The Experts below are selected from a list of 8151 Experts worldwide ranked by ideXlab platform
Jean-françois Molinari - One of the best experts on this subject based on the ideXlab platform.
-
adhesive wear and interaction of tangentially loaded micro contacts
International Journal of Solids and Structures, 2020Co-Authors: Son Phamba, Tobias Brink, Jean-françois MolinariAbstract:Abstract Current engineering wear models are often based on empirical parameters rather than built upon physical considerations. Here, we look for a physical description of adhesive wear at the microscale, at which the interaction between two surfaces comes down to the contact of asperities. Recent theoretical work has shown that there is a critical micro-contact size above which it becomes energetically favorable to form a wear Particle. We extend this model by taking into consideration the elastic interaction of multiple closely-spaced micro-contacts in 2D, with different sizes and separation distances. Fundamental contact mechanics solutions are used to evaluate the elastic energy stored by shearing the micro-contacts, and the stored energy is compared to the energy needed to detach a single joined Debris Particle or multiple Debris Particles under the micro-contacts. Molecular dynamics simulations are used to test the predictions of the outcome for various sets of parameters. Our model provides simple criteria to evaluate the energetic feasibility of the different wear formation scenarios. Those criteria can be used to rationalize the transition between mild and severe wear regimes and help define the notion of asperity.
-
A mechanistic model for the growth of cylindrical Debris Particles in the presence of adhesion
International Journal of Solids and Structures, 2020Co-Authors: Enrico Milanese, Jean-françois MolinariAbstract:Abstract The wear volume is known to keep increasing during frictional processes, and Archard notably proposed a model to describe the probability of wear Particle formation upon asperity collision in a two-body contact configuration. While this model is largely adopted in the investigations of wear, the presence of wear Debris trapped between the surfaces changes the system into a three-body contact configuration already since the early stages of the process. In such a configuration, a significant amount of wear is produced at the interface between the trapped Debris and the sliding bodies. Here, relying on analytical models, we develop a framework that describes crack growth in a three-body configuration at the Particle–surface interface. We then show that crack growth is favoured within the sliding surfaces, instead of within the Debris Particle, and test such result by means of numerical simulations with a phase-field approach to fracture. This leads to an increase in the wear volume and to Debris Particle accretion, rather than its breakdown. The effects of adhesion, coefficient of friction, and ratio of the applied global tangential and normal forces are also investigated.
-
adhesive wear and interaction of tangentially loaded micro contacts
arXiv: Soft Condensed Matter, 2019Co-Authors: Son Phamba, Tobias Brink, Jean-françois MolinariAbstract:Current engineering wear models are often based on empirical parameters rather than built upon physical considerations. Here, we look for a physical description of adhesive wear at the microscale, at which the interaction between two surfaces comes down to the contact of asperities. Recent theoretical work has shown that there is a critical micro-contact size above which it becomes energetically favorable to form a wear Particle. We extend this model by taking into consideration the elastic interaction of multiple nearby micro-contacts in 2D, with different sizes and separation distances. Fundamental contact mechanics solutions are used to evaluate the elastic energy stored by shearing the micro-contacts, and the stored energy is compared to the energy needed to detach a single joined Debris Particle or multiple Debris Particles under the micro-contacts. Molecular dynamics simulations are used to test the predictions of the outcome for various sets of parameters. Our model provides simple criteria to evaluate the energetic feasibility of the different wear formation scenarios. Those criteria can be used to rationalize the transition between mild and severe wear regimes and help define the notion of asperity.
-
Adhesive wear mechanisms uncovered by atomistic simulations
Friction, 2018Co-Authors: Jean-françois Molinari, Ramin Aghababaei, Tobias Brink, Lucas Frérot, Enrico MilaneseAbstract:In this review, we discuss our recent advances in modeling adhesive wear mechanisms using coarse-grained atomistic simulations. In particular, we present how a model pair potential reveals the transition from ductile shearing of an asperity to the formation of a Debris Particle. This transition occurs at a critical junction size, which determines the Particle size at its birth. Atomistic simulations also reveal that for nearby asperities, crack shielding mechanisms result in a wear volume proportional to an effective area larger than the real contact area. As the density of microcontacts increases with load, we propose this crack shielding mechanism as a key to understand the transition from mild to severe wear. We conclude with open questions and a road map to incorporate these findings in mesoscale continuum models. Because these mesoscale models allow an accurate statistical representation of rough surfaces, they provide a simple means to interpret classical phenomenological wear models and wear coefficients from physics-based principles.
