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This paper presents the results of estimation of fatigue strength dependingon the variable orientation of the critical plane for proportional and non-proportionalbending and torsion with regard to specimens made of aluminium alloy 2017A. Thealgorithm applied for the estimation of the fatigue strength was based on theCarpinteri-Spagnoli proposal and its subsequent modifications in accordance with theideas developed by the authors. The objective in this paper is to search for a modelwhich offers the best possible results of estimating fatigue strength of materials withproperties which are intermediate between elastic-brittle and elastic-plastic such asaluminium alloys and a further insight into them with regard to their fatigue strength.