List of Journal Publications
2019
5.
Irani, N., Nicola, L.
Modelling surface roughening during plastic deformation of metal crystals under contact shear loading Journal Article
In: MECHANICS OF MATERIALS, vol. 132, pp. 66–76, 2019.
Abstract | BibTeX | Tags: Contact shearing, Dislocations, Finite strains, size effects, Surface roughening | Links:
@article{Irani2019,
title = {Modelling surface roughening during plastic deformation of metal crystals under contact shear loading},
author = {N. Irani and L. Nicola},
doi = {10.1016/j.mechmat.2019.02.007},
year = {2019},
date = {2019-01-01},
journal = {MECHANICS OF MATERIALS},
volume = {132},
pages = {66–76},
publisher = {Elsevier B.V.},
abstract = {During plastic deformation, metal surfaces roughen and this has a deleterious impact on their tribological performance. It is therefore desirable to be able to predict and control the amount of roughening caused by subsurface plasticity. As a first step, we focus on modelling plastic deformation during contact shearing of an FCC metallic single crystal, employing a finite strain Discrete Dislocation Plasticity (DDP) formulation. This formulation allows us to capture the finite lattice rotations induced in the material by shearing and the corresponding local rotation of the crystallographic slip planes. The simulations predict a pronounced material pile-up in front of the contact and a sink-in at its rear, which are strongly crystal-orientation dependent. By comparing finite and small strain DDP, we can assess the effect of slip plane rotation on surface roughening and on metal plasticity in general. Results of the simulations are also compared with crystal plasticity, which is also capable of predicting a pile-up and sink-in, but not the crystal-orientation dependency of roughening.},
keywords = {Contact shearing, Dislocations, Finite strains, size effects, Surface roughening},
pubstate = {published},
tppubtype = {article}
}
During plastic deformation, metal surfaces roughen and this has a deleterious impact on their tribological performance. It is therefore desirable to be able to predict and control the amount of roughening caused by subsurface plasticity. As a first step, we focus on modelling plastic deformation during contact shearing of an FCC metallic single crystal, employing a finite strain Discrete Dislocation Plasticity (DDP) formulation. This formulation allows us to capture the finite lattice rotations induced in the material by shearing and the corresponding local rotation of the crystallographic slip planes. The simulations predict a pronounced material pile-up in front of the contact and a sink-in at its rear, which are strongly crystal-orientation dependent. By comparing finite and small strain DDP, we can assess the effect of slip plane rotation on surface roughening and on metal plasticity in general. Results of the simulations are also compared with crystal plasticity, which is also capable of predicting a pile-up and sink-in, but not the crystal-orientation dependency of roughening.
2016
4.
Malagù, Marcello, Lyulin, Alexey, Benvenuti, Elena, Simone, A.
A Molecular-Dynamics Study of Size and Chirality Effects on Glass-Transition Temperature and Ordering in Carbon Nanotube-Polymer Composites Journal Article
In: MACROMOLECULAR THEORY AND SIMULATIONS, vol. 25, no. 6, pp. 571–581, 2016.
Abstract | BibTeX | Tags: carbon nanotubes, crystallization, Molecular dynamics, polymer, size effects | Links:
@article{Malagu2016,
title = {A Molecular-Dynamics Study of Size and Chirality Effects on Glass-Transition Temperature and Ordering in Carbon Nanotube-Polymer Composites},
author = {Marcello Malagù and Alexey Lyulin and Elena Benvenuti and A. Simone},
doi = {10.1002/mats.201600041},
year = {2016},
date = {2016-01-01},
journal = {MACROMOLECULAR THEORY AND SIMULATIONS},
volume = {25},
number = {6},
pages = {571–581},
publisher = {Wiley-VCH Verlag},
abstract = {Molecular-dynamics simulations of single-walled carbon nanotubes (CNTs) embedded in a coarse-grained amorphous monodisperse polyethylene-like model system have been carried out. The roles of nanotube diameter and chirality on the physical and structural properties of the composite are thoroughly discussed for several CNTs with different diameter and chirality. It is shown that the glass-transition temperature of the polymer matrix increases with the diameter of the CNT while chirality effects are negligible. A denser and ordered layered region of polymer matrix is found in the vicinity of the nanotube surface. The density and ordering of this layer increases with the CNT diameter. All simulations indicate that chirality does not affect the atomic structure of the highly ordered layer surrounding the CNTs. Despite the simplicity of the polymer model, results of this study are qualitatively comparable with those obtained from experiments and numerical simulations that consider a chemically specific polymer matrix.},
keywords = {carbon nanotubes, crystallization, Molecular dynamics, polymer, size effects},
pubstate = {published},
tppubtype = {article}
}
Molecular-dynamics simulations of single-walled carbon nanotubes (CNTs) embedded in a coarse-grained amorphous monodisperse polyethylene-like model system have been carried out. The roles of nanotube diameter and chirality on the physical and structural properties of the composite are thoroughly discussed for several CNTs with different diameter and chirality. It is shown that the glass-transition temperature of the polymer matrix increases with the diameter of the CNT while chirality effects are negligible. A denser and ordered layered region of polymer matrix is found in the vicinity of the nanotube surface. The density and ordering of this layer increases with the CNT diameter. All simulations indicate that chirality does not affect the atomic structure of the highly ordered layer surrounding the CNTs. Despite the simplicity of the polymer model, results of this study are qualitatively comparable with those obtained from experiments and numerical simulations that consider a chemically specific polymer matrix.
