Effect of yarn cross-sectional shape on resin flow through inter-yarn gaps in textile reinforcements

Axial flow through gaps between aligned straight yarns with realistic cross-sectional shapes, described by power-ellipses, was analysed numerically. At a given fibre volume fraction, equivalent gap permeabilities have a maximum at minimum size of elongated tapering parts of the gap cross-section and a ratio of gap width to height near 1. When the yarn spacing is given in addition to the fibre volume fraction, calculated maximum and minimum values for the equivalent permeability of inter-yarn gaps, which occur at near-rectangular and lenticular cross-sections, differ by factors of up to 3.3. Novel approximations for the shape factor and the hydraulic diameter in Poiseuille flow were derived as a function of the fibre volume fraction, the yarn cross-sectional aspect ratio and the exponent describing the shape of the power-elliptical yarn cross-section. This allows the equivalent gap permeability to be predicted with good accuracy for any fibre volume fraction and yarn cross-section

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

Multiscale modeling of combined deterministic and stochastic fabric non-uniformity for realistic resin injection simulation

The local fiber arrangement in a bi-directional fabric formed to a complex shape was modeled considering the stochastic arrangement of filaments within yarns, which determines axial and transverse yarn permeabilities, and the stochastic arrangement of yarns in a fabric, which determines the dimensions of interyarn gap spaces locally. To mimic the uncertainty in fabric forming, drape simulation was randomized in terms of start point and yarn start orientations. From yarn permeabilities and simulated local yarn spacing distributions, local fiber volume fractions and fabric permeabilities were approximated. This allowed resin injection into a deformed fabric to be simulated for different drape scenarios with different probabilities and different degrees of fabric randomness. The results indicate that variability in fabric properties and the forming process affects flow front shapes and times for complete impregnation of the reinforcement

Inter-ply stitching optimisation of highly drapeable multi-ply preforms

An efficient finite element model has been developed in Abaqus/Explicit to solve highly non-linear fabric forming problems, using a non-orthogonal constitutive relation and membrane elements to model bi-axial fabrics. 1D cable-spring elements have been defined to model localised inter-ply stitch-bonds, introduced to facilitate automated handling of multi-ply preforms. Forming simulation results indicate that stitch placement cannot be optimised intuitively to avoid forming defects. A genetic algorithm has been developed to optimise the stitch pattern, minimising shear deformation in multi-ply stitched preforms. The quality of the shear angle distribution has been assessed using a maximum value criterion (MAXVC) and a Weibull distribution quantile criterion (WBLQC). Both criteria are suitable for local stitch optimisation, producing acceptable solutions towards the global optimum. The convergence rate is higher for MAXVC, while WBLQC is more effective for finding a solution closer to the global optimum. The derived solutions show that optimised patterns of through-thickness stitches can improve the formability of multi-ply preforms compared with an unstitched reference case, as strain re-distribution homogenises the shear angles in each ply

Modelling the permeability of random discontinuous carbon fibre preforms

A force-directed algorithm was developed to create representative geometrical models of fibre distributions in directed carbon fibre preforms. Local permeability values were calculated for the preform models depending on the local fibre orientation, distribution and volume fraction. The effect of binder content was incorporated by adjusting the principal permeability values of the meso-scale discontinuous fibre bundles, using corresponding experimental data obtained for unidirectional non-crimp fabrics. The model provides an upper boundary for the permeability of directed carbon fibre preform architectures, where predictions are within one standard deviation of the experimental mean for all architectures studied

Simulation of the forming process for curved composite sandwich panels

© 2019, The Author(s). For affordable high-volume manufacture of sandwich panels with complex curvature and varying thickness, fabric skins and a core structure are simultaneously press-formed using a set of matched tools. A finite-element-based process simulation was developed, which takes into account shearing of the reinforcement skins, multi-axial deformation of the core structure, and friction at the interfaces. Meso-scale sandwich models, based on measured properties of the honeycomb cell walls, indicate that panels deform primarily in bending if out-of-plane movement of the core is unconstrained, while local through-thickness crushing of the core is more important in the presence of stronger constraints. As computational costs for meso-scale models are high, a complementary macro-scale model was developed for simulation of larger components. This is based on experimentally determined homogenised properties of the honeycomb core. The macro-scale model was employed to analyse forming of a generic component. Simulations predicted the poor localised conformity of the sandwich to the tool, as observed on a physical component. It was also predicted accurately that fibre shear angles in the skins are below the critical angle for onset of fabric wrinkling

