Cognitive fatigue

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01 Understanding fatigue: Fatigue is a phenomenon in which the structural properties of a material (metal) decline under cyclic stress or strain, ultimately leading to failure. Fatigue failure is one of the most common forms of failure. According to data from literature, among various machines, parts that fail due to fatigue account for 60–70% of all failed parts. Fatigue fracture failure essentially belongs to the category of low-stress brittle fracture failure; significant plastic deformation is difficult to observe in fatigue, as it is primarily characterized by local plastic deformation that occurs mainly at the inherent defects in the structure. Although frequency has a certain impact on fatigue failure, in most cases it is primarily related to the number of cycles. ● Based on the characteristics of the stresses that cause fatigue failure, it can be divided into mechanical fatigue caused by mechanical stresses, and thermal fatigue caused by thermal stresses (alternating thermal stresses) ； ● Based on the number of cycles, it can be divided into: high-cycle, low-cycle, and ultra-high-cycle fatigue ； ● Based on the nature of the load, it can be classified into tensile-compressive fatigue, torsional fatigue, and bending fatigue, etc ； ● Based on the working environment of the workpiece, it can be classified into corrosion fatigue, low-temperature fatigue, and high-temperature fatigue. The strength of a material and structure before it suffers fatigue damage is generally defined as the \"fatigue limit\". 01 Impact fatigue refers to the fatigue caused by repeated impact loads. When the number of impacts N is less than 500–1000 and failure occurs, the fracture mode of the part is the same as that under a single impact ； When the number of impacts exceeds 105, the failure of the part is due to fatigue fracture, exhibiting typical characteristics of a fatigue fracture surface. In design calculations, when the number of impacts is greater than 100, the strength is calculated using a method similar to fatigue analysis. 02 Contact fatigue: The process in which a part suffers local, permanent cumulative damage under cyclic contact stress; after a certain number of cycles, pitting and superficial or deep delamination occur on the contact surface, and this is known as contact fatigue. Contact fatigue is a typical mode of failure for gears, rolling bearings, and camshafts. 03 Thermal fatigue: The fatigue of materials or components caused by cyclic thermal stresses resulting from temperature cycles is known as thermal fatigue. Temperature cycling causes cyclic changes in the volume of the material; when the free expansion or contraction of the material is restricted, cyclic thermal stresses or cyclic thermal strains are generated. There are mainly two situations that give rise to thermal stress: ▪ Thermal expansion and contraction of the component are constrained by external forces holding the component in place, resulting in thermal stress ； ▪Although there are no external constraints, the temperatures of different parts of the two pieces are not uniform, resulting in a temperature gradient; this leads to uneven expansion and contraction of the various parts, thereby generating thermal stress. In addition to generating thermal stress, temperature cycling also causes changes in the internal structure of the material, resulting in a decrease in its strength and ductility. The temperature distribution under thermal fatigue conditions is not uniform; in areas with large temperature gradients, plastic deformation is severe and thermal strain concentrations are high ； When the thermal strain exceeds the elastic limit, there is no linear relationship between thermal stress and thermal strain; in such cases, thermal stress must be calculated using elastoplastic relationships. Thermal fatigue cracks start from the surface and extend inward, in a direction perpendicular to the surface. The magnitude of thermal stress is proportional to the coefficient of thermal expansion; the larger the coefficient of thermal expansion, the greater the thermal stress. Therefore, when selecting materials, it is necessary to consider their compatibility, that is, the difference in thermal expansion coefficients of different materials should not be too large. Under the same thermal strain conditions, the greater the elastic modulus of the material, the greater the thermal stress ； The greater the temperature cycling, that is, the larger the difference between the upper and lower limits, the greater the thermal stress ； The lower the thermal conductivity of a material, the steeper the temperature gradient during rapid acceleration or cooling, and the greater the thermal stress. 04 Corrosion fatigue: Fatigue resulting from the combined effect of a corrosive environment and cyclic stress (strain) is known as corrosion fatigue. Corrosion damage caused by the combined effect of a corrosive medium and static stress is known as stress corrosion. The difference between the two is that stress corrosion occurs only in specific corrosive environments, whereas corrosion fatigue can lead to fracture under any corrosive environment as well as under the combined effect of cyclic stresses. Stress corrosion cracking has a critical stress intensity factor, KISCC; when the stress intensity factor KI ≤ KISCC, stress corrosion cracking does not occur ； Corrosion fatigue, on the other hand, does not have a critical stress intensity factor; as long as cyclic stress is present in a corrosive environment, fracture will always occur. The difference between corrosion fatigue and fatigue in air is that, during corrosion fatigue, the surface of mechanical components changes color, except for stainless steel and nitrided steel. Corrosion fatigue results in a large number of cracks, that is, a multi-crack pattern. The S-N curve for corrosion fatigue does not have a horizontal portion; therefore, when referring to the corrosion fatigue limit, it is necessary to specify the value at a certain life, meaning that only a conditional corrosion fatigue limit exists. The factors affecting corrosion fatigue strength are more numerous and complex than those for fatigue in air. For example, in air, when the fatigue testing frequency is below 1000 Hz, frequency has basically no effect on the fatigue limit, but corrosion fatigue is affected by frequency across the entire range. 