What makes for a good aerospace material?

Selecting the best material for an aircraft structure or engine component is an important task for the aerospace engineer. The success or failure of any new aircraft is partly dependent on using the most suitable materials. The cost, flight performance, safety, operating life and environmental impact from engine emissions of aircraft is dependent on the types of materials that aerospace engineers choose to use in the airframe and engines. It is essential that aerospace engineers understand the science and technology of materials in order to select the best materials. The selection of materials for aircraft is not guesswork, but is a systematic and quantitative approach that considers a multitude of diverse (and in some instances conflicting) requirements. The selection of materials is performed during the early design phase of aircraft, and has a lasting influence which remains until the aircraft is retired from service.

The key requirements and factors that aerospace engineers must consider in the selection of materials are listed below and in Table 1.2.

Table 1.2

Selection factors for aerospace structural materials

Costs	Purchase cost.
	Processing costs, including machining, forming, shaping and heat treatment costs.
	In-service maintenance costs, including inspection and repair costs.
	Recycling and disposal costs.
Availability	Plentiful, consistent and long-term supply of materials
Manufacturing	Ease of manufacturing.
	Low-cost and rapid manufacturing processes.
Density	Low specific gravity for lightweight structures.
Static mechanical properties	Stiffness (elastic modulus).
	Strength (yield and ultimate strength).
Fatigue durability	Resistance against initiation and growth of cracks from various sources of fatigue (e.g. stress, stress-corrosion, thermal, acoustic).
Damage tolerance	Fracture toughness and ductility to resist crack growth and failure under load.
	Notch sensitivity owing to cut-outs (e.g. windows), holes (e.g. fasteners) and changes in structural shape. Damage resistance against bird strike, maintenance accidents (e.g. dropped tools on aircraft), impact from runway debris, hail impact.
Environmental durability	Corrosion resistance.
	Oxidation resistance.
	Moisture absorption resistance.
	Wear and erosion resistance.
	Space environment (e.g. micrometeoroid impact, ionizing radiation).
Thermal properties	Thermally stable at high temperatures.
	High softening temperatures.
	Cryogenic properties.
	Low thermal expansion properties.
	Non/low flammability.
	Low-toxicity smoke.
Electrical and magnetic properties	High electrical conductivity for lightning strikes.
	High radar (electromagnetic) transparency for radar domes.
	Radar absorbing properties for stealth military aircraft.

Cost. The whole-of-life cost of aerospace materials must be acceptable to the aircraft operator, and obviously should be kept as low as possible. Whole-of-life costs include the cost of the raw material; cost of processing and assembling the material into a structural or engine component; cost of in-service maintenance and repair; and cost of disposal and recycling at the end of the aircraft life.

Availability. There must be a plentiful, reliable and consistent source of materials to avoid delays in aircraft production and large fluctuations in purchase cost.

Manufacturing. It must be possible to process, shape, machine and join the materials into aircraft components using cost-effective and time-efficient manufacturing methods.

Weight. Materials must be lightweight for aircraft to have good manoeuvrability, range and speed together with low fuel consumption.

Mechanical properties. Aerospace materials must have high stiffness, strength and fracture toughness to ensure that structures can withstand the aircraft loads without deforming excessively (changing shape) or breaking.

Fatigue durability. Aerospace materials must resist cracking, damage and failure when subjected to fluctuating (fatigue) loads during flight.

Damage tolerance. Aerospace materials must support the ultimate design load without breaking after being damaged (cracks, delaminations, corrosion) from bird strike, lightning strike, hail impact, dropped tools, and the many other damaging events experienced during routine operations.

Thermal properties. Aerospace materials must have thermal, dimensional and mechanical stability for high temperature applications, such as jet engines and heat shields. Materials must also have low flammability in the event of aircraft fire.

Electrical properties. Aerospace materials must be electrically conductive to dissipate the charge in the event of lightning strike.

Electromagnetic properties. Aerospace materials must have low electromagnetic properties to avoid interfering with the electronic devices used to control and navigate the aircraft.

Radar absorption properties. Materials used in the skin of stealth military aircraft must have the ability to absorb radar waves to avoid detection.

Environmental durability. Aerospace materials must be durable and resistant to degradation in the aviation environment. This includes resistance against corrosion, oxidation, wear, moisture absorption and other types of damage caused by the environment which can degrade the performance, functionality and safety of the material.