Spring makes our daily life common, mainly reflected in the electronic resistance of air, steam, the stainless steel spring water, corrosion and acid, alkali and salt corrosion corrosive medium, according to the mechanical properties of spring material can be divided into various types of spring, which is used in daily life, what is the cause of causes of spring fatigue, there are six main reasons.
1, the material yield strength yield strength and fatigue limit of the relationship between a certain, in general, the higher the yield strength of the material, the fatigue strength is higher, therefore, in order to improve the fatigue strength of the spring should try to increase the yield strength of the spring material, or the yield strength and tensile strength of the high ratio of materials. For the same material, the fine grain structure has a higher yield strength than the coarse grain.
2, the surface state stress occurs mostly in the surface of the spring material, so the surface quality of the spring has a great influence on the fatigue strength. The cracks, defects and wounds caused by the spring material in rolling, drawing and rolling are often caused by the fatigue fracture of the spring.
The smaller the surface roughness of the material, the smaller the stress concentration and the higher the fatigue strength. The effect of the surface roughness of the material on the fatigue limit. The fatigue limit decreases with the increase of surface roughness. Under the same roughness condition, the fatigue limits of different steel grades and different rolling methods are also different. For example, the degree of reduction of cold coil springs is smaller than that of thermal coil springs. Because the steel coil spring and its heat treatment, due to oxidation of the spring material surface rough and decarburization phenomenon, thus reducing the fatigue strength of the spring.
For grinding, pressing, blasting and rolling on the surface of the material. The fatigue strength of the spring can be improved.
3, the size of the size effect material is bigger. The more possibility of the defects caused by cold processing and hot working technology, the greater the possibility of surface defects. These reasons will lead to the decline of fatigue performance. Therefore, the effect of the size effect should be considered when calculating the fatigue strength of the spring.
4. Metallurgical defect metallurgical defects are non-metallic inclusions, bubbles, elements segregation in the material, and so on. The inclusion on the surface is the stress concentration source, which leads to premature fatigue cracks between the inclusions and the matrix interface. The quality of steel can be greatly improved by means of vacuum smelting and vacuum pouring.
5, when corrosive medium spring is working in corrosive medium, it will become a fatigue source due to pitting on the surface or corrosion on the surface grain boundary, which will gradually expand and cause fracture under the action of stress. For example, the fatigue limit of spring steel working in fresh water is only 10%~25% in the air. The effect of corrosion on the fatigue strength of the spring is not only related to the number of variable loads on the spring, but also on the working life. So when designing and calculating the spring that is affected by corrosion, the working life should be taken into account.
In order to ensure its fatigue strength, high strength corrosion resistant materials, such as stainless steel, non-ferrous metal, or protective coating on the surface, such as coating, oxidation, spray painting and painting, can be used to ensure fatigue strength. The practice shows that cadmium plating can greatly improve the fatigue limit of the spring.
6, the fatigue strength of temperature carbon steel decreased from room temperature to 120 degree temperature, and increased from 120 to 350. The temperature was lower than that at 350 C, and then there was no fatigue limit at high temperature. Heat resistant steel should be considered in the spring working under high temperature. The fatigue limit of steel is increased at less than room temperature.