C. Lekakou

An experimental study of the permeability and capillary pressure in resin-transfer moulding

This paper addresses issues of the inter-relationship between the permeability and capillary pressure in in-plane infiltration of assemblies of woven fabrics. Rectilinear infiltration experiments of assemblies of a plain-weave fabric at various low injection pressures were carried out to evaluate permeability and capillary pressure (Pc) for the woven fibre preform. Capillary pressure and the form factor, F, for the Young–Laplace equation were also estimated and a normalised capillary pressure was used to compare the Pc values found at different porosities. Regarding the estimation of permeability, it has been shown that the higher the injection pressure, the lower the associated error of not including Pc into the permeability calculation. When a corrected permeability was calculated taking into account the capillary pressure effect, the permeability proved to be independent of the injection pressure and the permeating fluid. An increase in permeability was found for pre-wetted fabrics in comparison to dry fabrics. # 2001 Elsevier Science Ltd. All rights reserved.

Shear deformation and micromechanics of woven fabrics

Abstract This paper includes an experimental study and a mathematical analysis of the shear deformation of woven fabrics by using picture-frame type shear testing. Four types of weaves were tested and compared: a loose plain weave, a tight plain weave, a twill and a satin weave. The locking shear angle was determined both in picture-frame tests and manual shear tests. The experimental data presented for each fabric include curves of shear load–shear stress as a function of either the shear angle or the shear rate, and measured locking shear angles. The shear deformation data were analysed by following elasticity principles and taking into account the effects of fibre inextensibility. A microstructural analysis was carried out in all four fabrics to investigate the shear locking on the basis of a geometrical approach and the maximum packing fibre fraction.

Compression in the processing of polymer composites 1. A mechanical and microstructural study for different glass fabrics and resins

This paper focuses on the compression of fibre reinforcements during the processing of polymer composites, covering a range of fabrics, namely a plain weave, a twill, a satin and a non-crimped, stitch-bonded fabric. The compression of assemblies of fabrics has been studied in both dry and wet states where, in the latter case, the fabrics were impregnated with three alternative resins: a non-Newtonian polyester of relatively high viscosity, a non-Newtonian polyester of relatively low viscosity, and a Newtonian epoxy of equally low viscosity. Investigations of mechanical behaviour included the influence of compression speed, type of fabric and viscosity and type of resin. Microstructural studies of laminates produced under different degrees of maximum compression elucidated further the results of mechanical compression and provided data of average area porosity (resin-rich areas), area pore structure, average area voidage and voids for the different types of fabrics.

Measurement techniques and effects on in-plane permeability of woven cloths in resin transfer moulding

The resin transfer moulding process studied in this paper involves the injection of a liquid polymer resin into a mould containing a pre-placed assembly of woven cloths. Two techniques of measuring the in-plane permeability of the reinforcement are considered. The first technique involves rectilinear flow of a model liquid, whereas the second involves radial flow of the liquid injected from a central gate. A comparison of the two techniques is given, including problems and differences in the obtained permeability values. The effects of a number of parameters are studied in each case, including the flow rate, porosity, pre-wetting the cloths and the number of layers of cloth.

Mathematical modelling of macro- and micro-infiltration in resin transfer moulding (RTM)

Abstract A mathematical model is proposed to describe the macro- and micro-infiltration through reinforcements of bimodal porosity distribution in resin transfer moulding. The model is based on Dareys law incorporating mechanical, capillary and vacuum pressures and covers three modes of infiltration. Permeabilities and capillary pressures are calculated at macro- and micro-levels. A numerical analysis is presented where at each numerical location the flowrate is split between three possible types of flow on the basis of mass balance and a combination of permeability magnitude and local flow potential. Parametric computational studies are carried out to study the flow of a model Newtonian fluid through woven cloths where the following parameters are varied: fibre volume fraction, fibre tow diameter and injection pressure. Predicted variables include micro- and macro-infiltration times and apparent global permeability. The latter was found to depend on the injection pressure, in the regime of relatively low injection pressures, where the variations also depended on the fabric architecture and fibre volume fraction.

