Creep-recovery behaviour of cork

Creep tests were performed in cork samples under compression, at different temperatures between 0 and 50 degrees C. Master curves for the creep behaviour along the radial and axial directions could be constructed from short-time creep experiments, that could be described by a simple empirical model. The temperature shift factors were used to build a relaxation plot, where, assuming an Arrhenius model, an activation energy of 172 kJ mol(-1) was obtained from the data in the two principal directions. Comparison with previous dynamic mechanical analysis (DMA) and dielectric results suggests that the relaxation behaviour observed around room temperature could play an active role in the molecular mechanisms underlying the creep process in cork. From preliminary recovery studies it was seen that a fraction of the strain resulting from creep is preserved permanently, especially if the load is applied along the axial direction. This irreversible strain exhibited a linear relationship with the logarithm of the creep time, thus not being related to the irreversible viscous flow intrinsic to the material. Scanning electronic microscopy observations showed that creep could, in fact, lead to the bucking of the cellular structure of cork, this process being an active and time-dependent contributor for the permanent deformation of cork upon creep. (c) 2006 Elsevier B.V. All rights reserved.