Piezoelectric actuators usually are analyzed using finite element programs. These programs are often restricted to linear constitutive equations. Furthermore, the coupling with electrical element types is very problematical. Experiments showed, that even at weak electric fields the piezoceramics show nonlinear behavior, if the structure is excited near a resonance frequency. In many applications actuators are excited in resonance, for instance in ultrasonic motors and therefore these nonlinearities are important in practice. In this paper a nonlinear model for piezoelectric materials is presented. Nonlinear constitutive equations are derived from Hamiltons principle to calculate the longitudinal oscillations of a piezoceramic slender rod. The oscillations are excited by a harmonic electric potential at the electrodes of the rod, which is polarized in longitudinal direction. The resulting nonlinear partial differential equation is approximated using a Rayleigh Ritz ansatz. This leads to a set of ordinary nonlinear differential equations. In the present analysis, the displacement-functions used for the approximation are the eigenfunctions of the linearized system. The resulting nonlinear differential equation is solved by the harmonic balance method. This leads to a set of equations, that can be solved numerically to calculate the amplitude of the oscillations. As a result it is shown, that the Duffing type nonlinearities found in measurements can be described with this model. In future investigations the focus will be on the identification of the parameters of the nonlinear model.
In piezoceramic actuators normally a linear behavior is observed if the excitation is done by a weak electric field. In this paper, however, it is shown that if the system is excited near resonance even for a weak electric field strong nonlinearities may occur. As an example a piezoceramic transformer is investigated. The transformer is simulated using linear constitutive equations. For a very low excitation voltage the simulated results agree well with experimental results. For a moderate excitation voltage the experiments show that linear theory cannot describe all results and nonlinear constitutive equations have to be used.
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