Experimental Study Of The Mechanics Properties And Multi-field Behaviors Of YBCO High Temperature Superconducting Tapes Under Fatigue Loading And Low-temperature Environment

At present,high-temperature superconducting materials and related technologies play an important role in scientific research,transportation,electric power,new energy,biomedicine,aerospace and many other fields,and have a broad application prospect in these fields,which is inseparable from the excellent performance of high-temperature superconducting materials including the high critical current density,wide temperature margin,strong resistance to particle irradiation and a series of other excellent performance.In particular,in the large particle accelerators,where high magnetic fields are required,there have been a large number of superconducting magnets made of high-temperature superconducting materials which possess high critical current and could generate intense magnetic fields to meet the practical needs of these areas.However,in general,the technology and application of high-temperature superconducting materials is still in the stage of gradual exploration,and the relevant theoretical and experimental results are far from meeting the design requirements,which is mainly related to the extremely complex working conditions and service conditions of superconducting materials.In the case of large particle accelerators,first,superconducting materials and corresponding equipment need to operate under extremely low temperatures.Second,the high magnetic field inside the large particle accelerators usually generate strong electromagnetic force and the superconducting materials working inside the accelerators are subjected to different kinds of loadings and deformations.Besides,due to the need of controlling the deflection path of particles,the electric and magnetic fields inside large particle accelerators need to change their directions at high frequencies.So the superconducting materials are inevitably subjected to fatigue loadings produced by the rapidly changing electromagnetic fields.Although the strong Hastelloy substrate layer of the high temperature superconducting tapes could enhance the yield strength and tensile strength of the high temperature superconducting tapes,the interaction between the low temperature,fatigue loadings and electromagnetic fields will still lead to some changes in the basic physical parameters of the superconducting material.The engineering design only considering a single kind of loading can no longer meet the requirements of the actual working conditions.To promote better development of superconducting application,it is important to fully understand the fatigue properties of superconducting materials.In order to solve the above-mentioned problems in the practical application of high temperature superconducting materials,this article includes the following sapects:(1)In chapter 1,we introduce a cryogenic fatigue test facility for high temperature superconducting tapes.In the first part of this paper,the main ideas in developing the sub-systems of the fatigue testing device and the functions of each sub-system are elucidated in detail.At present,the assembly of the room temperature —77 K part of the fatigue testing device has been completed.The testing device has been used to conduct fatigue experiments with YBCO superconducting tapes and experiment results display that the main functional indicators of the testing device have met the design requirements.(2)In chapter 2,based on the fatigue test facility for high temperature superconducting tapes,the tensile experiments of YBCO high temperature superconducting tapes subjected to fatigue loads were carried out at room temperature and 77 K.The mechanical properties,such as yield strength and elastic modulus,of fatigued YBCO high temperature superconducting tapes were obtained.The relationship between the mechanical properties of YBCO superconducting tapes under fatigue loading and stress ratio was also investigated.The experimental results show that the mechanical properties of the fatigued YBCO tapes both at room temperature and 77 K are non-linearly dependent on their fatigue stress ratio.(3)In chapter 3,with the cryogenic fatigue test facility for high temperature superconducting tapes,the critical current of YBCO high temperature superconducting tapes after fatigue loadings was tested.We also conduct experiments to investigate the electromechanical behavior of YBCO high temperature superconducting tapes subjected to fatigue loading.The effect of fatigue loading on the current-carrying characteristics of YBCO high temperature superconducting strips is presented,which is of importance to the design of superconducting structures.(4)In chapter 4,based on the scanning electron microscope technique,the microstructural changes of YBCO superconducting materials after fatigue loading and their damage characteristics are studied from a microscopic perspective.The microstructure of the surface of the superconducting layer and the distribution of Ba,Ni have been observed in detail.We elucidate how the fatigue cycles effect the microstructure of the surface of the superconducting layer.Besides,we also attempt to fully understand the relationship between the size and morphology of cracks and defects on the surface of the superconducting layer and the macroscopic electromechanical properties of the superconducting tapes.