PhD defence of Yoan Boussès

22 April 2021

Yoan Boussès defends his PhD in Numerical Mechanics and Materials on May 5th, 2021

Prediction and modeling of mechanical properties of dental composites – experimental, theoretical and numerical approach

Yoan Boussès conducted his PhD work in CSM team, under the supervision of Yannick Tillier, CEMEF and Nathalie Brulat, Associate Researcher, Unviersity Côte d'Azur, Dental Chirurgy Faculty. The project was financed in the framework of the ANR Toothbox Project by ARMINES. He defends his PhD in Numerical Mechanics and Materials on May 5, 2021 in front of the following juy:

Pr. Brigitte GROSGOGEAT (Univ. Lyon 1 LMI-Univ. Claude Bernard, Villeurbanne) : rapporteur
Pr. Daniel RITTEL (Inst. de Technologie d'Israël, Haifa, Israël) : rapporteur
Pr. Pierre COLON (Université de Paris, Paris) : examinateur
Pr. Piere-Olivier BOUCHARD (MINES ParisTech-CEMEF, Sophia Antipolis) : examinateur

Abstract:

As tooth decays are the most widespread pathologies, dental restoration is a global public health problem. Manufacturers of dental composite resins must therefore offer increasingly high performance and lasting materials while meeting health recommendations. The development process is costly but could be improved by a better understanding of their mechanical behaviour and the development of theoretical and numerical predictive models for their properties.

In this work, an experimental material is tested before and after accelerated ageing protocols (thermocycling). A detailed experimental protocol has been established so that the results are reproducible and comparable between them. Predictive models of the yield stress and the elastic modulus, initially proposed for other applicaions, are compared with experimental results and then adapted to the specificities of dental composites, with high filler ratios. The use of a numerical model of a composite material also makes it possible to enrich the experimental database by playing for instance on the modulus of the components, without having to produce the materials. Despite some limits of the innovative approach, the comparison of experimental, theorical and numerical results gives promising outlooks. They allow to predict essential characteristics of composites in an easier way and to make direct connections between macroscopic and microscopic properties. Finally the understanding of the ageing mechanisms was improved.

Keywords: Mechanical characterization of dental composites, Thermocycling, Interfaces, Numerical modelling, Elastic properties

MEB pictures of the two fracture surfaces of a broken composite sample. Broken particles are highlighted in red and debonded particles in green and blue.

PhD Defense of Emilie Forestier

29 March 2021

Emilie Forestier defends her PhD in Numerical Mechanics and Materials on March 29, 21

"Strain induced microstructure in PEF, a biobased polymer, upon uniaxial and biaxial stretching above the α-relaxation"

Emilie Forestier completed her thesis within the MPI team, under the direction of Noëlle Billon in collaboration with Nicolas Sbirrazzuoli, Professor at the University of the Côte d'Azur, Institute of Chemistry. Her thesis was funded by Adème.

Her jury is composed of the following personalities:

– Pr.U. Valérie GAUCHER (Univ. Lile-Unité Mat. et Transformations, Cité Scientifique, Villeneuve d'Ascq)

– Pr.U. Isabelle ROYAUD (Univ. de Lorraine-Inst. Jean Lamour, Nancy)

– D.R. Sylvain CAILLOL (Ecole Nat. Sup. de chimie de Montpellier-Inst. Charles Gerhardt, Montpellier)

– Pr.U. Allison SAITER (Univ. de Rouen Groupe Physique des mat.-EIRCAP,St Etienne du Rouvray)

– I.R. Christelle COMBEAUD (MINES ParisTech-CEMEF, Sophia Antipolis)

– M.C. Nathanaël GUIGO (Univ. Côte d'Azur- Inst. de Chimie de Nice, Nice)

– Pr.U. Nicolas SBIRRAZZUOLI (Univ. Côte d'Azur- Inst. de Chimie de Nice, Nice)

– I.R. Roy VISSER (Avantium Zekeringstraat, Amsterdam, Pays-Bas)

– Ing. Mikaël DERRIEN (Sidel, Octeville-sur-Mer)

Abstract:

The aim of this work is to provide a better understanding of the mechanical behaviour as well as of the associated microstructural development of a biobased polymer, named PEF. Indeed, this polymer is more and more evocated to replace one of the polymer used in food packaging, PET. PEF is identified as the biobased counterpart of PET. The difference between these two materials is the presence of a furan ring (composed of an oxygen atom with two non-binding electrons), instead of a benzene one, in PEF. This PEF specificity is responsible of a higher glass transition temperature and elastic modulus, a slower crystallization rate, a lower stability of the crystal and a lower melting temperature in comparison with PET.

