Mechanisms of fatigue failure in metallic materials

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I. Glossary of Terms 1. Alternating stress: When the stress at a certain point in a component changes over time, such stress is referred to as “alternating stress”” ； 2. Fatigue: The phenomenon of failure that occurs under the action of alternating stresses is known as “fatigue failure” or “fatigue damage”, simply referred to as “fatigue”. There is a fundamental difference between fatigue failure and strength failure under static loading. Under alternating stress, the strength properties of materials depend not only on the materials themselves, but also on factors such as the stress variation pattern, the shape and dimensions of the components, and the surface finish quality. II. Characteristics of fatigue failure 1. The failure stress value is much lower than the strength parameter of the material under static load. 2. It takes a process for a component to suffer fatigue failure at a certain stress level, that is, a certain number of stress cycles are required. 3. The members show no significant plastic deformation before or at the moment of failure; even materials that are highly plastic under static loads exhibit brittle fracture. 4. For the same fatigue fracture surface, there are generally two distinct regions: a smooth region and a granular region. III. Causes of fatigue failure. Taking polycrystalline metals as an example, they consist of numerous grains with varying strengths; at grain boundaries or in the presence of inclusions, the strength is even weaker. Under external forces, slip zones of dislocation will appear in the grains that are subjected to greater stress or have weaker strength, as well as at the grain boundaries. As the number of stress cycles increases, slip becomes more pronounced, the slip bands widen, and eventually they fracture along the slip bands, forming cracks. Most of these initially formed microcracks are origins of fatigue damage, known as “fatigue sources”. After several more cycles of stress alternation, the macroscopic cracks continue to expand, resulting in a weakening of the component’s cross-section, similar to creating a sharp \"cut\" in the component. As a result, the component fails at very low nominal stress levels (the stress calculated without considering stress concentration). The formation and propagation of cracks is a complex process that is greatly influenced by factors such as the shape and size of the component, the type of stress variation, and the medium in which the component is located. 1. The effect of stress concentration on the fatigue limit: At locations where there is a sudden change in cross-section of a component—such as the transition sections of stepped shafts, holes, or slots—stress concentration occurs; that is, in these localized areas, the stress can reach very high values. 2. Influence of component size on fatigue limit: Component size has a significant impact on the fatigue limit, which is one of the key differences between fatigue strength issues and static load strength issues. Experimental results show that when the stress is non-uniformly distributed across the cross-section of a component, the larger the component size, the lower its fatigue limit. 3. Influence of the surface finish quality of components on the fatigue limit: Rough machining can create scratches of varying depths on the component surface, and these scratches themselves serve as initial cracks. When the stress is high, crack propagation starts here first. Therefore, as the quality of surface finishing improves, the fatigue limit will increase. The effect of surface processing quality on the fatigue limit is measured by the “surface quality coefficient” β. In China, the quality coefficient β=1.0 for polished surfaces is used as a benchmark; the quality coefficients for all other surface processing methods (grinding, finish turning, etc.) are less than 1.0. 4. Fatigue limit of components under symmetric cyclic loading: Utilizing the fatigue limits ơ-1 and T-1 for bending and torsion under symmetric cyclic loading in smooth small specimens, and taking into account the effects of stress concentration, component size, and surface finish quality, the fatigue limits of components under bending and torsion symmetric cyclic loading can be determined. The coefficients Kơ, KT, εơ, εT, β, etc., which influence the fatigue limit, can be found in relevant handbooks. Appropriately increasing the transition radii at cross-section changes and taking other measures helps to alleviate stress concentration. IV. Ways to improve the fatigue strength of components
1. Alleviating stress concentration
