Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
6.2 Manufacture of composite materials
As stated above, fibre composite materials represent a major part of this
category: a wide range of possible fibres exists and Table 6.1 illustrates the
range of properties they possess. The most widely used fibres are of carbon,
glass and Kevlar and the method of their production is outlined below.
6.2.1 Carbon fibres
Carbon fibres consist of small crystallites of graphite, whose crystal structure
is shown in Fig. 1.1(b). The atoms in the basal planes are held together by
very strong covalent bonds, and there are weak van der Waals forces between
the layers. To obtain high modulus and high strength, the layer planes of the
graphite have to be aligned parallel to the axis of the fibre and the modulus
of carbon fibres depends on the degree of perfection of alignment of the
atom planes. This varies considerably with the particular manufacturing route
adopted, of which there are three main possibilities:
(a) The polymer PAN (polyacrylonitrile), which closely resembles
polyethylene in molecular conformation, is converted into a fibre and
then stretched to produce alignment of the molecular chains along the
fibre axis. When the stretched PAN fibre is heated, the nitrile groups
interact so that the flexible molecule is changed into a rigid ‘ladder’
molecule. While still under tension, it is heated in oxygen, which leads
to further cross-links between the ladder molecules. It is finally chemically
reduced to give (at high temperatures) a graphitic structure. The final
graphitization temperature determines whether the fibres have maximum
stiffness but a relatively low strength (Type I fibres), or whether they
develop maximum strength (Type II).
Table 6.1 Mechanical properties of some reinforcing fibres
Material Density ρ Young’s Tensile Fibre radius r
(Mg m –3 ) modulus E strength σ* (µm)
(GPa) (GPa)
E-glass fibres 2.56 76 1.4–2.5 10
Carbon fibres 1.75 390 2.2 8.0
(high modulus)
Carbon fibres 1.95 250 2.7 8.0
(high strength)
Kevlar fibres 1.45 125 3.2 12
Silicon carbide 3.00 410 8.6 140
(monofilament)
Silicon carbide 2.50 180 5.9 14
(Nicalon)
Alumina (Saffil) 2.80 100 1.0 3