Dual phase-field modeling of coupled delamination—Fracture and critical current degradation in REBCO coated conductors under combined mechanical loading

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.