Growth stresses and strains in cork

Abstract

Summary. A detailed study of the growth stresses and strait~s in the cork sheI1 of the cork-oak was undertaken based on experimentally determined constitutive relations for cork in tension and compression. The stresses depend on the thicknesses of the cork shell and of the back layer around the cork shell, on the radius of the trunk and on its increase due to growth. The circumferential stresses in the cork shell and back layer are tensile and increase with increasing distance to the tree axis. The radial stresses are compressive and decrease with increasing distance to the tree axis. The strains due to growth are not recovered when the cork boards are removed, unless the boards are heated, for example, by immersion in boiling water. Other consequences of the growth stresses are analysed, such as the occurrence of corrugations in the lateral cell walls of cork, the variation of width of the successive growth rings and the occurrence of cracks in the back layer and outer cork layers.

Tensile properties of cork in axial stress and influence of porosity, density, quality and radial position in the plank

“…Density is higher in the inner part of the plank, e.g., on average 0.175 g cm -3 and 0.156 g cm -3 , respectively in the inner and outer positions. This results from the fact that the annual rings of cork have decreasing width towards the inner part of the plank (Ferreira et al 2000, Fortes andRosa 1992). Thinner rings have a relatively larger proportion of latecork cells with thicker cell walls and smaller radial dimension (Pereira et al 1987), and thereby lead to larger values of density (Fortes and Rosa 1992).…”

“…This results from the fact that the annual rings of cork have decreasing width towards the inner part of the plank (Ferreira et al 2000, Fortes andRosa 1992). Thinner rings have a relatively larger proportion of latecork cells with thicker cell walls and smaller radial dimension (Pereira et al 1987), and thereby lead to larger values of density (Fortes and Rosa 1992). On the contrary, the wider annual rings in the outermost part of the cork plank contain cells with larger mean dimensions (Pereira et al 1992).…”

“…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 the cork. In the tree, the cork layers are also put under tension by the radial growth of the stem (Fortes and Rosa 1992) but in this case the load is applied tangentially. Fracture may occur in both cases.…”

Cork and Cork Stoppers: Quality and Performance

“…If the cork layer of the initial periderm (virgin cork) formed in the young cork oaks is removed (by an operation called cork stripping), a new phellogen is formed inside the phloem and rebuilds a traumatic periderm and its subsequent cork layer (second cork). At this time of life in the young cork oaks, the radial growth of the stem is still important, and the second cork external regions are subject to large tangential stress that may result into deep fractures of the cork [19]. If the second cork is removed, the process is repeated with the formation of a new phellogen and the production of a new cork layer (reproduction cork) that will endure few fractures due to the low tangential stress caused by the radial growth of the mature tree.…”

“…Cork is a cellular material with a compact three-dimensional structure of closed prismatic, on average hexagonal, cells that are assembled base to base creating rows that are aligned in the radial direction in the tree and disposed of in parallel, forming a honeycomb-type structure [19,26]. The structure of cork observed by scanning electron microscopy in the three principal sections is shown in Figure 2.…”