Fatigue (material)
Cyclic loading causes crack growth and eventual fracture.
Fatigue in materials is the initiation and propagation of cracks due to cyclic loading. It is a critical failure mechanism in engineering, historically associated with metal components such as railway axles, but now recognized in most materials including composites, plastics, and ceramics. The process begins with crack initiation at stress concentrations like holes, grain boundaries, or persistent slip bands, even when applied stresses are well below the material’s yield strength. In metals, initiation involves the development of cell structures and hardening, followed by the formation of persistent slip bands where localized slip creates surface intrusions and extrusions that act as stress risers. Cracks can also form directly at pre-existing defects such as inclusions or sharp internal corners. Once initiated, a crack grows incrementally with each loading cycle, often producing visible striations on the fracture surface. Most of the fatigue life is consumed during this growth phase, which is driven primarily by the range of cyclic loading but also influenced by mean stress, environment, and overloads or underloads. Growth may cease if loads fall below a critical threshold. When the crack reaches a critical size where the stress intensity factor exceeds the material’s fracture toughness, rapid propagation occurs, leading to sudden, often brittle-looking failure. Historically, nineteenth-century failures of metal railway axles were mistakenly attributed to crystallization of the metal due to the brittle fracture appearance, a theory later disproven. Fatigue is categorized into high-cycle fatigue, requiring more than ten thousand cycles with primarily elastic stress, and low-cycle fatigue involving significant plasticity; both follow the same stages of crack initiation, stage I growth along crystallographic planes, stage II growth perpendicular to the applied force, and ultimate failure.
- field
- Materials science
- known_for
- Crack initiation and growth under cyclic loading, leading to sudden brittle-like failure
- key_concept
- Fatigue life prediction via constant amplitude cyclic loading tests
- associated_materials
- Metals, composites, plastics, ceramics
Lore & Background
Fatigue has traditionally been associated with the failure of metal components, leading to the term 'metal fatigue.' In the nineteenth century, the sudden failing of metal railway axles was thought to be caused by the metal crystallising due to the brittle appearance of the fracture surface, but this has since been disproved. Most materials, such as composites, plastics and ceramics, seem to experience some sort of fatigue-related failure. Historically, fatigue has been separated into regions of high cycle fatigue (requiring more than 10^4 cycles to failure, where stress is low and primarily elastic) and low cycle fatigue (where there is significant plasticity). Experiments have shown that low cycle fatigue is also crack growth. Fatigue failures, both for high and low cycles, all follow the same basic steps: crack initiation, crack growth stages I and II, and finally ultimate failure. Crack initiation in metallic samples involves four discrete steps: development of cell structures and hardening, breakdown into persistent slip bands (PSBs), localized slip at PSBs, and nucleation of cracks. PSB-induced slip planes result in intrusions and extrusions along the surface, creating fine surface structures that act as stress concentrators. These steps can be bypassed if cracks form at pre-existing stress concentrators such as inclusions or geometric features like sharp internal corners.
Reader's Guide
Fatigue is a fundamental concept in materials science and engineering because it governs the failure of structures under repeated loading, often at stresses well below the material's yield strength. Its significance lies in the fact that most engineering failures—from aircraft components to bridges—are fatigue-related. The process is stochastic, with considerable scatter even in identical samples, and damage is irreversible. Understanding fatigue allows engineers to predict component life through tests using constant amplitude cyclic loading, measuring crack growth rates. Special cases such as short cracks, overloads, and underloads can significantly alter growth rates. The presence of a fatigue limit in some steels and titanium alloys means that below a certain stress threshold, failure does not occur. Fatigue is usually associated with tensile stresses, but cracks have been reported due to compressive loads. The legacy of fatigue research is the development of design practices that account for crack initiation and propagation, leading to safer, more reliable structures.
Did You Know?
- Fatigue cracks can grow from material or manufacturing defects as small as 10 μm.
- In aluminium, cracks generally grow from the surface where water vapour dissociates into atomic hydrogen, causing hydrogen embrittlement.
- Overloads initially cause a small increase in crack growth rate followed by a long reduction.
- Fatigue life scatter tends to increase for longer fatigue lives.
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