Experimental and theoretical investigations of visco-plastic deformation in structural steels considering mutual influence of plasticity and creep effects

This paper considers the developed mathematical model of inelastic deformation in structural steels, describing thermoviscoplastic deformation taking into account mutual influence of plasticity and creep effects. An integration algorithm for constitutive relations of thermoviscoplasticity has been developed. It consists in the formulation of constitutive relations in increments, depending on the selected time step. In difficult areas of deformation paths, time step can be adjusted throughout the whole estimation time in case of stability calculations. Stresses, plastic deformations and creep deformations are determined by integrating the defining relations of thermal creep by Runge-Kutta method with the correction of stress deviator and subsequent determination of stress according to thermal plasticity equations with regard to the average creep strain at the next sampling time. Experimental studies of influence between creep processes and plasticity under high temperature using 12H18N9 steel have been conducted. By numerical computer simulation of stress-strain state (SSS) kinetics in laboratory samples and by comparing the obtained results with field experiments, the authors carried out certification of the developed thermoviscoplastic model and integration algorithm of constitutive relations. All of these led to the conclusion about the reliability of model concepts and methods for determining material parameters under joint actions of fatigue and creep mechanisms. The authors have compared computer and physical tensile experiments of laboratory 12H18N9 steel samples with different histories of changes in temperature and mechanical deformation. It is shown that the developed thermoviscoplastic model qualitatively and quantitatively describes main effects of inelastic deformation in structural steels with different histories of mechanical deformation and changes in temperature. It is concluded that the defining relations of thermoviscoplasticity are reliable, and the above methods of integration are accurate.