The fatigue strength of materials is highly sensitive to various internal and external factors.
External factors include the part’s shape, size, surface finish, and service conditions, while internal factors include the material’s composition, microstructure, purity, and residual stress.
A small change in these factors can cause fluctuations or significant changes in the material’s fatigue performance. Understanding the impact of various factors on fatigue strength is crucial in fatigue research.
This research provides a foundation for the proper structural design of parts, the appropriate selection of materials, and the effective implementation of cold and hot processing technologies, ensuring that parts have high fatigue performance.
01 Effect of stress concentration
The conventional method of measuring fatigue strength involves using carefully processed smooth specimens.
However, in reality, mechanical parts often have various forms of gaps such as steps, keyways, threads, and oil holes.
These notches result in stress concentration, causing the maximum actual stress at the root of the notch to be much greater than the nominal stress of the part.
As a result, the fatigue failure of the part often initiates from these notches.
Theoretical stress concentration factor Kt:
Under ideal elastic conditions, the ratio of the maximum actual stress to the nominal stress at the root of the notch is calculated based on elastic theory.
Effective stress concentration factor (or fatigue stress concentration factor) Kf:
The fatigue limit of smooth specimens (σ-1) and the fatigue limit of notched specimens (σ-1n) are evaluated.
The effective stress concentration factor is impacted not only by the size and shape of the component, but also by the physical properties, processing, heat treatment, and other factors of the material.
The effective stress concentration factor increases with the increase of notch sharpness, but it is typically less than the theoretical stress concentration factor.
Fatigue notch sensitivity coefficient q:
The fatigue notch sensitivity coefficient represents the sensitivity of the material to the fatigue notch and is calculated by the following formula:
The range of the value of q is between 0 and 1. The smaller the value of q, the less sensitive the material being characterized is to the notch.
It has been demonstrated that q is not only a constant for the material, but also depends on the size of the notch.
The value of q is only considered to be independent of the notch when the radius of the notch is larger than a specific value, which varies for different materials or treatment states.
02 The influence of size factor
The inhomogeneity of material structure and the presence of internal defects result in an increased likelihood of failure as the size of the material increases, thereby lowering its fatigue limit.
The phenomenon of size effect is a significant issue when extrapolating fatigue data from small laboratory specimens to larger practical parts.
It is not possible to replicate the stress concentration and stress gradient of actual-sized parts on small samples, leading to a disconnect between the results obtained in the laboratory and the fatigue failure of certain specific parts.
03 Influence of surface processing state
The machined surface always contains uneven machining marks, which act like tiny gaps, leading to stress concentration on the material surface and reducing its fatigue strength.
Research shows that for steel and aluminum alloys, the fatigue limit of rough machining (rough turning) is reduced by 10% to 20% or more compared to longitudinal polishing.
Materials with higher strength are more sensitive to the surface finish.
04 The impact of loading experience
In reality, no part operates under a strictly constant stress amplitude.
Overloading and secondary loads can impact the fatigue limit of materials.
Studies show that overload damage and secondary load training are prevalent in materials.
Overload damage refers to a decrease in the fatigue limit of a material after it has undergone a certain number of cycles under a load that is higher than its fatigue limit.
The greater the level of overloading, the quicker the damage cycle occurs, as depicted in the figure below.
Overload damage boundary
In certain conditions, a limited number of instances of overloading may not cause damage to the material.
Due to the effects of deformation strengthening, crack tip passivation, and residual compressive stress, the material is also strengthened, thereby improving its fatigue limit.
Thus, the idea of overload damage should be revised and modified.
The phenomenon of secondary load training refers to an increase in the fatigue limit of a material after a certain number of cycles under stress that is below the fatigue limit but above a certain limit value.
The impact of secondary load training depends on the properties of the material itself.
In general, materials with good plasticity should have a longer training cycle and be subjected to higher training stress.
05 Influence of chemical composition
Fatigue strength and tensile strength have a strong correlation under certain conditions.
Consequently, under specific conditions, any alloy elements that enhance the tensile strength can also improve the fatigue strength of the material.
Among the various factors, carbon has the most significant impact on the strength of materials.
However, some impurities that form inclusions in steel can have a negative effect on the fatigue strength.
06 Effect of heat treatment on Microstructure
The effect of heat treatment on fatigue strength is largely the effect of microstructure, as different heat treatments result in different microstructures.
Although the same composition of materials can achieve the same static strength through various heat treatments, their fatigue strength can vary greatly due to different microstructures.
