Comparisons Between Load Controlled And Displacement Controlled Extension Of NiTi Wires
by
Crone, W. C., Leo, P. H., and Shield, T. W.
in
Scripta Materialia, Vol. 38, No. 12, pp. 1825–1828, 1998.
Category: Journal Article
Keywords: Shape Memory, NiTi, Mechanical Testing
Click here to request an electronic copy of this paper.
Abstract:
The behavior of the shape memory alloy NiTi is governed by a phase transformation between a high temperature cubic austenite phase and a low temperature monoclinic martensite phase. The transformation can be triggered either by 1) temperature cycling between the phases (shape memory effect), or 2) load cycling between the high strain martensite phase and the low strain austenite phase (pseudoelasticity). The phase transformation behavior of the material depends on temperature, volume fraction of the two phases and imposed loading.
Previous work by Leo, Shield and Bruno [2] and Shield, Leo, and Grebner [1] investigated this coupling by considering a detailed model of the latent heat release at the austenite-martensite interface, and comparing the results to a series of displacement and load controlled experiments. By assuming that a single interface dominates the transformation they found a direct relationship between the transformation stress and the strain rate imposed on the wire in displacement controlled tests and the load and rate of elongation of the wire in load controlled tests. The load control results also confirmed the prediction of Bruno, Leo and Reitich [3] that there is a maximum rate of transformation under load control conditions.
Shield, Leo, and Grebner [1] reported that NiTi wires transformed at lower stress levels in load control than in displacement control. This was explained by the different energetics of the loading devices. Shield, Leo, and Grebner [1] also reported that wires can be ‘damaged’ both by repeated cycling and by high strain rates. The high rate damage is manifested by a drop in the transformation stress and a decrease in the temperature dependent behavior of the wire. The cyclic damage displays only a drop in the transformation stress and no change in the temperature dependent behavior of the wire. Similar results have been seen in other experiments [4 –6].
In this note, it is shown that the differences observed by Shield, Leo, and Grebner [1] between load control and displacement control were actually caused by testing the wire at different points in its loading history. The transformation stresses in load control and displacement control are identical if the displacement controlled tests are run immediately after the load control tests. The behavior of the wire is completely independent of loading device and depends only on the number of times is has gone through a transformation cycle.
This entry has been accessed 959 times.