During the last decades critical plane models have been developed as tools for crack initiation analyses within fatigue. Compared to older methods, these models aim to improve fatigue life predictions in multiaxial and non- proportional loading situations. The task in this thesis was to study how to produce the material data required in the Fatemi and Socie critical plane model. A parameter handling the normal stress influence on the crack initiation life was especially studied. The study was performed under the assumption that results from torsion tests and tension tests could be utilized to represent the new data. First, structural analyses were performed in the commercial FE-code ANSYS for specimens exposed to tension and torsion, respectively. The stress states achieved were then used in crack initiation analyses in the in-house fatigue computer program fatsoc8.f90. The results show an increasing normal stress influence with increasing crack initiation life. However, the method utilized is very sensitive to changes and requires exact input data for the results to be reliable. A suitable continuation would be to perform a crack initiation analysis of a component, subjected to multiaxial and non-proportional loading, applying the new data and compare to experimental results. In addition, it would be interesting to investigate the possibility to use two different tension tests for calculation of the parameter.