Vesselin Yamakov | National Institute of Aerospace (original) (raw)
Papers by Vesselin Yamakov
Journal de Physique II, 1997
Physical Review E Statistical Physics Plasmas Fluids and Related Interdisciplinary Topics, Dec 1, 1997
We study conformational and dynamic properties of dilute polymer solutions drifting through a ran... more We study conformational and dynamic properties of dilute polymer solutions drifting through a random environment of obstacles at varying intensity of the external field B and of the host matrix density Cob using dynamic Monte Carlo simulation of an off-lattice bead-spring model. The presence of obstacles is found to influence strongly the conformational properties of the drifting chains: with growing strength of the field B and Cob≠0 the chain mean size (gyration radius), R2g, rapidly increases while the ratio between the end-to-end distance, R2ee, and R2g drops essentially below the usual value of 6, typical in the absence of drift, suggesting a hooflike shape of the chain with both ends directed along the external field vector. We confirm the finding of G. M. Foo and R. B. Pandey [Phys. Rev. E 51, 5738 (1995)] of a critical strength of the external field Bc above which the permeability of the host matrix sharply drops. A detailed study of this phenomenon suggests that Bc may be related to a dramatic growth of a specific ``capture'' time, characterizing the interaction of the chains with the obstacles, so that a simple model describing the drift of chains among obstacles may be shown to reproduce our findings.
Europhysics Letters, Sep 15, 1999
The escape transition of a flexible polymer chain of chain length N, endgrafted at a hard wall an... more The escape transition of a flexible polymer chain of chain length N, endgrafted at a hard wall and compressed by a piston of radius R, is studied by Monte Carlo simulation and by phenomenological arguments. In contrast to previous theories which have considered the transition as a function of a (fixed) height H of the piston above the wall, we consider the transition as a function of the conjugate variable, the force f acting on the piston. We find that the transition (which is sharp only for N → ∞) is characterized by a flat region of f in the f vs. H isotherm, i.e. a jump in the height occurs at the transition from Hesc,t to Himp,t, with (Himp,t - Hesc,t)/Hesc,t approx 0.26.
Physical Review Letters, Sep 1, 1997
Off-lattice Monte Carlo simulations show anomalous diffusion at intermediate times without a clea... more Off-lattice Monte Carlo simulations show anomalous diffusion at intermediate times without a clear Rouse regime, for the motion of chains through a random porous medium.
Physical Review E Statistical Physics Plasmas Fluids and Related Interdisciplinary Topics, Feb 1, 1997
Using an off-lattice Monte Carlo bead-spring model of a chain in a random environment, we study c... more Using an off-lattice Monte Carlo bead-spring model of a chain in a random environment, we study chain conformations and dynamic scaling of diffusivity and relaxation times with chain length N and density of the host matrix Cobs. Our simulational results show that with growing Cobs the mean size (gyration radius) of the polymer, R2g, initially slightly decreases and then rapidly increases as the macromolecule exceeds the size of the average entropic wells and stretches through bottlenecks into neighboring wells. The chain dynamics changes from a Rouse-like one into a reptational one as the permeability of the matrix decreases. Although at variance with some previous treatments [M. Muthukumar, J. Chem. Phys. 90, 4594 (1989)], these findings agree well with a recent analytic approach (S. V. Panyukov, Zh. Éksp. Teor. Fiz. 103, 1287 (1993) [Sov. Phys. JETP 76, 631 (1993)]) to chain conformations in random media. We also suggest a simple scaling analysis, based on a ``blob'' representation of the polymer chain, whereby the blob size is governed by the size of the cavities in the host matrix and yields a faithful description of our computer experiments.
European Physical Journal B, 1999
A coarse-grained off-lattice bead-spring model is used to reveal the complex dynamics of a polyme... more A coarse-grained off-lattice bead-spring model is used to reveal the complex dynamics of a polymer chain in a quenched porous medium in the presence of an external field B. The behavior of the mean square displacement (MSD) of the center chain bead and that of the center of mass of the chain as a function of time is studied at different values of the barrier concentration C, the field strength B and the chain length N. In a field, important information on the way in which chains move between obstacles and overcome them is gained from the MSD vs. time analysis in the directions parallel and perpendicular to the flow. Instead of a steady approach to uniform drift-like motion at low C, for sufficiently strong field B we observe logarithmic oscillations in the effective exponents describing the time dependence of the MSD along and perpendicular to field. A common nature of this phenomenon with oscillatory behavior, observed earlier for biased diffusion of tracers on random lattices, is suggested. PACS. 36.20.-r Macromolecules and polymer molecules -82.45.+z Electrochemistry and electrophoresis -87.15.-v Biomolecules: structure and physical properties
Materials Science Forum, 2000
Physical Review Letters, 2000
We study the size R g of random polyampholytes (i.e., polymers with randomly charged monomers) as... more We study the size R g of random polyampholytes (i.e., polymers with randomly charged monomers) as a function of their length N. All results of our extensive Monte Carlo simulations can be rationalized in terms of the scaling theory we develop for the Kantor-Kardar necklace model, although this theory neglects the quenched disorder in the charge sequence along the chain. We find ͗R g ͘~N 1͞2 . The elongated globule model, the initial predictions of both Higgs and Joanny (~N 1͞3 ) and Kantor and Kardar (~N), and previous numerical estimates are ruled out. 87.15.By
Physical Chemistry Chemical Physics, 1999
ABSTRACT
The Journal of Chemical Physics, 2000
A generic phenomenological theory of the glass transition is developed in the framework of a quas... more A generic phenomenological theory of the glass transition is developed in the framework of a quasilinear formulation of the thermodynamics of irreversible processes. Starting from one of the basic principles of this science in its approximate form given by de Donder's equation, after a change of variables the temperature dependence of the structural parameter xi(T), the thermodynamic potentials DeltaG~(T), the
Journal of Applied Crystallography, 1999
A simple solution accounting for the secondary-extinction correction of the integrated intensity ... more A simple solution accounting for the secondary-extinction correction of the integrated intensity of the diffracted beam in a mosaic crystal is derived for the case of symmetrical Bragg re¯ection from an`in®nitely thick' plane parallel plate. The solution of energytransfer equations contains a`thickness-dependent term' which enables further extension of the problem to the`thin' ®lm case. The new formulae are derived assuming a rectangular or triangular crystalline block distribution, which leads to exact integration of the diffracted intensity. In addition, a general term for Zachariasen [Acta Cryst. (1963), 16, 1139±1147] series expansion, assuming Gaussian domain distribution, is deduced. In fact, the new analytical results represent a variety of improved approximations which are simultaneously valid both for weak and for strong extinction effects usually observed in textured ®lms. The formulae are used for computing the pole density and secondary extinction in electrodeposited nickel ®lms having different texture sharpnesses. It may be anticipated that the precision in any X-ray diffraction characterization of ®lms could be enhanced using the improved secondaryextinction corrections.
The European Physical Journal E - Soft Matter, 2002
We use large scale Monte Carlo simulations to test scaling theories for the electrostatic persist... more We use large scale Monte Carlo simulations to test scaling theories for the electrostatic persistence length le of isolated, uniformly charged polymers with Debye-Hückel intrachain interactions in the limit where the screening length κ −1 exceeds the intrinsic persistence length of the chains. Our simulations cover a significantly larger part of the parameter space than previous studies. We observe no significant deviations from the prediction le ∝ κ −2 by Khokhlov and Khachaturian which is based on applying the Odijk-Skolnick-Fixman theory to the stretched de Gennes-Pincus-Velasco-Brochard polyelectrolyte blob chain. A linear or sublinear dependence of the persistence length on the screening length can be ruled out. We argue that previous numerical results pointing into this direction are probably due to a combination of excluded volume and finite chain length effects. The paper emphasizes the role of scaling arguments in the development of useful representations for experimental and simulation data.
The European Physical Journal B, 1999
A coarse-grained off-lattice bead-spring model is used to reveal the complex dynamics of a polyme... more A coarse-grained off-lattice bead-spring model is used to reveal the complex dynamics of a polymer chain in a quenched porous medium in the presence of an external field B. The behavior of the mean square displacement (MSD) of the center chain bead and that of the center of mass of the chain as a function of time is studied at different values of the barrier concentration C, the field strength B and the chain length N. In a field, important information on the way in which chains move between obstacles and overcome them is gained from the MSD vs. time analysis in the directions parallel and perpendicular to the flow. Instead of a steady approach to uniform drift-like motion at low C, for sufficiently strong field B we observe logarithmic oscillations in the effective exponents describing the time dependence of the MSD along and perpendicular to field. A common nature of this phenomenon with oscillatory behavior, observed earlier for biased diffusion of tracers on random lattices, is suggested. PACS. 36.20.-r Macromolecules and polymer molecules -82.45.+z Electrochemistry and electrophoresis -87.15.-v Biomolecules: structure and physical properties
Springer Proceedings in Physics, 2001
49th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference <br> 16th AIAA/ASME/AHS Adaptive Structures Conference<br> 10t, 2008
A concurrent multiscale modeling methodology that embeds a molecular dynamics (MD) region within ... more A concurrent multiscale modeling methodology that embeds a molecular dynamics (MD) region within a finite element (FEM) domain has been enhanced. The concurrent MD-FEM coupling methodology uses statistical averaging of the deformation of the atomistic MD domain to provide interface displacement boundary conditions to the surrounding continuum FEM region, which, in turn, generates interface reaction forces that are applied as piecewise constant traction boundary conditions to the MD domain. The enhancement is based on the addition of molecular dynamicsbased cohesive zone model (CZM) elements near the MD-FEM interface. The CZM elements are a continuum interpretation of the tractiondisplacement relationships taken from MD simulations using Cohesive Zone Volume Elements (CZVE). The addition of CZM elements to the concurrent MD-FEM analysis provides a consistent set of atomistically-based cohesive properties within the finite element region near the growing crack. Another set of CZVEs are then used to extract revised CZM relationships from the enhanced ESCM simulation of an edge crack under uniaxial loading.
48th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, 2007
Molecular dynamics (MD) methods are opening new opportunities for simulating the fundamental proc... more Molecular dynamics (MD) methods are opening new opportunities for simulating the fundamental processes of material behavior at the atomistic level. However, increasing the size of the MD domain quickly presents intractable computational demands. A robust approach to surmount this computational limitation has been to unite continuum modeling procedures such as the finite element method (FEM) with MD analyses thereby reducing the region of atomic scale refinement. The challenging problem is to seamlessly connect the two inherently different simulation techniques at their interface. In the present work, a new approach to MD-FEM coupling is developed based on a restatement of the typical boundary value problem used to define a coupled domain. The method uses statistical averaging of the atomistic MD domain to provide displacement interface boundary conditions to the surrounding continuum FEM region, which, in return, generates interface reaction forces applied as piecewise constant traction boundary conditions to the MD domain. The two systems are computationally disconnected and communicate only through a continuous update of their boundary conditions. With the use of statistical averages of the atomistic quantities to couple the two computational schemes, the developed approach is referred to as an embedded statistical coupling method (ESCM) as opposed to a direct coupling method where interface atoms and FEM nodes are individually related. The methodology is inherently applicable to three-dimensional domains, avoids discretization of the continuum model down to atomic scales, and permits arbitrary temperatures to be applied.