Processing of Composites: Simulations of the Draping of Fabrics with Updated Material Behaviour Law

In the processing of composites, fibre reinforcement is draped over complex three dimensional geometries, yielding local variations in the fibre volume fraction and fibre orientation with subsequent effects on the permeability of the reinforcement during resin infiltration and on the properties of the composite product. This study presents computer simulations of the draping of fabrics by following a mechanical, finite element approach. The fabric is considered as a solid continuum with mechanical properties and friction properties at interfaces. The problem is solved by applying an explicit dynamic finite element analysis. In order to incorporate the large shear deformation of fabric during draping, an updated material behaviour law was formulated on the basis of changing fibre directions. The model was implemented in a user computer subroutine and tested first in case-studies of simple in-plane shear where the predictions were also compared with experimental data from a picture frame type of shear test. Thereafter, numerical simulations focused on the draping of a fabric in a “hat” shaped mould, comprising a hemispherical cup with a wide flat rim. The predictions were compared with predictions from similar simulations without the updated material behaviour law, experimental data and predictions from a “fishnet” algorithm.

Activated carbon from phenolic resin with controlled mesoporosity for an electric double-layer capacitor (EDLC)

Activated carbon materials are prepared from phenolic resin precursors by physical activation to fabricate electrodes for electric double-layer capacitors (EDLCs). Pore size and surface area of the carbon materials are controlled during the synthesizing process and after the carbonization through activation in a CO2 atmosphere to different levels of burn-off. The resultant carbon materials were evaluated as EDLC electrodes, using electrochemical impedance spectroscopy (EIS) and galvanostatic charge–discharge (GCD) measurements with the organic electrolyte of spiro-(1,1′)-bipyrrolidinium tetrafluoroborate in propylene carbonate, SBPBF4/PC. The results of the study showed that the capacitance of carbon materials, as well as energy density of the EDLC cells, increased by increasing the level of burn-off (activation). The 46% activated carbon gave a capacitance of ∼160 F g−1 and an energy density of ∼35 W h kg−1, at a current density of 1 mA cm−2. The long term cycling tests showed high cycling stability of these carbon materials.

Mathematical modelling of capillary micro-flow through woven fabrics

The present work considers the capillary flow of Newtonian fluids along a single fibre yarn and through a plain-woven fabric. In the first case, one-dimensional Darcys flow is considered through the micro-pores of the fibre yarns. In the second case, two-dimensional in-plane network infiltration is considered through the micro-pores of the network of fibre yarns in the fabric. In both cases predictions include the infiltration length as a function of time, the apparent permeability and the capillary pressure. The latter case also includes the number of unsaturated transverse yarns and the degree of saturation. All predictions are compared to experimental measurements and the agreement is very good.

Solid-mechanics finite element simulations of the draping of fabrics: a sensitivity analysis

Abstract This paper covers numerical investigations of the draping of woven fabrics into a “hat” shape, combining a hemispherical cup with a wide flat rim. A mechanical approach is adopted using finite element analysis (FEA) methodology. In this, the fabric is considered as a solid sheet with mechanical properties and friction properties. In this study, a linear elastic anisotropic material model describes the deformation of fabrics. An explicit dynamic finite element analysis is applied and systematic parametric numerical studies are presented, which incorporate investigations of the effects of numerical parameters, material properties and processing conditions on the draping of fabrics. More specifically, the effects of the following variables and parameters are included: number of elements, number of time increments in the dynamic FEA analysis, punch speed, shear and tensile moduli of fabric, coefficient of friction for all interfaces and level of load on the fabric holder.

Experimental studies and analysis of the draping of woven fabrics

This study investigates and compares the draping and forming of four types of woven fabrics, namely a loose plain weave (basket weave), a tight plain weave, a satin and a twill weave. The fabrics were draped over a hat mould consisting of a hemispherical dome surrounded by a flat base. The draping of each fabric was examined in terms of wrinkle formation, boundary profile of the draped fabric, distribution of fibre orientation and local shear angles. A theoretical analysis of the experimental results involved the calculation of the distributions of the fibre volume fraction and mechanical properties, in terms of components of the reduced stiffness matrix, from the experimental data of local shear angles.