Material selection for any engineering product depends on its function, shape, manufacturability (i. e., process), and cost. For aircraft applications, there is the additional consideration of weight. Within the material classes, there are subclasses of alloys: composites that offer appropriate properties to suit a product – a large variety is available with ever-increasing newer types. In the conceptual design phase, engineers must screen and rank the types of materials that suit the requirements, listing the limitations and constraints involved and whether a change to another type is
aluminum + copper (e. g., 2014-T6 Alclad sheet,* 2024-T4 extrusion)
aluminum + manganese
aluminum + silicon
aluminum + magnesium
aluminum + magnesium + silicon
aluminum + zinc – high strength, heat treatable, prone to fatigue (e. g., 7076-T6, 7076-T6 extrusion)
Table 15.6. Typical composite material usage in various aircraft classes
* Some smaller aircraft, including the Bizjet, are constructed of all-composite structures. ** B787 has over 50% composite material by weight.
Several options are available for appropriate materials to make the best compromise. Thus, aircraft-weight estimation is more complex, and engineers must identify and compute numerous parts to estimate component weights before an aircraft is built; CAD 3D modeling helps.
Choice of material affects aircraft weight and cost. The semi-empirical relation for weight estimation in Chapter 8 considers all-metal construction and describes how to adjust the prediction if some parts are made of a lighter material. For a rapid method, the OEW may be factored accordingly – only the structural weight is affected; the remainder is unchanged. Composites may be used in secondary and tertiary structures, where loads are low and failure does not result in catastrophe.
In general, for the same Young’s Modulus, metals have higher density. However, when the strength-to-weight ratio (i. e., specific strength) is considered, then composites overtake metals; that is, engineers can obtain the same strength with lighter components even when the higher FS erodes the weight savings. Metals demonstrate a better Young’s Modulus for the same strength. Metals also show better fracture toughness for the same Young’s Modulus. Another important comparison is the relative cost per unit volume versus the Young’s Modulus when metal alloys are less costly.