The fatigue strength of materials is extremely sensitive to various external and internal factors. External factors include the shape and size of the part, surface finish and service conditions, etc. Internal factors include the composition of the material itself, organization state, purity and residual stress, etc. The subtle changes of these factors will cause the fluctuation of the fatigue performance of the material and even a large change.

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

Effect of stress concentration

Generally speaking, the fatigue strength is measured by carefully processed smooth samples. However, actual mechanical parts inevitably have different forms of notches, such as steps, keyways, threads and oil holes. The existence of these notches causes stress concentration, which makes the maximum actual stress at the root of the notch far greater than the nominal stress borne by the part, and the fatigue failure of the part often starts here.

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

Effective stress concentration factor (or fatigue stress concentration factor) Kf: fatigue limit of smooth specimen σ- 1 and notch specimen fatigue limit σ- 1n ratio.

The effective stress concentration factor is affected not only 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 is usually less than the theoretical stress concentration factor.

Influence of size factor

Due to the inhomogeneity of the structure of the material itself and the existence of internal defects, the increase in size will increase the probability of material failure, thus reducing the fatigue limit of the material. The existence of size effect is an important problem in applying the fatigue data measured by the small sample in the laboratory to the large size actual parts. Because it is impossible to reproduce the stress concentration, stress gradient, etc. existing on the actual size parts on the small sample in a completely similar way, resulting in the disconnection between the laboratory results and the fatigue failure of some specific parts.

Effect of surface processing state

There are always uneven machining marks on the machined surface, which are equivalent to tiny notches, causing stress concentration on the material surface, thus reducing 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 than that of longitudinal fine polishing. The higher the strength of the material, the more sensitive it is to the surface finish.

Impact of loading experience

In fact, no part works under the condition of absolutely constant stress amplitude, and the overload and secondary load in the actual work of the material will affect the fatigue limit of the material. The test shows that the phenomenon of overload damage and secondary load exercise is common in the material.

The so-called overload damage refers to the reduction of the fatigue limit of materials after the materials are operated for a certain number of cycles under the load higher than the fatigue limit. The higher the overload, the shorter the number of weeks required to cause damage.

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

The so-called secondary load exercise refers to the phenomenon that the fatigue limit of the material increases after the material is operated for a certain number of cycles under the stress level lower than the fatigue limit but higher than a certain limit value. The effect of secondary load exercise is related to the performance of the material itself. Generally speaking, the exercise cycle of the material with good plasticity should be longer and the exercise stress should be higher before it can take effect.

Effect of chemical composition

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

Effect of heat treatment and microstructure

Different heat treatment conditions will lead to 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 treatments, the fatigue strength can vary in a considerable range due to different microstructures.

At the same strength level, the fatigue strength of lamellar pearlite is significantly lower than that of granular pearlite. 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 microstructures, but also related to the grain size and the distribution characteristics of microstructures in the composite structure. Grain refinement can improve the fatigue strength of materials.

Influence of inclusions

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

The mechanical and physical properties of different types of inclusions are different, and the difference between the properties of the base metal is different, and the impact on the fatigue properties is also different. Generally speaking, plastic inclusions that are easy to deform (such as sulfides) have little impact on the fatigue properties of steel, while brittle inclusions (such as oxides, silicates, etc.) have great harm.

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

The tightness of inclusion and base metal will also affect the fatigue strength. Sulfide is easy to deform and is closely combined with the base metal, while oxide is easy to separate from the base metal, resulting in stress concentration. It can be seen from this that, in terms of the type of inclusion, the influence of sulfide is small, while oxides, nitrides and silicates are more harmful.

Under different loading conditions, the impact of inclusions on the fatigue properties of materials is also different. Under high load conditions, whether there are inclusions or not, the external load is sufficient to make the material produce plastic rheology, and the impact of inclusions is small. In the fatigue limit stress range of materials, the existence of inclusions causes local strain concentration to become the control factor of plastic deformation, thus strongly affecting the fatigue strength of materials. 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, purification smelting methods (such as vacuum smelting, vacuum degassing and electroslag remelting) can effectively reduce the impurity content in steel and improve the fatigue properties of materials.

Effect of surface property change and residual stress

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

Surface heat treatment such as carburizing, nitriding and carbonitriding can not only increase the wear resistance of the parts, but also improve the fatigue strength of the parts, especially improve the corrosion fatigue and 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 machined first and then subjected to chemical heat treatment, generally speaking, the sharper the notch is, the more the fatigue strength is improved.

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

Contrary to chemical heat treatment such as carburizing, nitriding and carbonitriding, if the part is decarburized during the heat treatment process and the strength of the surface layer is reduced, the fatigue strength of the material will be greatly reduced. Similarly, the fatigue strength of surface coatings (such as Cr and Ni plating) 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 hardening 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 surface of the sample and produce residual compressive stress on the surface.

Information source: Caiyitong

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