In accordance with the FKM-guideline “Analytical Strength Assessment of Components in Mechanical Engineering” the principal stress amplitudes have to be determined for a fatigue assessment of solid block-shaped, non-welded components. In case of non-proportional stresses the evaluation of principal stress amplitudes is complex due to time-dependent principal stress directions. In this connection an approximate assessment is recommended which may cause too conservative results. Therefore the aim of this thesis is the development of a novel approach to evaluate principal stress amplitudes under time-dependent principal stress directions. For this purpose a tensor transformation approach was developed and implemented into existing in-house post-processing software. Stress tensors determined by means of linear-elastic FE-calculations are transformed to incrementally rotated coordinate systems. In each of these rotation states fatigue strength assessment is carried out in accordance with the FKM-guideline. The degree of utilization for combined types of stresses is calculated by means of normal stress amplitudes based on the interaction approach for proportional or synchronous stresses and attains a maximum in a certain rotation state at which the most unfavourable interaction of normal stress amplitudes is marked. Hence the fatigue assessment is carried out basing on principal stress amplitudes. For a validation of the developed calculation concept experimental fatigue test results including proportional and non-proportional loads on cylindrical specimens from literature and the corresponding calculation results are compared. It is shown that the calculated results evaluated by the tensor transformation approach are conservative in case of non-proportional stresses. Compared to the recommended approximate assessment the difference can be reduced by about 40%. Thus the developed calculation concept provides an essential benefit for light-weight dimensioning solid block-shaped, non-welded components in accordance with the FKM-guideline.