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Numerical simulation of the mechanical behavior of the ITER cable-in-conduit conductoras,

Abstract : The ITER Cable-In-Conduit Conductors (CICC) are composed of an assembly of pure copper wires and composite superconducting strands (with embedded brittle Nb3Sn microfilaments) cabled together and inserted in a stainless steel jacket. If the current carrying capacities of individual ITER strand are clearly identified, by a dependence of the critical current on the applied strain and by a statistical quantification of possible microfilaments breakage, the characterization of cable-in-conduit is not yet fully achieved. What are the local strain values of the strands inside CICCs under operating conditions is still an open question. A deeper understanding of how local strains develop and where critical strains appear in complex cabled structures could help to optimize CICCs designs in term of the losses of conductivity.The present work aims at providing for a finite element model of conductors, able to predict local strains, especially the bending strain, at the scale of individual strands. The finite element software, Multifil, initially developed to model various kinds of entangled media, has been adapted to consider the specific issues related to the conductors. The Multifil’s main feature is basically to handle the evolution of contact-friction interactions between wires. In this study, the initial conductors’ geometry (trajectories of all individual wires), a priori unknown, is determined by a simulation of the shaping process by means of moving rigid tools. Starting from formed cables, both the thermal restraint and the transverse Lorentz loads are simulated through successive applications of proper loading. An important part of this thesis is dedicated to the implementation in the code of proper transverse boundary conditions that are relevant to the cable modelling. Moreover, the numerical work is supported by experiments performed at ECP regarding the characterization of the axial and transverse material constitutive law of the strands of the cable. The, experimental and numerical “Force/Displacements” curves, obtained on cables under standard axial and transverse loading, show good agreement. At last, the results of the full conductor simulations (from initial shaping to magnetic loading) are presented for various conductors design. Relevant information at the scale of strands (axial strains across and along the strands of the cable) can be retrieved from these simulations. The careful analysis of these data have led to highlight the high non-uniformity of the axial strains in loaded conductor with occurrence of localized critical strains that could explain the conductivity loss observed in ITER conductors. At last, the mechanical information provided by the Multifil have been put to good use by two different electromagnetic codes, CARMEN and JackPot in order to predict the superconducting properties of the conductor according to the axial strains measured along and across the strand.
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Hugues Bajas. Numerical simulation of the mechanical behavior of the ITER cable-in-conduit conductoras,. Other. Ecole Centrale Paris, 2011. English. ⟨NNT : 2011ECAP0016⟩. ⟨tel-00697000⟩

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