[{"content":"I participated, as a presenter, in the Contact Mechanics International Symposium 2026 in Lugano, and gave a talk on:\nEvolution of the Contact between Rough Viscoelastic Solids After Decreasing Loads: Memory Erasure and Monotonic Increase\nCo-authors: Zichen Li and Rennald Brenner\nThe presentation was recordeded, check out the link below.\n","date":"2026-04-23T00:00:00Z","image":"/img/lugano.jpg","permalink":"/post/conf_2026_cmis/","title":"Presentation at CMIS 2026, Lugano, Switzerland"},{"content":"April 1st 2026 marks the beginning for the ANR JCJC project I am leading (codename BADaS2), in collaboration with Renald Brenner (Institut Jean le Rond d\u0026rsquo;Alembert, Sorbonne Université), Juliette Cayer-Barrioz and Denis Mazuyer (Laboratoire de Tribologie et Dynamique des Systèmes, École Centrale de Lyon). The project will last 48 months, for a total of 330 k€.\nFriction is a phenomenon that takes root in the many, both in kind and in number, molecular interactions that occur when two surfaces are close enough. However, these interactions are greatly influenced by the roughness of these surfaces, and by the long distance dialog between micro-contact junctions that takes place through the medium of bulk interactions. This distortion of the molecular picture explains in part why macroscopic friction models, like Coulomb friction or the more general rate-and-state friction, remain empirical. Shortcomings of such models lie in lumping together different physical processes that contribute to the macroscopic friction force. For example, frictional aging, the increase of the force needed to slide an interface initially at rest, comes from different mechanisms (e.g. creep and structural aging) which may compete, leading to behavior that cannot be captured by these models. The aim of the proposed research is to develop and deploy a multiscale modeling strategy to make predictions on the interactions between roughness, surface structural properties, and bulk properties that underlie the competition between different sources of frictional aging within polymer glasses. The multiscale framework described in this proposal combines molecular models, which account for the friction behavior of contact junctions at the nanoscale, and continuum models of rough contacts with viscoelastoplastic bulk constitutive behavior, which, using the junction friction knowledge at the atomic level, will help upscale it to the macro-scale, thus combining the structural (glass-like) aging and geometric (viscoelastic) aging mechanisms observed in polymer frictional experiments. While aimed at polymers, this multiscale framework will break ground in a transferable approach to friction. Ultimately, this will impact how surfaces and materials are designed to frictional constraints, leading to better performance and lower energy consumption.\nThis is an amazing opportunity to work on challenging problems, train young researchers and contribute to open modeling tools for tribologists !\n","date":"2026-04-01T00:00:00Z","image":"/img/webps/bridge_logo.webp","permalink":"/post/anr_begins/","title":"Bridging Atom-scale Dissipation and Surfaces Sliding"},{"content":" Authors: Zichen Li, Renald Brenner, Lucas Frérot\nThe real area of contact governs, in part, the magnitude of the friction force, yet its time evolution in rough viscoelastic interfaces remains incompletely understood. In experiments of contact between polymethylmethacrylate blocks under decreasing normal loads, Dillavou and Rubinstein have shown that the true contact area exhibits, after unloading, a decreasing phase and long-term memory of the contact state prior to unloading. It is however unclear what modeling ingredients are necessary to reproduce these two features. Here, we investigate these effects using fractional viscoelastic rough contact models. By adapting existing contact theories and numerical simulation methods to fractional viscoelasticity, which induces a wide relaxation spectrum, we reproduce logarithmic aging under constant load, but show that memory of the contact state is erased upon unloading. Indeed, the contact area behaves as if it had always experienced the reduced load, even on short time-scales, contrasting with the response of a standard linear solid. Moreover, none of our results show a decreasing regime of the contact area after unload: we ultimately prove that this is the case for all linear viscoelastic models\u0026mdash;despite capturing logarithmic aging\u0026mdash;leading to the conclusion that additional local internal variables are required to explain both long-term contact memory and contact area reduction after unloading.\nCover art: made with Matplotlib, computed with Tamaas\n","date":"2025-11-25T00:00:00Z","image":"/img/webps/viscoelastic_unloading.webp","permalink":"/post/paper_viscoelastic_unload/","title":"Evolution of the contact between rough viscoelastic solids after decreasing loads: memory erasure and monotonic increase"},{"content":"I participated, as a presenter in the ICTAM 2024 organized in Daegu, South Korea, and gave a talk on:\nElastic shakedown and roughness evolution in repeated elastic-plastic contact\nCo-authors: Lars Pastewka\nICTAM is a four-year cycle conference on all areas of mechanics, with experimental, numerical and theoretical approaches showcased. The tribology and contact mechanics symposium was organized by Ramin Aghababaei and Marco Paggi.\n","date":"2024-08-27T00:00:00Z","image":"/img/jpgs/daegu.jpg","permalink":"/post/conf_2024_ictam/","title":"Presentation at ICTAM 2024, Daegu, South Korea"},{"content":"I presented, in the European Congress on Computational Methods in Applied Sciences and Engineering, some high-performance aspects and code design of Tamaas, in the open-source softwares in mechanics symposium co-organized by Vladislav Yastrebov (Mines PSL), Lukasz Kaczmarczyk (U. Glasgow), Tzanio Kolev (Laurence Livermore National Labs), Andrei Shvarts (U. Glasgow).\n","date":"2024-06-03T10:00:44+02:00","image":"/img/jpgs/lisbon.jpg","permalink":"/post/conf_2024_eccomas/","title":"Presentation at ECCOMAS 2024, Lisbon, Portugal"},{"content":"I participated, as a presenter, in the Contact Mechanics International Symposium (CMIS) organized in Lyon by LaMCoS and LTDS, and gave a talk on:\nElastic shakedown and roughness evolution in repeated elastic-plastic contact\nCo-authors: Lars Pastewka\nCMIS is a symposium dedicated to bringing together scientists working in all aspects of contact mechanics, from applied mathematicians to tribologists, with a good balance of theoretical, numerical and experimental approaches.\nCMIS Group Photo ","date":"2024-05-30T17:29:04+02:00","image":"/img/lyon_river.jpg","permalink":"/post/conf_2024_cmis/","title":"Presentation at CMIS 2024, Lyon, France"},{"content":" Authors: Nicolas Richart, Guillaume Anciaux, Emil Gallyamov, Lucas Frérot, David Kammer, Mohit Pundir, Marco Vocialta, Aurelia Cuba Ramos, Mauro Corrado, Philip Müller, Fabian Barras, Shenghan Zhang, Roxane Ferry, Shad Durussel, and Jean-François Molinari\nComplex, nonlinear, and transient phenomena are at the heart of modern research in mechanics of materials. For example, the buildup and release of elastic energy at geological fault is what causes earthquakes, and the intricate details of the slip zone, the propagation of slip fronts and waves radiated through the various geological media are still active areas of research (Kammer et al., 2012, 2014; Roch et al., 2022). Similarly, understanding fracture in heterogeneous materials such as concrete, masonry or ceramics necessitates the modeling of interaction of crack fronts with complex materials (A. I. Cuba Ramos et al., 2018; Taheri Mousavi et al., 2015; Yilmaz et al., 2017), the representation of residual shear stresses in the contact of newly-formed crack surfaces (Pundir \u0026amp; Anciaux, 2021; Zhang et al., 2017), and the accurate characterization of transient dynamics (Corrado \u0026amp; Molinari, 2016; Vocialta et al., 2018) and material structure evolution (A. I. Cuba Ramos et al., 2018; Gallyamov et al., 2020). The finite-element method is now ubiquitous in virtually all areas of solid mechanics. With meticulous care on code architecture and performance, we show that our finite-element library Akantu can handle the requirements mentioned above for state-of-the-art research in mechanics of materials. Akantu is designed from the ground up for high-performance, highly distributed computations, while retaining the necessary flexibility to handle:\ncrack propagation with cohesive elements non-local damage models plastic and visco-plastic constitutive laws large deformations contact constraints (including rate and state friction) interaction between contact and cohesive elements (residual crack shear strength) Cover art: dynamic fragmentation of a sheared interface simulated in large deformations with Akantu and rendered with Paraview\n","date":"2024-02-23T00:00:00Z","image":"/img/webps/cracks_akantu.webp","permalink":"/post/paper_akantu/","title":"Akantu: an HPC finite-element library for contact and dynamic fracture simulations"},{"content":" Authors: Petr Grigorev, Lucas Frérot, Fraser Birks, Adrien Gola, Jacek Golebiowski, Jan Grießer, Johannes L. Hörmann, Andreas Klemenz, Gianpietro Moras, Wolfram G. Nöhring, Jonas A. Oldenstaedt, Punit Patel, Thomas Reichenbach, Thomas Rocke, Lakshmi Shenoy, Michael Walter, Simon Wengert, Lei Zhang, James R. Kermode, Lars Pastewka\nBehaviour of materials is governed by physical phenomena that occur at an extreme range of length and time scales. Computational modelling requires multiscale approaches. Simulation techniques operating on the atomic scale serve as a foundation for such approaches, providing necessary parameters for upper-scale models. The physical models employed for atomic simulations can vary from electronic structure calculations to empirical force fields. However, construction, manipulation and analysis of atomic systems are independent of the given physical model but dependent on the specific application. matscipy implements such tools for applications in materials science, including fracture, plasticity, tribology and electrochemistry.\nCover art: made with Blender, BCC edge dislocation junction data provided by Petr Grigorev.\n","date":"2024-01-30T10:32:57+01:00","image":"/img/webps/dislocation_colors.webp","permalink":"/post/paper_matscipy/","title":"matscipy: materials science at the atomic scale with Python"},{"content":" Authors: Lucas Frérot, Lars Pastewka\nSurface roughness emerges naturally during mechanical removal of material, fracture, chemical deposition, plastic deformation, indentation, and other processes. Here, we use continuum simulations to show how roughness which is neither Gaussian nor self-affine emerges from repeated elastic-plastic contact of a rough and rigid surface on a flat elastic-plastic substrate. Roughness profiles change with each contact cycle, but appear to approach a steady-state long before true elastic shakedown of the substrate. We propose a simple dynamic collapse for the emerging power-spectral density, which shows that the multi-scale nature of the roughness is encoded in the first few indentations. In contrast to macroscopic roughness parameters, roughness at small scales and the skewness of the height distribution of the resulting roughness do not show a steady-state, with the latter vanishing asymptotically.\nCover art: made with Blender, computed with Tamaas, HDRI from Polyhaven.\n","date":"2024-01-17T00:00:00Z","image":"/img/webps/indentation_roughness.webp","permalink":"/post/paper_indentation_roughness/","title":"Elastic Shakedown and Roughness Evolution in Repeated Elastic-Plastic contact"},{"content":"Tamaas\u0026rsquo; footprint in the tribology community Tamaas, the open-source, high-performance rough contact simulation library, is slowly seeing adoption in the academic and industrial tribology communities. Since its first public release in 2019, after 4 years of git-recorded history, and longer prior apocryphal development, Tamaas has been used outside of its research \u0026ldquo;birthplace\u0026rdquo; to study contact of non-Gaussian surfaces, benchmark a spherical cap harmonic analysis method for non-flat rough surfaces, validate finite-element rough contact solutions, investigate conductivity of rough contacts, gain insights in elastic-plastic rock contacts and ice friction, probably among other uses that I am not aware of.\nThe December 2023 cover story of Tribology \u0026amp; Lubrication Technology, the magazine of the Society of Tribologists and Lubrication Engineers, mentions Tamaas as a positive example of open-source code in the tribology community, which is still very closed when it comes to modeling efforts.\nThis is the perfect occasion for me to talk about the evolution of Tamaas: what new features were added since the first public release, what is the state of Tamaas\u0026rsquo; support (human and financial), and what developments are planned for its future.\nRetrospective PhD work \u0026mdash; Development at LSMS on volumetric elastic-plastic contact When Guillaume Anciaux shared his code with the Computational Solid Mechanics Laboratory, and Ramin Aghababaei chose the name Tamaas, at the start of my PhD thesis in 2016, the library was only capable of solving normal elastic contact of rough surfaces. The first year of development saw the addition of adhesive and saturated contact solvers by Valentine Rey and associated friction solvers by myself (later reimplemented in the new core by Son Pham-Ba). In the middle of 2017 I started reimplementing the core of the library in anticipation of the 3D developments necessary for elastic-plastic contact. After the derivation of the Fourier-space Mindlin solution in December 2017, 2018 saw the implementation of the Fourier-accelerated volume integral method, which was presented at ECCOMAS 2018 in Glasgow.\nThe last year of my PhD was dedicated to polishing the code (e.g. with a symbolic, compile-time integration library, expolit, now part of Tamaas\u0026rsquo; source tree), creating the documentation, preparing a Python package installable with pip. In fall of 2019, a Zenodo release, coinciding with the submission of my thesis manuscript and the preprint submission of my article on crack nucleation in rough contacts, marked the first public release of Tamaas.\nPost-PhD work \u0026mdash; Improving usability and performance This first release, version 2.0.0, marks a soft feature freeze in terms of modeling abilities, but the start of significant improvements to quality of life and performance features.