Piezoelectric macro fiber composites (MFCs) have been implemented as actuators into an active composite wing. The goal of the project was the design of a wing for an unmanned aerial vehicle (UAV) with a thin profile and integrated roll control with piezoelectric elements. The design and its optimization were based on a fully coupled structural fluid dynamics model that implemented constraints from available materials and manufacturing. A scaled prototype wing was manufactured. The design model was validated with static and preliminary dynamic tests of the prototype wing. The qualitative agreement between the numerical model and experiments was good. Dynamic tests were also performed on a sandwich wing of the same size with conventional aileron control for comparison. Even though the roll moment generated by the active wing was lower, it proved sufficient for the intended roll control of the UAV. The active wing with piezoelectric flight control constitutes one of the first examples where such a design has been optimized and the numerical model has been validated in experiments.

The scientific community has put significant effort into the development and optimization of sensors and actuators manufactured as piezoelectric composites with interdigitated electrodes (IDEs), well known as active fiber composite (AFC) and macro fiber composite (MFC). The advantages of these elements are their higher actuation performance and flexibility as compared to monolithic piezoceramic (PZT) elements. In general, their mechanical properties are calculated based on the classical lamination theory and the uniform field model (UFM). These two theories are well suited for predicting the stiffness and piezoelectric strain constants of the AFC or MFC. Although there are a variety of numerical investigations related to their electromechanical properties, there are no appropriate tools for accessing the stresses within these piezoelectric elements (including the inhomogeneous electric field conditions as well as the change in material properties). Explanations are given for this situation indicating the problems in investigating these types of piezoelectric elements with respect to stress states. In this work a finite element modeling approach is presented, which shows the influence of the IDE on the mechanical properties of PZT fibers. Experimental evidence is presented, which affirms the location of critical stress predicted in this model and explains the reported cracking in AFC in past research.Key Words: piezoelectric fiber composite, active fiber composite (AFC), macro fiber composite (MFC), interdigital electrode concept (IDE), state of stress, material orientation, state of polarization.

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It is well known that the material properties of piezoelectric materials strongly depend on the state of polarization of the individual element. While an unpolarized material exhibits mechanically isotropic material properties in the absence of global piezoelectric capabilities, the piezoelectric material properties become transversally isotropic with respect to the polarization direction after polarization. Therefore, for evaluating piezoelectric elements the material properties, including the coupling between the mechanical and the electromechanical behavior, should be addressed correctly. This is of special importance for the micromechanical description of piezoelectric elements with interdigitated electrodes (IDEs). The best known representatives of this group are active fiber composites (AFCs), macro fiber composites (MFCs) and the radial field diaphragm (RFD), respectively. While the material properties are available for a piezoelectric wafer with a homogeneous polarization perpendicular to its plane as postulated in the so-called uniform field model (UFM), the same information is missing for piezoelectric elements with more complex electrode configurations like the above-mentioned ones with IDEs. This is due to the inhomogeneous field distribution which does not automatically allow for the correct assignment of the material, i.e. orientation and property. A variation of the material orientation as well as the material properties can be accomplished by including the polarization process of the piezoelectric transducer in the finite element (FE) simulation prior to the actual load case to be investigated. A corresponding procedure is presented which automatically assigns the piezoelectric material properties, e.g. elasticity matrix, permittivity, and charge vector, for finite element models (FEMs) describing piezoelectric transducers according to the electric field distribution (field orientation and strength) in the structure. A corresponding code has been realized for a commercial finite element program allowing for an automatic assignment of different material properties for two- and three-dimensional FEMs with arbitrary electrode configurations. Examples of piezoelectric transducers with complex electrode configurations are presented and the influence of the material description on the behavior of the modeled element is discussed. Furthermore, as an attempt at verification of the FEM simulation, a comparison of simulated stress concentrations with experimental investigations is presented.