Moisture-Induced Strains and Stresses in Wood

Sammanfattning: To design safe, reliable and durable timber structures subjected to varying natural outdoor or indoor climates, understanding the long-term behavior of wood when mechanically loaded or restrained to deform is crucial. The present thesis focuses on the numerical modeling of the long-term mechanical behavior of wood. The numerical analysis is divided in the moisture transport and the mechanical analyses. In the moisture analysis, the multi-Fickian moisture transport model is used to determine spatial and temporal moisture content fields over the analyzed domain due to changing relative humidity (RH) of the ambient air. The obtained moisture contents are taken into the mechanical analysis where a new mechanical model is applied for predicting rheological response of wood in three orthotropic directions simultaneously. Experimental results of different authors are used to support numerous numerical analyses performed for various wood species, deformation and loading modes in constant or changing RH conditions. The performed analyses show that the new mechanical model adequately predicts the viscoelastic behavior of hardwood and softwood species in two orthotropic directions simultaneously under a sustained load or deformation. A significant influence of grain orientation in relation to the applied mechanical load on the viscoelastic creep behavior of wood is observed. The mechanical model is also able to predict accurately the rheological behavior of hardwood subjected to a sustained compressive mechanical load and changing moisture content. Applying the moisture and the mechanical models to the glued-laminated timber specimens during wetting and drying shows good agreement with the experimental results. The magnitudes of moisture-induced stresses perpendicular to the grain indicate a possibility of crack initiation during drying. The influence of characteristic material parameters required in the models on the mechanical state of the analyzed specimens is also determined. A quantification of the viscoelastic and the mechanosorptive material parameters required in the mechanical model is the additional outcome of the performed numerical analyses. The mechanical model presented in this thesis in combination with the multi-Fickian moisture transport model enables a full two- or three-dimensional long-term mechanical analysis of timber members exposed to natural climate with RH variations.