Glossary term
Fatigue
The progressive damage and eventual failure of a material subjected to repeated or fluctuating loads.
Definition
phenomenonFatigue is the progressive structural damage that occurs in a material subjected to repeated or fluctuating loads, potentially leading to crack initiation, crack growth, and final fracture at stress levels well below the material's static strength.
Fatigue is the dominant failure mode for a large fraction of mechanical and structural components in service. It is treacherous because it produces no macroscopic warning signs — the material appears intact until the crack has grown to a critical size and sudden fracture occurs. Fatigue failure depends on the amplitude and mean of the cyclic stress, the number of loading cycles, the surface condition, residual stresses, material microstructure, environment, and the presence of stress concentrations.
Fatigue is the process by which a material accumulates damage under repeated or fluctuating loads, eventually developing a crack that grows until the remaining cross-section can no longer carry the applied load and fracture occurs. The critical characteristic of fatigue is that failure takes place at stresses far below the material’s yield strength or ultimate tensile strength — stresses that would cause no harm if applied once. It is the repetition that matters: each load cycle advances the damage a small amount, and failure results from the accumulation of millions or billions of such increments.
Three stages of fatigue failure
Fatigue failure develops in three sequential stages. In the first stage — crack initiation — a fatigue crack nucleates at a point of high local stress. Initiation sites are typically surface scratches, machining marks, corrosion pits, grain boundaries, inclusions, or geometric discontinuities such as fillets and holes. Even in a smooth, polished specimen, cyclic slip along crystallographic planes produces persistent slip bands that eventually become the embryo of a crack.
In the second stage — crack propagation — the crack advances a small increment with each load cycle. The stress singularity at the crack tip, quantified by the stress intensity factor K, drives further crack extension cycle by cycle. The rate of crack advance per cycle is described by the Paris–Erdogan law, treated in a dedicated entry.
In the third stage — final fracture — the crack has grown to the critical size at which the maximum stress intensity factor exceeds the fracture toughness K_{Ic} of the material. The remaining cross-section fails suddenly, often with little or no plastic deformation in high-strength materials.
Engineering significance
Fatigue is critical in aircraft structures, rotating machinery, bridges, ships, pressure vessels, and any component exposed to vibration or cyclic loading. Design against fatigue requires knowledge of the material’s resistance to cyclic loading — characterised by the S–N curve and the endurance limit — the stress amplitude and mean stress at the critical location, the effect of stress concentrations, and the expected load history over the component’s service life. When variable amplitude loading is involved, cumulative damage is assessed using the Miner rule. When cracks must be assumed to pre-exist, fracture mechanics and the Paris–Erdogan law govern the design. Mean stress effects on fatigue life are assessed using the Goodman criterion or related models. Each of these tools is treated in a dedicated entry.