\nSince version 2.0.0, feature improvements are documented in the CHANGELOG. The year 2020 marks the publication of the JOSS article, the transition to Gitlab, the improvement of ease of use of the build-system, and most importantly a working MPI implementation. In 2021, the Python interface of core objects was reworked to be more Pythonic, and input-output routines were improved. In 2022, a good number of utility functions were added, the scipy.sparse.linalg.LinearOperator interface was implemented for a number of Tamaas\u0026rsquo; integral operators, allowing seamless integration with Scipy\u0026rsquo;s linear solvers. Finally, new modeling features and performance improvements were added in 2023: among them the ability to simulate non-periodic contact, as well the possibility of modeling contact with heterogeneous materials. An Anderson-mixing iteration was implemented, drastically improving the reliability and performance of elastic-plastic contact simulations, and more of Tamaas\u0026rsquo; internals were exposed on the user-facing Python API, giving more flexibility in prototyping new simulation procedures.\nBuildup of roughness from elastic-plastic indentation, computed with Tamaas on 30 CPUs with MPI, made possible with Anderson acceleration Perspective Having found a permanent academic position in an institution supportive of high-performance mechanics simulation codes (Basilisk and the PARIS Simulator Code, among many others, are supported by ∂\u0026lsquo;Alembert), the future of Tamaas as an, open, active tribology research project is secure!\nI cannot discuss new modeling efforts in store for Tamaas yet (research proposals in the pipeline), but several major technical improvements are planned for the near future:\nIntegration with the PETSc matrix-free API and solvers (linear, non-linear and constrainted) is in the works. Tamaas\u0026rsquo; solvers are good but rather limited, and Scipy\u0026rsquo;s solvers are not parallel. PETSc implements robust solvers that can be used in an MPI context, and is used by much larger projects than Tamaas (e.g. Akantu, FEniCS and MoFEM). MFront is a tool that generates optimized code for constitutive laws written in a domain-specific language. It boasts a large number of tested constitutive laws, and has been successfully integrated in codes like FEniCS. With the recent rework of materials classes in Tamaas, an MFront integration could drastically improve the modeling capabilities of Tamaas. The MPI implementation has been very robust thus far (I have pushed elastic simulations upwards of 1 billion DOFs and elastic-plastic simulations larger than 150 million DOFs) and should soon make it out of experimental. In the long-term, consolidation efforts are planned with other open-source packages for contact simulations, like SurfaceTopography and ContactEngineering. The latter is an open-platform cojointly developed by the groups of Lars Pastewka and Tevis Jacobs for the multi-scale analysis of rough surface measurements. A recent plugin system should allow the integration of Tamaas among the analysis methods available! More than ever, contributions are welcome! Modeling research in tribology is becoming more and more diverse, and Tamaas aims to be a base platform to support this complex endeavor. I wholeheartedly encourage researchers using Tamaas to share their code with the community, or even better, contribute their code back to Tamaas.\n","date":"2023-11-28T10:30:55+01:00","image":"/img/tamaas_icon.svg","permalink":"/post/news_2023_tamaas_tlt/","title":"Tamaas update, press mentions and future work"},{"content":"I joined on September 1st the Solids \u0026amp; Structures (MISES) team of the ∂\u0026lsquo;Alembert Institute, at the Sorbonne Université in Paris. I\u0026rsquo;ll be working on multi-scale modeling of friction between rough surfaces. I\u0026rsquo;m looking forward to exploring this topic with future colleagues, and to bring my rough surface and contact expertise to their research projects, while contributing to free and open-source software for scientific computing, and promoting open-data and open-access science!\nI will also be teaching in the Bachelor and Master mechanical engineering curriculum at la Sorbonne, and the mechanical engineering major of the engineering school Polytech\u0026rsquo; Sorbonne.\n","date":"2023-09-01T00:00:00Z","image":"/img/jpgs/jussieu.jpg","permalink":"/post/event_2023_dalembert/","title":"Assistant Professor / Maître de Conférences at Institut ∂'Alembert"},{"content":"I participated, as a presenter, in the ECCOMAS 7th Young Investigators Conference in Porto, and gave a talk on:\nNon-linear elasticity and contact of Zinc-Phosphate tribofilms\nCo-authors: Lars Pastewka\nECCOMAS YIC is a europe-wide conference \u0026ldquo;with the main purpose of bringing together, within a relaxed environment, students and young researchers developing their work on all areas related with computational science and engineering.