Analysis of Wood-cutting Mechanism by Impact Fracture Theory

传统的静态弹塑性理论不能很好地分析圆锯等旋转刀具对木材的切割过程。在本研究中，我们旨在阐明木材在慢速和高速切割中的切割机理的差异，为此，我们研制了一台试验机，可以使用特制的尖锯在5~70m/s的速度范围内进行切割。扫描显微镜观察发现，低速切割时得到的切削面非常粗糙，伴随着细胞壁的变形和沿纤维的分裂，而高速切割时的切削面比较光滑，细胞壁的变形较小。为此，采用了悬臂形试件。在高速切割时，试件的悬臂部分被清晰地切割，但在低速切割时，观察到裂纹的扩展。这种切割类型的差异被怀疑与测试试件的表观硬度的变化有关。为此，在恒切削速度下，对不同刚度的试件进行了切割试验。在这种情况下，随着试件高度的增加，结果会发生以下变化：试件从刀上逃逸，然后在支撑处破裂，最后被刀截断。通过上述实验，得出了不同切割速度下切割类型的不同取决于试件的表观刚度的结论，并用简单的弹簧-质量模型对切割过程进行了数值分析。仿真结果定性地证实了我们的假设。

The traditional static elastic-plastic theory seemed to be inadequate for the analysis of the cutting process of wood by the rotating knife like a circular saw. In this research, we aimed to clarify the difference of cutting mechanism of wood between in the slow- and high- speed cutting.For this purpose, testing machine was prepared which enables the cutting in the velocity range from 5 to 70 m/sec using a special made tip-saw. It was found by scanning microscope that the obtained cut surfaces were very rough, accompanied the deformation of the cell walls and the splits along the fiber in the slow-speed cutting, whereas those were rather smooth with less deformation of the cell walls in the high-speed cutting.The cutting process was investigated by high-speed video camera. For this purpose, cantilever shaped test specimens were employed. In high-speed cutting, the cantilever part of the test specimen was clearly cut, but in slow-speed cutting, the propagation of splits was observed. This cutting type difference was suspected to have concern with the change of the apparent stiffness of the test specimens. Therefor, cutting test of test specimens with different stiffness was done under a constant cutting speed. In this case, the result changed as follows with the increase of the specimens hight : escape of the specimen form the knife, then the break down at the support, and finally the amputation by the knife.Through the above mentioned experiments, we concluded that the difference of the cutting type under the various cutting speed depends on the apparent stiffness of the specimen, and numerically analyzed the cutting process by a simple spring-mass model. The result of the simulation affirmed our supposition qualitatively.