PhD defence of Joe Khalil

22 November 2022

Joe Khalil will defend his PhD in Computational Mathematics, High Performance Computing and Data on Nov. 22, 22.

"Modeling fluid solid coupling with residual and thermoelastoplastic stresses analysis and optimization"

Joe Khalil conducted his PhD work in the CFL team under the supervision of Elie Hachem and Elisabeth Massoni. He will defend his PhD in Computational Mathematics, High Performance Computing and Data, on November 22, 22 in front of the following jury:

– Mr Julien Bruchon, Mines Saint Etienne, reviewer

– Mr Chady Ghnatios, Arts et Métiers Paris, reviewer

– Mrs Chantal David, SC&C

– Mr Lukasz Madej, AGH University of Science and Technology, Cracovie, Pologne

– Mr Elie Hachem, CEMEF Mines Paris – PSL

– Mrs Elisabeth Massoni, CEMEF Mines Paris – PSL

Abstract:

Quenching is a very important cooling process adopted nowadays by most of the industries, in particular automotive, aerospace and nuclear industries. The importance of this process comes from its ability to control the microstructure, to have better thermal properties as hardness and yield strength, and to release residual stresses. Nevertheless, it is a very complex process since it includes several physical phenomena on both the fluid (quenchant) and the solid. On the fluid level, because of the direct contact with a hot surface, the fluid will evaporate and reach the boiling point. On the solid level it exists stresses, deformations that changes the piece shape, and phase transformation which will generate latent heat. A boiling and evaporation was used to simulate what is happening in the surrounding of the solid. The importance of this model comes with its ability to give a real description of the heat transfer happening between the solid and the fluid. The temperature change in the solid will affect both the phase transformation and the mechanical response of the piece. In this project, a hybrid model is developed to solve the boiling and evaporation, at the same time phase transformation parameters and the mechanical response. In a Fluid-Solid domain, Navier-Stokes coupled with the heat equation is solved to give a temperature distribution in the solid. In a Solid domain only, the phase transformation parameters along with the stresses and deformations, using a thermo-elasto-plastic solver, are calculated based on the temperature distribution transported from the Fluid-Solid domain. The novelty of this model is its ability to work on two different domains simultaneously, and to give a better resolution on each domain, in addition to its completeness to simulate all the physics happening in the quenching process.

Keywords: fluid solid coupling, modeling, residual stresses, thermoelastoplastic stresses

PhD defence of Laura Montalban

8 November 2022

Laura Montalban defends her PhD in Computational Mechanics and Materials on Nov. 8, 22.

Experimental study of wear of elastomers at elevated temperature: application to power transmission belts

Laura Montalban conducted her PhD work in PSF team under the supervision of Pierre Montmitonnet and Imène Lahouij. She defends her PhD in "Computational Mechanics and Materials" on October 2022 (subject to the agreement of the reviewers) in front of the following jury:

– DELBÉ Karl, Ecole Nationale d'Ingénieurs de Tarbes, Reviewer

– LE HOUÉROU Vincent, Université de Strasbourg – IUT Robert Schuman Illkirch, Reviewer

– CHATEAUMINOIS Antoine, ESPCI

– JRIDI Nidhal, Hutchinson, guest

– MONTMITONNET Pierre, Mines Paris – PSL, CEMEF

– LAHOUIJ Imène, Mines Paris – PSL, CEMEF

Abstract:

