Aligned fibre composites mechanical propertie

The mechanical properties of composite materials under the influence of "rule of mixtures". The alignment or orientation of the fibres in the composite materials can be divided on three type one-dimensional reinforcement, two-dimensional (planar) reinforcement and three-dimensional(random) reinforcement. The random orientation type of the isotropic but has greatly decreased reinforcing value(about one-third of the one-dimentional reinforced value). As the fibre orientation becomes more random, the mechanical properties in any one direction become lower. 

One common characteristic of all C/SiC composites is their distinct anisotropy in the mechanical as well as thermophysical properties. Considerable lower values of the tensile strength and the strain to failure have to be considered for an appropriate design if the load direction and the fibre alignment are not congment. As the carbon fibres show a different physical behaviour in longitudinal and radial direction, the composite s properties like thermal conductivity and coefficient of thermal expansion differ widely with respect to the in-plane or transverse direction. 

Fabric composites have mechanical properties similar to those of laminates made from orthogonal uni-directional layers. However, fibre curvature arising from yarn twist and weave crimp makes fabric reinforcement less efficient than in the case of aligned straight fibres. 

S.H. Aziz, and M.P. Ansell, The effect of alkahzatlon and fibre alignment on the mechanical and thermal properties of kenaf and hemp bast fibre composites Part 1 - polyester resin matrix. Compos. Sci. Technol. 64,1219-1230 (2004). 

To maximise the mechanical properties of short fibre-reinforced thermoplastic composites, it is necessary to achieve effective stress transfer between fibre and polymer matrix and appropriate fibre alignment in the finished component relative to the direction of applied stress. The former requirement is governed primarily by the method of compounding, whilst fibre orientation is very dependent on moulding conditions [76]. 

In a transversally isotropic material, there is a plane in which all properties are isotropic. Perpendicular to this plane, the properties differ. One example for such a material is a hexagonal crystal which is transversally isotropic with respect to its mechanical properties.Other technically important materials may also be transversally isotropic, for example directionally solidified metals in which the grains have a preferential orientation (see also section 2.5), or composites (chapter 9) with fibres oriented in one direction, but aligned arbitrarily (or hexagonally) in the perpendicular plane. 

Some preliminary measurements have been made of the mechanical properties of composites consisting of aligned polyOCHO fibres ( 10 mm long) in an epoxy resin matrix [62). The... 

Since the basic concept behind the creation of high performance fibres is the exploitation of molecular alignment, the term self-reinforcement has also often been used to describe the creation of polymer structures with mechanical properties that are superior to those of the isotropic polymer, for example achieved by rapid extension of melts [11] or by flow-induced crystallisation [12-14]. These and the multitude of other routes to the orientation of polymers in a solid state, and thus increased mechanical properties [15-18], are no less valid, but are not the focus of this review and will not be further described here. This review will focus on the combination of fibrous or tape-like reinforcements and the technology used to consolidate these in thermoplastic matrices to create fibre-reinforced composites. 

Fig. 1 Process feasibility window for the thermal processing of a self-reinfOTced composite from the fibre and matrix precursor into a composite structure. The process-feasible window is dictated by four boundaries of temperature and pressure (based rui [22, 23]). (a) When excessive temperature is applied, molecular relaxatimi of the fibrous reinfOTcement can occur, resulting in a loss in mechanical properties and, at the miset of fibre melting, a loss of reinforcemcmt volume fraction. (b) In a similar way to excessive temperature, an excess in applied pressure can encourage flow, disrupting molecular alignment and resulting in a loss in mechanical properties. Conversely, the lower temperature and pressure houndaries of the process-feasible windows (c) and (d), respectively, are dictated by the need to apply a minimum temperature and pressure to achieve adequate composite consolidation...

Wood, then, is a foamed fibrous composite. Both the foam cells and the cellulose fibres in the cell wall are aligned predominantly along the grain of the wood (i.e. parallel to the axis of the trunk). Not surprisingly, wood is mechanically very anisotropic the properties along the grain are quite different from those across it. But if all woods are made of the same stuff, why do the properties range so widely from one sort of wood to another The differences between woods are primarily due to the differences in their relative densities (see Table 26.1). This we now examine more closely. 

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