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 material composition, microstructure, purity, and residual stress.
A small change in these factors can cause significant fluctuations or changes in the fatigue performance of the material. Understanding the impact of various factors on fatigue resistance is crucial in fatigue research.
This research provides a foundation for proper structural design of parts, proper material selection, and effective implementation of cold and hot processing technologies, ensuring parts have high fatigue performance.
Factors Affecting the Fatigue Resistance of Metallic Materials
Here is the content in table form:
| Factor | Description |
|---|---|
| Stress concentration | Stress concentration is one of the main causes of fatigue failure in materials. This can be avoided by optimizing the shape, selecting smooth transition radii, and using precision machining methods to improve the surface quality of components. |
| Size Factor | The larger the size of the material, the more difficult it is to control the manufacturing process, resulting in lower density and uniformity of material organization and more metallurgical defects, all of which affect fatigue resistance. |
| Surface processing state | The surface processing state, such as surface roughness and machining tool marks, affects fatigue resistance. Surface damage can cause stress concentration and reduce the fatigue limit. |
| Chemical composition | Chemical composition has a significant impact on fatigue resistance. For example, surface heat treatments such as carburizing and nitriding can improve the fatigue resistance of the material on the part surface. |
| Heat treatment | Proper heat treatment can improve the fatigue performance of materials. For example, quenching, carburizing, cyanidation and high-frequency nitriding can improve the fatigue resistance of springs. |
| Environmental Factors | Environmental humidity has a significant impact on the durability of high-strength chromium steel, and water vapor has an adverse effect on the fracture toughness of most metals and alloys. |
| Metallurgical defects | Metallurgical defects, such as the presence of inclusions, affect fatigue resistance. Brittle inclusions (such as oxides, silicates, etc.) pose a significant risk to the fatigue performance of steel. |
| Corrosion | Corrosion is also an important factor affecting fatigue resistance, and anti-corrosion measures need to be considered in the design and manufacturing process. |
| Microstructure | By subjecting metallic materials to severe plastic deformation (SPD), microstructures such as ultrafine grains (UFG) and nanocrystalline grains (NG) can be produced, which can improve the fatigue resistance of materials. |
| Load and environment | Fatigue testing can be divided into room temperature fatigue testing, high temperature fatigue testing, low temperature fatigue testing, etc. according to the load and environment. Different working conditions have different effects on fatigue resistance. |
01 And effect of stress concentration
The conventional method of measuring fatigue strength involves the use of carefully processed smooth samples.
However, in reality, mechanical parts often have various shapes 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, fatigue failure of the part often starts from these notches.
Theoretical stress concentration factor K t :
Under ideal elastic conditions, the relationship between the maximum real stress and the nominal stress at the notch root is calculated based on elastic theory.
Effective stress concentration factor (or fatigue stress concentration factor) K f :
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 increasing notch sharpness, but is normally smaller than the theoretical stress concentration factor.
Fatigue notch sensitivity coefficient q :
The fatigue notch sensitivity coefficient represents the sensitivity of the material to fatigue notch and is calculated by the following formula:
The range of q value is between 0 and 1. The smaller the q value, the less sensitive the material being characterized is to the notch.
It has been shown that q is not only a constant for the material, but also depends on the notch size.
The value of q is only considered independent of the notch when the notch radius is greater than a specific value, which varies for different materials or treatment states.
02 The influence of the size factor
The lack of homogeneity of the material structure and the presence of internal defects result in an increased probability of failure as the size of the material increases, thus decreasing its fatigue limit.
The size effect phenomenon is a significant problem when extrapolating fatigue data from small laboratory samples to larger practical parts.
It is not possible to replicate the stress concentration and stress gradient of full-size parts in small samples, leading to a disconnect between results obtained in the laboratory and fatigue failure of certain specific parts.
03 EU influence of surface processing state
The machined surface always contains irregular machining marks, which act as small gaps, leading to stress concentration on the surface of the material and reducing its fatigue resistance.
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 greater resistance are more sensitive to surface finish.