50th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, 2009
A concurrent multiscale modeling methodology that embeds a molecular dynamics (MD) region within ... more A concurrent multiscale modeling methodology that embeds a molecular dynamics (MD) region within a finite element (FEM) domain is used to study plastic processes at a crack tip in a single crystal of aluminum. The case of mode I loading is studied. A transition from deformation twinning to full dislocation emission from the crack tip is found when the crack plane is rotated around the [ 111 ] crystallographic axis. When the crack plane normal coincides with the [112] twinning direction, the crack propagates through a twinning mechanism. When the crack plane normal coincides with the [011] slip direction, the crack propagates through the emission of full dislocations. In intermediate orientations, a transition from full dislocation emission to twinning is found to occur with an increase in the stress intensity at the crack tip. This finding confirms the suggestion that the very high strain rates, inherently present in MD simulations, which produce higher stress intensities at the crack tip, over-predict the tendency for deformation twinning compared to experiments. The present study, therefore, aims to develop a more realistic and accurate predictive modeling of fracture processes.
Acta Mater, 2003
Grain growth during high-T deformation of a Pd polycrystal (25 columnar grains of average size ~ ... more Grain growth during high-T deformation of a Pd polycrystal (25 columnar grains of average size ~ 15nm) is examined and its coupling with Coble creep studied. Grain growth occurs via grain-boundary (GB) migration and grain-rotation-induced grain coalescence. Comparison with grain growth in response only to temperature reveals mechanisms by which external stress affects grain growth, e.g., both GB migration and grain rotation are accelerated. Also, topological changes during grain growth cause a (temporary) increase of the creep rate.
Using a recently developed, massively parallel molecular-dynamics (MD) code for the simulation of... more Using a recently developed, massively parallel molecular-dynamics (MD) code for the simulation of polycrystal plasticity, we analyze for the case of nanocrystalline Al the complex interplay among various dislocation and grain-boundary processes during low-temperature deformation. A unique aspect of this work, arising from our ability to deform to rather large plastic strains and to consider a rather large grain size, is the observation of deformation under very high grain-boundary and dislocation densities, i.e., in a deformation regime where they compete on an equal footing. We are thus able to identify the intra-and intergranular dislocation and grain-boundary processes responsible for the extensive deformation twinning observed in our simulations. This illustrates the ability of this type of simulations to capture novel atomic-level insights into the underlying deformation mechanisms not presently possible experimentally.
Journal de Physique II, 1997
Physical Review E Statistical Physics Plasmas Fluids and Related Interdisciplinary Topics, Dec 1, 1997
We study conformational and dynamic properties of dilute polymer solutions drifting through a ran... more We study conformational and dynamic properties of dilute polymer solutions drifting through a random environment of obstacles at varying intensity of the external field B and of the host matrix density Cob using dynamic Monte Carlo simulation of an off-lattice bead-spring model. The presence of obstacles is found to influence strongly the conformational properties of the drifting chains: with growing strength of the field B and Cob≠0 the chain mean size (gyration radius), R2g, rapidly increases while the ratio between the end-to-end distance, R2ee, and R2g drops essentially below the usual value of 6, typical in the absence of drift, suggesting a hooflike shape of the chain with both ends directed along the external field vector. We confirm the finding of G. M. Foo and R. B. Pandey [Phys. Rev. E 51, 5738 (1995)] of a critical strength of the external field Bc above which the permeability of the host matrix sharply drops. A detailed study of this phenomenon suggests that Bc may be related to a dramatic growth of a specific ``capture'' time, characterizing the interaction of the chains with the obstacles, so that a simple model describing the drift of chains among obstacles may be shown to reproduce our findings.
Europhysics Letters, Sep 15, 1999
The escape transition of a flexible polymer chain of chain length N, endgrafted at a hard wall an... more The escape transition of a flexible polymer chain of chain length N, endgrafted at a hard wall and compressed by a piston of radius R, is studied by Monte Carlo simulation and by phenomenological arguments. In contrast to previous theories which have considered the transition as a function of a (fixed) height H of the piston above the wall, we consider the transition as a function of the conjugate variable, the force f acting on the piston. We find that the transition (which is sharp only for N → ∞) is characterized by a flat region of f in the f vs. H isotherm, i.e. a jump in the height occurs at the transition from Hesc,t to Himp,t, with (Himp,t - Hesc,t)/Hesc,t approx 0.26.
