Mechanical consequences of dynamically loaded NiTi wires under typical actuator conditions in rehabilitation and neuroscience

TL;DR

This study investigates how dynamic loading affects the mechanical properties of NiTi shape memory alloy wires used in rehabilitation devices, revealing frequency-dependent behaviors and providing empirical models for device design.

Contribution

It provides experimental data on the thermomechanical behavior of NiTi wires under dynamic conditions and introduces an empirical relation to model their temperature-dependent storage modulus.

Findings

Loss factors are highly dependent on loading frequency.

Storage modulus is unaffected by loading frequency.

Force-controlled conditions reduce the storage modulus.

Abstract

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.