Poly-V Belts are multilayered flexible elements that enable power transmission between rotational shafts of automotive engine drives. Wear of the external coating of the belts is inevitable because friction at the interface between the pulley and the belt is required for the transmission of torque. The aim of this PhD thesis is to understand the tribological behavior of the TPV elastomer coating of the Poly-V belts in order to improve the wear resistance and increase their lifetime. First, the wear behavior of the Poly-V belts was studied by means of two industrial test rigs: electric and diesel test benches. Experiments were performed at different operating conditions such as: applied contact pressure, sliding velocity and slip (%). The abraded surface of the belts was also observed in order to identify the dominant wear mechanisms and determine their evolution with the aforementioned input parameters. Then, a high temperature tribometer was developed in order to perform wear tests of the material of the TPV coating at a laboratory scale. The main purpose was to reproduce the kinematical configuration found on industrial test rigs and to control the main input parameters. Due to the nature of the thermoplastic constituent of the TPV coating, an analytical model for predicting the contact temperature at the interface was also proposed. A good agreement between the contact temperature measured with the infrared camera and the numerical simulations was observed. Results suggests that an increase in contact pressure and sliding velocity affects the wear kinetics of the TPV coating. In this context, the contact temperature rise generated by frictional heating is detrimental for the wear resistance of the TPV coating. The presence of a non-woven layer of fibers decreases friction at the interface, however, it also seems to increase the wear rate of the TPV coating. The incorporation of a substrate with an elastic modulus E’ affects the wear performance of the TPV coating. The previous findings indicate that the mechanical properties (e.g rupture and fatigue) of the TPV coating and the substrate play a major role in the wear behavior of the Poly- V belts. Moreover, SEM images of the worn surface showed two main types of wear features characteristic of abrasive wear (fiber pull out – detachment of thermoplastic constituent of the non-woven layer and abrasion patterns). Lastly, wear trends and wear mechanisms obtained with the high temperature tribometer are similar to the ones reported on industrial test benches. The approach presented in this work provides a basis for the development of future material formulations that could enhance the wear performance of the coating of the Poly-V belts.

Keywords: Thermoplastic elastomer, high temperature tribometer, contact temperature, sliding wear, friction

PhD defence of Joël Keumo Tematio

27 October 2022

Joël Keumo Tematio defends his PhD in Computational Mechanics and Materials on Oct. 27, 22.

Efficient thermomechanical model of DED process by inherent strain method

Joël Keumo Tematio conducted his PhD work in the 2MS team under the supervision of Michel Bellet and Yancheng Zhang. He defends his PhD in "Computational Mechanics and Materials" on October 27^th, 2022 in front of the following jury:

– Anne-Marie Habraken, Université de Liège, Belgium, reviewer

– Pierre Joyot, ESTIA, reviewer

– Anthony Gravouil, INSA Lyon

– Daniel Weisz-Patrault, Ecole Polytechnique

Abstract:

A thermo-mechanical finite element simulation is developed for additive manufacturing by directed energy deposition (DED). The simulation is conducted at the scale of the part, by modelling the progressive deposition of matter and the energy source. A general expression of the consistent tangent modulus is derived and implemented in the finite element resolution, considering an elastic-viscoplastic behavior with both isotropic and non-linear kinematic hardening. To incrementally resolve the displacement, strain and stress fields, a theoretical formulation for the kinematic positioning is proposed to minimize the distortion of the non-constructed fraction by considering current displacement and strain in the constructed part. The convergence analysis of the developed simulation is verified by both temporal and spatial aspects. The validation is obtained by comparison with experimental results taken from the literature for the cases of a straight vertical wall and a turbine blade showing a strong curvature. To reduce the computational time, an incremental inherent strain method is first proposed, where the inherent strain is determined by an inverse method based on simulation results from the standard elastic-viscoplastic calculation applied to a few tracks. However, when applying this inherent strain in the case of a simulation of entire parts, the results are severely degraded with respect to the reference solution given by the standard simulation. This is confirmed, whatever the method to apply inherent strains: uniform, or spatially distributed in each new layer. Especially the results become worse for the highly curved turbine blade structure. To resolve such problems, a new "inherent strain rate" method is proposed, consisting of a linearization of the progressive calculation. This is obtained by considering the scalar equivalent viscoplastic strain rate as the inherent strain rate. During the process simulation, the inherent strain rate-based calculation is combined with the standard calculation which is kept to simulate the ends of each track. Thanks to this combination, and to a continuous updating of the inherent strain rate, perfect results are obtained for both the single wall and the turbine blade, with a time gain of 5. This makes the proposed inherent strain rate method very promising for additive manufacturing process simulation.

