The viscoplastic flow of magnetorheological fluid in a disc was analyzed based on the Navier–Stokes Momentum Equation, and the yielded and unyielded decomposition surfaces were obtained. For the shear-thinning phenomenon of magnetorheological fluid, the magnetorheological properties of the magnetorheological fluid were described based on the Herschel–Bulkley model. Then, the relationship between torque, magnetic field, material, size and motion was established, and the magnetic field and working gap were optimized and analyzed. The results show that in the unyielding region, the magnetorheological fluid flows rigidly. In the yielding region, it flows as a viscous fluid. The degree of error of the proposed torque equation decreased gradually with the increase to current, as observed by experimental comparison.
In this paper, an electrothermal shape memory alloy helical spring actuator constructed from shape memory alloy with copper-cored enameled wire is presented and fabricated. Based on the shear constitutive model of a shape memory alloy, the Thermo equilibrium equation and the geometrical equation of helical spring establish the thermomechanical theoretical model of helical spring actuator with electrothermal shape memory alloys under different scenarios. The thermomechanical behaviors of the actuator were verified by numerical simulation with experimental tests, and the actuator thermomechanical properties were derived from the analysis with current, temperature, response time, restoring force, and axial displacement as parameters. The experimental results show that the actuator produces a maximum recovery force of 70.2 N and a maximum output displacement of 7.7 mm at 100°C. The actuator response time is 26 s at a current of 3A. It is also demonstrated that the theoretical model can effectively characterize the complex thermo-mechanical properties of the actuator due to the strong nonlinearity of the shape memory alloy. The experimental temperature-force response and temperature-displacement response, as well as the force-displacement response at different temperatures, provide references for the design and fabrication of electrothermal shape-memory alloy coil spring actuators.
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