The Investigations On Some Mechanical-Electrical Properties Of Nb3Sn Materiais And Strands

Nb3Sn materials have excellent critical superconducting properties, so the Nb3Sn composite strands have important applications in ITER magnetic system and other high magnetic field engineering. Because the mechanical and electrical properties of Nb3Sn materials and structures, such as the critical current density, the effective elastic modulus, the statistical strength, the contact mechanical behavior, of Nb3Sn superconducting materials and structures, are strongly related to the safe and stable running of superconducting magnets, the prediction and evaluation of them are key problems in the high magnetic field science and technology. Hence, this dissertation presents theoretical investigations on the mechanical-electrical properties of Nb3Sn superconducting materials and strands.Based on the electron scattering pinning mechanism, this study proposes a model describing the grain size effects on the effective critical current density of Nb3Sn superconducting materials by using the collective pinning theory, where the electron effective mean free path which is affected by grain size and the grain boundary scattering properties is obtained through Boltzmann transport equation. The investigation indicates:the effective critical current density is not only dependent on the grain size but also on the electron transmistivity at grain boundaries; when the transmistivity is less than a specific value, there exists a best grain size for the effective critical current density. So, this clarifies the question about whether there exists a best grin size for the critical current density in the prior work. The results predicted by the proposed BT model- small vortex bundle pinning mechanism are in good agreement with the experimental results。Furthermore, we get a new critical current density scaling relation for Nb3Sn strands, which can describe the grain size effects.An analytical model is developed to simulate this degradation of bronze route strands induced by initial damage and filament fracture. The model contains three sub-models:the model of the effective modulus of a filament with initial damage; the model of the weakened stiffness of the superconducting layer; the model of the effective axial modulus of the strand. The results indicate that the stiffness reduction of a strand presents obvious nonlinear behavior and depends on the initial damage parameter and the evolution of total damage. The predicted effective axial modulus from the present model is in basic agreement with that determined from the first loading stress-strain curve.We use the shear lag model combined the Curtin-Zhou model which is based on the Weibull/Poisson statistics and GLS schema to describe the filament fragmentation in composite strands, and get the statistical average tensile strength and the failure probability distribution function of these superconducting composite strands. Furthermore, we propose a model to analyze the load factor of the periodic bending strand, and obtain the periodic bending strength which is related to the tensile strength by using the load factor. The study points out that the initial damage parameter affects evidently the tensile strength and the periodic bending strength which all decreases as the initial damage parameter value increases. Comparing to the results of previous research, the periodic bending load factor values and dependence on the Weibull modulus predicted by our model is correct. Our research fills the gap of previous work.We propose a discrete element frame and method to analyze the 2D contact mechanical behavior and characteristics of strands in the CICC cross-section under the applied transverse electromagnetic loads. The discrete element simulation finds: among strands in the CICC cross-section, the probability density function distributions of the magnitudes of the relative contact force, the relative normal contact force and the relative tangential contact force all decay as a negative exponential law; the probability density function distributions of the directions of the contact force, the normal contact force and the tangential contact force all are anisotropic, and all have six directions with relatively large probability density values. The simulated transverse load-displacement curves, where the treatment of applying pre-spring between the triplet strand disc elements is taken, agree well with the experimental test curves.After all, through the investigations in the dissertation, it is valuable and important to predict and assess the multi-field effective properties of Nb3Sn superconducting materials and structures, and can provide certain theoretical guidance to assess and promise the safety and stability of ITER superconducting magnets.