This master thesis is focused on the development of a mean stress strategy for an integral fatigue strength criterion. In the course of this, mean stress effects of both mean shear stresses and mean normal stresses on the fatigue strength of the aluminium wrought alloy AlCu4PbMgMn were characterised indicating no significant effect of shear mean stresses on the resulting torsional fatigue strength. Moreover, an existing and extensive fatigue test data bench for the QT-steel 34CrNiMo6 determined at the institute were considered in this thesis indicating a significant mean shear stress sensitivity. The fatigue test data was compared to diverse various strength criteria. The Safety Factor Intensity Hypothesis (SFIH) demonstrated the most reliable results. In the second part of this work, a new concept was developed, which considers mean stress effects in the SFIH. By means of this criterion it is now possible to take the shear mean stress effects into account as well as normal mean stress effects both under uniaxial and multiaxial load conditions. The lifetime simulation results indicated a good agreement to the test results both for mean shear stress sensitive and non sensitive materials.