Implementation of the ‘smart’ rotor concept - Wind Power Generation and Wind Turbine Design - page 482

There are also ways of increasing the defl ection of these benders. This is done
by introducing a geometrical non-linearity by axially compressing the bender [76]
or using the thermal mismatch between the plies to introduce an internal stress
state. Two types of benders that make use of the thermal mismatch are called
Thunders [77, 78] and lightweight piezo ceramic actuators (LiPCA's) [79-81].
The Thunder actuators consist of a piezo ceramic sheet that is laminated between
a thin steel plate and an even thinner aluminum foil by means of an adhesive. This
laminate is consolidated at about 300°C and when it cools, the difference in coef-
fi cient of thermal expansion (CTE) causes a bend, and even slightly domed shape
of the bender, depending on the aspect ratios and the presence of tabs. Aimmannee
presents a model [78] which accurately predicts the shape of the Thunder. LiPCAs
are based on the same principle, but they are produced by laminating a piezo patch
between glass-epoxy and carbon-epoxy plies.
Mulling et al . [82] makes an analysis of the Thunder under external mechanical
loading with different end conditions (e.g., clamping or simply supported). Hyer
and Jilani [83] focus on the geometrically non-linear deformation of the actuator
that results from the production method. But Haertling and co-workers [84] men-
tion another effect that might enhance the defl ection of Thunder-type actuators
under actuation. The residual stresses that occur after cooling from processing at
elevated temperature cause additional domain orientation. This increases the
piezoelectric charge constant d and thus the coupling between an applied fi eld and
the strain of the PZT in the actuator.
Haertling and co-workers [84] focuses mainly on this effect in the analysis of
Rainbow actuators. The concept of this actuator is similar to that of Thunders and
LiPCA's, but with Rainbow actuators the layered constitution is obtained by
'reducing' a PZT patch to a certain depth. In this procedure [85], the oxygen from
the PZT reacts with a carbon plate on which the PZT is placed at high temperature.
This reduced layer has no piezoelectric coupling and has different mechanical
properties and CTE than the unreduced layer. In addition, a volume change occurs
in the reduction process. This leads to residual stresses after cooling. A Rainbow
actuator is therefore not laminated but monolithic.
There are also other ways to mechanically amplify the defl ection of piezoelectric
actuators. Niezrecki et al . [86] presents an overview including concentric cylinder
telescopic actuators, leave springs and lever systems. These mechanical amplifi ers
are not feasible for integration into active surfaces, but should be regarded as optional,
low-wear, stand-alone actuators that could drive other mechanisms in turn.
3.2 Shape memory alloys
3.2.1 Material characteristics
SMAs derive their name from the fact that large, plastic deformations to which
the material is subjected can be recovered. At low temperatures the material is
in martensitic. The martensitic lattice structure has two variants, or twins. Load-
ing the martensite above a certain level will lead to 'detwinning'. In this process
one variant fl ips to the other over a certain stress range and the material can be