Fundamentals Of Wood Composites Properties: Mat Structure And Board Property Relationship

Stress-strain relationship and cell structure fracture mechanism of wood element during radial compression, as well as influence of compression ratio to element properties was tested and analyzed, A veneer strip computer model for simulating strand mat was developed, and also be used in this study. Four type simulated strand board with different horizontal density distributions were made in the Lab. Properties of simulated strand board were tested and discussed. The thesis consists of four chapters: (1) Background and Objectives, (2) Transverse compression behavior of wood element, (3) Relationship of horizontal density distribution and board property, and (4) General conclusions. The findings of this study are summarized as follows:In radial compression, the stress-strain behavior of hardwood (aspen) and softwood (spruce) were similar to the typical stress-strain responses for wood transverse compression. But result shows there was a different cell wall collapse mechanism between hardwood and softwood when subjected to radial compression. First collapse of cell wall of softwood occurs in earlywood cells with the thinnest cell wall. In the hardwoods, it happens in the largest vessels in the earlywood.Physical and mechanical properties of wood elements can be significantly influenced by compression ratio (CR). Increasing CR will benefit to increase parallel tensile strength of wood, but deteriorate its tensile strength perpendicular to the grain and thickness swelling.Aspen veneer stack presented different characteristics to solid aspen block when subjected to radial compression under same testing condition. Much lower MOE was observed in aspen veneer stack stress-strain curve. While veneer layers shows no influence to its compression behavior, veneer thickness was found to be an impact factor. Moisture content had a significant effect on wood compression behavior which was believed as a soften effect.Horizontal density variation (HDV) is an inherent characteristic of non-veneer wood composites. HDV is largely affected by element size and geometry and forming process. With the help of a computer model, simulated strand boards with different HDV can be simulated and pressed.The density property relationship of wood strand board composites is related to the influence ofdensification on strand contact and bonding, the development in internal stresses and wood damage and the existence and influence of horizontal density variation. Increasing density can increase strand to strand contact which can lead to better bonding. However, internal stresses increase with densification and even wood cell wall damage may occur at excessive densification. These contradicting effects suggest that the existence of optimum densification (CR) for bonding. The optimum IB strength can be clearly detected in elements with no or little horizontal density variation. With horizontal density variation, the density and IB relationships of strand boards become monotonically linear. Density variation also causes a constraining effect on thickness swell (TS) which may lead to lower overall TS. While it does not appear to have an effect on MOE, horizontal density variation causes a negative effect on MOR. The information generated from this study is useful for developing models for predicting board properties and low density strand products.