Numerical modeling and experiments on wood-strand composites

Graduation date: 2010In wood-based composites, the glue-line (interface) between wood-strands affects the stress transfer from one member to the next. The glue-line properties determine the rate of load transfer between phases and these properties depend on wood species, surface preparation, glue properties, glue penetration into wood cells, and moisture content of the wood. As a result, the strength and stiffness of the composites are significantly affected by the amount, distribution, and properties of the resin.\ud In the first part of this research, the glue-line stiffness between wood strands was determined by experiments. The interfacial properties were calculated from experimental data on double lap shear (DLS) specimens. The results showed that in both normal and densified wood strands, resin coverage has a positive effect on the interfacial stiffness, and consequently on stiffness properties of wood-based composites. As adhesive coverage increased from discrete droplets (1% coverage) to a continuous bondline (100% or fully glued) the stiffness of the interface increased significantly and could even become stiffer than the wood itself. \ud In the second part of this research, once the mechanical properties of individual strands and interfacial properties were determined by experiment, they were used as input to a numerical model for the mechanical properties of oriented strand board (OSB) panels. Modeling the compression of wood-strands and wood-based composites was done using a numerical method called the material point method (MPM). MPM was used to model wood-strand composite mechanical properties as a function of compaction (densification), compaction rate, strand geometry (strand length and strand size), strand undulations, strand properties, and adhesive properties. In addition, density profiles of the panels as a function of selected variables were studied. The various simulations were for either conventional OSB panels or for OSB panels with densified strands in the surface layers. \ud To demonstrate the importance of glue-line properties and undulating strands, a simple homogenized rule of mixtures (HROM) was developed for OSB and oriented strand lumber (OSL) structures. The results of MPM were compared to the HROM model. The results show that typical glue properties have a significant effect on mechanical properties of OSB. The role of the interface is a consequence of strand undulation in typical OSB structures and the length of the strands. Interfacial properties are most important for composites with short strands or for composites with imperfect alignment such as OSB with undulating or misaligned strands