Being monocotyledons, palms show distinct differences in anatomical structure compared to common wood species. Oil palm wood can be seen as a unidirectional long-fiber-reinforced bio-composite, if vascular bundles are considered reinforcements (fibers) and parenchymatous ground tissue the matrix. The elastomechanical properties in bending (fm, Em), compression parallel (fc,0) and perpendicular (fc,90) and tension parallel (ft,0, Et,0) and perpendicular (ft,90) directions to the vascular bundles of small-size test specimen show a much higher exponential increase in density, following power law relationships, than common wood species and a significant gradient over both trunk height and cross section. The rule-of-mixture cannot be confirmed for ft,0 and Et,0, because the concentration of vascular bundles, as well as the share of fibers within the bundles, is greater in the periphery of the stem than in the central tissue. Furthermore, the cell wall properties themselves are not constant; cell wall thickening is more pronounced in the peripheral than in the central tissue and more in the bottom of the trunk than near the top. The “fibers” of the composite material are not homogeneous nor regularly spaced, which leads to exponents > 1 of the power law relationship. Different from common wood species, the compression strength of oil palm wood exceeds the tensile strength: fc,0 : fm : ft,0 are 2.2 : 3.3…1.7 : 1. The performance indices for minimum weight design by Ashby et al. (1995) are comparable to that for coconut and date palm.
The wood from oil palm trunks exhibits significant variations in distribution of structural tissue, density and elastomechanical properties across and along the trunk. Its reliable, safe, and economic usage for loadbearing purposes, such as glued laminated timber (GLT), requires a precise definition of its elastomechanical properties through appropriate strength grading procedures. Oil palm lumber is strength graded according to its density using an X-ray technique in which 50 % of the lamellas are ripped, graded, edge glued and therefore density homogenized, and 50 % are cut only according to their geometry. Lamellas are tested in tension parallel to the vascular bundles; combined GLT is produced from strength-graded lamellas and tested in bending parallel and compression parallel and perpendicular to the vascular bundles. The characteristic strength values for C10 and C14 according to EN 338 are achieved. A correlation between density and elastomechanical properties is established. GLT from density-homogenized lamellas achieve higher bending properties than from lamellas with a "natural" density gradient across the width.
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