Testing internal friction and high-frequency response in pseudoelastic NiTi

This paper presents a method comprising Dynamic Mechanical Analysis (DMA) and high-frequency stress-strain testing to determine the damping behaviour of pseudoelastic NiTi alloys. In particular, specially-designed fixtures were built to subject wire samples to controlled pre-strain levels and thus measure the internal friction response of the material at various locations along the transformation stress-strain path. The DMA fixtures allow applying of a tensile pre-load on the wires, which are then tested in bending by a dual cantilever method. In addition, an ad hoc set-up comprising a dedicated motor, a load-cell and strain measurement equipment was developed in order to deploy high-speed mechanical testing of the wires and observe the evolution of stress-strain curves for different levels of cycling speed. This device was created in order to expand the capabilities of conventional universal testing instrumentation, at least in relation to this particular application. Some examples of obtainable results are presented for the two techniques.