EFFECT OF RING ANGLE ON SHEAR STRENGTH PARALLEL TO THE GRAIN OF WOOD

American Society for Testing and Material (ASTM) standard D 143 (1) described the test procedure for shear strength based on small, clear specimens. The standard recommends that the specimens have to be made in two different orientations: radial (the shear plane along the radial surface) and tangential (the shear plane along the tangential surface) (Fig. 1). The requirement on the shear test specimen implies that the orientation of growth rings on the shear plane must be considered in determining the shear strength of wood. The effect of ring angle on mechanical properties, especially on shear strength, has not been reported in the literature, except for tangential, radial, and 45-degree orientation. The only unpublished data on Dahurian larch (Larix dahurica) showed that there may not be any relationship between ring orientation and shear strength (Fig. 2). 1 Gupta, R. and R.L. Ethington. 1994. Mechanical proporties of Dahurian Larch. Dept. of Forest Prod. , Oregon State Univ., Corvallis, Oreg. FOREST PRODUCTS JOURNAL VOL. The objective of this study was to examine the effect of ring angle on shear strength of wood based on small, clear specimens. The secondary objective was to determine the effect of other variables such as specific gravity, percent latewood, and the number of growth rings per inch on the shear strength of wood. Some factors that might affect the shear strength, particularly for small clear specimens, have been examined by Okkonen and River (9). Among other factors, they investigated effects of growth ring orientation (only radial and tangential shear planes) on the apparent shear strength of Douglas-fir, southern pine, white oak, and hard maple. They reported that ring orientation was a strong but inconsistent factor affecting shear strength parallel to the grain. Tangential shear plane produced significantly higher shear strength than did radial shear plane for maple and oak. Conversely, in Douglas-fir and southern pine, radial shear plane produced significantly higher shear strength than did tangential shear plane. Bendtsen and Porter (6) determined radial and tangential shear strengths and shear strength at a 45-degree orientation to the annual ring. They found that the average radial shear strength was greater than the average tangential shear strength, and the average of the radial and tangential shear strength was only 2 to 3 percent greater than the shear strength at a 45-degree ring orientation. They attributed this slight difference to a slightly lower specific gravity for the 45-degree specimens. The Wood Handbook (15) describes the value of shear strength parallel to the grain as the average of equal numbers of specimens with 0and 90-degree growth ring orientations. It also mentions that for some species, there is no difference in 0and 90-degree orientation; other species exhibit slightly different shear strengths The authors are, respectively, Graduate Research Assistant and Assistant Professor, Dept. of Forest Products, Oregon State Univ., Corvallis, OR 97331. The authors would like to thank Frank Lumber Co, Inc., of Mill City, OR, for providing the sample beams for this project. Our appreciation also goes to Bill Galligan, Technical Director, and Clemant Frank, Quality Control Manager, for their advice on this project, and to Perum Perhutani, Indonesia, for financial support, This paper was received for publication in June 1995, Reprint No. 8364. © Forest Products Soceity 1996. Forest Prod. J, 46(7/8):87-92.