M. Kurek et alii, Frattura ed Integrità Strutturale, 37 (2016) 221-227; DOI: 10.3221/IGF-ESIS.37.29 221 Focussed on Multiaxial Fatigue and Fracture Estimation of fatigue strength under multiaxial cyclic loading by varying the critical plane orientation Marta Kurek, Tadeusz Łagoda Department of Mechanics and Machine Design, Opole University of Technology - Opole, Poland Andrea Carpinteri, Sabrina Vantadori Department of Civil-Environmental Engineering and Architecture, University of Parma - Parma, Italy A BSTRACT . The main purpose of this paper is to examine the influence of the critical plane orientation on the estimated fatigue strength of metals under multiaxial loading. The algorithm employed to evaluate fatigue strength implements the criterion of maximum normal and shear stress on a suitable damage plane (critical plane). The angle defining the critical plane orientation is measured with respect to the direction that maximises the applied normal stress. Eleven (11) structural materials under combined bending and torsion cyclic loading are examined. For each analysed material, the value of angle is selected so that the value of the scatter, defined by a root-mean-square value, is minimum. On the basis of such a calculation, an empirical expression for is proposed, that takes into account the values of bending and torsion fatigue strengths at a reference number of loading cycles. According to such an expression, is constant for a given material. K EYWORDS . critical plane; fatigue strength; multiaxial loading. I NTRODUCTION tructural components of machines and devices are subjected to service loads which often include multiaxial load conditions. The complex nature of the fatigue processes has produced several fatigue criteria which, implemented in algorithms, constitute a basic tool for estimating fatigue strength/life. These criteria generally reduce the spatial stress state to an equivalent uniaxial one. Among all multiaxial fatigue criteria, it is possible to distinguish a group based on the critical plane concept, which assumes that material fatigue failure is caused by stresses (strains) related to the critical plane. In 1935, Stanfield suggested the use of the critical plane to describe multiaxial fatigue [1]. Currently, such a concept gains an increasing interest. The paper presents both a model for estimating fatigue life and the analysis of the influence of the critical plane orientation on such an estimation. Particular attention is paid to the proposal of a new function to determine the critical plane orientation, based on both the analysis of scatters and the ratio between the fatigue strength for bending and that for torsion, at the given number of loading cycles. The calculation employs the criterion of maximum normal and shear stresses acting on the critical plane [2]. S

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