List of Journal Publications
2022
10.
Liu, Dy, Boom, Sj, Simone, A., Aragon, Am
An interface-enriched generalized finite element formulation for locking-free coupling of non-conforming discretizations and contact Journal Article
In: COMPUTATIONAL MECHANICS, vol. 70, no. 3, pp. 477–499, 2022.
Abstract | BibTeX | Tags: Contact, Enriched FEM, IGFEM, Lagrange multipliers, Multiple-point constraints, Non-conforming meshes | Links:
@article{Liu2022,
title = {An interface-enriched generalized finite element formulation for locking-free coupling of non-conforming discretizations and contact},
author = {Dy Liu and Sj Boom and A. Simone and Am Aragon},
doi = {10.1007/s00466-022-02159-w},
year = {2022},
date = {2022-01-01},
journal = {COMPUTATIONAL MECHANICS},
volume = {70},
number = {3},
pages = {477–499},
publisher = {SPRINGER},
abstract = {We propose an enriched finite element formulation to address the computational modeling of contact problems and the coupling of non-conforming discretizations in the small deformation setting. The displacement field is augmented by enriched terms that are associated with generalized degrees of freedom collocated along non-conforming interfaces or contact surfaces. The enrichment strategy effectively produces an enriched node-to-node discretization that can be used with any constraint enforcement criterion; this is demonstrated with both multi-point constraints and Lagrange multipliers, the latter in a generalized Newton implementation where both primal and Lagrange multiplier fields are updated simultaneously. We show that the node-to-node enrichment ensures continuity of the displacement field-without locking-in mesh coupling problems, and that tractions are transferred accurately at contact interfaces without the need for stabilization. We also show the formulation is stable with respect to the condition number of the stiffness matrix by using a simple Jacobi-like diagonal preconditioner.},
keywords = {Contact, Enriched FEM, IGFEM, Lagrange multipliers, Multiple-point constraints, Non-conforming meshes},
pubstate = {published},
tppubtype = {article}
}
We propose an enriched finite element formulation to address the computational modeling of contact problems and the coupling of non-conforming discretizations in the small deformation setting. The displacement field is augmented by enriched terms that are associated with generalized degrees of freedom collocated along non-conforming interfaces or contact surfaces. The enrichment strategy effectively produces an enriched node-to-node discretization that can be used with any constraint enforcement criterion; this is demonstrated with both multi-point constraints and Lagrange multipliers, the latter in a generalized Newton implementation where both primal and Lagrange multiplier fields are updated simultaneously. We show that the node-to-node enrichment ensures continuity of the displacement field-without locking-in mesh coupling problems, and that tractions are transferred accurately at contact interfaces without the need for stabilization. We also show the formulation is stable with respect to the condition number of the stiffness matrix by using a simple Jacobi-like diagonal preconditioner.
9.
Müser, Martin H, Nicola, L.
Modeling the surface topography dependence of friction, adhesion, and contact compliance Journal Article
In: MRS BULLETIN, vol. 47, no. 12, pp. 1221–1228, 2022.
Abstract | BibTeX | Tags: Adhesion, Contact, friction | Links:
@article{Mueser2022,
title = {Modeling the surface topography dependence of friction, adhesion, and contact compliance},
author = {Martin H Müser and L. Nicola},
doi = {10.1557/s43577-022-00468-2},
year = {2022},
date = {2022-01-01},
journal = {MRS BULLETIN},
volume = {47},
number = {12},
pages = {1221–1228},
publisher = {SPRINGER HEIDELBERG},
abstract = {The small-scale topography of surfaces critically affects the contact area of solids and thus the forces acting between them. Although this has long been known, only recent advances made it possible to reliably model interfacial forces and related quantities for surfaces with multiscale roughness. This article sketches both recent and traditional approaches to their mechanics, while addressing the relevance of nonlinearity and nonlocality arising in soft- and hard-matter contacts.},
keywords = {Adhesion, Contact, friction},
pubstate = {published},
tppubtype = {article}
}
The small-scale topography of surfaces critically affects the contact area of solids and thus the forces acting between them. Although this has long been known, only recent advances made it possible to reliably model interfacial forces and related quantities for surfaces with multiscale roughness. This article sketches both recent and traditional approaches to their mechanics, while addressing the relevance of nonlinearity and nonlocality arising in soft- and hard-matter contacts.
