Numerical Simulation Study Of Delamination Behavior Of HTS Tape,Its Joints And Coil Structures

Rare-earth-barium-copper-oxide(REBCO)high temperature superconducting(HTS)tapes have the advantages of high critical temperature and high critical current,and they have been widely applied to the fabrication of various superconducting electromagnetic devices such as superconducting motor,superconducting cable and future fusion magnets.Due to multi-layer composite structure,delamination is the main failure mode of HTS tape and its soldering joints under loading in practical applications.For superconducting electromagnetic devices,the delaminations in HTS tapes and their solder joints not only destroy the structural integrity and thus affect the mechanical properties,but also lead to significant degradation of electrical properties such as decrease of critical current and increase of joint resistance.Therefore,the research on the delamination behavior of HTS tapes and their joints is the basis for the functional design of superconducting magnet devices in extreme environments.The numerical simulation of corresponding delamination behavior can effectively reveal the mechanism of mechanical and electrical failure of superconducting materials and structures,and further optimize the superconducting structure.Therefore,in this dissertation,we systematically study the debonding behavior from single tape to related joints and coils.Firstly,the finite element numerical simulation is carried out for the anvil test measuring the transverse tensile strength of HTS tapes,which is closely related to the delamination.On this basis,the finite element numerical analysis is carried out on the tensile and bending tests of various types of joints welded by superconducting tapes,and the electrical performance degradation and debonding behavior are also studied.Finally,the delamination behavior of epoxy impregnated REBCO pancake coils considering thermal-mechanical coupling effect is numerically analyzed.The main contents and conclusions of this dissertation are as follows:(1)For delamination of HTS tapes,finite element models for transverse tensile strength test of HTS tape are established.The complete failure process of HTS tapes in anvil test is simulated by combining the continuous damage mechanical model and the cohesion model.The numerical results show that the tapes will first undergo delamination under the transverse load.If the load is increased,the copper layer around the anvil will suffer shear damage due to the localization of the shear band.In addition,the bending of HTS tapes along different shapes of formers is numerically simulated,and the bending strain of superconducting layers in the tapes is very close to the theoretical values.(2)Two-dimensional and three-dimensional finite element models are established for different configurations of the solder joints of HTS tapes: single-lap joint and single bridge joint.Firstly,the axial tension of single lap joint and single bridge joint is numerically simulated by using the established two-dimensional and three-dimensional finite element models.When the single lap joint is subjected to axial tensile load,both the force-displacement curve and the stress and strain distribution obtained by the finite element model are in good agreement with the existing experimental measurements or numerical simulations.Through finite element analysis,it is found that the axial tensile strength of single bridge joint can be improved by using the mixed solder.Secondly,two different bending loading methods are used in the established two-dimensional and three-dimensional finite element models to numerically simulate the bending of single-lap joints,and the influence of these methods is also compared.In addition,the three-dimensional finite element model is used to numerically simulate the bending of single-bridge and multi-bridge joints of superconducting tapes,and the influence of bending radius is discussed.In numerical simulations,the strain distribution of superconducting layer during bending process can be easily obtained through shell element.Moreover,the delamination behavior of various types of joints during bending can be effectively captured by cohesive elements.(3)Based on the reasonable simplification of HTS tapes,the finite element model of epoxy impregnated REBCO pancake coils is established.Firstly,the equivalent thermal conductivity of the superconducting tape is calculated and the radial stress distribution of the coil after cooling is obtained.The comparison with the existing numerical results verifies the effectiveness of the simplified model.Then,the multi-layer zero-thickness thermal-mechanical coupling cohesive element,as a user-defined element in ABAQUS,are inserted inside HTS tapes in the two-dimensional finite element model of the coil to determine the debonding sites during the cooling process.Considering the Weibull distribution of the cohesive strength of the cohesive element,the non-uniform damage characteristics of HTS tape along the length direction are simulated.In addition,the local heat generation in the damage region of the superconducting coil under overcurrent is further analyzed.The numerical results show that delamination leads to local higher temperature rise and the change of heat flux vector field in the coil.The copper bobbin inside the coil has a significant impact on the damage of the HTS tapes during the cooling process;the damage occurs in the edge region along the width direction,which is consistent with the reported experimental observations.In addition,compared with other turns in the coil,the innermost HTS tape in the coil model with bobbin will be more seriously damaged due to the shear effect.