New approach for fabrication of auxetic foam and determination of Poisson's ratio MOZAFAR Shokri Rad 1,a and ZAINI Ahmad 1,b 1 Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 Johor Bahru, Johor, Malaysia a shokri_rad1221@yahoo.com, b azaini@fkm.utm.my Keywords: auxetic, Poisson’s ratio, compression, fabrication Abstract. Conventional foam could be converted to auxetic foam under auxeticity process. A new and simple technique to fabricate auxetic foam and to further determine its Poisson’s ratio is described in this paper. It is evident that the present modified technique in fabricating auxetic foam could be adopted to produce desirable auxeticity characteristics. Moreover, the approach used for determination of Poisson’s ratio has considerably merit with great cost effectiveness. This method is, however, specific to auxetic foam sample under compression loading. Introduction Auxetic foam has been known conceptually as material that has negative Poisson’s ratio. Lakes [1] has discovered a process of transferring opened-cell conventional foam to auxetic one. The process comprises of three steps i) Compression of the foam specimen tri-axially ii) Heating the foam specimen up to just above softening temperature and iii) Cooling down at room temperature. However, there are some drawbacks of such process. One of them is a long-term instability which causes samples reverting back to their original shape, severe surface creasing, inability of isotropic auxetic production and limitation of complex shapes. In particular, the surface creasing and wrinkles observed in any deformable regions of foam specimen are due to the volumetric compression ratio applied during fabrication [2,3]. Chan and Evans [2] have proposed an applicable solution to surmount the formation of wrinkles on the sample surface by applying volumetric compression in several stages. It is evident that more homogeneous auxetic foam may be produced by using this technique. For decades, this production process has been applied by numerous researchers and a number of modifications have been reported [2, 3, 4-7, 8]. Nevertheless, the overall principle has remained the same, i.e. volumetric compression followed by heating and cooling [2, 6, 9]. The most possible modifications reported are the multi-phase auxetic fabrication [7, 9], solvent based auxetic fabrication, vacuum-bag auxetic fabrication and dual density auxetic fabrication. Fabrication of auxetic foam Despite numerous modification of the auxetic fabrication has been discovered, some of the abovementioned drawbacks still exist. In this present study, a new modification method has been proposed to enhance the stage of volumetric compression and fabrication of isotropic auxetic even for more complex shape. Fig. 1 depicts the fabrication technique of auxetic foam and the equipment used. A fabrication technique has been designed to apply pressure tri-axially on the conventional opened-cell foams, resulting inauxetic behaviour. The technique generally needs several equipment namely a hydraulic oil pump (a capacity of 200 bar), athick-walled aluminum cylinder (both closed- end), a high pressure ball valve, moulds and oven. Inlet and outlet valves on one-end have been devised for inward flow of pressurized oil and air removal, respectively, as shown in Fig. 2. The oil pump is connected to the aluminium cylinder and high pressure ball valve. Firstly, a bulk size of polyurethane foam has previously been cutin a cubic size of 22x22x22 mm. In order to prevent oil absorption into the foam specimen, it is enclosed with a layer of plastic and two layers of special