Plastic deformation of materials sensitive to a type of stress state

The paper considers main principles and equations of the plasticity theory for materials sensitive to stress state, i.e. materials which have different plastic deformation curves under uniaxial tension, compression and torsion (shear). Thus, such materials don’t have a unified plastic deformation curve under beam (simple) loading processes. The considered theory of plasticity refers to the plastic flow theory under combined hardening, in which yield surface radius is taken to be dependent on the first stress tensor invariant and parameter of the active stress state type. In this case the defining functions of the evolution equation for yield surface displacement are dependent on the parameter of the additional stress state type (state of microstresses). The parameter of the stress state type is determined as a ratio between the third and second invariants in the power of 3/2 of the corresponding deviators, and under compression it is equal to 1, under tension it is equal to +1, under shear it is equal to 0. Plastic volume change (loosening) is considered within this theory in case of dependence between the yield surface on the first stress tensor invariant. For damage accumulation processes, the kinetic equation is presented based on the work of microstresses on the field of plastic deformations. In this equation the destruction energy is considered to be dependent on the first stress tensor invariant and type parameter of microstresses’ state. Material functions, completing the theory and their determination method are presented. Results of theoretical and experimental researches have been analyzed regarding the elastoplastic deformation of aluminum alloy D16T samples along double-part deformation trajectories, as well as samples made of 30HGSA steel under loading along double-part orthogonal stress trajectories. A satisfactory compliance between the calculation and experimental results has been obtained. The effect of “deformations splitting” was explored that lead to the fact that beam trajectories of deformation (stresses) may correspond to non-beam trajectories of stresses (deformations), and plain trajectories may correspond to non-plain ones.