Advanced Structural Analysis/Part I - Theory/Failure Modes/Fatigue/Crack Initiation/The Wöhler Curve

thumb|left|alt=Wöhler curves of Steel and Aluminum|Wöhler curves of Steel and Aluminum The Wöhler curve, also referred to as the S-N curve, describes the function or . It is based on empirical results and often represents the median of the data scatter. A significant interval of the curve can be approximated by the Basquin relation Where is a constant specific to the test case. The Basquin relation is often presented in the form where cycles is the so called detail category number. Detail Category Number The fatigue detail number defines the Basquin relation and specifies a Wöhler curve. The property is often denoted FAT, C, or in mathematical expressions: . If there are more than one FAT tied to a geometry-type, they refer to different specifics such as weld quality, loading direction etc. Moreover, the FAT normally corresponds to a fatigue life of cycles. Statistical Properties