What are the causes of spring fatigue? There are six main reasons.
1. Yield strength there is a certain relationship between the yield strength and fatigue limit of the material. 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, try to improve the yield strength of the spring material, or use a material with a high yield strength and tensile strength ratio. For the same material, the fine grain structure has higher yield strength than the coarse and fine grain structure.
2. The surface state Z large stress mostly occurs in the surface layer of the spring material, so the surface quality of the spring has a great influence on the fatigue strength. Cracks, defects and scars caused by rolling, drawing and rolling of spring materials are often the causes of spring fatigue fracture.
The smaller the surface roughness of the material, the smaller the stress concentration and the higher the fatigue strength. Influence of material surface roughness on fatigue limit. The fatigue limit decreases with the increase of surface roughness. Under the same roughness, the reduction degree of fatigue limit is different for different steel grades and different rolling methods. For example, the reduction degree of cold coil spring is smaller than that of hot coil spring. When the steel hot coil spring and its heat treatment are heated, the surface of the spring material becomes rough and decarburized due to oxidation, which reduces the fatigue strength of the spring.
Grinding, forced pressing, shot blasting and rolling of the material surface. The fatigue strength of the spring can be improved.
3. The larger the size of the size effect material, the higher the possibility of defects caused by various cold processing and hot processing processes, and the greater the possibility of surface defects. These reasons will lead to the reduction of 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, etc. in materials. The inclusion on the surface is the source of stress concentration, which will lead to premature fatigue crack between the inclusion and the matrix interface. The quality of steel can be greatly improved by adopting measures such as vacuum smelting and vacuum pouring.
5. When the corrosion medium spring works in the corrosion medium, it will become a fatigue source due to pitting corrosion 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 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 the 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, 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 and non-ferrous metals, or protective layers, such as plating, oxidation, plastic spraying and painting, can be used. Practice shows that cadmium plating can greatly improve the fatigue limit of spring.
6. The fatigue strength of temperature 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. Heat resistant steel shall be considered for springs working under high temperature conditions. The fatigue limit of steel increases when the temperature is lower than room temperature.
