A Continual Model of Damage for Analyzing Long-Term Strength of Materials and Structural Elements

Abstract

The issue of evaluating the strength and service life of critical engineering facilities with the exploitation properties characterized by multi-parametrical nonstationary thermal–mechanical effects is discussed. The main degradation mechanisms of structural materials (metals and their alloys), characteristic of such facilities, are examined. Basic requirements imposed on mathematical models of the above processes are formulated. In the framework of mechanics of damaged media (MDM), a mathematical model is developed, which describes processes of inelastic deformation and damage accumulation in the presence of creep. The MDM model consists of three interrelated parts: relations defining inelastic behavior of the material, accounting for its dependence on the failure process; equations defining damage accumulation kinetics; a strength criterion of the damaged material. The results of experimentally studying short-term creep of the VZh-159 heat-resistant alloy at constant temperatures and different stresses applied to the specimens are presented. Deformation and damage accumulation processes have been numerically analyzed; the obtained numerical results are compared with the data of full-scale experiments. The results of numerically modeling the carrying capacity of a nuclear power plant (NPP) reactor vessel in the conditions of a hypothetical accident are presented. The emergency was modeled by applying pressure modeling the effect of meltdown, constant internal pressure and temperature varying within the limits of the analyzed part of the reactor vessel. The analysis of the obtained numerical results reveals a number of characteristic features accompanying the process of deformation and failure of such facilities, related with time and location of the forming macrocracks, history of the stressed–strained state (SSS), damage degree in the failure zone, etc. In general, the results of comparing the numerical and experimental data make it possible to conclude that the proposed defining relations of MDM are reliable in accounting for the degradation of initial strength properties of materials according to the long-term strength mechanism and can be effectively used for evaluating long-term strength of structural elements under thermal–mechanical loading.