Integration of monolithic piezoelectric damping devices into adaptive composite structures

Abstract

The first part of this thesis is dedicated to the problem of finding optimal positions for the placement of piezoelectric tranducers in complex composite structures. The proposed procedure is based on the maximization of the strain in the inclusion area of the piezoelectric element and restricts the placemement to regions of allowable maximum strain. From the harvested piezo strain energy in relation to the total strain energy of the structure, the generalized electromechanical coupling factor can be derived. A realistic finite element representation of an open rotor blade is developed and validated. The placement procedure is then tested on a composite blade structure. Evaluations are very fast, allowing for exhaustive search for placements in the design domain. Placement results can be displayed in contour plots which illustrate the influence of transducer placement on the coupling efficiency and according sensitivities. Placement layers and areas of equal coupling performance can be identified. The proposed procedure is then compared to experimental results. A full carbon composite rotor blade with structurally integrated piezo modules is manufactured. The generalized electromechanical coupling coeffcient is measured in a vibration test stand. Results are compared to calculated values. The major part of this thesis refers to the investigation of piezoceramic transducer patches with increased strain allowables. Different ways to introduce compressive prestrain in the piezoceramic material are investigated. Most promising is the use of mechanical preloading of the insulating polyimide foil which leads to a compression of the piezoceramic material. We propose a piezo transducer patch that consists of the monolithic piezoelectric material polarized in thicknes direction (d31), electrodes of thin copper foil, epoxy as matrix material and the mentioned polyimide foil as top and bottom insulating layer. The transducer design matches the demands for structural integration such as flat design for minimum laminate strength reduction, electrical insulation from the conducting CFRP (carbon fibre reinforced plastic) laminate and increased strain allowables.