Modelling of creep deformation mechanisms in wood: Dissertation

This study explores the explanatory clarification and quantitative modelling of the creep deformation mechanisms in wood, especially the mechano-sorptive phenomenon. The study concerns mainly the longitudinal, parallel to grain direction. The study contains short reviews of the structure of wood, observations and mathematical modelling of creep, especially mechano-sorptive behaviour. The results of two bending creep experiments of small size specimens in changing humidity conditions are presented. The purpose of the experiments was to compare the difference between creep accumulation in ad- and desorption and to clarify the viscoelastic and mechano-sorptive creep relationship. A new constitutive equation for the creep of wood is developed based on the idea that the rise of the mechano-sorptive effect can be explained by the non-linear coupling of the hygroexpansion (swelling/shrinkage) and viscoelastic creep. The fundamental assumption is that the same molecular (hydrogen) bonds are activated in a bond breaking and reforming process in the two phenomena. Apparently only a small ratio of the molecular bonds that are involved in hygroexpansion are also involved in creep, so that the above-mentioned coupling is only applicable to this smaller quantity. The mathematical development is accomplished by modifying the flow equation derived in the theory of deformation kinetics to account for creep flow, hygroexpansion and their combined effect, the mechano-sorptive effect. The applicability of the combined creep flow and hygroexpansion equation is extended by assigning it to a generalised Maxwell material type model. A sample of such a model with ten parallel elements in use is implemented into an FE-program which can solve the evolution of moisture content distribution and the development of stresses and strains in a cross-section of a wooden beam in real humidity conditions. Calculations with the model show good agreement with experiment results. The main results of this study are: the theoretical and experimental work gives insight into the physical backgrounds of the stress moisture deformation interaction in wood substance; the quantitative material model based on the theoretical assumptions and the developed structural analysis tool for the calculation of the response of wooden members in service provide the means for understanding the effects of the interaction in practical applications of wood construction.