Sorptive stress estimation: An important key to the mechano-sorptive effect in wood

The mechano-sorptive effect (MSE) can be classified as a very interesting, but not yet completely explained phenomenon of wood behaviour. We therefore decided to try to discover its most relevant mechanisms, by conducting two independent series of measurements: accurate measurements of bending defle ctions on small, clear specimens of spruce wood in changeable climate condi tions; and average moisture content (MC) in three parts of the cross-sectio n of unloaded dummy samples. The paper emphasises on the sorptive part of M SE. A computer simulation of the wetting experiment is done to determine th e space and time-dependent MC held in the cross-section of the sample. Usin g a so-called inverse problem identification method, where the equivalence of the computed and measured responses of the numerical model and real samp le is imposed, we identified unknown material properties. The solution of t he inverse problem enabled us simultaneously to estimate the moisture diffu sion coefficients and to determine the MC field. Two kinds of boundary cond itions were used in simulations. To estimate the magnitude of sorption stre sses a numerical analysis using Finite Element Method (FEM) was done. The s orptive stresses resulting from the nonhomogeneous MC distribution in the s ample were obtained. The decisive factor is the gradient of moisture conten t and resulting sorptive stresses must be taken into account because their magnitude is of the same order as the load induced bending stresses. Our ex periments and the calculations following enabled us to conclude that the re ason for MS behaviour could be the simultaneous action of permanent load st resses and the transient triaxial sorptive stress state.