Metal has always been the main force in mechanical manufacturing, because the strength and rigidity of metal are much better than non-metallic materials such as plastic, rubber, wood, etc., so for a long time, the role of metal is difficult to be replaced.
However, with the development of materials science, metal is no longer our only choice, and even a large number of non-metallic materials have been used in aeroengines because of their "special properties", such as ceramic materials, because of their It has excellent high temperature resistance and may be used in high temperature parts of aeroengines in the future.
Ceramic-based turbine blades, let`s not talk about ceramics today, but about another magical non-metallic material: carbon fiber
Why does the aerospace industry favor carbon fiber?
If we want the aero-engine to be light enough, how to reduce the density of the material as much as possible is a good method. The titanium alloy used to be lightweight and qualified for stiffness, strength and high temperature performance. It is now an excellent aerospace material widely used.
But the metal is a metal after all, even if it is a material like magnesium aluminum alloy, the density is only about 2000kg per cubic meter, and the metal material has a very serious problem, that is, the metal has fatigue problems, when the metal material When the internal force is changed, as long as the force is large enough and the action time is long enough, the material will generate micro cracks, and then such cracks will grow quickly until the entire material is cracked.
So this time a new material: carbon fiber, appeared in the eyes of engineers.
This material first has one of the most obvious advantages: light. The density of carbon fiber is generally 1400 ~ 2000kg per cubic meter, and the density of carbon fiber reinforced with epoxy resin is only 1600kg per cubic meter, which is far better than metal materials.
Moreover, this material has extremely high tensile strength. If you want to break the high-strength carbon fiber material, you need to use two to four times the force to break the steel with the same cross-sectional area.
In addition, carbon fiber is different from metal materials. Carbon fiber materials do not have the concept of "fatigue strength", which means that the changing force is a threat to metal materials and needs to be focused on, but it is not the case for carbon fiber. .
These advantages can be said to be crucial for aviation equipment, because these things are going to fly into the sky, so naturally the lighter the better, and devices such as aero engines need to withstand huge and complex loads. There will be complex and variable loads. Such a material with low density, high strength and strong fatigue resistance is simply helpful.
Carbon fiber is widely used in aircraft and rockets
Due to the above advantages, carbon fiber is widely used in the aerospace field. For example, carbon fiber can be used to make rocket fuel storage tanks, for example, it can be used to make aircraft shells, and so on. Especially in the manufacture of advanced civil passenger aircraft, the use of carbon fiber has exceeded 50%.
However, although carbon fiber has many advantages, it is indeed widely used in rockets and aircraft structures, but there are many problems in the use of aircraft engines.
Application of carbon fiber in aircraft engines: fan blades
Although carbon fiber, it is not so easy to use carbon fiber materials in aeroengines, because aeroengines have two very fatal problems in addition to large and complex loads:
The temperature of the engine working environment is too high
The engine has strict control of structural deformation
The operation of an aero engine is inseparable from the combustion of a large amount of fuel, so it will inevitably produce extremely high temperatures, for example, the temperature in front of the engine turbine can be as high as 2000 ℃. Although carbon fiber can also withstand a certain high temperature, it still seems powerless at such a high temperature, especially epoxy resin itself is difficult to withstand high temperatures above 200 ℃.
In addition, the aero engine is divided into two parts: a static and a dynamic, that is, a "static" and a "rotor". The static is not moving, the rotor rotates at high speed, and the dynamic and static are required to fit tightly. control".
The clearance between the rotating and stationary parts of the engine is very high
Although the carbon fiber material is said to have high strength, it has insufficient stiffness, which means that once the engine works, the parts made of carbon fiber will undergo greater deformation, bringing about a change in the matching state between the rotor and the stator. Especially in places where the pressure is relatively high, a little gap may cause safety problems and performance degradation.
Therefore, the application of carbon fiber materials in aircraft engines is relatively small, and the main location is: fan blades. That is the same thing that we can see when we fly on the plane.
The temperature of the fan blades farther away from the combustion chamber is very low (in fact, the ambient temperature), and the pressure in this position is relatively low, and the gap control is not so strict. At the same time, these blades are large enough to facilitate the processing of carbon fiber materials, and the use of carbon fiber materials can also maximize the characteristics of low material density.