Creep fatigue

Creep generally refers to the plastic strains in a material that occur under a constant load even below the yield point. If the stresses in the material are caused by an external force, creep or retardation leads to an increasing deformation of the component. However, if they originate from a given forced deformation, the tension in the material decreases, it relaxes while the shape is retained. The creep tendency depends on the material, time and temperature.

If the load causing the creep changes periodically and at the same time individual load levels are maintained for a sufficiently long time, this leads to recurring creep strains, which cause a corresponding damage in the material, creep fatigue. Depending on the type of load, the reason for the failure is a crack or breakage or an impermissibly high deformation.

Analogous to the evaluation of low cycle fatigue or thermomechanical fatigue, the strain history must also be tracked down sufficiently well in the simulation for creep fatigue. Since creep processes are relatively slow, a correspondingly long time horizon must be considered for the numerical evaluation. If the holding times for individual load levels are long, this is not a problem, as correspondingly large time steps can be expected. If, on the other hand, the holding times during the individual load changes are comparatively short, many cycles have to be run through in the simulation with a small increment. A correspondingly high numerical effort is the consequence. At the same time, the data volume of the results increases, so that an evaluation of creep fatigue is also associated with special requirements from this perspective.