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Bog'liq
Edward Le PhD Dissertation

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142 
Figure 6.19. Simulated density profile (half) for continuous layer (no gap) at different 
levels of compression in 3D.
Figure 6.18, shows that the density profile (half the panel) for 3D calculation that 
went up to 30% and 60% compaction. No increase in surface density was obtained. Even 
as the level of compaction went up to 60%, the density profile remained flat. This may be 
because in 3D, there is Poisson’s expansion in the width direction. The strands can 
therefore expand into empty spaces. This expansion into empty spaces may prevent 
densification of the surface strands.
To test for Poisson effects, a simulation was done where one surface layer had a 
continuous layer with no open space. Figure 6.19 shows small densification at the 
surface. Thus empty spaces in layers has an effect. Overall the 3D simulations are very 
different compared to 2D, suggesting the best study of VDP may require 3D simulations.
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143 
6.5.2. Effects of Surface Strand Properties
As in 2D simulation, the 3D simulations were repeated with altered surface 
properties. When the stiffness of the surface strands was reduced by half, the density of 
the surface was significantly increased (see Figure 6.20). As the stiffness of the surface 
strands reduce, the strand is softer and allows more compaction. The increased 
compaction of the surface strand resulted in increased density at the surface.
However, when yield stress of the surface strands was reduced by half, there was 
no increase in density of the surface (results not shown). This result may be controlled by 
the plasticity theory (see chapter 3). In the classical plasticity model (chapter 3 and Hill 
1986), there is no change in volume once yielding occurs. All volume changes 
(densification) are due to elastic deformation and thus only influenced by elastic stiffness.
In processing of wood-strand composites (OSB), heat and moisture may reduce 
the properties of the surface strands. These reduced properties allow more compaction at 
the surface, thus leading to higher density at the surface. In order to fully model density 
profiles, we need to incorporate moisture and temperature gradient information into the 
mat during the pressing of the panel. A better plasticity model to account for 
densification may also be needed. This is because the traditional plasticity models do not 
allow for plastic densification after it yields. All modeling may also need to be 3D 
modeling, which further complicates the task.


144 
Figure 6.20. Simulated density profile (half) for different levels of compression in 3D 
with reduce stiffness of the face. 
6.6 Homogenized Model Interpretation
6.6.1 Model Derivation
For another view of explicit numerical results, the calculated results of moduli are 
compared to a simplistic model based on homogenization of each layer, followed by 
simple and uniform compression.
The homogenized rule of mixture (HROM) is developed and this accounted for 
different levels of compaction, density, gap and strand length. The OSB composite 
representation by three layer structure, with density of each individual strand is combined 
and represented by a single number. For each layer, the axial density was replaced by a 
homogenized density by considering the volume of fraction of gaps within each layer. 
The density of the surface (
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layers in terms of initial density were 
replaced by: 
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