Issue 53

Y. Saadallah, Frattura ed Integrità Strutturale, 53(2020) 417-425; DOI: 10.3221/IGF-ESIS.53.32 418 Poisson's ratio have been determined. The results obtained show that cork is almost isotropic in non-radial directions. However, it has a larger Young's modulus and a zero Poisson's ratio when it is stressed in the radial direction. Oliveira et al. [7], studied the variability of the compressive properties of cork. The results show that the radial direction has the greatest compressive strength while the resistance in the axial and tangential directions is almost similar. Anjos et al. [8] investigated the effect of density on the properties of cork in compression. This results in an increase of the Young's modulus with the increase of the density especially beyond the elastic region. García et al. [9] have sought a compression model linking physical properties (porosity, density and test direction) to mechanical properties using a classical linear regression technique. The compressive strain stress curves reveal an elastic domain at (5-7) % followed by a large plateau for strains caused by the progressive buckling of the cell walls until a strain of (50-70) %, and a steep stress increase for higher strains corresponding to cell collapse [5, 7, 8, 10]. Cork has the distinction of having an insignificant Poisson's ratio [2]. This is explained by its nature and highly porous and cellular structure. Indeed, cork has the ability to undulate its cell walls when it is compressed and thus it can have a large longitudinal compression deformation without lateral expansion [10]. Complete densification corresponds to 85% strain without fracture [7]. Indeed, the fracture occurs in the case of a tensile stress. After the removal of the load, the cork recovers. The rate of recovery decreases over time [11]. For a strain level of 50%, half of the dimensions recover after the first day and end at 90% in the fifteenth day [8]. For a strain level of 30%, the recovery is total after 20 days of removal of the load [11] while it is not total for a level exceeding 80%. The behavior of the tensile cork in the tangential and axial direction has been studied respectively in the references [12, 13]. It follows that the tensile strength in the tangential direction is lower than that in the axial direction. In comparison with its compression behavior, cork has a lower tensile elastic domain. This area is around 2% in both tangential and axial directions. The fracture takes place respectively in the tangential and axial directions at 5% and 7.1%. On the other hand, in the radial direction, it corresponds to an 18% strain value [14]. It follows that the resistance of the cork is much greater in the radial direction as well in compression as in traction. Many studies have focused on the analysis of cork behavior through its parameters such as the Young's modulus, the elastic limit and the stresses corresponding to certain critical strain values [5, 7-9, 11-13]. However, few of them have focused on the proposal of models to predict the overall behavior of cork. The present work is a contribution in the modeling of the mechanical behavior of cork in compression. To do this, compression tests are conducted in the non-radial direction. On the basis of qualitative analysis of stress-strain curves, two models of behavior are proposed: a trilinear model and nonlinear model. The parameters that manage these models are identified. Model test comparisons are presented and discussed. E XPERIMENTAL P ROTOCOL Material of study he material of study is a cultivated breeding cork from the forests of the Jijel area in Algeria. The cork plank (reproduction cork) obtained are wetted in boiled water at atmospheric pressure for 1 h and left to air-dry to get rid of any impurities. It is a procedure widely applied in the cork industry. Specimens are cut into cubes of 20 mm on the side, with their faces perpendicular to each of the three main directions. (Fig. 1). This geometry is chosen for the sake of conformity with previous works, including, for example, references [7, 9]. The density was measured from the volume and weight in g.cm - 3 . Experimental procedure The compression tests in the non-radial direction were done at a constant crosshead speed of 2 mm.min -1 . The test machine is a Zwick 1476 with a capacity of 100 kN (Fig. 2) driven by software suitable for computer control of the test. The test conditions are summarized in a temperature of 26 °C and a relative humidity of 30%. Behavior modeling As illustrated in Fig. 3, the stress-strain curves obtained from a compression test on a sample show a three-domain behavior: first, an elastic domain; then a domain of buckling cells with low slope; finally, a densification area with a high slope. In the light of these findings, two approaches are proposed, in this work, to predict the behavior of cork in compression: linear and nonlinear. The linear approach considers that the behavior is linear throughout its evolution where the three domains are modeled by lines of different slopes. Average slopes are represented by the following formulas: T