Mechnical Analysis Of The Superconducting Cable Based On Discrete Element Method

Superconducting CICC(cable-in-conduit conductor) is the key function part of the ITER(International Thermonuclear Experimental Reactor), and is subjected to the coupled multi-field of large current, strong magnetic field and extremely low temperature. A great quantity studies have demonstrated that the superconducting performance(critical current, upper critical field and critical temperature) of CICC is directly related to the mechanical properties(stress and strain). Thus, the accurate quantitative predictions of their mechanical properties directly involve functions implementation and safe operation of ITER. Since cable is composed of the multistage twist of composite strands, the whole present the geometrical multiscale structure. Meanwhile, the strand posses the nonlinear plastic deformation characteristics and the cable show complex multi-body nonlinear contacts. Therefore, the mechanical properties of cable are difficult to predict accurately using the finite element method(FEM) based on the traditional continuum mechanics means, which is hardly able to consider the continent material specialties and craftsmanship. Especially, the whole cable contains some various functional structures(e.g., He channel, wraps, conduit, etc.) of the CICC, causing the poor generalization.Therefore, this doctoral thesis is focus on superconducting CICC. A multistage discrete dynamic model is established, by introduction the flexible authorization contact model, which can predict mechanical characteristics accurately of cable. In addition, multi-stage constitutive of the cable at different scales is built from bottom to top. Finally, this thesis carries out the investigations on the macroscopic mechanical properties and microscopic mechanism of superconducting cable under the change pitch, loads, temperature, other multiple structural parameters and numerous factors so on. The following presents the main content of this dissertation:Firstly, based on the discrete element method(DEM) that suitable for larger deformation, the multi-layer discrete dynamic models from the composite superconducting strand to the petal sub-cable(basic symmetry unit of the cable) are established by stepwise twisting process that is closer to the actual production process. The multiple nonlinear contacts constitutive for characteristics of the component materials are defined based on the secondary development to the DEM and the hybrid theory, successfully achieving the accurate prediction for the nonlinear stress and strain constitutive relation of the composite strand. With the equivalent continuous sliding treatment among multiple discrete element continuous systems considered, the mechanics properties prediction for the multi-stage twist cable with the continuous and discrete coupled characteristic are performed. DEM prediction of nonlinear stress-strain relationship of the multi-layer cable is in good agreement with the experimental data, which indicates that the bottom-up modeling process presents a useful approach to predicate the mechanical properties of strong nonlinear strands and complex twist models.Secondly, the superconducting properties of multi-stage twist cable are more sensitive to transverse contact stress compared with the axial stress. The contact stresses among the strands, which occur during twisting process, are still unclear due to the absence of craftsmanship. Based on the established multilayer discrete models, the influences of the pitch of the triplet and the pith ratio between succession twisted stages on the void fraction and the contact compressive stress are investigated. Furthermore, the distribution features of the local strain of strands in the petal cable under equivalent operating condition(axial shrinkage at low temperature and transverse compression under strong magnet field) are performed. These results provide guidance for the CICC optimal structure design.Thirdly, discrete dynamical model of the whole cable cross section including the various components with the different functions and performance(central helium channel with the continuous fixed support and thin walled wrap with free shrinkage) has been improved. CICC under transverse cyclic mechanical loads is realized by introducing the transverse displacement drive boundary conditions something like servo mechanism, and more cyclic transverse load-displacement curves have been numerically obtained. The effective transverse young’s modulus is closer to the experimental data compared with existing theoretical results. Meanwhile, the variation of macroscopic mechanical characters such as the plastic deformation and mechanical loss of cable with transverse loads and cyclic numbers are investigated. In addition, considering the macroscopic mechanical behaviors of cable with the multi-body contact characteristics are correlated with the microscopic mechanism such as the contact, friction and arrangement among the strands during the cyclical loading, the evolving properties of local void fraction of petal, residual deformation of strands, and the adjacent number among the strands are explored. Furthermore, the main factors influencing the transverse compression stress of the cable are analyzed.Finally, an additional degree of temperature freedom that can be characterized as strand temperature is introduced in DEM conveniently. The contact heat transfer among the strands is successfully achieved. And the convective heat transfer in liquid helium between the central helium channel and the annular space of the porous cable is taken into account. Thus DEM is used for cooling cable under two heat transfer mechanisms is established. The temperature variation of cable with time is obtained by DEM agrees well with the existing experimental results. In addition, the evolution features of the contact mechanical properties and the heat transfer characteristics of cable under different mass flow rates of liquid helium are performed. Finally, the evolving properties of effective thermal conductivity(ETC) of cable under cyclic transverse loads are presented.In conclusion, the relevant mechanical properties and influence factors of cable under complex operating environment are in-depth discussed based on DEM that is usually used for the study of the discrete material. The results consistently show that the prediction of the mechanical properties of the nonlinear composite material by the reasonable characterization for the contact constitution between the particles based on the DEM demonstrate great vitality. Meanwhile, the DEM can handle the contact and friction between discrete materials quickly and efficiently, making it as a powerful tool that can predict the mechanical properties of the multi-stage twist cable. In addition, the modeling process is greatly simplified and the computational efficiency is highly improved, convergence also shows clear advantage due to the meshless characteristics. All advantages mentioned above show that DEM excellent applied in modeling complex continuum problems and performing their mechanical behaviors.