Comparison of Standard Block Shear Test with the Panel Shear Test

The ASTM standard shear test for small, clear specimens of wood, developed about 1910, does not subject the specimen to uniformly distributed pure shear. This inadequacy applies in the elastic range only. Shear stress is not distributed uniformly across the shear plane; stresses other than shear act on this plane, and stress concentrations do occur at the reentrant corner and the opposite corner of the specimen. The panel shear specimen, developed for solid wood, has also been used successfully for solid wood. The specimen, described in Forest Products Laboratory Report No. 1328-C, consists of a wood panel cut in the form of a cross. Wood blocks are glued to the arms of the cross on which are mounted steel rollers centered on the inner surfaces of the blocks. In the test, wedge-shaped pieces apply forces to the rollers in the direction of the edges of the wood blocks. Thus the central square part of the sample is subjected to shear stress. Tests in which resistance-wire strain gages were attached to the surfaces of the samples showed that the shear strains are nearly uniformly distributed over the test area. Shear strength values obtained in this way are compared with values obtained by the block shear method in FPL Report No. 1801. Specimens tested were Douglas-fir, yellow-poplar, yellow birch, and sweetgum. Average values, obtained from panel-shear and block-shear tests are compared in tables. The agreement in the shear strength values obtained by the two test methods is in very close agreement. This is true for all strains, up to failure, in the plastic range as well as the elastic range. Results of these tests suggest that a state of nearly pure plastic shear stress is distributed approximately uniformly across the shear plane before failure.

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