Luigi Di Lillo

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.

Mechanical characterization of electro bonded laminates

This work reports on the coupled electro-mechanical simulation and mechanical characterization of electro-bonded laminates (EBL). In light of their conceivable implementation in morphing wings as leading/trailing edge reinforcement plates or as active elements for shear center position adaptation, values of shear strength comparable to those achievable by an epoxy resin, i.e. 10 - 15 [MPa], are needed. Finite element analysis routines have been implemented to gain insights into the shear behavior of EBL under high electric fields. Further, they allowed for the optimization of EBL in terms of electrode shaping in order to obtain a smooth introduction of axial stresses, avoiding thereby the appearance of shear stress singularities at the edges of the structures. Experimental verification was carried out through single lap shear tests. These experiments quantified the mechanical properties of electro bonded interfaces, assessed the importance of the thickness and alignment of electrodes and the degradation of their constitutive layers due to friction.

Energy harvesting module for the improvement of the damping performance of autonomous synchronized switching on inductance

Shunted piezoelectric transducers can be used to dissipate vibration energy of a host structure. The synchronized switch damping on inductance is a shunting technique characterized by a nearly rectangular shape of the resulting voltage on the transducer being in antiphase with the structure’s velocity. As for these systems, previous studies have reported the strong relationship between the dissipated energy and the slope of the voltage signal occurring during the switch. This implies that any electrical losses have to be minimized in order to increase the slope of the voltage signal and, thus, the damping performance. The rate of change of the voltage represents a critical issue for autonomous shunts, where the switch can be inefficient because the power for switching the circuit is not supplied by an external source but is supplied by the vibrating structure itself. In this study, a new technique for improving the damping performance of autonomous synchronized switch damping on inductance is proposed based on controlling the switch with a square wave signal that reduces its electrical losses. An experimental validation of the proposed shunting technique is carried out in order to assess the performance in both the cases of a single-tone and multimodal responses of the structure.

Piezoelectric vibration damping using autonomous synchronized switching on inductance

Shunted piezoelectric transducers can be used to dissipate vibration energy of a host structure. The Synchronized Switch Damping on Inductance (SSDI) is a shunting technique featured by a nearly rectangular shape of the resulting voltage on the transducer being in anti-phase with the structure’s velocity. As for these systems, previous studies have reported the strong connection of the dissipated energy on the slope of the voltage signal occurring during the switch. This implies that any electrical losses have to be minimized in order to increase the slope of the voltage signal and, thus, the damping performance of the shunt. Moreover, the rate of change of the voltage signal represents a critical issue for autonomous shunts, where the switch can be inefficient because the power for switching the circuit is not supplied by an external source but is supplied by the vibrating structure itself. In this study, a new technique for improving the damping performance of autonomous switching shunt is proposed based on reducing the electrical losses of the switch. Finally, an experimental validation of the novel shunting technique is carried out.Copyright

Frequency-dependent dielectric response model for polyimide-poly(vinilydenefluoride) multilayered dielectrics

A physical model for the frequency-dependent dielectric response of multilayered structures is reported. Two frequency regimes defined by the relative permittivities and volume resistivities of the layers have been analytically identified and experimentally investigated on a structure consisting of polyimide and poly(vinilydenefluoride) layers. The relative permittivity follows an effective medium model at high frequency while showing a dependence on the volume resistivity at low frequency. In this regime, relative permittivities exceeding those expected from effective medium model are recorded. These findings provide insights into inhomogeneous dielectrics behavior for the development of high energy density dielectric films.

Multilayered Structures for Electro Bonded Laminates

This paper reports on the investigation and optimization of the electric properties of a geometry based variable stiffness concept. The concept relies on the combination of electrostatic normal forces and friction between the layers of a multi-layer beam to develop structures, electro bonded laminates (EBL), with an actively tuneable bending stiffness. Previous studies have detailed the connection between the mechanical response of these system and the electrical properties of its polymer constitutive films. Particularly, they pointed out the need to improve both the dielectric and the insulating properties of these films in order to have an enhancement of the bending tunability range of the system. In this paper a multilayer polymer configuration is considered as a possible answer to this need and it is put forward together with the electric model and the relatively high frequency experimental evidence of the proposed solution.Copyright