Study On Elastic-Plastic Mechanical Behaviors Of Hierarchical Helical Structures

The International Thermonuclear Experimental Reactor program（ITER）is the largest international scientific and engineering cooperation project that China has participated in at present.Seven countries and regions including China,the European Union,Japan,South Korea and the United States have jointly participated in the construction of the ITER device and the experimental operation.The purpose of the project is to verify the scientific and engineering feasibility of the nuclear fusion energy,and to explore the way for the future commercial application of nuclear fusion energy.As the key device of the ITER,the superconducting magnet systems in superconducting Tokamak are usually composed of CICC（Cable in Conduit Conductor）conductor.The CICC conductor is made of superconducting cable with hierarchical helical structures,and the outer layer of the conductor is covered with a steel sleeve.Previous studies have shown that the superconducting properties of Nb₃Sn strands in CICC are extremely sensitive to the mechanical response.Due to the influence of Lorentz force and thermal stress,the performance of CICC will degrade.Therefore,the performance and service life of the conductors are directly affected by the mechanical response.The research on the mechanical behavior of superconducting cable is the basis for the safety and stability design of Tokamak device.This paper studies the linear elastic and elastic-plastic mechanical response of the triplet and double-helix structure under the tensile load and torsional load.The influence of tensile load on the axial mechanical response with different helical angles and boundary conditions is discussed.The main contents of this paper are as follows:In the first part,the elastic-plastic mechanical behavior of the triplet is studied.The geometric configuration of the triplet can be described by the curvature and twist per unit length of its centroidal axis.According to thin rod theory,the force analysis for the centroidal axis of the wire is carried out and the equilibrium equations are given.The linear elastic deformation can be expressed by the changes of the overall axial strain,the curvatures and the twist per unit length.Based on the plastic increment theory,the elastic-plastic stress distribution of the triplet under tensile and torsional loads can be obtained using the return-mapping method,and the elastic-plastic response of the triplet can also be obtained.The influence of contact deformation on the elastic-plastic mechanical response of the structure is considered to modify the model in this paper,which is equivalent to the strain generated by Poisson effect.Meanwhile,the non-hertzian contact constitutive model is used to describe the contact behavior of wires to expand and improve the model.In the second part,the elastic-plastic mechanical analysis of the double-helix structure is carried out.To obtain the elastic response of the two-level helical structure,the two-level helical structure is built from simple to complex and from the first stage to the second stage based on the thin rod theory.Three winded helical strands are treated as an equivalent helical strand.On the basis,the mechanical variables of cables can be solved and processed step by step,and the‘bottom-up’hierarchical modeling process can be achieved.Based on the incremental theory of plasticity,the elastic-plastic response of double-helix structure under the axial tensile and torsional loads can be obtained.