-
critical length scale controls adhesive wear mechanisms
Nature Communications, 2016Co-Authors: Ramin Aghababaei, D.h. Warner, Jean-françois MolinariAbstract:The adhesive wear process remains one of the least understood areas of mechanics. While it has long been established that adhesive wear is a direct result of contacting surface asperities, an agreed upon understanding of how contacting asperities lead to wear Debris Particle has remained elusive. This has restricted adhesive wear prediction to empirical models with limited transferability. Here we show that discrepant observations and predictions of two distinct adhesive wear mechanisms can be reconciled into a unified framework. Using atomistic simulations with model interatomic potentials, we reveal a transition in the asperity wear mechanism when contact junctions fall below a critical length scale. A simple analytic model is formulated to predict the transition in both the simulation results and experiments. This new understanding may help expand use of computer modelling to explore adhesive wear processes and to advance physics-based wear laws without empirical coefficients.
George K. Nikas - One of the best experts on this subject based on the ideXlab platform.
-
algebraic equations for the pile up geometry in Debris Particle indentation of rolling elastohydrodynamic contacts
Journal of Tribology-transactions of The Asme, 2016Co-Authors: George K. NikasAbstract:Metallic microParticles of 5–100 μm in size often contaminate elastohydrodynamic (EHD) contacts and indent surfaces. The geometrical characteristics of dents by such solid Particles are linked to the way surface damage may evolve and how it may affect the life of the damaged contacts. In many cases, Debris dents appear with shoulders raised above the original surface. Material piling-up this way causes high-pressure spikes when dents are over-rolled by an element such as a ball in a rolling bearing. This study introduces an approximate analytical method based on the so-called expanding cavity model (ECM) to calculate pile-up geometry with simple algebraic equations in thermoviscoplastic indentation of rolling EHD contacts by ductile spherical microParticles. Based on an experimentally validated Debris indentation model published by the author, the pile-up model is shown to give realistic predictions in a wide range of operating parameters. Upon experimental validation, the new model is used to study the effects of Particle size and hardness, Coulomb friction coefficient (CFC), strain hardening, and rolling velocity of EHD contacts on pile-up geometrical parameters including length, height, volume, and curvature.
-
Debris Particle indentation and abrasion of machine-element contacts: An experimentally validated, thermoelastoplastic numerical model with micro-hardness and frictional heating effects:
Proceedings of the Institution of Mechanical Engineers Part J: Journal of Engineering Tribology, 2012Co-Authors: George K. NikasAbstract:For a very long time, Debris Particles have been blamed to causing serious problems in machine-element contacts such as those of bearings and gears. This involves a huge number of mechanisms and machines worldwide. The financial cost associated with machinery failure under such circumstances is enormous. Past research has identified the main mechanisms governing damage from Debris Particles. A few theoretical models have been built on the experience accumulated on damage mechanics. The capabilities of said models vary a lot. The model originally developed by this author in the 1990s was recently expanded. The previous version of the model, which was published in this journal in May 2012, offered a number of innovative features to calculate spherical-Particle indentation and soft abrasion in lubricated rolling-sliding contacts. It was experimentally validated following a rigorous programme. However, it neglected frictional heating from Particle extrusion. This study significantly expands the previous model...
-
An experimentally validated numerical model of indentation and abrasion by Debris Particles in machine-element contacts considering micro-hardness effects
Proceedings of the Institution of Mechanical Engineers Part J: Journal of Engineering Tribology, 2012Co-Authors: George K. NikasAbstract:Indentation and abrasion of machine-element contacts by solid contamination Particles is a major problem in many industries and manufacturing processes involving the automotive, aerospace, medical and electronics industries among others. Published theoretical studies on indentation and soft abrasion of surfaces by ductile Debris Particles other than those of the author are based on several major simplifications concerning material properties, hardness, plasticity modelling, interfacial friction, kinematic conditions, etc. None of the studies published in the literature to date (2011) have those simplifications concurrently relaxed.In view of the shortcomings of existing numerical models on Debris Particle indentation and abrasion, and given the importance of dent geometry and size on fatigue life of machine elements, a greatly improved numerical model has been developed based on the previous studies of the author. The new model deals with elastoplastic indentation and abrasion of rolling–sliding, dry and ...
