What Does Fatigue Mean?
Fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. If the local stresses are high enough, this 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.
Many dramatic and severe failures are caused by fatigue, and it is essential that structural engineers include fatigue into design considerations. Some materials (such as some steel and titanium alloys) exhibit a theoretical fatigue limit below which continued loading does not lead to fatigue failure.
Corrosionpedia Explains Fatigue
In materials science, fatigue is the weakening of a material. It is a cumulative effect causing 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.
Corrosion fatigue is the result of the combined action of an alternating or cycling stresses and a corrosive environment. The fatigue process is thought to cause rupture of the protective passive film, upon which corrosion is accelerated. If the metal is simultaneously exposed to a corrosive environment, 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 begin to 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 initiate.
Fatigue failure occurs in three stages:
- Crack initiation
- Slow, stable crack growth
- Rapid fracture
The majority of engineering failures are caused by fatigue. The type of fatigue of most concern in circuit boards, gasoline, diesel, gas turbine engines and many industrial applications is thermal fatigue. Thermal fatigue can arise from thermal stresses produced by cyclic changes in temperature.