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

In veneer production, lathe checks created during the cutting process a#ect 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 di#ered due to the di#erence 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 ; +) Crack propagation in early wood showed any one or a combination of the following characteristics in both the radial and tangential directions : (+) Propagation by separation between cells ; (,) propagation by partial splitting of cell walls ; and (-) propagation by rupture of cell walls. ,) 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. -) The mode of crack propagation di#ered with propagation direction. .) 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.