Theoretical Study On The Complicated Mechanical Behaviours Of High-Cr Steels At Elevated Temperatures

In recent years,due to the rapid development of renewable energy,the conventional power plants are faced with great transition.During the frequent startup and shutdown processes,the high-Cr steel components are subjected to complex thermomechanical loads,which results in the creep,low-cycle fatigue,ductile,oxidation corrosion and other kinds of damages in the components.These different kinds of damages may occur simultaneously and interact with each other,therefore the residual lifetimes of high-Cr steel components can be influenced significantly.Thus,for the aims of safe operation of components,it is necessary to investigate the thermomechanical behavior and the damage evolution property of high-Cr steels and the components under complicated loading condition at elevated temperatures.Firstly,a three-stage constitutive model is proposed to simulate the creep behaviours of high-Cr steels under constant loading at elevated temperatures,in order to investigated the creep damage of high-Cr steels.The different features of the creep rates in the three creep stages will be taken into account.For the secondary and tertiary creep stages,two Larson–Miller parameters are adopted to predict the minimum creep rates and the average creep rupture times.A creep damage variable is also adopted to capture the acceleration of creep rate in the tertiary creep stage.The decrease of the creep rate during the primary creep stage is captured by introducing an internal variable representing the strain hardening effect.The material parameters of the model can be identified by fitting the conventional experimental results.Both the strainand stress-driven algorithms are designed to solve the constitutive evolution equations.Further implementing the model into a finite element software,the global creep behaviours of high-Cr components under realistic loading conditions can be simulated.Next,based on the approach of continuum damage mechanics and framework of thermodynamics,a unified multi-mechanism continuum viscoplastic damage model is established for the simulation of the thermomechanical behavior of high-Cr steels at elevated temperatures under complex loading condition.For isotropic damage case,a scalar damage variable is incorporated to represent the effects of material degradation,which is composed of the creep damage,lowfatigue damage and the ductile damage.During the formulation of the model,some kinematic assumptions are proposed and the concept of effective stress is adopted firstly.Then,based on a state potential with proper constitutive form,the constitutive equations can be derived from the second law of thermodynamics.By further considering the postulate of maximum dissipation,a Lagrangian functional is constructed through a regularization scheme.The stationary points of the Lagrangian functional yield the evolution equations of the dissipative variables.Thereout,the constitutive evolution equations of the multi-mechanism continuum damage model can be established.For the anisotropic damage,damage variable can be represented by a second order tensor,and the constitutive evolution equations in the model can be derived through the same way.To be prepared for practical applications,numerical integration algorithms are proposed to solve the constitutive evolution equations,and the material parameters in the model are identified based on the experimental data.With the identified material parameters,the current model can not only simulate the thermomechanical behaviors of high-Cr steels under different loading conditions,can also predict the damage evolution and the residual lifetimes of high-Cr steels.Final,a combined fatigue-ductile continuum damage model is presented,for the description of the complex mechanical behaviors of high-Cr steels in the high stress range.The central aim of this work is improving the accuracy of the model predictions by means of the parameter optimization.The initial value of the material parameters in the model can be identified based on the experimental data.To improve the accuracy of the model predictions,an efficient parameter optimization program is designed,in which the response data of uniaxial cyclic loading tests,uniaxial static loading tests and uniaxial loading-dwell tests are adopted as optimization objective.By adopting the optimized parameters,the mechanical responses of high-Cr steels under different loading conditions can be predicted well.Specifically,the minimum strain rates and the average rupture times of high-Cr steels under static loading condition in the high stress range,can be predicted by the Chaboche-type viscoplastic constitutive equation in the current model.The study result of this work can be applied for the safety design and lifetime evaluation of high-Cr steel components in practical applications,showing important value in sphere of learning and engineering application.