List of Journal Publications
2022
6.
Irani, N., Murugesan, Y., Ayas, C., Nicola, L.
Effect of dislocation core fields on discrete dislocation plasticity Journal Article
In: MECHANICS OF MATERIALS, vol. 165, 2022.
Abstract | BibTeX | Tags: Dislocation climb, Dislocation core, dislocation dynamics, Plasticity | Links:
@article{Irani2022,
title = {Effect of dislocation core fields on discrete dislocation plasticity},
author = {N. Irani and Y. Murugesan and C. Ayas and L. Nicola},
doi = {10.1016/j.mechmat.2021.104137},
year = {2022},
date = {2022-01-01},
journal = {MECHANICS OF MATERIALS},
volume = {165},
publisher = {Elsevier B.V.},
abstract = {Discrete dislocation plasticity is a modeling technique that treats plasticity as the collective motion of dislocations. The dislocations are described through their elastic Volterra fields, outside of a cylindrical core region, with a few Burgers vectors of diameter. The contribution of the core fields to the dislocation dynamics is neglected, because it is assumed that their range is too short to be of influence. The aim of this work is to assess the validity of this assumption. In recent ab-initio studies it has been demonstrated that the dislocation core fields are significant up to a distance of ten Burgers vector from the dislocation line. This is a longer range influence than expected and can give rise to changes in the evolving dislocation structure and in the overall response of a plastically deforming body. It is indeed experimentally observed that dislocations pile up against strong interfaces, and that the spacing between dislocations at the front of these pile-ups can be less than ten Burgers vectors. In this work, 2-D discrete dislocation plasticity simulations are performed to investigate the effect of core fields on edge dislocation interactions. The results of the simulations, which include core fields for the first time, show indeed that dislocations that are very closely spaced experience additional glide or climb due to core fields. The effect is however negligible when compared to glide and climb due to Volterra fields or due to the external load.},
keywords = {Dislocation climb, Dislocation core, dislocation dynamics, Plasticity},
pubstate = {published},
tppubtype = {article}
}
Discrete dislocation plasticity is a modeling technique that treats plasticity as the collective motion of dislocations. The dislocations are described through their elastic Volterra fields, outside of a cylindrical core region, with a few Burgers vectors of diameter. The contribution of the core fields to the dislocation dynamics is neglected, because it is assumed that their range is too short to be of influence. The aim of this work is to assess the validity of this assumption. In recent ab-initio studies it has been demonstrated that the dislocation core fields are significant up to a distance of ten Burgers vector from the dislocation line. This is a longer range influence than expected and can give rise to changes in the evolving dislocation structure and in the overall response of a plastically deforming body. It is indeed experimentally observed that dislocations pile up against strong interfaces, and that the spacing between dislocations at the front of these pile-ups can be less than ten Burgers vectors. In this work, 2-D discrete dislocation plasticity simulations are performed to investigate the effect of core fields on edge dislocation interactions. The results of the simulations, which include core fields for the first time, show indeed that dislocations that are very closely spaced experience additional glide or climb due to core fields. The effect is however negligible when compared to glide and climb due to Volterra fields or due to the external load.
2019
5.
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.
4.
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.
2016
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.
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.
2004
1.
Simone, A., Sluys, L. J.
The use of displacement discontinuities in a rate-dependent medium Journal Article
In: COMPUTER METHODS IN APPLIED MECHANICS AND ENGINEERING, vol. 193, no. 27-29, pp. 3015–3033, 2004.
Abstract | BibTeX | Tags: Damage, Displacement discontinuities, Partition of unity, Plasticity, Rate dependence, Regularised continuum | Links:
@article{Simone2004b,
title = {The use of displacement discontinuities in a rate-dependent medium},
author = {A. Simone and L. J. Sluys},
doi = {10.1016/j.cma.2003.08.006},
year = {2004},
date = {2004-01-01},
journal = {COMPUTER METHODS IN APPLIED MECHANICS AND ENGINEERING},
volume = {193},
number = {27-29},
pages = {3015–3033},
abstract = {This paper illustrates the use of displacement discontinuities in a rate-dependent elastoplastic-damage model. Rate-dependency is considered in the framework of Perzyna viscoplasticity. Displacement discontinuities are incorporated using the partition of unity property of finite-element shape functions. A combined continuous–discontinuous approach is proposed in which a discontinuity is only inserted after a certain amount of damage. Various examples illustrate the applicability of the combined approach to granular and quasi-brittle materials with and without fibre-reinforcement.},
keywords = {Damage, Displacement discontinuities, Partition of unity, Plasticity, Rate dependence, Regularised continuum},
pubstate = {published},
tppubtype = {article}
}
This paper illustrates the use of displacement discontinuities in a rate-dependent elastoplastic-damage model. Rate-dependency is considered in the framework of Perzyna viscoplasticity. Displacement discontinuities are incorporated using the partition of unity property of finite-element shape functions. A combined continuous–discontinuous approach is proposed in which a discontinuity is only inserted after a certain amount of damage. Various examples illustrate the applicability of the combined approach to granular and quasi-brittle materials with and without fibre-reinforcement.
