Study On Fabrication And Tensile Characteristics About Mechanical Joint Of High Temperature Superconducting Tapes

High temperature superconducting（HTS）coated tape using REBa₂Cu₃O₇（REBCO）as superconducting material has attracted more and more attention,due to its obvious advantages of high critical magnetic field strength,low operating cost and large elastic modulus.However,because of the limitation of production technology,it is difficult and expensive to manufacture tape with stable performance above 1000 meters,the joint is inevitable for the production of large magnets.Meanwhile,the joint is also the key for the segmented manufacture of large magnets.Mechanical joint is a very important connection mode with simple process,no welding defects and excellent mechanical characteristics.In this paper,the tape used to fabricate mechanical joint is REBCO tape from Super Power company and the model number is SCS4050.Firstly,the numerical simulation method was used to analyze the step of applying pressure in process of mechanical joint manufacturing,and it was found that the stress distribution of the joint was uneven when the pressure step was applied by bolt torque.Vacuum heat treatment could effectively improve the stress imbalance,thus reducing the resistance of the joint and improving the electrical properties of the joint.For the joint,it was necessary to consider not only the resistance of the joint,but also the AC loss of the joint under self-field and external field.Therefore,the AC loss of the joint in the self-field and the external field was explored.The results showed that the AC loss power increased with the gain of current and frequency,while the AC loss in one cycle decreased with the gain of frequency and increased with the gain of current.As the current amplitude and frequency increased,the ratio of resistance loss gradually decreased.When the mechanical joint was placed in perpendicular magnetic field,the trend of the AC loss of the joint was consistent with that of the self-field,and the AC loss increased with the increase of the perpendicular magnetic field.When the joint was placed in parallel magnetic field,the trend of the AC loss was consistent with that of the self-field,but the AC loss did not change significantly with the increase of the parallel magnetic field.Since superconducting joint was inevitably interfered by external magnetic field,it also need to keep good mechanical integrity.In this paper,the tensile and fatigue characteristics of the joint were studied experimentally.Firstly,we studied the monotone tensile properties of lap joints with and without jigs,and then the fatigue characteristics of lap joints under different cyclic loading conditions,such as the applied pressure in production process,stress amplitude,holding time and cyclic loading frequency,were studied.The monotonous experimental results showed that:（1）the critical current of the lap joint decreased gradually with the gain of tensile load in the first stage,and decreased sharply when the tensile value reached a certain value;At the same time,the joint resistance increased with the gain of tensile load after the permanent degradation of the critical current.（2）For mechanical joints with jig,when the stress exerted by jig on the joint was small,the interlayer metal was first damaged by shear,and then the tape terminal continued to produce tensile damage under the action of tensile force;When the stress exerted by the jig on the joint was large,the tape terminal was first destroyed by tensile,and the interlayer metal shear failure occured under the action of tensile force until the whole failure occurs.（3）The use of jig had advantages and disadvantages:the use of jig could maintain the stability of joint resistance,it might also cause stress concentration at the edge of the joint.Taking 5%critical current degradation as the joint failure criterion,the ultimate tensile force of the joint without jig was 195N at 77K,while that of the joint with jig was 170N,both of which were lower than the tensile limit of the tape.Cyclic loading tests showed that:（1）the lower the pressure applied during production,the shorter was the cycle life of the joint;（2）When the stress amplitude were larger,the joint resistance increased more obviously with the gain of loading cycles.（3）When the frequency was lower or the holding time under maximum stress was longer,the critical current degradation of the joint was more obvious,and the fatigue life of the joint was shorter.The research results of this paper have certain reference significance for joint design and system operation of large high temperature superconducting magnet.