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73 Advanced materials | Focus When hollow carbon fibre parts are to be manufactured, the enclosed core must be filled during curing using either an inflatable bladder or a structural foam. Structural foams are easy to machine but will remain inside the component once cured, while a bladder can be removed after manufacture. Structural foams add some weight to the components, but serve a dual purpose as an internal mould core during manufacture, but then contribute a degree of structural performance in service that may mitigate the weight penalty if the number of carbon plies can be reduced compared to a completely hollow core. Structural foams are easily machined from blocks of various densities depending on the application and load cases. Machining epoxy board and carbon fibre composites is quite a specialist task. Epoxy is a hard material and tends to abrade cutting tools quite quickly, so hard polycrystalline diamond-coated tooling has to be used to obtain a reasonable wear life. Finishing cuts are usually made by diamond-coated solid carbide tooling for the best accuracy and surface quality. Carbon fibre parts have to be machined with great care to minimise damage to the fibres, for example avoiding breakout when drilling holes and avoiding delamination if temperatures at the cutting interface become too high. When machining metallic materials, a liquid coolant can readily reduce temperatures at the cutting edge, but composites are always machined dry, so the only coolant available is compressed air, which is far less efficient. Composite inserts are often machined before bonding into a larger composite structure, in order to provide accurate location or mounting features, or complex geometry that would otherwise be difficult to mould or lay up. When assembling larger composite structures, bonding is by far the preferred joining technology, with many adhesives available with strengths higher than the composite matrix resin. Bonding relies on meticulous surface preparation but is generally reliable. Although mechanical fixings can be used, they are typically a back-up solution and have some inherent disadvantages, as they create stress concentrations and can potentially damage fibres. As well as the manufacture of flat sheet fabrics, composites can also be formed into nominally cylindrical geometries by winding-type processes. Filament winding for example offers a way of producing tubes or shapes such as pressure vessels by winding a continuous tow of carbon onto a mandrel. An alternative to filament winding is a variant of tape laying called preform winding, which places thin-ply composites to form cylindrical parts. The particular benefit of this is the ability to place longitudinal fibres rather than only winding around the circumference of the part, which allows greater flexibility in design for function, as different regions of a shaft can be given different properties according to their ideal function. Although the fundamentals of carbon fibre composite materials have not changed significantly in the past few years, the greatest developments in their use have been in the areas of manufacture and design. Many suppliers now offer consultancy services that enable customers to make far better use of composites by providing expertise in the design, analysis and testing of composite materials and structures. Specialists in UAV manufacture can also assist with functional design, for example by introducing controlled aeroelasticity by designing the lay- up direction and thicknesses such that certain surfaces will deform as the aerodynamic loads on them increase, and twisting or bending them into a more efficient shape for the higher speed implied by the higher aerodynamic loads. Conclusion Unmanned systems use a variety of advanced materials in the continuing quest to reduce weight, increase strength and stiffness and improve endurance Unmanned Systems Technology | December/January 2017 Filament-wound carbon fibre pressure vessels (Courtesy of Lentus Composites) Specialists in UAV manufacture can assist with functional design, for example by introducing controlled aeroelasticity