Research On Stress Distribution And Fracture Behavior Of Cylindrical High Temperature Superconductors

High-temperature superconductors(HTSs)are well known to hold large current densities and trap strong magnetic fields.and have been widely used in engineering fields such as: superconducting transformers,magneto-fluid generators,nuclear resonance spectrometers,magnetic energy storage,superconducting quantum interferometers.Since the high-temperature superconducting material is brittle ceramic material,defects such as inclusions,voids,and dislocations are inevitably present during the productive process,which reduce the use performance of the superconductor in the practical application process.Therefore,the mechanical properties of HTSs in the activation process have always been the focus of researchers.This dissertation is mainly based on the non-ideal second-type high-temperature superconducting bulk materials,the mechanical stability and crack propagation of critical current density homogeneous or inhomogeneous HTSs in the descending process are theoretically studied.And metal tube can improve the safety performance of the hollow cylindrical superconductor or not was also discussed in detail.Firstly,the analytic expression of stress distribution under electromagnetic force was deduced.the distribution of the magnetic flux density when the critical current density of the exponential model is uniform.According to the field-decreasing stage of the zero-field cooling and field-cooling process,the stress distribution is obtained.By comparing the critical state Bean and Kim model,the stress distribution of the exponential critical state model is different from other models.It is a good explanation that during the experimental process,the magnetic flux pinning force is increasing and then reducing during the drop process,which affect the stress distribution in the superconductor.Inhomogeneous exponential model of the critical current density along the radius of the HTSs is considered,and the influence of the critical current inhomogeneous on the stress distribution in the magnetization process of the HTSs is analyzed.The results show that the difference in inhomogeneous parameters has a significant influence on the distribution of magnetic flux density,which will lead to different stress distribution in the magnetization process.Secondly,the fracture behavior of a long cylindrical superconductor with a central penetrating crack is studied based on the exponential critical state model.the finite element integral is used to solve the stress intensity factor at the crack tip.The influence of different magnetization processes,external magnetic field and crack length on the stress intensity factor of a long cylindrical superconductors with central penetrating cracks under the exponential critical state model is discussed and compared with the critical state Bean model and the Kim model.And the difference in the stress intensity factor at the crack tip of the exponential model is analyzed.In order to fully discuss the effect of electromagnetic force on the cracking of superconductors,an inhomogeneous model of exponential distribution of critical-current density along the radius is introduced.The influence of non-uniform parameters on the crack stress intensity factor in the superconductor during magnetization is analyzed,which has certain significance for the safety of HTSs.Finally,the stress distribution of the hollow cylindrical HTSs restricted by metal tube in the magnetization process is discussed.Using the plane strain method,the expression of the stress distribution inside the superconductor and the contact stress expression between the metal tube and the superconductor are solved.Based on the field cooling magnetic flux density distribution of the exponential critical state model,the stress distribution in the superconductor during the magnetization process is obtained.The data shows that the Young's modulus ratio and thickness ratio of HTS and metal tube greatly affect the stress distribution in the superconductor.By comparing the hollow superconductors without metal tube restricted under the same magnetization conditions,the tensile stress of the hollow cylindrical superconductors constrained by the metal tube is significantly reduced.Therefore,choosing the appropriate material parameters and the size of the metal tube will improve the mechanical properties of superconductors.