Eight factors affecting fatigue strength in fatigue test of metal materials

Date：2022-07-28 14:30:00Views：753

Because fatigue fracture usually occurs from the weakest part of the machine or the stress concentration caused by external defects, fatigue fracture is very sensitive to many factors, such as cyclic stress characteristics, environmental media, temperature, surface state of the machine, internal structural defects, etc., which lead to the generation of fatigue cracks or rapid crack growth and reduce the fatigue life. In order to improve the fatigue resistance of machine parts and prevent the occurrence of fatigue fracture accidents, reasonable structural shapes should be selected during the design and processing of machine parts to prevent surface damage and stress concentration. There are many factors that affect the fatigue strength of metal materials. Here are some common factors that affect the fatigue strength.

External factors include the shape and size of parts, surface finish and service conditions, while internal factors include the composition of the material itself, organizational state, purity and residual stress. Subtle changes in these factors will cause fluctuations or even large changes in the fatigue properties of materials. 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 processes, so as to ensure the high fatigue performance of parts.

1. Effect of stress concentration

The conventional 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, so that the maximum actual stress at the root of the notch is far 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 and notch specimen fatigue limit σ- The ratio of 1n.

The effective stress concentration factor is not only affected by the size and shape of components, but also by the physical properties of materials, 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 indicates the sensitivity of the material to fatigue notch, which is calculated by the following formula.

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, but 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 this radius value is also different for different materials or treatment states.

2. Influence of size factor

Due to the inhomogeneity of the structure of the material itself and the existence of internal defects, the increase of the size will increase the failure probability of the material, thereby reducing the fatigue limit of the 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 size 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 tiny notches, causing stress concentration on the material surface, thereby reducing the fatigue strength of the material. The test shows that for steel and aluminum alloys, 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 the phenomenon of overload damage and secondary load exercise is common in materials.

The so-called overload damage refers to the reduction of the fatigue limit of materials after a certain cycle of operation under a load higher than the fatigue limit. The higher the overload, the shorter the cycle required to cause damage.

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 a certain cycle of operation 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. Generally speaking, the exercise cycle of the material with good plasticity should be longer and the exercise stress should be higher before it takes effect.

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. Comparatively speaking, 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.

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 structures.

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

The influence 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. Influence of inclusions

The inclusion itself or the holes generated 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, which will have an adverse impact on the fatigue performance of materials. 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 and the level and state of applied stress.

Different types of inclusions have different mechanical and physical properties, different properties from the 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.) have greater harm.

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

The tightness of the inclusion and the base metal will also affect the fatigue strength. Sulfide is easy to deform and closely integrate with the base metal, while oxide is easy to separate from the base metal, causing stress concentration. It can be seen from this that from the type of inclusions, sulfide has less influence, 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 to become the controlling factor of plastic deformation, which strongly affects 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, adopting 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 mechanical properties of the surface layer can be caused by the difference of chemical composition and structure of the surface layer, or by the deformation strengthening of the surface layer.

In addition to increasing the wear resistance of parts, surface heat treatment such as carburizing, nitriding and carbonitriding is also an effective means to improve the fatigue strength of parts, especially to improve the corrosion fatigue and biting corrosion resistance.

The influence of surface chemical heat treatment on fatigue strength mainly depends on 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 will be improved.

The effect of surface treatment on fatigue performance is also different under different loading modes. Under axial loading, since there is no uneven distribution of stress along the depth of the layer, the stresses in the surface layer and under the layer are the same. In this case, surface treatment can only improve the fatigue performance of the surface layer. Because the core material has not been strengthened, the improvement of fatigue strength is limited. Under the conditions of bending and torsion, the distribution of stress is concentrated on the surface. The residual stress formed by surface treatment and this additional stress are superimposed, which 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.

In contrast 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 hardening layer, and form favorable residual compressive stress on the surface, so it 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 specimen and produce residual compressive stress on the surface at the same time.

The above is the content related to the fatigue test of metal materials compiled by the core detection team. I hope it will be helpful to you. Shenzhen Chuangxin Online Testing Technology Co., Ltd. is a well-known professional testing institution for electronic components in China, with 3 standardized laboratories covering an area of more than 1000 square meters. The scope of testing services covers: electronic component testing and verification, IC authenticity identification, product design and material selection, failure analysis, function testing, factory incoming material inspection, component X-ray testing, taping and other test items.