Micromechanical Measurement of Wood Substructure Properties

The annual rings of softwoods are visually obvious and represent cylindrical layers of primarily cellulosic material that possess significantly different properties. For simplicity, wood construction products are designed assuming a material homogeneity that does not exist. As rapidly grown plantation trees are used for wood products, fewer rings are contained in an individual wood product with the outcome of each individual ring playing a greater role in product structural performance. The magnitudes of these earlywood and latewood property differences are largely unknown because of the difficulty in measuring small specimens. A cooperative study between the University of WisconsinMadison and USDA FS Forest Products Laboratory is underway to quantify the impact of variations in earlywood and latewood proportions. This paper discusses the background, approach, device used to determine modulus of elasticity (E) and shear modulus (G) for earlywood and latewood, difficulties faced with testing wood, and presents preliminary results. BACKGROUND A large portion of the nation’s future dimension lumber resource will come from genetically improved trees grown on intensively managed plantations. The focus of most tree breeders and forest managers is on increasing volume of wood fiber produced (i.e. “fast growth”). Fast growth can have negative impacts on the structural qualities of the wood products produced [1]. Traditional databases don’t always characterize this new wood resource. An unacceptably high variability in stiffness and dimensional stability limits the use of the fast growth plantation wood. Earlywood and latewood property differences are believed to be a key contributor to this variability. Current structural wood product design properties are developed from macroscale tests and visual inspection of products. Latewood percentage is one of the most widely used wood quality characteristics because it is highly correlated with wood specific gravity and provides a visual index of strength and structural properties, and is an important component in lumber and timber grading rules. No clearly defined relationships between latewood structure and solid wood properties exist. We know latewood tracheids can be over twice as strong as earlywood tracheids [2]. Latewood tracheids are also thicker walled with much smaller radial lumen diameters (Figure 1). But there currently are no means to translate these observations to wood product performance. The ratio of specific gravity of latewood to earlywood in loblolly pine is typically 2 to 1 or greater [3]. While the differences in earlywood and latewood specific gravities have been investigated, actual values of the mechanical properties of earlywood versus latewood are rare and at present we can only infer the consequences of these differences. We believe that in extreme cases, the latewood may be resisting most of the load, with the earlywood acting primarily as spacing material. Figure 1—Micrograph of latewood-earlywood transition in a piece of softwood The micro testing of earlywood and latewood reported in this paper is a portion of a cooperative study between the University of Wisconsin-Madison and USDA FS Forest Products Laboratory underway to quantify the impact of variations in earlywood and latewood proportions. APPROACH Ten loblolly trees in a southern pine plantation in Arkansas have been hand selected. The fertilization and pruning history of the plantation have been recorded as well as the location and directional orientation of each stem. Two 1.5m (5-ft) bolts were collected from each stem – one at breast height and the other beginning at approximately 5 m (20-ft) and shipped to the USDA Forest Service Forest Products Laboratory (FPL) in Madison, WI. Once at FPL the bolts were cut into toothpick size specimens, curved arcs, and full size boards (Figure 2) and stored under controlled environmental conditions.