5. Fatigue and creep damage and failure

5.1 Pure fatigue

The Wöhler curve is obtained by plotting the number of cycles to failure as a function of the amplitude of the cyclic stress applied (or measured at mid-life in the case of total strain amplitude control). It is commonly used for direct plotting in the case of imposed stress tests (fatigue with a large number of cycles). However, it cannot be used to unify experimental data, whatever the martensitic material being tempered.

The Manson-Coffin curve describes the evolution of the number of cycles to failure as a function of the amplitude of plastic deformation measured at mid-life. Unlike the Wöhler curve, the Manson-Coffin curve provides a master curve that is valid for all grades at 550°C, over a wide range of service lives (Figure