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Eight main influencing factors of fatigue strength of metal materials

The fatigue strength of metal material is extremely sensitive to a variety of extrinsic and intrinsic factors. Extrinsic factors include the shape and size of the part, surface finish and use conditions, etc. Intrinsic factors include the composition of the metal material itself, tissue state, purity and residual stresses. Small changes in these factors can cause fluctuations or even substantial changes in the fatigue performance of the metal material.
The influence of various factors on fatigue strength is an important aspect of fatigue research, this research will provide the basis for the reasonable structural design of the parts, as well as the correct choice of metal materials and the reasonable development of various hot and cold processing processes, in order to ensure that the parts have high fatigue performance.

1. The effect of stress concentration

Conventional fatigue strength, are measured with carefully processed smooth specimens, however, the actual mechanical parts are inevitably notched in different forms, 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 suffered by the part, and the fatigue damage of the part often starts here.
Theoretical stress concentration factor Kt: in ideal elastic conditions, derived from the theory of elasticity, the ratio of the maximum actual stress at the root of the notch to the nominal stress.
Effective stress concentration factor (or fatigue stress concentration factor) Kf: the ratio of the fatigue limit σ-1 of a smooth specimen to the fatigue limit σ-1n of a notched specimen.
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 metal material, processing, heat treatment and many other factors.
The effective stress concentration coefficient increases with the increase of notch sharpness, but is usually smaller than the theoretical stress concentration coefficient.
Fatigue notch sensitivity coefficient q: fatigue notch sensitivity coefficient indicates the sensitivity of the metal material to fatigue notch, calculated by the following formula.

20220417044524 52080 - Eight main influencing factors of fatigue strength of metal materials

The data range of q is 0-1, the smaller the value of q, the less sensitive the metal material is to the notch. Tests show that q is not purely a metal material constant, it is still related to the size of the notch, only when the notch radius is greater than a certain value, q value is basically independent of the notch, and for different metal materials or processing state, this radius value is also different.

2. The influence of dimensional factors

Due to the inhomogeneity of the metal material itself organization and the existence of internal defects, the increase in size causes an increase in the probability of metal material damage, thus reducing the fatigue limit of the metal material. The existence of the size effect is an important problem in applying the fatigue data measured by the laboratory on small specimens to the large size of the actual parts, because it is impossible to reproduce the stress concentration and stress gradient existing on the actual size of the parts exactly similarly on the small specimens, thus causing a disconnect between the laboratory results and the fatigue damage of some specific parts.

3. The influence of the surface processing state

Machined surfaces always have uneven machining marks, which are equivalent to tiny notches and cause stress concentrations on the metal material surface, thus reducing the fatigue strength of the metal material. Tests have shown that for steel and aluminum alloys, rough machining (rough turning) reduces the fatigue limit by 10%-20% or more compared to longitudinal fine polishing. The higher the strength of the metal material, the more sensitive it is to surface finish.

4. Effect of loading experience

In fact, no part is working under absolutely constant stress amplitude conditions, the actual work of the metal material overload and sub-load will have an impact on the fatigue limit of the metal material, the test shows that the metal material is commonly overload damage and sub-load exercise phenomenon.
The so-called overload damage refers to the metal material in higher than the fatigue limit of the load to operate under a certain number of weeks, will cause the metal material fatigue limit of the decline. The higher the overload, the shorter the number of weeks required to cause damage, as shown in Figure 1.
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Figure 1 damage line
In fact, under certain conditions, a small number of overloads will not only not cause damage to the metal material, but will also strengthen the metal material due to deformation strengthening, crack tip passivation and residual compressive stresses, thus increasing the fatigue limit of the metal material. Therefore, some additions and corrections should be made to the concept of overload damage.
The so-called sub-load exercise is a phenomenon that causes the fatigue limit of the metal material to increase after a certain number of weeks of operation at a stress level below the fatigue limit but above a certain limit. The effect of sub-load exercise and the performance of the metal material itself is related to the plasticity of the metal material, generally speaking, the exercise period should be longer, the exercise stress should be higher to be effective.

5. The influence of chemical composition

Fatigue strength of the metal material and tensile strength under certain conditions there is a closer relationship, therefore, under certain conditions where the tensile strength of the alloying elements can improve the fatigue strength of the metal material. Comparatively speaking, carbon is the most important factor affecting the strength of the metal material. And some impurity elements in the formation of inclusions in the steel has a negative impact on the fatigue strength.

6. Heat treatment and the effect of microstructure

Different heat treatment state will get different microstructure, therefore, the impact of heat treatment on fatigue strength, in essence, is the impact of microstructure. The same composition of the metal material, due to different heat treatment, although the same static strength can be obtained, but due to the different organization, fatigue strength can vary in a fairly large range.
At the same strength level, the fatigue strength of lamellar pearlite is obviously lower than that of granular pearlite. The same as granular pearlite, the finer the carburized particles are, the higher the fatigue strength is.
The influence of microstructure on the fatigue performance of the metal material is related to the mechanical property characteristics of various tissues, but also related to the grain size and the distribution characteristics of the tissues in the composite tissue. Refining the grain can improve the fatigue strength of the metal material.

