The fatigue strength of materials is extremely sensitive to various external and internal factors. External factors include the shape and size of parts, surface finish and service conditions. Internal factors include the composition, microstructure, purity and residual stress of materials themselves. The slight change of these factors will cause the fluctuation or even significant change of material fatigue properties.

The influence of various factors on fatigue strength is an important aspect of fatigue research. This research will provide basis for reasonable structural design of parts, correct selection of materials and reasonable formulation of various cold and hot machining processes, so as to ensure high fatigue performance of parts.

Eight factors affecting fatigue strength of metal materials

1. Effect of stress concentration

The conventional fatigue strength is measured by carefully processed smooth specimens. However, there are inevitably different forms of notches in actual mechanical parts, such as steps, keyways, threads and oil holes. The existence of these notches causes stress concentration, so that the maximum actual stress at the root of the notch is much greater than the nominal stress borne by the part, and the fatigue failure of the part often starts from here.

Theoretical stress concentration factor KT: the ratio of the maximum actual stress at the notch root to the nominal stress obtained from the elastic theory under ideal elastic conditions.

Effective stress concentration factor (or fatigue stress concentration factor) KF: fatigue limit of smooth specimen σ- 1. Fatigue limit of notched specimens σ- The ratio of 1n.

The effective stress concentration factor is not only affected by the size and shape of the component, but also by the physical properties of the material, processing, heat treatment and other factors.

The effective stress concentration factor increases with the increase of notch sharpness, but it is usually less 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, which is calculated by the following formula:

q=(Kf-1)/(Kt-1)

The data range of Q is 0 ~ 1. The smaller the value of Q, the less sensitive the characterization material is to the notch. The test shows that Q is not purely a material constant, it is still related to the notch size. Only when the notch radius is greater than a certain value, the Q value is basically independent of the notch, and the radius value is also different for different materials or treatment states.

2. Influence of size factor

Due to the non-uniformity of material structure and the existence of internal defects, the increase of size increases the failure probability of material, so as to reduce the fatigue limit of material. The existence of size effect is an important problem in applying the fatigue data measured by small samples in the laboratory to large-scale actual parts. Because it is impossible to reproduce the stress concentration and stress gradient on the actual parts on the small samples, it causes the disconnection between the laboratory results and the fatigue failure of some specific parts.

3. Influence of surface processing state

There are always uneven machining marks on the machined surface, which are equivalent to small notches, causing stress concentration on the material surface, so as to reduce the fatigue strength of the material. The test shows that for steel and aluminum alloy, the fatigue limit of rough machining (rough turning) is reduced by 10% ~ 20% or more compared with longitudinal fine polishing. The higher the strength of the material, the more sensitive it is to the surface finish.

4. Impact of loading experience

In fact, no part works under the condition of absolutely constant stress amplitude. The overload and secondary load in the actual work of materials will have an impact on the fatigue limit of materials. The test shows that there are generally overload damage and secondary load exercise phenomena in materials.

The so-called overload damage refers to the decrease of material fatigue limit after a certain cycle of operation under the load higher than the fatigue limit. The higher the overload, the shorter the cycle required to cause damage, as shown in the figure below.

Overload damage bound

In fact, under certain conditions, a small number of overloads will not damage the material, but also strengthen the material due to deformation strengthening, crack tip passivation and residual compressive stress, so as to improve the fatigue limit of the material. Therefore, the concept of overload damage should be supplemented and modified.

The so-called secondary load exercise refers to the phenomenon that the fatigue limit of materials increases after running for a certain cycle at a stress level lower than the fatigue limit but higher than a certain limit. The effect of secondary load exercise is related to the performance of the material itself. For materials with good plasticity, generally speaking, the exercise cycle should be longer and the exercise stress should be higher.

5. Influence of chemical composition

There is a close relationship between the fatigue strength and tensile strength of materials under certain conditions. Therefore, under certain conditions, any alloy element that can improve the tensile strength can improve the fatigue strength of materials. In comparison, carbon is the main factor affecting the strength of materials. However, some impurity elements forming inclusions in steel have an adverse effect on fatigue strength.

6. Effect of heat treatment and microstructure

Different heat treatment states will get different microstructures. Therefore, the effect of heat treatment on fatigue strength is essentially the effect of microstructure. Although the same static strength can be obtained for materials with the same composition due to different heat treatment, the fatigue strength can vary in a considerable range due to different microstructure.

At the same strength level, the fatigue strength of flake pearlite is obviously lower than that of granular pearlite. For the same granular pearlite, the finer the cementite particles, the higher the fatigue strength.

