Progress of key multi-field coupled mechanical issues in superconducting materials and structures

High-quality superconducting materials and structures play a crucial strategic role in the development of modern science and technology, as well as major national equipment such as nuclear magnetic devices, high-field superconducting magnets, and nuclear fusion magnetic confinement systems. The Bi₂Sr₂CaCu₂O_x (Bi-2212) round wires (RWs) and REBCO-coated conductors (CCs), among all practical superconducting materials, have emerged as the optimal choices for fabricating high-performance superconducting magnets due to their higher engineering critical current density and negligible decrease with an applied magnetic field. Previous studies have primarily focused on the effect of the Lorentz force resulting from the current-carrying operation of these conductors under an applied magnetic field. However, some basic experimental studies have found that the influence of deformation caused by various loads on the electromagnetic properties of materials is significant and cannot be ignored. Notably, applied strain leads to considerable degradation of critical parameters in superconductors. In other words, mechanical and electromagnetic factors are bidirectionally coupled in such materials and structures. This review specifically highlights the mechanical properties, mechano-electric coupling effects, and electro-thermal coupling phenomena observed in two types of superconducting materials: Bi-2212 RWs and REBCO CCs. Among these investigations, three key features deserve particular attention: i) the strain-sensitive dependence of the critical current for almost all of practical superconductors. So far, the underlying mechanism has not been completely clear even though the related experimental data have been accumulated for decades. ii) Quench, essentially electro-thermal coupling interaction. For superconducting structures of coils and magnets, the current challenge lies in detecting quench occurrence in real time and implementing effective protection. iii) Strength failure due to the mechanical, electromagnetic and thermal coupling. For example, the interface strength and delamination in REBCO CCs pose significant challenges, and currently more precise studies at the microscale are required to address these issues effectively. In the future, besides superconducting wires, for larger scale structures such as superconducting cables, coils and magnets, the key multi-field coupled mechanical issues need more attention.