1. Service Environment Aerospace Materials Aerospace Materials in addition to experiencing high stress, inertia force, the aerocraft is also subjected to impact load and alternating load caused by take-off and landing, engine vibration, high-speed rotation of rotating parts, maneuvering flight and gust. The engine gas and the solar radiation cause the aircraft to be in the high temperature environment, along with the flight speed enhancement, the aerodynamic heating effect highlights, produces “The thermal barrier”. In addition, it will be subjected to alternating temperature. When the stratosphere flies at subsonic speed, the surface temperature will drop to-50 °C, and the ambient temperature in the polar region in winter will be lower than-40 °C. Gasoline, kerosene and other fuels and various lubricants, hydraulic oil, most of the corrosion of metal materials, non-metal materials have a swelling effect, however, the aging process of the polymer material will be accelerated by the mold produced by the Sun Irradiation, wind and rain erosion and long-term storage in the underground moist environment.

2. The selection and application of aerospace materials spacecraft long-term operation in the atmosphere or in outer space, service in extreme environments with high reliability and safety, excellent flight performance and maneuverability, in addition to the optimization of the structure to meet the aerodynamic requirements, technological requirements and the use of maintenance requirements, but also depends on the excellent characteristics and functions of materials. 01 The principle of material selection is that structural members should bear all kinds of external forces in service, and the materials should not exceed the allowable deformation and not be broken within the prescribed time limit, and the aerospace structure should also reduce the structural size and weight as far as possible, early Aerospace components were designed with static strength without considering or little considering plastic toughness, which resulted in catastrophic accidents.

In order to ensure the safety of the structure and make full use of the properties of the materials, the design of aerospace structural components has been changed from “Strength design principle”to “Damage tolerance design principle”, and gradually transitioned to “Life cycle design principle”, all aspects of the product life cycle are taken into account during the design phase, and all relevant factors are comprehensively planned and optimized during the design phase. Materials are required not only to have high specific strength and stiffness, but also to have a certain degree of fracture toughness and impact toughness, fatigue resistance, high temperature resistance, low temperature resistance, corrosion resistance, aging resistance and mould resistance, and strengthen some performance index pertinently. In addition, different criteria are used for selecting materials for different load grades, and materials matching the components are selected according to their specific requirements. Strength criteria are used for large load areas and high strength materials are used for medium load areas, high Elastic modulus material is selected and the dimension stability is considered in the light load area to ensure the size of the member is larger than the minimum critical size.

When selecting and evaluating structural materials, a suitable test method of mechanical properties (tensile, compressive, impact, fatigue and low temperature impact) should be selected according to service conditions and stress states, according to different fracture modes (ductile fracture, brittle fracture, stress fatigue, strain fatigue, stress corrosion, hydrogen embrittlement, neutron irradiation embrittlement, etc. . The stress distribution on the surface and the center of the member subjected to tensile load is uniform. The material selected should have uniform structure and performance. The large member should have good hardenability. The stress difference between the surface and the center of the member subjected to bending and torsion is large, and the material with low hardenability can be used. The fatigue limit and notch sensitivity of members subjected to alternating load are important indexes for material selection. The corrosion resistance, hydrogen embrittlement sensitivity, stress corrosion cracking tendency and corrosion fatigue strength of components in corrosive medium are important evaluation indexes for material selection. The high temperature service material also needs to consider the organization stability, the low temperature service material also needs to consider the low temperature performance.

Weight reduction is of practical significance to improve the safety of aircraft, increase the payload and range, improve the maneuverability and range, reduce fuel or propellant consumption and flight cost. A 15% reduction in the weight of the fighter reduces the taxiing distance by 15% , increases range by 20% , and increases payload by 30% . For short-time single-use aircraft such as missiles or launch vehicles, the equivalent functions should be performed with minimum volume and mass, and the performance of materials should be maximized, select the smallest margin of safety as possible to achieve absolute reliability of the safe life.

02, the main aerospace materials play a more important role in reducing the structural mass, density by 30% , and strength by 50% . Aluminum Alloy, titanium alloy and composite are the main aerospace structural materials, which have high specific strength and stiffness, can improve the vehicle’s payload, maneuverability, range, 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 dosage of ultra-high strength steel is stable at 5% ~ 10% , the tensile strength is 600 ~ 1850 MPA, sometimes up to 1950 MPA, and the fracture toughness KIC is 78 ~ 91 MPA M1/2. High Strength corrosion-resistant steel is generally used for fuselage load-bearing components in active corrosion media, and carbon-free corrosion-resistant steel is used for aircraft equipped with hydrogen-fueled engines in liquid hydrogen and hydrogen media. The 21st century aircraft body structure material is mainly aluminum alloy, including 2XXX series, 7XXXX and aluminum lithium alloy. Adding lithium into aluminum alloy can increase the strength and decrease the density at the same time, and realize the goal of increasing the specific strength and specific stiffness. Al-li alloy has been used in large transport aircraft, fighter aircraft, strategic missiles, space shuttle and launch vehicle, mainly used in the head Shell, load-bearing components, liquid hydrogen and liquid oxygen storage tanks, pipelines, payloads adapter, etc. . The third-generation and the developing fourth-generation al-li alloys are not only pursuing low density, but also have better comprehensive properties. The fourth-generation al-li alloys have higher static strength (especially yield strength) and higher fracture toughness under the same conditions as the third-generation al-li alloys, such as crack growth rate, fatigue property, corrosion property and elastic modulus.

