Sorptive Stress Estimation: An Important Key to the Mechano-Sorptive Effect in Wood

Abstract

The mechano-sorptive effect (MSE) can be classified as a veryinteresting, but not yet completely explained phenomenon of woodbehaviour. We therefore decided to try to discover its most relevantmechanisms, by conducting two independent series of measurements:accurate measurements of bending deflections on small, clear specimensof spruce wood in changeable climate conditions; and average moisturecontent (MC) in three parts of the cross-section of unloaded dummysamples. The paper emphasises on the sorptive part of MSE. A computersimulation of the wetting experiment is done to determine the space andtime-dependent MC field in the cross-section of the sample. Using aso-called inverse problem identification method, where the equivalenceof the computed and measured responses of the numerical model and realsample is imposed, we identified unknown material properties. Thesolution of the inverse problem enabled us simultaneously to estimatethe moisture diffusion coefficients and to determine the MC field. Twokinds of boundary conditions were used in simulations. To estimate themagnitude of sorption stresses a numerical analysis using Finite ElementMethod (FEM) was done. The sorptive stresses resulting from thenonhomogeneous MC distribution in the sample were obtained. The decisivefactor is the gradient of moisture content and resulting sorptivestresses must be taken into account because their magnitude is of thesame order as the load induced bending stresses. Our experiments and thecalculations following enabled us to conclude that the reason for MSbehaviour could be the simultaneous action of permanent load stressesand the transient triaxial sorptive stress state.