Implementation of the ‘smart’ rotor concept - Wind Power Generation and Wind Turbine Design - page 491

superelastic behavior and the other about the ability of the materials to exert
work. Both test will provide points another set of points on the transitions' bor-
ders (the fi rst being the result of the DSC measurements).

Lagoudas further mentions it is also important to determine the stabilizing behav-
ior under cyclic loading, especially if more than one cycle are part of the function-
ality. Prahlad compares the results from a model, based on the characterization
experiments with experimental results for restrained recovery.
To be applied as an actuator, the SMA material must be prestrained and pre-
stressed and attached to, or embedded in the structure. When the SMA material is
heated it will start to recover its deformation. The structure will resist to the defor-
mation and the resulting stresses will postpone the formation of austenite. The
structure or a bias force (spring, mass) will also have to force it back to its original
position because typically the SMA is employed with one-way behavior. In addition,
two wires can be set to act against each other.
If the structure is stiff enough, the behavior of a SMA can be described as
restrained recovery: the strain remains negligible in comparison to the maximal
recoverable strain, and it reduces to a s , T -behavior. This still shows a considerable
amount of hysteresis and the behavior is non-linear. The restrained recovery force
can be used to determine the defl ection of structures.
Restrained SMA wires can exert high forces, up to several hundred MPa. The
force that can be exerted increases linearly for moderate amounts of prestraining,
but it fl attens off for high rates of prestraining [100]. Practical functionalities of
(embedded) SMA wires and ribbons include tuning of dynamic behavior [101]
and increasing aeroelastic stability [41], increasing critical buckling loads [102]
and increased impact resistance [103]. Practical applications are mentioned by
[19] and [20]. They mention pipe couplings that do not require fasteners and
deforming cheyfrons on jets in order to change the jet outlet from low noise con-
fi guration during landing and take off to optimal performance while cruising.
SMA material is also often implemented in bio-mechanical engineering, because
of its good bio-compatibility. Use of the SME in bio-mechanical engineering can
be found in stents to open arteries and in minimally invasive surgical equipment.
See also [104].
SMA materials seem very suitable for application for control surfaces on MW-
sized turbines because of their high power density, high actuation force and/or
strain capability and because their bandwidth is in the required range. However,
several drawbacks exist:

• Like all conductors, they are susceptible to lightning strike.

• The goal of the control system of which the actuator is a part is to alleviate fatigue

loads on the blade. However, SMA material itself shows poor fatigue properties.
Several options exist to increase the fatigue life:
Only subject the material to partial cycles -

- Implement materials that exhibit the R-phase transition

- Use special high fatigue alloys