In the aero engine design process, due to the structure, use and maintenance, etc., a considerable number of components and components need to be connected to each other. Therefore, a fastener for joining two or more zeros and components into one unit is an essential component in engine design. Due to the special working environment of the aero-engine, the fastening connectors used must have good reliability and sufficient life to withstand the harsh environmental conditions and load changes to ensure safe and efficient flight.
In recent years, with the rapid development of aerospace technology, the increase of foreign cooperation and exchange, many new foreign connection structures and the theory of forming and strengthening have been introduced and applied, and the fastener connection technology has developed rapidly. However, due to the limitations of domestic processing conditions and process levels, many new types of fasteners are not widely used, and most of them are purchased abroad. Until recently, specialized production has been continuously strengthened, new materials and new processes have been continuously adopted, the manufacturing level of fasteners has been greatly improved, and the design and application of new fasteners have become more extensive and diversified.
Threaded connectors are the most important type of fastener, and the reliability of the threaded connection is critical. The bolted connection is simple in construction, easy to install and widely used.
For bolts subjected to variable loads and impact loads, we use flexible structural elastic bolts and improve and optimize the material selection, structure, processing technology, etc., so as to improve the stress state of the bolts to adapt to the different structures and materials of the connected parts. Features and complex stress requirements. This paper elaborates the measures to improve the fatigue strength of elastic bolts from the aspects of rational material selection, structural improvement and surface strengthening.
1 The basic form of the elastic bolt and the material elastic bolt is a reduced-diameter type fatigue bolt. The cross-sectional area of ​​the rod is about 9C% of the area of ​​the thread core. The screw is flexible, which can improve the stress state of the bolt and improve the fatigue performance. Elastic bolts can not only reduce the influence of temperature on bolt strength, but also apply to parts with large alternating load and large vibration. The structure and form of fasteners are simple, but metallurgical quality and fatigue performance can not be ignored.
Reasonable material selection is one of the main measures for designing high reliability connecting bolts.
In recent years, the selection of fasteners has gradually developed towards high heat and high strength. The high temperature alloy of grade GH2132 has been widely used as a heat resistant stainless steel. It is characterized by high yield strength and long-lasting creep strength below 650C. , has a good processing plasticity. High-quality (YZ) GH2132 superalloy is based on GH2132 to improve the purity of the alloy and adjust the heat treatment system to improve the heat resistance and long-term performance of the alloy. The elastic bolts designed by the author use (YZ) GH2132 material, which has been tested and tested repeatedly and has excellent performance to meet the design needs.
2 Advantages of elastic bolt structure In addition to the preferred materials, we also effectively improve the fatigue strength of bolts by optimizing the elastic bolt structure and applying surface strengthening techniques. Specific measures: the radial dimension of the head of the twelve-point bolt is small. Can effectively reduce the width of the mounting side. It can reliably clamp and apply a large torque during installation, and disassemble the graduate student.
Therefore, the installation space required for the use of the twelve-point socket wrench is also small, which makes the engine compact and effectively reduces the weight.
The bolt head is made into a shape, the metal flow line is reasonable, and the fatigue strength is high. Seeing the relief hole at the end of the head not only reduces the weight, but also is advantageous for the head boring process.
Twisting bolt head flow line The twelve-point bolt head has a small radial dimension which is advantageous for reducing the additional bending stress of the bolt, thereby greatly improving the fatigue strength of the threaded connection.
Although we have strict requirements on the bolt head support against the optical rod axis runout in the design, it may still cause the bolt head and the screw to be non-perpendicular during manufacturing and installation, causing eccentric bending moment and additional bending stress on the bolt fatigue. The formation of the initial crack of the fracture has an important influence. The smaller outer diameter of the bolt head reduces the additional bending stress, greatly reduces the axial maximum tensile stress, and correspondingly increases the fatigue strength of the bolted joint.
