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The fatigue strength of materials is highly sensitive to various external and internal factors. External factors include the shape and size of the parts, surface finish, and operating conditions, while internal factors comprise the material’s composition, microstructural state, purity, and residual stresses. Even slight changes in these factors can cause fluctuations or even significant changes in the fatigue performance of the material. The influence of various factors on fatigue strength is an important aspect of fatigue research. Such research provides a basis for the rational structural design of components, as well as for the proper selection of materials and the appropriate development of various cold and hot processing techniques, in order to ensure that these components possess high fatigue performance. 01 Effects of stress concentration: The fatigue strength commonly referred to is measured using carefully processed, smooth specimens. However, actual mechanical parts inevitably have various types of notches, such as steps, keyways, threads, and oil holes. The presence of these notches causes stress concentration, resulting in the maximum actual stress at the root of the notch being much higher than the nominal stress applied to the part; fatigue failure of the part often begins at this location. Theoretical stress concentration factor Kt: Under ideal elastic conditions, it is the ratio of the maximum actual stress at the root of the notch to the nominal stress, as determined by elastic theory. Effective stress concentration factor (or fatigue stress concentration factor) Kf: the ratio of the fatigue limit σ-1 of a smooth specimen to the fatigue limit σ-1n of a notched specimen. The effective stress concentration factor is influenced not only by the size and shape of the component, but also by various factors such as the physical properties of the material, processing methods, and heat treatment. The effective stress concentration factor increases as the sharpness of the notch increases, but it is usually lower than the theoretical stress concentration factor. Fatigue notch sensitivity coefficient q: The fatigue notch sensitivity coefficient indicates the degree to which a material is sensitive to fatigue notches, and it is calculated using the following formula. The value of q ranges from 0 to 1; the lower the value of q, the less sensitive the material is to notches. Tests have shown that q is not purely a material constant; it is still related to the size of the notch. Only when the radius of the notch exceeds a certain value does the q value become essentially independent of the notch, and this radius value varies for different materials or treatment conditions. 02 Influence of size factors: Due to the unevenness in the material’s structure and the presence of internal defects, an increase in size raises the probability of material failure, thereby reducing the material’s fatigue limit. The existence of size effects is an important issue when applying fatigue data obtained from small laboratory specimens to large-scale actual components. Since it is not possible to reproduce stress concentrations and stress gradients present in components of actual size in small specimens in exactly the same manner, this leads to a disconnect between the laboratory results and the fatigue failure of certain specific components. 03 Influence of surface finishing condition: The machined surface always has uneven machining marks, which act like tiny notches and cause stress concentration on the surface of the material, thereby reducing its fatigue strength. Tests have shown that for steel and aluminum alloys, rough machining (rough turning) results in a fatigue limit that is 10%~20% or even more lower compared to longitudinal finish polishing. The higher the strength of the material, the more sensitive it is to surface finish. 04 The influence of loading history: In reality, no component operates under absolutely constant stress amplitudes. Both overloading and underloading during actual use affect a material’s fatigue limit. Tests have shown that materials generally suffer from overloading-induced damage as well as benefits resulting from underloading. The so-called overload damage refers to the decrease in a material’s fatigue limit that occurs after it is subjected to loads higher than its fatigue limit for a certain number of cycles. The higher the overload, the fewer weeks are required to cause damage. In fact, under certain conditions, a small number of overload cycles not only does not cause damage to the material; due to strain strengthening, crack tip blunting, and residual compressive stress, it actually strengthens the material, thereby increasing its fatigue limit. Therefore, some supplements and revisions to the concept of overload damage are necessary. The so-called secondary loading training refers to the phenomenon in which, after the material is subjected to stress levels below its fatigue limit but above a certain threshold for a certain number of cycles, its fatigue limit increases. The effect of secondary strain aging is related to the properties of the material itself; for materials with good plasticity, a longer aging period and higher strain levels are generally required to achieve desired results. 05 Influence of chemical composition: There is a close relationship between the fatigue strength and tensile strength of materials under certain conditions; therefore, any alloying element that can increase the tensile strength can also enhance the fatigue strength of the material under those same conditions. In comparison, carbon is the most significant factor affecting material strength. Some impurity elements that form inclusions in steel have an adverse effect on fatigue strength. 06 Effects of heat treatment and microstructure: Different heat treatment conditions result in different microstructures; therefore, the effect of heat treatment on fatigue strength is essentially the effect of the microstructure. For materials with the same composition, although the same static strength can be achieved due to different heat treatments, the fatigue strength can vary considerably owing to differences in the microstructure. At the same strength level, the fatigue strength of flake pearlite is significantly lower than that of granular pearlite. For granular pearlite as well, the finer the cementite particles, the higher the fatigue strength. The influence of microstructure on the fatigue properties of materials is related not only to the mechanical properties of the various microstructures themselves, but also to the grain size and the distribution pattern of the different structures within a composite structure. Refining the grain structure can improve the fatigue strength of the material. 