Theoretical Investigations On Fracture Behavior In Bulk Superconductors And Composite Wires Based On Peridynamics

The superconductors with excellent electromagnetic properties such as zeroresistance have a wide industrial application prospects,such as electric energy transmission,thermonuclear reactor and so on.Superconducting materials and large structures operate in extremely multi-field environments such as low temperature,large current and high magnetic field during long-term service,which are subjected to huge electromagnetic force and thermal stress,and large mechanical loads can easily cause cracking or even destruction of superconductors.It brings great safety risk in the magnet structure.Meanwhile,the local properties of materials exhibit a high nonuniformity,which leads to complex mechanical behavior under multi-fields.Therefore,the mechanical properties of superconducting materials have become the key problems for the stable and safe operation of superconducting structures.The description of fracture evolution and contact of superconducting structure in electromagnetic-thermal coupling fields is the key factors in superconducting applications.This dissertation is mainly based on the peridynamic theory to study the mechanical and electromagnetic behaviors of bulk superconductors and composite wires.The deformation and damage of bulk superconductors during the pulse field magnetization are discussed.The crack propagation inside the composite wires,the contact characteristics and performance degradation under the external magnetic field are investigated.Firstly,the fracture processes of bulk superconductors are simulated by the combination of H-formulation and peridynamic approach,the effects of voids,cross cracks and multiple inclusions are discussed on the crack propagation path.A finite element model based on the H-formulation is used to study the electromagnetic force in bulk GdBCO including defects under a pulsed field.Strong electromagnetic force concentration occurs in the area near the crack tip.The dynamic mechanical behavior and brittle fracture of bulk superconductors are analyzed by the bond-based peridynamic approach.The displacement distributions are obtained during the magnetization.Meanwhile,the crack initiation and propagation paths can also be predicted using peridynamic theory.Secondly,the fracture behavior of bulk superconductors in the electromagneticthermal coupled field is studied with considering the heat transfer under pulsed field magnetization.An electromagnetic-thermal model is established based on the Hformulation and heat transfer equation to obtain the electromagnetic force and thermal stress in the bulk with defects.Based on the ordinary state-based peridynamic approach and J-integral in fracture mechanics,the variation law of stress intensity factor in magnetization process is analyzed,and the crack propagation path is also predicted.The influences of eccentric crack,stainless steel ring,void,and inclusion on the mechanical stability of the bulk superconductors are discussed.Thirdly,a local damping is introduced into the ordinary state-based peridynamic equation of motion to study the bending deformation and fracture behavior of strands in Rutherford cable during quasi-static loading.The stress intensity factor of mixedmode crack is solved based on the interaction integral method.The numerical results of quasi-static problem are compared with the results of existing models to verify the accuracy.The crack propagation path under quasi-static problem is also verified.The displacement and stress distributions of stand during bending are obtained.The dynamic stress intensity factor of the mixed-mode crack tip is calculated,the crack initiation and propagation path is predicted,and the distribution of damage is simulated when the strand is subjected to a transverse compression force.Finally,the performance degradation of strands in CICC under transverse electromagnetic force is analyzed.According to the special structure of CICC,a 3D helix model based on the TEMLOP and FEMCAM is used to calculate the accumulated contact force among the strands.The maximum elongation linear strain and indentation depth induced by contact stress are calculated with two-dimensional incomplete contact model of cylinder.The effects of layers and cross angles on the indentation depth are discussed.The performance degradation caused by the indentation depth is discussed by using the relevant data of literatures.