Electromagnetic-thermal-structure multi-layer nonlinear elastoplastic modelling study on epoxy-impregnated REBCO pancake coils in high-field magnets

Abstract

• Multi-physics elastoplastic nonlinear FE model with main constituent materials of epoxy impregnated REBCO pancake winding is developed. • Discrete stresses appear on all the constituent materials. • Stress of each constituent material induced by thermal mismatch cannot be ignored. • Plastic failure mainly induced by hoop stress propagates from inner to outer turns with transport current increasing. • Failure of the superconducting layer occurs before yield in Hastelloy due to the direct action of Lorentz force. The elastoplastic mechanical behaviour of an epoxy-impregnated REBCO pancake winding under cryogenics and high magnetic field is investigated in the frame of finite element (FE) modelling. A two-dimensional axisymmetric electromagnetic-thermal-structure multi-physics multi-layer FE model with main layers of the coated conductor and insulation materials is developed. The radial stress and hoop stress on each constituent layer induced by thermal mismatch stress during cooling are investigated. The mechanical behaviour of each constituent material is also analysed by considering the thermal mismatch stress and electromagnetic force under 20 T background field. The results show that discrete stresses appear in all the constituent materials indicating that the multi-layer winding model containing the main constituent materials is necessary for the accurate stress analysis in an epoxy-impregnated REBCO winding. The stress of each constituent material induced by thermal mismatch during cooling process are too high to be ignored in the subsequent electromagnetic structure analysis. The mechanical-magnetic coupling analyses show that the stresses of all the constituent materials increase with the transport current. The plastic failure mainly induced by hoop stress successively occurs on copper stabilizer and Hastelloy substrate of the innermost turn, and the plastic region propagates from the inner turns to the outer turns with the increase of transport current. The failure of the superconducting layer occurs before the yield in Hastelloy due to the direct action of Lorentz force on the superconducting layers. The transverse tensile stress increases with the increasing transport current, indicating that the risk of transverse delamination failure increases with the increase of transport current. The mechanical failure modes including delamination within the conductor, plastic deformation in substrate and crack in superconducting layer should be seriously considered.