What are the factors that affect the fatigue strength of materials?

Fatigue fracture often starts from the weakest place. If there are defects inside the material, such as cracks, brittle inclusions, etc., fatigue may start from these internal weak places.

If the internal structure and performance of the material are uniform and there are no defects, fatigue may start from the surface stress concentration area. Of course, surface stress concentration may be caused by structural problems, processing damage, or defects such as surface cracks.

1. The influence of stress concentration. The biggest influence of the surface on fatigue strength is stress concentration, which is generally the cross-sectional mutation part, such as the root of the bolt, the keyway at the step of the shaft, the oil hole, etc.

2. The influence of surface roughness. The influence of surface roughness on fatigue strength is also great. These influencing factors include surface processing marks, scratches, installation damage, impact damage, abrasions, etc. These not only change the surface roughness, but also cause work hardening and stress concentration, thereby reducing the fatigue resistance of the material.

3. The influence of residual stress and surface strengthening. The surface residual stress will be superimposed with the external stress to produce a superimposed effect. If the residual stress is tensile stress, it will reduce fatigue strength. If the residual stress is compressive stress, it will increase fatigue strength.

Surface strengthening treatment can produce residual compressive stress on the surface of the material, thereby improving the fatigue resistance of the material. General surface strengthening methods include: shot peening, rolling, surface quenching, surface chemical heat treatment, etc.

4. The influence of alloy composition. Alloy composition is the basic element that determines the material structure. Structural steel has high fatigue resistance, and carbon is an important element that affects its fatigue resistance.

Carbon can be dissolved in steel to produce solid solution strengthening, or it can form dispersed carbides for dispersion strengthening, thereby preventing the initiation of fatigue cracks. Other alloying elements can be solid solution strengthened, or improve hardenability and toughness, thereby improving the fatigue strength of steel.

5. The influence of microstructure. The structure of structural steel also has an important influence on fatigue strength. The carbides of the normalized structure are flaky, and stress concentration is easily generated at the end of the carbide, and the fatigue strength is low; the carbides of the tempered structure after quenching are granular, with small stress concentration, and its fatigue strength is higher than that of the normalized structure.

If there is non-martensitic structure in the quenched structure, such as undissolved ferrite and residual austenite, the fatigue strength will be reduced because the mechanical properties in the structure are not uniform.

6. The influence of metallurgical defects. Metallurgical defects will reduce fatigue strength. Such defects include: non-metallic inclusions, pores, shrinkage cavities, segregation, white spots, folds, cracks, overburning, etc.