Advanced Composites

Reinforcements
The reinforcement in a composite material is used to increase the mechanical properties of the neat resin system. The many different fibres used in composites have different properties which changes the properties of the composite. For most applications, the fibres are arranged in a sheet, known as a fabric, to make handling possible. The four main fibre orientation categories are unidirectional, woven, Multiaxial and random.

A unidirectional (UD) fabric is one in which the majority of fibres run in one direction only. Unidirectional fabrics offer the ability to place fibre in the component exactly where it is required, and in the optimum quantity. Unidirectional fibres are also straight and uncrimped, resulting in the highest possible fibre properties from a fabric.

For applications where more than one fibre orientation is required, a fabric combining 0° and 90° fibre orientations is useful. Woven fabrics are produced by the interlacing of warp (0°) fibres and weft (90°) fibres in a regular pattern or weave style. Drape, which is the ability of a fabric to conform to a complex surface, surface smoothness and stability of a fabric are controlled primarily by the weave style. The commonly found weave styles include plain, twill and satin weaves.

Plain weave
The fabric is symmetrical, with good stability and reasonable porosity. However, it is the most difficult of the weaves to drape, and the high level of fibre crimp imparts relatively low mechanical properties compared with the other weave styles.

Twill weave
The weave produces the visual effect of a straight or broken diagonal 'rib' to the fabric. Superior wet out and drape is seen in the twill weave over the plain weave with only a small reduction in stability. With reduced crimp, the fabric also has a smoother surface and slightly higher mechanical properties.

Satin weave
Satin weaves are modified twill weaves with fewer intersections of warp and weft. The ‘harness’ number used in the designation (typically 4, 5 and 8) is the total number of fibres crossed and passed under, before the fibre repeats the pattern. Satin weaves are very flat, have good wet out and a high degree of drape. The low crimp gives good mechanical properties. Satin weaves allow fibres to be woven in the closest proximity and can produce fabrics with a close ‘tight’ weave. However, the style’s low stability and asymmetry needs to be considered. Care must be taken in assembling multiple layers of these fabrics to ensure that stresses are not built into the component through this asymmetric effect.

Multiaxial fabrics consist of one or more layers of long fibres held in place by a secondary non-structural stitching tread. The stitching thread is usually polyester due to its combination of appropriate fibre properties and cost. The stitching process allows a variety of fibre orientations, beyond the simple 0/90° of woven fabrics, to be combined into one fabric. The two key improvements with stitched multiaxial fabrics over woven types are:

1. Better mechanical properties, primarily from the fact that the fibres are always straight and non-crimped.
2. Improved component build speed as fabrics can be made thicker and with multiple fibre orientations.

• Laminate formulae - Calculate fibre volume fraction and cured ply thickness

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