Papers by Author: M. Ravi Shankar

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Authors: Michael Sevier, Seongeyl Lee, M. Ravi Shankar, Henry T.Y. Yang, Srinivasan Chandrasekar, W. Dale Compton
Abstract: The deformation field associated with chip formation in plane strain (2-D) machining has been simulated using the finite element method (FEM), with the objective of developing 2-D machining as an experimental technique for studying very large strain deformation phenomena. The principal machining parameters are the tool rake angle, cutting velocity and the friction at the toolchip interface while the deformation field parameters are strain, strain rate and temperature. The relation between rake angle and the shear strain in the deformation zone is studied for the low-speed cutting of lead. This correspondence is validated by comparison with measurements of the deformation parameters made by applying a Particle Image Velocimetry (PIV) technique to highspeed photographic image sequences of the deformation. It is shown that plastic strains in the range of 1-15 can be realized in a controlled manner by appropriate choice of the rake angle. The unique capabilities offered by 2-D machining for studying micro- and nano- mechanics of large strain deformation, and the creation of ultra-fine grained materials are highlighted in the context of these results.
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Authors: Shashank Shekhar, S. Abolghashem, S. Basu, J. Cai, M. Ravi Shankar
Abstract: During high rate severe plastic deformation (HRSPD), strain and strain-rate are not the only external factors that determine microstructural transformations in materials, temperature-rise due to heat generation from deformation processes, also plays an important role. Temperature may influence the microstructure directly by controlling grain growth kinetics and it may also have an indirect effect through the interactive effect on material behavior, which in turn, influences strain and strain-rate parameters. This complex thermomechanics of HRSPD can lead to myriad of microstructure and consequently, material properties and phenomenon. These deformation parameters can be utilized as a ‘fingerprint’ for the resulting microstructure, and the properties and phenomenon related to it. Here, we capture some of these microstructural transformations by relating grain and sub-grain sizes, to the deformation parameters. In doing so, we find evidence of continuous dynamic recrystallization operative under these HRSPD conditions, where the interplay of strain, strain rate and temperatures offer varying degrees of multimodality in the grain-size distributions.
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Authors: Tejas G. Murthy, C. Huang, M. Ravi Shankar, Srinivasan Chandrasekar, Kevin P. Trumble, John P. Sullivan
Abstract: The temperature and strain rate fields in severe plastic deformation (SPD) are measured using infra-red thermography and Particle Image Velocimetry (PIV), respectively. Plane strain machining is used as the method of SPD to impose controlled strains and strain rates. For metals such as titanium, the temperature rise is small at small strain rates and SPD occurs at near-ambient temperature. The possibility of exploring dynamic recovery/recrystallization phenomena using the Zener-Hollomon parameter in this SPD framework is briefly discussed.
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