2021
8.
Murugesan, Y., Venugopalan, S. P., Nicola, L.
On sub-surface stress caused by contact roughness in compressible elastic solids Journal Article
In: TRIBOLOGY INTERNATIONAL, vol. 159, 2021.
Abstract | BibTeX | Tags: Compressibility, Contact, Rough surfaces, Simulation | Links:
@article{Murugesan2021,
title = {On sub-surface stress caused by contact roughness in compressible elastic solids},
author = {Y. Murugesan and S. P. Venugopalan and L. Nicola},
doi = {10.1016/j.triboint.2021.106867},
year = {2021},
date = {2021-01-01},
journal = {TRIBOLOGY INTERNATIONAL},
volume = {159},
publisher = {Elsevier Ltd},
abstract = {Contact between elastic bodies with self-affine rough surfaces is mostly studied with a focus on determining surface fields, despite body fields are of great importance to establish, for instance, when and where elasticity breaks down. This work aims at analyzing the effect of contact roughness on the body fields of compressible frictionless solids modeled using Green's function molecular dynamics. Although area-load curves are insensitive to changes in the Hurst exponent as long as they are correctly normalized and are clearly not affected by compressibility, the Von-Mises stress is found to depend on both Hurst exponent and Poisson's ratio.},
keywords = {Compressibility, Contact, Rough surfaces, Simulation},
pubstate = {published},
tppubtype = {article}
}
Contact between elastic bodies with self-affine rough surfaces is mostly studied with a focus on determining surface fields, despite body fields are of great importance to establish, for instance, when and where elasticity breaks down. This work aims at analyzing the effect of contact roughness on the body fields of compressible frictionless solids modeled using Green's function molecular dynamics. Although area-load curves are insensitive to changes in the Hurst exponent as long as they are correctly normalized and are clearly not affected by compressibility, the Von-Mises stress is found to depend on both Hurst exponent and Poisson's ratio.
2018
7.
Vakis, A. I., Yastrebov, V. A., Scheibert, J., Nicola, L., Dini, D., Minfray, C., Almqvist, A., Paggi, M., Lee, S., Limbert, G., Molinari, J. F., Anciaux, G., Aghababaei, R., Restrepo, S. Echeverri, Papangelo, A., Cammarata, A., Nicolini, P., Putignano, C., Carbone, G., Stupkiewicz, S., Lengiewicz, J., Costagliola, G., Bosia, F., Guarino, R., Pugno, N. M., Müser, M. H., Ciavarella, M.
Modeling and simulation in tribology across scales: An overview Journal Article
In: TRIBOLOGY INTERNATIONAL, vol. 125, pp. 169–199, 2018.