\u0026rdquo;\n","date":"2023-06-21T00:00:00Z","image":"/img/porto.jpg","permalink":"/post/conf_2023_yic/","title":"Presentation at ECCOMAS YIC 2023, Porto, Portugal"},{"content":" Authors: Jan Grießer, Lucas Frérot, Jonas A. Oldenstaedt, Martin H. Müser, Lars Pastewka\nElastic moduli are among the most fundamental and important properties of solid materials, which is why they are routinely characterized in both experiments and simulations. While conceptually simple, the treatment of elastic is complicated by two factors not yet having been concurrently discussed: finite-strain and non-affine, internal displacements. Here, we revisit the theory behind zero-temperature, finite-strain elastic constants and extend it to explicitly consider non-affine displacements. We further present analytical expressions for second-order derivatives of the potential energy for two-body and generic many-body interatomic potentials, such as cluster and empirical bond-order potentials. Specifically, we revisit the elastic constants of silicon, silicon carbide and silicon dioxide under hydrostatic compression and dilatation. Based on existing and new results, we outline the effect of multiaxial stress states as opposed to volumetric deformation on the limits of stability of their crystalline lattices.\nPublisher\u0026rsquo;s version (Physical Review Materials, CC-BY): doi:10.1103/PhysRevMaterials.7.073603 Preprint (arXiv, Open Access): doi:10.48550/arXiv.2302.08754 Cover art: made with Blender with the Atomic Blender addon, amorphous silicon configuration from Matscipy, and assets from ambientCG\n","date":"2023-06-05T00:00:00Z","image":"/img/webps/aSi_raytraced.webp","permalink":"/post/paper_elastic_constants/","title":"Analytic elastic coefficients in molecular calculations: Finite strain, non-affine displacements, and many-body interatomic potentials"},{"content":" Authors: Lucas Frérot, Alexia Crespo, Jaafar A. El-Awady, Mark O. Robbins, Juliette Cayer-Barrioz, Denis Mazuyer\nThe tangential force required to observe slip across a whole frictional interface can increase over time under constant load, due to any combination of creep, chemical or structural changes of the interface. In macroscopic rate-and-state models, these frictional aging processes are lumped into an ad-hoc state variable. Here, we explain, for a frictional system exclusively undergoing structural aging, how the macroscopic friction response emerges from the interplay between the surface roughness and the molecular motion within adsorbed monolayers. The existence of contact junctions and their friction dynamics are studied through coupled experimental and computational approaches. The former provides detailed measurements of how the friction force decays, post-stiction-peak, to a steady-state value over a few nanometers of sliding distance, while the latter demonstrates how this memory distance is related to the evolution of the number of cross-surface attractive physical links, within contact junctions, between the molecules adsorbed on the rough surfaces. We also show that roughness is a sufficient condition for the appearance of structural aging. Using a unified model for friction between rough adsorbed monolayers, we show how contact junctions are a key component in structural aging, and how the infrajunction molecular motion can control the macroscopic response.\nPublisher\u0026rsquo;s version (ACS Nano, Open Access): doi:10.1021/acsnano.2c08435 Preprint (arXiv, Open Access): doi:10.48550/arXiv.2111.13588 École Centrale de Lyon press release (FR) Johns Hopkins Press release Johns Hopkins MechE press release Phys.org Azonano report RCF Interview of Juliette Cayer-Barrioz and Denis Mazuyer (FR) News CNRS INSIS (FR) News Nach Welt (GE) SDSC Story HPC Wire Cover art: made with Blender with the Atomic Blender addon, roughness generated with Tamaas from the LTDS AFM measurements (data available), assets from ambientCG, atomic trajectories computed with LAMMPS\n","date":"2023-01-24T00:00:00Z","image":"/img/webps/rough_atomistic_dual.webp","permalink":"/post/paper_stearic_friction/","title":"From molecular to multi-asperity contacts: how roughness bridges the friction scale gap"},{"content":"I participated, as a presenter, in the Multiscale Materials Modeling conference in Blatimore, and gave a talk on:\nFrom molecular to multi-asperity contacts: the role of roughness in the transient friction response\nCo-authors: Alexia Crespo, Jaafar El-Awady, Mark Robbins, Juliette Cayer-Barrioz, Denis Mazuyer\nMMM is a world-wide multi-scale material science meeting, with a strong focus on modeling and simulations.