Transverse compression performance of twisted stacked-tape double casing conductors under monotonic, cyclic loads and axial preloads with FBG strain diagnostics

RE-Ba 2 Cu 3 O 7 − δ (REBCO) coated conductors are promising candidates for next-generation high-field fusion magnets operating above 20 T. To meet the structural and electromagnetic demands of such environments, various cabling concepts based on REBCO coated conductors have been developed. However, inadequate mechanical protection can significantly degrade their transverse load tolerance under practical operating conditions. In particular, multiaxial stresses—such as axial pre-strain combined with transverse loading—may induce irreversible performance degradation, yet their coupled effects remain poorly understood, limiting reliable application of REBCO-based conductors in high-field fusion systems. In this study, the transverse compression behavior of a double-casing conductor (DCC) with a twisted stacked-tape cable was systematically investigated under monotonic, cyclic, and axial preload conditions. Real-time strain monitoring was enabled by fiber Bragg grating (FBG) sensors, rigorously calibrated to decouple mechanical, thermal, and clamping-induced strains. This approach allowed precise quantification of the actual strain state, demonstrating the strong potential of FBG diagnostics for in-situ integration into future conductor architectures. Experimental results show that the DCC strand sustained a critical transverse load of 1188 kN m − 1 without axial pre-strain. However, this tolerance dropped by over 52% when 0.25% axial pre-strain was applied. Finite element analysis revealed a peak compressive strain of 0.36% at the critical load. Under six-around-one contact, monotonic loading of DCC-8 exhibited a degradation threshold of 1095 kN m − 1 , while fully superconducting DCC-25 demonstrated superior fatigue resistance under cyclic loading, maintaining less than 5% degradation after 500 cycles at 1125 kN m − 1 . These findings highlight the necessity of incorporating multiaxial stress criteria into conductor design and confirm DCC architecture with FBG diagnostics as a robust candidate for next-generation high-field fusion applications.