Abstract | BibTeX | Tags: Adhesion, Contact, friction, Lubrication, Multiphysics modeling, Multiscale modeling, roughness, Tribochemistry, tribology, Wear | Links:
@article{Vakis2018,
title = {Modeling and simulation in tribology across scales: An overview},
author = {A. I. Vakis and V. A. Yastrebov and J. Scheibert and L. Nicola and D. Dini and C. Minfray and A. Almqvist and M. Paggi and S. Lee and G. Limbert and J. F. Molinari and G. Anciaux and R. Aghababaei and S. Echeverri Restrepo and A. Papangelo and A. Cammarata and P. Nicolini and C. Putignano and G. Carbone and S. Stupkiewicz and J. Lengiewicz and G. Costagliola and F. Bosia and R. Guarino and N. M. Pugno and M. H. Müser and M. Ciavarella},
doi = {10.1016/j.triboint.2018.02.005},
year = {2018},
date = {2018-01-01},
journal = {TRIBOLOGY INTERNATIONAL},
volume = {125},
pages = {169–199},
publisher = {Elsevier Ltd},
abstract = {This review summarizes recent advances in the area of tribology based on the outcome of a Lorentz Center workshop surveying various physical, chemical and mechanical phenomena across scales. Among the main themes discussed were those of rough surface representations, the breakdown of continuum theories at the nano- and microscales, as well as multiscale and multiphysics aspects for analytical and computational models relevant to applications spanning a variety of sectors, from automotive to biotribology and nanotechnology. Significant effort is still required to account for complementary nonlinear effects of plasticity, adhesion, friction, wear, lubrication and surface chemistry in tribological models. For each topic, we propose some research directions.},
keywords = {Adhesion, Contact, friction, Lubrication, Multiphysics modeling, Multiscale modeling, roughness, Tribochemistry, tribology, Wear},
pubstate = {published},
tppubtype = {article}
}
This review summarizes recent advances in the area of tribology based on the outcome of a Lorentz Center workshop surveying various physical, chemical and mechanical phenomena across scales. Among the main themes discussed were those of rough surface representations, the breakdown of continuum theories at the nano- and microscales, as well as multiscale and multiphysics aspects for analytical and computational models relevant to applications spanning a variety of sectors, from automotive to biotribology and nanotechnology. Significant effort is still required to account for complementary nonlinear effects of plasticity, adhesion, friction, wear, lubrication and surface chemistry in tribological models. For each topic, we propose some research directions.
2017
6.
Siang, Kelvin Ng Wei, Nicola, L.
Static friction of sinusoidal surfaces: a discrete dislocation plasticity analysis Journal Article
In: PHILOSOPHICAL MAGAZINE, vol. 97, no. 29, pp. 2597–2614, 2017.
Abstract | BibTeX | Tags: Contact, discrete dislocation plasticity, friction, Size effect | Links:
@article{NgWeiSiang2017,
title = {Static friction of sinusoidal surfaces: a discrete dislocation plasticity analysis},
author = {Kelvin Ng Wei Siang and L. Nicola},
doi = {10.1080/14786435.2017.1344785},
year = {2017},
date = {2017-01-01},
journal = {PHILOSOPHICAL MAGAZINE},
volume = {97},
number = {29},
pages = {2597–2614},
publisher = {Taylor and Francis Ltd.},
abstract = {Discrete dislocation plasticity simulations are carried out to investigate the static frictional response of sinusoidal asperities with (sub)-microscale wavelength. The surfaces are first flattened and then sheared by a perfectly adhesive platen. Both bodies are explicitly modelled, and the external loading is applied on the top surface of the platen. Plastic deformation by dislocation glide is the only dissipation mechanism active. The tangential force obtained at the contact when displacing the platen horizontally first increases with applied displacement, then reaches a constant value. This constant is here taken to be the friction force. In agreement with several experiments and continuum simulation studies, the friction coefficient is found to decrease with the applied normal load. However, at odds with continuum simulations, the friction force is also found to decrease with the normal load. The decrease is caused by an increased availability of dislocations to initiate and sustain plastic flow during shearing. Again in contrast to continuum studies, the friction coefficient is found to vary stochastically across the contact surface, and to reach locally values up to several times the average friction coefficient. Moreover, the friction force and the friction coefficient are found to be size-dependent.},
keywords = {Contact, discrete dislocation plasticity, friction, Size effect},
pubstate = {published},
tppubtype = {article}
}
Discrete dislocation plasticity simulations are carried out to investigate the static frictional response of sinusoidal asperities with (sub)-microscale wavelength. The surfaces are first flattened and then sheared by a perfectly adhesive platen. Both bodies are explicitly modelled, and the external loading is applied on the top surface of the platen. Plastic deformation by dislocation glide is the only dissipation mechanism active. The tangential force obtained at the contact when displacing the platen horizontally first increases with applied displacement, then reaches a constant value. This constant is here taken to be the friction force. In agreement with several experiments and continuum simulation studies, the friction coefficient is found to decrease with the applied normal load. However, at odds with continuum simulations, the friction force is also found to decrease with the normal load. The decrease is caused by an increased availability of dislocations to initiate and sustain plastic flow during shearing. Again in contrast to continuum studies, the friction coefficient is found to vary stochastically across the contact surface, and to reach locally values up to several times the average friction coefficient. Moreover, the friction force and the friction coefficient are found to be size-dependent.
