The many variables associated with material type, sample geometry and in service use of a part or component complicates the design and implementation of an appropriate fatigue testing regime.
Since 1850 we have known that metal subjected to fluctuating stresses will fail at a stress much lower than required to cause fracture on a single quasi static pull to break. Failure usually occurs without warning and results in a brittle appearing fracture with no significant deformation. Metal fatigue is a multi-step process and is often described as having four stages.
Stage 1: Crack Initiation: A part becomes damaged as a micro crack forms at a point of high stress concentration. High stress concentration points are usually located at notches, sharp edges or corners. Annealing is used to repair metals damaged in stage 1.