02 Fatigue life: When a material or mechanical component fails, its total life is typically composed of three parts. 01 Crack initiation life: Numerous engineering practices have shown that, during actual operation, the crack initiation life of mechanical components accounts for the vast majority of their fatigue life (often up to 90% of the total life). 02 Crack stable growth life: In most cases, when the depth of a microcrack reaches this size (about 0.1 mm), it begins to grow steadily along the cross-section of the material or component. 03 Instability propagates to fracture life. 03 Forms of fatigue in metallic materials. There are mainly the following types of fatigue in metallic materials: general plastic deformation ； Plastic deformation under low-cycle fatigue ； Plastic deformation under high-cycle fatigue ； Microscopic plastic deformation of grain dimensions under ultra-high cycle fatigue. 04 Factors Affecting the Fatigue Strength of Materials and Structures 01 Mean Stress As the mean stress (statistical stress) increases, the material’s dynamic fatigue resistance decreases. For forces of the same property, the greater the average stress σm, the smaller the stress amplitude σa for a given service life. 02 Stress concentration: Due to working conditions or processing requirements, parts often have steps, small holes, keyways, etc., which cause sudden changes in the cross-section and thus lead to local stress concentration. This significantly reduces the material’s fatigue limit; however, experiments show that the degree of reduction in the fatigue limit is not proportional to the stress concentration factor. However, to accurately predict the fatigue behavior of mechanical components, it is necessary to estimate the crack initiation life in high-stress areas or those containing manufacturing defects. 03 Residual Stress: Literature research indicates that studying the effect of residual stress on the fatigue strength of metals is meaningful only under high-cycle fatigue conditions. Under low-cycle fatigue, with its high strain amplitudes, residual stress relaxes significantly; therefore, it has little effect under such conditions. Surface residual compressive stress is beneficial for components subjected to axial loads, where fatigue cracks originate from the surface; however, attention must be paid to the issue of residual stress relaxation caused by yielding in the core region due to the superposition of residual tensile stress and external loads. Residual stress has a significant effect on the notch fatigue strength of parts, due to the stress concentration that also occurs in residual stress, as well as its greater influence on the propagation of fatigue cracks. However, the stress concentration of residual stress is related not only to the geometric factors of the notch but also to the material properties. 04 Size effect: The fatigue limit σ-1 value of materials is usually determined using small specimens, with a diameter of generally 7–12 mm, whereas the cross-sections of actual components are often larger than this size. Tests indicate that as the diameter of the specimen increases, the fatigue limit decreases. Among them, steel with high strength decreases more rapidly than steel with low strength. 05 Surface condition of the component: The surface of a component is a place where fatigue cracks tend to form, and in components subjected to alternating bending or alternating torsional loads, the surface stress is at its highest. The surface roughness of the component and the tool marks from machining can both affect fatigue strength. Surface defects such as knife marks and abrasion marks are themselves surface notches that cause stress concentration, thereby reducing the fatigue limit. The higher the strength of the material, the more sensitive it is to such notches, and the greater the impact of the surface finish quality on the fatigue limit. 06 Environmental factors: The fatigue properties of metal materials are also affected by the surrounding environment, such as liquid or gas phases. Corrosion fatigue refers to the response of metal materials under the combined action of a corrosive environment and cyclic loading; it is commonly used to describe the fatigue behavior of materials in aqueous environments. Corrosion fatigue, low-temperature fatigue, high-temperature fatigue – different pressure and humidity environments are all examples of fatigue phenomena resulting from the interaction between materials and environmental factors. In an atmospheric environment, the number of failure cycles for the same material is also much lower than in a vacuum environment. The crack initiation life in a vacuum environment is much longer than that in an atmospheric environment. When the pressure in the working environment of the workpiece approaches Pcr (the pressure at the life inflection point is defined as the critical pressure), the fatigue life of the material becomes extremely sensitive. The fatigue life of materials in an atmospheric environment (generally shorter than in a vacuum environment) decreases with increasing temperature, accelerating crack propagation. Environmental humidity has a significant impact on the durability of high-strength chromium steel. Water vapor (especially in room-temperature environments) has an adverse effect on the fracture resistance of most metals and alloys; this adverse effect depends on 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 significantly influences the fracture morphology and dislocation sliding mechanisms. There is also an interaction between the environment and crack closure, especially in the near-threshold region. The degree of environmental influence depends on the morphology of the crack surface, especially in the depth direction. At low temperatures, the strength of metals increases while their ductility decreases. Therefore, the high-cycle fatigue strength of smooth specimens at low temperatures is higher than that at room temperature, whereas their low-cycle fatigue strength is lower than that at room temperature. For specimens with notches, toughness and plasticity decrease even more. Notches and cracks are more sensitive to low temperatures; that is, the critical fatigue crack length at the time of fracture decreases sharply at low temperatures. Broadly defined, high-temperature fatigue refers to the phenomenon of fatigue at temperatures above room temperature. But usually, since the operating temperature of some components is higher than room temperature, yet not excessively so. Only when the temperature is above 0.5Tm (where Tm is the melting point expressed in thermodynamic temperature) or above the recrystallization temperature does fatigue resulting from a combination of creep and mechanical fatigue occur, and this is then referred to as high-temperature fatigue. 