| Summary: | The subject of modelling the wood composite manufacturing process has recently
attracted more research attention due to the increasing need for a quantitative understanding
of the product formation process. The existing models provide little information about wood
composite mat structure, which is, like that of any other material, very important to the
material properties. In this study, a mathematical model of wood flake mat formation has
been developed using geometric probability theory. This model has been applied to further
predict the compression behaviour of a wood flake mat during pressing. The model is
verified through both experimental observations and computer simulations.
The results show that flake mat formation approximately follows a random process
with random flake positions and orientations. This allows a flake mat structure to be fully
characterized on a probability basis. Mat structural parameters such as flake centroids, flake
coverage and between-flake void sizes are random variables, which are in essence Poisson
distributed. Non-uniform flake coverage distribution is an inherent feature of a randomly-formed
flake mat. This is why a horizontal density variation always exists in a random
flakeboard panel. Due to point-to-point spatial correlation of local flake coverage, the
variation of flakeboard density averages in finite sampling zones depends on the zone size,
flake size, flakeboard thickness and compaction ratio. Such a relationship is known
rigorously through the derivation of a mathematical model and through the visual
presentation of the density variation image created by a computer simulation program.
The structural model of mat compression behaviour shows that the pressure applied
in a mat during pressing is mainly supported by areas with higher wood coverage. Because
of the random distribution of local flake coverage, a wide stress variation exists among the
constituent wood elements from location to location in the mat field. Considering wood as
a porous material, the void volume in a mat is composed of two components: within- and
between-flake voids. Their relative volumetric change during mat compression is
considerably different. Equations derived for calculating inter-flake bonded area change
indicate a highly nonlinear relationship between the relative bonded area and mat compaction
ratio. Because of the viscoelasticity of wood, a wood flake mat also exhibits time-dependent
compression behaviour during pressing. Using the structure model developed, the stress
relaxation of a flake mat has been explicitly related to that of wood. To correlate the creep
response of a flake mat to that of wood, a new creep evaluation terminology -- relative creep
compaction ratio, should be employed instead of the more common relative creep strain. The
creep of a flake mat seems to be affected by its constituents through their average
viscoelastic responses. === Forestry, Faculty of === Graduate
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