Unmanned Systems Technology 001 | UAV Factory Penguin C | Real-time operating systems | Hirth S1218 two-stroke twin | Base stations | ASV C-Enduro | Composites | Datacomms

64 cured, but that its surface is not over-cured. An alternative to RIM or autoclave approaches is to use hot pressing or moulding techniques. Here a metallic tool of the desired part geometry is heated and a press is then used to form the correct shape and achieve the curing process. Hot pressing is typically suited to moderately flat geometries where enough of the same components are required to justify the outlay on the initial tooling. The process does of course require some tooling design to achieve the correct component shape and to control the temperature distribution in the part. The pressing will usually form a blank, which can then be trimmed to shape and/or joined to a larger assembly. Apart from conventional fabric- based composite manufacturing, there are also additive manufacturing (AM) technologies such as laser sintering of polyamide plastics, which often use small glass beads or carbon fibres (about 100 µm in size) mixed into the powder feedstock to improve strength and stiffness. The layer-based approach does of course mean the improvements to strength are anisotropic, lending strength preferentially within the layer and not across layers, as with any composite system. However, the geometric freedom it gives for creating complex shapes can be attractive, as there is no tooling cost, making it ideal for one-off prototypes or short-run productions. Joining When joining composite materials, issues usually arise with mechanical fixings such as rivets or bolts when cutting the composite in order to insert the fixing. Drilling composites is not only expensive (owing to high tool wear) but risks damaging the reinforcement fibres and their interface with the matrix. Compounding this problem, a mechanical fixing will then concentrate the stress in the component into this damaged region when in service. Notably, the end condition of machined thin-ply composites is of a much higher quality than regular fibres when machining or drilling – indeed, one manufacturer queried in relation to this article has produced thin-ply based components that have then been fully machined with tapped holes. Adhesive technologies are a sensible candidate for joining composite components, as they can achieve a very strong serviceable bond for most PMCs. An alternaitve is to integrate components into a single composite part that can be formed in one piece, with the fibre lay-up designed to match the expected loads – an essential principle of composite design. Inspection The inspection of composites for manufacturing defects or damage received in service is a huge area of interest, with a wide variety of methods available to allow this. Acoustic emission testing is one method that can be readily applied to composite materials. If the acoustic signature of a known good component can be recorded, the signature of each subsequent part can be compared and faulty parts identified. Standard ultrasonic inspection methods are widely used to detect flaws as well. Radiographic inspection is also regularly applied to composite parts, either using 2D shadow X-rays or (increasingly) full 3D Computed X-ray Tomography (CT Scanning). Although radiographic inspection gives high-quality results, the cost of the equipment is high and turnround times for full scanning are slow. Infrared thermography (thermal cameras) is another interesting technique. Voids or other defects typically contain moisture either captured during manufacture or acquired during service if damage occurs. IR thermography can detect the moisture by heating the component on test; any difference in temperature due to the presence of moisture can then be easily detected and indicates the presence of a defect. With modern high- resolution cameras, this provides a very quick turnround along with the ability Focus | Composites November 2014 | Unmanned Systems Technology Automated response helicopter, using composite AM materials for several parts (Courtesy of CRP) The area of the inspection of composites for defects in manufacturing or damage in service is of huge interest