuous fibre composites using a force-directed algorithm

A geometrical modelling scheme is presented to produce representative architectures for discontinuous fibre composites, enabling downstream modelling of mechanical properties. The model generates realistic random fibre architectures containing high filament count bundles (>3k) and high (~50%) fibre volume fractions. Fibre bundles are modelled as thin shells using a multi-dimension modelling strategy, in which fibre bundles are distributed and compacted to simulate pressure being applied from a matched mould tool. FE simulations are performed to benchmark the in-plane mechanical properties obtained from the numerical model against experimental data, with a detailed study presented to evaluate the tensile properties at various fibre volume fractions and specimen thicknesses. Tensile modulus predictions are in close agreement (less than 5% error) with experimental data at volume fractions below 45%. Ultimate tensile strength predictions are within 4.2% of the experimental data at volume fractions between 40%-55%. This is a significant improvement over existing 2D modelling approaches, as the current model offers increased levels of fidelity, capturing dominant failure mechanisms and the influence of out-of-plane fibres

A macroscale finite element approach for simulating the bending behaviour of biaxial fabrics

A macroscale finite element (FE) model was developed to simulate the forming behaviour of biaxial fabrics, incorporating the effects of bending stiffness to predict fabric wrinkling. The dependency of the bending stiffness on the fibre orientation was addressed by extending a non-orthogonal constitutive framework previously developed for biaxial fabric materials. The nonlinear bending behaviour of a biaxial non-crimp fabric (NCF) with pillar stitches was characterised by a revised cantilever test using structured light scanning to measure specimen curvature, providing input data for the material model. Simulations were performed to replicate the bias-extension behaviour of the NCF material, showing good agreement with experimental data. Wrinkles were observed within the central area of the specimen at low extension, which consequently affect the uniformity of the shear angle distribution in the region where pure shear is expected.EPRSC Doctoral Training Partnership awar

Finite element study of the microdroplet test for interfacial shear strength: Effects of geometric parameters for a carbon fibre/epoxy system

A 3D finite element model has been developed to identify the main causes of variability in the microdroplet test, which is commonly used to characterise the interfacial shear strength between polymer matrices and single filaments. A more realistic droplet shape and test configuration, including meniscus details and prismatic shear blades, have been modelled for a carbon fibre/epoxy system to simulate a more representative set up than is commonly used in the literature. The interfacial behaviour has been modelled using a cohesive surface contact and fibre breakage has been captured using a maximum stress criterion. A statistical study has been performed to systematically evaluate the influence of key geometrical test parameters on the variability in the measured interfacial shear strength values and the likelihood of fibre breakage. Parameters studied are fibre embedded length, fibre diameter, shear blade radial opening distance and shear blade axial misalignment. Results of the studied carbon fibre /epoxy system suggest that fibre embedded length and the combined effects of the shear blade radial distance and the shear blade axial misalignment are the most significant sources of variability for the measured interfacial shear strength. However, fibre embedded length and the shear blade radial distance are the most significant variables contributing to fibre breakage

A macroscale finite element approach for simulating the bending behaviour of biaxial fabrics

A macroscale finite element (FE) model was developed to simulate the forming behaviour of biaxial fabrics, incorporating the effects of bending stiffness to predict fabric wrinkling. The dependency of the bending stiffness on the fibre orientation was addressed by extending a non-orthogonal constitutive framework previously developed for biaxial fabric materials. The nonlinear bending behaviour of a biaxial non-crimp fabric (NCF) with pillar stitches was characterised by a revised cantilever test using structured light scanning to measure specimen curvature, providing input data for the material model. Simulations were performed to replicate the bias-extension behaviour of the NCF material, showing good agreement with experimental data. Wrinkles were observed within the central area of the specimen at low extension, which consequently affect the uniformity of the shear angle distribution in the region where pure shear is expected