-
Thermoelastic Distortion of EHD Line Contacts During the Passage of Soft Debris Particles
Journal of Tribology, 1999Co-Authors: George K. Nikas, R. S. Sayles, E. IoannidesAbstract:During the passage ofa Debris Particle through an EHD contact, mechanical stresses due to Particle compression and thermal stresses due to Particle frictional heating produce a thermoelastic/plastic stress field, which governs the way a possible damage is generated. In the present paper, the complete three-dimensional solution of the thermoelastic distortion of surfaces due to the compression of a soft, ductile Debris Particle in an EHD line contact is presented both theoretically and through a realistic example. It is found that thermal stresses increase the likelihood of yielding and produce a characteristic omega shaped thermoelastic displacement. The important outcome of this work is the construction of a map which shows the critical Particle size to cause damage (plastic deformations) in combination with operational parameters as the lubricant film thickness and relative sliding velocity of the contact.
-
Thermal Modeling and Effects From Debris Particles in Sliding/Rolling EHD Line Contacts—A Possible Local Scuffing Mode
Journal of Tribology, 1999Co-Authors: George K. Nikas, E. Loannides, R. S. SaylesAbstract:The damage caused by Debris Particles in concentrated contacts has been studied extensively in the past, both theoretically and experimentally. Most of the theoretical studies, in which the damage on the surfaces was calculated in the form of dents, were performed isothermally. It is known that sliding asperity contacts, which resemble third body contacts, reach high local temperatures that can affect local material properties which, in turn, will affect the way damage is generated on the surfaces of machine elements. In the present work the heat transfer of lubricated, rolling/sliding line contacts in the presence of a ductile spherical Particle is modeled. The Particle is assumed to be significantly softer than the counterfaces that squash it. The local flash temperatures due to the combined sliding and squashing of a Debris Particle are calculated. It is found that high temperatures caused from small and soft Particles are rather the rule than the exception.
Nadim J. Hallab - One of the best experts on this subject based on the ideXlab platform.
-
Implant Debris Particle size affects serum protein adsorption which may contribute to Particle size-based bioreactivity differences
Journal of Long-Term Effects of Medical Implants, 2014Co-Authors: Anand Reddy, Marco S. Caicedo, Lauryn Samelko, Joshua J. Jacobs, Nadim J. HallabAbstract:Biologic reactivity to orthopedic implant Debris mediates long-term clinical performance of total joint arthroplasty implants. However, why some facets of implant Debris are more pro-inflammatory remains controversial such as Particle size, shape, base material etc. This precludes accurate prediction and optimal design of modern total joint replacements. We hypothesized that Debris Particle size can influence adsorbed protein film composition and affect subsequent bioreactivity. We measured size-dependent protein film-adsorption, and adsorbed protein film-dependent cytokine release using equal surface areas of different sized cobalt-chromium-alloy (CoCr-alloy) Particle and in vitro challenge of human macrophages (THP-1 and human primary). Smaller 5μm vs 70μm sized Particles preferentially adsorbed more serum protein in general (p
-
implant Debris Particle size affects serum protein adsorption which may contribute to Particle size based bioreactivity differences
Journal of Long-term Effects of Medical Implants, 2014Co-Authors: Anand Reddy, Marco S. Caicedo, Lauryn Samelko, Joshua J. Jacobs, Nadim J. HallabAbstract:Biologic reactivity to orthopedic implant Debris mediates long-term clinical performance of total joint arthroplasty implants. However, why some facets of implant Debris are more pro-inflammatory remains controversial such as Particle size, shape, base material etc. This precludes accurate prediction and optimal design of modern total joint replacements. We hypothesized that Debris Particle size can influence adsorbed protein film composition and affect subsequent bioreactivity. We measured size-dependent protein film-adsorption, and adsorbed protein film-dependent cytokine release using equal surface areas of different sized cobalt-chromium-alloy (CoCr-alloy) Particle and in vitro challenge of human macrophages (THP-1 and human primary). Smaller 5μm vs 70μm sized Particles preferentially adsorbed more serum protein in general (p<0.03), where higher molecular weight serum proteins consistent with IgG were identified. Additionally, 5μm CoCr-alloy Particles pre-coated with different protein biofilms (IgG vs albumin) resulted in differential cytokine expression where albumin-coated Particles induced more TNF-α and IgG-coated Particles induced more IL-1β release from human monocyte/macrophages. In these preliminary in vitro studies we demonstrated the capability of equal surface areas of different Particle sizes to influence adsorbed protein composition and that adsorbed protein differences on identical Particles can translate into complex differences in bioreactivity. Together this suggests adsorbed protein differences on different sized Particles of the same material may be a contributing mechanism by which different sized Particles induce differences in reactivity.
