Fibres volume fraction

This design study has shown that it is possible to design a sectional composite that will reproduce both the vibrational frequencies and the mass of a traditional wooden soundboard. For a soundboard made out of spruce the equivalent composite is a sandwich of cardboard glued between two identical layers of aligned CFRP with a fibre volume fraction of 0.13. If the wooden soundboard is 3 mm thick the replacement composite must be 1.98 mm thick with a cardboard core of 1.25 mm. 

A unidirectional glass fibte/epoxy composite has a fibre volume fraction of 60%. Given the data below, calculate the density, modulus and thermal conductivity of the composite in the fibre direction. 

By use of this functionally gradient coating the carbon-fibre-reinforced aluminium composites (C/Al) exhibit excellent mechanical properties. The ultimate tensile strength reaches 1250 MPa when the fibre volume fraction is 35%. 

It is also found that there is a general increase in strength of the composites with increasing fibre volume fraction. However, at low values of Vf the strength is below that of the pure resin until sufficient fibres are present to produce reinforcement (29) and at high values of Vf (> 60%) the strength falls off because there is not enough resin to wet the fibres (37). 

The important process-control parameters in pultrusion manufacturing are the pull speed, fibre volume fraction, viscosity, temperature settings for die heating zones, and the preform plate area ratio (compaction ratio). Key process variables include die pressure and temperature. 

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%. 

In Figure 1 are presented the main mechanical results. We show the evolution of the ratio (composite value/matrix value) versus fibre volume fraction. The... 

Figure 1 PE and TPS-based composites. From top to the bottom modulus, maximum strength and elongation at break ratios versus fibre volume fraction...

The mean fibre length determines the fibre aspect ratio. One theory considers the effects of inclusions, of a given aspect ratio, surrounded by a material with the average properties of the composite. Figure 4.29a shows predictions for E-glass inclusions ( = 73GPa, p = 0.22) in an epoxy matrix (E = 5.35 GPa, v = 0.34) there is a steady increase in the composite modulus Ell in the direction of the perfectly aligned inclusions, as the aspect ratio and the fibre volume fraction increase. 

Based on the yam volume description, the fibrous stmcture of the yarn, or, more generally, the fibrous stmcture of the unit cell, is described as follows. Consider a point P and the fibres in the vicinity of this point. The fibrous assembly can be characterised by physical and mechanical parameters of the fibres near the point (which are not necessarily the same in all points of the fabric), fibre volume fraction Vf and direction /of them. If the point does not lie inside a yam, then Ff = 0 and/is not defined. For a point inside a yam, fibrous properties are easily calculated, providing that the fibrous stmcture of the yam/ply in the virgin state is known and its dependency of local compression of the yarn/ply, bending and twisting of the yam are given. Searching the cross-sections of the yams, cross-sections S, = S(si) and S,+i =5 (s,+i) (s is a... 

Local fibre volume fractions. Fibres are distributed nonuniformly within the yams. Experimental measurements show that distribution of fibres in a yam can vary and be up to 15% lower at the yam edges than in the middle zone [25—27]. In contrast, the local FVF in the overlap configurations may be up to 30% higher at the edges. Introduction of the local FVF variation within given iy-FVF constraints reduces o-FVF. This is why accounting for FVF variation is often treated as an unnecessary luxury. On the other hand, introduction of FVF variation may be useful for FEA to smooth down artificial stress concentrations imposed by the edges. 

Impregnated yams are considered as composites with unidirectional reinforcement (twist of the yarns used in composite reinforcement is normally negligible) with the given fibre volume fraction Vj this value is determined by the dimensions of the cross-section of the yam (which can differ from point to point in the unit cell, but most often assumed to be constant) and the amount of fibres inside the yam, given by the fibre count or yam linear density ... 

First, elastic properties of each impregnated yam segment are calculated using homogenisation formulae (e.g. the aforementioned Chamis formulae) for the unidirectional array of fibres, using local fibre volume fraction at the segment, properties of the fibres and elastic properties of the matrix. The result is the stiffuess matrix C], expressed in the local 123 coordinate system. Then each yarn segment is represented by an ellipsoidal inclusion with axis... 

Figure 2.6 Dependencies of mechanical properties of angle interlock reinforced composite on parameters of the reinforcement (normalised values) (a) Young s modulus in the warp direction, Ff = 58% (b) Young s modulus in the weft direction, Ff = 58% (c) shear modulus, Ff=58% (d) Young s moduli versus fibre volume fraction, PSl and PS2 refer to two values of the weft spacing.

Finally, Figure 2.6(d) shows a linear dependence of the modulus with fibre volume fraction, and this is for the two different weft spacings. 

All these mechanical results demonstrate the ability of 3D woven fabrics to be adjusted to requirements and cover a wide range of mechanical properties by modifying weaving or manufacturing parameters such as spacing between yams or fibre volume fraction of the composite. Databases of materials can be built in order to help design or selection of these 3D textile reinforced composites. 

Abraham and McDhagger (1996) [21] compared the RTM and autoclave processing routes and found that the main advantage of the autoclave route was that the higher consolidation pressure used led to higher values of fibre volume fraction which favourably influenced component mecbanical properties. They also confirmed that the RTM method offers materials and labour cost savings compared with conventional autoclaving as well as operational, health and safety benefits. 

RFI In this process, instead of injecting the film into the mould, sheets of resin are produced as thin films and placed on the bottom or top or both of the preform. Tool inserts are placed around the preform and the assembly placed in vacuum bag. When the tool is heated and pressurised, the resin film or films melt, flow into the preform and are cured. Advantages include higher fibre volume fractions and less expensive moulds. 

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