04 The impact of loading experience
In reality, no part operates under a strictly constant voltage amplitude.
Overloads and secondary loads can impact the fatigue limit of materials.
Studies show that damage from overload and secondary load training is prevalent in materials.
Overload damage refers to a decrease in the fatigue limit of a material after it has gone through a certain number of cycles under a load greater than its fatigue limit.
The higher the overload level, the faster the damage cycle occurs, as illustrated in the figure below.
Overload Damage Limit
Under certain conditions, a limited number of overload occurrences may not cause damage to the material.
Due to the effects of strain strengthening, crack tip passivation and residual compressive stress, the material is also reinforced, thus improving its fatigue limit.
Therefore, the idea of overload damage must be reviewed 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 threshold 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 greater training stresses.
05 Influence of chemical composition
Fatigue strength and tensile strength have a strong correlation under certain conditions.
Consequently, under specific conditions, any alloying elements that increase tensile strength can also improve the fatigue resistance 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 fatigue resistance.
06 Effect of heat treatment on microstructure
The effect of heat treatment on fatigue strength is largely the effect of microstructure, since different heat treatments result in different microstructures.
Although the same material composition can achieve the same static strength through various heat treatments, its fatigue resistance can vary greatly due to different microstructures.
At a similar strength level, the fatigue strength of flaked perlite is noticeably lower than that of granular perlite.
The smaller the cementite particles, the greater the fatigue resistance.
The impact of microstructure on the fatigue properties of materials is not only related to the mechanical properties of various structures, but also to the grain size and distribution characteristics of structures in the composite structure.
Grain refinement can increase the fatigue resistance of the material.
07 Influence of inclusions
The presence of inclusions or holes created by them can act as small notches, causing concentration of stresses and deformations under alternating loads, and become a source of fatigue fractures, negatively impacting the fatigue performance of the materials.
The impact of inclusions on fatigue strength depends on several 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 applied stress.
Different types of inclusions have unique mechanical and physical properties and their effects on fatigue properties vary. 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 a coefficient of expansion greater than the matrix, such as sulfides, have a lower impact due to the compressive stress in the matrix, while inclusions with a coefficient of expansion lower than the matrix, such as alumina, have a greater impact due to the tensile stress in the matrix. headquarters. Compaction of the inclusion and base metal also affects fatigue resistance.
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 its 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 fatigue resistance.
In other words, inclusions primarily impact the fatigue limit of the material and have little effect on fatigue strength under high stress conditions. To improve the fatigue performance of materials, purification smelting methods such as vacuum casting, vacuum degassing and electroslag remelting can be used to effectively reduce the impurity content in steel.
08 Influence of change in surface properties and residual stress
In addition to the surface finish mentioned above, the influence of surface condition also encompasses changes in surface mechanical properties and the effect of residual stress on fatigue strength.
The change in the mechanical properties of the surface layer may be due to different chemical compositions and microstructures of the surface layer, or the reinforcement of surface deformation.
Surface heat treatments such as carburizing, nitriding and carbonitriding can not only increase the wear resistance of components, but also improve their fatigue resistance, particularly their resistance to corrosion, fatigue and corrosion.
The impact of surface chemical heat treatment on fatigue resistance largely depends on the mode of loading, the concentration of carbon and nitrogen in the layer, the surface hardness and gradient, the ratio of surface hardness to core hardness, the depth layer and 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, in general, the sharper the notch, the greater the improvement in fatigue resistance.
The effect of surface treatment on fatigue properties varies depending on the loading mode.
Under axial loading, there is no uneven distribution of stress throughout the depth of the layer, which means that the stress at the surface and below the layer is the same.
In this scenario, surface treatment can only improve the fatigue performance of the surface layer since the core material is not reinforced, thus limiting the improvement in fatigue resistance.
Under bending and torsion conditions, the stress is concentrated in the surface layer, and the residual stress from the surface treatment and the external stress are superimposed, reducing the actual stress on the surface.