The lower rounded corners and the supporting surface are one of the weak points of the bolt because the head supporting surface and the polished rod transfer point are one of the weak points of the bolt. In addition to appropriately increasing the rounded corner under the head, the rounded corner must be heat treated. After the cold rolling process, the fatigue strength of the adapter is increased by surface strengthening.
(2) The surface quality of the lower bearing surface of the elastic bolt head is high, which can reduce the deformation caused by the local stress increase when the bolt is pre-tightened, and reduce the eccentric load of the bolt.
3 The reduction of the diameter of the flexible rod of the flexible rod portion reduces the variation of the cross section of the thread to the polished rod, which effectively alleviates the stress concentration at the end of the thread, thereby improving the static strength and fatigue strength of the bolt.
Reducing the effect of temperature on fatigue strength In order to reduce weight and increase the thrust-to-weight ratio, the engine often uses a large amount of lightweight materials such as aluminum and magnesium alloys. When the bolt and the material of the connecting side are different, especially when the linear expansion coefficients of the two materials are different, a difference in thermal expansion occurs, resulting in an increase in the bolt load and fatigue damage. If elastic bolts are used, the flexibility of the bolts is increased, and the load variation due to different expansions in the elastic range is reduced, so that the first and second buckle weak threads from the nut support surface can be safely adapted and effectively Improve the threaded joint strength.
Reduce the stress amplitude of the alternating load When the working tension changes between 0 and F, the bolt tension changes between Qp and Q, and the tensile force of the bolt is f. When the bolt stress level is fixed, the fatigue strength of the part is affected. The main factor is the stress amplitude. The smaller the stress amplitude, the higher the fatigue strength.
When the working tension and the remaining pre-tightening force are fixed, the bolt stiffness is reduced or the rigidity of the connected member is increased, and the bolt pulling force can be reduced, thereby reducing the screw stress: the total bolt pulling force Q can be expressed as: Qp bolt Pre-tightening force Ch bolt tensile stiffness Cm is connected by compression stiffness F bolt working tension assumed bolt dangerous section diameter is 'the maximum tensile minimum tensile stress of bolt dangerous section: by formula (1) (2) (3) The obtained stress amplitude is as follows: It can be known from the formula (4) that reducing the bolt stiffness or increasing the rigidity of the connected member can achieve the purpose of reducing the bolt stress amplitude and improving the fatigue strength.
4 Strengthen the thread to reduce the stress concentration. Use the M-reinforced thread of the large arc bottom to improve the thread fatigue strength. After the heat treatment, the roller-rolled thread has a better fatigue performance than the cutting thread. This is because when the thread is rolled, the thread is threaded. The surface, especially the thread bottom, forms the residual compressive stress of the surface layer. When the part is under the external load, the residual stress on the surface of the bolt and the tensile stress in the external alternating stress cancel each other out, so that the actual tensile stress is reduced, thereby reducing the part's Actual stress level. In addition, the compressive stress region is more difficult to slip between metal atoms than the tensile stress region, thereby improving the fatigue life of the bolt.
Cold work hardening of the threaded surface enhances the resistance of the surface layer to deformation and fracture.
Better surface integrity, improved surface quality of the threads and reduced stress concentration.
The effect of decarburization on strength is avoided.
After the thread is rolled and then heat treated, stress relief occurs, which loses the benefit of rolling the thread. Practice has proved that the rolling thread can obtain the full benefits of regenerative compressive stress after heat treatment, and the tensile fatigue performance can be increased by 5~8 times compared with the rolling thread before heat treatment.
The elastic bolt is a bolt structure that is reduced in shape and can effectively improve fatigue strength. The use of elastic bolts can reduce the influence of temperature and vibration on the bolt strength. It is suitable for connecting parts with large difference in linear expansion coefficient between two materials, and also for parts with large alternating load and large vibration. In addition, the economic benefits of modern aircraft due to the weight reduction of fasteners far exceed the cost of manufacturing and processing. Therefore, elastic bolts have broad prospects for development.
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