A detailed explanation of the uses of composite materials, including classification, characteristics, and significance for aviation development. Composite materials are materials with new properties composed of two or more materials with different properties through physical or chemical methods at the macro (micro) level. All kinds of materials learn from each other in performance and produce synergistic effect, so that the comprehensive performance of composite materials is better than that of raw materials to meet different requirements. The amount of composite materials has become an important symbol to measure the progressiveness of military equipment. The rise of composite materials has enriched the modern material family. In particular, the appearance of carbon fiber reinforced composites with high strength, high modulus and low specific gravity makes them one of the important candidate materials for various military and civilian equipment.

Composite materials can be classified according to the types of matrix materials, the morphology of reinforcement materials, and the functions of composite materials.

1. According to the category of matrix materials, composites can be divided into metal matrix, organic non-metallic matrix and inorganic non-metallic matrix, such as resin matrix, aluminum matrix, titanium matrix composites, etc;

2. According to the shape of reinforcement materials, composites can be divided into fiber reinforcement, particle reinforcement, short fiber reinforcement, sheet reinforcement, etc., such as carbon nanotubes, carbon fiber composites, etc;

3. According to material functions, composites can be divided into structural composites, functional composites and intelligent composites, such as conductive composites, optical fibers, shape memory alloys, etc.

Properties of composite materials

Compared with ordinary materials, composite materials have many characteristics, which can improve or overcome the weakness of a single material, give full play to the advantages of each material, and endow materials with new properties; According to the structure and stress requirements of components, the matching performance with reasonable distribution can be given and the best performance design of materials can be carried out.

Specifically

1. High specific strength and high specific modulus. The outstanding advantage of composites is their high specific strength and specific modulus. For example, the specific modulus of carbon fiber reinforced resin composite is 5 times higher than that of steel and aluminum alloy, and the specific strength is more than 3 times higher than that of steel and aluminum alloy.

2. High fatigue resistance. Fiber composites, especially resin matrix composites, are less sensitive to notch and stress concentration, and the interface between fiber and matrix can blunt or change the direction of the growing crack tip, that is, prevent the rapid growth of the crack, so the fatigue strength is high. The fatigue limit of carbon fiber unsaturated polyester resin composites can reach 70%~80% of their tensile strength, while that of metal materials is only 40%~50%.

3. Strong fracture resistance. There are a large number of independent fibers in the fiber composite materials, generally thousands to tens of thousands of fibers per square centimeter. They are combined into a whole by a tough matrix. When a few fibers in the fiber composite component break due to overload or other reasons, the load will be redistributed to other unbroken fibers, so that the component will not be suddenly damaged in a short time. Therefore, the composites have relatively high fracture toughness.

4. Good damping performance. The natural frequency of a structure is related to the mass and shape of the structure itself, and is proportional to the square root of the specific modulus of the material. If the natural vibration frequency of the material is high, resonance and early damage caused by it can be avoided under working conditions.

5. Good high temperature resistance and creep resistance. The heat resistance of fiber reinforced composites, such as carbon fiber reinforced resin composites, is significantly improved than that of resin matrix because fiber materials can still maintain high strength at high temperatures. However, metal matrix composites show their advantages in heat resistance. For example, the strength of aluminum alloy decreases rapidly with the increase of temperature, while the strength of aluminum matrix composites reinforced with quartz glass can maintain 40% of the room temperature strength at 500 ° C. The composite of silicon carbide fiber, aluminum oxide fiber and ceramic can withstand the high temperature of 1200~1400 ° C in air, which is more than 100 ° C higher than the heat resistance of all ultra-high temperature alloys.

6. Good corrosion resistance. Many kinds of composite materials are resistant to acid and alkali corrosion, such as glass fiber reinforced phenolic resin composite materials, which can be used for a long time in acidic media containing chloride ions, and can be used to manufacture chemical pipes, pumps, valves, containers, agitators and other equipment resistant to strong acids, salts, esters and some solvents.

7. Excellent antifriction, wear resistance and self lubrication. Composite components are suitable for integral molding because of their simple manufacturing process and good process performance. At the same time of manufacturing composite materials, parts are also obtained, thus reducing the number of parts, fasteners and joints, and saving raw materials and labor hours.

The development of composite materials is of great significance to the development of aviation equipment. The performance of an aircraft depends partly on its design and partly on its materials. The quality of materials has an undoubted significant impact on the speed, height, range, mobility, stealth, service life, safety, reliability, maintainability and other properties. According to statistics, 70% of aircraft weight loss is contributed by technological progress of aviation materials. The aircraft using carbon fiber reinforced resin matrix composites has obvious advantages in reducing aircraft weight, reducing fuel, reducing maintenance costs and extending aircraft service life.