Keywords: Additive manufacturing, Numerical simulation, Finite element, Inherent strain, Inherent strain rate

PhD defence of Corentin Perderiset

14 October 2022

Corentin Perderiset defends his PhD in Computational Mechanics and Materials on Oct. 14, 22.

Study of the adhesion mechanisms in a titanium / composite bonded joint with consideration of environmental aging

Corentin Perderiset conducted his PhD work under the supervision of Pierre Montmitonnet, Frédéric Georgi (PSF team) and Jean-Luc Bouvard (MPI team). He defends his PhD in "Computational Mechanics and Materials" on October 14, 22 (subject to the agreement of the reviewers) in front of the following jury:

– Mme Valérie Nassiet, ENIT

– M. Eric Paroissien, Isae-Supaero

– M. Romain Creachcadec, ENSTA Bretagne

– M. Maelënn Aufray, INPT-ENSIACET

– M. Anthony Grunenwald, SAFRAN

– M. Pierre Montmitonnet, Mines Paris – PSL

– M. Jean-Luc Bouvard, Mines Paris – PSL

– M. Frédéric Georgi, Mines Paris – PSL

Abstract:

Structural bonds today offer a convincing alternative to raw metals for the purpose of reducing the mass of structures in the aeronautical field. Also, titanium reinforcements are now bonded to composite matrices. However, metallic materials do not have good predispositions to bonding, which is why it is necessary to carry out a surface preparation to promote adhesion.

The objective of this thesis is to contribute to the understanding of the adhesion mechanisms after bonding a titanium alloy part with an organic matrix composite, involving a surface treatment and an adhesion primer. The main point of interest is in particular the study of titanium/primer and primer/adhesive interfaces. Their behavior over time is also studied. For this, they were subjected to accelerated aging in damp heat (70 °C, 80 % RH, up to 1000h).

Initially, a shifted-TAST test was chosen to stress the assembly at these interfaces and thus make it possible to identify key parameters of the surface treatment process. Thanks to the exploitation of mechanical tests in conjunction with the study of fracture facies at several scales, it was possible to determine that several adhesion mechanisms are involved in this assembly. The parameters that most influence the adhesion properties of the interface are the initial microstructure of the TA6V and the thickness of primer deposited. Too thick a primer degrades the adhesion properties after aging by limiting the mechanical anchoring of the adhesive on the titanium surface because it reduces its roughness. The absence of a primer also reduces the adhesion properties because the adhesive wets the surface less well than the primer. The latter also makes it possible to delay the degradation of the interface in humid conditions.

Regarding the initial microstructure of TA6V, the presence of α nodules allows the creation of a rough surface that serves to promote the mechanical anchoring of the adhesive. On the other hand, these areas are often poorly covered with primer and the latter has a better affinity with the adhesive, especially after aging. The presence of lamellar grains, of smaller dimensions (α lamellae of the order of 200 to 400 nm) offers better wettability for the primer and thus allows these areas to better resist environmental aging.

Keywords: Surface treatment, adhesion, titanium-composite bonding, TAST testing, interfaces, aging

PhD defence of Lucas Ravix

2 September 2022

Lucas Ravix defends his PhD in Computational Mechanics and Materials on Sept. 2nd, 22.

Multi-scales modelisation of Wire Arc Additive Manufacturing : from CMT process to large dimensions parts

Lucas Ravix conducted his PhD work in the 2MS team under the supervision of Michel Bellet, Charles-André Gandin, Yancheng Zhang and Gildas Guillemot. He defends his PhD in "Computational Mechanics and Materials" on September 2nd, 22 in front of the following jury:

– Mrs Muriel CARIN, Laboratoire IRDL

– Mr. Remy DENDIEVEL, SIMAP/GPM2

– Mr. Frederic DESCHAUX-BEAUME, Université de Montpellier

– Mr. Christophe TOURNIER, ENS Paris-Saclay

– Mr. Michel BELLET, CEMEF – Mines Paris – PSL

– Mr. Charles-André GANDIN, CEMEF – Mines Paris – PSL

– Mr. Gildas GUILLEMOT, CEMEF – Mines Paris – PSL

– Mr. Yancheng ZHANG, CEMEF – Mines Paris – PSL

Abstract:

