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

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Fatigue fracture usually starts at the weakest point. If there are defects within the material, such as cracks or brittle inclusions, fatigue may start from these internal weak points. If the internal microstructure of the material is uniform and free of defects, fatigue may begin at the stress concentration sites on the surface. Of course, surface stress concentration can be caused by structural issues, processing defects, or imperfections such as surface cracks. 1. The effect of stress concentration. The factor that has the greatest impact on fatigue strength at a surface is stress concentration, which generally occurs at areas where the cross-section changes, such as the root of bolts, keyways at the steps of shafts, oil holes, etc., as shown in Figure 1. Using arc transitions at the sections with variable cross-sections reduces the sensitivity of the material to notches, which can effectively prevent the occurrence of such fatigue fractures. Figure 12: Effect of surface roughness. Surface roughness also has a significant impact on fatigue strength; these influencing factors include surface machining marks, scratches, installation-induced damage, impact damage, abrasions, and so on. These not only change the surface roughness but also cause work hardening and stress concentration, thereby reducing the material’s fatigue resistance. 3. Effects of residual stress and surface strengthening. The surface residual stress will combine with the applied stress, resulting in a combined effect. If the residual stress is tensile, it will reduce fatigue strength; if it is compressive, it will increase fatigue strength. Surface strengthening treatment can generate residual compressive stress on the material surface, thereby improving the material’s fatigue resistance. Common surface strengthening methods include: shot blasting, rolling, surface quenching, and surface chemical heat treatment. A schematic diagram of shot blasting is shown in Figure 2. 4. Influence of alloy composition. Alloy composition is the fundamental factor that determines the microstructure of a material. Structural steel has high fatigue resistance, and carbon is an important element that affects its fatigue resistance. Carbon can be dissolved in steel to achieve solid solution strengthening, or it can form dispersed carbides, which also provides solid solution strengthening and thus helps to prevent the initiation of fatigue cracks. Other alloying elements can provide solid solution strengthening, or enhance hardenability and toughness, thereby improving the fatigue strength of the steel. 5. Influence of microstructure. The microstructure of structural steel also has a significant impact on fatigue strength. In the normalized microstructure, the carbides are flaky in shape, which leads to stress concentration at the ends of these carbides, resulting in lower fatigue strength ； In the tempered structure after quenching, the carbides are granular, resulting in small stress concentrations; therefore, its fatigue strength is higher than that of the normalized structure. Figure 3 shows tempered martensite. Figure 3 shows that the presence of non-martensitic phases in the quenched structure, such as undissolved ferrite and residual austenite, reduces fatigue strength due to the uneven mechanical properties within the structure. 6. Impact of metallurgical defects. Metallurgical defects reduce fatigue strength. Such defects include: non-metallic inclusions, pores, shrinkage cavities, segregation, white spots, folds, cracks, overburning, and so on.