Actuators in the form of a helical spring made from shape memory alloy are attractive due to light weight, large recoverable deformation, high energy density and manufacturing simplicity. For their optimal design and control detailed information on evolution of phase and stress distribution within the material during operation is advantageous. In this work a constitutive model tailored for non-proportionally loaded shape memory alloys exhibiting R-phase transition, transformation strain anisotropy, tension-compression asymmetry is employed to reveal and interpret relation between macroscopic response of such an actuator and microscopic state within the shape memory material. Numerical simulations confirm good predictive capability of the model and demonstrate that because of naturally non-proportional loading mode, phase and stress distributions within cross-section of the wire may be rather complex and counterintuitive.

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