What Does Fatigue Loading Mean?
Fatigue loading is the changes observed in a material under the influence of stress generated during cyclic loading. This is generally represented by plotting a stress cycle curve (S-N curve), where S represents stress and N represents the number of cycles to failure.
There is progressive and localized structural damage when fatigue loading occurs, and if the local stresses are high enough leads to the initiation of a crack, the growth of the crack and finally fracture.
Fatigue can exacerbate structural damage in the presence of corrosive environments.
Corrosionpedia Explains Fatigue Loading
In materials science, fatigue is the weakening of a material, and fatigue loading means continuously loading a material with some force until it experiences a crack. It is a cumulative effect that causes a material to fail after repeated applications of stress, none of which exceeds the ultimate tensile strength. The term fatigue is based on the concept that a material becomes "tired" and fails at a stress level below the nominal strength of the material.
In a corrosive environment, if a material experiences fatigue loading, it leads to corrosion fatigue due to the combined action of cycling (or alternating) stresses and a corrosive environment. The fatigue process is thought to rupture the protective passive film, after which corrosion is accelerated. If the metal is simultaneously exposed to a corrosive environment, then the failure can take place at even lower loads and after a shorter time.
Fatigue occurs when a material is subjected to repeat loading and unloading. If the loads are above a certain threshold, microscopic cracks form at the stress concentrators such as the surface, persistent slip bands (PSBs) and grain interfaces. Eventually, when a crack reaches a critical size, the crack propagates suddenly, and the structure fractures. The shape of the structure significantly affects the fatigue life — square holes or sharp corners lead to elevated local stresses where fatigue cracks can begin.
Fatigue loading causes fatigue failure, which occurs in three stages:
- Crack initiation
- Slow, stable crack growth
- Rapid fracture
The majority of engineering failures are caused by fatigue. The greatest concern in circuit boards, engines and many industrial applications is thermal fatigue. Thermal fatigue can arise from thermal stresses produced by cyclic changes in temperature.