Magnetic Cloak And Mechanical Properties Of High Temperature Superconducting Composite Structures

High temperature superconductors(HTSs) have important applications in the areas such as scientific research and engineering applications. The electromagnetic behaviors of the superconducting materials are complex due to the special characteristics which are different from the ordinary materials. On the one hand, it can be used as the electromagnetic shielding devices to achieve invisible in electromagnetic field because of the anti-magnetic properties. On the other hand, the HTSs suffer the electromagnetic body forces produced by the flux pinning during magnetization process which can lead to stress and deformation in superconductors. The HTSs are brittleness materials and have many defects so that it may result in the extending of crack and even structural damage due to the complicated electromagnetic body forces in strong magnetic field. Guaranteeing the safety and stability of the superconducting devices in electromagnetic field is always the long-term fundamental subjects during the study on the application of the superconductors. The thesis focuses on the study of magnetic cloak and electromagnetic behaviors as well as the fracture problem of the HTSs composite structures under magnetic and mechanical loads.Firstly, the magnetic cloak characteristics of the superconductor-ferromagnetic bilayer structure is studied. Supposing superconducting materials have completely anti-magnetic properties and based on the magnetomechanical coupling constitutive relationship of ferromagnetic materials. In order to achieve perfect cloak in homogeneous magnetic field the relationship of the permeability for ferromagnetic material which needs to meet is derived. Numerical simulation results show that the electromagnetic response of the superconductors can be changed by applying a shear stress only on the ferromagnetic material. The mechanical loads can affect the magnetic cloak performance of the structural in non-uniform magnetic field situation. The study of this section provides a new method to adjust superconductor electromagnetic behaviors by applying proper mechanical loads.Secondly, the magnetomechanical coupling constitutive relationship of ferromagnetic materials is considered. Based on the linear elastic piezomagnetic coupling model and anti-plane shear deformation, the effect of mechanical loads on the electromagnetic response of the superconductor-ferromagnetic structure is studied. By solving the magnetomechanical coupling equation the distribution of the magnetic field for ferromagnetic materials and the space around can be obtained under the mechanical loads. The distribution of current density and magnetic field are obtained based on the critical state Bean model. We further discuss the impact of shear stress on the magnetization of superconducting tape. Besides, we study the critical current density of the superconducting tape based on the critical state Kim model. It’s is discovered that the critical current density is closely related to the mechanical loads and the geometry by comparing different structures. For different superconductor-ferromagnetic structures, the critical current density can be improved with proper shear loads.Finally, the crack tip strain energy release rate of the stacked structure for the superconducting coated conductors with interface crack in magnetic field is studied. A classic interface crack problem is solved by the virtual crack closure technique(VCCT). The high precision proves that the method is reliable. The impact of crack on the induced current and the magnetic field is ignored. After setting up a stacked structure model of the superconducting coated conductors and based on the critical state Bean model the problem is studied with the Abaqus finite element software. The VCCT is used in post processing to calculate the strain energy release rate near the crack tip. Then we analyze the fracture behavior of stacked structure of the superconducting coated conductors and get the strain energy release rate near the tip of the interface crack in different magnetization processes, the crack lengths, locations of the crack and substrate thickness et al, so as to provide some references for the design of safety and stability of the structure.