Mechanical Behavior Of CORC Winding Process And Investigations On Mechanical-Electro Properties Under Multi-field Conditions

The second-generation high-temperature-superconducting(HTS)tape will gradually replace traditional low-temperature-superconducting(LTS)materials such as Nb Ti and Nb₃Sn to produce HTS magnets due to its high current carrying,high magnetic field producing and low cooling cost.General large superconducting magnets,such as thermonuclear fusion magnets and large particle accelerator magnets require far more current than a single HTS tape,so it is necessary to make the HTS tape into coils or cables.CORC cables are the most potential because of their advantages of simple cabling process,high current-carrying capacity and low AC loss.In the real winding process,the cooling process and the running process,the CORC cable is inevitably affected by mechanical and electrical deformation,thus resulting in transport performance degradation,mechanical structure damage and even superconducting quench,these problems seriously influence the safety of HTS magnet equipment and hindered the CORC cable application and development.Therefore,it is essential to study the mechanical behavior of the CORC cable during the winding process and the mechanical-electro properties under multi-field conditions.Based on this,from theoretical analysis and numerical calculation,the mechanical and electrical behavior of the winding process and stretching process of CORC cable are systematically studied in this paper.Firstly,based on the cylindrical shell theory,a winding mechanical model for the Pancake coiling and CORC cabling of the HTS tape was established.This model takes into account the relaxation phenomenon caused by the bending stiffness of the HTS tape and the anticlastic deformation caused by the Poisson effect during the winding process.From this model,it is pointed out that the Re BCO layer in the HTS tape should face the winding core.And the formula for calculating the minimum pre-tension force to ensure winding success is given.The axial strain of the Re BCO layer and the contact force between the tape and the winding core is calculated.State the three kinds of contact form between the tape and the winding core with different pre-tension forces and different the radius of the winding core.An analytical formula for calculating the characteristic distance of the"ear"shape near the tape edge is presented.Finally,the interaction and axial strain level between layers of multi-layer Pancake coil and multi-layer CORC cable are calculated based on the compression cylinder theory.Secondly,considering the nonlinear and complex contact form caused by anticlastic deformation in the real Pancake coiling and CORC cabling,it is difficult to find analytical solutions for theoretical models.Therefore,the three-dimensional dynamic winding model of Pancake coiling and CORC cabling was established by the finite element method(FEM).Then,the axial strain distribution of the Re BCO layer in tapes and the nonlinear contact form between the tape and the winding core were studied in detail.On this basis,a tape winding experimental platform is built,the experiments of different pre-tension forces and different radius of winding core were carried out,and the winding mechanics theory and numerical model is verified.It was proved that in the process of winding,if the winding pre-tension is insufficient,the relaxation will inevitably occur,and the relaxation on both sides of the tape is different in the process of winding CORC cable.In addition,the winding process of multi-layer Pancake coil and multi-layer CORC cable are established by the FEM model.The comparison results show that the theoretical model,the FEM model and the experimental results are in agreement.The results show that:the axial strain distribution of the Re BCO layer in the tape after the winding process is directly affected by the relaxation effect and the Poisson effect.Different winding parameters(winding pre-tension force,the radius of the winding core and winding helical angle)will have an obvious influence on the strain level of the Re BCO layer and the contact form between the tape and core after winding.Thirdly,the theoretical model of CORC cables is established to calculate the tape strain level under axial tensile load,and the FEM model of single and multi-layer cables is established based on the geometric structure of real CORC cables.The strain level of the Re BCO layer under axial tensile load and the normal contact stress and tangential friction stress between the tape and winding core of single-layer CORC cable with different winding parameters are studied.The irreversible tensile strain of CORC cable is given when the winding helical angle and Poisson's ratio of core change.Then,the mechanical behavior of the two-layer CORC cable when the winding helical angle of the inner and outer tapes are changed is calculated.And explains the reason why the critical current of the seventh layer tapes degenerates most obviously for the CORC cable made of 12 layers and 30 HTS tapes produced by ACT Company under axial tensile load.The most obvious reason for the degradation of the critical characteristics of the seventh layer HTS tape is that the helical angle of the seventh layer is too large on the one hand,and the helical angle of the sixth layer to the seventh layer jumps on the other hand.By comparing the force-strain curve calculated by the FEM model with the experimental results,it is shown that the initial contact from the winding process is the reason for the yield limit increase in the experimental results.The optimization CORC cabling scheme is given through theoretical analysis,and the degradation of critical characteristics of CORC cable under axial tensile load is compared before and after optimization.It shows that the optimized CORC cable can still maintain excellent current transport capacity under extremely high axial tensile strain based on analyzed in this paper.Finally,by combining the mechanics module and the T-A method module in COMSOL software,a set of three-dimensional FEM models is established for analyzing the electromagnetic degradation of CORC cable in mechanic-electromagnetic-magnetic multi-field conduction.Based on considering the cabling parameters,the strain level of the Re BCO layer and the layer interaction,the T-A method was first used to simulate the electromagnetic characteristics(critical current degradation,current distribution and hysteresis loss)after mechanical loading(axial tensile load,bending process,transverse load).This model can provide a theoretical basis and technical support for the optimization design of CORC HTS cabling.