Implementation of the ‘smart’ rotor concept - Wind Power Generation and Wind Turbine Design - page 481

Figure 8: The principle of active fi ber composites (fi gure adapted from [72]).
Figure 9: The build-up of a MFC (fi gure adapted from [74]).
A special type of these active fi ber composites are called macro fi ber composites
(MFC's) [74] (Macro Fiber Composites), developed by NASA. MFCs are pro-
duced by sawing very fi ne strips from a patch and embedding those in epoxy
between Kapton fi lm (see Fig. 9).
Another way to amplify the displacement of a piezo-based actuator is to apply
sheet material in a bender, either as a unimorph, with a patch on one side, or as a
bimorph with a patch on each side. This way, the small strain of the piezo patch
can be used to induce relatively large defl ections of the bender. The behavior of
such a bender is quite accurately described by extending the classical laminate
theory (CLT) with piezoelectric effects analogous to thermal expansion. But other
models which incorporate out-of-plane shear have also been developed [72]. When
applying piezo patches in laminates, one must be aware that a piezo patch strains
in all in-plane directions when a fi eld is applied. However, usually straining the
construction or substrate in one direction is wanted. Barrett et al . [ 75 ] therefore
introduced the enhanced directionally attached piezoelectric (EDAP). With the
concepts, narrow strips or special adhesion strategies are used to exploit the
induced strain in only one direction.