Nobuhiro Takezawa

Brittle-Ductile Transition Phenomena in Microindentation and Micromachining

Abstract A generalized hypothesis for the brittle to ductile transition in micromachining and microindentation of brittle materials is proposed. By the hypothesis, complicated transition phenomena observed in practical machining processes are well explained. Experimental results on microturning, ELID grinding of monocrystalline Si and LiNbO 3 support the applicability of the hypothesis. Microindentation testing is shown to evaluate the intrinsic ductility and critical scale of machining for ductile mode machining. To analyze the machining process in extremely small scale, molecular dynamics computer simulations of microindentation and cutting are made on a defect-free surface. These results suggest that any material, in spite of their ductility, can be machined in ductile mode under the sufficiently small scale of machining.

Atomic-scale cutting in a computer using crystal models of copper and diamond

Summary A method of computer simulation based on a quasi-static computation has been proposed to analyze nanoscale cutting. The results of simulation show that, though the cutting mechanism depends on both the crystalline orientation of a workpiece and the interatomic potential between the tool and workpiece materials, discontinuous change of cutting force as well as impulses in the cutting temperature are commonly observed in all case. The results obtained by a polycrystal workpiece indicate that the plastic deformation in metals first occurs along grain boundaries and then propagates into each grain.

Mechanics and energy dissipation in nanoscale cutting

Summary The Methods by which to evaluate energy dissipation and stress/strain distribution in nanoscale cutting have been presented and applied to the results of atomic-model-based, simulation of nanoscale cutting. The results thus obtained show that the rate of energy dissipation in plastic deformation under a tool is large compared with that in macroscale cutting hit that, the value for surface generation is still small. The results also show that the stress-and-strain distribution in nanoscale cutting is consistent with that of macroscale cutting expect that there is almost no concentrated shear stress in the primary shear zone.

Brittle/Ductile Transition Phenomena Observed in Computer Simulations of Machining Defect-Free Monocrystalline Silicon

Abstract By using renormalized molecular dynamics(RMD) proposed by the authors, computer simulations of machining defect-free monocrystal silicon of various sizes have been carried out to investigate crack initiation process. The results show that a defect-free monocrystal silicon can be machined in ductile mode to any scale in an absolute vacuum but exhibits brittle-ductile transition depending on the scale of machining under normal atmosphere. In this paper, detailed mechanism of the process of crack initiation is discribed together with the discussion of micro dynamics on why ductile mode machining is always possible either in small scale or in case of f.c.c. metals.

On a Possible Mechanism of Shear Deformation in Nanoscale Cutting

Summary Based on the method of transformation from an atomic model to a corresponding continuum model, the stress and strain distributions in nanoscale cutting have been evaluated. The results show that a workpiece is subjected to concentrated compressive and shear strain at the primary shear zone, though the area along the rake face of the tool is strained tensilely. The results also show that the interior of the workpiece is. however, exposed to high, almost constant compressive stress. A possible mechanism of these different stress and strain distributions is discussed as well as its interpretation on a macroscale.

Crack Initiation in Machining Monocrystalline Silicon

Abstract Based on the discussion in which the defect as a source of cracks must be created during cutting a silicon monocrystal, the renormalization group molecular dynamics has been proposed to simulate the defect initiation process. The method can be applied to a model of micrometer size, which is necessary to bring about brittle mode cutting, and yet permit the observations of the defect initiation process of an atomic size. The result of the simulation shows that a microcrack-like defect can be initialed during cutting through the interaction between a local static stress distribution and global dynamic stress associated with acoustic waves

Effect of Surface Oxidation on Micromachinability of Monocrystalline Silicon

Abstract Microcutting experiments are carried out under an atomic force microscope (AFM) using workpieces of silicon monocrystals that have been exposed to air for various lengths of time before cutting. The results are observed under the same AFM with decreased tip force. The results show that difficult-to-cut areas appear locally after 24 hours of exposure time and these areas extend with increasing exposure time until the whole surface is covered after 120 hours. It is also found that exposure of workpieces to air produces a SiO 2 surface layer in which residual compressive stress is generated and whose hardness and/or elastic constant are lower than those of bulk Si. The molecular dynamics simulations carried out based on the above results show that the deterioration of machinability of monocrystalline silicon is caused by the viscoelastic/plastic properties of SiO 2

Molecular dynamics simulation of dimple formation process on ductile fracture surface

On the basis of the drawbacks of the existing theoretical and/or simulation methods, a new coupled analytical/MD method has been proposed to study void and dimple formation in the ductile fracture of a defect-free monocrystal copper. The result of the simulation shows that void and dimple formation in a defect-free monocrystal copper occurs, first, through a phase change from a monocrystal structure to a polycrystal structure, and then by a force system that produces relative rotations of grains.

Molecular Dynamics Simulation of Micro Mechanisms in Slip Deformation Theory of Crystals

Abstract Based on the discussion that there should be a micromechanism that causes a macroscopic slip of mono-crystal copper, molecular dynamics simulations with the analytical displacement feld around a crack tip have been carried out. The result of the simulation shows that macroscopic shear slip in an f.c.c. mono-crystal copper occurs as discrete time events. This is because cross-slips occur in many places in a material such that a macroscopic shear slip is blocked until some critical state of deformation. A macroscopic shear slip then occurs suddenly at the critical state in which the area of disordered atomic arrangement has stretched from one end of a crystal to the other end. The reason why macroscopic shear slips occur in the directions of the slip planes of a crystal is attributed to the fact that the areas of disordered atomic arrangement develop only along those directions.

Importance of Micro/Macro Interaction in the Mechanism of Brittle Mode Cutting

Abstract The mechanism of brittle mode cutting has been investigated from the viewpoint of the micro/macro interaction. The concept of micro/macro interaction is first described briefly, and the method to incorporate the concept into the Variable Resolution Molecular Dynamics (VRMD) has been discussed, together with the explanation of the VRMD itself. The results of the VRMD without, with incomplete and with complete micro/macro interaction have been compared and the crucial role of the interaction has been clarified.