Theoretical Research On Mechanical Properties Of High-Temperature Bulk Superconductors

The friability of the high-temperature superconductors (HTS) limits their broad applications in engineering. Therefore, there are many important mechanical problems need to be solved in HTS' practical application. Based on the mechanism of electromagnetic force in HTS, the distributions of stress and displacement in a superconductor with a central elliptic hole are investigated in this dissertation. As an important aspect of the studies of mechanical properties, the crack problems of the superconductor subjected to the electromagnetic force were investigated.Firstly, based on Bean model of critical state and the finite element method, the distributions of stress and displacement in a superconductor with different elliptic hole are investigated. The effects of the ratio of the major axis and the minor axis on the distributions of stress and displacement in superconductor are discussed, and the effects of different conditions of magnetic field on the distributions of stress and displacement are analyzed. Numerical results obtained show that due to the effect of stress concentration, as the ratio of the major axis and the minor axis becomes large, the radial stress and the hoop stress in superconductor just increases significantly in the vicinity of the hole. However, the displacement changes very little all along. As magnetic field becomes small, the maximum of radial stress and displacement increase first and then decrease, but the maximum of hoop stress increases gradually.Secondly, based on Kim model of critical state, the finite element method and different values of the dimensionless parameter p, the distributions of stress and displacement in the same superconductor are investigated. The effects of the parameter p on the distributions of stress and displacement in superconductor are discussed, and the results obtained from two different models of critical state are compared. In Kim model, when the parameter p is biggish (larger than 0.1), the radial stress, hoop stress and displacement in superconductor all increase with increasing p , and all increase significantly away from the hole. However, when the parameter p is lesser (less than 0.1), the effects of p on the stress and displacement results are very small. And the results of Kim model when the parameter p is lesser are very close to the results of Bean model.Finally, based on the process of zero-field cooling (ZFC) the assumption that the crack forms a perfect barrier to the flow of current, the stress intensity factor in a long cylindrical superconductor containing an internal crack are investigated for both Bean and Kim model. The complex potential method and the boundary collocation method are applied in the calculation. When the superconductor subjected synchronously to tensile and compressive stress, the stress intensity factor first increases with the increase in the crack length, and then decreases after it reaches a certain peak value. This is due to the fact that, the effect of the crack dominates the stress intensity factor as the crack is small, and them the resultant force decided by the crack length and the magnetic field exerts a significant influence on the stress intensity factor when the crack becomes longer. When the superconductor subjected merely to tensile stress, the stress intensity factor decreases when increasing the crack length because the resultant force always has dominant effect on the stress intensity factor. The trend of variation in the stress intensity factor obtained from Bean and Kim model are generally the same. It is worth pointing out that the variation in the stress intensity factor with respect to the crack length is relatively small, whereas the magnetic field exerts a significant influence on the stress intensity factor.