Shear Strength of Wood Obtained by Torsion Test

Abstract

塑性変形を考慮し，矩形棒のねじり試験から，木材のせん断強度を求めた。試験にはシトカスプルース（Picea sitchensis Carr.），アガチス（Agathis sp.），カツラ（Cercidiphyllum japonicum Sieb. and Zucc.）およびブナ（Fagus crenata Bl.）を用いた。これらの試験体を，繊維方向を中心軸としてねじり，まさ目面および板目面のせん断応力－せん断ひずみ関係を得た。このせん断応力－せん断ひずみ関係をべき乗関数を用いて定式化し，破壊発生時のひずみの値をこの式に代入することでせん断強度を求めた。一方，ねじり負荷では破壊まで弾性状態を保つという従来より行われている方法でねじりせん断強度を求め，それぞれの値を比較した。その結果，樹種によってせん断応力－せん断ひずみ関係は多様であったが，いずれの樹種においても定式化によって得られた真のせん断強度の値は従来の方法で得られた強度の約70％程度となった。In this paper, we tried to obtain the real shear strength of wood by torsion tests with considering the plastic deformation. Sitka spruce (Picea sitchensis Carr.), agathis (Agathis sp.), katsura (Cercidiphyllum japonicum Sieb. and Zucc.) and buna (Japanere beech, Fagus crenata Bl.) were used for the specimens. These specimens were twisted around the longitudinal axis, and the shear stress-shear strain relationships of LR- (longitudinal-radial) planes and LT- (longitudinal-tangential) planes were obtained. These relationships were formulated by n-power functions, and the shear strengths were predicted by putting the shear strain at the occurrence of failure on the formula. On the other hand, the shear strength was independently calculated by the conventional method which is based on the hypothesis that the specimen keeps its elastic stress condition under the torsional loading, and the strengths obtained by the different methods were compared with each other. Although the shear stress-shear strain relationships varied by the species, the values of elastic-plastic strengths were about 70% of those obtained by the conventional method. There are several results of mechanical tests which show the coincidence between the testing data and theoretical analyses more precisely when the shear strength is evaluated as about 70% of that obtained by the conventional solution