Abstract
The aim of the study was to predict the long-term creep behaviour of green wood in the longitudinal direction for a period corresponding to the life of a tree and to explore the variability of this behaviour. The study took place in two steps commencing with a screening of vibration properties on a large sample of ten tropical species including different wood types, the reaction wood included. Next, a small subsample was used for an in depth analysis of the long-term creep properties. Further, the relation of viscoelastic properties with structural parameters such as basic density, microfibril angle and percentage of anatomical elements was investigated. A testing procedure was developed to assess the long-term viscoelastic properties based on shortterm creep tests performed at different temperatures. Occurrence of physical aging subsequent to a quench from a temperature above the glassy transition was evidenced. Direct applicability of the timetemperature equivalency was questioned by discrepancies observed in the approximated complex plane (ACP). Additional assumption of a temperature-dependent initial compliance, similar to the entropic elasticity in amorphous polymers, was suggested and successfully applied to obtain reliable long-term creep predictions. Thermo-activated creep behaviour along with physical aging was described by a parabolic Maxwell model identified from the representation of experimental data in the ACP. The creep behaviour was revealed not to be related to the damping coefficient measured by the vibration method, indicating that different rheological mechanisms govern the viscoelastic behaviour at acoustic time scales (hundreds of Hz) and biologic scales (several years). In conclusion, the hypothesis of the middle lamella playing a key role in the long-term creep was proposed to explain the weakness of the correlations observed between the amount of relative creep and the structure of the cell wall.