The effect of microstructure on the fatigue properties of materials is not only related to the mechanical properties of various structures, but also related to the grain size and the distribution characteristics of structures in the composite structure. Grain refinement can improve the fatigue strength of materials.

7. Effect of inclusions

The inclusion itself or the hole generated by it is equivalent to a small notch. Under the action of alternating load, it will produce stress concentration and strain concentration, become the crack source of fatigue fracture, and have an adverse impact on the fatigue performance of the material. The influence of inclusions on fatigue strength depends not only on the type, nature, shape, size, quantity and distribution of inclusions, but also on the strength level of materials, the level and state of applied stress.

Different types of inclusions have different mechanical and physical properties, different properties from base metal, and different effects on fatigue properties. Generally speaking, deformable plastic inclusions (such as sulfide) have little effect on the fatigue properties of steel, while brittle inclusions (such as oxide, silicate, etc.) do great harm.

Inclusions with larger expansion coefficient than the substrate (such as sulfide) have less influence due to compressive stress in the matrix, while inclusions with smaller expansion coefficient than the substrate (such as alumina) have greater influence due to tensile stress in the matrix.

The compactness of inclusion and base metal will also affect the fatigue strength. Sulfide is easy to deform and closely combined with the base metal, while oxide is easy to separate from the base metal, resulting in stress concentration. Therefore, from the type of inclusions, sulfide has little effect, while oxides, nitrides and silicates are more harmful.

Under different loading conditions, the influence of inclusions on the fatigue properties of materials is also different. Under high load conditions, whether there are inclusions or not, the external loading is enough to make the material produce plastic rheology, and the influence of inclusions is small. In the fatigue limit stress range of materials, the existence of inclusions causes local strain concentration, which becomes the controlling factor of plastic deformation, Thus, the fatigue strength of the material is strongly affected. In other words, the existence of inclusions mainly affects the fatigue limit of materials, and has no obvious effect on the fatigue strength under high stress conditions.

The purity of materials is determined by the smelting process. Therefore, the use of purification smelting methods (such as vacuum smelting, vacuum degassing and electroslag remelting) can effectively reduce the impurity content in steel and improve the fatigue performance of materials.

8. Change of surface properties and influence of residual stress

In addition to the surface finish mentioned above, the influence of surface state also includes the change of surface mechanical properties and the influence of residual stress on fatigue strength. The change of surface mechanical properties can be caused by the difference of surface chemical composition and structure, or by deformation strengthening.

Surface heat treatment such as carburizing, nitriding and carbonitriding can not only increase the wear resistance of parts, but also improve the fatigue strength of parts, especially the corrosion fatigue and bite corrosion resistance.

The effect of surface chemical heat treatment on fatigue strength mainly depends on the loading mode, carbon and nitrogen concentration in the carburized layer, surface hardness and gradient, the ratio of surface hardness to core hardness, layer depth, and the size and distribution of residual compressive stress formed by surface treatment. A large number of tests show that as long as the notch is processed first and then treated by chemical heat treatment, generally speaking, the sharper the notch is, the more the fatigue strength is improved.

Under different loading modes, the effect of surface treatment on fatigue properties is also different. Under axial loading, the stresses in the surface layer and under the layer are the same because there is no uneven distribution of stress along the layer depth. In this case, surface treatment can only improve the fatigue performance of the surface layer. Because the core material is not strengthened, the improvement of fatigue strength is limited. Under the conditions of bending and torsion, the stress distribution is concentrated on the surface. The superposition of the residual stress formed by surface treatment and this external stress reduces the actual stress on the surface. At the same time, due to the strengthening of the surface material, it can effectively improve the fatigue strength under the conditions of bending and torsion.

Contrary to chemical heat treatment such as carburizing, nitriding and carbonitriding, if parts decarburize during heat treatment and reduce the strength of the surface layer, the fatigue strength of the material will be greatly reduced. Similarly, the fatigue strength of surface coatings (such as Cr, Ni, etc.) is reduced due to the notch effect caused by cracks in the coating, the residual tensile stress caused by the coating in the base metal, and the hydrogen embrittlement caused by the immersion of hydrogen in the electroplating process.

Induction quenching, surface flame quenching and thin shell quenching of low hardenability steel can obtain a certain depth of surface hardness layer and form favorable residual compressive stress on the surface, which is also an effective method to improve the fatigue strength of parts.

Surface rolling and shot peening are also effective ways to improve fatigue strength because they can form a certain depth of deformation hardening layer on the sample surface and produce residual compressive stress on the surface.