Aerospace Materials: Types, Selection and Applications

Service environment of aerospace materials

In addition to withstanding high stress and inertial force, aerospace materials also have to withstand impact load and alternating load caused by takeoff and landing, engine vibration, high-speed rotation of rotating parts, maneuvering flight, sudden wind and other factors.

The engine gas and solar radiation cause the aircraft to be in a high temperature environment. With the increase of flight speed, the aerodynamic heating effect is prominent, resulting in “thermal barrier”.

In addition, it is subject to alternating temperature.

When flying at subsonic speed in the stratosphere, the surface temperature will drop to about – 50 ℃, and the severe winter ambient temperature within the polar circle will be lower than – 40 ℃.

Metal components or rubber tires are prone to embrittlement.

Gasoline, kerosene and other fuels, as well as various lubricants and hydraulic oils, mostly have corrosive effects on metal materials and swelling effects on non-metal materials.

However, mold produced by solar irradiation, wind and rain erosion and long-term storage in underground humid environment will accelerate the aging process of polymer materials.

Selection and application of aerospace materials

Aerospace vehicles have been operating in the atmosphere or outer space for a long time.

They also have to have high reliability and safety, excellent flight performance and maneuverability in extreme environments.

In addition to optimizing the structure to meet the aerodynamic requirements, technological requirements and use and maintenance requirements, they also depend on the excellent characteristics and functions of materials.

1. Material selection principle

In service, structural members must bear various forms of external forces, and the materials are required to not exceed the allowable deformation and not break within the specified period.

In addition, aerospace structures should try to reduce the size and weight of the structure.

In the early aerospace components, static strength design is adopted, and plastic toughness is not considered or rarely considered, resulting in catastrophic accidents.

Aerospace Materials: Types, Selection and Applications 1

Main structural components of mainline aircraft

In order to ensure structural safety and make full use of the performance of materials, the design of aerospace structural parts has been changed from “strength design principle” to “damage tolerance design principle”, and gradually transferred to “full life cycle design principle”.

In the design stage, all links of product life history are considered, and all relevant factors are comprehensively planned and optimized in the product design stage.

The material is required not only to have high specific strength and specific stiffness, but also to have certain fracture toughness and impact toughness, fatigue resistance, high temperature resistance, low temperature resistance, corrosion resistance, aging resistance and mold resistance, and to strengthen some performance indicators.

In addition, different material selection criteria are adopted for different levels of load areas, and matching materials are selected according to the specific requirements of components.

Strength criteria are adopted for large load areas, and high-strength materials are selected;

In the middle load area, the stiffness criterion is adopted and the material with high elastic modulus is selected;

In the light load area, the dimensional stability is mainly considered to ensure that the component size is larger than the minimum critical size.

When selecting and evaluating structural materials, appropriate test methods for mechanical properties (tensile, compression, impact, fatigue, low-temperature series impact) shall be selected according to service conditions and stress states, and the reasonable combination of material strength, plasticity and toughness shall be comprehensively considered for different fracture modes (ductile fracture, brittle fracture, stress fatigue, strain fatigue, stress corrosion, hydrogen embrittlement, neutron irradiation embrittlement, etc.).

For the member bearing tensile load, the stress distribution on the surface and the core shall be uniform, the selected material shall have uniform structure and performance, and the large member shall have good hardenability.

For members bearing bending and torsion loads, the stress difference between the surface and the core is large, and materials with low hardenability can be used.

Fatigue limit and notch sensitivity are important evaluation indexes for material selection of components bearing alternating load.

For components serving in corrosive medium, corrosion resistance, hydrogen embrittlement sensitivity, stress corrosion cracking tendency, corrosion fatigue strength, etc. are important assessment indicators for material selection.

The structure stability should be considered for high-temperature service materials, and the low-temperature performance should be considered for Low-Temperature Service materials.

Weight reduction has practical significance for improving the safety of the aircraft, increasing the payload and endurance, improving the maneuverability and range, and reducing the fuel or propellant consumption and flight cost.

The faster the speed of the aircraft, the greater the significance of weight reduction.

If the weight of the fighter is reduced by 15%, the taxiing distance of the fighter can be shortened by 15%, the range can be increased by 20%, and the payload can be increased by 30%.

For short-time disposable aircraft such as missiles or launch vehicles, it is necessary to exert equivalent functions with the minimum volume and mass, strive to exert the material performance to the limit, and select the smallest possible safety margin to achieve an absolutely reliable safety life.

2. Main aerospace materials

For reducing the structural mass, the density is reduced by 30%, which is greater than the strength by 50%.

Aluminum alloy, titanium alloy and composite materials are the main aerospace structural materials, with high specific strength and specific stiffness, which can improve the payload, mobility and endurance of the aircraft and reduce the flight cost.

The amount of ultra-high strength steel (yield strength > 1380MPa) used in aerospace engineering will not exceed 10%.

