The Analysis For Mechanical Behavior Of Superconductors With Defects

In recent years,due to the unique properties,high-temperature superconductors have been used to manufacture various components,such as frictionless bearings,motor components and magnetic separators.As in the process of manufacturing and using the high-temperature superconductors often produce the defects such as crack and hole et al.,and these defects will seriously affect the stability of the high-temperature superconductors,the crack problem of high-temperature superconductors(HTSs)have been a focus of research.On the basis of previous studies,the mechanical properties of high temperature superconductors with cracks and holes of different shapes under the electromagnetic force are investigated in this paper.The specific content is as follows:A square central crack problem is investigated for an infinitely long square cross section superconducting column under electromagnetic forces.The generalized Irie-Yamafuji critical current densities model and the magnetically impermeable crack surface condition are adopted for both the zero-field cooling(ZFC)and the field cooling(FC)activation processes.The distributions of magnetic field and current density are derived in the superconductor,where the crack affected region is introduced.Based on the finite element method(FEM),the stress intensity factors(SIFs)at the edge of square crack in the process of field descent are numerically calculated.Numerical results show that the SIFs at the corner points of square crack are generally larger than the values of other dots at the four edges of square crack in the process of magnetization.In the meantime,the FC process is usually easier to promote crack propagation than the ZFC activation.In addition,both the crack sizes and the introduced index ? in the generalized Irie-Yamafuji critical current density model have important and different effects on the crack growth in the ZFC and FC activation processes.This part investigates the flux-pinning-induced stress behaviors in a long superconducting slab with a central cuboid hole.The distribution of flux density is analytically derived for the critical current density of Kim model by considering the effects of the hole.The concentration of stress for both of the zero-field cooling(ZFC)and the field cooling(FC)magnetization processes are investigated through numerical simulation by using FEM.The results indicate the potential failure mode varies with the height-width ratio and the dimensions of the hole.Different to the case of infinite hole height,the stress concentration behavior for the FC process is more critical when the dimensions of the hole are similar.Finally,the stress of a cylindrical superconductor with double cylindrical holes under the electromagnetic force is studied.In the calculation process,the shielding effect of the hole on the current and magnetic field is considered,and the Kim critical current densities model is used outside the affected region of the hole.Firstly,the magnetic field distributions in the cylindrical superconductor under field cooling(FC)and zero field cooling(ZFC)magnetization processes are deduced.Then,the stress concentration coefficient of the cylindrical hole is calculated and analyzed by using the finite element method with the field reduced.The numerical results show that,similar to the central rectangular hole problem,the magnetization process of FC is more likely to cause the destruction of the superconductor.In addition,the external magnetic field,magnetization process,hole radius,hole aspect ratio,hole spacing and the coefficient p in Kim's critical model all have effects on the stress concentration factor.