Materials fatigue is the process by which materials fail or fracture when subjected to repetitive stress / strain. Fatigue failures are normally studied in the context of modern or manufactured materials, however the concepts apply to natural materials and fibers as well.

History The mechanism of fatigue was first studied when early railroad car or engine axles failed unexpectedly. Engineers of the time had a good understanding of simple stress failures however the resulting designs did not take into account the then unknown mechanisms of fatigue failure.

Mechanisms When metals are stressed the lattice structure deforms along slippage planes. Within the elastic limit, most of slippage is fully recovered on unloading. However under cyclic deformations some of the slippages are permanent and these begin to form microscopic cracks.

These small cracks can act as both stress concentrations and to effect stress relief, depending on the details of design and loading. At the micro-level stress cracks develop low pH at the crack-tip which acts to accelerate crack growth and is a factor in accelerating fatigue rate in aqueous, saline or corrosive environments.

Design implications

Many structural metals (iron, steel, titanium exhibit a distinct 'endurance limit' which is a level of cyclic stress which can continue for an infinite number of cycles without causing failure. Some materials (e.g. aluminum, magnesium) do not have a fixed endurance limit and will eventually fail at any level of repeated stress, no matter how low. Generally where there is a known endurance limit, if a structure survives one million cycles, it will last indefinitely.

The study of fatigue is a substantial branch of materials science. Because most reciprocating or rotating machinery sees well over a million cycles in many modes the economics of understanding fatigue are compelling.

Historically, when fatigue was not well understood, many catastrophic failures resulted, including for example early iron and steel ships sometimes broke in half due to repetitive stresses due to wave and storm motions.

Today many large structures such as bridges are inspected for fatigue cracks. In situations where it is not practical to completely prevent fatigue failure, the designer must accommodate the anticipate crack growth rate, crack-size and ensure that the inspection plan will find fatigue cracks at or above the critical size for the design.

High-stress, low weight components, such as engine connecting rods must be produced and maintained without transverse scratches, which will induce early failure by creating a stress concentration where a fatigue crack will be initiated at a lower level than in a smooth surface.

The fatigue endurance of critical components may also be increased by creating residual stress in the surface of the part. Usually this is accomplished by bead blasting the surface, which leaves the surface in a state of residual compressive stress. By pre-stressing in compression, the effective tension stress in the surface is reduced.