Based on the stretching of amorphous PET and on its microstructural development, the stretching of amorphous PEF is investigated. Considering the differences in the chain architecture, it appears that PEF and PET cannot be stretched with the same process parameters (strain rate and temperature). In order to perform efficient PEF uniaxial and biaxial tests, it is necessary to define the forming range and some suitable stretching parameters. In this way, a specific stretching protocol, based on the building and on the reading of a master curve at a reference temperature for each material, has been defined and applied on PEF and PET. This master curve allows to know the initial physical state of the material in relation with the couples strain rate/temperature. The stretching mastering and the use of optimize stretching settings lead to the creation of a crystal that increases the thermal stability as well as the rigidity of the material. The mechanical behaviour of PEF reveals that, after the optimization of the stretching settings, this material acts in a similar way as PET. In the first stages of the stretching, both materials mechanical responses are really close. A major difference exists between PEF and PET concerning the apparition of the crystal upon stretching. Indeed, it is found that PEF must form the crystal to strain harden. Up to the end of the stretching, this crystal does not evolve. On the contrary, PET forms firstly a mesophase. The crystal existence is dependent on the final strain and on the quenching protocol.

The microstructure induced upon stretching has been widely analysed and compared to the microstructure existing when a sample is crystallised in static conditions. A high similarity exists between the two ways of crystallization, especially for the crystal definition. It seems that the microstructure induced during the stretching is more constrained than the one obtained from a static crystallization. It has to be noted that the aliphatic part of the chain is the first one impacted by the stretching. This work has also highlighted that the microstructure induced in PEF under stretching is relatively close whatever the stretching conditions are. Concerning PET, the microstructural development seems to be more dependent on the stretching settings.

Uniaxial stretching of PEF (biosourced material) and PET. Influence of stretching on the aliphatic part of the chain

Keywords: Strain induced crystallization, uniaxial and biaxial stretching, biobased polymer, microstructural analysis, PEF, thermomechanical behaviour

PhD defence Prashanth Thirunavukkarasu

4 March 2021

Prashanth Thirunavukkarasu defends his PhD in Numerical Mechanics and Materials on March 31, 21

Analysis of the interfacial flow behavior of polymers along the walls of an internal mixer

Prashanth Thirunavukkarasu completed his thesis within the CFL team, under the supervision of Edith Peuvrel-Disdier and Rudy Valette. His PhD work was funded by Michelin.

Abstract:

The internal mixing process is crucial to the manufacturing of rubber compounds as it is instrumental to the final properties of the product. Understanding the interfacial phenomena during mixing is crucial to the evaluation of the process, determination of process parameters and numerical prediction of the same. These phenomena include wall slip, adhesion and movement of free surfaces. The evolution of wall slip velocities was characterized with classical indirect rheological techniques. An eccentric counter-rotating Couette cell was designed and developed during this PhD to observe the free surfaces of viscous fluids under shear flow in steady-state conditions. A small volume of silicone fluid was used to study the flow behavior and observe the free surfaces in counter-rotating conditions. Steady state conditions were investigated for different conditions of cylinder velocities, volumes of fluid, surface roughness and nature of cylinder surfaces. The contribution of the adhesion energy to the stabilization velocities appears to be negligible. Finally, the influence of shear flow on the movement and shape of free surfaces was explored with the help of finite element method. Numerical simulations with the integration of an adhesion boundary condition show a clear effect of the adhesion energy on the free surface shape and movement. But the level of adhesion energy necessary in the case of viscous fluids was found to be far too high to be observed experimentally.

Keywords: Internal mixing, Rubber, Wall slip, Movement of free surfaces, Adhesion

PhD Defense of Pierre-François Mougard-Camacho

1 March 2021

Pierre-François Mougard-Camacho defends his PhD in Numerical Mechanics and Materials on March 8, 21

Rheological studies on ceramic pastes for the shaping of nuclear fuels by extrusion

Pierre-François Mougard-Camacho completed his PhD thesis under the supervision of Rudy Valette and Romain Castellani, in the CFL team in the framework of a project with the CEA. He will defend his PhD thesis in front of the following jury:

– Christophe LANOS, Professeur, Université de Rennes

– Fabrice ROSSIGNOL, Directeur de Recherche, CNRS, Limoges

– Elisabeth LEMAIRE, Directrice de Recherche CNRS, Nice

– Julie BOURRET, Maîtresse de conférences, ENSCI, Limoges

– Romain CASTELLANI, Ingénieur de Recherche, Mines ParisTech

– Arnaud POULESQUEN, Chercheur Ingénieur, CEA Marcoule

– Franck DOREAU, Chercheur Ingénieur, CEA Marcoule

– Rudy VALETTE Professeur, Mines ParisTech

– Joumana YAMMINE-MALESYS, Directrice de Recherche, Weber St-Gobain

Abstract:

This work deals with the rheology of ceramic pastes, surrogate of MOX nuclear fuels (U,Pu)O2.. The thesis falls in the scope of ongoing research towards innovative processing methods to produce nuclear fuels. Fuels that have been produced by powder pressing so far.