Military aircraft

At present, the amount of composite materials used in the world's advanced military aircraft accounts for 20% - 50% of the total structural weight of the aircraft. The parts mainly used for composite materials include fairing, flat tail, vertical tail, flat tail wing box, wing, middle and front fuselage, etc. If composite materials account for about 50% of the total weight of the aircraft, most of the structural parts of the aircraft are made of composite materials, such as the B-2 stealth bomber.

civil aircraft

The application of composite materials in civil aircraft has gone through four processes.

In the first stage, in the mid-1970s, composite materials were mainly applied to the leading edge, flap, fairing, spoiler and other components with less stress.

In the second stage, in the mid-1980s, composite materials were mainly used in elevator, flap aileron and other components with less stress.

In the third stage, composite materials are applied to vertical tail, flat tail and other components with large stress. For example, the vertical and horizontal tails of the Boeing 777 aircraft are made of composite materials, which account for 11% of the total weight of the structure.

In the fourth stage, composite materials were applied to the wing and fuselage, the most important force bearing parts of the aircraft. The composite material consumption of Boeing 787 Dreamliner is 50%, which exceeds the total weight of aluminum, steel, titanium and other metal materials.

helicopter

Carbon fiber composites are widely used in military, civil and light helicopters, and the amount of composite materials used in helicopters has reached 40% - 60% of the structural weight. For example, the use of composite materials for the American armed helicopter RAH-66 is 50%; The use of composite materials for European NH-90 helicopters has reached 80%, close to the full composite structure.

UAV

Military UAVs have an urgent need for weight reduction, so composites are widely used in UAVs. For example, the amount of composite materials used in the US X-45 series aircraft is more than 90%; The X-47 series aircraft are basically all composite aircraft, and the composite material consumption of the "Global Hawk" unmanned reconnaissance aircraft reaches 65%, of which the wing, tail, rear fuselage, large radome, etc. are all made of composite materials; The same is true of the European test drone "Barracuda" and the American long-range attack drone "Skunk".

aircraft engine

The amount and proportion of composite materials have also become a measure of the advanced level of aeroengines. The excellent specific strength and specific modulus properties of resin matrix composites are of great significance for the weight reduction, improvement of propulsion efficiency, reduction of noise and emissions, and cost reduction of high thrust ratio aeroengines. They are mainly used in cold end components of aeroengines with operating temperatures below 150-200 ℃, such as compressor blades, guide vanes and their frame components of turbofan engines, nose cones and rectification devices of turbofan engines.

In hot end components, metal matrix, ceramic matrix and carbon/carbon composites have important applications due to the requirements of high temperature and other special conditions.

SiC long fiber reinforced titanium matrix composite (Ti MMC) has the advantages of high specific strength, high specific stiffness, high temperature resistance, good fatigue resistance and good creep performance. Ti MMC blade ring can reduce the weight of parts by 70% instead of compressor disk. In the future, metal matrix composites will be used to manufacture aeroengine compressor blades and mirror blades, integral blade rings, casings and turbine shafts. Ceramic matrix composites have always been the focus of research on high-temperature materials. Engine components made of fine ceramics and silicon nitride can work at 1371 ℃, and their performance is even better than that of superalloys.

At present, almost all military aircraft in service in China use composite materials on different parts. At present, a composite material system mainly composed of epoxy, bismaleimide and polyimide, and a composite material forming technology system mainly composed of hot-melt prepreg manufacturing technology and autoclave forming technology have been formed in China. The mechanical properties of aviation resin matrix composites in China have preliminarily met the requirements of the main load-bearing structure. The integration of structure and function is still under pre research, and the low-cost technology is still weak. The application of aviation resin matrix composites in active aircraft includes front fuselage, vertical tail, flat tail, aileron, rudder, canard, ventral fin, wing bulkhead, various flaps, etc. The applications on helicopters mainly include rotor, vertical tail, fuselage and tail rotor.

In the new generation of military aircraft, composite materials are mainly used in wings, canards, tail fins, vertical tails, mid fuselage wallboards, ventral fins, weapon doors, etc., with the amount up to 19% of the structural weight. The amount of composite materials used in large transport aircraft is about 10%, mainly used in vertical tail, flat tail, rudder, elevator, flap, aileron, distillation sheet, etc. The amount of composite materials used in the new generation of helicopters is about 34%, mainly including inclined beams, flat tails, fairings, skin, tail beams, side panels of the middle aircraft body, etc. The use of composite materials in some load-bearing structures has fundamentally changed the application scale of composite materials.

It is estimated that with the breakthrough of relevant composite materials and structural materials technology, the amount of composite materials used in domestic military aircraft in the future will increase to about 25%, and the weight reduction efficiency will increase from 20% to 30%. More composite materials will be used in the main load-bearing structures such as wings and fuselage, which will give full play to the advantages of composite materials such as corrosion resistance, stealth, conformal antennas, etc. while reducing weight.

Source: Military Industry Association

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