Six factors affect the fatigue strength of spring

Several factors influencing the fatigue strength of spring

1. Generally speaking, the higher the yield strength of the material, the higher the fatigue strength, therefore, in order to improve the fatigue strength of the spring should try to improve the yield strength of the spring material, or use the yield strength and tensile strength ratio of high materials. For the same material, the fine grain structure has higher yield strength than the coarse and fine grain structure.

2. Surface state stress occurs mostly on the surface of spring material, so the surface quality of spring has a great influence on the fatigue strength. The cracks, defects and scars caused by spring materials during rolling, drawing and winding are often the causes of fatigue fracture of spring.

The smaller the surface roughness of the material, the smaller the stress concentration and the higher the fatigue strength. Effect of material surface roughness on fatigue limit. With the increase of surface roughness, the fatigue limit decreases. In the case of the same roughness, different steel grades and different winding methods have different degrees of fatigue limit reduction, such as the reduction degree of cold coil spring is smaller than that of hot coil spring. Because when the steel hot coil spring and its heat treatment are heated, the surface of the spring material becomes rough and decarbonized due to oxidation, which reduces the fatigue strength of the spring.

The surface of the material is ground, pressed, shot blasting and rolling. Can improve the fatigue strength of the spring.

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

4. Metallurgical defects Metallurgical defects refer to non-metallic inclusions, bubbles, segregation of elements in materials, and so on. The inclusion on the surface is the source of stress concentration, which leads to premature fatigue cracks between the inclusion and the matrix interface. Vacuum smelting, vacuum casting and other measures can greatly improve the quality of steel.

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

In order to ensure the fatigue strength of the spring working under corrosion conditions, materials with high corrosion resistance can be used, such as stainless steel, non-iron metal, or the surface of the protective layer, such as coating, oxidation, spraying, painting, etc. The practice shows that cadmium plating can greatly improve the fatigue limit of spring.

6. The fatigue strength of carbon steel decreases from room temperature to 120 ° C, increases from 120 ° C to 350 ° C, and decreases after the temperature is higher than 350 ° C, and there is no fatigue limit at high temperature. For springs working under high temperature conditions, heat-resistant steel should be considered. At temperatures below room temperature, the fatigue limit of steel increases.

For the specific values of the above factors affecting fatigue strength, see the relevant information.