Page 249 - Mechanics of Microelectromechanical Systems

236 Chapter 4
The other important feature of certain SMAs, the superelasticity
(sometimes called pseudoelasticity), is depicted in Fig. 4.48. Figure 4.48
shows the force-temperature characteristics of four different SMA
compositions, each of them corresponding to a temperature which is relevant
to either the martensitic transformation or the reversed one. The temperatures
denoted by and symbolize the start of the martensitic transformation
and the end (finish) of it, respectively. Similarly, and represent the
same points for the austenite phase. For temperatures smaller than the
entire composition is martensite, whereas for temperatures higher than the
SMA is completely in its austenitic phase, in the absence of loading.
Obviously, for temperatures within the range, the SMA contains both
phases. The SE effect, as suggested in this figure, consists in heating the
SMA over the point (where only the austenite exists in stable condition),
and loading the mechanical component at constant temperature (iso-
thermally) – direction 1 in Fig. 4.48. In doing so, a final state can be reached
where the martensite fraction predominates and where large superelastic
deformations of 15-18% can be achieved easily, since the plateau region
permits it. By downloading the mechanical component, along direction 2 in
the same figure, it is possible to reach the initial state. However, the
generation of the SE effect is more complex and manifests itself as a
spontaneous, stress-free phenomenon, which takes place in certain shape
memory alloys after many cycles of so-called training. Training consists of
combined thermal and mechanical loading which alters the crystallographic
structure of an SMA in order to favor SE behavior – Otsuka and Wayman
[10].
The mechanics of shape memory alloy actuation/sensing are exemplified
by the simple experiment illustrated in Fig. 4.49 where a weight is attached
to a SMA wire.

Figure 4.49 SMA transformation as a source for actuation/sensing