Researches On Crack Problems In High Temperature Superconducting Bulk Based On Extended Finite Element Method

High-temperature superconducting bulks have been widely used in medical treatment,electric power,environmental protection,transportation and other fields owing to their high critical current density and the ability to generate a magnetic field much higher than that of conventional permanent magnets.However,the bulks face key mechanical difficulties and challenges in practical applications.The cracks,holes and inclusions will appear during the fabrication of bulk superconductors,and under the combined action of the electromagnetic force and thermal strain during the high-field magnetization process,the cracks inside the bulk will propagate,which directly affects the stability and reliability of the bulk superconductor in stable operation.Therefore,the study of fracture behavior in HTS bulk is of great significance for its engineering application,which can effectively reveal the key factors affecting the failure of bulk superconductor,and provide guidance for the preparation of bulk superconductor.In this dissertation,a numerical study on the fracture behavior of HTS bulk is carried out aiming at the above-mentioned key mechanical problems.The main contents are as follows:Firstly,based on the Maxwell's equations solved with the finite difference method,the magnetic field and electromagnetic force of a rectangular bulk superconductor during field-cooling magnetization are calculated.A computational model of stress intensity factor(SIF)for cracked HTS bulk is established by employing extended finite element method(XFEM).The effects of crack length and angle on the stress intensity factor under the action of electromagnetic force are calculated for the bulks with different critical current densities and sizes by the interaction integral method involving body force.In addition,the maximum hoop stress criterion is used to simulate the crack propagation.Secondly,the dynamic stress intensity factors(DSIFs)variation of cracks in bulk superconductor during pulsed field magnetization(PFM)is studied.Due to the short duration of the PFM process and the large temperature rise inside the bulk induced by the drastic change of the magnetic field during the magnetization process,it is necessary to consider the thermal strain during the magnetization process.A numerical model of DSIF of a cracked bulk superconductor under the combined action of electromagnetic force and thermal strain is established by interaction integration method involving thermal strain and body force.Then,the different peak magnetic fields during single and multiple pulsed field magnetization in the circular bulk superconductor,and the DSIFs of different cracks in the bulk superconductor are simulated.Finally,in view of the influence of cracks and holes in the bulk on its internal magnetic field distribution,a method for detecting internal cracks in bulk superconductors based on magnetic field is designed with the genetic algorithm(GA).Numerical results show that edged cracks and central oblique cracks inside the bulk can be effectively detected using the magnetic field detection method.In order to improve the detection effectiveness,the electromagnetic force inside the bulk is derived from the obtained magnetic field.The displacement distribution of the bulk is calculated with the XFEM,therfore a model for crack detection based on the displacement distribution is given.By comparing the detection methods based on the distribution of magnetic field and displacement respectively,it is found that the displacement detection can effectively detect the horizontal cracks inside the bulk.