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68 A dvanced materials are used in a variety of applications in unmanned vehicles, where an ‘advanced’ material is one that delivers some improvement in properties or functionality compared to its conventional equivalent. Better performance of a material or structure can be achieved either through the composition of the material itself or, more often, the manner in which it is processed. In general, the performance drivers for the unmanned sector are a desire to reduce overall system weight while having high strength and stiffness. Other properties such as corrosion resistance, transparency to EM radiation and damage resistance are also often key factors in selecting a material for a given application. Materials used in the construction of unmanned vehicles can be broadly broken down into two groups, metallic and non-metallic. At this point it is worth defining an important metric for advanced materials – specific strength. While a material may be particularly strong, it is actually the strength-to-weight ratio rather than the absolute strength that is generally of more interest. For example, a material that provides a 10% increase in strength for a 30% increase in weight is in real terms inferior to a weaker but much lighter material. Metal alloys In performance applications, alloys using iron, aluminium, titanium or nickel are most often used for their strength and stiffness, or in the case of nickel-based alloys, for their strength at high temperatures. The metals themselves are almost always alloyed to deliver reasonable properties, with small percentages of alloying elements such as carbon, silicon, magnesium, vanadium, chromium, niobium or molybdenum (among others) increasing the properties through strengthening mechanisms that generally impede the movement of defects through the material when a load is applied. Iron alloys such as high-strength or stainless steels are often used where Dr Dave Cooper gives an overview of the material options and their advantages for unmanned systems Matters of substance This sub-4 kg carbon composite airframe has a wingspan of 2.7 m and provides mountings for VTOL capability (Courtesy of Carbonix) December/January 2017 | Unmanned Systems Technology