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PhD in experimental and numerical physics/mechanics of contacts in soft matter (M/F)

This offer is available in the following languages:
Français - Anglais

Date Limite Candidature : lundi 12 décembre 2022

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General information

Reference : UMR5513-DAVDAL-003
Workplace : ECULLY
Date of publication : Monday, November 21, 2022
Scientific Responsible name : Davy DALMAS
Type of Contract : PhD Student contract / Thesis offer
Contract Period : 36 months
Start date of the thesis : 2 January 2023
Proportion of work : Full time
Remuneration : 2 135,00 € gross monthly

Description of the thesis topic

-The WEEL project:
The PhD work will be part of a broader collaborative project (acronym WEEL) between Laboratoire de Tribologie et Dynamique des Systèmes (LTDS), Ingénierie des Matériaux Polymères (IMP) and Solvay Silica company, funded by the French National Agency for Research (ANR). The project aims at bringing new insights in the fundamental understanding of the abrasion wear mechanisms of reinforced elastomers that will set the basis for future improvement in the prediction of durability of such materials, and in particular car or truck tires. Understanding abrasion wear remains an unsolved scientific challenge, that we propose to tackle by combining for the first time (i) proper wear experiments with (ii) advanced experimental and numerical investigation of the in-operando stress/strain field at a sliding interface. Indeed, our main research hypothesis is that the latter mechanical fields are the true local driving forces responsible for wear evolution. Our strategy will thus be threefold:
1. preparation of various types of carefully controlled reinforced elastomer samples by Solvay company
2. use of a home-made wear test device to characterize the wear properties of those samples, including the evolutions of their shape and of the local wear patterns (ridges, rolls,…)
3. use of a recent, cutting-edge opto-mechanical device to analyse the contact stress/strain state and dynamics, at different stages of the wear evolution.
4. development of an improved, comprehensive model of wear of reinforced elastomers incorporating the local contact conditions.

The “contact mechanics” PhD project
While point 2 above will be carried out by another PhD student, point 3 and point 4 are the focus of the present PhD project. In recent years, the consortium developed an internationally recognized expertise in the contact mechanics of sheared elastomer contacts. Under increasing shear, those contacts undergo important changes in the average contact pressure and in the micro-contact morphology, even under constant normal loading [R. Sahli et al. PNAS 115, 471 (2018)]. Such changes are expected to affect all macroscopic responses of the interface, including wear. But, so far, those insights have not been correlated to the observed spatial patterns of wear in reinforced elastomers
The main experimental task of the PhD candidate (point 3) will be to perform single-loading-cycle tribo-tests with complex kinematics, on either pristine or previously worn samples from wear tests that will be performed by the consortium. He/she will measure not only the classical evolution of macroscopic normal and tangential forces but also that of the true contact area and interfacial stress/strain field at different stages during the wear process. Both latter quantities will be obtained through advanced analysis of in-situ contact images using particle tracking [J. Lengiewicz, et al. J. Mech. Phys. Solids 143, 104056 (2020)] or digital image correlation (DIC) [A. Prevost et al. Eur. Phys. J. E 36, 17 (2013)]. We will aim at extracting the local stress/strain state, at different scales, to understand both the global morphology of the wear scar and origin of the local conditions associated with different wear mechanisms.
In parallel, a Finite Element based approach will be developed (point 4) to quantitatively correlate the simulated stress/strain fields to the experimental ones and to interpret the wear data. The primary objective is not only to simulate the experiments but also to use the flexibility of a numerical model to increase our understanding and elucidate the occurrence of the wear mechanisms. From a modelling standpoint, this is a challenging task because modelling soft contacts typically means dealing with severe non-linearities such as geometrical, material or friction-based. This usually leads to convergence issues, long computation time, mesh distortions as well as numerical locking phenomena due to the incompressible nature of elastomers.
Finally, this project will follow a methodology that we have recently used successfully on the same topic [J. Lengiewicz, et al. J. Mech. Phys. Solids 143, 104056 (2020)]: quantitative comparison between experimental and numerical data incorporating all the material behavior laws and boundary conditions at play in the experiments. Quantitative matching at local scales is currently the best available proof that the correct dominant physical ingredients have been used in the simulations, thus providing unique insights into the elementary mechanism occurring in the contact and invaluable information about the loading conditions responsible for the various observed wear processes.
Available resources:
Many experimental set-ups and numerical facilities available in LTDS laboratory will be made available for this project. This includes in particular a world class, new generation opto-mechanical contact mechanics device. Many more facilities of the LTDS laboratory will be used for specific friction tests, sample characterization (microscopy (optical, SEM, AFM ...), camera, fast camera ...) or numerical modelling, among others. The PhD will be located at LTDS (Sites of Ecully and Saint-Etienne) in Lyon Region but will involve close collaborative work with other partners within the WEEL project (IMP and Solvay, both also very close to Lyon), specifically with the other PhD student dedicated to wear experiments.

Work Context

-The PhD thesis will be attached to the Ecole Doctorale Mécanique, Energétique, Génie Civile et Acoustique (MEGA). The student will work within a multidisciplinary team of the LTDS including two supervisors (physicists), two engineers (one in surface sciences and one in instrumentation) and also in connection with the IMP laboratory. The work will include an important experimental component: preparation / physico-chemical characterization of samples, their mechanical and optical characterization. It will also involve processing and analyzing a large amount of data, not only experimental but also digital. Finally, the doctoral student will have to synthesize these results, place them in the context of international literature and use them to write articles for international scientific journals. The work also includes participation in national or international congresses.

Constraints and risks

not applicable

Additional Information

The candidate must hold a master's degree and / or an engineering degree in experimental/numerical physics or mechanics. The position requires solid knowledge in continuum solids mechanics, in particular for elastomers. The following specific skills are not required but will be a plus:
- Practical experience in Tribology and in particular in Contact mechanics
- Practical experience with elastomers
- Practical experince in numerical simulations (FEM)
- Practical experience in image analysis
- Ability to develop or modify an experimental device
- Ability to run and anlayse simulations
A good command of at least one programming language (ideally Matlab) is necessary. A good command of English is required. Experience of at least 3 months in a non-French speaking country is highly desirable. Candidates must demonstrate excellent writing skills, and have demonstrated (internships for example) their ability to lead a scientific project. They should be able to work in an international and multidisciplinary team. A great deal of autonomy, a proven organizational capacity and a good capacity to report are expected. Candidates must show a high level of critical thinking, ease in scientific argumentation and the ability to take initiative. Applications should include a detailed CV; if possible two references (people likely to be contacted); a one-page cover letter; a one-page summary of the master's thesis or end of study thesis; Master 1 or 2 or engineering school grades).

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