Eight factors affecting the fatigue strength of metal materials
The fatigue strength of the material is extremely sensitive to various external and internal factors. The external factors include the shape and size of the part, the surface finish and the conditions of use, etc. The internal factors include the composition of the material itself, the state of the organization, the purity and the residual stress Wait. The subtle changes of these factors will cause fluctuations or even substantial changes in the fatigue properties of the 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 parts, the correct selection of materials and the rational formulation of various cold and hot processing techniques to ensure that the parts have high fatigue performance. .
- The influence of stress concentration. Conventional fatigue strength is measured with carefully processed smooth specimens. However, actual mechanical parts inevitably have gaps of different forms, such as steps, keyways, threads and oil. Kong etc. The existence of these gaps causes stress concentration, so that the maximum actual stress at the root of the gap is much greater than the nominal stress borne by the part, and the fatigue failure of the part often starts here. Theoretical stress concentration factor Kt: Under ideal elastic conditions, the ratio of the maximum actual stress at the root of the notch to the nominal stress obtained by the theory of elasticity. 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 material, processing, heat treatment and other factors. The effective stress concentration factor increases with the sharpness of the notch, but it is usually less than the theoretical stress concentration factor. Fatigue notch sensitivity coefficient q: The fatigue notch sensitivity coefficient indicates the sensitivity of the material to fatigue notches, calculated by the following formula: the data range of q is 0~1, the smaller the value of q, the less sensitive the material is to the notch. Experiments show that q is not purely a material constant, it is still related to the gap size, only when the gap radius is greater than a certain value, the q value is basically independent of the gap, and for different materials or processing conditions, the radius value is also different.
- The influence of size factors. Due to the inhomogeneity of the material itself and the existence of internal defects, the increase in size will increase the probability of material damage, thereby reducing the fatigue limit of the material. The existence of the size effect is an important problem in applying the fatigue data measured by the small sample in the laboratory to the actual large-size parts. Because it is impossible to completely resemble the stress concentration and stress gradient existing on the actual-size parts. Reproduced on a small sample, causing a disconnect between the laboratory results and the fatigue failure of certain specific parts.
- The influence of the surface processing state. There are always uneven processing marks on the machined surface. These marks are equivalent to tiny gaps, which cause stress concentration on the surface of the material, thereby reducing the fatigue strength of the material. Tests have shown that for steel and aluminum alloys, rough machining (rough turning) can reduce the fatigue limit by 10% to 20% or even more than that of longitudinal fine polishing. The higher the strength of the material, the more sensitive it is to surface finish.
- The effect of loading experience. In fact, no part works under absolutely constant stress amplitude. 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 material is generally exposed to overload damage and Second load exercise phenomenon. The so-called overload damage refers to the decrease of the fatigue limit of the material after the material runs for a certain number of cycles under a load higher than the fatigue limit. The higher the overload, the shorter the cycles required to cause damage, as shown in the figure below. In fact, under certain conditions, a small number of overloads will not only cause no damage to the material, but will also strengthen the material due to the effects of deformation strengthening, crack tip passivation and residual compressive stress, thereby increasing the fatigue limit of the material. Therefore, some supplements and amendments should be made to the concept of overload damage. The so-called sub-load exercise refers to the phenomenon that the material fatigue limit rises after the material runs for a certain number of times under the stress level below the fatigue limit but higher than a certain limit. The effect of sub-load exercise is related to the performance of the material itself. Generally speaking, for materials with good plasticity, the exercise period is longer and the exercise stress is higher to be effective.
- The 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, all alloy elements that can increase the tensile strength can increase the fatigue strength of the material. In comparison, carbon is the most important factor affecting material strength. Some impurity elements that form inclusions in steel have an adverse effect on fatigue strength.
- The influence of heat treatment and microstructure Different heat treatment states will result in different microstructures. Therefore, the effect of heat treatment on fatigue strength is essentially the effect of microstructure. The material of the same composition can obtain the same static strength due to different heat treatment, but due to the difference of the structure, the fatigue strength can vary within a considerable range. At the same strength level, the fatigue strength of flaky pearlite is significantly lower than that of granular pearlite. The same is granular pearlite, the smaller the cementite particles, the higher the fatigue strength. The influence of the microstructure on the fatigue properties of materials is not only related to the mechanical properties of various organizations themselves, but also related to the grain size and the distribution characteristics of the structure in the composite structure. Refining the grains can improve the fatigue strength of the material.
- The influence of inclusions The inclusion itself or the holes created by it are equivalent to tiny gaps, which will cause stress concentration and strain concentration under the action of alternating loads, which will become the source of fatigue fracture and have a negative impact on the fatigue performance of the material. . The influence of inclusions on fatigue strength not only depends on the type, nature, shape, size, number and distribution of inclusions, but also depends on the strength level of the material, the level and state of applied stress and other factors. Different types of inclusions have different mechanical and physical properties, and the difference between the properties of the base metal and the impact on the fatigue properties are also different. Generally speaking, easily deformable plastic inclusions (such as sulfides) have little effect on the fatigue performance of steel, while brittle inclusions (such as oxides, silicates, etc.) have greater harm. Inclusions with a larger expansion coefficient than the matrix (such as sulfides) have a small effect due to compressive stress in the matrix, while inclusions with a smaller expansion coefficient (such as alumina, etc.) have a greater impact due to tensile stress in the matrix. The tightness of the combination of inclusions and base metal will also affect the fatigue strength. Sulfides are easy to deform and bond closely with the base material, while oxides are easy to separate from the base 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, inclusions have different effects on the fatigue performance of materials. Under high load conditions, regardless of the presence of inclusions, the external load is sufficient to cause the material to produce plastic rheology, and the influence of inclusions is small. The fatigue limit stress range of the material, the presence of inclusions causes the local strain concentration to become the controlling factor of plastic deformation, which strongly affects the fatigue strength of the material. In other words, the presence of inclusions mainly affects the fatigue limit of the material, and has little effect on the fatigue strength under high stress conditions. The purity of the material is determined by the smelting process. Therefore, the use of purification smelting methods (such as vacuum smelting, vacuum degassing and electroslag remelting, etc.) can effectively reduce the impurity content in the steel and improve the fatigue performance of the material.
- The effect of surface performance changes and residual stress. In addition to the surface finish mentioned above, the surface condition also includes changes in surface mechanical properties and the effect of residual stress on fatigue strength. The change in the mechanical properties of the surface layer can be caused by the chemical composition and organization of the surface layer, or it can be caused by the deformation strengthening of the surface layer. Surface heat treatments such as carburizing, nitriding and carbonitriding can increase the wear resistance of the parts and are also an effective means to improve the fatigue strength of the parts, especially to improve the corrosion fatigue and seizure resistance.
The effect of surface chemical heat treatment on fatigue strength mainly depends on the loading method, the carbon and nitrogen concentration in the infiltration layer, the surface hardness and gradient, the ratio of surface hardness to core hardness, the depth of the layer, and the magnitude and magnitude of the residual compressive stress formed by the surface treatment. Factors such as distribution. A large number of tests have shown that as long as the notch is processed first and then subjected to chemical heat treatment, generally speaking, the sharper the notch, the more the fatigue strength will increase. Under different loading methods, the effect of surface treatment on fatigue performance is also different. During axial loading, since there is no uneven distribution of stress along the depth of the layer, the stresses on the surface 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 has not been strengthened, the improvement of the fatigue strength is limited. Under bending and torsion conditions, the stress distribution is concentrated on the surface. The residual stress formed by the surface treatment and this applied stress are superimposed to reduce the actual stress on the surface. At the same time, due to the strengthening of the surface material, it can effectively improve the bending and Fatigue strength under torsion conditions. Contrary to chemical heat treatments such as carburizing, nitriding and carbonitriding, if the parts are decarburized during the heat treatment, the strength of the surface layer is reduced, and the fatigue strength of the material will be greatly reduced. Similarly, the surface coating (such as Cr, Ni, etc.) due to the notch effect caused by the cracks in the coating, the residual tensile stress caused by the coating in the base metal, and the hydrogen embrittlement caused by the infiltration of hydrogen during the electroplating process, reduce the fatigue strength. . Induction hardening, surface flame hardening and thin shell hardening of low hardenability steel can obtain a certain depth of surface hardness 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 treatments can form a certain depth of deformation hardening layer on the surface of the sample, and at the same time generate residual compressive stress on the surface, which is also an effective way to improve the fatigue strength.
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