There is a trend in the selection of materials in the structural design of aerospace vehicles, that is, aluminum alloys are preferred, titanium alloys are appropriately selected, and composite materials are reasonably selected. Because these materials have their own distinct advantages. Aluminum alloys are light in weight and their specific strength is getting better and better with the development of pre-stretching, heat treatment technology and material purification technology. The advantages of titanium alloys are light weight, high specific strength, and superior corrosion resistance. Composite materials also have the above advantages. So to sum up, within the allowable range of strength, we must choose light-weight materials.
Material selection should consider the following points in terms of performance
1. Specific static strength;
2. Specific stiffness;
3. Corrosion and embrittlement;
4. Fatigue;
5. Crack growth characteristics and fracture toughness;
6. Environmental stability.
Materials for some special parts should also be considered：
1. Corrosion and wear;
2. Compatibility with other materials;
3. Abrasion;
4. Temperature characteristics and potential characteristics.
According to the force and use characteristics of the parts in the structure, the parts mainly include three types of parts designed according to static strength requirements, parts designed according to rigidity requirements and parts designed according to fatigue fracture requirements.
Aerospace parts designed according to static strength requirements
1. For the main stress structure in the tensile stress area, if it is a key part of non-fatigue fracture, the tensile and shear ultimate strength and the composite strength according to the strength theory are mainly considered.
2. The main consideration for the static strength of the parts in the compression and compressive shear composite stress area is the stability of the structure. The main factor affecting the elastic stability is the elastic modulus E.
3. For structures whose working stress exceeds the proportional limit, the buckling stress of stability shall take into account the influence of plasticity. In addition to the elastic modulus E, the plastic stability buckling stress is also related to the secant modulus Es, the tangent modulus Et and the Poisson’s ratio ν. The solution is to choose a material with a higher compressive yield strength σ−0.2.
Aerospace parts designed according to stiffness requirements
Stiffness is the ability of a structure to resist deformation under load. The stiffness is related to the force form of the part (tension, compression, shear, bending, etc.), the size of the part and the elastic modulus E and shear modulus G of the material. It is not to say that the smaller the deformation in any place, the better. In some places, such as the wings, a moderate amount of deformation can improve the comfort of the cabin. Therefore, the stiffness should be selected according to the specific situation of the corresponding material. Parts that need to limit the flexibility of the structure must have rigidity requirements. For example, the skin of the wing box, the tie rod of the control system, and the rocker arm are often designed according to the rigidity.
Aerospace parts designed according to fatigue fracture requirements
Fatigue is the study of crack formation life under the action of environment and load spectrum. The anti-fatigue fracture design focuses on the key parts of the key parts of fatigue fracture, and the key parts are generally in:
1. High tensile and shear stress areas;
2. There is a large stress concentration in the structural details and a large fastener load transfer in the analysis of the connector;
3. Structural discontinuity on the force transmission route or sudden change in stiffness on the force transmission route;
4. Test or use experience proves that the parts are easy to crack;
5. Flight safety structure.
Therefore, such parts should be made of materials with good plasticity and high structural strength. For example, Ti-6Al-4V, HY-180 steel, 30CrMnSiA, etc.
There are many structural components of aircraft and aerospace vehicles, and even some finished products, pipelines and their brackets within the scope of each system are considered structural components. The types of materials used for these various and different parts of the parts are still quite diverse. The above-mentioned classification according to the force and use characteristics in the structure is for the body structure, and does not go into the details of each system. Therefore, when designing the system, it is not limited to the above. The load situation is simpler than the airframe. But it can also be used as a reference, because the above theories are common in all structural design links, and the alternative is to choose materials from the perspective of low cost and light weight.
In fact, there are still many things to pay attention to in the selection of materials in the structural design of aircraft and aerospace vehicles. In order to more effectively calculate the compliance of materials with operating conditions, special software is sometimes used to calculate simulation analysis, or to arrange actual tests for verification. I will not introduce too much space here, focusing on the accumulation of practical experience, the above is used as a reference.
