Implementation of the ‘smart’ rotor concept - Wind Power Generation and Wind Turbine Design - page 484

The transitions from martensite to austenite and vice versa do not occur suddenly
at a certain temperature but they rather occur gradually over a certain temperature
range. The temperatures at which the martensite-to-austenite (MA) transition
begins ( A s ) and fi nishes ( A f ) are different then those of the reverse AM transition.
The latter transition occurs at lower temperatures (see Fig. 12). Something else
that can be observed in Fig. 12 is that the MA transition is postponed by
tensile stresses.
When the temperature is raised until T > A f , this causes the material to also exhibit
pseudo-elasticity: stretching a piece of austenitic SMA causes the formation of mar-
tensite, so-called 'stress induced martensite' or SIM. The deformation that is obtained
under the formation of martensite is recovered as the material transformed back into
austenite when the tension is released (see Fig. 10). The behavior is apparently elas-
tic since all the deformation is recovered, but the physical behind this process is a
reversible change in lattice structure, not atomic bond stretching. Moreover, the
stress-strain loop shows a considerable amount of hysteresis.
Therefore the SMA is said to have two observable effects during thermal and
mechanical load cycles: super- or pseudo-elasticity when A f is below operating
temperature and the SME when the material is deformed at T < M f and then
reheated, or a combination of both in between M f and A f . A third effect that is
reported [19, 20] is the rubber-like effect, exhibited by some alloys. With this, the
material shows recovery of deformation below M f . This is usually regarded as an
anomaly.
3.2.2 SMA behavior modeling
Usually the mechanical properties of a SMA are described as an e , s , T -behavior,
but actually the underlying, connecting parameter is the martensite fraction x . Three
types of models can be distinguished to model this behavior: the thermodynamical,
Figure 12: The displacement characteristics of an SMA wire as a function of the
temperature, under constant loading.