At the same time, the reinforcement of the surface material improves fatigue resistance under bending and torsion conditions.
In contrast, chemical heat treatments such as 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 the heat treatment.
Similarly, the fatigue strength of surface coatings such as Cr and Ni decreases due to the notching effect caused by cracks in the coatings, tensile residual stress caused by coatings on the base metal, and hydrogen embrittlement caused by hydrogen absorption during electroplating process. .
Induction quenching, flame surface quenching and shell quenching of low hardenability steel can result in a certain depth of the surface hardness layer and form a favorable residual compressive stress in the surface layer, making it an effective method for improve the fatigue resistance of components.
Surface rolling and shot peening can also create a certain depth of deformation and hardening layer on the surface of samples and produce residual compressive stress, which is also an effective way to increase fatigue resistance.
How can the fatigue resistance of materials be improved by optimizing surface processing?
Improving the fatigue resistance of materials by optimizing surface processing conditions can be achieved in several ways:
Introducing residual compressive stress: Near the end of component processing, methods such as shot peening are used to introduce a certain magnitude and depth of compressive stress. This can effectively improve surface integrity and increase service life and fatigue resistance. It is widely accepted that residual compressive stress is a critical strengthening mechanism for increasing the fatigue and stress corrosion resistance of engineering materials.
Optimizing surface deformation: The pursuit of nanoscale processing hardening may sacrifice ductility for strength, but it accelerates crack propagation, which is detrimental to fatigue. Therefore, excessive pursuit of surface strain hardening effect should be avoided to avoid adverse impacts on fatigue performance.
Bearing strengthening: As one of the mechanical surface strengthening techniques, the rolling strengthening process can effectively improve the fatigue performance, wear resistance, corrosion resistance and damage tolerance of materials. This technique has been applied to surface modification treatments, such as those for aircraft engine blades.
Surface modification technologies: Through surface modification technologies, the material's surface hardness, wear resistance and corrosion resistance can be improved, while reducing the probability of fatigue damage. Optimizing material integrity can reduce internal defects and residual stresses, thereby improving its fatigue performance.
Influence of heat treatment and microstructure: Different states of heat treatment produce different microstructures, so the effect of heat treatment on fatigue strength is essentially the influence of microstructure. By controlling the heat treatment process, a microstructure that is more conducive to increasing fatigue resistance can be obtained.
What is the specific impact of environmental humidity on the fatigue strength of different materials?
The specific impact of environmental humidity on the fatigue resistance of different materials is mainly reflected in the following aspects:
For high-strength chrome steel, environmental humidity has a significant impact on its durability. Under certain conditions of humidity and heat, the fatigue life of the material is affected, accelerating the propagation of cracks.
The fatigue performance of metallic materials is also affected by the surrounding environment, especially in the case of corrosion fatigue. This refers to the response of metallic materials under the interactive effects of corrosive media and cyclic loading, often used to describe the fatigue behavior of materials in aqueous environments.
Studies on the tensile fatigue performance of carbon fiber composite laminates under different environmental conditions (such as dry state at room temperature, dry state at low temperature and wet state at high temperature) indicate that humid and hot environments are one of the main factors that affect mechanical properties. of these composite materials.
Research into the degradation tendency and fatigue performance mechanism of CFRP (Carbon Fiber Reinforced Polymer) in a humid and hot environment shows that such conditions cause different forms and degrees of damage to the matrix, fibers and fiber-fiber interface. CFRP matrix, leading to degradation of CFRP mechanical properties.
Under an environment of 60°C/95% relative humidity, the fatigue performance of CFRP/aluminum alloy adhesive joints decreases with increasing aging time, and the decline in fatigue resistance is most noticeable in the early stages of aging. aging.
Studies have found that humid environments have a significant impact on both the mechanism and degree of fatigue damage. The higher the relative humidity, the more severe the fatigue damage.
What are the effects of corrosion on the fatigue strength of metals and alloys under different environmental conditions?
