Head of Research
Deputy Director of CEMEF
Director of CNRS UMR7635
Surfaces & Tribology group
Phone +33 (0)4 93 95 74 14
Room : BR 09c
(pdf) Publications list
Birth date : 23 September 1957
- Engineer from the "Ecole Centrale des Arts et Manufactures (Centrale Paris)", Option Material Physics (1979).
- Doctor-Ingineer, Ecole des Mines de Paris (1983)
- PhD, UniversitÚ de Besanšon (1985)
Works and publications :
24 PhD supervised + 5 currently running.
55 papers in refereed journals, 63 conference Proceedings.
Application of solid mechanics to metal forming processes:
- Mechanical modelling and numerical simulation of metal forming processes.
- Surface and interface deformation processes for multimaterials bonding.
- Compressible Plasticity (soil mechanics, powder metallurgy).
Tribology of forming and bonding processes:
- Lubrication, lubricants, tribochemistry
- Modelling of mixed regime lubrication (monophasic or diphasic lubricants)
Our basic purpose is to build friction models and laws starting from a detailed understanding of elementary, microscopic phenomena behind friction forces. But tribology means a systemic approach of the behaviour of sliding interfaces. Therefore, in forming and bonding processes, understanding the transformations at interfaces and their impact on adhesion or friction is intimately connected to the mechanical analysis of the system as a whole. If necessary, the elastic tool deformation is coupled to the elastic or plastic deformation of the workpieces to form or join. This leads from the microscopic, even nanometric scale of elementary interactions between the first and third bodies, to the millimetric or metric scale of the workpiece or machine tool.
A major part of my activity consists in developing macroscopic, predictive thermomechanical models of forming processes, mainly rolling processes (of strips, bars, rings, tubes…). I therefore participate in the development of FEM numerical methods and their exploitation to the study of forming processes.
At a microscopic scale, I work on internal state variable friction models (mixed lubrication, solving Reynolds equation and roughness evolution equations), or on the deformation and fracture of coatings and superficial layers, more generally on the measurement, analysis and understanding of microscopic tribological phenomena, whatever their nature, chemical or mechanical.
Whenever possible, I try to couple interfacial microscopic models and global process modelling. The most complete example is mixed lubrication modelling in cold strip rolling.
Ongoing research, a few examples (2008)
- A thick oxide layer is formed on steel (“oxide scale”) during hot rolling. Just upstream the roll contact, a longitudinal tension state at the surface causes the oxide to crack periodically. Then, in the roll bite, these cracks may open more or not depending on local conditions, and give way to “micro-extrusion” – just as in roll-bonding. Moreover, more cracks may form in the bite, due to differential flow tendencies between the hard and brittle oxide and the softer, but more ductile metal. Even at higher temperatures, where the oxides become plastic, other phenomena, such as roll roughness imprinting, plastic interface instabilities, or oxide fragment delamination, may lead to similar geometrical interface defects. All this may result in a rough oxide – metal interface, leading to defective pickling before cold rolling, with consequences down to the finished product. These defects also change the roll / strip contact conditions, such as friction or heat transfer properties.
We are studying all these phenomena by diverse experimental (Plane Strain Compression, 4-Point Hot Bending, Hot Hardness Testing) and numerical simulation tools, in collaboration with P.-O. Bouchard (M3P group). We first concentrated on cracks opening upstream of the bite, building a “danger map” which is a function of rolling conditions (oxide thickness, temperature, rolling speed and reduction). We are now working to extend it to other interfacial deformation mechanisms.
The same approach of course applies to any case where a thin, hard and brittle superficial layer undergoes stresses which may either deform it or crack it by yielding of the softer, plastic substrate.
- In bonding of sandwich strips, a polymer film is inserted between two thin steel strips using warm rolls. Thermal transfer has been shown to be essential: contact time, Heat Transfer Coefficients, determine the surface temperature reached during the process, hence the molten polymer thickness, which both condition adhesion. We are not after physical and chemical bonding mechanisms in this case, but we study numerically (Forge3® from Cemef and Transvalor) contact temperature and its coupling with surface mechanics, to try and build (i) a refined understanding of the melting process, of the polymer surface deformation and its wetting the rough metal surface, (ii) an analysis of potential actuators, leading to evolutions of the manufacturing tools, (iii) numerical tools with their application procedures, to be used by the industrial partners in the following.
- High C-steel profiles are formed by wire drawing and rolling of rods. Damage and fracture may occur during manufacturing or, worse, during further processing at the steelmaker’s customers. Numerical simulation (Forge3®), together with an experimental study of damage modes, has disclosed the major causes and allowed us to propose a remedy based on slight changes in the tools geometry; this solution has been tested with success on an industrial scale. We are starting to apply automatic FEM inverse modelling to go further into this process optimisation, as is done by L. Fourment in forging. Damage should be part of the objective-function, together with geometry and final mechanical properties, provided an adequate damage model is built for this anisotropic material. The parameters of the optimisation need be both continuous ones (tools shapes) and discrete ones (number of passes).
- We have proposed the challenge to refund numerical simulation of rolling processes of flat products. Our in-house solution, the FEM software Lam3, 14 years old, remains at the top in terms of performance with its quasi-eulerian, steady state formulation, its low-dimensionality elastic model for roll and stand deformation. But certain cases and certain problems remain inaccessible, strip flatness namely. But the evolution capacities of this software are limited, due to critical choices made for the steady state formulation. This forbids to import modern features developed in Forge3®, such as parallelism. Therefore, we are presently building next generation of strip rolling software based on Forge3®. An essential requirement is to preserve the major advances of Lam3 (locally refined, highly anisotropic mesh giving high precision at a low cost, very fast solution times thanks to the steady state character and the very efficient roll deformation model). This has oriented us towards an ALE formulation with adaptive meshing, already existing in Forge3®, which we are optimising for strip rolling.
- We have carried out a study on flatness problems, still within Lam3, without waiting for the conclusion of the abovementioned work. Mechanical instability specialists are associated to this work (M. Potier-Ferry and H. Zahrouni, from LPMM Metz). We have first implemented a buckling criterion thanks to which a complete model coupling in-bite strip deformation, roll deformation and out-of-bite, out-of plane buckling (flatness defect) prediction has been built. As a second part (in Metz), the buckling and post-buckling behaviour is being examined in more details using the Asymptotic Numerical Method (ANM). Comparisons with experiments and applications to the mills will conclude this work.