Study On The Mechanical Behaviour And Multi-Mechanism Damage Of High-Cr Steels Under Complex Load Conditions At Elevated Temperatures

Due to the rapid development of renewable energy,conventional power plants need to start and stop frequently to meet changes in power supply.Besides,coal-fired power plants operate in a high temperature and high-pressure environment to improve fossil energy efficiency.High-Cr steel components are subjected to complex thermal-mechanical loads,resulting in creep,fatigue,ductile,oxidation corrosion and other types of damage.Different types of damage may co-occur and interact with each other,exposing the structural integrity and operational reliability to a great threat.Therefore,it is necessary to study the thermodynamic response of high-Cr steel to ensure the safe operation of high-Cr steel components.Firstly,the mechanical response of P92 steel in the stress-controlled cyclic tensile-loading tests with holding periods is studied.Here,some typical experimental results of P92 steel samples obtained in the previous works are introduced.A three-stage creep model is adopted to simulate the mechanical behaviors of P92 steel samples in the tests.The material parameters in the model are identified by using the experimental data of pure creep tests,where the lowand high-stress ranges are considered separately.The constitutive equation in the model has a relatively simple form,and it can be solved through a conventional numerical approach.With the obtained numerical solutions to the constitutive equations,the responses of P92 steel samples in the cyclic tensile-loading tests with holding periods are simulated,which shows good consistency with the experimental results.Some key features of the mechanical behaviors and damage evolution properties of the P92 steel samples can be revealed by virtue of the model.With the determined model parameters and numerical algorithm,The mechanical responses and lifetimes of P92 samples in cyclic tensile-loading tests can be evaluated accurately by the model.Next,a damage tensor variable and a microcrack closure parameter are introduced into the unified multi-mechanism continuum viscoplastic damage model to simulate the mechanical responses of P92 steel under complex loading.By reviewing the damage mechanism under different kinds of failures,isotropic creep damage,anisotropic ductile damage and anisotropic low-cycle fatigue damage are adopted in the multi-mechanism damage model,and the parameters in the damage evolution equation are identified.Besides,a microcrack closure parameter is introduced into the model to simulate the tension-compression asymmetry accurately.A system of equations with damage tensor variable and microcrack closure parameter is concluded,and the numerical integration algorithms are designed for stress-driven and strain-driven problems respectively.Finally,the multi-mechanism damage model is used to simulate the mechanical behaviors of P92 steel under biaxial creep and biaxial low-cycle fatigue at elevated temperatures.Multiple biaxial creep tests with different stress states are simulated at the same von Mises stress,while the biaxial low-cycle fatigue tests are simulated with the different loading phases.A 3D visualization scheme of stress tensor and damage tensor is proposed to show the stress and damage states of different directions.Furthermore,by analyzing the simulation results,the effect on P92 mechanical behaviors of isotropic damage,anisotropic damage and microcrack closure parameter are discussed,providing an insight into the failure mode of P92 steel under different biaxial loading conditions.