Analysis of Wire Position and Operating Conditions on Functioning of NiTi Wires for Shape Memory Actuators

In this work an experimental-numerical approach was used to analyze the thermo-mechanical behavior of thin NiTi wires, electrically heated, finalized to defining the influence of both wire position and the operating conditions of the actuator functioning. Tests were carried out on wires having diameters of 80 and 150 μm, loaded by constant stresses of 100 and 200 MPa and characterized by DSC and strain/temperature hysteresis measurements. Two wire positions (horizontal and vertical) were adopted in single cycle tests and designed to obtain different typologies of the heating and cooling transients. In general, the heating time was selected to reach a steady state condition while the cooling time always allowed decreasing the wire temperature to the ambient one. Data concerning strain, applied current and voltage were simultaneously acquired during the tests. Moreover, for the optimization and validation of a numerical model, for the 150 μm wire in diameter was used, its temperature was recorded by IR thermographic system. On the basis of the collected experimental data, a simple model was tested to reproduce the experimental results and data regarding the heat exchange coefficient and wire electrical resistivity dependence on temperature were obtained. The influence of the experimental wire positioning and wire diameter on the free convection coefficient is reported and the results indicate that the heating transient is associated with different convection coefficients depending on the heating modalities.