Study On Nonlinear Mechanical Behaviors Of Nb₃Sn Superconducting Composite Strands And First-stage Cables

Nb₃Sn superconducting material has high critical temperature,upper critical magnetic field and critical current,and is one of the most ideal materials to manufacture high-field superconducting magnets around 10 T.In order to reduce the AC loss during the process of operation,nearly 1000 Nb₃Sn superconducting filaments with diameter less than 100μm are embedded into the copper substrate to form the Nb₃Sn/Cu superconducting composite wire.The cable-in-conduit conductors（CICC）is multistage structure and basic unit in the International Thermonuclear Experimental Reactor（ITER）.The central solenoid（CS）magnet of ITER Tokmak device is 13.5 m high,4.1 m wide,and has an operating temperature of 4.2 K and a maximum magnetic field of 13 T.It is mainly used for pulsating heating plasma,also known as ITER’s beating heart.In order to meet the safety and function of CS coil,it is necessary to understand the key mechanical behaviors of its basic unit,namely Nb₃Sn superconducting composite strand wire,such as energy dissipation and fatigue life under room,low temperature and cyclic loading.Existing studies on Nb₃Sn superconducting composite strand wires mainly face problems such as inaccurate energy dissipation prediction under quasi-static conditions,lack of basic low-temperature fatigue experiments,and unclear fatigue mechanical behaviors.This doctoral thesis constructed a low-temperature fatigue experimental device to study the mechanical response of Nb₃Sn superconducting composite strand and first–stage cable during fatigue process at room and low temperature.Meanwhile,the mechanical models of energy dissipation and fatigue life under quasi-static and fatigue loading are quantitatively consistent with the experimental results.The main achievements are:Firstly,the nonlinear mechanical behaviors of Nb₃Sn superconducting wires have been investigated during the quasi-static loading-unloading process at the temperature of 77 and 300 K.The quasi-static loading-unloading tensile tests were conducted at the temperature of 77 and 300 K.and the experimental results indicate that the quasi-static stress-strain curves and energy dissipations exhibit strong nonlinearity,and linear unloading modulus degrades with the increase of plastic strain.Meanwhile,a double-surface elastic-plastic model and constitutive equations considering linear unloading modulus degradation were presented,and the nonlinear and linear unloading modulus are derived by considering the damage degradation,and the new nonlinear and linear unloading modulus were derived.Finally,the new model and constitutive equation can be embedded into the finite element method by user subroutine UMAT,and were used to predict the stress-strain curves and energy dissipation of Nb₃Sn superconducting wire during loading-unloading process.The comparison between the experimental results and theoretical results indicated that there is a good agreement.The theoretical results show that the nonlinear energy dissipation in quasi-static loading and unloading process is caused by the modulus degradation,and the modulus degradation can cause more energy dissipation.Secondly,the fatigue damage properties of Nb₃Sn superconducting wires have been investigated during strain cycling at the temperature of 77 K and 300 K.A fatigue device of liquid nitrogen soaking was constructed to study the fatigue life,damage degradation and energy dissipation of Nb₃Sn superconducting wire by strain cycling test at 300K and 77K.The experimental results showed that the energy dissipation of Nb₃Sn superconductor composite wire shows two different mechanisms during fatigue at room temperature and low temperature.At room temperature,the energy dissipation increases with the fatigue cycles that the increasing speed decreases gradually,and the fatigue fracture shows a ductile fracture mode.However,the damage degradation of Nb₃Sn superconducting composite strand at low temperature presents three stages:rapid damage at the early fatigue stage,slow steady damage at the middle fatigue stage and accelerated damage at the end of fatigue stage.That is,the energy dissipation at the early fatigue stage is large and rapidly decreases,the energy dissipation at the middle fatigue stage is small and stable,and the energy dissipation at the end fatigue stage is rapidly increased.Moreover,the fatigue fracture presents a brittle fracture mode.According to the experimental phenomena,the damage evolution model related to component damage was established,and the damage factors and energy dissipation of Nb₃Sn superconducting wire were derived during the strain cycling process.The comparison between the experimental results and theoretical results indicated that there is a good agreement at room and low temperature.Again,the nonlinear mechanical behaviors of the first-stage Nb₃Sn superconducting cables have been investigated during the quasi-static loading-unloading process at the temperature of 77 and 300 K.The quasi-static loading-unloading tensile tests were conducted at the temperature of 77 and 300 K,and the influence of pitches on the stress-strain curve,energy dissipation of the Nb₃Sn first-stage cables was investigated during the quasi-static loading-unloading process.The experimental results show that Nb₃Sn first-stage cables with short pitches have less energy dissipation and damage factor degradation during the quasi-static loading-unloading process at room and low temperature.Based on the double-surface elastic-plastic model and constitutive equations of a single Nb₃Sn superconducting wire in Chapter 2,the finite element model of the triplet has been established.By comparing the experimental results with the simulated results,it is found that the proposed model can well predict the influence of pitches on the stress-strain curve and energy dissipation of Nb₃Sn first-stage cable during the quasi-static loading-unloading process.Finally,the fatigue damage properties of Nb₃Sn first-stage cables have been investigated at the room and low temperature.The fatigue life,damage degradation and energy dissipation of Nb₃Sn first-stage cable were investigated by strain cycling test at the room and low temperature,and the influence of pitches on fatigue life,energy dissipation,damage degradation of Nb₃Sn first-stage cable during fatigue.The experiment results showed that the Nb₃Sn first-stage cable with long-pitches has longer fatigue life,slower damage degradation and less energy dissipation at the process of stress cycling at room temperature,and the Nb₃Sn first-stage cable with short-pitch has longer fatigue life,slower damage degradation and less energy dissipation at the process of stress cycling at 77 K temperature.Morever,the damage factor and energy dissipation are positively correlated with the strain cycling range.Based on the relationship of geometry between the Nb₃Sn first-stage cable and single Nb₃Sn superconducting composite strand,stress damage evolution model and strain damage evolution model considering twist pitches were established to derive the damage factors and energy dissipation.By comparing the experimental results with the theoretical ones,it is found that the theoretical model can well predict the damage factors and energy dissipation of Nb₃Sn first-stage cable during fatigue process at room and low temperature.Finally,the reasons for the good fatigue performance of the Nb₃Sn first-stage cable with short-pitches during the strain cycling process at low temperature were discussed.