Effect of density on the compression behaviour of cork

Abstract The compression properties of cork were studied for samples of different density. The densities were grouped into three classes: low density (0.13–0.15), mid density (0.15–0.19) and high density (0.19–0.25). The porosity of the cork samples increased from the low to the high density class, with porosity coefficients of 5.1%, 6.9% and 9.4%, respectively. The difference in the porosity was associated with structural features, namely the presence of thick walled cork cells and the presence of lignified cells lining the pores. The stress–strain curves were similar for all cases, showing an elastic compression up to a yield point of about 5% strain, followed by a plateau with a small slope. The cork strength was higher in the radial direction than in the other directions. The density influenced the compression such that the corks with high density presented higher stiffness in compression in three directions: Young’s modulus was 17.4, 22.6 and 26.1 MPa for low, mid and high density corks respectively. This density effect was more evident in the plateau region of the progressive buckling of the cell walls (σ 30 was respectively 1.07, 1.29 and 1.54 MPa for the three density classes). The recovery of dimensions after compression in each direction was also studied following compression to 50% strain. The recovery was on average 50% of the initial deformation on the first day, and almost total after 15 days. The recovery was higher for corks with low density and in non-radial directions.

[1]  M. T. Nogueira,et al.  The poison effect in cork , 1989 .

[2]  Helena Pereira,et al.  Tensile properties of cork in axial stress and influence of porosity, density, quality and radial position in the plank , 2011, European Journal of Wood and Wood Products.

[3]  H. Pereira,et al.  Chemical composition and variability of cork from Quercus suber L. , 1988, Wood Science and Technology.

[4]  Helena Pereira,et al.  The Effect of Long Term Treatment at 100°C–150°C on Structure, Chemical Composition and Compression Behaviour of Cork , 1994 .

[5]  H. Pereira,et al.  Natural variability of surface porosity of wine cork stoppers of different commercial classes , 2012 .

[6]  M. Ashby,et al.  The structure and mechanics of cork , 1981, Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences.

[7]  João F. Mano,et al.  The viscoelastic properties of cork , 2002 .

[8]  Helena Pereira,et al.  Variability of the Chemical Composition of Cork , 2013 .

[9]  Helena Pereira,et al.  Effect of quality, porosity and density on the compression properties of cork , 2008, Holz als Roh- und Werkstoff.

[10]  Helena Pereira,et al.  Cork : biology, production and uses , 2007 .

[11]  Helena Pereira,et al.  Tensile properties of cork in the tangential direction: Variation with quality, porosity, density and radial position in the cork plank , 2010 .

[12]  Helena Pereira,et al.  Characterization of radial bending properties of cork , 2011, European Journal of Wood and Wood Products.

[13]  J. García-Olmo,et al.  Discriminant Analysis of Geographical Origin of Cork Planks and Stoppers by Near Infrared Spectroscopy , 2012 .

[14]  M. A. Fortes,et al.  Rate effects on the compression and recovery of dimensions of cork , 1988 .

[15]  Helena Pereira,et al.  The Evaluation of the Quality of Cork Planks by Image Analysis , 1996 .

[16]  Helena Pereira,et al.  The Cellular Structure of Cork from Quercus Suber L. , 1987 .

[17]  Helena Pereira,et al.  The Effect of Growth Rate on the Structure and Compressive Properties of Cork , 1992 .

[18]  M. Ashby,et al.  The mechanics of three-dimensional cellular materials , 1982, Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences.