Stress relaxation and creep of cork
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The stress relaxation and creep behaviour of cork under compression were characterized in tests done with the compression axis parallel to each of the three principal directions in the tree (radial, tangential and axial). All stress relaxations lead to a linear variation of stress with the logarithm of time, the slopes being nearly independent of stress and direction of compression. Creep stresses in the range 0.36 to 1.72 MPa were used. The strain rate continuously decreases during creep, from initial values around 10−4sec−1 to ∼ 10-7 sec−1 after 8 h, but its dependence on the creep stress and direction of compression is not simple, mainly because different deformation regimes may operate during a single creep test. Compression loading-relaxation-unloading cycles were imposed on specimens, with compression either in the radial or in the tangential direction, with the purpose of simulating the performance of a cork stopper. A “work softening” is observed, i.e. the resistance decreases in successive compressions, particularly between the first two. This is explained in terms of an increased undulation of the cell walls produced in the first compression.
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