Creep at high temperature (120 °C) of 'green' wood and modelling by thermo-activated Kelvin's elements.

This work involved creep tests performed on green wood samples at high temperature (up to 120 degrees C). The main problem to be solved lies in the boiling point of water, which induces internal vaporisation as soon as the temperature attains 100 degrees C. The solution consists in using an autoclave which maintains the overpressure required to keep saturated vapour conditions. The whole experiment (support and sensors) has been designed to withstand the severe conditions of temperature, pressure and relative humidity. Numerous experiments are now available, mainly for oak. Results are highly reproducible and depict a dramatic deformation of green wood when the temperature approaches 100-120 degrees C. The constitutive equation used to fit the experimental curves is made up of Kelvin elements with thermal activation placed in series. The fitted parameters depend on the species (spruce and oak) as well as on the experimental procedure. Nevertheless, the predicted deflexion simulated using these fitted parameters is in rather good agreement with the experiment. ((C) Inra/Elsevier, Paris.).