RELATIONSHIP BETWEEN LOBLOLLY PINE SMALL CLEAR SPECIMENS AND DIMENSION LUMBER TESTED IN STATIC BENDING
Keywords:mechanical properties, modulus of elasticity, modulus of rupture, nondestructive testing, southern pine, wood quality
AbstractPrior to the 1980s the allowable stresses for lumber in North America were derived from testing of small clear specimens but the procedures changed because these models were found to be inaccurate. Nevertheless, small clear testing continues to be used around the world for allowable stress determinations and in studies that examine forest management impacts on wood quality. Using small clears and nondestructive technologies is advantageous because of the ease of obtaining and testing small clear specimens compared to lumber. The objective of this study was to compare the mechanical properties in bending of small clear specimens with lumber specimens for loblolly pine. Eight hundred and forty-one pieces of lumber in the No. 1 to No. 3 grades and 2×4 to 2×10 sizes were collected from a forest-thru-mill study and tested in static bending. A small clear specimen (25 x 25 x 410 mm) was prepared from each piece of lumber and tested in static bending. The effect of growth ring orientation was explored and overall samples tested on the radial or rift face did a better job of explaining the variation in lumber than samples tested on the tangential face; however, the relationships were generally poor for the modulus of elasticity (MOE) (R2 = 0.22) and modulus of rupture (MOR) (R2 = 0.11) pooled data. A lumber-based multiple regression model explained 44% and 37% of the variability for MOE and MOR, respectively; whereas a stand-based multiple regression model explained 41% and 29% of the variability for MOE and MOR, respectively.
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