5.
Dikken, R. J., Thijsse, B. J., Nicola, L.
Impingement of edge dislocations on atomically rough contacts Journal Article
In: COMPUTATIONAL MATERIALS SCIENCE, vol. 128, pp. 310–319, 2017.
Abstract | BibTeX | Tags: Atomic scale roughness, Contact, Dislocation pile-up, Molecular dynamics | Links:
@article{Dikken2017a,
title = {Impingement of edge dislocations on atomically rough contacts},
author = {R. J. Dikken and B. J. Thijsse and L. Nicola},
doi = {10.1016/j.commatsci.2016.11.038},
year = {2017},
date = {2017-01-01},
journal = {COMPUTATIONAL MATERIALS SCIENCE},
volume = {128},
pages = {310–319},
publisher = {Elsevier B.V.},
abstract = {The impingement of edge dislocations on nano-scale interfaces formed when bringing in contact aluminum crystals is investigated using molecular dynamics simulations. Dislocations, inserted in the bottom crystal, glide towards the contact when the two crystals are pressed together. There, dislocations are absorbed and upon further loading new dislocations are nucleated from the impinging site. Absorption and nucleation are events that affect the length of dislocation pile-ups and therefore the plastic behavior of crystals under contact loading. While it is possible to track absorption and nucleation at the nano-scale with molecular dynamics simulations, larger scale models, which are suitable to study plasticity, do not have the right resolution and neglect these events. The goal of this work is to gain a better understanding of dislocation impingement and to assess to which extent absorption and renucleation would play a role at the larger scale. The contacts are here characterized by their initial atomic scale roughness for which a simple, novel definition is introduced. Results show for the first time that roughness controls dislocation nucleation from the contact. This is true for both dislocation-free crystals and for crystals containing one or more dislocations before application of contact loading. In dislocation-free crystals nucleation occurs at decreasing load for increasing roughness. When a dislocation impinges on the contact, it affects its local roughness, by that decreasing the load necessary for dislocation nucleation. Only when the initial roughness of the contact is above a given threshold, dislocation impingement does not affect the load required for nucleation. If instead of a single dislocation, a train of dislocations impinges on the same site, dislocation nucleation is even more facilitated. However, even in this case the contact pressure required to nucleate dislocations is in the order of one GPa, rather high compared with the pressure required to sustain plastic deformation when macro-scale bodies are in contact.},
keywords = {Atomic scale roughness, Contact, Dislocation pile-up, Molecular dynamics},
pubstate = {published},
tppubtype = {article}
}
The impingement of edge dislocations on nano-scale interfaces formed when bringing in contact aluminum crystals is investigated using molecular dynamics simulations. Dislocations, inserted in the bottom crystal, glide towards the contact when the two crystals are pressed together. There, dislocations are absorbed and upon further loading new dislocations are nucleated from the impinging site. Absorption and nucleation are events that affect the length of dislocation pile-ups and therefore the plastic behavior of crystals under contact loading. While it is possible to track absorption and nucleation at the nano-scale with molecular dynamics simulations, larger scale models, which are suitable to study plasticity, do not have the right resolution and neglect these events. The goal of this work is to gain a better understanding of dislocation impingement and to assess to which extent absorption and renucleation would play a role at the larger scale. The contacts are here characterized by their initial atomic scale roughness for which a simple, novel definition is introduced. Results show for the first time that roughness controls dislocation nucleation from the contact. This is true for both dislocation-free crystals and for crystals containing one or more dislocations before application of contact loading. In dislocation-free crystals nucleation occurs at decreasing load for increasing roughness. When a dislocation impinges on the contact, it affects its local roughness, by that decreasing the load necessary for dislocation nucleation. Only when the initial roughness of the contact is above a given threshold, dislocation impingement does not affect the load required for nucleation. If instead of a single dislocation, a train of dislocations impinges on the same site, dislocation nucleation is even more facilitated. However, even in this case the contact pressure required to nucleate dislocations is in the order of one GPa, rather high compared with the pressure required to sustain plastic deformation when macro-scale bodies are in contact.
