FATIGUE FAILURE ASSESSMENT UNDER MULTIAXIAL LOADING Andrea Carpinteri and Andrea Spagnoli Department of Civil Engineering, University of Parma, Parco Area delle Scienze 181/A, 43100 Parma, Italy ABSTRACT According to the so-called critical plane approach, the plane where the fatigue failure assessment should be performed is determined by maximising the amplitudes and/or values of some stress components. In the present paper, the critical plane orientation is proposed to be correlated with the averaged principal stress directions deduced through the weight function method. Then the fatigue failure assessment is performed by considering a function of the stress components acting on the critical plane. The results derived by applying this criterion or some other critical plane criteria commonly used are compared with experimental data related to different brittle (hard) metals under in-phase or out-of-phase sinusoidal biaxial normal and shear stress states. INTRODUCTION Several criteria developed during the last decades to predict whether fatigue failure under multiaxial loading may occur or not are generally aimed at reducing a given multiaxial stress state to an equivalent uniaxial stress condition (e.g. see review in Ref. 1 ). Some of these criteria are based on the so-called critical plane approach, according to which the fatigue failure assessment is performed in a plane where the amplitude or the value of some stress components or a combination of them attains its maximum [2-5]. Alternatively, the position of the critical plane may be correlated with that of the principal stress directions, but, since such directions under fatigue loading are generally time-varying, averaged principal stress directions should be considered [6-8]. In the following, a new criterion is proposed which correlates the critical plane orientation with the mean principal stress directions determined through the weight function method. Then the fatigue failure assessment is performed by considering a quadratic function of the shear amplitude and the maximum normal stress acting on the critical plane. Finally, such a criterion is applied to some experimental tests on brittle (hard) metals under in-phase or out-of-phase sinusoidal biaxial normal and shear stress states. For these materials, the ratio between the endurance limit under fully reversed torsion and that under fully reversed bending falls within the following range : 1 3 1 af af . FATIGUE CRITERIA BASED ON THE CRITICAL PLANE APPROACH Let us consider the plane stress condition of biaxial normal and shear stresses at the generic point P of a cylindrical body (Figure 1a) subjected to synchronous out-of-phase sinusoidal loading :