Physical Review Letters, Sep 1, 1997
Off-lattice Monte Carlo simulations show anomalous diffusion at intermediate times without a clea... more Off-lattice Monte Carlo simulations show anomalous diffusion at intermediate times without a clear Rouse regime, for the motion of chains through a random porous medium.
Physical Review E Statistical Physics Plasmas Fluids and Related Interdisciplinary Topics, Feb 1, 1997
Using an off-lattice Monte Carlo bead-spring model of a chain in a random environment, we study c... more Using an off-lattice Monte Carlo bead-spring model of a chain in a random environment, we study chain conformations and dynamic scaling of diffusivity and relaxation times with chain length N and density of the host matrix Cobs. Our simulational results show that with growing Cobs the mean size (gyration radius) of the polymer, R2g, initially slightly decreases and then rapidly increases as the macromolecule exceeds the size of the average entropic wells and stretches through bottlenecks into neighboring wells. The chain dynamics changes from a Rouse-like one into a reptational one as the permeability of the matrix decreases. Although at variance with some previous treatments [M. Muthukumar, J. Chem. Phys. 90, 4594 (1989)], these findings agree well with a recent analytic approach (S. V. Panyukov, Zh. Éksp. Teor. Fiz. 103, 1287 (1993) [Sov. Phys. JETP 76, 631 (1993)]) to chain conformations in random media. We also suggest a simple scaling analysis, based on a ``blob'' representation of the polymer chain, whereby the blob size is governed by the size of the cavities in the host matrix and yields a faithful description of our computer experiments.
European Physical Journal B, 1999
A coarse-grained off-lattice bead-spring model is used to reveal the complex dynamics of a polyme... more A coarse-grained off-lattice bead-spring model is used to reveal the complex dynamics of a polymer chain in a quenched porous medium in the presence of an external field B. The behavior of the mean square displacement (MSD) of the center chain bead and that of the center of mass of the chain as a function of time is studied at different values of the barrier concentration C, the field strength B and the chain length N. In a field, important information on the way in which chains move between obstacles and overcome them is gained from the MSD vs. time analysis in the directions parallel and perpendicular to the flow. Instead of a steady approach to uniform drift-like motion at low C, for sufficiently strong field B we observe logarithmic oscillations in the effective exponents describing the time dependence of the MSD along and perpendicular to field. A common nature of this phenomenon with oscillatory behavior, observed earlier for biased diffusion of tracers on random lattices, is suggested. PACS. 36.20.-r Macromolecules and polymer molecules -82.45.+z Electrochemistry and electrophoresis -87.15.-v Biomolecules: structure and physical properties
Materials Science Forum, 2000
Physical Review Letters, 2000
We study the size R g of random polyampholytes (i.e., polymers with randomly charged monomers) as... more We study the size R g of random polyampholytes (i.e., polymers with randomly charged monomers) as a function of their length N. All results of our extensive Monte Carlo simulations can be rationalized in terms of the scaling theory we develop for the Kantor-Kardar necklace model, although this theory neglects the quenched disorder in the charge sequence along the chain. We find ͗R g ͘~N 1͞2 . The elongated globule model, the initial predictions of both Higgs and Joanny (~N 1͞3 ) and Kantor and Kardar (~N), and previous numerical estimates are ruled out. 87.15.By
Physical Chemistry Chemical Physics, 1999
ABSTRACT
The Journal of Chemical Physics, 2000
A generic phenomenological theory of the glass transition is developed in the framework of a quas... more A generic phenomenological theory of the glass transition is developed in the framework of a quasilinear formulation of the thermodynamics of irreversible processes. Starting from one of the basic principles of this science in its approximate form given by de Donder&amp;#39;s equation, after a change of variables the temperature dependence of the structural parameter xi(T), the thermodynamic potentials DeltaG~(T), the
Journal of Applied Crystallography, 1999
A simple solution accounting for the secondary-extinction correction of the integrated intensity ... more A simple solution accounting for the secondary-extinction correction of the integrated intensity of the diffracted beam in a mosaic crystal is derived for the case of symmetrical Bragg re¯ection from an`in®nitely thick' plane parallel plate. The solution of energytransfer equations contains a`thickness-dependent term' which enables further extension of the problem to the`thin' ®lm case. The new formulae are derived assuming a rectangular or triangular crystalline block distribution, which leads to exact integration of the diffracted intensity. In addition, a general term for Zachariasen [Acta Cryst. (1963), 16, 1139±1147] series expansion, assuming Gaussian domain distribution, is deduced. In fact, the new analytical results represent a variety of improved approximations which are simultaneously valid both for weak and for strong extinction effects usually observed in textured ®lms. The formulae are used for computing the pole density and secondary extinction in electrodeposited nickel ®lms having different texture sharpnesses. It may be anticipated that the precision in any X-ray diffraction characterization of ®lms could be enhanced using the improved secondaryextinction corrections.
