Technical Report Tensile properties of cork in the tangential direction: Variation with quality, porosity, density and radial position in the cork plank Ofélia Anjos a,b,d, * , Helena Pereira b , M. Emília Rosa c a Unidade Departamental de Silvicultura e Recursos Naturais, Escola Superior Agrária de Castelo Branco, Apartado 119, 6001-909 Castelo Branco, Portugal b Centro de Estudos Florestais, Instituto Superior de Agronomia, Universidade Técnica de Lisboa, 1349-017 Lisboa, Portugal c Departamento de Engenharia de Materiais, Instituto Superior Técnico, Universidade Técnica de Lisboa, 1049-001 Lisboa, Portugal d CERNAS – Centro de Estudos de Recursos Naturais, Ambiente e Sociedade, 3040-316 Coimbra, Portugal article info Article history: Received 20 August 2009 Accepted 22 October 2009 Available online 25 October 2009 abstract The behaviour of cork under tensile stress in the tangential direction was studied using cork planks of two commercial quality classes and samples taken at three radial positions in the cork planks. The stress–strain curves showed a linear elastic region approximately until 2% strain (stress at 0.4 MPa), followed by a region of decreasing slope up to the fracture at an average stress of 0.6 MPa (strain of 5%). The radial position in the cork plank was a highly significant factor of variation in the tensile properties. The tensile properties showed statistically significant correlations with cork density and porosity. Published by Elsevier Ltd. 1. Introduction Cork, the raw-material for wine stoppers and insulation prod- ucts, is a natural material of cellular structure with an unusual combination of properties: low density, very low permeability to liquids, high compressibility with dimensional recovery, low con- ductivity, chemical stability and durability [1]. The aesthetical character and overall feeling of comfort of cork are also valued in many applications [2,3], while combination of cork with other materials may improve performance [4]. The properties of cork result from the characteristics of its cel- lular structure and biological formation as the outer bark of the cork oak tree (Quercus suber L.) [5]. Cork is made up of small closed cells (4–20 Â 10 7 cells/cm 3 ) forming a highly ordered honeycomb with cell walls made up of suberin in association with lignin and, to a lesser extent, of cellu- lose and hemicelluloses [1,6,7]. Cork presents variability in struc- ture and properties caused by the presence of lenticular channels that cross the cork planks along a radial direction, linking the out- side to the internal living tissues and allowing gas exchanges [8]. The lenticular channels contain a filling material of loosely bound cells of different chemical composition and are occasionally lined by heavily lignified cells. The mechanical behaviour of cork in compression shows stress– strain curves that are linear elastic up to about 7% strain, at which point elastic collapse gives a rather horizontal plateau which ex- tends to nearly 70% strain when complete cell collapse occurs [9–11]. The compressive properties of cork were found to vary with density [12], cellular dimensions [13] and quality class [11]. The behaviour of cork under tensile stress is less documented. Cork is put under tensile stress when a stopper is pulled out of a wine bottle; in this case the load is applied in the axial direction of cork. In the tree, the cork layers are also put under tension by the radial growth of the stem [14]; in this case the load is applied tangentially. In both cases, fracture may occur. In tension, the cork cell walls straighten and align in the direction of tension, and sub- sequently stretch until fracture [15]. The tensile behaviour of cellular materials depends on the homogeneity of the structure and is very sensitive to the presence of defects. For instance, the random removing of cell walls has a dramatic effect on the stiffness and strength of honeycombs and foams [16], and 10% removal of struts reduces the modulus and strength of a honeycomb by about 60–70% [17,18]. A previous work [19] found that tensile properties of cork stressed in the axial direction, namely fracture stress and strain, depend on the irregu- larities that are present in the cork structure and on their specific characteristics. The aim of this study is to characterize the behaviour of cork under tangential tension using stress–strain curves, and to analyze the influence of porosity, density and commercial quality of cork planks on the Young’s modulus and fracture resistance, as well as the influence of the sample’s radial position in the cork plank. Fur- ther a comparison with the results obtained previously regarding the tensile properties under axial stress is also made. 0261-3069/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.matdes.2009.10.048 * Corresponding author. Address: Unidade Departamental de Silvicultura e Recursos Naturais, Escola Superior Agrária de Castelo Branco, Apartado 119, 6001-909 Castelo Branco, Portugal. Tel.: +351 272339900; fax: +351 272339901. E-mail address: ofelia@esa.ipcb.pt (O. Anjos). Materials and Design 31 (2010) 2085–2090 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes