Andrea Bergamini

Broadband vibration energy harvesting based on cantilevered piezoelectric bi-stable composites

Bi-stable composites are considered for vibration based energy harvesting, thanks to the broadband nature of their dynamic response. In this letter, a cantilevered piezoelectric bi-stable composite concept is introduced for broadband energy harvesting. The proposed configuration allows for exploiting the large strains developed close to the clamped root, significantly enhancing the harvesting effectiveness in comparison to previous settings. A simple model is used for designing the dynamic response aiming to maximise broadband oscillations. Experimental results reveal wide bands of high power conversion. Additionally, a shunting circuit suitable for broadband conversion is employed, further increasing the effectiveness of the proposed concept.

Measurement of insulating and dielectric properties of acrylic elastomer membranes at high electric fields

This work reports on the investigation of VHB 4910 acrylic elastomer insulating and dielectric properties. This material is widely exploited for the realization of actuators with large deformations, dielectric elastomer actuators (DEA), and belongs to the group of so-called electroactive polymers (EAP). Extensive investigations concerning its mechanical properties are available in literature while its electric behavior at working conditions has not received the same level of attention. In this work, the relative permittivity and the volume resistivity have been measured on VHB 4910 membranes under different fixed stretch conditions (λ1, λ2 = 3, 3.6, 4, 5) using circular gold electrodes sputtered onto both sides of the specimens. The measured values of relative permittivity are in fairly good agreement with the results previously published by other groups. The volume resistivity, at field values close to the operational ones, has shown a field-dependent behavior revealing dissipative properties that should...

A sandwich beam with electrostatically tunable bending stiffness

The tuning of the bending stiffness of structural elements is of interest for, among other things, the suppression of vibrations related to resonance phenomena. For a given cross-sectional area and geometry, the variation of the elastic properties of the material composing the structure provides a viable approach to this task. Only very limited options are available for such changes in material properties. The use of NiTi shape memory alloys has been proposed for this purpose. A new, energetically less expensive method for the modification of the bending stiffness of sandwich beams is presented. The proposed method makes use of electrostatic forces to modify the transfer of shear stresses at the interface between the faces and the core of the sandwich. Changes in bending stiffness of up to 18 times could be obtained for a prototype beam. A simple model for describing the behavior of the beam is presented.

Electrostatically tunable bending stiffness in a GFRP?CFRP composite beam

The suppression of vibrations in structures is commonly considered a useful measure for the extension of their lifetime, when high amplitude vibrations are observed. In the experiments presented in this work, the modification of the stiffness of a beam as a means to suppress vibrations due to resonance is proposed as an alternative to the introduction of discrete damping devices. The stiffness of a beam is modified by applying an electric field between the main element of the structure and additional stiffening elements applied to its surface, thus coupling the latter to the former by transfer of shear stresses. The effect of electrostatic tuning of the bending stiffness (and consequently of its eigenfrequencies) of a large size GFRP–CFRP beam is shown by the shift of the resonance peak for the first bending mode to higher frequencies. The discrete character of the stiffness increase in multi-layer beams (n≥3) is postulated.

Passive damping of composite blades using embedded piezoelectric modules or shape memory alloy wires: a comparative study

Emission reduction from civil aviation has been intensively addressed in the scientific community in recent years. The combined use of novel aircraft engine architectures such as open rotor engines and lightweight materials offer the potential for fuel savings, which could contribute significantly in reaching gas emissions targets, but suffer from vibration and noise issues. We investigated the potential improvement of mechanical damping of open rotor composite fan blades by comparing two integrated passive damping systems: shape memory alloy wires and piezoelectric shunt circuits. Passive damping concepts were first validated on carbon fibre reinforced epoxy composite plates and then implemented in a 1:5 model of an open rotor blade manufactured by resin transfer moulding (RTM). A two-step process was proposed for the structural integration of the damping devices into a full composite fan blade. Forced vibration measurements of the plates and blade prototypes quantified the efficiency of both approaches, and their related weight penalty.

