1. It is recognized that fatigue is a phenomenon that when materials (metals) are subjected to cyclic stress or strain, their structural properties decline and eventually lead to failure. Fatigue failure is one of the most common failure forms. According to the data provided in the literature, the fatigue failure parts account for 60 ~ 70% of the failure parts in all kinds of machines. In principle, fatigue fracture failure is low stress brittle failure, and it is difficult to observe obvious plastic deformation in fatigue, because it is mainly local plastic deformation and mainly occurs on the inherent defects of structure. Although frequency has some influence on fatigue failure, in most cases fatigue failure is mainly related to the number of cycles.

According to the stress characteristics of fatigue failure, it can be divided into mechanical fatigue caused by mechanical stress and thermal fatigue caused by thermal stress (alternating thermal stress) It can be divided into tension-compression fatigue, torsion fatigue and bending fatigue according to the load property. It can be divided into corrosion fatigue, low temperature fatigue and high temperature fatigue according to the work environment. The strength before fatigue damage of materials and structures is generally defined as “Fatigue limit”.

01 impact fatigue is the fatigue caused by repeated impact load. When the number of Impact Times N is less than 500 ~ 1000, the fracture form of the part is the same as that of one impact, and when the number of impact times is more than 105, the part fracture belongs to fatigue fracture and has typical fatigue fracture characteristics. In the design calculation, when the number of shocks is more than 100, the strength is calculated by a method similar to fatigue. 02 contact fatigue parts produce local permanent accumulative damage under the action of cyclic contact stress. After a certain number of cycles, the contact surface has the process of pitting, shallow or deep peeling off, which is called contact fatigue. Contact fatigue is a typical failure mode of gears, rolling bearings and CAMSHAFTS. 03 thermal fatigue the fatigue of materials or parts caused by the thermal stress of temperature cycling is called thermal fatigue. When the free expansion or contraction of the material is restrained, the cyclic thermal stress or cyclic thermal strain is produced.

2. There are two main types of thermal stress. The expansion and contraction of the parts are subject to the external restraint of the fixed parts, which results in the thermal stress, the thermal stress is caused by the inconsistency of expansion and contraction of different parts. In addition to the thermal stress, the alternating action of temperature will lead to the change of the internal structure of the material, which will reduce the strength and plasticity. Under the condition of thermal fatigue, the temperature distribution is not uniform, where the temperature gradient is large, the plastic deformation is serious and the thermal strain concentration is large; when the thermal strain exceeds the elastic limit, the thermal stress and the thermal strain are not linear, in this case, the solution of thermal stress should be treated as elastoplastic relation. The thermal fatigue crack propagates from the surface to the interior, and the direction is perpendicular to the surface. The thermal stress is proportional to the Coefficient of thermal expansion, and the greater the coefficient of thermal expansion, the greater the thermal stress. So in the selection of materials to consider the material matching, that is, different materials can not be too different Coefficient of thermal expansion. Under the same thermal strain condition, the greater the elastic modulus of the material, the greater the thermal stress; the greater the temperature cycle change, that is, the greater the temperature difference between the upper and lower limits, the greater the thermal stress; the lower the thermal conductivity of the material, the faster the acceleration or cooling process, the steeper the temperature gradient, the greater the thermal stress.

04 corrosion fatigue the fatigue caused by the combined action of corrosion medium and cyclic stress (strain) is called corrosion fatigue. The corrosion damage caused by the interaction of corrosion medium and static stress is called stress corrosion. The difference between them is that stress corrosion only occurs in a specific corrosion environment, and corrosion fatigue will occur under any corrosion environment and cyclic stress. Stress Corrosion cracking (SCC) has a critical Stress intensity factor KISCC , when Stress intensity factor Ki ≤ KISCC , SCC will not occur, and corrosion fatigue does not have a critical Stress intensity factor, as long as the cyclic stress continues to act in the corrosive environment, fracture will always occur. The difference between corrosion fatigue and fatigue in air is that the surfaces of mechanical parts are discolored except for stainless steel and nitrided steel during corrosion fatigue. The number of corrosion fatigue cracks is more, that is, there are many cracks. There is no horizontal part in the corrosion fatigue S-N curve, so it must be pointed out that the corrosion fatigue limit is a value under a certain life, that is, only the conditional corrosion fatigue limit exists. The factors affecting the corrosion fatigue strength are more and more complicated than that in air. For example, when the fatigue test frequency is less than 1000HZ in air, the frequency has no effect on the fatigue limit, but corrosion fatigue has an effect on the whole range of frequency. Fatigue life when a material or mechanical component fails, the total life usually consists of three parts: 01 crack initiation life, in actual service, crack initiation life of mechanical parts accounts for most of the fatigue life (even reaches 90% of the total life) . 02. In most cases, when the depth of a microcrack reaches that size (about 0.1 mm) , it propagates along the cross-section of the material or component. 03 instability extends to fracture life.

