Numerical simulation of damage and critical current degradation behaviors of RRP and PIT Nb3Sn wires under transverse pressure

Abstract

The state-of-the-art Nb₃Sn wire still exhibits discrepancies in both mechanical and superconducting critical properties when juxtaposed with the target of the requirement for superconducting wire set for future circular colliders (FCC). Intensive efforts are currently underway to develop high performance Nb₃Sn wires with enhanced mechanical and superconducting properties. In parallel, developing convenient and efficient numerical methods for comprehensively evaluating the mechanical and superconducting properties of various Nb₃Sn wires is of critical significance. Given the relevant context, the analysis of critical current degradation and fracture damage behavior of Nb₃Sn superconducting wires under transverse pressure is of significant importance. In this paper, the detailed strand numerical models are established based on the meso‑structure characteristics of Restacked-Rod-Process (RRP) and Powder-in-tube (PIT) Nb₃Sn wires at first. These models can be used to simulate stress distribution and mechanical behaviors under various external loading conditions. Subsequently, the damage and superconducting critical current degradation of epoxy-impregnated RRP and PIT wires under transverse pressure are evaluated in numerical simulations, taking into account the irreversible degradation caused by the residual plastic stress of the annealed copper matrix. This numerical method encodes all superconducting filaments/sub-elements of the two-dimensional numerical model by considering the periodic positional changes of the twisted filaments/sub-elements along the wire axis. The results indicate that within a transverse pressure range of 100 to 200 MPa, the plastic residual stress of the copper matrix is the primary cause of irreversible critical current degradation. At transverse pressure exceeding 200 MPa, the influence of damage on the critical current cannot be ignored due to the emergence of damage. When the transverse pressure reaches 413 MPa, through-thickness damage has occurred in the Nb₃Sn phase of all types RRP and PIT wires. Compared to PIT B215 wires, the critical current of RRP 108/127 wires have better strain tolerance. The numerical method with a low computational burden in this paper can conveniently evaluate the fracture and damage as well as degradation of critical properties of various Nb₃Sn wires under transverse pressure.