Abstract
The effect of time on mechanical behaviour of wood is decisive for biomechanical study of trees as well as response of timber structures or improvement of transformation process and preservation of this material. Originating from the polymeric nature of its constituents, wood presents a viscoelastic behaviour highly dependent on humidity and temperature of the environment. A moisture content change combined with mechanical loading creates an increase of creep or relaxation, known as mechanosorptive effect. Mechanosorptive effects are usually considered as time-independent. However the contributions of time and moisture content change are delicate to separate. An experimental phenomenological approach has been implemented based on long term creep test to describe longitudinal viscoelastic kinetics of evolution and extrapolate using parabolic models. The results obtained allow to quantify a finite value of compliance to infinite time. Mechanosorptive effects have been compared to this viscoelastic description and figure out irreversible phenomena. To understand the underlying mechanisms a specific analysis of in-situ cellulose behaviour by X-ray diffraction experiments has been conducted. The behaviour of crystalline cellulose reveals a proportional relation to macroscopic strain until a limit value is reached where the cellulose does not deform any more. This work aims at gathering fundamental elements for a better understanding of deformation mechanisms of wood, setting actual standards against the long term creep prediction of timber (Eurocode 5) and developping constitutive equations of wood.