  In the field of rehabilitation and neuroscience shape memory alloys play a
crucial role as lightweight actuators. Devices are exploiting the shape memory
effect by transforming heat into mechanical work. In rehabilitation
applications, dynamic loading of the respective device occurs, which in turn
influences the mechanical consequences of the phase transforming alloy. Hence
in this work, dynamic thermomechanical material behavior of temperature
triggered phase transforming NiTi shape memory alloy wires with different
chemical compositions and geometries is experimentally investigated. Storage
modulus and mechanical loss factor of NiTi alloys at different temperatures and
loading frequencies are analyzed under force controlled conditions.
Counterintuitive storage modulus and loss factor dependent trends regarding the
loading frequency dependency of the mechanical properties on the materials
composition and geometry are hence obtained. It could be revealed that loss
factors show a pronounced loading frequency dependency, whereas the storage
modulus was not affected. It is shown that force controlled conditions lead to
a lower storage modulus than expected. Further it turned out that a simple
empirical relation can capture the characteristic temperature dependency of the
storage modulus, which is an important input relation for modeling the
rehabilitation device behavior under different dynamic and temperature loading
conditions, taking directly into account the material behavior of the shape
memory alloy.
