Classifying by particle geometry

If we characterise composites by particle geometry, we can distinguish fibres and particles (in the narrow sense). In fibres, one dimension is larger than the others by at least one order of magnitude, thus they are shaped like long and slender cylinders. In particles, the extension is approximately the same in aU directions. Other structures are also possible the phases may, for example, also be arranged in a sandwich structure or laminate with alternating layers of different materials. 

The length of the fibres not only determines the mechanical properties, but is also important for the manufacturing process because long fibres have to be processed different from short fibres. This will be discussed now. 

Because the fibres are usually very thin, they are bundled in so-called rovings, comprising several thousand single fibres. The diameter of these rovings is thus in the range of millimetres. Alternatively, the fibres can be spun to form yarns. 

One disadvantage of the uniaxial structure is that the stiffness and the strength in the transversal direction are markedly inferior (see sections 9.2.2 and 9.3.6). For this reason, different orientations of the fibres may be combined. Mats containing uniaxial fibres are stacked on top of each other to form laminates, or the fibres can be woven to fabrics that can be laminated again. The orientation of the fibres within the layers can be chosen orthogonal to each other or in more complex ways (for example, with relative orientations of 45°). The properties of such a material can be isotropic within the plane of the fibres, but they are weaker perpendicular to them because there are no fibres in this direction. To make the material isotropic, it is necessary to arrange the fibres in all three directions, but this is rather difficult to achieve with continuous fibres because the fibres have to be woven in three directions. 

Long fibres or fabrics have to be positioned correctly within the manufactured component. This can be achieved most easily if the melting point of the matrix material is well below that of the fibres, as in polymer matrix composites. If duromers are used as matrix, the fibres can be laid down in an uncured mixture of resin and hardener. On curing, the resin hardens to form the duromer. 

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