The European Physical Journal E - Soft Matter, 2002
We use large scale Monte Carlo simulations to test scaling theories for the electrostatic persist... more We use large scale Monte Carlo simulations to test scaling theories for the electrostatic persistence length le of isolated, uniformly charged polymers with Debye-Hückel intrachain interactions in the limit where the screening length κ −1 exceeds the intrinsic persistence length of the chains. Our simulations cover a significantly larger part of the parameter space than previous studies. We observe no significant deviations from the prediction le ∝ κ −2 by Khokhlov and Khachaturian which is based on applying the Odijk-Skolnick-Fixman theory to the stretched de Gennes-Pincus-Velasco-Brochard polyelectrolyte blob chain. A linear or sublinear dependence of the persistence length on the screening length can be ruled out. We argue that previous numerical results pointing into this direction are probably due to a combination of excluded volume and finite chain length effects. The paper emphasizes the role of scaling arguments in the development of useful representations for experimental and simulation data.
The European Physical Journal B, 1999
A coarse-grained off-lattice bead-spring model is used to reveal the complex dynamics of a polyme... more A coarse-grained off-lattice bead-spring model is used to reveal the complex dynamics of a polymer chain in a quenched porous medium in the presence of an external field B. The behavior of the mean square displacement (MSD) of the center chain bead and that of the center of mass of the chain as a function of time is studied at different values of the barrier concentration C, the field strength B and the chain length N. In a field, important information on the way in which chains move between obstacles and overcome them is gained from the MSD vs. time analysis in the directions parallel and perpendicular to the flow. Instead of a steady approach to uniform drift-like motion at low C, for sufficiently strong field B we observe logarithmic oscillations in the effective exponents describing the time dependence of the MSD along and perpendicular to field. A common nature of this phenomenon with oscillatory behavior, observed earlier for biased diffusion of tracers on random lattices, is suggested. PACS. 36.20.-r Macromolecules and polymer molecules -82.45.+z Electrochemistry and electrophoresis -87.15.-v Biomolecules: structure and physical properties
Springer Proceedings in Physics, 2001
49th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference <br> 16th AIAA/ASME/AHS Adaptive Structures Conference<br> 10t, 2008
A concurrent multiscale modeling methodology that embeds a molecular dynamics (MD) region within ... more A concurrent multiscale modeling methodology that embeds a molecular dynamics (MD) region within a finite element (FEM) domain has been enhanced. The concurrent MD-FEM coupling methodology uses statistical averaging of the deformation of the atomistic MD domain to provide interface displacement boundary conditions to the surrounding continuum FEM region, which, in turn, generates interface reaction forces that are applied as piecewise constant traction boundary conditions to the MD domain. The enhancement is based on the addition of molecular dynamicsbased cohesive zone model (CZM) elements near the MD-FEM interface. The CZM elements are a continuum interpretation of the tractiondisplacement relationships taken from MD simulations using Cohesive Zone Volume Elements (CZVE). The addition of CZM elements to the concurrent MD-FEM analysis provides a consistent set of atomistically-based cohesive properties within the finite element region near the growing crack. Another set of CZVEs are then used to extract revised CZM relationships from the enhanced ESCM simulation of an edge crack under uniaxial loading.