\n","date":"2022-10-03T00:00:00Z","image":"/img/baltimore.jpg","permalink":"/post/conf_2022_mmm/","title":"Presentation at MMM 2022, Baltimore, USA"},{"content":"I participated, as a presenter, in the World Tribology Congress in Lyon, and gave two talks on:\nFrom molecular to multi-asperity contacts: the role of roughness in the transient friction response\nCo-authors: Alexia *Crespo, Jaafar El-Awady, Mark Robbins, Juliette Cayer-Barrioz, Denis Mazuyer\nCrack nucleation in the adhesive wear of an elastic-plastic half-space\nCo-authors: Guillaume Anciaux, Jean-François Molinari\nWTC is a world-wide tribology meeting with experimentalists and theoreticians alike.\n","date":"2022-07-11T00:00:00Z","image":"/img/lyon.jpg","permalink":"/post/conf_2022_wtc/","title":"Presentation at WTC 2022, Lyon, France"},{"content":"I participated, as a presenter, in the ECCOMAS conference in Oslo, for which I gave a talk on:\nFrom molecular to multi-asperity contacts: the role of roughness in the transient friction response\nCo-authors: Alexia Crespo, Jaafar El-Awady, Mark Robbins, Juliette Cayer-Barrioz, Denis Mazuyer\nECCOMAS is the largest european conference in the field of computational mechanics, with a strong orientation towards engineering.\n","date":"2022-06-07T00:00:00Z","image":"/img/oslo.jpg","permalink":"/post/conf_2022_eccomas/","title":"Presentation at ECCOMAS 2022, Oslo, Norway"},{"content":"Today I am starting a postdoctoral fellowship at Freiburg University. I\u0026rsquo;ll be working with prof. Lars Pastewka on mechanical properties of phosphate glasses and ZDDP tribofilms.\n","date":"2021-11-01T00:00:00Z","image":"/img/freiburg.jpg","permalink":"/post/event_2021_freiburg_start/","title":"Started post-doc at Freiburg University"},{"content":"As per the official announcement, I was appointed to Tribology Letters\u0026rsquo; Early Career Editorial Board. Tribology Letters is the leading journal in Tribology and publishes impactful fundamental results from both experimental and computational tribology. As an ECEB member I\u0026rsquo;ll be doing reviews in my expertise areas and promoting publication of early career tribologists.\nCover art: illustration from unDraw by Katerina Limpitsouni\n","date":"2021-10-13T00:00:00Z","image":"/img/book_reading.svg","permalink":"/post/event_2021_tribo_editor/","title":"Member of Tribology Letters' Early Career Editorial Board"},{"content":"For my PhD work, I was awarded the SWICCOMAS Prize 2021 (archive link).\nCover art: illustration from unDraw by Katerina Limpitsouni\n","date":"2021-02-23T00:00:00Z","image":"/img/undraw_awards_fieb.svg","permalink":"/post/event_2021_swiccomas_award/","title":"Winner of the SWICCOMAS Award 2021!"},{"content":"For my PhD work, I was awarded the EPFL Outstanding Ph.D Thesis Distinction in Civil and Environmental Engineering” 2020 (archive link).\nCover art: illustration from unDraw by Katerina Limpitsouni\n","date":"2020-12-21T00:00:00Z","image":"/img/undraw_winners_ao2o.svg","permalink":"/post/event_2021_edce_award/","title":"Winner of the EDCE Distinction 2020 (EPFL)!"},{"content":" Authors: Tobias Brink, Lucas Frérot, Jean-François Molinari\nIn order to develop predictive wear laws, relevant material parameters and their influence on the wear rate need to be identified. Despite decades of research, there is no agreement on the mathematical form of wear equations and even the simplest models, such as Archard’s, contain unpredictable fit parameters. Here, we propose a simple model for adhesive wear in dry sliding conditions that contains no fit parameters and is only based on material properties and surface parameters. The model connects elastoplastic contact solutions with the insight that volume detachment from sliding surfaces occurs in the form of wear particles, the minimum size of which can be estimated. A novelty of the model is the explicit tracking of the sliding process, which allows us to meaningfully connect particle emission rates and sizes to the macroscopic wear rate. The results are qualitatively promising, but we identify the necessity for more controlled wear experiments and the parameters needed from such work in order to fully verify and improve our model.