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Chapter 2
2.2 Subsurface deformation
It is well known that a newly generated surface is plastically deformed (see Fig. 2.7) during micro-cutting leading to an increase in cutting forces. Thomsen et al. [58] noted that there is usually a subsurface deformation during metal cutting that requires additional energy. They argued that the pushing by a cutting tool, similar to that of a punch shearing a sheet metal is always involved. They attributed this phenomenon to the size effect as there is a constant force involved in plastic deformation of the subsurface. A considerable research to understand the nature of plastic deformation of subsurface in micro-cutting exists [59-62].
Researchers found that the magnitude of plastic deformation, plastic strain and hardness in the subsurface region changes with cutting parameters. Nakayama and Tamura [17] observed size effect during orthogonal micro-cutting experiments of brass. To avoid thermal and strain rate effects, experiments were conducted at very low speeds. They attributed the size effect to the subsurface deformation due to the fact that the energy consumed in plastic deformation is not proportional to the uncut chip thickness (see Fig. 2.8) and the extra work done for subsurface deformation has to be supplied by the cutting forces. The main cause of the subsurface deformation was recognized as an extension of PDZ below the machined surface at a small uncut chip thickness due to the ploughing effect by negative rake angled tools.
Shimada et al. [19] used molecular dynamic simulations to predict minimum chip thickness obtained in a micro-cutting operation with 5 nm edge radius tool. They found that while the copper and aluminum workpieces show identical chip thickness, the minimum chip thickness in micro cutting can be expected to be about 5 to 10 percent of the radius of the cutting edge. They also showed that depth of the deformed layer around the cutting edge radius is difficult to reduce to the level less than the size of tool edge radius. Therefore, leading to the size effect during micro-cutting. Moriwaki et al. [20] suggested that the depth of the damaged layer beneath a surface finished in diamond cutting becomes about 10 times the edge radius of the tool. Therefore, it influences the specific cutting energy during micro-cutting. Komanduri et al. [21] carried out molecular dynamics simulation studies by varying the tool edge radius (3.62-21.72 nm), uncut chip thickness (0.362-2.172 nm) and maintaining the ratio of uncut chip thickness to tool edge radius constant (0.1, 0.2, and 0.3). Variations in cutting and thrust forces, the force ratio, the specific energy, and the subsurface deformation with the tool geometry and depths of cut were investigated and found significant.
2.4 Microcrack formation in the shear zone
2.5 Gross fracture phenomenon ahead of tool-tip
2.6 Workpiece microstructure effect
2.7 Conclusions from the literature review
2.8 Objective and scope of the research
