Hardware knowledge: six factors affect spring fatigue strength

Several factors affecting spring fatigue strength

1. There is a certain relationship between the yield strength of the material and the fatigue limit. Generally speaking, the higher the yield strength of the material, the higher the fatigue strength. Therefore, in order to increase the fatigue strength of the spring, we should try to increase the yield strength of the spring material. Or use materials with a high ratio of yield strength to tensile strength. For the same material, the fine grain structure has a higher yield strength than the coarse and fine grain structure.

2. The maximum stress of the surface state mostly occurs on the surface of the spring material, so the surface quality of the spring has a great influence on the fatigue strength. Defects such as cracks, flaws and scars caused by spring materials during rolling, drawing and coiling are often the causes of spring fatigue and 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. As the surface roughness increases, the fatigue limit decreases. In the case of the same roughness, different steel grades and different coiling methods have different degrees of fatigue limit reduction. For example, the degree of reduction of cold-coiled springs is smaller than that of hot-coiled springs. Because the steel hot coil spring and its heat treatment are heated, the surface of the spring material becomes rough due to oxidation and decarburization occurs, which reduces the fatigue strength of the spring.

Grinding, pressing, shot blasting and rolling the material surface. Both 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 and hot working processes, and the greater the possibility of surface defects. These reasons will all lead to a decrease in fatigue performance. Therefore, the influence of the size effect should be considered when calculating the fatigue strength of the spring.

4. Metallurgical defects Metallurgical defects refer to the segregation of non-metallic inclusions, bubbles, elements, etc. in the material. The inclusions present on the surface are the source of stress concentration, which will cause premature fatigue cracks between the inclusions and the substrate interface. The use of vacuum smelting, vacuum casting and other measures can greatly improve the quality of steel.

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

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

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