NUMERICAL STUDY OF THE PROCESS OF HIGH-TEMPERATURE CREEP AND LONG-TERM STRENGTH OF STRUCTURAL ALLOYS UNDER UNIAXIAL TENSION

Abstract

The main regularities of deformation processes are consideredand degradation of the initial strength properties of structural materials (metals and their alloys) by the mechanism of long-term strength. To describe the processes of high-temperature creep and long-term strength of polycrystalline structural alloys, an approach based on the concept of “hidden” or “internal” parameters is used, which can give a qualitative and quantitative description of experimental data. This approach has two important advantages: it allows you to cover a wide range of behavior of structural materials and at the same time it is very convenient for analyzing the stress-strain state. The mathematical model of the mechanics of a damaged medium used in this work, describing the processes of inelastic deformation and damage accumulation during creep, consists of three interrelated components: evolutionary relations that determine the inelastic behavior of the material, considering the dependence on the destruction process; kinetic equations describing the process of damage accumulation; criteria for the strength of the damaged material. The variant of the determining relations of viscoplastic deformationof polycrystalline structural alloys is based on the idea of the existence of a family of equipotential creep surfaces in the stress space and the principle of gradiency of the creep strain rate vector to the corresponding surface at the loading point.This version of the equations of state reflects the main regularitiesthe process of viscoplastic deformation of the material under proportional and disproportionate modes of combined thermomechanical loading. The variant of the kinetic equations of damage accumulation is based on theintroduction of a scalar damage parameter, is based on energy principles, and considers the main effects of the formation, growth and fusion of microdefects for arbitrary complex modes of thermomechanical loading. As a criterion of the strength of the damaged material, the condition for reaching the critical value of the damage value is used. The results of experimental studies of short-term high-temperature creep of several structural alloys (copper, stainless steel X18H10T) at constant temperature values and various levels of forces set in the samples are presented. To assess the degree of reliability and determine the limits of applicabilitymodels of the mechanics of the damaged medium numerical studies of the process of high-temperature creep and long-term strength of these structural alloys are carried out and the numerical results obtained are compared with the data of field experiments. The results of comparing the calculated and experimental data allow us to conclude that the proposed determining relationships are reliable when the initial strength properties of structural materials are degraded by the mechanism of long-termstrength. It is shown that the model used qualitatively and with the accuracy necessary for practical calculations quantitatively describes the main effects of the process of viscoplastic deformation and damage accumulation in polycrystalline structural alloys under high-temperature thermomechanical loading.