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In this paper, a previously developed modification of the Sainz model of multiaxial fatigue is reduced to an invariant form. Model constants were determined for different sets of setup experiments. It was supposed to introduce an additional summand to account for the phase shift between loading modes. The model is used to describe the fatigue behavior of the D16T aluminum alloy. Low-cycle fatigue tests under biaxial loading conditions are presented, with one mode changing cyclically and the other mode remaining constant in magnitude throughout the test. The results of cyclic durability prediction by the modified model provide good convergence.

The work is aimed at experimental and theoretical study of the regularities and effects of the behaviour of metallic materials at the stage of postcritical deformation, which is of both applied and fundamental interest from the point of view of understanding preceding of the fracture processes and the possibility of predicting the survivability and kinetics of structural failure processes. An experimental assessment was made of the influence of the strain rate in a wide range on the postcritical behaviour of X15CrNi12-2 steel samples at the temperature of 500 °C. Experimental data of rheological processes of the steel at various levels of achieved postcritical deformation was obtained. It was shown that during creep holding at the postcritical stage of deformation, a transition to the final creep stage and subsequent destruction of the samples almost immediately were occurs. An analysis of the experimental dependences obtained during exposition in the relaxation mode demonstrates the independence of the stress level at the end of exposition on the speed of preliminary deformation and, therefore, on the initial value of the stresses, which reached at the beginning of the exposition. The data obtained are necessary to develop models of softening media, taking into account the rheological processes that occur in metallic materials under conditions of postcritical deformation at high temperatures.

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