2016
4.
Siang, Kelvin Ng Wei, Nicola, L.
Discrete dislocation plasticity analysis of contact between deformable bodies of simple geometry Journal Article
In: MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING, vol. 24, no. 4, 2016.
Abstract | BibTeX | Tags: Contact, discrete dislocation plasticity, Size effect | Links:
@article{NgWeiSiang2016,
title = {Discrete dislocation plasticity analysis of contact between deformable bodies of simple geometry},
author = {Kelvin Ng Wei Siang and L. Nicola},
doi = {10.1088/0965-0393/24/4/045008},
year = {2016},
date = {2016-01-01},
journal = {MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING},
volume = {24},
number = {4},
publisher = {Institute of Physics Publishing},
abstract = {A contact mechanical model is presented where both metal bodies can deform by discrete dislocation plasticity. The model intends to improve on previous dislocation dynamics models of contact, where only a plastically deformable body was considered, flattened by a rigid platen. The effect of the rigid platen was mimicked through boundary conditions acting on the deformable body. While the formulation is general, the simulations presented here are only performed for contact between a plastically deforming body with sinusoidal surface and a flat body that is either elastic or rigid. Results show that the contact conditions, i.e. frictionless and full stick, affect the morphology of the contact as well as the contact pressure distribution. This is because dislocations can glide through the frictionless contact and fragment it, but do not penetrate a sticking contact. Average quantities like mean apparent contact pressure and total plastic slip are, instead, independent of contact conditions and of the details of the contact area. A size dependence is observed in relation to the onset of plastic deformation, where surfaces with smaller wavelength and amplitude require a larger contact pressure to yield than self similar surfaces with larger wavelength. The size dependence is very pronounced when the flat body is rigid, but fades when the compliance of the flat body is large.},
keywords = {Contact, discrete dislocation plasticity, Size effect},
pubstate = {published},
tppubtype = {article}
}
A contact mechanical model is presented where both metal bodies can deform by discrete dislocation plasticity. The model intends to improve on previous dislocation dynamics models of contact, where only a plastically deformable body was considered, flattened by a rigid platen. The effect of the rigid platen was mimicked through boundary conditions acting on the deformable body. While the formulation is general, the simulations presented here are only performed for contact between a plastically deforming body with sinusoidal surface and a flat body that is either elastic or rigid. Results show that the contact conditions, i.e. frictionless and full stick, affect the morphology of the contact as well as the contact pressure distribution. This is because dislocations can glide through the frictionless contact and fragment it, but do not penetrate a sticking contact. Average quantities like mean apparent contact pressure and total plastic slip are, instead, independent of contact conditions and of the details of the contact area. A size dependence is observed in relation to the onset of plastic deformation, where surfaces with smaller wavelength and amplitude require a larger contact pressure to yield than self similar surfaces with larger wavelength. The size dependence is very pronounced when the flat body is rigid, but fades when the compliance of the flat body is large.
3.
Siang, Kelvin Ng Wei, Nicola, L.
Contact between two plastically deformable crystals: a discrete dislocation dynamics study Journal Article
In: PHILOSOPHICAL MAGAZINE, vol. 96, no. 25, pp. 2583–2599, 2016.
