The eight main factors affecting the fatigue strength of metal materials

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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 include the composition of the material itself, its 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. 1 Effect 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 a notch to the nominal stress, as determined by elasticity 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, and heat treatment. The effective stress concentration factor increases with the increase in notch sharpness, but is generally smaller 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 data range for q in the image is 0-1; the lower the q value, the less sensitive the material is to notches. Experiments show that q is not purely a material constant; it still depends on the notch size. Only when the notch radius exceeds a certain value does the value of q become essentially independent of the notch. Moreover, this critical radius value varies depending on the material or its treatment state. 2 Influence of size factors: Due to the unevenness in the material’s structure and the presence of internal defects, an increase in size leads to a higher probability of material failure, thereby reducing the material’s fatigue limit. The existence of size effects is a major issue when applying fatigue data obtained from small laboratory specimens to full-sized actual components. Since it is impossible to perfectly replicate stress concentrations, stress gradients, and other factors present in real-sized components on small specimens, there is often a disconnect between laboratory results and the actual fatigue failure of certain components. 3 Influence of surface finish: Machined surfaces always exhibit uneven machining marks; these marks act as tiny notches, causing stress concentration on the material surface and 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. 4. Influence of loading history: In reality, no component operates under absolutely constant stress amplitudes. Both overloading and underloading during actual operation have an impact on the material’s fatigue limit. Tests have shown that overloading damage and underloading conditioning are common phenomena in materials. The so-called overload damage refers to the reduction 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 it takes to cause damage. In fact, under certain conditions, a small number of overload occurrences not only do not damage the material; due to strain hardening, crack tip blunting, and the effect of residual compressive stress, they can also strengthen the material, thereby increasing its fatigue limit. Therefore, some supplements and corrections should be made to the concept of overload injury. 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 results. 5 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 main factor affecting the strength of materials. Meanwhile, some impurity elements that form inclusions in steel have an adverse effect on fatigue strength. 6 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, the finer the cementite particles, the higher the fatigue strength. The effect 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 characteristics of the microstructures within composite structures. Refining the grain structure can improve the fatigue strength of the material. 7 Effects of inclusions: The inclusions themselves or the voids generated by them act as tiny notches; under cyclic loading, they cause stress and strain concentrations, becoming sources of cracks for fatigue fracture, thereby adversely affecting the fatigue properties of the material. 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 applied stress level and state. Different types of inclusions have different mechanical and physical properties; the differences between these properties and those of the base material also vary, resulting in different effects on fatigue performance. Generally speaking, deformable plastic inclusions (such as sulfides) have little impact on the fatigue performance of steel, whereas brittle inclusions (such as oxides, silicates, etc.) cause significant harm. Inclusions with a larger expansion coefficient than that of the matrix (such as sulfides) have little effect, as they generate compressive stress in the matrix; whereas inclusions with a smaller expansion coefficient than that of the matrix (such as alumina) have a significant effect, as they generate tensile stress in the matrix. The degree of bonding between inclusions and the base metal also affects the fatigue strength. Sulfides are easily deformable and bond tightly to the base metal, while oxides tend to detach from the base metal, causing stress concentration. Thus, in terms of the types of inclusions, sulfides have a relatively minor impact, while oxides, nitrides, silicates, etc., are more harmful. 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 external 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 localized 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, the use of purification techniques such as vacuum melting, vacuum degassing, and electroslag remelting can effectively reduce the impurity content in steel and improve the material’s fatigue performance. 8 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 cyaniding not only increase the wear resistance of parts but also enhance their fatigue strength; they are particularly effective means of improving corrosion fatigue and galling resistance. 　 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 created first followed by chemical heat treatment, generally the sharper the notch, the greater the increase 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 across the layer thickness, the stress at the surface layer is the same as that 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 platings) result in reduced fatigue strength due to factors such as the notch effect caused by cracks in the coating, the residual tensile stress exerted by the coating on the base metal, and hydrogen embrittlement resulting from the infiltration 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 hardness, as well as favorable residual compressive stress 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.