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1. Understanding of Residual Stress and Subsurface Damage by 2- Step Machining of Single Crystal Sapphire

   Nagaraj, Aditya; Min, Sangkee (December 2022, 19th International Conference on Precision Engineering)

   The Japan Society for Precision Engineering (Ed.)

   Machining is in general conducted in multiple paths and thus residual stress and subsurface damage formed by previous cut may influence subsequent cutting. Ceramics materials are extremely brittle and prone to cracks. Ultra-precision machining with very small depth of cut enables ductile mode cutting. There have been various reports that critical depth of cut (CDC) for single crystal sapphire exists, where the ductile to brittle transition occurs. However, the CDC of subsequent cutting changes due to the influence of residual stress and subsurface damage by previous cut. This study investigates the indirect effect of residual stress and subsurface damage on the critical depth of cut of the second cut by analyzing the plastic deformation mechanisms activated during 2-step machining on A-plane of sapphire. It was found that the [1#100] machining orientation was most suitable since the critical depth of cut remained fairly constant due to dominant rhombohedral twinning activation during subsequent machining operations. 

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2. Prediction of crack initiation in single-crystal sapphire during ultra-precision machining using MD simulation-based slip/fracture activation model

   https://doi.org/10.1016/j.precisioneng.2023.12.007  

   Kwon, Suk Bum; Nagaraj, Aditya; Kim, Dae Nyoung; Xi, Dalei; Du, Yiyang; Kim, Woo Kyun; Min, Sangkee (March 2024, Precision Engineering)

   In this paper, material deformation during ultra-precision machining (UPM) on the C-, R-, and A-planes of sapphire was investigated using the slip/fracture activation model where the likelihood of activation of individual plastic deformation and fracture systems on different crystallographic planes was calculated. The stress data obtained from molecular dynamics (MD) simulations were utilized, and the slip/fracture activation model was developed by incorporating the principal stresses in calculating the plastic deformation and fracture cleavage parameters. The analysis methodology was applied to study material deformation along various cutting orientations in sapphire. The stress field at crack initiation during UPM on C-, R-, and A-planes of sapphire was calculated using molecular dynamics (MD) simulations. An equation describing the relationship between crack initiation and its triggering parameters was formulated considering the systems’ plastic deformation and cleavage fractures. The model can qualitatively predict the crack initiations for various cutting orientations. The proposed model was verified through ultra-precision orthogonal plunge cut experiments along the same cutting orientations as in the MD simulations. 

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3. In Situ Observations of Shear Localization and Fracture in Machining

   https://doi.org/10.1115/MSEC2024-124723  

   Yadav, Shwetabh; Chawla, Harshit; Sagapuram, Dinakar (June 2024, American Society of Mechanical Engineers)

   Using direct high-speed imaging, we study the transition between different chip formation modes, and the underlying mechanics, in machining of ductile metals. Three distinct chip formation modes — continuous chip, shear-localized chip, and fragmented chip — are effected in a same material system by varying the cutting speed. It is shown using direct observations that shear-localized chip formation is characterized by shear band nucleation at the tool tip and its propagation towards the free surface, which is then followed by plastic slip along the band without fracture. The transition from shear-localized chip to fragmented chip with increasing cutting speed is triggered by crack initiation at the free surface and propagation towards the tool tip. The extent to which crack travels towards the tool determines whether the chip is partially fragmented or fully fragmented (discontinuous). It is shown that shear localization precedes fracture and controls the crack path in fragmented chip formation. Dynamic strain and strain-rate fields underlying the each chip formation mode are quantified through image correlation analysis of high-speed images. Implications for using machining as an experimental tool for fundamental studies of localization and shear fracture in ductile metals are also discussed. 

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4. Face Turning of Single Crystal (111)Ge: Cutting Mechanics and Surface/Subsurface Characteristics

   https://doi.org/10.1115/1.4057054  

   Zare, A.; Tunesi, M.; Harriman, T. A.; Troutman, J. R.; Davies, M. A.; Lucca, D. A. (July 2023, Journal of Manufacturing Science and Engineering)

   Abstract Single crystal Ge is a semiconductor that has broad applications, especially in manipulation of infrared light. Diamond machining enables the efficient production of surfaces with tolerances required by the optical industry. During machining of anisotropic single crystals, the cutting direction with respect to the in-plane lattice orientation plays a fundamental role in the final quality of the surface and subsurface. In this study, on-axis face turning experiments were performed on an undoped (111)Ge wafer to investigate the effects of crystal anisotropy and feedrate on the surface and subsurface conditions. Atomic force microscopy and scanning white light interferometry were used to characterize the presence of brittle fracture on the machined surfaces and to evaluate the resultant surface roughness. Raman spectroscopy was performed to evaluate the residual stresses and lattice disorder induced by the tool during machining. Nanoindentation with Berkovich and cube corner indenter tips was performed to evaluate elastic modulus, hardness, and fracture toughness of the machined surfaces and to study their variations with feedrate and cutting direction. Post-indentation studies of selected indentations were also performed to characterize the corresponding quasi-plasticity mechanisms. It was found that an increase of feedrate produced a rotation of the resultant force imparted by the tool indicating a shift from indentation-dominant to cutting-dominant behavior. Fracture increased with the feedrate and showed a higher propensity when the cutting direction belonged to the <112¯> family. 

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5. Studying mechanism of anisotropic crack generation on C-, R-, A-, and M-planes of sapphire during ultra-precision orthogonal cutting using a visualized slip/fracture activation model

   https://doi.org/10.1063/10.0026318  

   Kwon, Suk Bum; Min, Sangkee (December 2024, Nanotechnology and Precision Engineering)

   Duan, Xuexin; Fu, Richard; Guan, Weihua; Guan, Yingchun; Sun, Shuhui (Ed.)

   With the growing demand for the fabrication of microminiaturized components, a comprehensive understanding of material removal behavior during ultra-precision cutting has become increasingly significant. Single-crystal sapphire stands out as a promising material for microelectronic components, ultra-precision lenses, and semiconductor structures owing to its exceptional characteristics, such as high hardness, chemical stability, and optical properties. This paper focuses on understanding the mechanism responsible for generating anisotropic crack morphologies along various cutting orientations on four crystal planes (C-, R-, A-, and M-planes) of sapphire during ultra-precision orthogonal cutting. By employing a scanning electric microscope to examine the machined surfaces, the crack morphologies can be categorized into three distinct types on the basis of their distinctive features: layered, sculptured, and lateral. To understand the mechanism determining crack morphology, visualized parameters related to the plastic deformation and cleavage fracture parameters are utilized. These parameters provide insight into both the likelihood and direction of plastic deformation and fracture system activations. Analysis of the results shows that the formation of crack morphology is predominantly influenced by the directionality of crystallographic fracture system activation and by the interplay between fracture and plastic deformation system activations. 

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