Abstract | BibTeX | Tags: Contact, equivalent system, Plasticity, Size effect | Links:
@article{NgWeiSiang2016a,
title = {Contact between two plastically deformable crystals: a discrete dislocation dynamics study},
author = {Kelvin Ng Wei Siang and L. Nicola},
doi = {10.1080/14786435.2016.1209311},
year = {2016},
date = {2016-01-01},
journal = {PHILOSOPHICAL MAGAZINE},
volume = {96},
number = {25},
pages = {2583–2599},
publisher = {Taylor and Francis Ltd.},
abstract = {It is customary to simplify the analysis of contact between two elastically deformable bodies by treating an equivalent problem where only one body is deformable and the other is rigid. This is possible provided that the gap geometry and the effective elastic modulus of the bodies in the simplified problem are the same as in the original problem. However, the question arises on whether - and to which extent - the simplification is still valid even when (size-dependent) plasticity occurs. Studies using discrete dislocation plasticity have also, so far, addressed simple contact problems where only one body can deform plastically. Here, we extend the analysis to two bodies in contact that can both deform by dislocation plasticity and investigate under which conditions the response agrees with that of an equivalent simplified problem. The bodies in contact are metal single crystals with sinusoidal and flat surface. It is found that the response of two plastically deformable bodies in contact can be simplified to an equivalent problem where one body is rigid and the other can deform plastically. Also, a plasticity size effect is observed, but the effect fades when the platen becomes more plastically deformable.},
keywords = {Contact, equivalent system, Plasticity, Size effect},
pubstate = {published},
tppubtype = {article}
}
It is customary to simplify the analysis of contact between two elastically deformable bodies by treating an equivalent problem where only one body is deformable and the other is rigid. This is possible provided that the gap geometry and the effective elastic modulus of the bodies in the simplified problem are the same as in the original problem. However, the question arises on whether - and to which extent - the simplification is still valid even when (size-dependent) plasticity occurs. Studies using discrete dislocation plasticity have also, so far, addressed simple contact problems where only one body can deform plastically. Here, we extend the analysis to two bodies in contact that can both deform by dislocation plasticity and investigate under which conditions the response agrees with that of an equivalent simplified problem. The bodies in contact are metal single crystals with sinusoidal and flat surface. It is found that the response of two plastically deformable bodies in contact can be simplified to an equivalent problem where one body is rigid and the other can deform plastically. Also, a plasticity size effect is observed, but the effect fades when the platen becomes more plastically deformable.
2015
2.
Dikken, Robbert Jan, Giessen, Erik Van Der, Nicola, L.
Plastic shear response of a single asperity: A discrete dislocation plasticity analysis Journal Article
In: PHILOSOPHICAL MAGAZINE, vol. 95, no. 34, pp. 3845–3858, 2015.
Abstract | BibTeX | Tags: Contact, discrete dislocation plasticity, friction, single asperity | Links:
@article{Dikken2015,
title = {Plastic shear response of a single asperity: A discrete dislocation plasticity analysis},
author = {Robbert Jan Dikken and Erik Van Der Giessen and L. Nicola},
doi = {10.1080/14786435.2015.1102982},
year = {2015},
date = {2015-01-01},
journal = {PHILOSOPHICAL MAGAZINE},
volume = {95},
number = {34},
pages = {3845–3858},
publisher = {Taylor and Francis Ltd.},
abstract = {We investigate the plastic shear response during static friction of an asperity protruding from a large FCC single crystal. The asperity is in perfectly adhesive contact with a rigid platen and is sheared by tangentially moving the platen. Using discrete dislocation plasticity simulations, we elucidate the plastic shear behaviour of single asperities of various size and shape, in search for the length scale that controls the plastic behaviour. Since plasticity can occur also in the crystal, identification of the length scale that controls a possible size-dependent plastic behaviour is far from being trivial. It is found that scaling down the dimensions of an asperity results in a higher contact shear strength. The contact area is dominant in controlling the plastic shear response, because it determines the size of the zone, in and below the asperity, where dislocation nucleation can occur. For a specific contact area, there is still a dependence on asperity volume and shape, but this is weaker than the dependence on contact area alone.},
keywords = {Contact, discrete dislocation plasticity, friction, single asperity},
pubstate = {published},
tppubtype = {article}
}
We investigate the plastic shear response during static friction of an asperity protruding from a large FCC single crystal. The asperity is in perfectly adhesive contact with a rigid platen and is sheared by tangentially moving the platen. Using discrete dislocation plasticity simulations, we elucidate the plastic shear behaviour of single asperities of various size and shape, in search for the length scale that controls the plastic behaviour. Since plasticity can occur also in the crystal, identification of the length scale that controls a possible size-dependent plastic behaviour is far from being trivial. It is found that scaling down the dimensions of an asperity results in a higher contact shear strength. The contact area is dominant in controlling the plastic shear response, because it determines the size of the zone, in and below the asperity, where dislocation nucleation can occur. For a specific contact area, there is still a dependence on asperity volume and shape, but this is weaker than the dependence on contact area alone.
