<論文>木材の横圧縮大変形(II) : 応力-ひずみ繰返し図

Abstract

スギ(Cryptomeria japonica)材とイヌシデ(Carpinus tschonoskii)材について,放射方向の圧縮試験を行った。 20℃・気乾状態,20℃・飽水状態,100℃・飽水状態のそれぞれについて,接線方向への試験片の膨張を拘束した場合と拘束しない場合について,ひずみを逐次増大させながら,負除荷を繰り返して,応力-ひずみ図を求めた。気乾状態では,降伏点を過ぎて高原域に入ると,除荷後に大きな変形が残留した。側面拘束の有無により,スギ材では,応力-ひずみ図にほとんど差異は認められなかったが,イヌシデ材では,拘束がない場合はある場合に比べ,降伏応力が小さくなり,応力-ひずみ図がひずみ軸に沿って引伸ばされた形となった。これらの原因は,スギ材では,圧縮に伴って細胞が座屈し,細胞壁が内腔に折りだたまれて,試験片の側面への膨張が生じないのに対し,イヌシデ材では,ポアソン効果により,膨張を生じるためと考えられた。飽水状態における結果は,気乾状態のそれと著しく異なった。特に,残留変形が著しく減少し,負除荷繰返しによる変形過程に非可逆性が認められた。このことから,細胞壁の内部で何らかの破壊が生じているものと推察された。

The compressive tests with increasing loading cycles in the radial direction for sugi (Cryptameria japanica) and inuside (Carpinus tschonoskii) woods were carried out in three conditions: air-dry at 20℃, wet at 20℃, wet at 100℃. The deformation in the tangential direction was either free or restricted. In air-dry condition at 20℃, a large residual strain remainded after unloading above the yield point. The difference in the stress-strain diagrams with and without lateral restriction for sugi wood was slight, showing that most of the radial compressive strain was absorbed by internal buckling of the cell walls. However, the diagram without lateral restriction for inuside wood showed a lower yield stress and higher strain levels. This was related with the Poisson's ratio at large deformation. In wet conditions at 20℃ and 100℃, an almost complete recovery of strain was observed whatever maximum stress level had been reached. However, subsequent loadings never followed the previous path and a new plateau with a much lower yield stress was observed. These results suggested that some ultrastructural damages in the cell walls by loading history occurred, although there were no indications of the damages through SEM observation.