2009
3.
Kumar, R., Nicola, L., der Giessen, E. Van
Density of grain boundaries and plasticity size effects: A discrete dislocation dynamics study Journal Article
In: MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, vol. 527, no. 1-2, pp. 7–15, 2009.
Abstract | BibTeX | Tags: Discrete dislocations, Grain boundary, size effects, Thin films | Links:
@article{Kumar2009,
title = {Density of grain boundaries and plasticity size effects: A discrete dislocation dynamics study},
author = {R. Kumar and L. Nicola and E. Van der Giessen},
doi = {10.1016/j.msea.2009.08.072},
year = {2009},
date = {2009-01-01},
journal = {MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING},
volume = {527},
number = {1-2},
pages = {7–15},
abstract = {Discrete dislocation dynamics simulations are carried out to systematically investigate the microstructural and geometrical size dependence of films under tension that have a varying number of grains through their thickness. By varying film thickness, grain size and aspect ratio, more insight is gained into the competition between grain boundary hardening and film thickness effects. This provides a seamless link between previous dislocation plasticity studies and qualitative agreement with experimental data. In the simulations, plasticity arises from the collective motion of discrete dislocations of edge character. Their dynamics is incorporated through constitutive rules for nucleation, glide, pinning and annihilation. Grain boundaries are treated as impenetrable to dislocation motion. The numerical results show that the grain size dependence of yield in thin films as well as in bulk polycrystals is controlled by the density of grain boundaries.},
keywords = {Discrete dislocations, Grain boundary, size effects, Thin films},
pubstate = {published},
tppubtype = {article}
}
Discrete dislocation dynamics simulations are carried out to systematically investigate the microstructural and geometrical size dependence of films under tension that have a varying number of grains through their thickness. By varying film thickness, grain size and aspect ratio, more insight is gained into the competition between grain boundary hardening and film thickness effects. This provides a seamless link between previous dislocation plasticity studies and qualitative agreement with experimental data. In the simulations, plasticity arises from the collective motion of discrete dislocations of edge character. Their dynamics is incorporated through constitutive rules for nucleation, glide, pinning and annihilation. Grain boundaries are treated as impenetrable to dislocation motion. The numerical results show that the grain size dependence of yield in thin films as well as in bulk polycrystals is controlled by the density of grain boundaries.
2006
2.
Nicola, L., Xiang, Y., Vlassak, J. J., der Giessen, E. Van, Needleman, A.
Plastic deformation of freestanding thin films: Experiments and modeling Journal Article
In: JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, vol. 54, no. 10, pp. 2089–2110, 2006.
