EVALUATION OF RESOURCE CHARACTERISTICS OF POWER EQUIPMENT PARTS UNDER THERMAL CYCLIC LOADING

Abstract

The problem of assessing the strength and resource of critical engineering facilities, the operational properties of which are characterized by multiparametric non-stationary thermo-mechanical effects, is discussed. The main degradation mechanisms of structural materials (metals and their alloys) characteristic of such objects are considered. The basic requirements for mathematical models of these processes are formulated. A mathematical model describing the processes of thermoplastic deformation and accumulation of fatigue damage during degradation of the material by the mechanism of thermal fatigue has been developed from the position of mechanics of the damaged medium (MPS). The IPU model consists of three interrelated parts: relations that determine the cyclic thermoplastic behavior of the material, taking into account the dependence on the destruction process; equations describing the kinetics of fatigue damage accumulation; strength criteria of the damaged material. The variant of the defining relations of thermoplasticity is based on the idea of the yield surface and the principle of gradiency of the velocity vector of plastic deformations to the yield surface at the loading point. This version of the equations of state reflects the main effects of the process of cyclic thermoplastic deformation of the material for arbitrary complex trajectories of combined thermomechanical loading. The variant of kinetic equations of fatigue damage accumulation is based on the introduction of a scalar damage parameter, is based on energy principles and takes into account the main effects of formation, growth and fusion of microdefects under arbitrary complex loading conditions. As a criterion of the strength of the damaged material, the condition of reaching the critical value of the damage value is used. To assess the degree of reliability and determine the limits of applicability of the developed defining ratios of MPS, calculations of the processes of thermoplastic deformation and accumulation of fatigue damage were carried out and the numerical results obtained were compared with the data of field experiments on the example of a specific applied problem. A numerical analysis of the characteristic features of the thermal fatigue durability of a compact sample with stress concentrators simulating the operation of parts in the nozzle box of a steam turbine of a nuclear power plant (NPP) is carried out. The results of calculations of fatigue damage accumulation processes during thermal pulsations are compared with experimental data. It is shown that the developed MPS model qualitatively and with the accuracy necessary for engineering calculations quantitatively describes experimental data and can be effectively used to assess the thermocyclic fatigue durability of structures with multiaxial disproportionate paths of combined thermomechanical loading.