Analysis Of Mechanical Behavior Of Nb₃Sn Superconducting Strands

Because of their high critical magnetic field and unimpeded current-carrying properties,superconducting materials have been highly concerned since the discovery and are widely used in many fields including energy.The core device of the International Thermonuclear Experimental Reactor consists of a large number of CICC superconductors.The CICC is a composite structure obtained by twisting multiple-stages Nb₃Sn superconducting strands,the scale of which ranges from a micron-scale superconducting filament to a millimeter-order superconducting strand,and then from a strand to a macro scale CICC conductor,which makes the structure of the CICC superconductor quite complicated.At the same time,the working environment of the CICC conductor,which includes extremely low temperature,high current and high magnetic field,inevitably leads to the coupling between the mechanical deformation and the physical fields.Moreover,Nb₃Sn is a typical brittle material and the superconducting critical properties are very sensitive to strain.However,it is unavoidable that the performance of Nb₃Sn strands will be seriously affected due to thermal strain and applied load during the fabrication and operation.Therefore,the study of the mechanical behavior of Nb₃Sn superconducting strands of CICC conductors becomes sufficient important on the design process of magnets.Firstly,we have made some extensions and studied the strain distributions in the Nb₃Sn superconducting strand deeply based on the three-dimensional twisted structure,and a multi-level micromechanical model is used to characterize the mechanical behavior of the superconducting strand with twisted filaments.On the one hand,an equivalent model is used to simplify the filament bundle to reduce the amount of calculations.On the other hand,the twisted filament bundles are discretized.And then,based on the Mori-Tanaka method in micromechanics,we can predict the equivalent modulus in Nb₃Sn strands and analyze the strain distributions of filament bundles in the superconducting strands subjected to external mechanical loads and thermal strain.A simple model is established by finite element software and the effect of pitch on the effective modulus is analyzed.The results are basically consistent with the theoretical results,thus confirming that the theoretical model is feasible.The effect of changes in material parameters with temperature is taken into account by means of piecewise calculation.At the same time,the influence of the pitch on the strain distribution in the filament bundle is analyzed.In addition,the distribution of strain and the change of equivalent modulus with temperature in Hitachi Nb₃Sn strands and EAS-TFAS Nb₃Sn strands are compared,and the influence of different structures on the properties of strands is also discussed.Secondly,the elastic-plastic behavior of EAS-TFAS Nb₃Sn superconducting strands is analyzed based on the finite element method.A?representative volume element??RVE?in the strand is selected as the research object,and the stiffness matrix of the material is predicted with different displacement boundary conditions,and then the equivalent moduli in the composite strand are obtained.The variation of each equivalent modulus in superconducting strands with temperature is given.The results are basically consistent with the theoretical results.At the same time,the influence of the plastic deformation of the bronze matrix on the stress and strain in the Nb₃Sn strands is considered,and the stress-strain curves of the strands at different temperatures are given.Not only does this RVE take into account the twisted properties of the strand,but also can effectively reduce the huge calculations caused by the complicated structure of the strands.