Akademik Arşiv / Academic Archive
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Publication Comparison of Implicit Time Integration Schemes for Nonlinear Dynamic Problems(ASMEDC, 2010-01-01) Murat DemiralImplicit time integration schemes are used to obtain stable and accurate transient solutions of nonlinear problems. Methods that are unconditionally stable in linear analysis are sometimes observed to have convergence problems as in the case of solutions obtained with a trapezoidal method. On the other hand, a composite time integration method employing a trapezoidal rule and a three-point backward rule sequentially in two half steps can be used to obtain accurate results and enhance the stability of the system by means of a numerical damping introduced in the formulation. To have a better understanding of the differences in the numerical implementation of the algorithms of these two methods, a mathematical analysis of dynamic equilibrium equations is performed. Several practical problems are studied to compare the implicit methods.Publication Finite-Element Simulations of Split Hopkinson Test of Ti-Based Alloy(Trans Tech Publications, Ltd., 2011-04-19) Murat Demiral; Tõnu Leemet; Mikko Hokka; Veli Tapani Kuokkala; Anish Roy; Vadim SilberschmidtTi-based alloys are extensively used in aerospace and other advanced engineering fields due to their high strength and toughness, light weight, excellent corrosion resistance and ability to withstand extreme temperatures. Since these alloys are hard to machine, there is an obvious demand to develop simulation tools in order to analyse the material's behaviour during machining and thus optimise the entire machining process. The deformation processes in machining of Ti-alloys are typically characterized by high strains and temperatures. A Split Hopkinson Pressure Bar (SHPB) technique is a commonly used experimental method to characterize a material`s behaviour at high strain rates; a stress-strain relation of the material is derived from the obtained experimental data. A computational study on a three-dimensional finite element model of the SHPB experiment is performed to assess various features of the underlying mechanics of deformation processes at high-strain and -strain-rate regimes. In the numerical analysis, an inhomogeneous deformation behaviour is observed in the workpiece at the initial stages of compression contrary to a standard assumption of stress and strain homogeneity in the specimen.Publication Computational Study of Ultrasonically-Assisted Turning of Ti Alloys(Trans Tech Publications, Ltd., 2011-04-19) Riaz Muhammad; Naseer Ahmed; Murat Demiral; Anish Roy; Vadim SilberschmidtIndustrial applications of titanium alloys especially in aerospace, marine and offshore industries have grown significantly over the years primarily due to their high strength, light weight as well as good fatigue and corrosion-resistance properties. The machinability of these difficult-to-cut metallic materials with conventional turning (CT) techniques has seen a limited improvement over the years. Ultrasonically-assisted turnning (UAT) is an advanced machining process, which has shown to have specific advantages, especially in the machining of high-strength alloys. In this study a three-dimensional finite element model of ultrasonically-assisted oblique cutting of a Ti-based super-alloy is developed. The nonlinear temperature-sensitive material behaviour is incorporated in our numerical simulations based on results obtained with split-Hopkinson pressure bar tests. Various contact conditions are considered at the tool tip-workpiece interface to get an in-depth understanding of the mechanism influencing cutting parameters. The simulation results obtained are compared for both CT and UAT conditions to elucidate main deformation mechanisms responsible for the observed changes in the material’s responses to cutting techniques.Publication Dynamic Behavior of Advanced Ti Alloy under Impact Loading: Experimental and Numerical Analysis(Trans Tech Publications, Ltd., 2011-08-18) Murat Demiral; Anish Roy; Vadim SilberschmidtIndustrial applications of Ti-based alloys, especially in aerospace, marine and offshore industries, have grown significantly over the years primarily due to their high strength, light weight as well as good fatigue and corrosion-resistance properties. A combination of experimental and numerical studies is necessary to predict a material behavior of such alloys under high strain-rate conditions characterized also by a high level of strains accompanied by high temperatures. A Split Hopkinson Pressure Bar (SHPB) technique is a commonly used experimental method to characterize a dynamic stress-strain response of materials at high strain rates. In a SHPB test, the striker bar is shot against the free end of the incident stress bar, which on impact generates a stress pulse propagating in the incident bar towards the specimen sandwiched between the incident and transmitted bars. An experimental study and a numerical analysis based on a three-dimensional finite element model of the SHPB experiment are performed in this study to assess various features of the underlying mechanics of deformation processes of the alloy tested at high-strain and -strain-rate regimes.Publication Indentation in F.C.C. Single Crystals(Trans Tech Publications, Ltd., 2012-05-14) Abolfazl Zahedi; Murat Demiral; Anish Roy; Vladimir I. Babitsky; Vadim SilberschmidtA three-dimensional crystal-plasticity finite element model of nano-indentation is developed in this paper to analyze deformation of a face-centred cubic (f.c.c.) high-purity single crystal of copper. This model was implemented as a user-defined subroutine in the commercial finite element software ABAQUS/Standard and used to study cases with different crystallographic orientations of the single crystal. The effects of various factors – crystallographic orientation of the indented material, an indenter angle and contact conditions between the indenter and workpiece material – on the load-displacement characteristics are studied. The obtained results show an anisotropic nature of surface topography around the obtained indents.Publication FE/SPH modelling of orthogonal micro-machining of f.c.c. single crystal(Elsevier BV, 2013-10) S. Abolfazl Zahedi; Murat Demiral; Anish Roy; Vadim V. SilberschmidtPublication Indentation studies in b.c.c. crystals with enhanced model of strain-gradient crystal plasticity(Elsevier BV, 2013-11) Murat Demiral; Anish Roy; Vadim V. SilberschmidtPublication Numerical modelling of micro-machining of f.c.c. single crystal: Influence of strain gradients(Elsevier BV, 2014-11) Murat Demiral; Anish Roy; Tamer El Sayed; Vadim V. SilberschmidtPublication Influence of strain gradients on lattice rotation in nano-indentation experiments: A numerical study(Elsevier BV, 2014-07) Murat Demiral; Anish Roy; Tamer El Sayed; Vadim V. SilberschmidtPublication Thermally enhanced ultrasonically assisted machining of Ti alloy(Elsevier BV, 2014) Riaz Muhammad; Agostino Maurotto; Murat Demiral; Anish Roy; Vadim V. Silberschmidt
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