Aligned fibre composites properties

Example 3.8 A thin unidirectional carbon fibre composite is loaded as shown in Fig. 3.14 and has the properties listed below. If the fibres are aligned at 35° to the x-axis, calculate the stresses parallel and perpendicular to the fibres. 

S.H. Aziz, M.P. Ansell, The effect of alkalization and fibre alignment on the mechanieal and thermal properties of kenaf and hemp bast fibre composites part 1-polyester resin matrix. Compos. Sci. Technol. 64(9), 1219-1230 (2004)... 

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. 

Example 3.6 A unidirectional composite consisting of carbon fibres in a PEEK matrix has the fibres aligned at 25° to the loading axis. If the fibres and matrix have the properties indicated below, calculate Ex, Ey, Gxy, Vxy, and Vyx. 

A single ply of carbon/epoxy composite has the properties listed below and the fibres are aligned at 25° to the x-direction. If stresses of... 

A single ply glass/epoxy composite has the properties Usted below. If the fibres are aligned at 30° to the x-direction, determine the value of in-plane stresses, a, which would cause failure according to (a) the Maximum Stress criterion (b) the Maximum Strain criterion and (c) the Tsai-Hill criterion. 

It is worth pointing out that carbon fibre itself has anisotropic thermal expansion properties, and therefore this mismatch between the carbon fibres and the a-sialon matrix should be considered in both the radial and axial directions when carbon fibres are unidirectionally aligned in the composite. The thermal stress caused by thermal expansion differences between the carbon fibres and the matrix in the radial (cr) and axial (oa) directions can be estimated from the formulae (Chawla, 1993 Kerans and Parthasarathy, 1991) ... 

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. 

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. 

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