Investigations On Fracture Behaviors Of Thin Superconducting Film Subjected To Electro-Magnetic Forces

As a new sort of smart materials, superconducting materials have been widely applied in electric power technology, communication technology, magnetic levitation devices, as well as other advanced smart structures. Since high temperature superconductors(HTSs) are susceptible to mechanical failure due to their brittleness and drawbacks, it is necessary to investigate the fracture behavior of superconductors, especially the superconducting films. This dissertation presents some analysis of fracture behavior of superconducting films subjected to electromagnetic force. The stress intensity factors(SIFs) and/or energy density factors(EDFs) are analytically obtained and numerically calculated. Prediction for the propagation of crack is made based on numerical results. Work of the author is as follows:Double exponential model is established to investigate the central crack problem for a functionally graded superconducting film with filed dependent critical current. The stress intensity factors(SIFs) are analytically obtained and numerically calculated. Numerical results show the effects of applied magnetic field, model parameters, and crack length on the SIFs. Among others, in the process of field descent, the crack in the superconducting film easily propagates in the mode-I form. Increasing the graded parameter of shear modulus can inhibit crack propagation. For a fixed reduced field(especially for a larger magnetic field), both the mode-I and mode-II SIFs firstly increase, then decrease with the increasing of introduced non-dimensional exponent parameter. This study should be useful for the application of superconducting devices.The strain energy density theory is used to investigate a central crack problem for a functionally graded superconducting film with the applied transport currents, where the Kim critical state model is adopted and the shear modulus is assumed to vary along the film’s width in a form of hyperbolic function. The flux and current densities, the stress intensity factors(SIFs) and energy density factors(EDFs) are all analytically obtained. Numerical results show the effects of a pplied transport currents, model parameters, and crack length on the EDFs and/or SIFs. Among others, in the process of descending transport current, increasing the graded parameter of shear modulus can inhibit crack propagation, and in general, the crack will propagate and grow into the field of shear modulus decreasing. Moreover, the fracture angle is independent of the applied transport currents, and the absolute-valued fracture angle generally increases slightly with either the increasing of material gra ded parameter or the increasing of crack length. This study should be useful for the application of superconducting devices.The finite element method(FEM) is applied to investigate the fracture behavior of superconducting films with rhombus cross section. Based on the Bean model, the distributions of current density and flux density are obtained analytically. Numerical results show the effects of crack length, thickness of the film and the magnitude of applied transport current on the stress intensity factors(SIFs). For a fixed reduced field, the SIFs increase with the increasing of crack length and maximum applied field, whilst decrease with the increasing of thickness of the film. This study should be useful for the application of superconducting devices.