03 The literature study of residual stress indicates that it is significant to study the effect of residual stress on the fatigue strength of metals only under high cycle fatigue, because the residual stress will be relaxed greatly under high strain amplitude of low cycle fatigue, so it doesn’t show much effect under low cycle fatigue. The residual compressive stress in the surface layer is beneficial to the parts subjected to axial load and the fatigue cracks originate from the surface, but the residual stress relaxation caused by the yield of the residual tensile stress in the core region should be paid attention to. The effect of residual stress on the notch fatigue strength is very significant, which is due to the stress concentration and the effect of residual stress on the fatigue crack growth. However, the stress concentration of residual stress is related not only to the notch geometry, but also to the material properties. The fatigue limit σ-1 value of size-effect materials is usually measured with small specimens, and the specimen diameter is usually from 7 mm to 12 mm, but the cross-section of the actual member is often larger than this size. It is pointed out that the fatigue limit decreases with the increase of sample diameter. Among them, the high strength steel than the low strength of the steel fall fast. 05 member surface state the surface of the member is the place where the fatigue crack is easy to occur, and the surface stress of the member subjected to alternating bending or alternating torsion is the biggest. The surface roughness of the component and the tool mark of machining can affect the fatigue strength. The surface damage (knife mark, abrasion mark, etc.) is itself a surface notch, which will produce stress concentration and reduce the fatigue limit, and the higher the material strength, the more obvious the notch sensitivity, the greater the influence of the machined surface quality on the fatigue limit.

06 environmental factors the fatigue properties of metallic materials are also affected by the surrounding liquid or gas phase environment. Corrosion fatigue is the response of metal materials under the interaction of corrosive medium and cyclic loading. It is usually used to describe the fatigue behavior of materials in aqueous phase. Corrosion fatigue, low temperature fatigue, high temperature fatigue, different air pressure environment and humidity environment are all the fatigue phenomena of materials and environment. In the atmospheric environment, the cycle time of the same material destruction is also much less than the vacuum environment. The crack initiation life in vacuum is much longer than that in atmosphere. The fatigue life of the material becomes very sensitive when the working environment pressure of the workpiece is close to the PCR (the air pressure at the inflexion of the life is defined as the critical air pressure) . The fatigue life of materials in atmospheric environment (generally lower than that in vacuum environment) will decrease with the increase of temperature and accelerate the crack growth. The environmental humidity has a great influence on the durability of high strength chromium steel. Water Vapor (especially at room temperature) has an adverse effect on the fracture resistance of most metals and alloys, which depends on the loading conditions such as stress level, load ratio and amplitude. There is a strong interaction between the microstructure and the environment. The gas-phase environment has a significant effect on the fracture morphology and the dislocation slip mechanism. The degree of environmental impact depends on the crack surface morphology, especially in the depth direction.

At low temperatures, the strength of the metal increases while the plasticity decreases. Therefore, the high-cycle fatigue strength of smooth specimens at low temperature is higher than that at room temperature, and the low-cycle fatigue strength is lower than that at room temperature. For the notched specimens, the toughness and plasticity decrease more. The notch and crack are sensitive to low temperature, that is, the critical fatigue crack length will decrease sharply at low temperature. The generalized high temperature fatigue is the fatigue phenomenon which is higher than normal temperature. But usually, because some parts of the working temperature, although higher than room temperature, but not too high. Only when the temperature is higher than 0.5 Tm (TM is the melting point expressed by Thermodynamic temperature) or above the recrystallization temperature, the creep fatigue and mechanical fatigue appear, which is called high temperature fatigue.