2008
1.
Nicola, L., Bower, A. F., Kim, K. -S., Needleman, A., der Giessen, E. Van
Multi-asperity contact: A comparison between discrete dislocation and crystal plasticity predictions Journal Article
In: PHILOSOPHICAL MAGAZINE, vol. 88, no. 30-32, pp. 3713–3729, 2008.
Abstract | BibTeX | Tags: Contact, Dislocation, Plasticity, Size effect | Links:
@article{Nicola2008,
title = {Multi-asperity contact: A comparison between discrete dislocation and crystal plasticity predictions},
author = {L. Nicola and A. F. Bower and K. -S. Kim and A. Needleman and E. Van der Giessen},
doi = {10.1080/14786430802566372},
year = {2008},
date = {2008-01-01},
journal = {PHILOSOPHICAL MAGAZINE},
volume = {88},
number = {30-32},
pages = {3713–3729},
abstract = {Plane strain indentation of single crystals by a periodic array of flat rigid contacts is analyzed. The calculations are carried out, with the mechanical response of the crystal characterized by conventional continuum crystal plasticity or by discrete dislocation plasticity. The properties used in the conventional crystal plasticity description are chosen so that both theories give essentially the same response in uniform plane strain compression. The indentation predictions are then compared, focusing in particular on the effect of contact size and spacing. The limiting cases of frictionless contacts and of perfectly sticking contacts are analyzed. Conventional continuum plasticity predicts a size-independent response. Unless the contact spacing to size ratio is very small, the predicted deformation mode under the contacts is a wedging mechanism of the type described by slip line theory, which is only weakly sensitive to friction conditions. For the micron scale contacts analyzed, discrete dislocation plasticity predicts a response that depends on the contact size as well as on the contact spacing to size ratio. When contacts are spaced sufficiently far apart, discrete dislocation plasticity predicts that the deformation is localized beneath the contacts, whereas for more closely spaced contacts, deformation occurs by shear bands extending relatively far into the crystal. Unless the contacts are sufficiently close together so that the response is essentially one of plane strain compression, the mean contact pressure predicted by discrete dislocation plasticity is substantially greater than that predicted by conventional continuum crystal plasticity and is more sensitive to the friction conditions.},
keywords = {Contact, Dislocation, Plasticity, Size effect},
pubstate = {published},
tppubtype = {article}
}
Plane strain indentation of single crystals by a periodic array of flat rigid contacts is analyzed. The calculations are carried out, with the mechanical response of the crystal characterized by conventional continuum crystal plasticity or by discrete dislocation plasticity. The properties used in the conventional crystal plasticity description are chosen so that both theories give essentially the same response in uniform plane strain compression. The indentation predictions are then compared, focusing in particular on the effect of contact size and spacing. The limiting cases of frictionless contacts and of perfectly sticking contacts are analyzed. Conventional continuum plasticity predicts a size-independent response. Unless the contact spacing to size ratio is very small, the predicted deformation mode under the contacts is a wedging mechanism of the type described by slip line theory, which is only weakly sensitive to friction conditions. For the micron scale contacts analyzed, discrete dislocation plasticity predicts a response that depends on the contact size as well as on the contact spacing to size ratio. When contacts are spaced sufficiently far apart, discrete dislocation plasticity predicts that the deformation is localized beneath the contacts, whereas for more closely spaced contacts, deformation occurs by shear bands extending relatively far into the crystal. Unless the contacts are sufficiently close together so that the response is essentially one of plane strain compression, the mean contact pressure predicted by discrete dislocation plasticity is substantially greater than that predicted by conventional continuum crystal plasticity and is more sensitive to the friction conditions.
