Theoretical Investigations On Fracture Behavior Of High-Temperature Bulk Superconductors Subjected To Electro-magnetic Forces

It has been known that some fracture or crack is induced, which plays an important role in the safety design when a bulk superconductor is used in practice. In practical applications, the superconductor is often subjected to several forces like electromagnetic force, thermal stress, which maybe lead to fracture of superconductors. This dissertation presents some analysis of fracture behavior of the superconductors subjected to the environment of electromagnetic fields and low temperature, where the bulk superconductors have either infinite or finite size.Firstly, the crack problem of the infinite superconductor is considered in the process of zero-field cooling (ZFC) and field cooling (FC). The superconductor contains three type of cracks, namely, Mode-â… , Mode-â…¡and mixed Mode. The fracture behaviors change with the magnetic field when the applied field is decreased. Both the stress intensity factors and J integral can be used to describe the trend of crack growth.Next, the fracture behaviors of finite superconductors are simulated under electromagnetic force by using the numerical method. A model for the calculation of the fracture behavior of finite type-â…¡superconductors is presented. The demagnetization effect is considered by using a simple approximation of a constant demagnetization factor. The stress intensity factors of finite superconductors are calculated in the process of zero-field cooling (ZFC) and field cooling (FC). Numerical results obtained show that the stress intensity factors increase with the increase of applied field. The effects of the susceptibility and ratio of length to diameter are presented. From the results, the stress intensity factors increase with the decrease of applied field, comparing to the Mode-â…¡, a larger change is found in the stress intensity factors of the Mode-â… , these results display that the crack growth is the Mode-â… fracture mainly.Finally, the fracture behaviors of infinite superconductors are displayed in the process of pulse magnetic field and temperature change. The fracture trend is predicted in pulse field magnetization. The lower the magnetic field is, the larger the stress intensity factors. When pulse magnetic field is maximum, the superconductor subject to maximal pressure so that the stress intensity factors have minimum value. The change of stress intensity factors is faster for the larger pulse field. Additional, the effects of temperature are considered. A theoretical analysis is used to investigate the fracture behavior of a large single domain YBCO superconductor under thermal stress based on the two-dimensional theory of anisotropic thermoelasticity. The thermal stress intensity factors are obtained due to a uniform heat flux by a line crack in a generally half plane superconductor. It is found that the thermal stress intensity factors decrease with the decrease of temperature, and while the longer the crack length is, the larger the stress intensity factors. Furthermore, the J integral at the crack tip is also investigated, a similar behavior to the thermal stress intensity factors is found. These results are benefit for us to understand the fracture mechanism of superconductor both in theory and application.