The term fatigue is used to describe the phenomenon where a component fails after repeated loadings and unloadings, even though the magnitude of each individual load may be much smaller than the ultimate stress of the material. The term was coined in the middle of the nineteenth century when several railroad accidents drew attention to the subject. Even today, the vast majority of all structural failures are attributed to fatigue, which means that designing against fatigue is of utmost importance.

During a large number of load cycles (repeated loadings and unloadings), damage is accumulated on the micromechanical scale and after some time a crack of macroscopic size is formed.

Usually, the details of the last two stages are considered within the topic of fracture mechanics; the term fatigue applies mainly to Stage 1. Because the largest part of the life of the component is spent before it is possible to observe a macroscopic crack, most designs aim to avoid ever getting such a crack. There is, however, some overlap between the disciplines and the measured number of cycles to fatigue often includes Stages 2 and 3. In the Fatigue Module the Darveaux model explicitly addresses stages 1 and 2.