The Analysis For Fracture Properties Of Superconductors With Different Cracks

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 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 achievements, the fractures of high-temperature superconductor with different crack models are investigated under electromagnetic forces. The specific content as follows:The central cross crack problem is investigated for an infinite long cylindrical high-temperature superconductor under electromagnetic forces. In both the Bean and the Kim critical state model, based on the finite element method, the stress intensity factors at the crack tips for decreasing magnetic fields are calculated. Numerical results obtained show that, the field cooling activation process has more significant influence on the stress intensity factors than the zero-field cooling activation process. And as the applied field decreases, the superconducting cylinder is most dangerous when the currents in the crack region are just influenced. Additionally, the dimensionless parameter p in the Kim model has different effects on the SIFs obtained from different external field(the most dangerous field and/or the trapped field).In the Kim critical state model and based on the finite element method, the fracture problem of a long cuboid superconducting slab with a central kinked crack is investigated. As the applied field decreases, the distributions of both the current density and the magnetic flux density in the slab are obtained for either the zero-field cooling or the field cooling magnetization processes. And at the same time, the stress intensity factors at the crack tips are calculated. Numerical results obtained show that the zero-field cooling activation process generally has more significant influence on the stress intensity factors than the field cooling activation process, and the superconducting slab is most dangerous when the currents in the crack region are just influenced. In general, both the maximal mode-I SIFs and mode-II SIFs decrease with the increasing of either the introduced dimensionless parameter p in the Kim model or the crack length. However, the effects of kinked angles on the SIFs are complex.The fracture problem of an infinite long superconducting cylinder with a penny-shaped crack is investigated under electromagnetic forces. The distributions of magnetic flux density in the superconducting cylinder are obtained analytically for both the ZFC and the FC activation processes, where the magnetically impermeable crack surface condition and the Bean model outside the crack region are adopted. Based on the finite element method, the stress intensity factor and energy release rate at the crack tips in the process of field descent are further numerically calculated. Numerical results obtained show that according to the maximal energy release rate criterion, the FC process is generally easier to enhance crack initiation and propagation than the ZFC activation process. On the other hand, for the FC activation process, the larger the maximal applied magnetic field, more likely the crack propagates. Additionally, crack size has important effects on the crack extension.Based on the problem of penny-shaped crack, the fracture problem of an infinite long superconducting cylinder with an inner annular crack is further investigated under electromagnetic forces. Based on the finite element method, and the magnetically impermeable crack surface condition and the Bean model outside the crack region are adopted, the stress intensity factors at both the inner and outer crack tips are calculated. Numerical results show that in the process of field ascent, the state corresponding to the applied maximal magnetic is the most dangerous, and the direction of the crack propagating depends on the crack size. In the process of field descent, for the FC case, the crack always propagates and grows from the inner crack tip. However, for the ZFC case, the fracture properties of the superconductor are much complicated.