Effect of Growth Ring Orientation and Placement of Earlywood and Latewood on Moe and Mor of Very-Small Clear Douglas-Fir Beams

Amy T. Grotta, Robert J. Leichti, Barbara L. Gartner, G. R. Johnson


ASTM standard sizes for bending tests (either 50 x 50 mm or 25 x 25 mm in cross-section) are not always suitable for research purposes that characterize smaller sections of wood. Moreover, the ASTM standards specify loading the sample on the longitudinal-tangential surface. If specimens are small enough, then the effects of both growth-ring orientation and whether earlywood or latewood is on the upper and lower surfaces could affect values of modulus of elasticity (MOE) and modulus of rupture (MOR). The objectives of this study were to assess the effects of growth-ring orientation and latewood/earlywood location on bending properties of Douglas-fir specimens (10 x 10 x 150 mm). MOE did not differ with ring orientation, and MOR was about 5% higher when specimens were loaded on the radial rather than the tangential surface (MOE-LT vs. MOE-LR, respectively). The choice of growth-ring orientation did not affect the relative ranking of trees with respect to MOR or MOE. As expected, the variation of MOR and MOE was lower if the loads were applied to the longitudinal-radial surface than the longitudinal-tangential surface. Thus, rather than following the ASTM standard, within-tree variation measured on very small bending specimens can be minimized if loads are applied to the longitudinal-radial surface. When specimens were loaded on the longitudinal-tangential surfaces, there was an effect on both MOE-LR and MOR-LR of whether the top and/or bottom surfaces were earlywood or latewood. The wood type had a large effect on both MOE-LR and MOR-LR when it was the compression surface rather than the tension surface. This result suggests that variance in MOE and MOR measurements in very small specimens can be reduced by tracking whether the top and bottom surfaces are earlywood or latewood.


Size effect;growth-ring orientation;MOE;MOR;Douglas-fir;bending;orthotropic material;small bending specimens

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