The future success of the aerospace industry both in terms of the cost-effective manufacture of new aircraft and the cost-effective extension of the operating life of existing aircraft is reliant on on-going improvements to existing materials and the development of new materials. Advances in materials technology is classified as evolutionary or revolutionary. Evolutionary advances mean that small, incremental improvements are made to existing materials, such as a new alloy composition, processing method or heat treatment. Examples include the addition of new alloying elements to nickel superalloys to increase the creep resistance and maximum operating temperature or the development of a new thermal ageing treatment for aluminium alloys to increase their resistance to stress corrosion. The evolutionary approach is often preferred by the aerospace industry since past experience has shown that almost every new material has some initial problems. The aerospace industry is more comfortable with incremental improvements on conventional materials, for which they have good knowledge of the design, manufacture, maintenance and repair issues.
Revolutionary advances are the application of new materials to structures or engines that are different to previously used materials. An example was the first-time that carbon-fibre composite was used to fabricate a primary load-bearing structure (the tail section) on a commercial airliner (B777) in the mid-1990s. Another example was the use of GLARE in the A380 in 2005, which was the first application of a fibre-metal laminate in an aircraft fuselage. Revolutionary materials usually have had limited success in being directly incorporated into aircraft owing to the high costs of manufacturing, qualification and certification. The cost and time associated with developing a new material and then testing and certifying its use in safety-critical components can cost an aerospace company hundreds of millions of dollars and take 5–10 years or longer. The introduction of new material can require major changes to the production infrastructure of aircraft manufacturing plants as well as to the in-service maintenance and repair facilities. For example, some suppliers of structural components to the Boeing 787 had to make major changes to their production facilities from metal to composite manufacturing, which required new design methods, manufacturing processes and quality control procedures as well as reskilling and retraining of production staff. As another example, the introduction of new radar absorbing materials on stealth aircraft such as the F-35 Lightning II require new repair methods in the event of bird strike, hail impact, lightning strikes and other damaging events. Despite the challenges, revolutionary materials are introduced into new aircraft when the benefits outweigh the potential problems and risks.
Evolutionary and revolutionary advances across a broad range of materials technologies are on-going for next-generation aircraft. It is virtually impossible to give a complete account of these advances because they are too numerous. Some important examples are: development of high-temperature polymers for composites capable of operating at 400 °C or higher; new polymer composites that are strengthened and toughened by the addition of nano-sized clay particles or carbon nanotubes; new damage tolerant composites that are reinforced in the through-thickness direction by techniques such as stitching, orthogonal weaving or z-pinning; multifunctional materials that serve several purposes such as thermal management, load-bearing strength, self-assessment and health monitoring, and self-actuation functions; bio-inspired polymer materials with self-healing capabilities; sandwich materials that contain high-performance metal-foam cores, truss or periodic open-cell cores; new tough ceramic materials with improved structural capabilities; rapidly solidified amorphous metals with improved mechanical properties and corrosion resistance; and new welding and joining processes for dissimilar materials. The aerospace materials for this century are sure to be just as ground-breaking and innovative as the materials used in the past century. On-going improvement in structural and engine materials is essential for the advancement of aerospace engineering. After many years of commercial service it might be expected that structural and engine material technology would be approaching a plateau, and the pace of innovation would decline. However, customer demands for higher performance, lower operating costs and more ‘environmentally friendly’ propulsion systems continue to drive materials research to ever more challenging goals. although they do occur when the benefits outweigh the cost and risk.
Leave a Reply