Observation of Crack Propagation in Wood at the Cellular Level Considering the Occurrence of Lathe Checks

単板切削において単板に生ずる裏割れは単板の品質に,また単板をエレメントとする合板の品質に大きく影響を及ぼす。裏割れの発生は切削に伴って工具刃先近傍に発生する諸切削応力のうち,主として引張応力に起因することが明らかになっている。単板内での裏割れの発生および成長過程を細胞レベルにおいて検証するべく,小さい試験片(厚さ2.5mm)の曲げ試験を実施した。試験片の引張側に亀裂を事前に施し,負荷に伴う亀裂の伝播を細胞の変形および破壊挙動を観察した。また,二次元切削を実施し,得られた切屑の横断面(木口面)で確認できる裏割れの微視的観察を行い,曲げ試験による亀裂の伝播状況と比較した。

In veneer production, lathe checks created during the cutting process affect the veneer qualities. Previous studies have clarified that lathe checks are generated due to tensile stress in bending at the rake face of the knife. To compare lathe checks propagated inside veneers in orthogonal cutting, small thin wood specimens with minute cracks inserted beforehand on the tension side were bended. Deformation and fracture of the wood structures due to propagation of the cracks were observed continuously through a digital microscope at the cellular level. The modes of crack propagation on early wood and late wood of Japanese cedar, sugi, (Cryptomeria japonica D. Don) were differed due to the difference in hardness between early wood and late wood, and classified into three patterns in early wood and twos in late wood according to the anatomy of the wood. The results obtained in this study were as follows ; 1) Crack propagation in early wood showed any one or a combination of the following characteristics in both the radial and tangential directions : (1) Propagation by separation between cells ; (2) propagation by partial splitting of cell walls ; and (3) propagation by rupture of cell walls. 2) Crack propagation in the radial direction in late wood almost always proceeded by separation between cells. This was also true in the tangential direction, but in rare cases, cracks propagate by the rupture of cell walls. 3) The mode of crack propagation differed with propagation direction. 4) The development of lathe checks in orthogonal cutting of wood perpendicular to grain, lathe checks developed chips inside from the loose side of the chips almost same modes as crack propagation observed in fundamental bending tests.

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KJ00004587985

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