Materials used in Aircraft – How to Select?

In modern aircraft construction, the main materials used are steel, aluminum alloys, titanium alloys, and fiber-reinforced composites. Here we mentioned also technical specification of materials used in aircraft.

1. Titanium Alloys:

Titanium alloys have excellent properties such as high strength-to-weight ratio, good fatigue strength, and resistance to corrosion. They can maintain their strength even at high temperatures of up to 400-500°C. However, they are relatively heavy and expensive compared to other materials like aluminum. They are primarily used in critical components of jet engines, such as turbine blades, due to their exceptional performance under extreme conditions.

2. Steels:

Steel is a material made by combining iron with carbon. In the past, steels were widely used for primary and secondary structural elements in aircraft. However, they were gradually replaced by aluminum alloys due to their high density, which made them heavier. Despite this, steel still finds applications in aircraft construction, particularly for small components requiring high strength, stiffness, and resistance to damage. Examples include landing gear pivot brackets, wing-root attachments, and fasteners.

3. Aluminum alloys

Aluminum alloys have been a cornerstone of aircraft construction since World War II, replacing steel due to their lightweight and improved mechanical properties when alloyed with other metals. Pure aluminum lacks strength and is highly flexible, but alloying it with elements like copper, magnesium, manganese, silicon, zinc, and lithium enhances its mechanical properties while maintaining its low weight, which is crucial for aviation.

There are four main groups of aluminum alloys used in aircraft construction:

1. Al-Cu (2000 series)
2. Al-Mg (5000 series)
3. Al-Mg-Si (6000 series)
4. Al-Zn-Mg (7000 series)

More recently, aluminum-lithium (Al-Li, 8000 series) alloys have also been introduced and widely adopted in the aerospace industry.

Each group of alloys offers unique combinations of properties, including strength, ductility, ease of manufacture (e.g., extrusion and forging), resistance to corrosion, and suitability for protective treatments like anodizing. However, improving one property often comes at the expense of others, presenting engineers with a challenging trade-off.

Despite their long-standing use, aluminum alloys are gradually being replaced by fiber-reinforced composite materials, particularly in secondary structures, and increasingly in primary structural elements. Aircraft like the Airbus A350 and Boeing 787 Dreamliner are leading this shift towards composite materials due to their superior strength-to-weight ratio and other desirable properties.

4. Fibre-reinforced composite materials

Fiber-reinforced composite materials are a key innovation in aircraft manufacturing, offering superior strength-to-weight ratios and other advantageous properties compared to traditional materials like metals. These composites are made by combining strong fibers such as glass or carbon with a matrix of plastic or epoxy resin, creating a material with unique characteristics that cannot be achieved by the individual components alone.

One notable aspect of fiber-reinforced composites is their anisotropic nature, meaning that their properties vary depending on the direction of the fibers. To maximize their effectiveness in handling loads, multiple sheets of composite material are often layered together in a specific orientation to align with the major stresses experienced by the structure. The matrix material, typically made of plastic or epoxy resin, binds the fibers together and provides structural integrity to support bending and shear stresses.

In the early stages of development, glass fibers embedded in an epoxy resin matrix were commonly used, known as glass-reinforced plastic (GRP). While effective for certain applications such as helicopter blades, GRP had limitations in stiffness, restricting its use in fixed-wing aircraft components. However, advancements led to the introduction of new fiber reinforcements like Kevlar, known for its stiffness and toughness but limited compression strength. Another breakthrough came with boron fiber composites, which offered sufficient strength and stiffness for primary structures but were eventually surpassed by carbon-fiber-reinforced plastics (CFRP).

CFRP, derived from carbon fibers embedded in a resin matrix, has become the material of choice for many aircraft structures due to its exceptional properties. It boasts a high Young’s modulus (stiffness), strength comparable to aluminum alloy, and a significantly lower weight. While CFRP offers numerous benefits, it is also brittle and susceptible to impact damage, which can compromise its strength. Additionally, the epoxy resin matrix may absorb moisture over time, affecting certain properties like compressive strength, especially at higher temperatures.

Despite these drawbacks, CFRP offers substantial weight savings compared to traditional materials. Replacing aluminum alloy structures with CFRP can result in significant reductions in overall structural weight, contributing to improved fuel efficiency and performance. In modern aircraft like the Airbus A350XWB, fiber-reinforced composites make up a significant portion of the structural components, including the empennage, wings, nose, and fuselage. This widespread adoption underscores the importance and versatility of composite materials in the aerospace industry.