Modeling Wood Strands as Multi-Layer Composites: Bending and Tension Loads
Keywords:Intra-ring properties, earlywood, latewood, mechanical properties
AbstractWood strands are composed of distinct layers of earlywood and latewood material. Previous research has demonstrated that latewood mechanical properties may be two to three times greater than earlywood mechanical properties. However, wood composite modeling assumes strands are uniform, homogenous elements. This paper investigated the effect of considering wood strands as two-layer composites consisting of earlywood and latewood, or intra-ring, layers. Experimental measurement of the intra-ring properties of loblolly pine (P. taeda) provided inputs to finite element models using both solid layers and cellular layers to represent longitudinal tracheids. The models were compared with three different types of strands cut at various orientations (flatsawn, quartersawn, non-aligned cut) in both tension and bending loadings. The model prediction of strand stiffness greatly improved by considering the strands as 2-layer composites compared to homogenous sections. Further improvements of the prediction of stiffness were made from modeling cellular layers rather than solid layers. The rule of mixtures predictions of stiffness produced good agreement for the non-aligned strands in both tension and bending loading, but only good agreement in the tension loading for the flatsawn strand. Examining the stress distributions of the strands from the finite element model, the solid models showed distinct stress changes at the edges of the intra-ring layers, indicating stress concentrations at the boundaries of the intra-ring layers. The cellular models showed a much more gradual stress transition between the intra-ring layers, which is a more realistic scenario.
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