Numerical analysis on the thermal mismatch stress of quasi-isotropic strand with square cross section during cool-down

The quasi-isotropic strands (Q-IS) with high current-carrying capacity can effectively improve the anisotropy of the critical current in the magnetic field. However, when a quasi-isotropic strand is cooled from room temper-ature (300 K) to the liquid helium temperature (4.2 K), it may cause the irreversible degradation of the critical current, the plastic deformation, and even mechanical damage. In order to achieve the safe and stable operation of quasi-isotropic strands, it is important to analyze the thermal mismatch stress during cool-down. In this study, two mechanical models (i.e. a bulk model and a contact model) were developed by considering different material elastoplastic characteristics to study the mechanical behaviors of quasi-isotropic strands with square cross sec-tion in the cooling process. Compared with the bulk model, the contact model can simulate the contact and separation between adjacent REBCO conductors in quasi-isotropic strands. Therefore, the contact model should be chosen to estimate the thermal mismatch stress of quasi-isotropic strands during cool-down. In addition, plastic deformation was clearly observed in the Cu and Ag layers with peak equivalent plastic strains of 0.016% and 0.171%, respectively. When the stainless steel sheath is replaced by the copper sheath to encase a quasi -isotropic strand, the interlaminar normal tensile stress between the superconducting layer and Hastelloy sub-strate was decreased from 101 MPa to 16.2 MPa. Therefore, a quasi-isotropic strand encased in a copper sheath could reduce the risk of delamination during cool-down.