Appl Compos Mater (2010) 17:543–556 DOI 10.1007/s10443-010-9142-2 Characterisation by Inverse Techniques of Elastic, Viscoelastic and Piezoelectric Properties of Anisotropic Sandwich Adaptive Structures Aurelio L. Araújo · Cristovao M. Mota Soares · Carlos A. Mota Soares · Jose Herskovits Received: 30 May 2009 / Accepted: 30 March 2010 / Published online: 22 April 2010 © Springer Science+Business Media B.V. 2010 Abstract In this article we present recent developments regarding parameter esti- mation in sandwich structures with viscoelastic frequency dependent core and elastic laminated skin layers, with piezoelectric patch sensors and actuators bonded to the exterior surfaces of the sandwich. The frequency dependent viscoelastic properties of the core material are modelled using fractional derivative models, with unknown parameters that are to be estimated by an inverse technique, using experimentally measured natural frequencies and associated modal loss factors. The inverse problem is formulated as a constrained minimisation problem, and gradient based optimiza- tion techniques are employed. Applications are presented and discussed, focused on the identification of viscoelastic frequency dependent core material properties. Keywords Inverse problems · Optimization · Sandwich structures · Hybrid damping 1 Introduction Sandwich structures are used in a variety of structural applications wherever strong, stiff and light structures are required. Passive damping might be efficiently incor- porated in a sandwich material configuration if the core is made of a viscoelastic material, while active damping is easily implemented by attaching piezoelectric patches to the face layers, using an appropriate control strategy. The combination of passive and active damping is often called hybrid damping, allowing for a broader control band due to the high capacity of viscoelastic materials to dissipate energy A. L. Araújo (B ) · C. M. Mota Soares · C. A. Mota Soares IDMEC/IST, Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisbon, Portugal e-mail: aurelio.araujo@ist.utl.pt J. Herskovits COPPE, Federal University of Rio de Janeiro, Caixa Postal 68503, 21945-970, Rio de Janeiro, Brazil