Study On The Stress Distributions Of High Temperature Superconductor Bulks During Magnetization Process

Due to its high critical current density,high-temperature superconductor(HTS)bulk can trap high magnetic fields at low temperature.Such excellent properties make it widely available for application prospects in many fields.However,because of its material properties,the mechanical properties of the HTS bulks are poor,which can endure large compressive stress but easily subject to large tensile stress.In fact,during the magnetization process,the HTS bulks may be degraded or damaged by the induced electromagnetic force due to the rapid change rate of the applied field,which leads to failure of the HTS bulks or weakens its applications.Therefore,in the magnetization process,the research of mechanical properties of HTS bulks is one of the important topics in the field.At present,the study of the mechanical properties of the HTS bulks mainly uses the critical state model or the method which considers Bean model and viscous flux flow simultaneously.In this thesis,Kim model and viscous flux flow are simultaneously considered to study the of stress distributions inside the bulk under different unloading rates of applied fields.The main points of this thesis are as follows:Firstly,the Kim model and viscous flux flow are combined to deduce the rate expression of applied field(db_a/dt)and the relationships among db_a/dt,viscous flux velocity and applied field are obtained by the expression,which is different from the expression of db_a/dt deduced by combining Bean model and viscous flux flow.In fact,the expression of db_a/dt by Bean model only contains viscous flux velocity.However,the expression of db_a/dt by Kim model contains not only viscous flux velocity but also applied field.Based on the expression of db_a/dt by Kim model,the stress distributions in a rectangular HTS bulk are discussed by different db_a/dt during field cooling(FC)and zero field cooling(ZFC)magnetization process in this thesis.The results show that,during ZFC,the stresses in the active part of the bulk increase with the increasing db_a/dt and with the increasing db_a/dt,the maximum stress remains constant,but the position of maximum stress points is moving towards the edge of the bulk during FC.Secondly,by analyzing the influence of the characteristic parameter p of Kim model on the stress distribution in a cylindrical HTS bulk during ZFC magnetization process,the differences between the Bean and Kim models are compared in this thesis.As p is equal to0.1,the stress distributions under Bean and Kim models are almost the same.However,as p is equal to 1,the stress by Kim model is larger than that by Bean model.Based on the Kim model,both the radial and hoop stresses in a cylindrical HTS bulk are discussed by different db_a/dt during FC and ZFC magnetization process.The results show that,during ZFC,both the radial and hoop stresses increase with the increasing db_a/dt and the radial stress is always no less than the hoop stress.During FC,the maximum radial and maximum hoop stresses increase with the increasing db_a/dt and their stress position points are moving towards the edge of the bulkFinally,the first,second and molar strength theories are used to analyze the strength of rectangular and cylindrical HTS bulks by different db_a/dt.The results show that,during FC and ZFC magnetization process for rectangular and cylindrical HTS bulks,the equivalent stress obtained by the molar strength theory is larger than that obtained by the first and second strength theory.Therefore,the molar strength theory is more suitable for the strength analysis of rectangular and cylindrical HTS bulks.