Geometric modeling of 3D woven preforms in composite T-joints

A common method to fabricate net-shaped three-dimensional (3D) woven preforms for composite T-joints is to weave flat 3D preforms via a standard weaving machine with variation in binder yarn path and then separate the preform in the form of a bifurcation. Folding introduces fiber architecture deformation at the 3D woven bifurcation area. In this paper, a geometric modeling approach is proposed to represent the realistic fiber architecture, as a preprocessor for finite element analyses to predict composite structural performance. Supported by X-ray micro-computed tomography (mCT), three important deformation mechanisms are observed including yarn stack shifting, cross-section bending, and cross-section flattening resulting from the folding process. Furthermore, a set of mathematical formulae for simulation of the deformations in the junction region are developed and satisfactory agreement is observed when compared with mCT scan results

Analysis of contact area in a continuous application-and-peel test method for prepreg tack

The relationship between prepreg tack and the degree of intimate contact (DoIC) between prepreg and a rigid substrate was explored in the context of a continuous application-and-peel test method. Tack for a unidirectional prepreg tape was characterised for different surface combinations and varying test parameters (material feed rate, temperature) at a constant compaction pressure. Application of the prepreg to a transparent rigid substrate (glass), was carried out at matching test conditions to the prepreg tack measurements. Optical microscopy was utilised to acquire images of the contact area at the prepreg-glass interface. Image analysis of the micrographs enabled quantification of the contact area. The time- and temperature-dependent viscoelastic behaviour of the resin was explored directly on the prepreg using oscillatory parallel plate rheometry, and time-temperature superposition was applied to construct both tack and DoIC master curves. The shifted DoIC data showed that true contact area increases with decreasing shifted feed rates, until maximum contact area is achieved. Similarly, tack increases with decreasing shifted feed rates. However, at a critical feed rate, the bond failure mechanism switches from adhesive to cohesive failure. In cohesive failure, tack decreases with decreasing feed rate despite the high levels of DoIC

Characterisation of tack for uni-directional prepreg tape employing a continuous application-and-peel test method

Employing a test method with coupled application and peel phases, tack was characterised for a UD prepreg tape. Different aspects of tack were explored by varying test parameters and material condition. In addition, different surface combinations were studied. In general, the test parameters, feed rate and temperature, affect the balance between cohesion within the resin and adhesion between resin and substrate. Exploring a range of parameters is required to understand the effect of viscoelastic resin properties on tack. The application pressure determines the true contact area between prepreg and substrate and hence affects tack. Changes in molecular mobility in the resin related to specimen conditioning, i.e. ageing or moisture uptake, result in maximum tack to occur at lower or higher feed rates, respectively. Differences in tack for different material combinations can be attributed to different molecular interactions at the contact interfaces and different resin distributions on the prepreg surfaces

Optimisation of local in-plane constraining forces in double diaphragm forming

Rigid blocks (risers) were introduced in the double diaphragm forming (DDF) process to change the local in-plane strain distribution in the diaphragms, aimed at reducing wrinkling defects in the production of fabric preforms. A two-step optimisation method was developed to determine the position and dimension of each riser. In Step I, optimisation of the riser position was conducted using a simplified finite element (FE) model coupled with a genetic algorithm (GA). The height of each riser was optimised in Step II using a detailed FE model with the optimised riser positions from Step I. For demonstration, a hemisphere preform was manufactured by DDF using the optimum riser arrangement established by the optimisation routine. Results indicate that the optimum riser pattern (shape and position relative to the component boundary) can dramatically improve the preform quality through reduction of out-of-plane wrinkles, validating the feasibility of the two-step routine