A water-based formulation yielding defectless extrudates and mimicking a 30%at PuO2 fuel was engineered through the use of polymeric additives. The rheology of the paste was studied, followed by a characterization of the mechanical properties of extrudated and fired rods, in order to assess the feasibility of the process. Conventional capillary rheometry as well as an empirical model derived from ceramic extrusion, were combined to provide with satisfactory pressure predictions over wide operating conditions. Squeeze flow rheometry was also implemented to improve the formulation of pastes with adequate behaviour. Furthermore, it also enables a better control of the slipping conditions, as well as can enlighten liquid phase migration.

As a conclusion of this work, the production of nuclear MOX fuels by extrusion seems a promising alternative, which would be worthy of further studies in the future.

Keywords: Rheology, Ceramic, Extrusion, MOX fuel, Nuclear

PhD defense of Malik Durand

18 January 2021

Malik Durand defends his PhD in Numerical Mechanics and Materials on Jan. 18, 2021

Metallurgical mechanisms upon stress relaxation annealing of the AD730TM superalloy

Malik Durand conducted his PhD work in MSR team, under the supervision of Nathalie Bozzolo in the framework of Industrial Chair ANR SAFRAN OPALE. He defends his PhD in Numerical Mechanics and Materials on January 18, 2021 in front of the following juy:

– Alain JACQUES, IJL

– Bernard VIGUIER, IRIMAT-ENSIACET

– Rodrigue DESMORAT, ENS CACHAN

– Fabien PAUMIER, PHYMAT

– Jonathan CORMIER, ISAE-ENSMA

– Jean-Michel FRANCHET, SafranTech

Abstract:

Multi-scale microstructural analyses have been performed to identify the microstructural mechanisms controlling stress relaxation during annealing of the AD730TM turbine disk superalloy. After cooling at 100°C.min-1 and for 500 MPa initial stress value, the relaxation testing at 760°C shows atypical behavior with sluggish relaxation in the first 25 hours before relaxation occurs in a more classical manner. To understand this unusual behavior, isothermal dilatometry tests were used to decouple the effects of stress and of temperature. The later revealed a contraction of the specimens when subjected to a constant temperature. This contraction induces a tendency for an increase in stress during the relaxation test to meet the imposed condition of constant total deformation. Relaxation is then controlled by the competition between the classical relaxation mechanisms (vacancies diffusion and/or dislocation gliding depending on stress level) which tend to lower the stress and the contraction of the specimen which on the contrary tends to increase the stress level. To understand the origin of such macroscopic contraction, polycrystalline and single crystalline samples of the AD730TM alloy were characterized by different techniques, at various scales. Macroscopic contraction arises from the redistribution of chemical species between the matrix and the hardening precipitates which leads to a decrease in the lattice parameters of both phases. This work carried out on the alloy AD730TM also confirmed a similar behavior of the Rene 65 alloy. The described mechanisms most likely could apply also to many other Nickel base superalloys for which similar macroscopic contraction has been reported. Furthermore, X-ray diffraction results also allowed to estimate the lattice misfit evolution, from about +0.03% at room temperature to -0.007% at 900°C, with a close to zero value at typical stress relaxation annealing temperatures or those targeted for in-service life of the alloy.

Keywords: Nickel-based superalloy, relaxation, stress, microstructure, misfit

Coraline Chartier research awarded at the EPNOE IJM

8 February 2021

Coraline Chartier is a 2nd year doctoral student in the BIO team at CEMEF. She has just received the Best Poster Award at the Virtual Conference organised by the EPNOE association. The 4th EPNOE Junior Scientist Meeting took place on 3 and 4 February 2021. Among the 38 posters presented, the jury rewarded three young researchers, including Coraline Chartier.

Coraline Chatier works on chitosan aerogel and more precisely on the influence of processing conditions on chitosan aerogel structure and properties.

Her objective focuses on biomedical applications, and solutions for wound healing, tissue engineering or controlled drug release.

Coraline is doing her PhD work at CEMEF with Tatiana Budtova, in the framework of a project funded by the CNRS.

>> More details on the Conference site

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