The fatigue strength of materials is a critical performance indicator for the design and longevity of components. It is influenced by both external and internal factors, and understanding these variables is essential for improving the service life and reliability of materials in various applications.

1. Key Factors Affecting Fatigue Strength

1.1 Stress Concentration

Mechanical components often feature shapes and structural elements, such as steps, keyways, threads, and oil holes, which can cause stress concentration. These irregularities result in a localized increase in stress, often at the notch or flaw, making it the point of initiation for fatigue failure.

　· Theoretical Stress Concentration Factor (Kt): The ratio of maximum stress at the root of the notch to the nominal stress, calculated under ideal elastic conditions.

　· Effective Stress Concentration Factor (Kf): The ratio of the fatigue limit of a smooth specimen to that of a notched specimen, influenced by material properties, processing, and heat treatment.

　· Fatigue Notch Sensitivity Factor (q): This measures the material’s sensitivity to notches—smaller values of q indicate lower sensitivity.

1.2 Size Effect

Fatigue strength is closely related to the size of the material. As the size of a component increases, internal material inhomogeneity and defects become more prominent, often leading to reduced fatigue strength. The size effect is especially important when applying laboratory data from small specimens to larger components, as real-world components may have different geometries, stress concentrations, and loading conditions that cannot be perfectly replicated in a test specimen.

1.3 Surface Finish

The surface condition plays a significant role in fatigue strength. Surface roughness can create small notches, which act as stress concentrators, thereby lowering fatigue strength. Studies show that finely finished surfaces, especially in high-strength materials, are less prone to fatigue failure compared to rough surfaces.

1.4 Loading History

In real-world applications, fatigue strength is influenced not only by constant stress amplitudes but also by overload and subload conditions.

　· Overload Damage: Loads exceeding the fatigue limit can reduce the fatigue strength of materials after a certain number of cycles.

　· Subloading Effect: Stress levels below the fatigue limit but above a certain threshold may lead to an increase in fatigue strength after repeated loading.

1.5 Chemical Composition

The fatigue strength of materials is strongly linked to their tensile strength. Alloying elements, such as carbon, can improve tensile strength and, in turn, enhance fatigue performance. However, impurities like sulfur and phosphorus can lead to the formation of harmful inclusions that degrade fatigue properties.

1.6 Heat Treatment and Microstructure

Heat treatment processes alter the microstructure of a material, significantly affecting its fatigue strength. For materials with the same chemical composition, different heat treatments can result in varying fatigue performance. Processes such as grain refinement and achieving a uniform microstructure can notably enhance fatigue strength. For instance, fine carbide particles can improve the material's resistance to fatigue.

1.7 Surface Properties and Residual Stresses

Fatigue strength is not only influenced by surface finish but also by the mechanical properties and residual stresses present at the surface. Surface treatments such as carburizing, nitriding, and shot peening increase surface hardness and corrosion resistance, significantly enhancing fatigue strength. Additionally, the distribution of residual stresses plays a critical role in a material's fatigue performance.

1.8 Inclusions

Inclusions are microscopic defects in the material that can act as stress concentrators under cyclic loading, initiating fatigue fractures. The type, size, quantity, and bonding of inclusions with the base material can all affect fatigue strength. Brittle inclusions, such as oxides and silicates, tend to have a more significant impact on fatigue performance.

2. Surface Treatments to Improve Fatigue Strength

Surface heat treatments, such as carburizing and nitriding, modify the surface hardness and microstructure of components. These processes not only enhance wear resistance but also substantially improve fatigue strength by introducing a hardened surface layer and residual compressive stresses. This is especially beneficial under complex loading conditions like bending and torsion.

However, processes such as decarburization during heat treatment can significantly reduce fatigue strength. Surface coatings like chrome or nickel plating may also negatively affect fatigue strength due to issues such as interlayer cracking or hydrogen embrittlement.

Conclusion

The fatigue strength of materials is a complex result of various factors, including external loading, component shape and size, and internal material characteristics such as chemical composition, microstructure, inclusions, and residual stresses. By carefully optimizing component designs, selecting appropriate materials, and implementing effective surface treatments, the fatigue strength of components can be significantly enhanced. This not only extends their service life but also minimizes the risks of fatigue failure, improving both safety and performance.