Fatigue (material)

Lore & Background

Fatigue has traditionally been linked to metal components, leading to the term 'metal fatigue.' In the nineteenth century, the sudden failure of metal railway axles was thought to be caused by the metal crystallizing due to the brittle appearance of the fracture surface, but this has since been disproved. Most materials, such as composites, plastics, and ceramics, experience some form of fatigue-related failure. Fatigue failures follow basic steps: crack initiation, crack growth stages I and II, and ultimate failure. Cracks nucleate at stress risers or high void density areas. Stage I growth occurs along crystallographic planes where shear stresses are highest, while stage II growth is perpendicular to the applied force. The process can be bypassed if cracks form at pre-existing stress concentrators like inclusions or sharp corners. Crack growth rate is driven by cyclic loading range, with factors like mean stress, environment, overloads, and underloads affecting it. Striations on the fracture surface mark crack tip position per cycle. When stress intensity exceeds fracture toughness, rapid fracture occurs. Fatigue life scatter tends to increase for longer lives, and damage is irreversible.

Reader's Guide

Fatigue is a fundamental failure mechanism in materials science, critical for engineering design and safety. It explains why components fail under repeated loading even at stresses below the yield strength. The concept evolved from nineteenth-century railway axle failures, where brittle fracture surfaces were mistakenly attributed to crystallization. Modern understanding shows fatigue involves crack initiation from persistent slip bands or stress concentrators, followed by slow crack growth and eventual fast fracture. Fatigue testing uses constant amplitude cyclic loading on coupons to measure crack growth rates. Special cases like short cracks, overloads, and underloads alter growth rates. High-cycle fatigue (over 10⁴ cycles) involves low stress and elastic behavior, while low-cycle fatigue involves significant plasticity. Both follow the same stages: initiation, stage I shear-driven growth, stage II perpendicular growth, and ultimate failure. Fatigue is stochastic, with scatter in identical samples. It is usually associated with tensile stresses but can occur under compressive loads. Factors like temperature, surface finish, microstructure, and environment influence fatigue life. Some steels and titanium alloys exhibit a theoretical fatigue limit below which failure does not occur. Understanding fatigue allows prediction of component life and prevention of catastrophic failures in structures like aircraft, bridges, and machinery.

Did You Know?

• Fatigue cracks can grow from defects as small as 10 μm.
• Striations on fracture surfaces mark the crack tip position for each loading cycle.
• In aluminium, water vapour from the atmosphere can cause hydrogen embrittlement at the crack tip.