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90 That can cause an aluminium tool for example to deteriorate over time. A steel tool would last longer but cost far more. Both bladder moulding and compression moulding are examples of techniques that can result in complex monocoque parts, of various sizes, rather than two shells bonded together. In parts produced in an autoclave, the split (or bond) line between two parts can occur at or around a site of significant load, something that can be avoided by using these techniques. Joining When monocoque parts cannot be used, however, manufacturers must choose a joining method. Adhesive bonding remains a logical first choice; synthetic adhesives provide a cost-effective and relatively quick solution here. They can bond dissimilar materials together, provide smooth contours around joint areas to preserve airflow and are immune to galvanic corrosion. Options include acrylic adhesives, epoxies, polyurethanes, silicones, phenolics and polyimides. All of them have varying temperature requirements, versatility, curing requirements and durability. For example, polyimide adhesive can be difficult to process but gives good thermal stability at high heat, being able to tolerate temperatures between -50 C and 300 C. Polyurethanes can withstand temperatures as low as -200 C, and are a particularly durable choice, with high resistance to fatigue and impact damage relative to other adhesives. Acrylic and epoxy adhesives, while not notable for wide temperature stability compared to other types, can still tolerate roughly between -40 C and 120 C. They cure faster and at lower temperatures than other adhesives, and are inexpensive and easy to use. When adhesives are not sufficient, however – as in high-load applications – simple mechanical joints such as riveting can be used. Two-piece bolts and blind fasteners are also potential choices, in materials such as aluminium or stainless steel. However, metallic fasteners can be subject to galvanic corrosion, and their expansion and contraction in extremes of temperature can damage the composites and alter the clamping load. Drilling holes in composites can also be problematic, raising the risk of delamination and fibre break-out. The former is also a distinct possibility if an interference fit is used. Where the need to bear significant stress outweighs the risks though, fasteners with large heads should be used, to spread the load distribution over a wider surface area and reduce any impact on the composite. There are other proprietary mechanical techniques for handling greater loads than can be managed with adhesives or simple mechanical bonds. For thin-ply materials, for example, a ‘scarf joint’ can be used. This involves using layers of composite tape attached to an adjacent piece of tape, over an offset joining area, cut back in a step-wise fashion to avoid any unnecessary build-up of material or abrupt transition in thickness. This strengthens the area by not merely overlapping one preform with another but instead spreading the area of the joint lengthways across the layers of each thin-ply part. The manufacturer may choose a completely flat ‘step/butted’ joint, or opt for a ‘step/overlap’ form, which increases the surface area (and therefore the strength) of the joint at the cost of having a slight overlap of usually one layer of tape over the top of the adjoining part. Thin plies Thin-ply materials have grown in popularity over the past few years as a result of government and commercial interest in solar-powered pseudo-satellite HALE UAVs. Thin plies weigh less than 100 g per square metre (gsm) of fibre, and can be as light as 15 gsm depending on the fibre and resin used. They are about February/March 2018 | Unmanned Systems Technology There is a wide range of core materials to meet the mechanical and chemical needs of sandwich-structured or compression-moulded composites (Courtesy of Diab)