International Journal of Innovation and Scientific Research ISSN 2351-8014 Vol. 15 No. 2 Jun. 2015, pp. 228-235 © 2015 Innovative Space of Scientific Research Journals http://www.ijisr.issr-journals.org/ Corresponding Author: Kévin M. Tsapi Tchoupou 228 Improvements of two fatigue criteria based on material parameters Kévin M. Tsapi Tchoupou and Bertin D. Soh Fotsing Department of Mechanical Engineering, IUT Fotso Victor–University of Dschang, P.O. Box 134 Bandjoun, Cameroon Copyright © 2015 ISSR Journals. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ABSTRACT: Most of mechanical components in the engineering are frequently subjected to the fatigue damaging process because of the great number of stress cycles they have to undergo. This paper presents an elaboration of two of the most studied methods for the computation of fatigue life in multiaxial fatigue. We describe the reformulation of Sines and Crossland fatigue criteria, which have been adapted so that they could preserve the observed detrimental influence of a tensile mean stress and the observed beneficial effect of a compressive mean bending stress. The proposed reformulation of Crossland and Sines criteria is applied to a general sinusoidal in-phase or out-of-phase bending and torsion stress state, for which analytical formulae can be derived. From the theoretical results calculated according to the present propositions, the criterion proposed by Sines was found to be the most precise in preserving the detrimental influence of a tensile mean stress and the observed beneficial effect of a compressive mean bending stress. On the other hand, the criterion proposed by Crossland was found to be precise in the multiaxial fatigue limit prediction. This analysis shows that the proposed procedure is very efficient, suggesting that Sines and Crossland fatigue criteria remain valuable fatigue evaluation tools for the mechanical design industry. KEYWORDS: high-cycle fatigue, mean stress effect; fatigue limit; proportional loading, non-proportional loading. 1 INTRODUCTION A major part of mechanical components in the engineering is subjected to fluctuating loads, which can lead to sudden fatigue failure phenomenon. The durability analysis of these components and structures against fatigue is nowadays a main checking point of the design-engineering field [1], [2], [3]. Multiaxial fatigue criteria are generally associated to the estimate of the fatigue strength under complex loading. Hence, efficient and accurate methodologies for the account of the main factors influencing the fatigue strength of materials (type of loading, temperature, microstructural heterogeneities, residual stresses) under multiaxial stress states are required for use in engineering design application [4], [5], [6]. Many formulation of fatigue criteria have been proposed over years [2], [7]. Two of them that have been widely studied are those proposed by Sines [8] and Crossland [9]. These criteria are well-known criteria and are attractive for engineering design of high cycle fatigue components because easy-to-use; however these solutions are generally the weakest methods; unsafe when used for complex stress states. The main limitation in using the best solutions see [2], [7], [10], in situations of practical interest is that their application requires the definition of nominal parameters such as reference section, nominal stress, equivalent amplitude, etc. Lengthy and quite complicated calculations are required. Crossland and Sines criteria had over years been widely studied for a large number of loading cases: tension, bending, torsion, and combined tension/torsion, in phase or out of phase. Sines [8] postulated that a mechanical component is in its fatigue limit condition when the following condition is assured: