The Influence of Small Grain Angle Variation on Toughness
Keywords:Toughness, redwood, slope-of-grain
AbstractToughness of solid wood as determined by ASTM D143-52 is known to be sensitive to slope-of-grain in the specimen. Although samples with observed sloping grain are routinely rejected, localized grain deviations in the center of nominally straight-grained specimens may escape detection. The acceptance of this sample is often aided by the difficulty in identifying the grain orientation at a point and by the usual large number of toughness tests that are conducted in any one experiment. In this study 226 standard toughness specimens of redwood were carefully machined to be straight-grained. However, close scrutiny of the center of the specimens (often after they had been failed) showed that only 10 of the 226 samples had truly straight grain and that localized grain deviations ranged from zero to 15°. Regression analysis showed that the slope-of-grain on the specimen face parallel to the impact direction explained a significant amount of variation in toughness. If this slope was accounted for, the coefficients of variation of toughness were reduced from 33 and 30% to 22 and 18% for impact in the radial and tangential direction, respectively. The results of the study showed that toughness was very sensitive to small localized grain deviations and that truly straight-grained toughness samples are rare. A significant amount of the usual high variation in toughness data could be attributed to these small grain deviations.
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