Appropriately increasing the transition fillets at points where there is a sudden change in cross-section, along with other measures, helps to alleviate stress concentration. 2. Improving the surface quality of components: In situations where stress is non-uniformly distributed (such as in bending and torsion), fatigue cracks mostly initiate and propagate from the surface of the components. Therefore, strengthening the surface of components through mechanical or chemical methods to improve the quality of the surface layer will significantly enhance their fatigue strength. Surface heat treatment and chemical treatments (such as surface high-frequency quenching, carburizing, nitriding, and cyaniding), as well as cold pressing mechanical processing (such as surface rolling and shot blasting), all help to improve the quality of the surface layer of components. The fatigue failure mechanism of metal materials is divided into three main stages: 1. Formation of fatigue cracks ； 2. Fatigue crack propagation ； 3. When the crack propagation reaches a critical size, final fracture occurs. 1. Formation of fatigue microcracks: Fatigue microcracks are caused by uneven sliding and microscopic cracking. ①Cracking of surface slip zones ； Phase two: fracture at the interface between the inclusion and the matrix phase, or within the inclusion itself ； Cracking occurs at grain boundaries or subgrain boundaries. ②Under cyclic loading, even if the cyclic stress does not exceed the yield strength, slip zones form on the surface of the specimen, known as cyclic slip zones. ③The slip bands formed during stretching are distributed more evenly, whereas cyclic slip bands concentrate in certain local areas. Moreover, extrusion and intrusion occur in the cyclic slip zones, thereby forming micro-cuttings on the surface of the specimen. 2. Cracking of surface slip zones: Persistence of cyclic slip zones: ● Slip zones appear in the early stages of fatigue. As the number of cycles increases, the slip zones increase. ● After removing the slip zone and reloading it, the slip zone reappears in its original location. ● This type of slip zone is called a Persist Slip Band. ● Fatigue cracks appear in the persistent slip zone. The formed microcracks will continue to grow under cyclic loading. When the tip of the microcrack approaches the grain boundary, its growth rate decreases or even stops growing. This must be due to the different orientations of the slip systems within adjacent grains. ● Only by crossing grain boundaries can microcracks connect with microcracks in adjacent grains, or extend into those adjacent grains to form fatigue cracks on a macroscopic scale. Since grain boundaries inhibit the growth and connection of microcracks, they help to extend the life until fatigue cracks form and the overall fatigue life. 3. Formation of fatigue microcracks ● Larger inclusions or second phases can cause microcracks to form due to cracking at the interface between the inclusion and the matrix. ● Under cyclic loading, a second phase will result in intergranular cracks. 4. Fatigue resistance indicators: Classified by stress condition, they include bending fatigue, torsional fatigue, tension-compression fatigue, contact fatigue, and combined fatigue. The fatigue failure mechanism of metal materials is divided into three main stages: 1. Formation of fatigue cracks ； 2. Fatigue crack propagation ； 3. When the crack propagation reaches a critical size, final fracture occurs. 1. Formation of fatigue microcracks: Fatigue microcracks are caused by uneven sliding and microscopic cracking. ①Cracking of surface slip zones ； Phase two: fracture at the interface between the inclusion and the matrix phase, or within the inclusion itself ； Cracking occurs at grain boundaries or subgrain boundaries. ②Under cyclic loading, even if the cyclic stress does not exceed the yield strength, slip zones form on the surface of the specimen, known as cyclic slip zones. ③The slip bands formed during stretching are distributed more evenly, whereas cyclic slip bands concentrate in certain local areas. Moreover, extrusion and intrusion occur in the cyclic slip zones, thereby forming micro-cuttings on the surface of the specimen. 2. Cracking of surface slip zones: Persistence of cyclic slip zones: ● Slip zones appear in the early stages of fatigue. As the number of cycles increases, the slip zones increase. ● After removing the slip zone and reloading it, the slip zone reappears in its original location. ● This type of slip zone is called a Persist Slip Band. ● Fatigue cracks appear in the persistent slip zone. The formed microcracks will continue to grow under cyclic loading. When the tip of the microcrack approaches the grain boundary, its growth rate decreases or even stops growing. This must be due to the different orientations of the slip systems within adjacent grains. ● Only by crossing grain boundaries can microcracks connect with microcracks in adjacent grains, or extend into those adjacent grains to form fatigue cracks on a macroscopic scale. Since grain boundaries inhibit the growth and connection of microcracks, they help to extend the life until fatigue cracks form and the overall fatigue life. 3. Formation of fatigue microcracks ● Larger inclusions or second phases can cause microcracks to form due to cracking at the interface between the inclusion and the matrix. ● Under cyclic loading, a second phase will result in intergranular cracks. 4. Fatigue resistance indicators: Classified by stress condition, they include bending fatigue, torsional fatigue, tension-compression fatigue, contact fatigue, and combined fatigue.