Issue 49

P. Trusov et alii, Frattura ed Integrità Strutturale, 49 (2019) 125-139; DOI: 10.3221/IGF-ESIS.49.14 130 where    T ω r r is the spin determining the rate of rotation for the moving coordinate system connected with the material symmetry axes of a meso-II element,     1 e e e l f f is the rate of elastic distortions of the lattice. The model constitutive relations for a meso-II element As the model is intended to describe the processes of thermo-mechanical treatment being characterized by large displacement gradients, geometrically nonlinear kinematic and constitutive relations are used in its structure [18–19]. The rate statement of the problem is done in the current configuration and the following constitutive relations are used [20–21]:                о ρ ; ; ; ˆρ cr p tr cr T Π k : l ω l l l k k k ω ω k k σ (5) where Π is the tensor of elastic properties for the meso-II element, defined by the constant components in the basis of the crystallographic coordinate system of the current phase in the initial configuration;   ˆ l v is the transposed velocity gradient for the material particles of the meso-II element in the current configuration, transmitted from the meso-I; p l is the plastic part of the relative velocity gradient connected with the shears on the slip systems inside the meso-II element in the deformation process;  l is the thermal part of the transposed relative velocity gradient; tr l is the transposed velocity gradient of the transformation deformation, associated with the phase transformation in the material; , cr k k are the weighted Kirchhoff stress tensor and its corotational derivative; о ˆρ, ρ are the densities of the meso-II element's material in the initial (unloaded) and current configurations (the density depends on the phase the element at the considered moment is in); σ is the Cauchy stress tensor of the meso-II element; ω is the spin tensor of the meso-II element (to define it, any physically based model of rotation can be used, for example, it can be the Teylor's model of turning in a fully constrained conditions [22] or the model of lattice rotation [18]). Wherein, at each moment of the process, the rotation of the rigid moving coordinate system's axes of the meso-II element, connected with the lattice of the element in its current phase, is considered. The plastic part of the velocity gradient p l is determined by shears on the slip systems in the meso-II element:            1 ; K k k k p k l b n (6) where k is the number of the slip system. Herewith, the shear rate on each slip system in the meso-II element is considered as a function of the acting stresses    k , critical shear stresses    k с and temperature  :               , , k k k с f ; (7) in general case, any physically valid model can be used to define it. In particular, a non-linear viscoplastic model [21, 23] can be used:                               1 0 . m k k k k с k с H (8) The thermal part of the transposed relative velocity gradient in the meso-II element is defined using the following relation:     l α , (9) where α is the thermal expansion tensor for the material of the meso-II element. Herewith, a simplification can be accepted for cubic crystals, and the relation for the thermal component of the velocity gradient can be written in the following form: