List of Journal Publications
2023
13.
Rafols, F Perez, Dokkum, JS Van, Nicola, L
On the interplay between roughness and viscoelasticity in adhesive hysteresis Journal Article
In: JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, vol. 170, 2023.
Abstract | BibTeX | Tags: Adhesion and adhesives, Adhesive hysteresis, contact mechanics, Viscoelastic material | Links:
@article{PerezRafols2023,
title = {On the interplay between roughness and viscoelasticity in adhesive hysteresis},
author = {F Perez Rafols and JS Van Dokkum and L Nicola},
doi = {10.1016/j.jmps.2022.105079},
year = {2023},
date = {2023-01-01},
urldate = {2023-01-01},
journal = {JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS},
volume = {170},
publisher = {PERGAMON-ELSEVIER SCIENCE LTD},
abstract = {Viscoelasticity and roughness are among the possible causes of the adhesive hysteresis displayed by soft contacts. Viscoelasticity causes an increased effective work of adhesion due to stiffening of the contact, while roughness is responsible for elastic instabilities. Herein, we explore the interplay between viscoelasticity and roughness by simulating in two dimensions the retraction of a rigid cylinder, with wavy surface profile, from a viscoelastic half-space. The wave amplitude and length are varied to induce instabilities in the load-to-area response, while the retraction velocity is increased to promote viscoelasticity. Results show that, in the regime where viscoelasticity is confined to the edges of the wavy contact, the contributions of viscoelasticity and waviness to adhesive hysteresis are nearly independent and additive. At low retraction rates, the instabilities in the load-area curve typical of rough elastic contacts are suppressed by viscoelasticity: the contact stiffens to promote a stable decrease of the contact area with load. This occurs with a minimal change in work of adhesion. However, when the instantaneous limit is met at high retraction rates, mechanical instabilities appear.},
keywords = {Adhesion and adhesives, Adhesive hysteresis, contact mechanics, Viscoelastic material},
pubstate = {published},
tppubtype = {article}
}
Viscoelasticity and roughness are among the possible causes of the adhesive hysteresis displayed by soft contacts. Viscoelasticity causes an increased effective work of adhesion due to stiffening of the contact, while roughness is responsible for elastic instabilities. Herein, we explore the interplay between viscoelasticity and roughness by simulating in two dimensions the retraction of a rigid cylinder, with wavy surface profile, from a viscoelastic half-space. The wave amplitude and length are varied to induce instabilities in the load-to-area response, while the retraction velocity is increased to promote viscoelasticity. Results show that, in the regime where viscoelasticity is confined to the edges of the wavy contact, the contributions of viscoelasticity and waviness to adhesive hysteresis are nearly independent and additive. At low retraction rates, the instabilities in the load-area curve typical of rough elastic contacts are suppressed by viscoelasticity: the contact stiffens to promote a stable decrease of the contact area with load. This occurs with a minimal change in work of adhesion. However, when the instantaneous limit is met at high retraction rates, mechanical instabilities appear.
12.
Civiero, R, Rafols, F Perez, Nicola, L
Modeling contact deformation of bare and coated rough metal bodies Journal Article
In: MECHANICS OF MATERIALS, vol. 179, 2023.
Abstract | BibTeX | Tags: contact mechanics, dislocation dynamics, Self-affine surfaces, Strain hardening | Links:
@article{Civiero2023,
title = {Modeling contact deformation of bare and coated rough metal bodies},
author = {R Civiero and F Perez Rafols and L Nicola},
doi = {10.1016/j.mechmat.2023.104583},
year = {2023},
date = {2023-01-01},
journal = {MECHANICS OF MATERIALS},
volume = {179},
publisher = {ELSEVIER},
abstract = {The effect of the presence of a passivation layer on a metal rough surface during contact loading is investigated by means of dislocation dynamics simulations. The metal body is modeled as an FCC single crystal with a self-affine rough surface that is either bare, or covered by a thin coating, impenetrable to dislocations. This analysis permits to isolate the effect of surface roughening driven by dislocation motion: when the surface is bare the dislocations can glide out, leaving crystallographic steps at the surface that modify the local roughness; when the surface is passivated, dislocations are stopped by the interface.},
keywords = {contact mechanics, dislocation dynamics, Self-affine surfaces, Strain hardening},
pubstate = {published},
tppubtype = {article}
}
The effect of the presence of a passivation layer on a metal rough surface during contact loading is investigated by means of dislocation dynamics simulations. The metal body is modeled as an FCC single crystal with a self-affine rough surface that is either bare, or covered by a thin coating, impenetrable to dislocations. This analysis permits to isolate the effect of surface roughening driven by dislocation motion: when the surface is bare the dislocations can glide out, leaving crystallographic steps at the surface that modify the local roughness; when the surface is passivated, dislocations are stopped by the interface.
2022
11.
Aramfard, Mohammad, RAFOLS, Francisco PEREZ, Nicola, L.
A 2D dual-scale method to address contact problems Journal Article
In: TRIBOLOGY INTERNATIONAL, vol. 171, 2022.
Abstract | BibTeX | Tags: contact mechanics, friction, Indentation, Multiscale modeling | Links:
@article{Aramfard2022,
title = {A 2D dual-scale method to address contact problems},
author = {Mohammad Aramfard and Francisco PEREZ RAFOLS and L. Nicola},
doi = {10.1016/j.triboint.2022.107509},
year = {2022},
date = {2022-01-01},
journal = {TRIBOLOGY INTERNATIONAL},
volume = {171},
publisher = {ELSEVIER SCI LTD},
abstract = {A seamless 2D dual-scale computational scheme is developed to study contact problems. The model consists of an atomistic domain close to the contact, coupled with an elastic continuum domain away from the contact. The atomistic formulation provides a description of the contact interaction through interatomic potentials and permits to capture atomic wear and defect formation in the contact region. The fields in the continuum domain are calculated by an efficient FFT-based Green's function method. The novel scheme is validated against full atomistic simulations and applied to study the effect of adhesion on the scratching of a rough copper surface by a rigid smooth spherical tip.},
keywords = {contact mechanics, friction, Indentation, Multiscale modeling},
pubstate = {published},
tppubtype = {article}
}
A seamless 2D dual-scale computational scheme is developed to study contact problems. The model consists of an atomistic domain close to the contact, coupled with an elastic continuum domain away from the contact. The atomistic formulation provides a description of the contact interaction through interatomic potentials and permits to capture atomic wear and defect formation in the contact region. The fields in the continuum domain are calculated by an efficient FFT-based Green's function method. The novel scheme is validated against full atomistic simulations and applied to study the effect of adhesion on the scratching of a rough copper surface by a rigid smooth spherical tip.
2019
10.
Venugopalan, S. P., Nicola, L.
Indentation of a plastically deforming metal crystal with a self-affine rigid surface: A dislocation dynamics study Journal Article
In: ACTA MATERIALIA, vol. 165, pp. 709–721, 2019.
Abstract | BibTeX | Tags: contact mechanics, dislocation dynamics, Plasticity, Self-affine surfaces | Links:
@article{Venugopalan2019,
title = {Indentation of a plastically deforming metal crystal with a self-affine rigid surface: A dislocation dynamics study},
author = {S. P. Venugopalan and L. Nicola},
doi = {10.1016/j.actamat.2018.10.020},
year = {2019},
date = {2019-01-01},
journal = {ACTA MATERIALIA},
volume = {165},
pages = {709–721},
publisher = {Acta Materialia Inc},
abstract = {Although indentation of elastic bodies by self-affine rough indenters has been studied extensively, little attention has so far been devoted to plasticity. This is mostly because modeling plasticity as well as contact with a self-affine rough surface is computationally quite challenging. Here, we succeed in achieving this goal by using Green's function dislocation dynamics, which allows to describe the self-affine rough surface using wavelengths spanning from 5 nm to 100 micron. The aim of this work is to gain understanding in how plastic deformation affects the contact area, contact pressure and hardness, gap profile and subsurface stresses, while the roughness of the indenter is changed. Plastic deformation is found to be more pronounced for indenters with larger root-mean-square height and/or Hurst exponent, and to be size dependent. The latter means that it is not possible to scale observables, as typically done in elastic contact problems. Also, at a given indentation depth (interference) the contact area is smaller than for the corresponding elastic contact problem, but gap closure is more pronounced. Contact hardness is found to be much larger than what reported by classical plasticity studies. Primarily, this is caused by limited dislocation availability, for which the stiffness of the deforming crystal is in between that of a linear elastic and an elastic-perfectly plastic material. When calculating hardness and nominal contact pressure, including very small wavelength in the description of the surface is not necessary, because below a given wavelength the subsurface stresses become invariant to a further decrease in true contact area. This is true for both elastic and plastic materials. Considering small wavelengths is instead required to capture accurately roughening and contact stress distribution.},
keywords = {contact mechanics, dislocation dynamics, Plasticity, Self-affine surfaces},
pubstate = {published},
tppubtype = {article}
}
Although indentation of elastic bodies by self-affine rough indenters has been studied extensively, little attention has so far been devoted to plasticity. This is mostly because modeling plasticity as well as contact with a self-affine rough surface is computationally quite challenging. Here, we succeed in achieving this goal by using Green's function dislocation dynamics, which allows to describe the self-affine rough surface using wavelengths spanning from 5 nm to 100 micron. The aim of this work is to gain understanding in how plastic deformation affects the contact area, contact pressure and hardness, gap profile and subsurface stresses, while the roughness of the indenter is changed. Plastic deformation is found to be more pronounced for indenters with larger root-mean-square height and/or Hurst exponent, and to be size dependent. The latter means that it is not possible to scale observables, as typically done in elastic contact problems. Also, at a given indentation depth (interference) the contact area is smaller than for the corresponding elastic contact problem, but gap closure is more pronounced. Contact hardness is found to be much larger than what reported by classical plasticity studies. Primarily, this is caused by limited dislocation availability, for which the stiffness of the deforming crystal is in between that of a linear elastic and an elastic-perfectly plastic material. When calculating hardness and nominal contact pressure, including very small wavelength in the description of the surface is not necessary, because below a given wavelength the subsurface stresses become invariant to a further decrease in true contact area. This is true for both elastic and plastic materials. Considering small wavelengths is instead required to capture accurately roughening and contact stress distribution.
9.
Dokkum, J. S. Van, Nicola, L.
Green's function molecular dynamics including viscoelasticity Journal Article
In: MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING, vol. 27, no. 7, 2019.
Abstract | BibTeX | Tags: contact mechanics, Green's function, viscoelasticity | Links:
@article{VanDokkum2019,
title = {Green's function molecular dynamics including viscoelasticity},
author = {J. S. Van Dokkum and L. Nicola},
doi = {10.1088/1361-651X/ab3031},
year = {2019},
date = {2019-01-01},
journal = {MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING},
volume = {27},
number = {7},
publisher = {Institute of Physics Publishing},
abstract = {The contact mechanical response of various polymers is controlled by the viscoelastic behavior of their bulk and the adhesive properties of their interface. Due to the interplay between viscoelasticity and adhesion it is difficult to predict the contact response, even more when surfaces are rough. Numerical modeling could be of assistance in this task, but has so far mostly dealt with either adhesion or viscoelasticity and focused on simple geometries. Ideally, one would need a model that can concurrently describe viscoelasticity, surface roughness, and interfacial interactions. The numerical technique named Green's function molecular dynamics (GFMD) has the potential to serve this purpose. To date, it has been used to model contact between adhesive elastic bodies with self-affine surfaces. Here, as a first step, we extend the GFMD technique to include the transient contact response of frictionless viscoelastic bodies. To this end, we derive the constitutive equation for a viscoelastic semi-infinite body in reciprocal space, then integrate it using the semi-analytical method, and find the quasi-static solution through damped dynamics of the individual modes. The new model is then applied to study indentation as well as rolling of a rigid cylinder on a frictionless isotropic half-plane that follows the Zener model when loaded in shear. Extension of the method to a generalized viscoelastic model is straightforward, but the computational effort increases with the number of time-scales required to describe the material. The steady-state response of the rolling cylinder was provided analytically by Hunter in the sixties. Here, we use his analytical solution to validate the steady-state response of our model and provide additionally the transient response for bodies with various shear moduli.},
keywords = {contact mechanics, Green's function, viscoelasticity},
pubstate = {published},
tppubtype = {article}
}
The contact mechanical response of various polymers is controlled by the viscoelastic behavior of their bulk and the adhesive properties of their interface. Due to the interplay between viscoelasticity and adhesion it is difficult to predict the contact response, even more when surfaces are rough. Numerical modeling could be of assistance in this task, but has so far mostly dealt with either adhesion or viscoelasticity and focused on simple geometries. Ideally, one would need a model that can concurrently describe viscoelasticity, surface roughness, and interfacial interactions. The numerical technique named Green's function molecular dynamics (GFMD) has the potential to serve this purpose. To date, it has been used to model contact between adhesive elastic bodies with self-affine surfaces. Here, as a first step, we extend the GFMD technique to include the transient contact response of frictionless viscoelastic bodies. To this end, we derive the constitutive equation for a viscoelastic semi-infinite body in reciprocal space, then integrate it using the semi-analytical method, and find the quasi-static solution through damped dynamics of the individual modes. The new model is then applied to study indentation as well as rolling of a rigid cylinder on a frictionless isotropic half-plane that follows the Zener model when loaded in shear. Extension of the method to a generalized viscoelastic model is straightforward, but the computational effort increases with the number of time-scales required to describe the material. The steady-state response of the rolling cylinder was provided analytically by Hunter in the sixties. Here, we use his analytical solution to validate the steady-state response of our model and provide additionally the transient response for bodies with various shear moduli.
8.
Venugopalan, S. P., Irani, N., Nicola, L.
Plastic contact of self-affine surfaces: Persson's theory versus discrete dislocation plasticity Journal Article
In: JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, vol. 132, 2019.
Abstract | BibTeX | Tags: contact mechanics, dislocation dynamics, Persson's theory, Plasticity, Self-affine surfaces | Links:
@article{Venugopalan2019a,
title = {Plastic contact of self-affine surfaces: Persson's theory versus discrete dislocation plasticity},
author = {S. P. Venugopalan and N. Irani and L. Nicola},
doi = {10.1016/j.jmps.2019.07.019},
year = {2019},
date = {2019-01-01},
journal = {JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS},
volume = {132},
publisher = {Elsevier Ltd},
abstract = {Persson's theory allows for a fast and effective estimate of contact area and contact stress distributions when a flat and a self-affine rough surface are pressed into contact. For elastic bodies, the results of the theory have been shown to be in very good agreement with rather costly simulations. The theory has also been extended to plastic bodies. In this work, the results of Persson's theory for plastic bodies are compared with those of discrete dislocation plasticity. The area-load curves obtained by theory and simulations are found to be in good agreement when the rough surface has a very small root-mean-square (rms) height. For larger rms heights, which are more realistic for metal surfaces, the agreement is no longer good unless in the theory, instead of a size-independent material strength, one uses a rms height- and resolution-dependent yield strength. A modification of this type, i.e., the use of a yield strength dependent on size, does however not lead to agreement between the probability distributions of the contact stress, which is much broader in the simulations than in the theory. The most likely reason for this discrepancy is that the theory, apart from neglecting plasticity size dependence, only applies to elastic-perfectly plastic bodies and therefore, neglects strain hardening.},
keywords = {contact mechanics, dislocation dynamics, Persson's theory, Plasticity, Self-affine surfaces},
pubstate = {published},
tppubtype = {article}
}
Persson's theory allows for a fast and effective estimate of contact area and contact stress distributions when a flat and a self-affine rough surface are pressed into contact. For elastic bodies, the results of the theory have been shown to be in very good agreement with rather costly simulations. The theory has also been extended to plastic bodies. In this work, the results of Persson's theory for plastic bodies are compared with those of discrete dislocation plasticity. The area-load curves obtained by theory and simulations are found to be in good agreement when the rough surface has a very small root-mean-square (rms) height. For larger rms heights, which are more realistic for metal surfaces, the agreement is no longer good unless in the theory, instead of a size-independent material strength, one uses a rms height- and resolution-dependent yield strength. A modification of this type, i.e., the use of a yield strength dependent on size, does however not lead to agreement between the probability distributions of the contact stress, which is much broader in the simulations than in the theory. The most likely reason for this discrepancy is that the theory, apart from neglecting plasticity size dependence, only applies to elastic-perfectly plastic bodies and therefore, neglects strain hardening.
2017
7.
Venugopalan, Syam P., Müser, Martin H., Nicola, L.
Green's function molecular dynamics meets discrete dislocation plasticity Journal Article
In: MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING, vol. 25, no. 6, 2017.
Abstract | BibTeX | Tags: contact mechanics, dislocation dynamics, Green's functions | Links:
@article{Venugopalan2017,
title = {Green's function molecular dynamics meets discrete dislocation plasticity},
author = {Syam P. Venugopalan and Martin H. Müser and L. Nicola},
doi = {10.1088/1361-651X/aa7e0e},
year = {2017},
date = {2017-01-01},
journal = {MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING},
volume = {25},
number = {6},
publisher = {Institute of Physics Publishing},
abstract = {Metals deform plastically at the asperity level when brought in contact with a counter body even when the nominal contact pressure is small. Modeling the plasticity of solids with rough surfaces is challenging due to the multi-scale nature of surface roughness and the length-scale dependence of plasticity. While discrete-dislocation plasticity (DDP) simulations capture size-dependent plasticity by keeping track of the motion of individual dislocations, only simple two-dimensional surface geometries have so far been studied with DDP. The main computational bottleneck in contact problems modeled by DDP is the calculation of the dislocation image fields. We address this issue by combining two-dimensional DDP with Green's function molecular dynamics. The resulting method allows for an efficient boundary-value-method based treatment of elasticity in the presence of dislocations. We demonstrate that our method captures plasticity quantitatively from single to many dislocations and that it scales more favorably with system size than conventional methods. We also derive the relevant Green's functions for elastic slabs of finite width allowing arbitrary boundary conditions on top and bottom surface to be simulated.},
keywords = {contact mechanics, dislocation dynamics, Green's functions},
pubstate = {published},
tppubtype = {article}
}
Metals deform plastically at the asperity level when brought in contact with a counter body even when the nominal contact pressure is small. Modeling the plasticity of solids with rough surfaces is challenging due to the multi-scale nature of surface roughness and the length-scale dependence of plasticity. While discrete-dislocation plasticity (DDP) simulations capture size-dependent plasticity by keeping track of the motion of individual dislocations, only simple two-dimensional surface geometries have so far been studied with DDP. The main computational bottleneck in contact problems modeled by DDP is the calculation of the dislocation image fields. We address this issue by combining two-dimensional DDP with Green's function molecular dynamics. The resulting method allows for an efficient boundary-value-method based treatment of elasticity in the presence of dislocations. We demonstrate that our method captures plasticity quantitatively from single to many dislocations and that it scales more favorably with system size than conventional methods. We also derive the relevant Green's functions for elastic slabs of finite width allowing arbitrary boundary conditions on top and bottom surface to be simulated.
6.
Venugopalan, Syam P., Nicola, L., Müser, Martin H.
Green's function molecular dynamics: Including finite heights, shear, and body fields Journal Article
In: MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING, vol. 25, no. 3, 2017.
Abstract | BibTeX | Tags: contact mechanics, Green's function, tribology | Links:
@article{Venugopalan2017a,
title = {Green's function molecular dynamics: Including finite heights, shear, and body fields},
author = {Syam P. Venugopalan and L. Nicola and Martin H. Müser},
doi = {10.1088/1361-651X/aa606b},
year = {2017},
date = {2017-01-01},
journal = {MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING},
volume = {25},
number = {3},
publisher = {Institute of Physics Publishing},
abstract = {The Green's function molecular dynamics (GFMD) method for the simulation of incompressible solids under normal loading is extended in several ways: shear is added to the GFMD continuum formulation and Poisson numbers as well as the heights of the deformed body can now be chosen at will. In addition, we give the full stress tensor inside the deformed body. We validate our generalizations by comparing our analytical and GFMD results to calculations based on the finite-element method (FEM) and full molecular dynamics simulations. For the investigated systems we observe a significant speed-up of GFMD compared to FEM. While calculation and proof of concept were conducted in two-dimensions only, the methodology can be extended to the three-dimensional case in a straightforward fashion.},
keywords = {contact mechanics, Green's function, tribology},
pubstate = {published},
tppubtype = {article}
}
The Green's function molecular dynamics (GFMD) method for the simulation of incompressible solids under normal loading is extended in several ways: shear is added to the GFMD continuum formulation and Poisson numbers as well as the heights of the deformed body can now be chosen at will. In addition, we give the full stress tensor inside the deformed body. We validate our generalizations by comparing our analytical and GFMD results to calculations based on the finite-element method (FEM) and full molecular dynamics simulations. For the investigated systems we observe a significant speed-up of GFMD compared to FEM. While calculation and proof of concept were conducted in two-dimensions only, the methodology can be extended to the three-dimensional case in a straightforward fashion.
2015
5.
Sun, Fengwei, Giessen, Erik Van Der, Nicola, L.
Interaction between neighboring asperities during flattening: A discrete dislocation plasticity analysis Journal Article
In: MECHANICS OF MATERIALS, vol. 90, pp. 157–165, 2015.
Abstract | BibTeX | Tags: contact mechanics, dislocation dynamics, Rough surface, Size effect | Links:
@article{Sun2015,
title = {Interaction between neighboring asperities during flattening: A discrete dislocation plasticity analysis},
author = {Fengwei Sun and Erik Van Der Giessen and L. Nicola},
doi = {10.1016/j.mechmat.2015.04.012},
year = {2015},
date = {2015-01-01},
journal = {MECHANICS OF MATERIALS},
volume = {90},
pages = {157–165},
publisher = {Elsevier},
abstract = {Discrete dislocation plasticity simulations are performed to investigate the role of interaction between neighboring asperities on the contact pressure induced by a rigid platen on a rough surface. The rough surface is modeled as an array of equispaced asperities with a sinusoidal profile. The spacing between asperities is varied and the contact pressure necessary to flatten the surface to a given strain is computed. Plasticity in the asperities and in the crystal below is described by the collective glide of dislocations of edge character. Results show that the mean contact pressure necessary to flatten closely spaced asperities is larger than that required to flatten widely separated asperities. A small dependence on asperity density is already observed for a purely elastic material, but it is enhanced for small asperities, in the presence of dislocation plasticity. Plastic strain gradients, dislocation limited plasticity and interaction between neighboring plastic zones all contribute to what we will call the asperity density effect. Since dislocation limited plasticity plays a dominant role, the asperity density effect will mainly be relevant for surfaces having small asperity roughness.},
keywords = {contact mechanics, dislocation dynamics, Rough surface, Size effect},
pubstate = {published},
tppubtype = {article}
}
Discrete dislocation plasticity simulations are performed to investigate the role of interaction between neighboring asperities on the contact pressure induced by a rigid platen on a rough surface. The rough surface is modeled as an array of equispaced asperities with a sinusoidal profile. The spacing between asperities is varied and the contact pressure necessary to flatten the surface to a given strain is computed. Plasticity in the asperities and in the crystal below is described by the collective glide of dislocations of edge character. Results show that the mean contact pressure necessary to flatten closely spaced asperities is larger than that required to flatten widely separated asperities. A small dependence on asperity density is already observed for a purely elastic material, but it is enhanced for small asperities, in the presence of dislocation plasticity. Plastic strain gradients, dislocation limited plasticity and interaction between neighboring plastic zones all contribute to what we will call the asperity density effect. Since dislocation limited plasticity plays a dominant role, the asperity density effect will mainly be relevant for surfaces having small asperity roughness.
4.
Song, H., Dikken, R. J., Nicola, L., Giessen, E. Van Der
Plastic ploughing of a sinusoidal asperity on a rough surface Journal Article
In: JOURNAL OF APPLIED MECHANICS, vol. 82, no. 7, 2015.
Abstract | BibTeX | Tags: contact mechanics, dislocation dynamics, friction | Links:
@article{Song2015,
title = {Plastic ploughing of a sinusoidal asperity on a rough surface},
author = {H. Song and R. J. Dikken and L. Nicola and E. Van Der Giessen},
doi = {10.1115/1.4030318},
year = {2015},
date = {2015-01-01},
journal = {JOURNAL OF APPLIED MECHANICS},
volume = {82},
number = {7},
publisher = {American Society of Mechanical Engineers (ASME)},
abstract = {Part of the friction between two rough surfaces is due to the interlocking between asperities on opposite surfaces. In order for the surfaces to slide relative to each other, these interlocking asperities have to deform plastically. Here, we study the unit process of plastic ploughing of a single micrometer-scale asperity by means of two-dimensional dislocation dynamics simulations. Plastic deformation is described through the generation, motion, and annihilation of edge dislocations inside the asperity as well as in the subsurface. We find that the force required to plough an asperity at different ploughing depths follows a Gaussian distribution. For self-similar asperities, the friction stress is found to increase with the inverse of size. Comparison of the friction stress is made with other two contact models to show that interlocking asperities that are larger than ∼2 μm are easier to shear off plastically than asperities with a flat contact.},
keywords = {contact mechanics, dislocation dynamics, friction},
pubstate = {published},
tppubtype = {article}
}
Part of the friction between two rough surfaces is due to the interlocking between asperities on opposite surfaces. In order for the surfaces to slide relative to each other, these interlocking asperities have to deform plastically. Here, we study the unit process of plastic ploughing of a single micrometer-scale asperity by means of two-dimensional dislocation dynamics simulations. Plastic deformation is described through the generation, motion, and annihilation of edge dislocations inside the asperity as well as in the subsurface. We find that the force required to plough an asperity at different ploughing depths follows a Gaussian distribution. For self-similar asperities, the friction stress is found to increase with the inverse of size. Comparison of the friction stress is made with other two contact models to show that interlocking asperities that are larger than ∼2 μm are easier to shear off plastically than asperities with a flat contact.
3.
Sun, Fengwei, Giessen, Erik Van Der, Nicola, L.
Effect of plastic flattening on the shearing response of metal asperities: A dislocation dynamics analysis Journal Article
In: JOURNAL OF APPLIED MECHANICS, vol. 82, no. 7, 2015.
Abstract | BibTeX | Tags: contact mechanics, discrete dislocation plasticity | Links:
@article{Sun2015a,
title = {Effect of plastic flattening on the shearing response of metal asperities: A dislocation dynamics analysis},
author = {Fengwei Sun and Erik Van Der Giessen and L. Nicola},
doi = {10.1115/1.4030321},
year = {2015},
date = {2015-01-01},
journal = {JOURNAL OF APPLIED MECHANICS},
volume = {82},
number = {7},
publisher = {American Society of Mechanical Engineers (ASME)},
abstract = {Discrete dislocation (DD) plasticity simulations are carried out to investigate the effect of flattening and shearing of surface asperities. The asperities are chosen to have a rectangular shape to keep the contact area constant. Plasticity is simulated by nucleation, motion, and annihilation of edge dislocations. The results show that plastic flattening of large asperities facilitates subsequent plastic shearing, since it provides dislocations available to glide at lower shear stress than the nucleation strength. The effect of plastic flattening disappears for small asperities, which are harder to be sheared than the large ones, independently of preloading. An effect of asperity spacing is observed with closely spaced asperities being easier to plastically shear than isolated asperities. This effect fades when asperities are very protruding, and therefore plasticity is confined inside the asperities.},
keywords = {contact mechanics, discrete dislocation plasticity},
pubstate = {published},
tppubtype = {article}
}
Discrete dislocation (DD) plasticity simulations are carried out to investigate the effect of flattening and shearing of surface asperities. The asperities are chosen to have a rectangular shape to keep the contact area constant. Plasticity is simulated by nucleation, motion, and annihilation of edge dislocations. The results show that plastic flattening of large asperities facilitates subsequent plastic shearing, since it provides dislocations available to glide at lower shear stress than the nucleation strength. The effect of plastic flattening disappears for small asperities, which are harder to be sheared than the large ones, independently of preloading. An effect of asperity spacing is observed with closely spaced asperities being easier to plastically shear than isolated asperities. This effect fades when asperities are very protruding, and therefore plasticity is confined inside the asperities.
2012
2.
Sun, Fengwei, der Giessen, Erik Van, Nicola, L.
Plastic flattening of a sinusoidal metal surface: A discrete dislocation plasticity study Journal Article
In: WEAR, vol. 296, no. 1-2, pp. 672–680, 2012.
Abstract | BibTeX | Tags: contact mechanics, dislocation dynamics, Rough surface, Size effect, Surface topography | Links:
@article{Sun2012,
title = {Plastic flattening of a sinusoidal metal surface: A discrete dislocation plasticity study},
author = {Fengwei Sun and Erik Van der Giessen and L. Nicola},
doi = {10.1016/j.wear.2012.08.007},
year = {2012},
date = {2012-01-01},
journal = {WEAR},
volume = {296},
number = {1-2},
pages = {672–680},
publisher = {Elsevier Ltd},
abstract = {The plastic flattening of a sinusoidal metal surface is studied by performing plane strain dislocation dynamics 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. By analyzing surfaces with constant amplitude we found that the mean contact pressure is inversely proportional to the wavelength. For small wavelengths, due to interaction between plastic zones of neighboring contacts, the mean contact pressure can reach values that are about 1/10 of the theoretical strength of the material, thus significantly higher than what is predicted by simulations that do not account for size dependent plasticity. Surfaces with the same amplitude to period ratio have a size dependent response, such that if we interpret each period of the sinusoidal wave as the asperity of a rough surface, smaller asperities are harder to be flattened than large ones. The difference between the limiting situations of sticking and frictionless contacts is found to be negligible.},
keywords = {contact mechanics, dislocation dynamics, Rough surface, Size effect, Surface topography},
pubstate = {published},
tppubtype = {article}
}
The plastic flattening of a sinusoidal metal surface is studied by performing plane strain dislocation dynamics 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. By analyzing surfaces with constant amplitude we found that the mean contact pressure is inversely proportional to the wavelength. For small wavelengths, due to interaction between plastic zones of neighboring contacts, the mean contact pressure can reach values that are about 1/10 of the theoretical strength of the material, thus significantly higher than what is predicted by simulations that do not account for size dependent plasticity. Surfaces with the same amplitude to period ratio have a size dependent response, such that if we interpret each period of the sinusoidal wave as the asperity of a rough surface, smaller asperities are harder to be flattened than large ones. The difference between the limiting situations of sticking and frictionless contacts is found to be negligible.
2007
1.
Nicola, L., Bower, A. F., Kim, K. -S., Needleman, A., der Giessen, E. Van
Surface versus bulk nucleation of dislocations during contact Journal Article
In: JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, vol. 55, no. 6, pp. 1120–1144, 2007.
Abstract | BibTeX | Tags: contact mechanics, discrete dislocation plasticity, Indentation, Residual stress, Size effect | Links:
@article{Nicola2007,
title = {Surface versus bulk nucleation of dislocations during contact},
author = {L. Nicola and A. F. Bower and K. -S. Kim and A. Needleman and E. Van der Giessen},
doi = {10.1016/j.jmps.2006.12.005},
year = {2007},
date = {2007-01-01},
journal = {JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS},
volume = {55},
number = {6},
pages = {1120–1144},
abstract = {The indentation of single crystals by a periodic array of flat rigid contacts is analyzed using discrete dislocation plasticity. Plane strain analyses are carried out with the dislocations all of edge character and modeled as line singularities in a linear elastic solid. The limiting cases of frictionless and perfectly sticking contacts are considered. The effects of contact size, dislocation source density, and dislocation obstacle density and strength on the evolution of the mean indentation pressure are explored, but the main focus is on contrasting the response of crystals having dislocation sources on the surface with that of crystals having dislocation sources in the bulk. When there are only bulk sources, the mean contact pressure for sufficiently large contacts is independent of the friction condition, whereas for sufficiently small contact sizes, there is a significant dependence on the friction condition. When there are only surface dislocation sources the mean contact pressure increases much more rapidly with indentation depth than when bulk sources are present and the mean contact pressure is very sensitive to the strength of the obstacles to dislocation glide. Also, on unloading a layer of tensile residual stress develops when surface dislocation sources dominate.},
keywords = {contact mechanics, discrete dislocation plasticity, Indentation, Residual stress, Size effect},
pubstate = {published},
tppubtype = {article}
}
The indentation of single crystals by a periodic array of flat rigid contacts is analyzed using discrete dislocation plasticity. Plane strain analyses are carried out with the dislocations all of edge character and modeled as line singularities in a linear elastic solid. The limiting cases of frictionless and perfectly sticking contacts are considered. The effects of contact size, dislocation source density, and dislocation obstacle density and strength on the evolution of the mean indentation pressure are explored, but the main focus is on contrasting the response of crystals having dislocation sources on the surface with that of crystals having dislocation sources in the bulk. When there are only bulk sources, the mean contact pressure for sufficiently large contacts is independent of the friction condition, whereas for sufficiently small contact sizes, there is a significant dependence on the friction condition. When there are only surface dislocation sources the mean contact pressure increases much more rapidly with indentation depth than when bulk sources are present and the mean contact pressure is very sensitive to the strength of the obstacles to dislocation glide. Also, on unloading a layer of tensile residual stress develops when surface dislocation sources dominate.
