What Is Fatigue Limit?

Fatigue limit refers to the maximum stress value when no stress occurs after infinite stress cycles. It is also called the endurance limit. The fatigue limit of a material is an inherent property of the material, which varies depending on the cyclic characteristics, the form of deformation of the test piece, and the environment in which the material is located, and so on. The measurement requires several smooth small-sized samples to be tested on a dedicated fatigue tester.

The fatigue limit is

Dividing the specimen into several groups, generally the maximum stress on the first specimen

0.70

The fatigue limit of smooth small specimens is not the fatigue limit of components. The fatigue limit of a component is related to the condition of the component and the working conditions. Component status includes factors such as stress concentration, size, surface processing quality, and surface strengthening treatment; working conditions include factors such as load characteristics, media, and temperature, among which

The so-called increasing fatigue strength generally refers to improving the fatigue strength of a component by reducing the influence of the factors affecting the fatigue limit of the component without changing the basic dimensions and materials of the component. That is, the fatigue limit of the component is mainly improved by eliminating or reducing stress concentration and additional stress on the part, etc.

(1) In stress reduction concentration, one should pay attention to the change of the section on the part, such as holes, keyways, transition fillets, threads, etc. The change of the section should not be sudden, the edges of the holes and transition fillets should be smooth, and the surface should be smooth. For example, the radius of the crank transition fillet should not be less than 5% of the diameter of the crank pin diameter, otherwise serious bending stress concentration will occur.

(2) Enhancing the strength of the surface layer-Surface chemical heat treatment such as carburizing, nitriding, and carbonitriding can effectively improve the surface fatigue strength of the part: cold machining of the surface of the component, such as shot peening, surface rolling, etc. The strengthening process can form a pre-stress layer on the surface, reduce the surface tensile stress that is prone to fatigue cracks, and significantly increase the fatigue of the surface of the part.

(3) Reduce the surface roughness-the surface processing quality of components has a great influence on fatigue strength. Components requiring higher fatigue strength should have lower surface roughness. High strength is more sensitive to surface roughness, and only after finishing can it be beneficial to exert its high strength performance. Otherwise, the durability limit will be greatly reduced, and the significance of using high-strength steel will be lost. In use, the surface of the component should be protected from mechanical damage (such as scratches) or chemical damage (such as corrosion, rust, etc.).