Novel generation of auxetic open cell foams for curved and arbitrary shapes M. Bianchi a , F. Scarpa a,⇑ , M. Banse a , C.W. Smith b a Advanced Composites Centre for Innovation and Science, University of Bristol, Bristol BS8, UK b School of Engineering, Computing and Mathematics, University of Exeter, EX4 4QF, UK Received 10 September 2010; received in revised form 3 October 2010; accepted 3 October 2010 Abstract We describe the manufacturing and mechanical properties of a novel class of auxetic (negative Poisson’s ratio) foams, which can assume arbitrary and complex shapes, and can be produced in large bulk quantities. Samples of sheets produced following the new man- ufacturing technique show a more uniform distribution of the Poisson’s ratio behaviour under tensile loading compared to classical neg- ative Poisson’s ratio foams, and up to one order of magnitude higher energy dissipated per unit volume under cyclic tensile–tensile loading. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Foams; Thermomechanical processing; Tension test 1. Introduction Materials having a negative Poisson’s ratio (NPR, i.e. auxetic materials) expand when subjected to tensile load- ing, behaving therefore in an opposite manner to the majority of solids used in technological applications. A first example of auxetic material reported in literature was the single crystal of pyrite described by Love [1], reported to have a Poisson’s ratio of 0.14 in the [1 1 0] plane. In 1987, Lakes [2] manufactured the first sample of negative Poisson’s ratio foam from commercially available open cell thermoplastics (Scott Inc, USA). Lake’s seminal work, together with pioneering activities in molecular auxetics of Evans and co-workers [3] were instrumental to open the field of auxetics in materials science and engineering applications [4–12]. The production of auxetic open cell foams is classically performed with a three-step process: volumetric compres- sion of the pristine foam with the use of a mould, annealing and release of the foam from the production chamber [2,13–15]. The volumetric compression induces the buck- ling of the cell struts and the re-entrant topology of the auxetic foams, while the annealing fixes the shape of the pores, thus providing the NPR effect. The modality of the extraction from the production chamber can affect the mechanical properties of the foams produced, with a more pronounced NPR effect either in compression or ten- sion [16]. A scale-up manufacturing process for open cell negative Poisson’s ratio polymeric foams was developed by Lakes and Loureiro in 1997 [17], based on the already patented concept proposed in Ref. [18]. In Ref. [17],a mould made by opposing plates moving toward each other is used to compress the foam along three orthogonal direc- tions, before subjected to heating and cooling processes. A revised technique for converting thermoplastic foam into auxetic was described by Chan and Evans [13]. Scarpa et al. [14] subsequently modified the process to obtain aux- etic specimens with high resilience, improved stiffness and high energy dissipation per unit volume under compressive cyclic fatigue loading [19,20]. Alderson et al. [15] described another manufacturing route where the foam is com- pressed along two axes before being heated and subse- quently cooled, with the possibility of repeating the 1359-6454/$36.00 Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2010.10.006 ⇑ Corresponding author. Tel.: +44 1173315306; fax: +44 117 9272771. E-mail address: f.scarpa@bristol.ac.uk (F. Scarpa). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 59 (2011) 686–691