Disputation i byggteknik: Winston Mmari

Abstract

Mmari, Winston (2023). Multiphase continuum modeling of wood: A hybrid
mixture theory approach, Linnaeus University Dissertations No 485/2023,
ISBN: 978-91-8082-003-5 (print), 978-91-8082-004-2 (pdf).
Wood has been used as a construction material for a very long time. The
development of efficient industrial production processes of wood has
expanded the use of the material with the introduction of new products, such
as engineered wood products. Considering the adversely changing climate,
the use of wood in construction is advocated due to its environmental
benefits, such as its low carbon footprint. As a naturally growing material,
however, wood has a high moisture content when harvested. Additionally,
the chemical composition of wood fibers together with its porous structure,
gives wood a strong affinity towards moisture, throughout the whole life
cycle of the material. The moisture content in wood strongly influences its
physical and mechanical properties, such as strength, stiffness, shape
stability and durability properties. Further, it requires energy-intensive
drying processes to bring wood to the desired moisture content for structural
use.
The task of predicting the moisture content and transport of moisture in
wood is challenging. It involves multiple phases, i.e., liquid water, gaseous
vapor and the solid wood fibers, and it also engages a number of physical
processes such as evaporation/ condensation, adsorption/ desorption,
diffusion and seepage of the fluids, heat conduction and swelling/shrinkage
of the wood fibers.
This thesis investigates the interplay between heat, moisture and their
associated transport mechanisms in wood. The mechanics of the solid wood
material is also studied. The primary goal of this thesis is to develop a
thermodynamically consistent continuum model that is capable of
predicting the macroscopic behavior of wood subjected to varying climate
conditions and mechanical loading. The hybrid mixture theory is used to
develop a multiphase continuum model for wood, in which, at the
macroscale, the wood material is considered to contain immiscible solid,
liquid and gaseous phases. Constitutive relations are derived by fulfillment
of the entropy inequality at the macroscopic scale. Interaction processes involving phase changes through sorption and evaporation/ condensation,
and diffusive transport mechanisms are described using the macroscale
chemical potential as defined by the hybrid mixture theory.
The thesis starts with introductory chapters describing the overall properties
of wood of importance in this context and the interactions between wood
and moisture. A summary of the mixture theory as applied to this work is
also presented. The thesis contains four attached papers, Paper I, Paper II,
Paper III and Paper IV. In Paper I a model describing moisture transport
and sorption processes in wood below the saturation point of the wood fibers
is presented. The model is developed further, in Paper II and Paper III, to
incorporate wood-water interactions below and above the fiber saturation
point. Shrinkage/swelling and non-linear elastic deformations are also
implemented. A drying test simulation of wood starting from the green state
is performed and compared to experimental results. The model presented in
Paper II and Paper III is complemented in Paper IV by considering damage
associated with anisotropic cracking of the solid wood material. The phase
field fracture modeling approach is used for this purpose. The resulting nonlinear coupled partial differential equations governing the macroscopic
behavior of the material are solved numerically using the finite element
method. Simulations are performed to check the overall performance of the
theoretical framework behind the proposed models and they are compared
to experimental results for the identification of some of the material
parameters of the models.