List of Journal Publications
2017
9.
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.
2016
8.
Sun, Fengwei, Giessen, Erik Van Der, Nicola, L.
Dry frictional contact of metal asperities: A dislocation dynamics analysis Journal Article
In: ACTA MATERIALIA, vol. 109, pp. 162–169, 2016.
Abstract | BibTeX | Tags: dislocation dynamics, friction, Rough surface, Size effect | Links:
@article{Sun2016,
title = {Dry frictional contact of metal asperities: A dislocation dynamics analysis},
author = {Fengwei Sun and Erik Van Der Giessen and L. Nicola},
doi = {10.1016/j.actamat.2016.02.033},
year = {2016},
date = {2016-01-01},
journal = {ACTA MATERIALIA},
volume = {109},
pages = {162–169},
publisher = {Elsevier Ltd},
abstract = {Discrete dislocation plasticity simulations are performed to investigate the static frictional behavior of a metal asperity on a large single crystal, in contact with a rigid platen. The focus of this study is on understanding the relative importance of contact slip opposed to plasticity in a single asperity at the micrometer size scale, where plasticity is size dependent. Slip of a contact point is taken to occur when the shear traction exceeds the normal traction at that point times a microscopic friction coefficient. Plasticity initiates through the nucleation of dislocations from Frank-Read sources in the metal and is modeled as the collective motion of edge dislocations. Results show that plasticity can delay or even suppress full slip of the contact. This generally happens when the friction coefficient is large. However, if the flattening depth is sufficiently large to induce nucleation of a large dislocation density, slip is suppressed even when the friction coefficient is very small. This study also shows that when self-similar asperities of different size are flattened to the same depth and subsequently loaded tangentially, their frictional behavior appears size independent. However, when they are submitted to the same contact pressure, smaller asperities slip while larger asperities deform plastically.},
keywords = {dislocation dynamics, friction, Rough surface, Size effect},
pubstate = {published},
tppubtype = {article}
}
Discrete dislocation plasticity simulations are performed to investigate the static frictional behavior of a metal asperity on a large single crystal, in contact with a rigid platen. The focus of this study is on understanding the relative importance of contact slip opposed to plasticity in a single asperity at the micrometer size scale, where plasticity is size dependent. Slip of a contact point is taken to occur when the shear traction exceeds the normal traction at that point times a microscopic friction coefficient. Plasticity initiates through the nucleation of dislocations from Frank-Read sources in the metal and is modeled as the collective motion of edge dislocations. Results show that plasticity can delay or even suppress full slip of the contact. This generally happens when the friction coefficient is large. However, if the flattening depth is sufficiently large to induce nucleation of a large dislocation density, slip is suppressed even when the friction coefficient is very small. This study also shows that when self-similar asperities of different size are flattened to the same depth and subsequently loaded tangentially, their frictional behavior appears size independent. However, when they are submitted to the same contact pressure, smaller asperities slip while larger asperities deform plastically.
7.
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.
6.
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
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.
2013
4.
Benvenuti, E., Simone, A.
One-dimensional nonlocal and gradient elasticity: Closed-form solution and size effect Journal Article
In: MECHANICS RESEARCH COMMUNICATIONS, vol. 48, pp. 46–51, 2013.
Abstract | BibTeX | Tags: Closed-form solutions, Gradient elasticity, Nonlocal elasticity, Size effect | Links:
@article{Benvenuti2013,
title = {One-dimensional nonlocal and gradient elasticity: Closed-form solution and size effect},
author = {E. Benvenuti and A. Simone},
doi = {10.1016/j.mechrescom.2012.12.001},
year = {2013},
date = {2013-01-01},
journal = {MECHANICS RESEARCH COMMUNICATIONS},
volume = {48},
pages = {46–51},
publisher = {PERGAMON-ELSEVIER SCIENCE LTD},
abstract = {The equivalence between nonlocal and gradient elasticity models is investigated by making reference to one-dimensional boundary value problems equipped with two integral stress-strain laws proposed by Eringen (Nonlocal Continuum Field Theories (2002)). Corresponding closed-form solutions are derived through a procedure for the reduction of integral to differential equations. The reproduction of size effects in micro/nano rods is discussed. The differential formulation associated with the local/nonlocal model is shown to correspond to the strain-gradient formulation proposed by Aifantis (Mech. Mater. 35 (2003) 259-280).},
keywords = {Closed-form solutions, Gradient elasticity, Nonlocal elasticity, Size effect},
pubstate = {published},
tppubtype = {article}
}
The equivalence between nonlocal and gradient elasticity models is investigated by making reference to one-dimensional boundary value problems equipped with two integral stress-strain laws proposed by Eringen (Nonlocal Continuum Field Theories (2002)). Corresponding closed-form solutions are derived through a procedure for the reduction of integral to differential equations. The reproduction of size effects in micro/nano rods is discussed. The differential formulation associated with the local/nonlocal model is shown to correspond to the strain-gradient formulation proposed by Aifantis (Mech. Mater. 35 (2003) 259-280).
2012
3.
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.
2008
2.
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.
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.
