Implementation of the ‘smart’ rotor concept - Wind Power Generation and Wind Turbine Design - page 483

deformed heavily - up to 8% - under a small increase in loading. Upon unloading
the deformation remains. This can be observed in Fig. 10. Here no elaboration into
metallurgy is made, but Otsuka and Ren [20] offer a comprehensive review.
The nearly fl at part of the tensile e , s -curve is called the stress plateau. The defor-
mation is plastic, but recoverable by raising the temperature above a certain thresh-
old. Above this threshold the material is austenitic. Austenite has a cubic lattice
structure and therefore no variants. The material 'remembers' its undeformed shape
because the net shape of the austenite is the same as of the undeformed variants of
the martensite state. When the material is cooled down again the lattice structure
becomes multi variant martensite again. This deformation and heating cycle can be
observed in Fig. 11 and is called the shape memory effect (SME).
Figure 10: Stress-strain curves of a SMA wire at low and high temperatures.
Figure 11: The SME cycle in SMA (fi gure adapted from [87]).