1. There is a certain relationship between the yield strength of a material and fatigue. 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. Surface state Stress 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.
New energy spring
The smaller the surface roughness of the new energy spring material, the smaller the stress concentration and the higher the fatigue strength. The effect of material surface roughness on fatigue. As the surface roughness increases, fatigue decreases. In the case of the same roughness, different steel grades and different coiling methods have different degrees of fatigue 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 on the surface of the material. Both can improve the fatigue strength of the spring.
3. Size effect 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. 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, and so on in the material. The inclusions 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. Corrosive medium When the spring is working in a corrosive medium, it becomes a source of fatigue 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, the fatigue of spring steel working in fresh water is only 10% to 25% of that in air. The effect 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 related 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 improve the fatigue of the spring.
6. Temperature The fatigue strength of 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 at high temperatures. For springs that work under high temperature conditions, heat-resistant steel should be considered. Under conditions below room temperature, the fatigue of steel increases.