Influence of the Phase Transformation Behaviour of NiTi Shape Memory Alloy Wires on the Predictability of Strain During Operation

Abstract

Abstract Predicting the stroke (respectively the strain) of binary Nickel-Titanium (NiTi) shape memory alloy (SMA) wires is a challenging, but interesting task for designers of SMA components. Due to fatigue the resulting stroke decreases with an increasing cycle count. In the past several approaches were investigate to control the stroke of an SMA component based on models resulting from the energy conservation equation. By heating the wire energy goes into the system. Via heat content, convection, radiation the heat conduction via mechanical power (lifting a weight) and phase transformation energy leaves the system. In 2004 Oelschläger proposed a model to calculate the resulting stroke of an SMA wire based on the electric energy. The presented work aims to extend Oelschläger’s model and takes the fatigue of the phase transformation temperature (PTT) into account, by monitoring parameters such as voltage, current, electric resistance, force, temperature and the maximum strain. The leading question for this work is: How well does this model perform for SMA wires generating a full stroke within small temperature ranges? The developed model is tested based on 8 pre-cycled binary NiTi wires (diameter 0.25 mm, 350 MPa stress, 4 % strain and PTT between 120 °C and 125 °C) using two different test setups (test rigs, input parameters). Finally, all results are described and discussed in detail. The fatigue tests are analyzed using statistical methods. Peak values of all recorded parameters are extracted of the fatigue tests. Furthermore, the deviation between the experiments and the simulations are determined to compare the simulation model with the real measurement series of the performed fatigue tests. The simulation model is suitable to predict the stroke of a wire if the deviation is less than e.g. 10 %. In this context, the influence of the PTT is the focus of the analysis. The results show that the PTT has a significant impact. The narrower the PTT range is, the more acurate the simulation model and the initial parameters of the simulation have to be.