\nPublisher\u0026rsquo;s version (JMPS, Open Access): doi:10.1016/j.jmps.2020.104238 Cover art: Tobias Brink, CC-BY\n","date":"2020-11-20T00:00:00Z","image":"/img/pngs/brink_2020.png","permalink":"/post/paper_sliding_wear/","title":"A parameter-free mechanistic model of the adhesive wear process of rough surfaces in sliding contact"},{"content":" Authors: Lucas Frérot, Guillaume Anciaux, Jean-François Molinari\nThe detachment of material in an adhesive wear process is driven by a fracture mechanism which is controlled by a critical length-scale. Previous efforts in multi-asperity wear modeling have applied this microscopic process to rough elastic contact. However, experimental data shows that the assumption of purely elastic deformation at rough contact interfaces is unrealistic, and that asperities in contact must deform plastically to accommodate the large contact stresses. We therefore investigate the consequences of plastic deformation on the macro-scale wear response using novel elastoplastic contact simulations. The crack nucleation process at a rough contact interface is analyzed in a comparative study with a classical J2 plasticity approach and a saturation plasticity model. We show that plastic residual deformations in the J2 model heighten the surface tensile stresses, leading to a higher crack nucleation likelihood for contacts. This effect is shown to be stronger when the material is more ductile. We also show that elastic interactions between contacts can increase the likelihood of individual contacts nucleating cracks, irrespective of the contact constitutive model. This is supported by a statistical approach we develop based on a Greenwood–Williamson model modified to take into account the elastic interactions between contacts and the shear strength of the contact junction.\nPublisher\u0026rsquo;s version: doi:10.1016/j.jmps.2020.104100 ArXiv: 1910.05163 Cover art: made with matplotlib, computed with Tamaas\n","date":"2020-08-18T00:00:00Z","image":"/img/jmps_2020.svg","permalink":"/post/paper_crack_nucleation/","title":"Crack nucleation in the adhesive wear of an elastic-plastic half-space"},{"content":" Authors: Lucas Frérot, Guillaume Anciaux, Valentine Rey, Son Pham-Ba, Jean-François Molinari\nPhysical phenomena that happen at solid contact interfaces, such as friction and wear, are largely entwined with the roughness of the surfaces in contact. For example, the fact that the friction force between two solids in contact is independent of their apparent contact area is due to roughness, as the solids are only in contact over a smaller “true contact area” which only depends on the normal force (Archard, 1957). Roughness occurs on most man-made and natural surfaces (Persson, Albohr, Tartaglino, Volokitin, \u0026amp; Tosatti, 2005) and can span many orders of magnitude, from the nanometer scale to the kilometer scale (Renard, Candela, \u0026amp; Bouchaud, 2013). This poses a serious challenge to conventional numerical approaches in solid mechanics such as the finite-element method (FEM).\nBoundary integral methods (Bonnet, 1995) are commonly employed in place of the FEM for rough elastic contact because of an inherent dimensionality reduction: the computational effort is focused on the contact interface whereas the FEM requires discretization of the volume of the solids in contact. In addition, the use of a half-space geometry provides a translational invariance: the computation of periodic equilibrium solutions can then be accelerated with the fast-Fourier Transform (Stanley \u0026amp; Kato, 1997).\nHowever, because of the roughness, the total contact load is distributed over a small area and local contact pressures are expected to cause non-linear material behavior, such as plasticity. In this case, volume integral methods can be employed to account for plastic deformation (Telles \u0026amp; Brebbia, 1979). These enjoy properties analogous to boundary integral methods and can also be accelerated with a Fourier approach (Frérot et al., 2019b). Taking plasticity into account is necessary in the accurate description of contact interfaces for the understanding of friction and wear. Moreover, high performance implementations are needed to model realistic rough surfaces with roughness spanning many orders of magnitude in scale.\nTamaas is a C++ library with a Python interface (Jakob, Rhinelander, \u0026amp; Moldovan, 2017), developed in the Computational Solid Mechanics Laboratory at EPFL, that implements a unique Fourier-accelerated volume integral formulation of equilibrium (Frérot et al., 2019b) for the solution of elastic-plastic rough contact problems. The use of C++ allows for a particular focus on performance: most loops are parallelized using Thrust/OpenMP and the fast-Fourier transforms are computed with FFTW3/OpenMP. Thanks to this, it can handle simulations with upwards of 100 million degrees of freedom on a single compute node (Frérot et al., 2019b). Tamaas is aimed at researchers and practitioners wishing to compute realistic contact solutions for the study of interface phenomena.\nPublisher\u0026rsquo;s version (JOSS, Open Access): doi:10.21105/joss.02121 Cover art: made with matplotlib, computed with Tamaas\n","date":"2020-07-28T00:00:00Z","image":"/img/tamaas_icon.svg","permalink":"/post/paper_tamaas/","title":"Tamaas: a library for elastic-plastic contact of periodic rough surfaces"},{"content":"Today I am starting my postdoctoral fellowship at Johns Hopkins University, funded by SNSF. I\u0026rsquo;ll be working with prof. Mark Robbins on nanoscale wear of polymers and friction mechanisms of fatty acid monolayers.