List of Journal Publications
2019
4.
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.
2014
3.
Zhang, Yunhe, Thijsse, Barend, Nicola, L.
Competition between dislocations and cracks in molecular dynamics simulations of metal nanoimprinting Journal Article
In: COMPUTATIONAL MATERIALS SCIENCE, vol. 94, no. C, pp. 95–105, 2014.
Abstract | BibTeX | Tags: Crack, Dislocations, Molecular dynamics simulations, Nanoimprinting, Thermal fluctuations | Links:
@article{Zhang2014,
title = {Competition between dislocations and cracks in molecular dynamics simulations of metal nanoimprinting},
author = {Yunhe Zhang and Barend Thijsse and L. Nicola},
doi = {10.1016/j.commatsci.2014.02.016},
year = {2014},
date = {2014-01-01},
journal = {COMPUTATIONAL MATERIALS SCIENCE},
volume = {94},
number = {C},
pages = {95–105},
publisher = {Elsevier B.V.},
abstract = {Direct metal nanoimprinting of a gold thin layer is studied by means of quasi-static molecular dynamics simulations. The aim of this study is to understand if it is possible to obtain a reproducible nanopattern with features of a few nanometers, that closely resembles the shape of the template. The majority of our simulations show an unexpected competition between crack formation and dislocation plasticity upon retraction of the template, which leads in some cases to an imprint and in other cases to a flat surface. These results are at odds with previous simulations of metal nanoimprinting, which always predicted formation of an imprint. The reason for this discrepancy lies in the much lower (and thus more realistic) imprinting velocity used in this work. The most interesting finding of this paper is that the competition between crack and dislocations for certain loading conditions and geometry of the crystals is driven by thermal fluctuations of the atomic velocities. Local events, namely atomic fluctuations and dislocation nucleation, determine the global mechanical response of the system, i.e. whether an imprint is obtained or not. The relevance of thermal fluctuations is confirmed by the fact that any of the simulations presented here, if repeated at 10 K, leads to a brittle material behavior.},
keywords = {Crack, Dislocations, Molecular dynamics simulations, Nanoimprinting, Thermal fluctuations},
pubstate = {published},
tppubtype = {article}
}
Direct metal nanoimprinting of a gold thin layer is studied by means of quasi-static molecular dynamics simulations. The aim of this study is to understand if it is possible to obtain a reproducible nanopattern with features of a few nanometers, that closely resembles the shape of the template. The majority of our simulations show an unexpected competition between crack formation and dislocation plasticity upon retraction of the template, which leads in some cases to an imprint and in other cases to a flat surface. These results are at odds with previous simulations of metal nanoimprinting, which always predicted formation of an imprint. The reason for this discrepancy lies in the much lower (and thus more realistic) imprinting velocity used in this work. The most interesting finding of this paper is that the competition between crack and dislocations for certain loading conditions and geometry of the crystals is driven by thermal fluctuations of the atomic velocities. Local events, namely atomic fluctuations and dislocation nucleation, determine the global mechanical response of the system, i.e. whether an imprint is obtained or not. The relevance of thermal fluctuations is confirmed by the fact that any of the simulations presented here, if repeated at 10 K, leads to a brittle material behavior.
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.
2005
1.
Nicola, L., der Giessen, Erik Van, Gurtin, Morton E.
Effect of defect energy on strain-gradient predictions of confined single-crystal plasticity Journal Article
In: JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, vol. 53, no. 6, pp. 1280–1294, 2005.
Abstract | BibTeX | Tags: Crystal plasticity, Dislocations, Non-local plasticity | Links:
@article{Nicola2005,
title = {Effect of defect energy on strain-gradient predictions of confined single-crystal plasticity},
author = {L. Nicola and Erik Van der Giessen and Morton E. Gurtin},
doi = {10.1016/j.jmps.2005.02.001},
year = {2005},
date = {2005-01-01},
journal = {JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS},
volume = {53},
number = {6},
pages = {1280–1294},
abstract = {Gurtin recently proposed a strain-gradient theory for crystal plasticity in which the gradient effect originates from a defect energy that characterizes energy storage due to the presence of a net Burgers vector. Here we consider a number of different possibilities for this energy: specifically, working within a simple two-dimensional framework, we compare predictions of the theory with results of discrete-dislocation simulations of stress relaxation in thin films. Our objective is to investigate which specific defect energies are capable of capturing the size-dependent response of such systems for different crystal orientations.},
keywords = {Crystal plasticity, Dislocations, Non-local plasticity},
pubstate = {published},
tppubtype = {article}
}
Gurtin recently proposed a strain-gradient theory for crystal plasticity in which the gradient effect originates from a defect energy that characterizes energy storage due to the presence of a net Burgers vector. Here we consider a number of different possibilities for this energy: specifically, working within a simple two-dimensional framework, we compare predictions of the theory with results of discrete-dislocation simulations of stress relaxation in thin films. Our objective is to investigate which specific defect energies are capable of capturing the size-dependent response of such systems for different crystal orientations.
