Electro-thermo-mechanical coupled torsional characteristics and active variable-stiffness mechanism of SMA/PA56 hybrid cord-reinforced rubber tubes

To address the limited scope for active stiffness regulation in conventional fabric-reinforced rubber tubes, this study proposed a rubber tube reinforced with shape memory alloy (SMA)/bio-based polyamide 56 (PA56) hybrid cords and investigated its torsional response and energy-dissipation characteristics under electro-thermo-mechanical coupling. Seven interfacial treatment routes were compared using H-pull-out tests. The selected combination of surface deoxidation, PA56 hybrid twisting, and 6150/KJ-252 coating achieved a peak pull-out load of 79.7 N and was used in tube fabrication. Tubes with different hybrid ratios and cord laying angles were fabricated, and hybrid-cord tensile tests and tube torsion tests characterized the reinforcement and structural responses before and after electrothermal activation. The martensite-to-austenite transformation of the SMA increased the torsional stiffness of the hybrid structure. Among the tested cord laying angles of 30°, 45°, and 60°, the 45° configuration exhibited the highest initial torsional stiffness, consistent with cord alignment with the principal tensile-strain direction under pure torsion. The activated 1:1 hybrid configuration at 45° exhibited an 82.3% enhancement in torsional stiffness relative to the pure-PA56 reference. The 45° configuration also exhibited the highest relative energy-dissipation ratio in the tested angle comparisons, whereas electrothermal activation increased initial torsional stiffness while decreasing the relative energy-dissipation ratio. These results link interfacial treatment selection to the combined evaluation of stiffness and energy dissipation in unpressurized straight rubber tubes, providing a basis for selecting cord laying angles and activation states in actively tunable rubber structures.