Interaction between long term viscoelastic and mechanosorptive response of wood
Résumé
Since the first-published work in the 50's the time dependent behaviour raised an increasing interest in wood material research. Creep research has a decisive influence in timber structures where prediction of total deflection is necessary for durable structural design. The nature of polymeric constituents induces viscoelastic behavior to wood. In constant environment the representation of creep against logarithm of time could be composed of two straight-line parts joined by a curved transition [1]. Such a linear description of kinetic suggests an infinite strain level in long-term prediction. However as a semi-crystalline polymer wood is expected to tend toward a limit of creep level. A simplified rheological approach using parabolic models has been proposed by Huet [2] to model timedependent behavior of wood in longitudinal direction. Sorption processes are suspected to accelerate time by increasing compliance [3]. This phenomenon called mechanosorptive effect interacts with ongoing time-dependent evolution and induces a slope decrease after a cycle in the logarithm description [4]. Matched spruce specimens (L=100 x R=10 x T=2mm) have been submitted to four-point bending creep test (inner span = 40 mm and outer span = 80 mm) in controlled climatic environment. The strain has been measured on lower and upper surface of each specimen using electric gages. Four specimens were loaded at high humidity level (w ! 27%) below the limit of linearity of viscoelastic and mechanosorptive response [5]. Two specimens were kept unloaded for free swelling and shrinkage determination. Three specimens were weighted occasionally for moisture content determination w. Parabolic models have been used to estimate creep limit of the viscoelastic description. Submitted to sorption cycles (from w ! 27% to w ! 7%) the increase of compliance and the decrease of slope evolution have been observed. Comparison between viscoelastic evolution and mechanosorptive increment during a sorption cycle seems to support the hypothesis that mechanosorptive can induce an apparent time-lag but also a non recoverable strain.