7. The effect of inclusions

The inclusions themselves or the holes produced by it are equivalent to tiny notches, which will produce stress concentration and strain concentration under the action of alternating load and become the crack source of fatigue fracture, causing adverse effects on the fatigue properties of the metal material. The effect of inclusions on fatigue strength depends not only on the type, nature, shape, size, number and distribution of inclusions, but also on the strength level of the metal material and the applied stress level and state.
Different types of inclusions have different mechanical and physical properties, and the difference between the properties of the parent metal material, the impact on fatigue performance is also different. Generally speaking, easy to deform the plastic inclusions (such as sulfides) on the fatigue properties of steel less, while brittle inclusions (such as oxides, silicates, etc.) have a greater risk.
Than the matrix expansion coefficient of inclusions (such as sulfides) due to compressive stress in the matrix and the impact is small, while the matrix expansion coefficient than the small inclusions (such as aluminum oxide, etc.) due to tensile stress in the matrix and the impact is large.
The fatigue strength is also affected by the tightness of the bond between the inclusions and the base metal material. Sulfides are easily deformed and tightly bonded with the base metal material, while oxides are easily separated from the base metal material, causing stress concentration. It can be seen that, from the type of inclusions, sulfides have less influence, while oxides, nitrides and silicates are more harmful.
Under different loading conditions, the impact of inclusions on the fatigue properties of the metal material is also different, in the high load conditions, regardless of the presence of inclusions, the applied load is sufficient to produce plastic rheology of the metal material, the impact of inclusions is small, while in the fatigue limit stress range of the metal material, the presence of inclusions cause local strain concentrations become plastic deformation control factors, thus strongly affecting the fatigue strength of the metal material. In other words, the presence of inclusions mainly affects the fatigue limit of the metal material and has an insignificant effect on the fatigue strength under high stress conditions.
The purity of the metal material is determined by the melting process, therefore, the use of purification smelting methods (such as vacuum melting, vacuum degassing and electroslag remelting, etc.) can effectively reduce the content of impurities in the steel and improve the fatigue properties of the metal material.

8. Surface property changes and the impact of residual stress

In addition to the surface finish mentioned before, the effect of surface condition also includes the change of surface mechanical properties and the effect of residual stress on fatigue strength. The change of surface mechanical properties can be caused by the different chemical composition and organization of the surface layer, or the surface layer can be strengthened by deformation.
Carburizing, nitriding and carbonitriding surface heat treatment in addition to increasing the wear resistance of the parts, but also to improve the fatigue strength of the parts, especially to improve corrosion fatigue and bite an effective means of corrosion resistance.
The effect of surface chemical heat treatment on fatigue strength mainly depends on the loading method, the concentration of carbon and nitrogen in the carburized layer, surface hardness and gradient, the ratio of surface hardness to heart hardness, layer depth and the size and distribution of residual compressive stresses formed by the surface treatment and other factors. Numerous tests have shown that as long as the notch is machined first and then chemically heat treated, the sharper the notch is, the more the fatigue strength is improved.
The effect of surface treatment on fatigue performance also varies under different loading methods. When loading axially, the stresses are the same in the surface layer and the layer below because there is no uneven distribution of stresses along the layer depth. In this case, the surface treatment can only improve the fatigue performance of the surface layer, and the fatigue strength improvement is limited because the core metal material is not strengthened. Under bending and torsion conditions, the distribution of stresses is concentrated in the surface layer, and the residual stresses formed by the surface treatment and this applied stress are superimposed, so that the actual stresses on the surface are reduced, and at the same time, the fatigue strength under bending and torsion conditions can be effectively improved due to the strengthening of the surface layer metal material.
And carburizing, nitriding and carbonitriding and other chemical heat treatment, if the parts in the heat treatment process decarburization, so that the strength of the surface layer is reduced, the fatigue strength of the metal material will be significantly reduced. Similarly, surface plating (such as plating Cr, Ni, etc.) due to the notch effect caused by cracks in the plating, residual tensile stresses caused by the plating in the base metal and the immersion of hydrogen in the plating process leads to hydrogen embrittlement, etc., so that the fatigue strength is reduced.
The use of induction quenching, surface flame quenching and thin shell quenching of low hardenability steel, can obtain a certain depth of surface hardening layer, and the formation of a favorable residual compressive stress in the surface layer, and therefore is also an effective way to improve the fatigue strength of the parts.

Surface tumbling and shot blasting and other treatments, as the surface of the specimen to form a certain depth of deformation hardening layer, while the surface to generate residual compressive stress, and thus is also an effective way to improve fatigue strength.

Source: Network Arrangement – China Metal Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at [email protected]

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