Multiscale analysis of coupled mechanical and moisture behavior of wood

Knowledge of mechanical and moisture properties of wood are crucial, since they determine to a great extent the durability and failure of wood elements exposed to varying mechanical and environmental conditions. For predictive purposes, continuum mechanics computational models are used to forecast the relation between the applied loads (mechanical and/or moisture) and the respective hygrothermal and mechanical response. However, when we realize that the particular behavior of wood is driven by phenomena occurring at the cellular level, it is obvious that a continuum approach is a simplification of the real behavior. Macroscopic mechanical and moisture properties of wood highly depend on lower level features. These features span the whole range of spatial scales, from nanoscopic (cell wall fibril aggregates), over microscopic (cell wall layers, cell geometry), and mesoscopic (growth ring of early- and latewood) to the macroscopic scale (wood). Therefore, macroscopic material properties, such as elasticity modulus and moisture sorption isotherms (or moisture capacity), should be considered as apparent material parameters, which incorporate actual physical constants such as the properties of the chemical constituents of wood, and their spatial arrangement in nano- and microscale geometry of cells and in growth rings composed of late- and earlywood. The relationship between the macroscopic apparent properties and the microscopic features is not fully understood to date. Furthermore, mechanical properties and moisture capacity are interrelated, and such interaction is often not consistently taken into account in continuum models. As a consequence, the available continuum models have a limited range of physical validity. In cases where microscopic details are important, a multiscale modeling approach is more appropriate than a continuum modeling approach, in order to understand the relative importance of microscopic features on the overall mechanical performance of wood. The objective of this research program is to investigate the relation between macroscopic mechanical and moisture properties of wood and its microscopic structure using a multiscale approach. Expected contributions are improvements in the micro-meso-macro modeling and upscaling of a complex porous material, validation of the models at different scales by advanced experimental techniques, development of advanced experimental approaches and improvement of a macromodel to include creep and damage phenomena. The better understanding of the role of microfeatures is crucial in developing solutions for the durability of wood components exposed to varying environmental and mechanical loads.

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