Study On Delamination Behavior Of The REBCO Tape And Joint Under Transverse Load

The second-generation high temperature superconducting(HTS)tape has the advantages of higher critical temperature,critical current and critical magnetic field,which makes it widely concerned in the application of superconducting technology.The lap joint of REBCO(RE rare earth elements and Y)coated conductors plays a vital role in the application of high temperature superconducting tapes,and changes in its mechanical and electromagnetic properties will affect the stable operation of the entire superconducting device.For example,in the epoxy-impregnated REBCO superconducting coil,the factors that may produce a large transverse tensile force include the mechanical force during winding,the temperature stress during cooling,and the Lorentz force.These forces can cause delamination of the superconducting coil.Hence,in the application of second-generation high temperature superconducting tapes,delamination will frequently occur at the joints as a representative phenomenon.Studying the delamination behavior of REBCO high-temperature superconducting tapes and lap joints under transverse load can provide theoretical guidance for the design and application of the tapes and joints.In this paper,a transverse tensile experiment of REBCO high-temperature superconducting tapes and lap joints was carried out.Combined with the experiment,a 2D finite element numerical model of REBCO high-temperature superconducting tape and its lap joint based on the bilinear cohesion criterion was established by COMSOL.This model was used to study the stress distribution and interfacial delamination behavior of soldered lap joints between REBCO high temperature superconducting tape under transverse load.First,in order to calculate the equivalent delamination strength and penalty stiffness of the Gd BCO high-temperature superconducting tapes and joints,transverse tensile experiments were carried out by using the anvil tensile test method.Through the force-displacement curve of the sample recorded in the experiment,the required value can be calculated.After that,the Energy Dispersive Spectrometer(EDS)and Scanning Electron Microscope(SEM)were used to study the specific location and situation of the delamination interface.Through morphology and element analysis,it can be determined that the delamination of REBCO coated conductors mainly occurs at the interface between the superconducting layer and the buffer layer and was accompanied by the brittle fracture of the superconducting layer.Cohesive layers and their main parameters were set at the superconducting tape and solder according to the results of the tensile experiment when establishing the finite element model of REBCO superconducting tape and joints.The comparison between the calculated results and the experimental results showed that the finite element model constructed in this paper was in good agreement with the experimental results and can accurately simulate the delamination behavior of superconducting tapes and joints.On this basis,the model was used to simulate the delamination behavior of lap joints under different welding strength and tape damage degree when subjected to transverse loads.The simulation results showed that when the welding strength of the REBCO high-temperature lap joint was less than the delamination strength of tape,the delamination of the joint layer occurred in the welding position,and the overall delamination strength increased with the increase of the welding strength.When the welding strength was greater than delamination strength of the tapes,the delamination mainly occurred in the upper superconducting tape,and the overall delamination strength did not increase either.Meanwhile,when the damage degree was small,the welding site was delaminated,and the overall delamination strength of the joint remained the same,and the crack initiation displacement increases as the damage increases.When the damage inside the tape exceeded a certain value,the position of the delamination layer of the lap joint was transferred to the inside of the tape.After that,the delamination strength decreased with the increase of the damage,and the crack initiation displacement of the joint reached the maximum at this time.And it can be judged that the initiation position of the interface delamination was at the edge of the load through the interface damage and stress distribution of the joint during delamination.