1 EXPERIMENTAL RESPONSE OF ADDITIVELY MANUFACTURED METALLIC PENTAMODE MATERIALS CONFINED BETWEEN STIFFENING PLATES A. Amendola 1 , C.J. Smith 2 , R. Goodall 2 , F. Auricchio 3 , L. Feo 1 , G. Benzoni 4 and F. Fraternali 1 1 Department of Civil Engineering, University of Salerno, Via Giovanni Paolo II 132, 84084 Fisciano (SA), Italy. adaamendola1@unisa.it (A. Amendola), l.feo@unisa.it (L. Feo), f.fraternali@unisa.it (F. Fraternali) 2 Department of Materials Science and Engineering, University of Sheffield, Mappin Street, Sheffield, S1 3JD, UK. chrisjsmith@sheffield.ac.uk (C.J. Smith), r.goodall@sheffield.ac.uk (R. Goodall) 3 Department of Civil Engineering and Architecture, University of Pavia, Via Ferrata 3, 27100 Pavia, Italy. auricchio@unipv.it (F. Auricchio) 4 Department of Structural Engineering, University of California at San Diego, La Jolla, California 92093- 0411, USA. gbenzoni@ucsd.edu (G. Benzoni) Keywords: Pentamode Lattices, Additive Manufacturing, Effective Elastic Moduli, Inelastic Response. Abstract An experimental investigation on the mechanical response of confined pentamode lattices in the elastic and post-yield regimes is presented. An Electron Beam Melting facility is employed to additively manufacture pentamode lattices confined by terminal plates in a titanium alloy. The given experimental results show that the geometry of the microstructure, and the macroscopic aspect ratio of the confined lattices strongly influence the lateral and vertical stiffness properties of the structure. The post-elastic response of the analyzed materials features acceptable energy dissipation capacity. The presented results highlight several analogies between the mechanical response of confined pentamode lattices and that of elastomeric bearings formed by soft rubber pads and stiffening steel or fiber-reinforced composite layers. They pave the way to future studies on the use of pentamode materials for the fabrication of innovative seismic isolation devices and/or shear-wave band gap systems. 1. INTRODUCTION Extremal materials that are receiving increasing interest are the so-called pentamode lattices, which consist of diamond-like lattices featuring five soft modes of deformation (unit cell with four rods meeting at a point) [1]. Such lattices exhibit very low shear moduli (theoretically equal to zero) [1], and may be able to stop or dramatically attenuate shear waves [2]. Physical models of pentamode materials have been fabricated through additive manufacturing (AM) techniques over the last few years, both at the macro- [3] and at the micro-scale [4]. Schittny et al. [3] have studied the experimental behavior of macroscopic, polymeric samples of pentamode lattices in the elastic regime: the results obtained by such authors prove that the elastic moduli of pentamode materials are strongly related to the geometry of the lattice micro-structure, being markedly affected by the dimensions of the rods forming the lattice, and particularly sensitive to the ratio between the diameter d of the connections between the rods and the lattice constant a . The experimental Young’s modulus E of the lattice has been found approximately three times stiffer than the experimental shear modulus G . The results presented in [3] also show that the ratio between the bulk modulus B and the shear modulus G strongly increases by reducing the contact area between