07 under different loads, the order of fatigue limit is: rotation bending < plane bending < compression load < torsion load. In corrosive medium, the effect of loading frequency on crack propagation is obvious. Conventional Frequency (0.1 ~ 100HZ) has almost no effect on crack propagation in steel and brass at room temperature and in test environment. In general, if the test loading frequency is lower than 250HZ, the frequency has less effect on the fatigue life of metal materials.

08 materials, such as welds (apertures) , cast steels (porosity) or sub-surfaces (large inclusions change local strain fields) , but rarely initiate internally. Crack initiation also depends on the number, size, properties and distribution of inclusions, and on the loading direction of external forces. In addition, the bonding strength between inclusion and Matrix can not be neglected. Microcracks are the most dangerous defects in materials with one million cycles, and micrographs control the life of materials with one billion cycles. The probability of crack initiation is higher than that of surface in the case of ultra-high cycle fatigue loading, because the probability of defect is much higher than that of surface in the case of micro-scale. There is no stress reduction or work hardening in brittle materials. Once the notch appears, fracture may occur under the condition of smaller nominal stress. Experience shows that the fatigue limit of metal decreases with the existence of notch, and the worse the plasticity is, the greater the influence of notch on fatigue limit is.

09, it is pointed out in the literature that the preparation of fatigue test specimen is the most important link which leads to the discreteness of test data, such as turning, milling and straightening, which are all related to the final preparation quality of specimen. It is precisely because the preparation method and the heat treatment factors will affect the fatigue properties of the materials, especially the heat treatment has a greater influence, therefore, it is difficult to completely repeat the fatigue test results even if the tests are of the same batch and have the same size and appearance. It can be seen that the production and processing factors of the workpiece will cause the actual fatigue life of the components to deviate from the expected life value calculated by the analysis. The property of high cycle fatigue strength (N > 106) is related to the hardness of the material, and the toughness is an important index for low and middle cycle fatigue. Under the condition of high stress, the fatigue property of high strength steel is low because of its poor toughness, but under the condition of low stress, it has better fatigue resistance. Low Strength Steel, in contrast, medium strength steel is in the middle. In general, the higher the elastic modulus is, the lower the crack growth rate is. The effect of grain size on crack propagation only exists in two extreme cases: △ K →△ Kth and △ Kmax →△ KC. The fracture toughness KIC (or KC) is related to the growth rate. It is generally believed that the increase of material toughness will reduce the rate of crack propagation.

5. The discreteness of fatigue test data is caused by the discreteness of fatigue test equipment and test sample itself. According to the analysis and introduction in the literature, when determining the fatigue life of components, the nominal load has 3% error relative to the actual load, will cause the fatigue life to have 60% error, the extreme case may cause the life error of 120% . For fatigue testing machine, 3% error is completely allowed. However, it is also mentioned that in static failure tests, even for cast materials and glass with high strength dispersion, there is not such a serious dispersion as in fatigue life. The discreteness of the fatigue test results is related to the properties of the material, including the intrinsic properties of the material, the preparation process of the test and the external environment of the test. Among them, the test preparation process is the most important step leading to the data discreteness, especially the heat treatment. The inclusion and the second phase particles in the material are the essential reasons for the dispersion of the test data, and the mechanism is still not clear.

6. Development of structural fatigue design method safety life method: The design stress is lower than the fatigue limit and no defects are considered in the structure. Failure safety method: The design stress is related to the residual strength in the case of plane defects, and the design method allows acceptable defects. ● Safe crack method: The existence of deterministic and predictable propagating cracks is permitted. Partial failure method: it can solve some problems in metal fatigue analysis, and is widely used in France. The rise of ultra-high cycle fatigue testing technology in the 1990s fully shows that some micro-defects (such as slag inclusion, porosity, large grain size formed by forging, etc. . In the absence of fatigue test data for steel, the S-N curve can be approximated by the tensile strength limit. It is an accurate method to estimate the fatigue limit in relation to the tensile strength and the elongation at break of specimen. In the fatigue analysis of materials and structures, it is necessary to give priority to the conclusions drawn from experiments rather than to blindly believe in elastoplastic calculations in order to ensure the reliability of the data. 

Source: CAITONG

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