Creep‐fatigue analysis of the steel AISI type 316 component failure

Abstract

The purpose of this work is to extend a typical creep-damage model in order to describe material behavior under variable thermal and mechanical loading in wide stress range. The model basis is creep constitutive law in form of hyperbolic sine stress response function proposed by Nadai. The constitutive law is extended to assume the damage process under creep and fatigue by the introduction of scalar damage parameters and appropriate evolution equations according to Kachanov-Rabotnov concept. The material constants for model are identified by fitting the experimental creep and low-cycle fatigue data for the steel AISI type 316 at the range of temperatures 500• C -750• C. The development of such model is motivated by the well described failure case study of high-temperature components at unit 1 of Eddystone power plant, which have operated during 130520 hours under creep-fatigue interaction conditions. The main steam piping (MSP) from this power plant is selected for thermo-mechanical creep-fatigue analysis applying the proposed material model. The estimated values of damage parameters comply with the real location of the component failure and a scatter of experimental data on creep-fatigue interaction diagram. Although high-temperature components of such engineering structures as boilers, turbines and chemical facilities are designed to resist creep, thermal-fatigue, corrosion and environmental-degradation processes, the failures are regularly documented in literature sources devoted to engineering failure analysis. One of the common reasons for happened failure cases is an imperfection of available material models employed in design procedures for high-temperature components. The reported failure case study [1] of the steel AISI type 316 components at unit 1 of Eddystone power plant, which have totaly operated during 130520 hours, indicated, that their failure had been caused by the damage accumulated under variable thermomechanical loading conditions inducing complex interaction of different processes. Therefore, the material model assuming not only irreversible creep strains, but also significant plastic strains caused by high thermal stresses, which simultaneously result in low-cycle fatigue, have to be employed for thermo-mechanical creep-fatigue analysis of these components. The main steam piping (MSP) from this power plant is selected for numerical FE-analysis in CAE-software ABAQUS, as illustrated in Fig. 1, in order to obtain a correct life-time prediction confirming the applicability of the proposed material model.All basic thermal and mechanical properties of the steel AISI type 316 show significant temperature dependence [2]. Plastic deformations are described by isotropic hardening model using experimental true stress-strain curves [3] also at different temperatures derived under 4 · 10 −5 (1/s) strain rate. And the creep behavior is characterized by the following constitutive equation based on hyperbolic sine stress response function proposed by Nadai and extended with Arrhenius-type functions:where...