Molinari Jean-francois - One of the best experts on this subject based on the ideXlab platform.
-
A mechanistic model for the growth of cylindrical Debris Particles in the presence of adhesion
'Elsevier BV', 2020Co-Authors: Milanese Enrico, Molinari Jean-francoisAbstract:The wear volume is known to keep increasing during frictional processes, and Archard notably proposed a model to describe the probability of wear Particle formation upon asperity collision in a two-body contact configuration. While this model is largely adopted in the investigations of wear, the presence of wear Debris trapped between the surfaces changes the system into a three-body contact configuration already since the early stages of the process. In such a configuration, a significant amount of wear is produced at the interface between the trapped Debris and the sliding bodies. Here, relying on analytical models, we develop a framework that describes crack growth in a three-body configuration at the Particle-surface interface. We then show that crack growth is favoured within the sliding surfaces, instead of within the Debris Particle, and test such result by means of numerical simulations with a phase-field approach to fracture. This leads to an increase in the wear volume and to Debris Particle accretion, rather than its breakdown. The effects of adhesion, coefficient of friction, and ratio of the applied global tangential and normal forces are also investigated. (C) 2020 Elsevier Ltd. All rights reserved
-
Adhesive wear and interaction of tangentially loaded micro-contacts
'Elsevier BV', 2019Co-Authors: Pham-ba Son, Brink Tobias, Molinari Jean-francoisAbstract:Current engineering wear models are often based on empirical parameters rather than built upon physical considerations. Here, we look for a physical description of adhesive wear at the microscale, at which the interaction between two surfaces comes down to the contact of asperities. Recent theoretical work has shown that there is a critical micro-contact size above which it becomes energetically favorable to form a wear Particle. We extend this model by taking into consideration the elastic interaction of multiple closely-spaced micro-contacts in 2D, with different sizes and separation distances. Fundamental contact mechanics solutions are used to evaluate the elastic energy stored by shearing the micro-contacts, and the stored energy is compared to the energy needed to detach a single joined Debris Particle or multiple Debris Particles under the micro-contacts. Molecular dynamics simulations are used to test the predictions of the outcome for various sets of parameters. Our model provides simple criteria to evaluate the energetic feasibility of the different wear formation scenarios. Those criteria can be used to rationalize the transition between mild and severe wear regimes and help define the notion of asperity.Comment: 9 pages, 11 figure
-
Fractal surfaces in adhesive wear processes.
