DEFORMATION OF WOOD IN A BIAXIAL-STRESS STATE .1. EXAMINATION OF THE BIAXIAL-COMPRESSION TESTING METHOD

In this study, we examined the stress-strain relationship of wood duri ng biaxial-compression loading. The material used for the experiment w as Sitka spruce (Picea sitchensis Carr.). To apply the compression for ces biaxially, one direction is obliged to be loaded partially even wh en the other direction is full-surface loaded. Hence, previous to the biaxial compression tests, partial uniaxial-compression tests and fini te element calculations simulating this loading condition were made fo r determining the specimen's shape. Then the biaxial-compression tests were made under various testing conditions. The results obtained were summarized as follows: (1) In biaxial-compression, the direction with the weaker stiffness should be loaded partially, and the full-surface compression load should be applied along the direction with the stron ger stiffness. (2) The frictional forces which disturb the ideal biaxi al-stress condition could be reduced effectively by inserting a Teflon plate between the compressing plate and the test specimen. (3) Althou gh the frictional forces were reduced as much as possible, the yield a nd failure surfaces were distorted in the tangential direction in the stress space. This phenomenon could not be expressed by the Hill-type yield (failure) criteria which are often used.