Investigation of the Fatigue Strength of Shape Memory Wires With Different Diameters

The properties of shape memory alloy wires (SMA wires) have still not been sufficiently investigated, which is the reason why shape memory-based actuators can hardly be found in serial or commercial applications in industry. The most important parameter for industrial applications is the fatigue strength of SMA wires under cyclic activation with mechanical stress. So far only isolated and application-specific investigations have been carried out. Thus there are no reliable statements on fatigue strength or reliable design calculations for design and application. Among other things, this circumstance is considered to be the reason why the majority of companies is still not ready or willing to use SMA technology in series production. This paper deals with a series of tests to determine the fatigue life of shape memory wires of different diameters by means of the design of experiments. Primarily the results aim at making more reliable predictions about the fatigue strength of shape memory wires, which are subject to statistical safety. Besides, the focus is on the influence of the wire diameter on the fatigue strength. The interdependency parameters are strain, stress and wire diameter. The fatigue strengths are shown in standardized Wohler diagrams, which should serve as a basis for future fatigue tests of shape memory wires. The main influence on the fatigue strength is the strain as expected. Another interesting tendency can be seen in the dependence on the wire diameter. Thus, this paper makes a contribution to the further application of this technology in both industrial and scientific environments.