For modern aircraft such as supersonic fighter, the amount of ultra-high strength steel is stable at 5% ~ 10%, the tensile strength is 600 ~ 1850MPa, sometimes as high as 1950mpa, and the fracture toughness KIc = 78 ~ 91MPa · m1 / 2.

High strength corrosion-resistant steel is generally used for the fuselage load-bearing structure used in active corrosion medium, and carbon free corrosion-resistant steel is used as the component material for the aircraft equipped with hydrogen fuel engine in liquid hydrogen and hydrogen medium.

The structural materials of aircraft fuselage in the 21st century are mainly aluminum alloy, including 2XXX series, 7XXXX and aluminum lithium alloy.

Adding lithium to the aluminum alloy can improve the strength and reduce the density, and achieve the goal of improving the specific strength and specific stiffness of the component.

Aluminum lithium alloy has been used in large transport aircraft, fighter planes, strategic missiles, space shuttles and launch vehicles, mainly used in head shells, load bearing components, liquid hydrogen and liquid oxygen storage tanks, pipelines, payload adapters, etc., and is known as an aerospace material with great development prospects.

The third generation and the developing fourth generation aluminum lithium alloys no longer pursue low density unilaterally and have better comprehensive properties.

Under the condition that the crack growth rate, fatigue performance, corrosion performance and elastic modulus are equivalent to those of the third generation aluminum lithium alloys, the fourth generation aluminum lithium alloys have higher static strength (especially yield strength) and higher fracture toughness.

The specific strength of titanium alloy is higher than that of aluminum alloy.

It has been applied to aircraft frame, flap guide rail and bracket, engine base and landing frame parts, and also used for exhaust hood, fire shield and other heating parts.

The surface temperature of supersonic aircraft with Ma > 2.5 can reach 200 ~ 350 ℃, and titanium alloy can be used as skin.

The high-purity and high density titanium alloy prepared by rapid solidification / powder metallurgy method has good thermal stability.

The strength at 700 ℃ is the same as that at room temperature.

The developed high-strength and high toughness β-type titanium alloy has been selected by NASA as the matrix material of SiC / Ti composite material, which is used to manufacture aircraft fuselage and wing panels.

The application proportion of titanium alloy in aircraft is gradually increasing, and the use amount in civil aviation fuselage will reach 20%, and the use amount in military aircraft fuselage will reach 50%.

Aerospace Materials: Types, Selection and Applications 2

Metal matrix composites, high temperature resin matrix composites, ceramic matrix composites and carbon / carbon composites have played an increasingly important role in the aerospace field.

Carbon / carbon composite material combines the refractory property of carbon with the high strength and rigidity of carbon fiber.

It has excellent thermal stability and excellent thermal conductivity.

It still has high strength and toughness at 2500 ℃, and the density is only 1 / 4 of that of high-temperature alloy.

Hybrid composites have received more and more attention.

For example, adding glass fiber to carbon fiber composites can improve its impact performance, while adding carbon fiber to glass fiber reinforced plastics can increase its stiffness.

In addition, layered composite materials are more and more widely used in aerospace engineering.

For example, A380 uses 3% of glare, which is a new laminate.

Laminate is a composite material made of two different kinds of materials stacked together by pressure.

It is usually composed of upper panel, upper glued layer, core material, lower glued layer and lower panel.

Its strength and rigidity are higher than that of individual panel material or core material.

It has been applied to transport aircraft and fighter aircraft.

The glare laminate is formed by hot pressing multi-layer thin aluminum plate and unidirectional glass fiber prepreg (impregnated with epoxy adhesive) through pressure (or hot pressing tank), as shown in Fig. 1.

The aluminum plate shall be properly pretreated to make it easier to adhere to the fiber prepreg layer.

Table 1 shows the types of commercially produced glare laminates, which can be made into plates of different thicknesses according to needs.

The fibers can be 2, 3, 4 layers, etc.

The fiber content and direction can meet the requirements in the table.

Each type of glare laminate can have different forms and can be adjusted according to specific needs.

Aerospace Materials: Types, Selection and Applications 3

Fig. 1 Schematic diagram of glare laminate

Table 1 types of commercially available glare laminates

Laminate

 

 

Aluminum layer

Fibrous layer

Typical density / (g / cm 3

aluminium product

Single layer thickness / mm

orientation

Single layer thickness / mm

GLARE1

GLARE2

GLARE3

GLARE4

GLARE5

GLARE6

7475-T76

2024-T3

2024-T3

2024-T3

2024-T3

2024-T3

0.3~0.4

0.2~0.5

0.2~0.5

0.2~0.5

0.2~0.5

0.2~0.5

Unidirectional

Unidirectional

0°/ 90° orthogonal

0°/ 90°/0° orthogonal

0°/ 90°/90°/0° orthogonal

+45 ° / – 45 ° orthogonal

0.25

0.25

0.25

0.375

0.5

0.25

2.52

2.52

2.52

2.45

2.38

2.52

The splicing technology of glare laminates solves the problem of limited width of aluminum plates.

As shown in Fig. 2, there is a narrow seam between the same layer of aluminum plates, and the joints between different layers of aluminum plates are at different positions.

