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

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... ining charges is investigated in both testing configurations. The findings herein lay the foundation for the implementation of electro-adaptive components in structural applications. V C 2013 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4809728] The development of new aerospace and automotive products has been enabled by remarkable progress in lightweight and composite structures disciplines. 1,2 Despite the adoption of new materials, which have allowed increasing their stiffness and strength to weight ratio, typically their mechanical behaviour is (a) time-invariant and (b) design driven. Specifically, any external load exerted on them is expected to yield the same response at all times. The resulting designs are necessarily sub-optimal tradeoffs, as they are sized for a wide range of operating conditions. These conditions often include very rare and unlikely operational states that are, however, critical and thus constitutive for the design, although compromising the performance during most of the operating time. The appearance of adaptive structures provides new degrees of freedom to designers since adaptivity allows the structure to respond differently to each operating conditions and to perform optimally in each one of them. 3 In this framework, airfoils having adaptive capabilities, i.e., morphing airfoils, are the object of considerable research efforts in civil and military applications. 4-6 The benefit of morphing capabilities is also postulated for wind turbine blades of the upcoming generations (with blade lengths exceeding 100 m). 7 Elements with variable bending/torsional stiffness capabilities are in demand for realizing such morphing concepts. Electro bonded laminates (EBL) are capacitor-like multilayered structures able to switch, on demand and reversibly (off/on-voltage), their capability of shear stress transfer and, consequently, their stiffness components like the bending stiffness T. In particular, they can also directly meet the need for elements with adaptive shear strength arising from structural concepts for morphing based on this feature. 8 The application of a voltage signal on EBL steers charges migration from one conductive plate to the other determining the appearance of charges with opposite sign on two facing electrodes. This capacitor-charging process, in turn, determines the onset of Coulomb attractive forces between the conductive plates. 9 Molecular adhesion and surface roughness cause resistive forces of friction to take place. 10 It is postulated, theoretically, 11 that the presence of friction combined with the appearance of Coulomb attractive forces lead to the formation of a physically bonded state and allow, by means of voltage control, (a) to adjust the shear strength of the bonding and (b) to tune the bending stiffness of the structure (by controlling the number of interfaces electro-mechanically connected). The shear stress s ?k transferable at the interfaces of an EBL is given in Eq. (1). It is dependent on the normal stress r ? (Maxwell stress 12 ) that it exhibits under the influence of an applied electric field E, In Eq. (1), e 0 is the vacuum permittivity, e r is the relative permittivity of the dielectric layers constituting the EBL, and l is the static coefficient of friction (COF) between the layers. The relative change in bending stiffness of an EBL consisting of n electrode layers and (n -1) dielectric layers, following voltage application, is (in a purely geometric formulation) expected to be proportional to the number of layers squared. 13 Considering a difference in electrodes and dielectric thicknesses (t e and t d , respectively) and assuming the Young's modulus of the dielectric layers Y d to be much smaller than the one of the electrodes Y e , a more general expression of the stiffness ratio reads, Previous works have implemented, experimentally, EBL in vibration suppression applications. 13 A detailed electromechanical characterization of EBL, showing their effective shear strength and bending stiffness behaviour as a function of layers' number and voltage intensity, is still missing but greatly needed. The purpose of this work is to focus on these aspects in order to fully exploit the potential of EBLs, define their limitations, and predict their behaviour when introduced in structural applications. This work investigates also the impact of charge injection at the dielectric/electrode interface on preventing the realization of a fully compliant state (0-stiffness) when the voltage is turned off. a)