Creep and plastic flow of a spherical viscoelastic layer material at its loading and unloading

The purpose of the research is to investigate the processes of accumulation of irreversible deformations through different mechanisms: creep and plastic flow. Initially irreversible deformations are produced due to the viscous properties of solid material as creep deformations. The mechanism of the production of irreversible deformations changes to plastic when stress state reach the yield surface. On the contrary, this mechanism changes from fast plastic to slow viscous during unloading. The continuity in such increase in irreversible deformations is provided by the corresponding set of creep and plasticity potentials. These processes are considered in the framework of the mathematical theory of small deformations on the example of a one-dimensional problem of the deforming of a viscoelastoplastic hollow sphere under the influence of volumetric pressure changing with time. The processes of creep and plastic flow under increasing pressure, plastic flow at constant pressure, the medium unloading at decreasing pressure and the repeated plastic flow at the unloading were considered. The regularities of the motion of elastic-plastic boundaries in the material of the hollow sphere were established. The parameters of the stress-strain state of the medium were calculated, stress relaxation after the unloading was investigated.