Study On Creep Damage Constitutive Model And Life Prediction Of T91 Heat-resistance Steel

The energy issue has always been the top priority of human development.Under the background of the continuous depletion of natural resources and the immature technology of new energy development and utilization,the continuous improvement of energy utilization efficiency has always been an important research topic in various countries.Nuclear energy is representative of new energy and one of the main pillars of the world energy supply,which has occupied an important position in the world energy structure.The integrity of nuclear reactor structure is a prerequisite for nuclear energy safety application.In all kinds of nuclear reactor components,the condition of the fuel element cladding is the most severe.The service environment temperature is very high,and thus the creep is prone to occur at elevated temperatures in the long-term reaction process,resulting in the phenomenon of creep fracture failure of materials.In view of this phenomenon,it is of great significance to analyze the creep damage law of fuel cladding material T91 heat-resistance steel at elevated temperatures and avoid accidents caused by the creep damage.Therefore,in view of the T91heat-resistance steel cladding structure material,it is necessary to conduct creep experiments at elevated temperatures,establish a creep damage constitutive model to predict the experimental results,and compare it with different creep failure life prediction methods,which will provide theoretical basis and technical support for the creep damage evolution and failure life prediction of T91 heat-resistance steel at elevated temperatures.The main work of this paper is summarized as follows:(1)The uniaxial tensile and creep experiments on T91 heat-resistance steel were carried out at elevated temperatures,and the dependence of creep strain evolution on temperature and stress level was revealed.It can be seen from the creep experimental results that the creep strain presents three stages,that is,with the increase of time,the creep strain rate decreases continuously in the first stage,keeps constant in the second stage,and increases rapidly in the third stage until the creep failure.The creep failure law of T91 heat-resistance steel at different temperatures was compared and analyzed.It is found that with the increase of stress level,the second stage of creep gradually shortens,the creep strain rate increases,and the third stage appears earlier.The creep fracture morphology of T91 heat-resistance steel at different temperatures was observed.It is found that under the action of high temperature and high stress,the creep pores form in the grain,and as the creep progresses,the pores grow and combine to form cracks,and the transgranular creep fracture occurs.The recrystallization process is accompanied by the occurrence of fracture,and the intergranular fracture of the material is inhibited by finer grains.The grain size at the necking of the material is small,and macroscopically,the area of the cross-section is gradually reduced.(2)Based on the classical Norton constitutive equation and the microscopic characterization mechanism analysis,the material was assumed to be a multiphase damage complex,and the damage evolution parameters were introduced in the second stage of creep to construct an improved multiphase creep damage constitutive model.The experimental creep curves of T91 heat-resistance steel at different temperatures were simulated by using different creep damage multiphase constitutive models.Combined with the discussion of creep parameters,the simulated results of creep at elevated temperature of T91heat-resistance steel were discussed,and the influence of damage parameters on the prediction ability of creep at elevated temperature with different models was clarified.(3)The coupled-damage creep models were used to analyze the creep failure life of T91heat-resistance steel at elevated temperatures,it is found that the multiphase coupled-damage creep model provide best predicted results comparing to the experimental results.Based on the traditional extrapolation method of creep life prediction at elevated temperature,the isotherm method,L-M parameter method and M-H parameter method were used to predict and analyze the experimental lives of creep failure of T91 heat-resistance steel at elevated temperature.It is found that the isotherm method is the most simple and convenient,but the prediction accuracy is low at the same time.The parameters of the M-H method are more complex,but it is the most accurate for life prediction analysis.Besides,the creep failure life of T91 heat-resistance steel at elevated temperature is predicted based on the ASME code,the results show that its prediction accuracy is lower than the SVM method,and all data is located in the twice error band.Using the SVM and BP neural network machine learning methods of T91 heat-resistance steel creep failure life at elevated temperature was predicted,and compared with the results by the creep life extrapolation method and the creep damage multiphase constitutive model,the prediction accuracy of creep failure life of T91heat-resistance steel by different methods was quantified.The results show that,compared with the multiphase coupled-damage creep model and creep life extrapolation method,the machine learning method has higher accuracy in life prediction.