2019Co-Authors: Milanese Enrico, Brink Tobias, Aghababaei Ramin, Molinari Jean-francoisAbstract:Wear of materials plays a key role in the durability of manifactured objects and it has thus an economical importance in our society. In spite of the efforts, a unified picture of the underlying physics of the phenomenon is still far from being reached, because of the numerous mechanisms and processes that constitute the complexity of wear. Moving one step closer to the understanding of the physics of wear would mean enhancing our understanding of all the related fields that it affects, like friction and contact mechanics. The evolution of the surfaces morphology upon sliding is one of the aspects that need further investigations. In fact, while it is known that newly created surfaces by means of crack propagation are self-affine, what happens to existing surfaces rubbed against one another is still unclear. What is known from experimental studies is that self-affinity is preserved for highly abrasive wear and that in other cases the worn surface undergoes asymmetric changes with respect of its original mean plane. Surface roughness is also linked to the wear rate, as rough surfaces are found to deteriorate more in the running-in, up to the point that they have been smoothed enough and the wear rate becomes smaller and constant (the opposite happening for surface that are initially too smooth). As atomistic simulations have been proved to be an effective means of investigation for the understanding of the physics of wear, we adopt a recently developed approach to investigate the surface roughness evolution under adhesive wear processes (i.e, when the material loss is mainly due to transfer of Particles from one body to another). The simulated system is two-dimensional, with periodic boundary conditions along the horizontal direction to allow for continuous sliding at constant velocity of the top surface over the bottom one. In the early stages of contact, the two surfaces form a Debris Particle that continuously rolls between them, interacting with them by removing material and deforming them. The peculiarity of the performed simulations is that they are long enough to allow for an investigation of the surfaces geometry evolution over time. The simulations display the two-regime evolution of the surfaces, characterized by high wear rate at running- in and lower wear rate in later stages of the process. Once the running-in is over, the morphology of the sliding surfaces and of the rolling Debris Particle are analyzed by means of their PSD. All the analyzed surfaces appear self-affine, and their heights are positively correlated. This type of roughness is consistent with the ones found in faults and, by analogy with gradient percolation models, hints that short range interactions prevail on long range elasticity. In some cases it is also observed that, after the running-in, the Debris Particle can heavily perturb the process increasing again the roughness of the mating surfaces, similarly to the running-in conditions. Despite the fact that this is not the picture commonly expected, it can be explained analyzing the Debris Particle geometry and the stress state at the contact interface. In particular, roughness enhancement happens by removal of material from the opposing surfaces when the Particle is approximately round, while an irregular shape favours the presence of stress singularities in the Particle itself and thus the deposit of material from the Particle on the opposing surfaces. In this presentation we analyze different initial setups (i.e. different morphologies, system size, materials), seeing how the final self-affine morphology is independent of the initial state and how the shape of the rolling Particle can alter the wear process
Milanese Enrico - One of the best experts on this subject based on the ideXlab platform.
-
Surface roughness evolution in adhesive wear processes
Lausanne EPFL, 2020Co-Authors: Milanese EnricoAbstract:Geological faults movements generating earthquakes, a vehicles' tyres rolling on the pavement, and a chalk writing on a blackboard are all different examples of frictional systems. In these systems, which are everywhere around us, two separate bodies are in contact and in relative motion, interacting with one another. Under these conditions, several phenomena arise at the interface: friction, wear, and lubrication being the main ones. Wear, in particular, is the loss of material from the surface of one (or both) the moving bodies. This phenomenon is of interest as it leads to loss of usefulness of manufactured objects and health concerns for patients with implants, to name just a few of important consequences due to material degradation. Yet, our knowledge on the topic of wear is still scattered, with many observations and models that are system dependent. How the surface morphology changes during wear processes is one of those aspects that are not well understood, and that at the same time affect significantly wear itself. Therefore, the aim of this dissertation is to investigate the role of surface roughness in wear processes upon dry sliding. The work focuses on wear of the adhesive type, as it is the most common one (together with the abrasive type). The first part of the thesis addresses the topic with numerical investigations. Two-dimensional systems are modelled with a discrete approach, where two surfaces are rubbed against one another. In this setup, a wear Debris Particle is eventually formed and it is constrained to roll between the sliding surfaces. It is shown that the method reproduces the evolution of the surface roughness into the self-affine morphology that is observed for different frictional surfaces. Furthermore, the interplay between surface roughness, adhesion, and wear Debris Particle size is investigated, and a minimum size for the Debris Particle is determined, based on a critical length scale recently derived for adhesive wear processes. These sets of simulations bring further observations on the wear process, like the evolution of the work due to the tangential forces and of the wear volume. The latter in particular displays an overall increase: Debris Particle accretion is favoured over its break down. This leads to the second part of the dissertation, where an analytical framework is presented that allows to explain the tendency of the Debris Particle to grow in volume, instead of depositing material onto the mating surfaces. The general approach of the work aims at uncovering underlying mechanisms of wear processes, and it is not restrained to some specific application. While on one hand this means that the work cannot cover the specificity of some frictional systems, on the other hand it leads to fundamental insights that are relevant from the nano- to the geological-scale
