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Modelling structure and processing characteristics of a randomly-formed wood-flake composite mat Dai, Chunping


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.

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