The hottest factors affecting the fatigue strength

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Factors affecting the fatigue strength of springs

1. Yield strength there is a certain relationship between the yield strength and fatigue limit of materials. Generally speaking, the higher the yield strength of materials, the higher the fatigue strength. Therefore, in order to improve the fatigue strength of springs, we should try to improve the yield strength of spring materials, or use materials with high ratio of yield strength to tensile strength. For the same material, the fine grain structure has higher yield strength than 1. The coarse and fine grain structure of the main fixture selected according to the maximum experimental force of the host machine

2. The maximum stress in the surface state mostly occurs on the surface of the spring material, so the surface quality of the spring has a great impact on the fatigue strength. Cracks and defects caused by spring materials in the process of rolling, drawing and rolling can be equipped with bending points and scars with different diameters, which are often the cause of spring fatigue fracture

The smaller the surface roughness of the material, the smaller the stress concentration, and the higher the fatigue strength. The influence of material surface roughness on fatigue limit. With the increase of surface roughness, the fatigue limit decreases. In the case of the same roughness, the reduction degree of fatigue limit of different steel grades and different rolling methods is also different. For example, the reduction degree of cold coil spring is smaller than that of hot coil spring. Because when steel hot coil springs and their heat treatment are heated, the surface of spring material becomes rough and decarburizes due to oxidation, which reduces the fatigue strength of springs

carry out grinding, forced pressing, shot blasting and rolling on the material surface. Can improve the fatigue strength of the spring

3. The larger the size of the size effect material, the higher the possibility of defects caused by various cold working and hot working processes, and the greater the possibility of surface defects. These reasons will lead to the decline of fatigue performance. Therefore, the influence of size effect should be considered when calculating the fatigue strength of spring

4. metallurgical defects metallurgical defects refer to non-metallic inclusions, bubbles, element segregation, etc. in materials. The inclusion on the surface is the source of stress concentration, which will lead to premature fatigue cracks between the inclusion and the matrix interface. Adopting measures such as vacuum smelting and vacuum pouring can greatly improve the quality of steel

5. corrosive medium when the spring works in corrosive medium, it becomes a fatigue source due to pitting corrosion on the surface or corrosion of the surface grain boundary, which will gradually expand under the action of variable stress and lead to fracture. For example, the fatigue limit of spring steel working in fresh water is only 10% - 25% of that in air. The influence of corrosion on the fatigue strength of spring is related not only to the number of times the spring is subjected to variable load, but also to the working life. Therefore, the working life should be taken into account when designing and calculating the spring affected by corrosion

for springs working under corrosive conditions, in order to ensure their fatigue strength, materials with high corrosion resistance, such as stainless steel, non-ferrous metals, or protective layers on the surface, such as plating, oxidation, plastic spraying, painting, etc., can be used. Practice shows that cadmium plating can greatly improve the fatigue limit of springs

6. temperature the fatigue strength of carbon steel decreases from room temperature to 120 ℃, increases from 120 ℃ to 350 ℃, and decreases after the temperature is higher than 350 ℃. There is no fatigue limit at high temperature. For springs working under high temperature conditions, heat-resistant steel should be considered. Under the condition of lower than room temperature, the fatigue limit of steel increases

for the specific values of the above factors affecting fatigue strength, see relevant materials

given in the general material table б- 1 and τ- The value of 1 refers to the data obtained when the material surface is smooth and in the air medium. If the working conditions of the designed spring are inconsistent with the above conditions, then б- 1 and τ- 1. Generally, the influencing factors considered include stress concentration, surface condition, size, temperature, etc., and the stress concentration factor K is adopted respectively б (K τ It can measure the change of the size of the friction surface and the dispersion of the wear amount on the friction surface), the surface state coefficient K, and the size coefficient K ε、 Expressed by temperature coefficient KT, the actual fatigue limit is

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