Abstract | BibTeX | Tags: Bauschinger effect, Computer simulation, Dislocations, size effects, Thin films | Links:
@article{Nicola2006,
title = {Plastic deformation of freestanding thin films: Experiments and modeling},
author = {L. Nicola and Y. Xiang and J. J. Vlassak and E. Van der Giessen and A. Needleman},
doi = {10.1016/j.jmps.2006.04.005},
year = {2006},
date = {2006-01-01},
journal = {JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS},
volume = {54},
number = {10},
pages = {2089–2110},
abstract = {Experimental measurements and computational results for the evolution of plastic deformation in freestanding thin films are compared. In the experiments, the stress-strain response of two sets of Cu films is determined in the plane-strain bulge test. One set of samples consists of electroplated Cu films, while the other set is sputter-deposited. Unpassivated films, films passivated on one side and films passivated on both sides are considered. The calculations are carried out within a two-dimensional plane strain framework with the dislocations modeled as line singularities in an isotropic elastic solid. The film is modeled by a unit cell consisting of eight grains, each of which has three slip systems. The film is initially free of dislocations which then nucleate from a specified distribution of Frank-Read sources. The grain boundaries and any film-passivation layer interfaces are taken to be impenetrable to dislocations. Both the experiments and the computations show: (i) a flow strength for the passivated films that is greater than for the unpassivated films and (ii) hysteresis and a Bauschinger effect that increases with increasing pre-strain for passivated films, while for unpassivated films hysteresis and a Bauschinger effect are small or absent. Furthermore, the experimental measurements and computational results for the 0.2% offset yield strength stress, and the evolution of hysteresis and of the Bauschinger effect are in good quantitative agreement.},
keywords = {Bauschinger effect, Computer simulation, Dislocations, size effects, Thin films},
pubstate = {published},
tppubtype = {article}
}
Experimental measurements and computational results for the evolution of plastic deformation in freestanding thin films are compared. In the experiments, the stress-strain response of two sets of Cu films is determined in the plane-strain bulge test. One set of samples consists of electroplated Cu films, while the other set is sputter-deposited. Unpassivated films, films passivated on one side and films passivated on both sides are considered. The calculations are carried out within a two-dimensional plane strain framework with the dislocations modeled as line singularities in an isotropic elastic solid. The film is modeled by a unit cell consisting of eight grains, each of which has three slip systems. The film is initially free of dislocations which then nucleate from a specified distribution of Frank-Read sources. The grain boundaries and any film-passivation layer interfaces are taken to be impenetrable to dislocations. Both the experiments and the computations show: (i) a flow strength for the passivated films that is greater than for the unpassivated films and (ii) hysteresis and a Bauschinger effect that increases with increasing pre-strain for passivated films, while for unpassivated films hysteresis and a Bauschinger effect are small or absent. Furthermore, the experimental measurements and computational results for the 0.2% offset yield strength stress, and the evolution of hysteresis and of the Bauschinger effect are in good quantitative agreement.
2003
1.
Nicola, L., der, Giessen Van, Needleman, E.
Discrete dislocation analysis of size effects in thin films Journal Article
In: JOURNAL OF APPLIED PHYSICS, vol. 93, no. 10, pp. 5920–5928, 2003.
Abstract | BibTeX | Tags: discrete dislocation plasticity, size effects, Thin films | Links:
@article{Nicola2003,
title = {Discrete dislocation analysis of size effects in thin films},
author = {L. Nicola and Giessen Van der and E. Needleman},
doi = {10.1063/1.1566471},
year = {2003},
date = {2003-01-01},
journal = {JOURNAL OF APPLIED PHYSICS},
volume = {93},
number = {10},
pages = {5920–5928},
abstract = {A discrete dislocation plasticity analysis of plastic deformation in metal thin films caused by thermal stress is carried out. The calculations use a two-dimensional plane-strain formulation with only edge dislocations. Single crystal films with a specified set of slip systems are considered. The film-substrate system is subjected to a prescribed temperature history and a boundary value problem is formulated and solved for the evolution of the stress field and for the evolution of the dislocations structure in the film. A hard boundary layer forms at the interface between the film and the substrate, which does not scale with the film thickness and thus gives rise to a size effect. It is found that a reduction in the rate of dislocation nucleation can occur abruptly, which gives rise to a two-stage hardening behavior.},
keywords = {discrete dislocation plasticity, size effects, Thin films},
pubstate = {published},
tppubtype = {article}
}
A discrete dislocation plasticity analysis of plastic deformation in metal thin films caused by thermal stress is carried out. The calculations use a two-dimensional plane-strain formulation with only edge dislocations. Single crystal films with a specified set of slip systems are considered. The film-substrate system is subjected to a prescribed temperature history and a boundary value problem is formulated and solved for the evolution of the stress field and for the evolution of the dislocations structure in the film. A hard boundary layer forms at the interface between the film and the substrate, which does not scale with the film thickness and thus gives rise to a size effect. It is found that a reduction in the rate of dislocation nucleation can occur abruptly, which gives rise to a two-stage hardening behavior.