48th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, 2007
Molecular dynamics (MD) methods are opening new opportunities for simulating the fundamental proc... more Molecular dynamics (MD) methods are opening new opportunities for simulating the fundamental processes of material behavior at the atomistic level. However, increasing the size of the MD domain quickly presents intractable computational demands. A robust approach to surmount this computational limitation has been to unite continuum modeling procedures such as the finite element method (FEM) with MD analyses thereby reducing the region of atomic scale refinement. The challenging problem is to seamlessly connect the two inherently different simulation techniques at their interface. In the present work, a new approach to MD-FEM coupling is developed based on a restatement of the typical boundary value problem used to define a coupled domain. The method uses statistical averaging of the atomistic MD domain to provide displacement interface boundary conditions to the surrounding continuum FEM region, which, in return, generates interface reaction forces applied as piecewise constant traction boundary conditions to the MD domain. The two systems are computationally disconnected and communicate only through a continuous update of their boundary conditions. With the use of statistical averages of the atomistic quantities to couple the two computational schemes, the developed approach is referred to as an embedded statistical coupling method (ESCM) as opposed to a direct coupling method where interface atoms and FEM nodes are individually related. The methodology is inherently applicable to three-dimensional domains, avoids discretization of the continuum model down to atomic scales, and permits arbitrary temperatures to be applied.
50th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, 2009
A concurrent multiscale modeling methodology that embeds a molecular dynamics (MD) region within ... more A concurrent multiscale modeling methodology that embeds a molecular dynamics (MD) region within a finite element (FEM) domain is used to study plastic processes at a crack tip in a single crystal of aluminum. The case of mode I loading is studied. A transition from deformation twinning to full dislocation emission from the crack tip is found when the crack plane is rotated around the [ 111 ] crystallographic axis. When the crack plane normal coincides with the [112] twinning direction, the crack propagates through a twinning mechanism. When the crack plane normal coincides with the [011] slip direction, the crack propagates through the emission of full dislocations. In intermediate orientations, a transition from full dislocation emission to twinning is found to occur with an increase in the stress intensity at the crack tip. This finding confirms the suggestion that the very high strain rates, inherently present in MD simulations, which produce higher stress intensities at the crack tip, over-predict the tendency for deformation twinning compared to experiments. The present study, therefore, aims to develop a more realistic and accurate predictive modeling of fracture processes.
Acta Mater, 2003
Grain growth during high-T deformation of a Pd polycrystal (25 columnar grains of average size ~ ... more Grain growth during high-T deformation of a Pd polycrystal (25 columnar grains of average size ~ 15nm) is examined and its coupling with Coble creep studied. Grain growth occurs via grain-boundary (GB) migration and grain-rotation-induced grain coalescence. Comparison with grain growth in response only to temperature reveals mechanisms by which external stress affects grain growth, e.g., both GB migration and grain rotation are accelerated. Also, topological changes during grain growth cause a (temporary) increase of the creep rate.
Using a recently developed, massively parallel molecular-dynamics (MD) code for the simulation of... more Using a recently developed, massively parallel molecular-dynamics (MD) code for the simulation of polycrystal plasticity, we analyze for the case of nanocrystalline Al the complex interplay among various dislocation and grain-boundary processes during low-temperature deformation. A unique aspect of this work, arising from our ability to deform to rather large plastic strains and to consider a rather large grain size, is the observation of deformation under very high grain-boundary and dislocation densities, i.e., in a deformation regime where they compete on an equal footing. We are thus able to identify the intra-and intergranular dislocation and grain-boundary processes responsible for the extensive deformation twinning observed in our simulations. This illustrates the ability of this type of simulations to capture novel atomic-level insights into the underlying deformation mechanisms not presently possible experimentally.