1. There is a certain relationship between the yield strength and the fatigue limit of the yield strength material. In general, the higher the yield strength of the material, the higher the fatigue strength. Therefore, in order to improve the fatigue strength of the spring, the yield strength of the spring material should be improved. Or use a material with a high yield strength and tensile strength ratio. For the same material, the fine-grained structure has a higher yield strength than the coarse-grained structure.
2. The surface state The maximum stress occurs in the surface layer of the spring material, so the surface quality of the spring has a great influence on the fatigue strength. The defects such as cracks, flaws and flaws caused by the spring material during rolling, drawing and rolling 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. Effect of surface roughness of material 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 the cold coil spring is smaller than that of the hot coil spring. Because the steel coil spring and its heat treatment are heated, the surface of the spring material is roughened due to oxidation and decarburization occurs, which reduces the fatigue strength of the spring.
The surface of the material is ground, pressed, shot blasted and rolled. All can increase the fatigue strength of the spring.
compressed spring
3. Size effect The larger the size of the material, the higher the likelihood of defects due to various cold and hot working processes and the greater the potential for surface defects, all of which can lead to reduced fatigue performance. Therefore, the effect of size effect must be considered when calculating the fatigue strength of the spring.
4. Metallurgical defects Metallurgical defects refer to the segregation of non-metallic inclusions, bubbles, and elements in the material, and so on. Inclusions present on the surface are stress concentration sources that can cause premature fatigue cracks between the inclusions and the substrate interface. Vacuum smelting, vacuum casting and other measures can greatly improve the quality of steel.
5. Corrosion medium When the spring is working in a corrosive medium, it will become a source of fatigue due to pitting or surface grain boundary corrosion of the surface, and it will gradually expand under the effect of stress and cause fracture. For example, in spring steel working in fresh water, the fatigue limit is only 10% to 25% in the 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 the spring affected by corrosion, the working life should be taken into consideration.
For springs operating under corrosive conditions, in order to ensure their fatigue strength, materials with high corrosion resistance, such as stainless steel, non-ferrous metals, or surfaces with protective layers such as plating, oxidation, spray, and paint, may be used. Practice shows that cadmium plating can greatly increase the fatigue limit of the spring.
6. Temperature The fatigue strength of carbon steel decreases from room temperature to 120°C and rises from 120°C to 350°C. After the temperature is higher than 350°C, it decreases again, and there is no fatigue limit at high temperatures. For springs operating at high temperatures, heat-resistant steels should be considered. Below room temperature, the fatigue limit of the steel increases.
For detailed information on these factors affecting fatigue strength, refer to the relevant information.
The values of σ-1 and τ-1 given in the general material table refer to the data obtained on the smooth surface of the material and in the air medium. If the working conditions of the designed spring are not consistent with the above conditions, then б-1 and τ-1 should be corrected. Generally considered influencing factors are stress concentration, surface conditions, size, temperature, etc., and the stress concentration factor K ( ( ( Kτ), the surface state coefficient Kß, the size factor Kε, the temperature coefficient Kt, etc. are expressed, and the actual fatigue limit is
Б'-1=(Kß KεKt/Kб )б'-1