-
A mechanistic model for the growth of cylindrical Debris Particles in the presence of adhesion
'Elsevier BV', 2020Co-Authors: Milanese Enrico, Molinari Jean-francoisAbstract:The wear volume is known to keep increasing during frictional processes, and Archard notably proposed a model to describe the probability of wear Particle formation upon asperity collision in a two-body contact configuration. While this model is largely adopted in the investigations of wear, the presence of wear Debris trapped between the surfaces changes the system into a three-body contact configuration already since the early stages of the process. In such a configuration, a significant amount of wear is produced at the interface between the trapped Debris and the sliding bodies. Here, relying on analytical models, we develop a framework that describes crack growth in a three-body configuration at the Particle-surface interface. We then show that crack growth is favoured within the sliding surfaces, instead of within the Debris Particle, and test such result by means of numerical simulations with a phase-field approach to fracture. This leads to an increase in the wear volume and to Debris Particle accretion, rather than its breakdown. The effects of adhesion, coefficient of friction, and ratio of the applied global tangential and normal forces are also investigated. (C) 2020 Elsevier Ltd. All rights reserved
-
Fractal surfaces in adhesive wear processes.
2019Co-Authors: Milanese Enrico, Brink Tobias, Aghababaei Ramin, Molinari Jean-francoisAbstract:Wear of materials plays a key role in the durability of manifactured objects and it has thus an economical importance in our society. In spite of the efforts, a unified picture of the underlying physics of the phenomenon is still far from being reached, because of the numerous mechanisms and processes that constitute the complexity of wear. Moving one step closer to the understanding of the physics of wear would mean enhancing our understanding of all the related fields that it affects, like friction and contact mechanics. The evolution of the surfaces morphology upon sliding is one of the aspects that need further investigations. In fact, while it is known that newly created surfaces by means of crack propagation are self-affine, what happens to existing surfaces rubbed against one another is still unclear. What is known from experimental studies is that self-affinity is preserved for highly abrasive wear and that in other cases the worn surface undergoes asymmetric changes with respect of its original mean plane. Surface roughness is also linked to the wear rate, as rough surfaces are found to deteriorate more in the running-in, up to the point that they have been smoothed enough and the wear rate becomes smaller and constant (the opposite happening for surface that are initially too smooth). As atomistic simulations have been proved to be an effective means of investigation for the understanding of the physics of wear, we adopt a recently developed approach to investigate the surface roughness evolution under adhesive wear processes (i.e, when the material loss is mainly due to transfer of Particles from one body to another). The simulated system is two-dimensional, with periodic boundary conditions along the horizontal direction to allow for continuous sliding at constant velocity of the top surface over the bottom one. In the early stages of contact, the two surfaces form a Debris Particle that continuously rolls between them, interacting with them by removing material and deforming them. The peculiarity of the performed simulations is that they are long enough to allow for an investigation of the surfaces geometry evolution over time. The simulations display the two-regime evolution of the surfaces, characterized by high wear rate at running- in and lower wear rate in later stages of the process. Once the running-in is over, the morphology of the sliding surfaces and of the rolling Debris Particle are analyzed by means of their PSD. All the analyzed surfaces appear self-affine, and their heights are positively correlated. This type of roughness is consistent with the ones found in faults and, by analogy with gradient percolation models, hints that short range interactions prevail on long range elasticity. In some cases it is also observed that, after the running-in, the Debris Particle can heavily perturb the process increasing again the roughness of the mating surfaces, similarly to the running-in conditions. Despite the fact that this is not the picture commonly expected, it can be explained analyzing the Debris Particle geometry and the stress state at the contact interface. In particular, roughness enhancement happens by removal of material from the opposing surfaces when the Particle is approximately round, while an irregular shape favours the presence of stress singularities in the Particle itself and thus the deposit of material from the Particle on the opposing surfaces. In this presentation we analyze different initial setups (i.e. different morphologies, system size, materials), seeing how the final self-affine morphology is independent of the initial state and how the shape of the rolling Particle can alter the wear process
