The paper presents the results of experimental studies of tube polycrystalline specimens made of pure NP2 grade nickel for active (uniaxial tension after preliminary compression) and passive (torsion with tension/compression within the boundaries of the yield surface) loading. The results of the experiments clearly demonstrate the relationship between the microstructure of the specimens and their mechanical properties. The aim of the study is to validate and identify the parameters of the developed microstructural elastoplastic model under various loadings. It is implied that a polycrystalline object is considered as a set of arbitrarily oriented crystallites included in it, so the comparison of the model with an experiment is carried out in terms of microstress and strain averaged over a representative volume. The proposed model is a significant extension of the theory of Taylor and Cailletaud single crystals strain hardening and takes into account up to 10 different slip mechanisms that are activated when the stress value of the local yield strength are reached. During the parameter identification process, a universal algorithm for finding the minimum functional is used, based on the gradient descent method, which allows to search for parameters in a given range of values. The proposed identification criterion is accompanied by wellknown statistical indicators: the method of least squares, which characterizes the minimization of the standard deviation of the calculated curve from the experimental one; proximity to unity of the value of the linear correlation coefficient between these curves; minimization of the difference in areas under the compared curves (energy criterion). In computations of the stress-strain state of a polycrystalline aggregate, the finite element homogenization method is used for cases of different slip mechanisms and crystallite orientations. The comparison of computation results with experimental data showed that the proposed microstructural model of polycrystalline material allows to describe with good accuracy the process of plastic strain accumulation under complex passive loading.

Ilnar R. Murtazin, Research Engineer at the National Research Center "Digital Engineering in Nuclear and Thermonuclear Power Engineering", Assistant at the Higher School of Mechanics and Control Processes; e-mail: murtazin_ir@spbstu.ru; https://orcid.org/0000-0002-7580-5669; AuthorID: 1059960

Artem S. Semenov, Doctor of Physical and Mathematical Sciences, Professor of the Higher School of Mechanics and Control Processes; e-mail: semenov_as@spbstu.ru; https://orcid.org/0000-0002-8225-3487; AuthorID: 6571

Anton V. Emelianov, Head of the laboratory of the Research Institute "Politechtest KSM"; e-mail: em-ant1990@yandex.ru; https://orcid.org/0000-0002-7736-5652; AuthorID: 966108