\n","date":"2020-02-13T00:00:00Z","image":"/img/webps/jhu.webp","permalink":"/post/event_2020_baltimore_start/","title":"Started post-doc at Johns Hopkins University"},{"content":"I successfully obtained my PhD diploma by defending my thesis titled:\nBridging scales in wear modeling with volume integral methods for elastic-plastic contact\nI am proud of the work I have accomplished during these four years with Jean-François Molinari, Guillaume Anciaux and the whole of LSMS!\nManuscript: doi:10.5075/epfl-thesis-7640 ","date":"2020-01-13T00:00:00Z","image":"/img/webps/diploma.webp","permalink":"/post/event_2020_defense/","title":"Successfully defended PhD thesis!"},{"content":" Authors: Lucas Frérot, Marc Bonnet, Jean-François Molinari, Guillaume Anciaux\nThe contact of solids with rough surfaces plays a fundamental role in physical phenomena such as friction, wear, sealing, and thermal transfer. However, its simulation is a challenging problem due to surface asperities covering a wide range of length-scales. In addition, non-linear local processes, such as plasticity, are expected to occur even at the lightest loads. In this context, robust and efficient computational approaches are required. We therefore present a novel numerical method, based on integral equations, capable of handling the large discretization requirements of real rough surfaces as well as the non-linear plastic flow occurring below and at the contacting asperities. This method is based on a new derivation of the Mindlin fundamental solution in Fourier space, which leverages the computational efficiency of the fast Fourier transform. The use of this Mindlin solution allows a dramatic reduction of the memory imprint (as the Fourier coefficients are computed on-the-fly), a reduction of the discretization error, and the exploitation of the structure of the functions to speed up computation of the integral operators. We validate our method against an elastic–plastic FEM Hertz normal contact simulation and showcase its ability to simulate contact of rough surfaces with plastic flow.\nPublisher\u0026rsquo;s version (CMAME, paywalled): doi:10.1016/j.cma.2019.04.006 ArXiv: 1811.11558 Cover art: made with Paraview, computed with Tamaas\n","date":"2019-04-11T00:00:00Z","image":"/img/webps/plastic_zones_large.webp","permalink":"/post/paper_fourier_accelerated/","title":"A Fourier-accelerated volume integral method for elastoplastic contact"},{"content":" Authors: Jean-François Molinari, Ramin Aghababaei, Tobias Brink, Lucas Frérot, Enrico Milanese\nIn 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.\nPublisher\u0026rsquo;s version (Friction, Open Access): doi:10.1007/s40544-018-0234-6 Cover art: made with Blender with the Atomic Blender addon\n","date":"2018-09-06T00:00:00Z","image":"/img/webps/asperities_bg.webp","permalink":"/post/paper_friction_review/","title":"Adhesive wear mechanisms uncovered by atomistic simulations"},{"content":" Authors: Lucas Frérot, Ramin Aghababaei, Jean-Fraçois Molinari\nSliding contact between solids leads to material detaching from their surfaces in the form of debris particles, a process known as wear. According to the well-known Archard wear model, the wear volume (i.e. the volume of detached particles) is proportional to the load and the sliding distance, while being inversely proportional to the hardness. The influence of other parameters are empirically merged into a factor, referred to as wear coefficient, which does not stem from any theoretical development, thus limiting the predictive capacity of the model. Based on a recent understanding of a critical length-scale controlling wear particle formation, we present two novel derivations of the wear coefficient: one based on Archard’s interpretation of the wear coefficient as the probability of wear particle detachment and one that follows naturally from the up-scaling of asperity-level physics into a generic multi-asperity wear model. As a result, the variation of wear rate and wear coefficient are discussed in terms of the properties of the interface, surface roughness parameters and applied load for various rough contact situations. Both new wear interpretations are evaluated analytically and numerically, and recover some key features of wear observed in experiments. This work shines new light on the understanding of wear, potentially opening a pathway for calculating the wear coefficient from first principles.\nPublisher\u0026rsquo;s version (JMPS, paywalled): doi:10.1016/j.jmps.2018.02.015 Manuscript (CC-BY-NC-ND): PDF Cover art: made with Paraview, computed with Tamaas\n","date":"2018-02-28T00:00:00Z","image":"/img/jmps_2018.svg","permalink":"/post/paper_mechanistic_wear/","title":"A mechanistic understanding of the wear coefficient: From single to multiple asperities contact"}]