Wire and arc additive manufacturing (WAAM) process allows complex geometries parts to be built, or functionalities to be added to existing components, by depositing the material in successive beads using a welding torch. The high material feeding rate, the accessible raw material price and its use with anthropomorphic robots in almost unlimited spaces make it a relevant industrial complement to the already industrially viable powder bed processes (LPBF). However, its development is still limited, due to the numerous physics and phenomena involved, the latter being highly dependent on process parameters. More specifically, the flows in the molten metal bath following the transfer of material and heat have a direct impact on the adhesion to the previous layer and its solidification. This leads to specific morphologies and a microstructure oriented in the direction of construction, all of which are dependent on the thermo-mechanical history of the complete part, due to heat accumulation. These coupled multi-scale interactions, inaccessible to a single numerical model, then guide the development of two multi-physics models in this research, which are based on previous work in welding and in the LPBF. The first model, called mesoscopic and at the bead scale, describes the complex cycle of the CMT process in a level set approach. Without considering electromagnetism, a contact and controlled loop model is developed to reproduce the cycle of the deposition. The process, stabilised by the electrode in contact with the pool, seems to be governed at first order by the surface tension, the shape of the previous deposit and its temperature. The second model, called macroscopic, describes the thermo-mechanical cycles of a part of industrial dimensions for times of several hours. Its speed, in a quiet element approach, is based on an extreme discretization of the material and heat transfer using material segments, adapted to use directly complex CAD parts. The mechanical resolution, combined with an unclamping model, allows the observation of residual deformations. In the end, the meso and macro models are confronted with numerous experimental resources and show good coherence in their own scales. The project therefore proposes a basis for future projects for multi-scale couplings, where numerical modelling could represent the behaviour of the material, from the inter-cordon roughness governed by the flows to the residual stresses in larges pieces.

Keywords: Additive manufacturing, Finite elements, Wire & arc process, CMT Cycle, Hydro-thermo-mecanic resolution, Multi-scale simulation

PhD defence of Clément Laügt

12 July 2022

Clément Laügt defends his PhD in Computational Mechanics and Materials on July 12th, 22

Mechanical behavior of a polyamide 6,6 subjected to thermo-hydro-glycol ageing ; Microstructure-property relationships

Clément Laügt conducted his PhD work in the MPI team. He defends his PhD in Computational Mechanics and Materials on July 12th, 2022 in front of the following jury:

– DR. Sylvie Castagnet, Institut PPrime

– Prof. Valérie Gaucher, Université de Lille, UMET

– Prof. Fodil Meraghini, ENSAM Metz, LEM3

– Prof. Bruno Fayolle, ENSAM Paris, PIMM

– Dr. Gilles Robert, DOMO Chemicals Polytechnyl

– Prof. Jean-Luc Bouvard, Mines Paris, CEMEF

– Prof. Noëlle Billon, Mines Paris, CEMEF, examinateur

Abstract :

The aim of this work is to assess the impact of a thermo-hydro-glycol ageing on the microstructure and the mechanical behavior of an injection molded polyamide 6,6. During ageing, polyamides 6,6 are immerged in a mixture of water and ethylene glycol, at temperatures ranging from 120°C to 140°C.

Concerning the microstructure, a decrease of the average molar mass of the polymer is measured by steric exclusion chromatography. This decrease results from the hydrolysis reaction. However, most of the measurements concern the crystalline phase of the polymer. Several quantities are measured, especially a crystallinity index, the lamellae thickness, the crystallites apparent size, and a crystal perfection index. A core-skin microstructural gradient is pointed out. It is the consequence of the injection molding process.

The mechanical behavior is assessed by tensile testing. The true strain fields are measured by a digital image correlation system. The loading conditions are chosen by referring to the time-temperature superposition principle. Microstructure-properties relationships are brought out.

Finally, the mechanical behavior is integrated in a model inspired from physics. The polymer is related to an equivalent network. This model allows the modelling of complex loading.

Keywords: PA66, thermo-hydro-glycol ageing, microstructure, mechanical behavior