Failure due to material fatigue is an important cause of structural failure. But what is material fatigue exactly? And which factors influence the fatigue strength or life of components and structures?
Description of Fatigue
Metal fatigue is the gradual development of the damage in a structure or component subjected to cyclic loads, eventually leading to the complete failure of the structure. Remarkable is that the material stress level causing fatigue failure is considerably lower than the maximum allowable stresses for a single, static applied load. The loads responsible for fatigue failure are called fatigue loads and are cyclic by nature.
The description of metal fatigue can be divided in two groups: a metallurgical and a mechanical description.
The metallurgical description considers the state of a material before, during and after the application of the fatigue loads and contains also the study of the fatigue mechanisms.
The mechanical description considers the mechanical response of a set of loading conditions, like predicting the number of load cycles leading to fatigue failure at a given stress level. The mechanical description is more useful from an engineering point of view, for example to predict the fatigue life of components and based on that, work out an inspection or maintenance strategy.
Phases of Metal Fatigue
Fatigue failure typically develops in three different phases:
- 1. Crack initiation
- Crack initiation originates in general at the surface of a component and on locations with elevated material stress. The size of the crack in this phase is usually not larger than 0.5 mm and is not visible by the naked eye.
- 2. Crack propagation
- The crack propagates with every dynamic load cycle. Initially the crack growth goes slowly, but accelerates when the material stress in the undamaged part of the component starts to rise.
- 3. Termination
- Once the material section of the undamaged part of the component has become too small to support the forces, the part completely ruptures in one load cycle as a brittle failure.
Factors Influencing Fatigue
The most important factors influencing metal fatigue are:
- Mean Stress
- Surface Roughness
- Residual Stress
Stress values can have a positive or a negative sign. By convention, compressive stresses have a negative sign and tensile stresses a positive sign. Since mainly tensile stresses are responsible for fatigue failure, a higher mean stress results in earlier failure. A higher mean stress means that a load cycle contains more or higher tensile than compressive stresses. With a constant amplitude loading, a mean stress of 0 MPa is caused by a load cycling between -P and +P.
Fatigue cracks in a metal develop usually at the surface of a component. Metal fatigue is therefore considered as a surface phenomenon. It also appears that the quality of the surface plays a major role in the fatigue life of a structure. A rougher surface results in faster fatigue failure. Read more about the Surface Roughness Factor KR.
Notches cause local stress concentrations. Fatigue usually occurs at those locations, but the fatigue strength is often a bit higher than the local stresses would imagine.
Residual stresses are stresses often originated from a fabrication of after-treatment process. Residual stresses can increase or decrease the fatigue strength of a part. Residual tensile stresses lower the fatigue strength, while residual compressive stresses increase the fatigue strength of a material. Shot peening is an after-treatment process that introduces local compressive stresses and therefore increases the fatigue strength.
The fatigue strength of some materials can also be influenced by temperature. At temperatures above 200°C many materials start to display structural changes, resulting in reduced fatigue strength.