THE EFFECT OF GROWTH-RATE ON THE STRUCTURE AND COMPRESSIVE PROPERTIES OF CORK

Cork samples with an age of 9 years and with different growth rates, corresponding to calibre classes `small', `medium' and `large', were observed by scanning electron microscopy and tested mechanically in compression. Differences in calibre correspond to different widths of annual growth rings; widths differ because both the number of cells produced per year and cell dimensions differ. For instance, small and large calibre cork had, respectively, 50 and 151 cells in one row of an annual growth ring, with an average prism height of 32.6 and 39.8 mum. The stress-strain curves obtained in compression parallel to the three main directions showed that cork with the higher growth rate (large calibre) has lower strength in compression for all strains and lower Young's moduli.