| Solid oxide fuel cell(SOFC)has the advantages of high energy conversion efficiency,flexible fuel and high waste heat utilization capacity,and has a good application prospect.SOFC is used in high temperature environment,and the creep damage of the material will destroy the structural integrity of SOFC,which is one of the main reasons for restricting the commercialization of SOFC.The Ni-based anode-supported SOFC is selected as the research object,the multi-axial creep damage constitutive model is developed based on continuous damage mechanics by a combination of theory and numerical simulation,and the Weibull failure probability calculation method that considers creep damage is developed;Furthermore,the influences of geometry,operating temperature and anode oxidation on the creep damage and failure probability of SOFC are studied systematically.Finally,based on the established high-temperature creep failure prediction model which considers the design parameters,the design method of SOFC high-temperature creep strength is proposed.The structure design of SOFC that meets a certain high temperature strength is realized accurately and efficiently.The main research contents and conclusions of this paper are summarized as follows:(1)The conventional mechanical properties and creep properties of the experimental research are carried out for the materials of SOFC,and Norton related creep parameters are fitted.The metal frame material Crofer 22 APU has no deceleration creep stage,while the cell and sealant ceramic materials have no accelerated creep fracture stage.According to the different creep deformation behaviors and evaluation methods of various materials in the SOFC structure,the ductility exhaustion creep damage models suitable for SOFC are established to accurately predict the creep deformation behavior of Crofer22 APU and Ni-YSZ;and for probabilistic failure of ceramic materials,the Weibull failure probability calculation model based on creep strain and damage-stress are established to realize the accurate prediction of the failure probability under the coupling action of chemical-mechanical,which provide a theoretical basis for the follow-up study of the high-temperature creep failure and influence law of the SOFC structure.(2)Considering the effects of package residual stress and the evolution of mechanical properties during anode reduction,the creep damage behavior of the full-scale SOFC during service at 800 ℃ for 50,000 hours and its influence on the failure probability are analyzed numerically.It is found that the sealant is the key component that affect the creep life of the SOFC overall structure.The creep damage and failure probability of the sealant are the largest in the SOFC structure.The maximum damage value is located at the outer corner of the sealant facing the frame,which is the most dangerous part of creep failure.(3)The factors which affecting the high-temperature creep strength of SOFC are analyzed.First,the influences of the design parameters such as the geometric dimensions and operating temperature of the SOFC on the creep damage and failure probability are discussed.Reducing the side length of the cell and the thickness of the anode and metal frame,and increasing the thickness of the sealant can reduce the creep damage and failure probability of the sealant.After 50,000 hours of creep at 600°C,the failure probability of the sealant is reduced by 88.71% compared to the failure probability at 800°C.Then based on the gradient distributions of the material characteristics and oxidation strain during the inhomogeneous anode oxidation,a finite element program for inhomogeneous anode oxidation analysis is developed to calculate the influence of the oxidation-creep coupling effect on the thermal stress and failure probability of the SOFC overall structure accurately.It is found that after inhomogeneous oxidation,a large stress gradient and bending occurred in the anode.The failure probabilities of the cathode,electrolyte,anode,and sealant increase sharply with the increase of the oxidation strain.The critical oxidation strain is about 9.4e-4,1.1e-3,2.3e-3and 7.2e-4,respectively.(4)Based on the response surface method,the interaction between the geometrical dimensions is studied,and the correlation between the objective function of sealant failure probability and the geometrical design variables is obtained.Combining the influence of design parameters such as creep time and working temperature on the failure probability of sealant,a high-temperature creep failure prediction model for the SOFC stack is proposed,and the model is verified by test.The results show that the model can predict the failure probability of SOFC stacks of any geometric size after creeping for t hours at a certain operating temperature,and can greatly reduce the calculation time.After optimizing the SOFC stack structure according to the optimal geometry obtained by the response surface model,it can meet the creep design life of 50,000 h. |
