Fatigue is a process in which damage is accumulated in the material undergoing fluctuating loading. Damage takes the form of cracks in the material that develop slowly at early stages of loading and accelerate very quickly toward the end (Figure 11.1). Microcracks start to develop at points of stress concentration at nominal stresses that may be well below the elastic limit. These cracks grow slowly under continuing fatigue loading but start to accelerate when the local stresses near the crack front increase due to cross-sectional reduction. Fracture occurs when the remaining section area is not able to support the applied load. Evidently, tension stresses are more significant than compression stresses. Fatigue is a local phenomenon that takes place at regions of stress concentration such as rapid changes of cross sections, at section reductions due to bolted connections or in welding regions, where the material undergoes metallurgic changes (see also Section 11.11).