Hybrid dispersive media with controllable wave propagation: A new take on smart materials

In this paper, we report on the wave transmission characteristics of a hybrid one dimensional (1D) medium. The hybrid characteristic is the result of the coupling between a 1D mechanical waveguide in the form of an elastic beam, supporting the propagation of transverse waves and a discrete electrical transmission line, consisting of a series of inductors connected to ground through capacitors. The capacitors correspond to a periodic array of piezoelectric patches that are bonded to the beam and that couple the two waveguides. The coupling leads to a hybrid medium that is characterized by a coincidence condition for the frequency/wavenumber value corresponding to the intersection of the branches of the two waveguides. In the frequency range centered at coincidence, the hybrid medium features strong attenuation of wave motion as a result of the energy transfer towards the electrical transmission line. This energy transfer, and the ensuing attenuation of wave motion, is alike the one obtained through internal resonating units of the kind commonly used in metamaterials. However, the distinct shape of the dispersion curves suggests how this energy transfer is not the result of a resonance and is therefore fundamentally different. This paper presents the numerical investigation of the wave propagation in the considered media, it illustrates experimental evidence of wave transmission characteristics and compares the performance of the considered configuration with that of internal resonating metamaterials. In addition, the ability to conveniently tune the dispersion properties of the electrical transmission line is exploited to adapt the periodicity of the domain and to investigate diatomic periodic configurations that are characterized by a richer dispersion spectrum and broader bandwidth of wave attenuation at coincidence. The medium consisting of mechanical, piezoelectric, and analog electronic elements can be easily interfaced to digital devices to offer a novel approach to smart materials.

Tuning the mechanical behaviour of structural elements by electric fields

This work reports on the adoption of electric fields to tune the mechanical behaviour of structural elements. A mechanical characterization procedure, consisting of double lap joint and 3-point bending tests, is conducted on copper-polyimide laminates while applying electric fields of varying intensity. Field dependence and, thus, adaptability of shear strength and bending stiffness are shown as a function of the overlapping length and interfaces number, respectively. Further, the impact of remaining charges is investigated in both testing configurations. The findings herein lay the foundation for the implementation of electro-adaptive components in structural applications.

Morphing wing structure with controllable twist based on adaptive bending-twist coupling

A novel semi-passive morphing airfoil concept based on variable bending–twist coupling induced by adaptive shear center location and torsional stiffness is presented. Numerical parametric studies and upscaling show that the concept relying on smart materials permits effective twist control while offering the potential of being lightweight and energy efficient. By means of an experimental characterization of an adaptive beam and a scaled adaptive wing structure, effectiveness and producibility of the structural concept are demonstrated.

Quasi-static electric properties of insulating polymers at a high voltage for electro-bonded laminates

This paper reports on a high voltage measurement set-up for determining the relative permittivity and the volume resistivity of dielectric polymers. These properties were evaluated on thin films at electric fields up to 80?V? ?m?1 and in the quasi-static regime at frequencies lower than 1?Hz. It is found that the high field properties of FEP, PFA, polyimide and Mylar are comparable to the respective low field values, while for ferroelectric PVDF poling behavior becomes evident at high fields. High field properties of dielectric polymers are of particular importance in the design of devices relying on electrostatic attraction, such as electro-bonded laminates applied in tunable bending stiffness structures.

Identification of electromechanical parameters in piezoelectric shunt damping and loss factor prediction

Shunted piezoelectric elements have been studied for several years as promising devices for vibration damping. Different shunting techniques have been developed to deal with the vibration energy in the appropriate way. The energy dissipated by all these techniques, expressed in terms of loss factor or damping ratio, mainly depends on two different contributions: the electromechanical coupling and the shunt design. Therefore, an accurate prediction of the damping is based on a reliable identification of the generalized coupling coefficient that completely describes the electromechanical coupling. In this study, a robust method for the measurement of this coefficient is proposed, where the influence of the inherent damping of the structure is also considered. This method is based on the analysis of the dynamic response of the structure when the piezoelectric patch is connected to a resonant shunt. The proposed method is applied to different sample structures, and the measured generalized coupling coefficients are used for predicting the values of damping attainable with different shunting techniques (such as the resonant shunt or the synchronized switching damping). Vibration tests are then carried out on the same shunted structures, and the analytical prediction of the damping is compared with the experimental results.