It is well known that the parameters of Nb₃Sn superconducting strands are strain sensitive, and the internal brittle Nb₃Sn filament can easily break under deformations. A temperature difference from the preparation temperature of about 1000 K to the cryogenic working environment of 4.2 K damages brittle Nb₃Sn fibers before working. Based on the Curtin–Zhou model, the damage theory for fiber-reinforced composites is utilized to study the influence of filament fractures caused by thermal stress. According to the typical multi-scale geometric of the EAS-Nb₃Sn strand (European Advanced Superconductor, EAS), an efficient hierarchical homogenized calculation model considering filament fracture and matrix plasticity was established. In this work, we took the filament fracture caused by both thermal stresses and mechanical loads into consideration using the secant modulus and simultaneously had the impact of the plastic constitutive of the bronze matrix and the copper protective layer. Mechanical parameters, such as the homogenized secant modulus, shear modulus, and Poisson’s ratio in different directions of level scale, were predicted at various temperatures. The elastoplastic mechanical behavior of the strands subjected to axial load was analyzed, and the results were in good agreement with the experiment. The initial thermal fiber fracture has non-negligible effects on the mechanical properties of the EAS-Nb₃Sn superconducting strand and play the role in accelerating the increase in fiber breakage.

Rare-earth barium copper oxide (REBCO) coated conductors serve as second-generation high-temperature superconductors for high-field magnets and power transmission. Their multi-layer thin-film architecture accumulates stress at interfaces and within layers under mechanical loading. Delamination and fracture then cut the superconducting current path. The result is irreversible degradation of current-carrying capacity. Existing studies have focused mainly on delamination. The dominant role of fracture damage in current degradation has received less attention. This work builds a coupled mechanical–electrical–magnetic numerical framework. It combines higher-order shear deformation theory with a dual phase-field damage model. The framework predicts mechanical response, coupled damage evolution, and critical current degradation in REBCO tapes. Loading conditions include uniaxial tension, torsion, and combined tension-torsion loading. Levinson's third-order theory supplies the displacement and rotation fields. Two phase-field variables track delamination and fracture separately. Bidirectional coupling connects the two damage modes. The T–A formulation links mechanical damage to critical current density through a degradation function. Results show that normalized critical current drops sharply beyond ∼0.55% tensile strain. It reaches ∼0.3 at 0.9% strain. Under torsion, degradation accelerates past a critical threshold of ∼4°/mm. This rate exceeds that under pure tension. Combined loading produces strong coupling. This coupling further lowers the critical strain threshold. Edge shear stress concentration drives delamination. The model predictions match experimental observations. This framework supports reliability assessment and structural optimization of REBCO tapes under complex operating conditions.