Overall fibre volume fraction

The CRTM process is described schematically in Fig. 11.1. The filling stage comprises a combination of injection and compression driven flows. Unlike RTM, the mould is not completely closed prior to the initiation of filling. The mould is closed to some predetermined position, which results in lower overall fibre volume fraction, and hence a lower global resin flow resistance offered by the preform. It is also possible to leave a small empty cavity on... 

Overall the model combines mass and force balances incorporating the chemorheological model (above), considerations for viscous flow and flow through flbre bundles, to simulate loss of resin during prepreging, the fibre volume fraction, the pressure distribution, temperature effects and part dimensions. Verification experiments show that the model predicts experimental results to within 8%. 

The main purpose of middle layers is to provide additional (thermal) insulation. Nowadays, these layers are often made of fleece materials with good air entrapment properties. Their thermal conductivity (typically 0.03-0.04 W/mK) is near from air (0.026 W/mK). The thermal resistance of such layers is directly correlated with their thickness, provided that no air movement occurs within the fabric. Thermal conductivity and air permeability also are generally dependent on the fabric density (Yip and Ng, 2008). Conduction has been shown to be the main heat transfer mechanism through textile layers as long as the fibre volume fraction is higher than 9% (Woo et al., 1994). However, materials with very low density (like spacer materials) allow radiant and convective heat transfer. This was demonstrated by Das et al. (2012) who compared a spacer fabric middle layer with two non-woven middle layers and showed that the contribution of this spacer fabric to the overall insulation was higher than the two other samples in a non-convective mode, while it was the lowest in a forced convective mode. The positive effect of metallised interlayers with low emissivity on the reduction of... 

To make reinforced plastics objects, the resin and fibres must be combined intimately, so that they adhere together and distribute the fibres uniformly, with the desired orientation, with a minimum of porosity and with no dry, resin-free reinforcement. The correct fibre volume fraction must be ensured. In the case of thermosetting resins, there is an additional requirement to ensure that the hardening reaction, or cure, occurs at the right stage of the process and is eventually completed. The methods available depend on the size, overall shape and complexity of the article being produced, the materials involved and the number of mouldings required. 

Inter-yarn fibre and yam volume fractions. Within the meso-scale modelling paradigm, yams are considered as continuous soUd medium with effective properties that are primarily governed by the fibre volume firaction (FVF). The correct overall FVF (o-FVF) is an obligatory feature of any acceptable model as it has to match fabric areal density, resin content and specific composite weight. The o-FVF is determined as the product of the yam volume firaction (YVF) and the local intra-yam FVF (iy-FVF). The latter depends on yam compressibility, pressure used for preform consolidation and even matrix viscosity. 

For most applications, resistance to sfiesses in more than one direction is essential, and in these cases, a unidirectional composite is not acceptable. Strength and stiffness in both dimensions of a plane, as in a car body panel, or isotropically in all three dimensions is normally required. Consequently, the orientation or the fibres must be modified to provide resultants in the required directions. This may be achieved in two directions by the use of woven or knitted cloths or with non-woven, random felts or mats. In three dimensions, random orientation of the reinforcement is usually the only possibility. However, the maximum strength attainable drops sharply in multidirectional composites, not only because of the smaller fraction of fibres contributing to resisting the stress but also becanse the maximum packing density of the reinforcing fabric decreases and hence the overall volume fraction of reinforcement decreases. Despite this fall-off, the strength of the fibres is snch that a very substantial reinforcement may still result. 

Porosity provides information on the overall pore volume of a porous material. Fabric porosity is defined as the ratio of the nonsolid volume (voids) to the total volume of nonwoven fabric, and the volume fraction of solid material is defined as the ratio of solid fibre materials to the total volume of fabric. While the fibre density is the weight of a given volume of the fibre solids only (ie, not containing other materials), the porosity can be calculated as follows by using the fabric bulk density and the fibre density ... 

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