Theoretical Study On Damage And Fracture Behavior Of High Temperature Superconducting Bulks

The second-generation high-temperature superconducting bulks have received widespread attention and application in various fields such as medical,national defense,and transportation because of their excellent and unique electromagnetic properties.However,cracks and defects will inevitably appear inside the superconducting bulks when bulks are prepared and operated.On other hand,superconducting bulks often undergo irreversible damage and even overall fracture failure in extreme high magnetic fields and extremely low temperatures environments.Therefore,the study of damage and fracture behavior of high-temperature superconducting bulks is of great significance for their engineering applications,which can improve the mechanical reliability and operational stability of superconducting bulks.This article carries out numerical modeling and research on the mechanical problems of superconducting bulks,and the main research content is as follows:First of all,the magnetic field and the radial stress distributions are calculated in the multi-seeded superconducting bulks,which are used to fabricate large size hightemperature superconductors in engineering.The results show that whether it is the two seeds bulk or four seeds bulk,the damage is most likely to occur around the seed.Then,we discuss the effect of different seed spacings and find that the seed spacing has a small impact on the central magnetic field and the maximum magnetic field of the bulk.The key point of manufacturing high-performance multi-seeded superconducting bulks in engineering is ensuring that the angle of each seed crystal does not deviate during preparation.Subsequently,based on the phase field method,we construct a electromechanical coupling model for superconducting bulks during magnetization,and calculate the fracture behavior of single domain,double seeds,and four seeds YBCO hightemperature superconducting bulks under large electromagnetic forces,as well as the influence of accumulated damage on the electromagnetic performance degradation of the bulks.The results show that the coupled model can describe the nucleation and propagation of cracks in the bulk during magnetization well,especially for the merging and branching of multiple cracks under complex conditions.Finally,this thesis studies the protective effect of stainless steel rings on superconducting bulks,which shows that protective sleeves can effectively reduce the radial stress during magnetization,thereby effectively reduce the speed of crack propagation and suppress multiple bifurcation behavior.