07 Effect of inclusions: The inclusions themselves, or the pores they cause, act as tiny notches; under alternating loads, these lead to stress concentration and strain concentration, thereby becoming sources of cracks that cause fatigue fracture and having a negative impact on the material’s fatigue properties. The effect of inclusions on fatigue strength depends not only on the type, properties, shape, size, quantity, and distribution of the inclusions, but also on factors such as the strength level of the material, as well as the level and condition of the applied stress. Different types of inclusions have varying mechanical and physical properties, as well as differences in their properties compared to the base material; these factors also result in different effects on fatigue performance. Generally speaking, deformable plastic inclusions (such as sulfides) have little impact on the fatigue properties of steel, whereas brittle inclusions (such as oxides, silicates, etc.) pose a greater hazard. Inclusions with an expansion coefficient greater than that of the matrix (such as sulfides) have a minor impact due to the compressive stress they generate in the matrix, whereas inclusions with an expansion coefficient lower than that of the matrix (such as alumina) have a greater impact due to the tensile stress they induce in the matrix. The degree of bonding between the inclusions and the base metal also affects fatigue strength. Sulfides are prone to deformation and bond tightly with the base material, whereas oxides tend to separate from the base material, causing stress concentration. It can be seen that, in terms of the type of impurities, sulfides have a relatively minor impact, whereas oxides, nitrides, and silicates pose a greater hazard. Under different loading conditions, the effect of inclusions on the fatigue properties of materials varies as well. Under high load conditions, whether or not there are inclusions present, the applied load is sufficient to cause plastic flow in the material, and the influence of inclusions is minimal. However, within the range of the material’s fatigue limit stress, the presence of inclusions leads to local strain concentration, which becomes the controlling factor for plastic deformation and thus has a significant impact on the material’s fatigue strength. In other words, the presence of inclusions mainly affects the fatigue limit of the material, with little impact on its fatigue strength under high-stress conditions. The purity of the material is determined by the melting process; therefore, employing purification melting methods (such as vacuum melting, vacuum degassing, and electroslag remelting) can effectively reduce the impurity content in steel and improve the fatigue properties of the material. 08 Effects of changes in surface properties and residual stresses. In addition to the surface finish mentioned earlier, the effects of the surface condition also include changes in the mechanical properties of the surface layer as well as the influence of residual stresses on fatigue strength. Changes in the mechanical properties of the surface layer can be caused by differences in its chemical composition and microstructure, or by strain strengthening of the surface layer. Surface heat treatments such as carburizing, nitriding, and carbonitriding can not only enhance the wear resistance of components, but also serve as an effective means of improving their fatigue strength, particularly their resistance to corrosion fatigue and fretting. The effect of surface chemical heat treatment on fatigue strength depends mainly on factors such as the loading mode, the carbon and nitrogen concentrations in the coated layer, surface hardness and its gradient, the ratio of surface hardness to core hardness, the depth of the coating, as well as the magnitude and distribution of the residual compressive stress generated by the surface treatment. Numerous tests have shown that as long as notches are machined first and then subjected to chemical heat treatment, generally speaking, the sharper the notch, the greater the improvement in fatigue strength. Under different loading conditions, the effect of surface treatment on fatigue performance also varies. Under axial loading, since there is no uneven distribution of stress along the layer thickness, the stress is the same in the surface layer and beneath it. In this case, surface treatment can only improve the fatigue performance of the surface layer; since the core material is not strengthened, the improvement in fatigue strength is limited. Under bending and torsional conditions, the stress distribution is concentrated in the surface layer. The residual stresses resulting from surface treatment, combined with these applied stresses, reduce the actual stress experienced by the surface. Meanwhile, the strengthening of the surface layer material enables an effective increase in fatigue strength under bending and torsional conditions. Unlike chemical heat treatments such as carburizing, nitriding, and carbonitriding, if a part loses carbon during heat treatment, resulting in a decrease in the strength of its surface layer, this will significantly reduce the material’s fatigue strength. Similarly, surface coatings (such as Cr or Ni plating) result in reduced fatigue strength due to factors such as the notch effect caused by cracks in the coating, the residual tensile stress induced by the coating in the base metal, and hydrogen embrittlement resulting from the penetration of hydrogen during the electroplating process. Induction hardening, surface flame hardening, and thin-shell hardening of low hardenability steels can all produce a surface layer with a certain depth of hardened area, as well as favorable residual compressive stresses in the surface layer; thus, they are also effective methods for improving the fatigue strength of parts. Treatments such as surface rolling and shot blasting are also effective ways to improve fatigue strength, as they can create a deformation-hardened layer of a certain depth on the specimen’s surface while simultaneously inducing residual compressive stress in it.