Factors affecting the strength of block-shear specimens

The ASTM D 143 and D 905 block-shear tests are commonly used for measuring the shear strength of solid wood and adhesively bonded joints. The test methods are similar, but they result in different apparent strengths. Two factors that contribute to this difference are specimen shape and shear tool design. ASTM D 143 uses single-notched specimens and a shear tool with a 1/8-inch offset; ASTM D 905 uses double-notched specimens and a shear tool without an offset. This report compares the effects of specimen form (solid or bonded with phenol-resorcinol formaldehyde adhesive), specimen shape (single- or double-notched), shear tool (with or without offset), specimen size (full or reduced), and grain orientation (tangential or radial) on the apparent strength of southern pine, Douglas-fir, white oak, and hard maple block-shear specimens. The ASTM D 905 shear tool (no offset) produced higher strengths than the ASTM D 143 shear tool (offset). Full-size double-notched specimens had higher shear strengths than full-size single-notched specimens when other factors were the same. When other factors were pooled, solid and adhesively bonded specimens had the same strength regardless of shape (single- or double-notched). Reduced-size specimens consistently had higher shear strengths than full-size specimens when tested in the D 905 shear tool. Tangential grain in the shear plane produced higher shear strengths than radial grain in maple and oak specimens. Radial grain produced higher shear strengths than tangential grain in Douglas-fir and southern pine specimens. All factors had statistically significant effects on strength and there were significant interactions among the factors. Therefore, comparisons of the strength of solid wood and adhesively bonded joints should be approached with caution.

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