Linking molecular level mechanisms with macroscopic performance
Predicting the performance of wood for decades ahead is important when using the material for structural purposes. The performance is closely related to the hierarchical material structure of wood and the dependent interaction with water in the structure. Accurately predicting wood performance therefore requires an understanding of material structure from molecular to macroscopic level as well as of the impact of water molecules. The objective of this work is to investigate the performance of wood in terms of mechanical response of the material and effect of water. To understand the latter, one must first know in which parts of the wood structure, water is located. If parts of the water in wood are held in capillaries in the wood structure, these water molecules interact with the material differently than those held within wood cell walls. In this study, the occurrence of capillary water in wood is investigated at high levels of relative humidity (RH), where capillary water might be present. Three different techniques are employed in overlapping RH regimes. The three techniques give similar results and show that the amount of capillary water is insignificant up to at least 99.5 % RH. Thus, for wood in equilibrium with surrounding climate in the RH range 0-99.5 %, water is only significantly present within cell walls. A structural model of a wood cell is developed in this study using Finite Element Method for predicting the mechanical performance of wood. The starting point for the model is the physical behaviour on the molecular level since water interferes with wood at this level. The elastic material properties of the wood cell wall are explained by the organisation of wood constituents and their properties. The effect of water as well as temperature is incorporated by considering the amount of hydrogen bonds between wood constituents and the stiffness of these bonds. The mechanical response of wood includes a substantial time-dependent response, which previously has been explained by sliding between wood constituents on the molecular level. In this study, this is incorporated in the model as time-dependent shearing of the material planes of the cell wall. The calculated results of the model is verified against various experimental results from literature as well as from measurements presented in this work. It is shown that the structural model is able to describe a diverse range of mechanical responses of wood cells in both elastic and time-dependent domains. Furthermore, comparison of results from experiments and model suggests that the mechanical response of wood tissue, i.e. the hierarchical level above single wood cells, is a sum of responses from both wood cells and intercellular layer, i.e. the middle lamella.