Study On Delamination And Crack Propagation Of High Temperature Superconducting Coated Conductors

Due to its high critical current density,high magnetic field and wide application range of temperature region,Rare earth barium copper oxide(REBCO)second-generation high temperature superconducting(2G HTS)coated conductor(CC)has a wide application prospect in superconducting cable,superconducting high field magnet,nuclear magnetic resonance and magnetic levitation technology.However,due to the unique laminated composite structure and the ceramic brittleness of the superconducting layer itself,superconducting tapes are prone to debonding and even cracks in the superconducting layer under the effects of electromagnetic force and thermal stress in extremely complex application environments,resulting in a significant degradation of the critical current and seriously destroying the safety of the superconducting device.Based on this,this paper combines the delamination and fracture behavior of superconducting tapes to carry out the following research.Firstly,focusing on the delamination problem of REBCO superconducting tapes,a finite element model(FEM)of tape delamination based on the bilinear cohesive zone model(CZM)is established.The delamination behaviors of tapes under transverse stretching,splitting and axial stretching-torsional action were simulated respectively.The calculation results show that,in the case of transverse tension,choosing a zero-thickness cohesive layer can improve the calculation efficiency and convergence.The cohesive shapes have not large effects on the load responses,expect that the bilinear CZM shows better convergence than the exponential CZM.For delamination under cleavage force,the effects of different tape widths and thicknesses of copper layer and Hastelloy layer on cleavage strength are analyzed.The results show that increasing the tape width and increasing the thickness of the copper layer can improve the cleavage strength,and 20-30 ?m thick Hastelloy layer also can improve the cleavage strength,but the effect is not obvious.For the delamination of the tape under the combined action of tension and torsion,it is found that the superconducting tape is prone to delamination under the action of torsion,and the damage degree of twisting and then stretching is greater than that of twisting,while the tapes that are stretched and then twisted do not delaminate.Secondly,aiming at the problems of delamination of REBCO superconducting tape and crack propagation of superconducting layer,a mixed model of interlayer debonding and superconducting layer crack propagation based on bilinear cohesive zone model(CZM)and extended finite element method(XFEM)is established.The delamination process and crack propagation behavior of the tape under the action of axial tension and torsion are analyzed respectively.In the case of axial stretching,when the tensile displacement is within a certain range,the tape will not delaminate,but the crack will rapidly expand until it penetrates the superconducting layer.With the gradual increase of the thickness of the copper layer,the crack propagation can be suppressed to a certain extent.For the torsion case,the effects of different prefabricated crack lengths,prefabricated crack angles and Hastelloy layer thicknesses on delamination and crack propagation are analyzed.The results show that delamination is observed at the edge of the tape and the crack through the superconducting layer.The crack is in the form of a curve.As the length of the prefabricated crack increases gradually,the total distance of the crack propagation in the superconducting layer increases correspondingly,and the superconducting layer is more difficult to fracture completely.With the gradual increase of the prefabricated crack angle,the superconducting layer is easier to break.As the thickness of the Hastelloy layer gradually increases,the superconducting layer is more difficult to fracture.Under the combined action of axial tension and torsion,delamination and crack propagation are mutually promoted,and the superconducting layer is more likely to be completely fractured.