Titanium alloys, which have higher specific strength than aluminum alloys, have been used in aircraft frames, flap guides and struts, engine mounts and landing gear components, as well as in the heating parts of hoods and flameproof plates. The surface temperature of Ma & GT; 2.5 supersonic aircraft can reach 200 ~ 350 °C, and titanium alloy can be used as skin. High Purity and high density titanium alloys prepared by rapid Powder metallurgy method have good thermal stability and the strength at 7 °C is the same as that at room temperature, the high strength and high toughness Beta titanium alloy developed by NASA has been designated as the Matrix material for the SIC/TI composite used to make the fuselage and wing panels of aircraft. The application proportion of titanium alloy in aircraft is increasing gradually, the amount of titanium alloy used in civil airframe will reach 20% , and the amount of titanium alloy used in military airframe will reach 50% . Metal Matrix composite, high temperature resin based composites, Ceramic Matrix composite, and carbon carbon composites have played an increasingly important role in aerospace. Carbon/carbon composites combine the fusibility of carbon with the high strength and rigidity of carbon fiber, have excellent thermal stability and excellent thermal conductivity, and still have fairly high strength and toughness at high temperature of 2500 °C, and only 1/4 the density of superalloys. More and more attention has been paid to hybrid composites, such as adding glass fiber to carbon fiber composites can improve its impact properties, and adding carbon fiber to glass fiber reinforced plastics can increase its stiffness.

In addition, laminated composites are increasingly used in aerospace engineering, such as the A380 using 3% GLARE, a new type of laminate. A laminate is a composite material made of two different kinds of materials laminated together by pressure, usually consisting of an upper panel, an upper laminate, a core, a lower laminate, and a lower laminate, its strength and stiffness is higher than that of individual panel materials or core materials, has been used in transport aircraft and fighter aircraft. Glare laminates are formed by hot pressing a multilayer thin aluminum sheet with unidirectional glass fiber prepreg (epoxy impregnated adhesive) , as shown in figure 1. The aluminum sheet shall be properly pretreated to make it adhere more easily to the fiber prepreg layer. Table 1 is a commercially available type of Glare laminates which can be made in different thicknesses as required. The fibers can be 2,3,4 layers, etc. . The fiber content and orientation should be in accordance with the table, each type of GLARE laminates may take different forms and may be adjusted according to specific requirements.

GLRE splicing solves the problem of limited width of aluminum sheets. As shown in figure 2, there is a narrow seam between the same aluminum sheets, and the joints between different aluminum sheets are in different positions, these joints can be joined by a fiber layer and other layers of aluminum, making it possible to make large body panels or integral skins, and have excellent fatigue, corrosion, and fire resistance properties, thus, the rivet holes and the resulting stress concentration are eliminated. In order to ensure the safe transmission of loads, a reinforcement layer can be added at the joint, that is, an additional layer of metal plate or a layer of glass fiber preg.

Honeycomb Sandwich composite material is composed of sandwich layer and skin (panel) , the skin can be aluminum, carbon/epoxy composite material, etc. , kong, Ivory Coast is a series of hexagons, quadrangles, and other shapes made of metal, fiberglass, or composite materials that are rebonded (or brazed) to thinner panels on both sides of the sandwich. The core material of the aluminum honeycomb sandwich composite material is bonded by aluminum foil in different ways and made into honeycomb of different specifications by drawing. The performance of the core material is mainly controlled by the thickness and the size of the hole lattice of the aluminum foil, compared with the riveted structure, the efficiency of the structure can be increased by 15% ~ 30% . Honeycomb Sandwich construction materials can be used to make a variety of wallboards, used for wing, hatch, hatch cover, floor, engine cover, muffler plate, heat insulation plate, satellite Shell, parabolic antenna, rocket propellant, etc. . However, the honeycomb sandwich structure composite is easy to corrode in some environments. When impacted, the Honeycomb Sandwich will deform permanently, and the honeycomb sandwich will be separated from the skin.

03 Aerospace Materials Analysis Table 2 shows the percentage of structural materials used in US military aircraft. The general trend is that the amount of composite materials and titanium alloys increases gradually, while the amount of aluminum alloys decreases.

The proportion of B787 composites and A350 composites is 50% and 52% respectively. It will be the development trend of aerospace field to apply composite materials extensively. The Composite Material has good weight reduction effect, 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 regeneration and utilization. Therefore, the A320neo and B737MAX composites did not use more than A320 and B737.
The structural materials of the modules of the manned spacecraft are mostly aluminum alloy, titanium alloy and composite materials. For example, the orbiter of the space shuttle is mostly made of aluminum alloy, and the thrust structure supporting the main engine is made of Chin Alloy, the main frame of part of the fuselage is made of Boron fiber reinforced aluminum alloy Metal matrix composite, and the cargo door is made of a special paper honeycomb sandwich structure, with graphite fiber reinforced epoxy composite panels. The ablation material should be used on the missile head, the spacecraft external surface and the internal surface of the rocket engine. Under the action of heat flow, the ablation material can decompose, melt, evaporate, sublimate, corrode and other physics and chemical reaction, mass consumption on the surface of the material takes away a great deal of heat in order to prevent heat transfer from re-entering the atmosphere and to cool the rocket engine combustor and nozzle. In order to maintain a suitable working temperature in the cabin, radiation heat protection measures should be taken in the re-manned cabin 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 Qin Alloy, quartz fiber, glass fiber composite ceramics and other materials with good thermal insulation properties are filled between the outer skin and the inner structure. With the development of manned spaceflight, lunar exploration and deep space exploration, high-resolution satellites, hypersonic vehicles, reusable launch vehicles and space mobile vehicles, new and more demanding requirements for materials have been put forward, providing new opportunities and impetus for the development of new space materials, in the field of materials, great breakthroughs must be made in material system innovation, independent support of key raw materials and engineering application.

 Source: CAITONG, reference: Advanced Materials for aerospace. Li Hongying, Wang Bingfeng, etc.