Piezo-actuated structural waves for delaminating surface accretions

Abstract

In recent years substantial attention has been given to the problem of unwanted accretions on structures of which the foremost example is aircraft icing. A potentially promising alternative to the traditional thermal methods for tackling ice build-ups is a mechanical approach in which the accreted layer is removed with the aid of structural waves induced by piezoelectric actuators. This thesis originates from research questions regarding this concept with a twofold motivation: assessment of the feasibility of the method and development of a wave-based modelling strategy applicable to a waveguide that contains layers made of a material with electromechanical coupling. The thesis starts with a broad theoretical study on free plane wave propagation in structures with accretions. The dispersion curves of a weakly coupled bilayer are analysed highlighting the aspects important from the viewpoint of the application. Conditions promoting high interface shear stress are identified and supported with a discussion on energy distribution patterns and their importance if a lossy accretion is considered. This part is concluded with a parametric study where accretions of different Young's moduli and different thicknesses are analysed with respect to the achievable interface shear stress yielding some implications for practical application.

In the next part the mechanical forcing is introduced in the analysis. Two different approaches, namely analytical and semi-analytical, are compared and the latter is validated with an experiment. In the light of injected power partitioning between the propagating waves, a similar parametric study is conducted aimed at broadening the insight gained from free wave analysis. Then, as a practical excitation strategy piezoelectric actuators are introduced. The common approaches for modelling piezoelectric excitation are reviewed and their strengths and limitations discussed. The background application requires high induced strain, therefore the actuators are expected to work near their resonances which violates the applicability conditions of the well-established models. In order to circumvent these limitations and avoid expensive conventional finite element model, a new methodology is developed in this thesis.

The approach consists of the development of a piezoelectric semi-analytical finite element that enables wave-based modelling of piezoelectric materials and calculation of the response to a distributed voltage excitation. The coupling of a finite length actuator to a mechanical waveguide is done with the aid of the analytical wave approach. A validation experiment on a beam with anechoic terminations is conducted showing very good agreement with the numerical results. The presented approach is very general and can be used for many 'smart structure' applications. The wave model is then used to investigate power and energy propagation in waveguides including the electrical system driving the actuator. The efficiency of the piezoelectric excitation is assessed with respect to the consumed electrical power and power loss in the driving system transmission line.

The parametric study shows the dependence of power transmission coefficients on the dimensions of the actuator and the thickness of the bonding layer. In the last part the methodology is applied to predict the interface shear stress generated by the piezoelectric excitation. The stress recovery routine is explained and validated with the conventional finite element method. The final parametric study is conducted showing the interface shear stress achievable for a structure with different accretions excited by a piezoelectric actuator and yielding the electrical power requirements. The concept of delaminating surface accretions with piezo-actuated structural waves is demonstrated in an experiment on a waveguide with emulated anechoic terminations. The corresponding numerical predictions for power requirements are found to be in a good agreement with experimental observations. The thesis is concluded with recommendations on the applicability of the wave-based concept and the design of the system that enables exploiting the described physical phenomena.

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