Are you asking about performing a non-linear fatigue study
using SW simulation? If so, the current version (2009 premium) does
not have a module to perform the non-linear fatigue study. The
fatigue study module is only applicable for linear static studies.
But you can perform a non-linear fatigue study using COSMOSM
interface(GeoStar). You can mesh your solids and carry over the
mesh file (*.geo) to GeoStar interfaace and perform a fatigue