At a similar strength level, the fatigue strength of flake pearlite is noticeably lower than that of granular pearlite.
The smaller the cementite particles, the higher the fatigue strength.
The impact of microstructure on the fatigue properties of materials is not just related to the mechanical properties of various structures but also to the grain size and distribution characteristics of the structures in the composite structure.
Grain refinement can enhance the fatigue strength of the material.
07 Influence of inclusions
The presence of inclusions or holes created by them can act as tiny notches, causing stress and strain concentration under alternating load, and become the source of fatigue fractures, negatively impacting the fatigue performance of materials.
The impact of inclusions on fatigue strength is dependent on various factors, including the type, nature, shape, size, quantity, and distribution of the inclusions, as well as the strength level of the material and the state and level of the applied stress.
Different types of inclusions have unique mechanical and physical properties, and their effect on fatigue properties varies. Plastic inclusions, such as sulfides, tend to have little impact on the fatigue properties of steel, while brittle inclusions, such as oxides and silicates, have a significant adverse effect.
Inclusions with an expansion coefficient larger than the matrix, such as sulfides, have less impact due to compressive stress in the matrix, while inclusions with a smaller expansion coefficient than the matrix, such as alumina, have a greater impact due to tensile stress in the matrix. The compactness of the inclusion and the base metal also affects fatigue strength.
The type of inclusion can also influence its impact. Sulfides, which are easy to deform and well-combined with the base metal, have less impact, while oxides, nitrides, and silicates, which are prone to separation from the base metal, result in stress concentration and have a greater adverse effect.
The impact of inclusions on the fatigue properties of materials varies under different loading conditions. Under high load, the external load is sufficient to induce plastic flow in the material, regardless of the presence of inclusions, and their impact is minimal.
However, in the fatigue limit stress range of the material, the presence of inclusions causes local strain concentration and becomes the controlling factor of plastic deformation, significantly affecting the fatigue strength.
In other words, inclusions primarily impact the fatigue limit of the material and have little effect on the fatigue strength under high stress conditions. To improve the fatigue performance of materials, purification smelting methods, such as vacuum smelting, vacuum degassing, and electroslag remelting, can be used to effectively reduce the impurity content in steel.
08 Influence of surface property change and residual stress
In addition to the surface finish mentioned previously, the influence of surface state also encompasses changes in surface mechanical properties and the effect of residual stress on fatigue strength.
The alteration of mechanical properties of the surface layer may be due to different chemical composition and microstructure of the surface layer, or from deformation strengthening of the surface.
Surface heat treatments, such as carburizing, nitriding, and carbonitriding, can not only increase the wear resistance of components but also improve their fatigue strength, particularly their resistance to corrosion fatigue and pitting.
The impact of surface chemical heat treatment on fatigue strength depends largely on the loading mode, the concentration of carbon and nitrogen in the layer, surface hardness and gradient, the ratio of surface hardness to core hardness, the depth of the layer, and the size and distribution of residual compressive stress formed during surface treatment.
Numerous tests have shown that, as long as a notch is machined first and then treated with chemical heat treatment, generally speaking, the sharper the notch, the greater the improvement in fatigue strength.
The effect of surface treatment on fatigue properties varies based on the loading mode.
Under axial loading, there is no uneven stress distribution along the depth of the layer, meaning that the stress on the surface and below the layer is the same.
In this scenario, surface treatment can only improve the fatigue performance of the surface layer, as the core material is not strengthened, thus limiting the improvement in fatigue strength.
Under bending and torsion conditions, stress is concentrated on the surface layer and the residual stress from surface treatment and external stress are superimposed, reducing the actual stress on the surface.
At the same time, the strengthening of the surface material improves the fatigue strength under bending and torsion conditions.
In contrast, chemical heat treatments like carburizing, nitriding, and carbonitriding can greatly reduce the fatigue strength of the material if the surface strength of the component is reduced due to decarburization during heat treatment.
Similarly, fatigue strength of surface coatings, such as Cr and Ni, decreases due to the notch effect caused by cracks in the coatings, residual tensile stress caused by the coatings in the base metal, and hydrogen embrittlement caused by hydrogen absorption during the electroplating process.
Induction quenching, surface flame quenching, and shell quenching of low hardenability steel can result in a certain depth of surface hardness layer and form favorable residual compressive stress on the surface layer, making it an effective method for improving fatigue strength of components.
Surface rolling and shot peening can also create a certain depth of deformation hardening layer on the surface of specimens and produce residual compressive stress, which is also an effective way to enhance fatigue strength.