Fatigue (material)

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.