These joints can be connected with other layers of aluminum plates through fiber layers, which makes it possible to manufacture the wall plate or the overall skin of the mainframe, and has excellent fatigue resistance, corrosion resistance and flame retardancy.

Thereby eliminating the rivet hole and the stress concentration caused thereby.

In order to ensure the safe transmission of load, a reinforcing layer can be added at the splice, that is, a layer of metal plate or a layer of glass fiber prepreg can be added.

Aerospace Materials: Types, Selection and Applications 4

Fig. 2 splicing diagram of glare laminate

Honeycomb sandwich composite material is composed of sandwich and skin (panel).

The skin can be aluminum, carbon / epoxy composite material, etc. And the sandwich is like a honeycomb.

It is a series of hexagonal, quadrilateral and other shaped cells made of metal material, glass fiber or composite material.

The upper and lower surfaces of the sandwich are bonded (or brazed) to the thinner panel.

The core material of aluminum honeycomb sandwich composite is glued by aluminum foil in different ways and made into honeycomb of different specifications by stretching.

The performance of the core material is mainly controlled by the thickness of aluminum foil and the size of cell.

It has the advantages of high specific strength and specific stiffness, good impact resistance, vibration reduction, microwave transmission and strong designability.

Compared with riveted structure, the structural efficiency can be increased by 15% ~ 30%.

Honeycomb sandwich structure materials can be used to make various wall panels, such as wing surface, cabin surface, cabin cover, floor, engine cover, muffler plate, heat insulation plate, satellite star shell, paraboloid antenna, rocket propellant storage tank bottom, etc.

However, the honeycomb sandwich structure composite material is easy to corrode in some environments.

When impacted, the honeycomb sandwich will undergo permanent deformation, and the honeycomb sandwich will be separated from the skin.

3. Analysis of aerospace materials

The following table 2 shows the percentage of structural materials for military aircraft in the United States.

The general trend is that the consumption of composite materials and titanium alloys gradually increases, while the consumption of aluminum alloys decreases.

Table 2 percentage of structural materials for US military aircraft

TypeSteelAlloyTitanium alloyCombined material
F-165823
F-18A/B1550129.5
F-18C/D16501310
F-18E/F14312123
F-225164124
F-352736
F1175202510
B-19412129
B26192638
C1712.369.310.38.1

The following table 3 shows the proportion of typical mainline aircraft materials.

The composite materials of B787 account for 50%, and the composite materials of A350 account for 52%.

A large number of composite materials will become the development trend in the aerospace field.

The composite material has good weight reduction effect, good damage resistance, corrosion resistance and durability, and is suitable for smart structures.

However, the composite material has high cost, poor impact resistance, no plasticity, increased technical difficulty, poor maintainability and poor recycling.

Therefore, the amount of composite material of A320neo and B737MAX is not increased than that of A320 and B737.

Table 3 proportion of typical mainline aircraft materials (%)

TypeAluminium alloySteelTitanium alloyCombined materialOther
B3737912531
B7477913431
B7577812631
B7678014231
B77770117111
B787201015505
A3007613452
A310748567
A3206665158
A330/A3406655168
A3806155227
A35020714527
MD-8274.51267.5
MS-2133519385
C919638821

Most of the structural materials of the cabin sections of the manned spacecraft are aluminum alloy, titanium alloy and composite materials.

For example, most of the orbiters of the space shuttle are made of aluminum alloy, the thrust structure supporting the main engine is made of Qin alloy, the main frame of the middle fuselage is made of metal matrix composite materials reinforced with boron fiber reinforced aluminum alloy, and the cargo compartment door is made of special paper honeycomb sandwich structure, with graphite fiber reinforced epoxy resin composite material as the panel.

Ablative materials shall be used for the outer surface of the missile head, the re-entry capsule of the spacecraft and the inner surface of the rocket engine.

Under the action of heat flow, the ablative materials can undergo physical and chemical changes such as decomposition, melting, evaporation, sublimation and erosion.

The mass consumption of the material surface takes away a large amount of heat, so as to prevent the heat flow during re-entry into the atmosphere from passing into the aircraft and cooling the combustion chamber and nozzle of the rocket engine.

In order to maintain the proper working temperature in the cabin, radiation heat protection measures shall be taken for the re-entry section.

The outer skin is made of high-temperature resistant nickel base alloy or beryllium plate, and the inner structure is made of heat-resistant alloy.

The outer skin and the inner structure are filled with materials with good thermal insulation properties such as quartz fiber and glass fiber composite ceramics.

Aerospace Materials: Types, Selection and Applications 5

Conclusion

With the implementation and continuous development of space projects such as manned spaceflight, lunar exploration and deep space exploration, high-resolution satellites, high-speed vehicles, reusable vehicles, and space mobile vehicles, new and more stringent requirements are put forward for materials, which provides new opportunities and power for the development of new aerospace materials.

In the field of materials Major breakthroughs have been made in the independent guarantee of key raw materials and engineering applications.

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