07 Load types: The order of the fatigue limits under different loads is: rotating bending < planar bending < compressive load < torsional load. In corrosive media, the effect of loading frequency on crack propagation is quite evident. At room temperature and under test conditions, conventional frequencies (0.1–100 Hz) have almost no effect on crack propagation in steel and brass. In general, in tests, if the test loading frequency is below 250 Hz, the effect of frequency on the fatigue life of metallic materials is relatively minor. 08 Material defects: Cracks generally originate on the surface, such as in welds (porosities), cast steel (porosity), or on the subsurface (large inclusions that alter the local strain field), and rarely originate inside. Crack initiation also depends on the quantity, size, nature, and distribution of inclusions, as well as the direction of the applied external force. Furthermore, the bonding strength between the inclusions and the matrix cannot be ignored either. Microcracks are the most dangerous defects in materials with a lifespan of millions of cycles, while microcurves determine the lifespan of materials with a lifespan of billion cycles. Since the probability of defects existing within a material at the microscale is much higher than on its surface, the likelihood of cracks forming internally under ultra-high cycle fatigue loading is naturally greater than that on the surface. Fragile materials do not exhibit stress relaxation or work hardening; once a notch is present, they can fracture under relatively low nominal stress levels. Experience shows that when a notch is present, the fatigue limit of metal decreases, and the worse the plasticity, the greater the impact of the notch on the fatigue limit. 09 Processing methods: Literature indicates that the preparation of fatigue test specimens is the most important factor contributing to the variability in the test data; mechanical processing methods such as turning, milling, and straightening are all related to the final quality of the specimens. It is precisely because the preparation method and heat treatment factors affect the fatigue properties of materials, with heat treatment having a particularly significant impact, that it is difficult to fully replicate previous fatigue test results even for tests using the same batch and specimens with identical dimensions and morphology. It can be seen that the manufacturing factors of the workpiece cause the actual fatigue life of the components to deviate from the expected life value calculated through analysis. 10 Material properties: The high-cycle fatigue strength (when N > 106) is related to the hardness of the material, whereas for low- and medium-cycle fatigue, toughness is an important parameter. Under high stress conditions, high-strength steel has poor fatigue performance due to its low toughness, whereas it exhibits better fatigue resistance under low stress conditions. Low-strength steel is the opposite, while medium-strength steel lies in between. Generally speaking, the higher the elastic modulus, the lower the crack propagation rate. The effect of grain size on crack propagation is only present in the two extreme propagation scenarios: △K→△Kth and △Kmax→△KC, and it has no significant impact on the characteristics of moderate-speed crack propagation. Fracture toughness KIC (or KC) is related to the strain rate. It is generally believed that an increase in material toughness reduces the crack propagation rate. 05 Discrepancy in fatigue test data: The testing equipment and the specimens themselves are the fundamental causes of the discrepancy in fatigue test data (or results). According to analyses in the literature, when determining the fatigue life of a zero-component, an error of 3% in the nominal load relative to the actual load can result in a 60% error in the fatigue life; in extreme cases, this error can reach 120%. For fatigue testing machines, a 3% error is completely acceptable. However, the text also mentions that in static failure tests, even for casting materials and glass, which have a large variation in strength, there is no such severe variation as in fatigue life. The discreteness of fatigue test results is related to the material properties, specifically: the inherent characteristics within the material ； The preparation process of the experiment, the external environment of the experiment. Among them, the experimental preparation process is the most important factor contributing to data dispersion, especially heat treatment. Inclusions and second-phase particles in the material are the fundamental reasons for the discreteness of the test data, and their mechanism of action remains not fully understood at present. 06 Development of structural fatigue design methods ● Safety life method: The design stress is kept below the fatigue limit, assuming that there are no defects in the structure. ● Fail-safe principle: The design stress is related to the residual strength under planar defects; this design approach allows for the presence of acceptable defects. ● Safety crack method: Allows for the existence of deterministically predictable propagating cracks. ● Local failure method: It can address some issues in metal fatigue analysis and is currently widely used in France. The emergence of ultra-high cycle fatigue testing techniques in the 1990s clearly demonstrates that certain microdefects (such as inclusions, pores, and large grains formed during forging) also have a significant impact on a material’s fatigue life. For steel materials, in the absence of fatigue test data for that material, an approximate S-N curve can be derived from the material’s ultimate tensile strength. Linking the fatigue limit to tensile strength and the elongation at fracture of the specimen is a highly accurate estimation method. In the fatigue analysis of materials and structures, it is essential to draw conclusions from experiments first rather than blindly relying on elastoplastic calculations; only in this way can the reliability of the data be ensured.