Advanced Structural Analysis/Part I - Theory/Failure Modes/Fatigue/Crack Initiation/Additional Influences
An engineering structure may contain internal stress components that are independent of external loads and body loads. Such internal stresses are commonly called Residual Stresses and may be substantial in magnitude.
As was established in the section Influence of Mean Stress", a perturbation of the mean stress may have a significant positive or negative impact on the fatigue life. Residual stresses are in effect perturbations of mean stress, when cyclic loading is present. Thus, predictions of the fatigue life can be thrown off by the residual stresses, which do not relate to the external loads.
The following are sources of residual stress:
- Pre-tensioned design elements such as bolts
- Inhomogeneous plastic deformation
- Thermal effects
- Residual stress
Fretting corrosion is the combined effect on fatigue life caused by fretting and corrosion. The mechanism is a surface effect and can have a detrimental impact on the crack initiation period, and consequently the high-cycle fatigue life. However, it has little or no effect on crack growth and consequently plays only a small role in low-cycle fatigue.
Note that fretting corrosion may reduce the fatigue limit, .
Fretting occurs when two surfaces are rubbing against each other during cyclic loading. Fretting corrosion can be significant even when rubbing displacements are very small, -displacements in the order of the elastic deformation of the rubbing parts can have a dramatic effect on the fatigue life. Corrosion has an important role even in normal air.
The effects of fretting corrosion can be reduced or even avoided by:
- Separating surfaces where fretting would otherwise occur
- Making sure the fretting zone is not also a stress concentration
Corrosion may reduce the fatigue life and fatigue limit significantly. The presence of liquids such as salt-water magnifies the effect and should be avoided if at all possible.