Partially oriented fibre composites

The theoretical estimation of the elastic constants of a partially oriented short fibre reinforced composite is most simply undertaken by considering that it consists of an aggregate of units of structure, each of which consists of a perfectly aligned short fibre composite. This approach is analogous to the aggregate model for oriented polymers described in Section 8.6.1. It was first proposed by Brody and Ward [15] and subsequently by Advani and Tucker [16], all these workers setting out the tensor equations for the elastic constants, as described in Section 8.6.3. 

Up to 200 C, the polyimide-based C/C composites have negative coefficients of thermal expansion parallel to the fibre axis (a = -1 10 °C . This expansion characteristic, generally typical of high-performance carbon fibres, is due to the carbon matrix, which has a partially preferred orientation structure and an expansion behaviour similar to C fibres. A proof of the preferred orientation can be seen in the fact that the coefficient of thermal expansion at room temperature is more negative for composites with higher final heat-treatment temperatures. Between the temperatures of 400 and 1400°C, the thermal expansion is linear, with a = 2 10 C-. ... 

Hybrid composite materials (HCM) represent the newest group of various composites where more than one type of fibre is used to increase cost-performance effectiveness, i.e., in a composite system reinforced with carbon fibres the cost can be minimised by reducing its content while maximising the performance by optimal partial replacement with an another fibre or by changing the orientations. HCM include nanocomposites [31], functionally gradient materials [32], Hymats (hybrid materials) [33], interpenetrating polymer networks (IPN) [34], and liquid crystal polymers [35]. 

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