The effects of corrosion on the fatigue resistance of metals and alloys under different environmental conditions are mainly reflected in the following aspects:
Impact of pre-corrosion: Pre-corrosion can significantly affect the fatigue SN curve and fatigue crack initiation behavior of aluminum alloys, but has no impact on the crack propagation behavior. Crack initiation life after pre-corrosion represents only less than 20% of the total life, leading to a sharp decline in fatigue life.
Deformation in corrosive environments: Medical metallic materials undergo certain deformation during fatigue processes in conventional air environments. However, this deformation is exacerbated in corrosive environments, thus affecting fatigue performance.
Wear and corrosion in marine environments: Studies on the corrosion and wear properties of metallic materials in marine environments indicate that the wear mechanism gradually transitions from abrasive wear to a mechanism dominated by wear-accelerated corrosion fatigue.
Decrease in fatigue strength in erosive environments: In erosive environmental conditions, the degree of decrease in fatigue strength of metals or alloys depends on the environmental conditions and test conditions. For example, the apparent fatigue strength limit observed in steel in air is no longer apparent in corrosive environments.
Corrosion fatigue characteristics in aggressive atmospheric corrosion environments: There are still many questions to be studied in the field of fatigue failure and corrosion of aluminum alloys under the coupling of aggressive atmospheric corrosion environments and dynamic loading conditions of high-speed rail . This indicates that corrosion has a significant impact on the fatigue resistance of metals and alloys in these specific environments.
Characteristics of the SN corrosion fatigue curve: The SN corrosion fatigue curve has no horizontal part, indicating that the corrosion fatigue limit is the value below a certain life, that is, there is only a conditional corrosion fatigue limit . This suggests that the factors affecting fatigue resistance in a corrosive environment are more complex than in air.
Fatigue performance in specific corrosive environments: Studies on the fatigue performance of aerospace aluminum alloy materials in a 3.5% NaCl corrosion environment indicate that fatigue performance in a corrosive environment has a significant impact on the resistance to corrosion. fatigue of metals and alloys.
What is the mechanism of influence of microstructure (ultrafine grains, nanograins) on the fatigue resistance of metallic materials?
The influence of microstructure (ultrafine grains, nanograins) on the fatigue resistance of metallic materials is mainly reflected in the following aspects:
The relationship between grain size adjustment and fatigue strength:
Research indicates that for materials of a specific composition, when the grain size is adjusted over a wide range to change strength, the fatigue strength of the material will increase with increasing tensile strength and then decrease. This implies that within a certain range, reducing grain size can increase the fatigue resistance of the material, but when the grain size is reduced to a certain extent, excessively refined grains can lead to a decline in fatigue resistance.
The effect of grain boundary volume fraction:
Ultrafine-grain and nanometallic materials have small grains and a large grain boundary volume fraction, which gives them unique and excellent properties. However, these materials produce a large number of defects during the grain refinement process, leading to significant reductions in toughness and plasticity, the disappearance of work hardening capacity, and thus affecting low-cycle fatigue performance.
The impact of plastic deformation on fatigue strength:
By subjecting metallic materials to severe plastic deformation (SPD), microstructures such as ultrafine grains and nanograins can be produced, thus improving the fatigue resistance of the material. However, after being treated with SPD, the fatigue strength of pure metals represented by copper appears to reach a saturation value, indicating that there are certain limitations to further improve the fatigue strength through optimizing the SPD process.
The role of fault energy stacking:
During cyclic deformation, with decreasing stacking fault energy, the microstructural instability caused by grain growth and highly localized shear bands show remarkable improvement. This suggests that the microscopic fatigue damage mechanism of the material will gradually change from grain growth dominated by grain boundary migration to other forms, affecting fatigue performance.
The difference in fatigue performance under stress control and strain control:
Refining the grains of metallic materials into ultrafine grains or nanograins can improve their high-cycle fatigue performance under stress-controlled conditions, but generally reduces their low-cycle fatigue performance under strain-controlled conditions. This is mainly because after grains are refined to the submicron or nanometer level, the microstructure of the material changes, affecting fatigue performance.
