RESEARCH FIELDMathematics › Computational mathematics
RESEARCHER PROFILEFirst Stage Researcher (R1)
APPLICATION DEADLINE20/03/2020 22:00 - Europe/Brussels
LOCATIONFrance › chasseneuil du poitou
TYPE OF CONTRACTTemporary
HOURS PER WEEK35
OFFER STARTING DATE01/10/2020
IS THE JOB RELATED TO STAFF POSITION WITHIN A RESEARCH INFRASTRUCTURE?Yes
The proposed thesis work aims to study the degradation and decomposition of structural composite materials subjected simultaneously to thermal aggression of the fire type and mechanical load. The wish is to model the various modes of damage (due to mechanics, thermal, combustion) of components subjected to thermomechanical loadings and to propose a coupled simulation tool for the calculation of structures.
From the results of coupled thermomechanical tests existing at the P 'Institute [P'1, P'2], a model able to simulate the strong interactions between heat transfers and thermal degradation on the one hand, and the mechanical behavior of composite structures under any load on the other hand, will be developed. Work has already been undertaken in the research unit in order to couple models of mechanical damage (of the Hashin type) and models of decomposition due to pyrolysis [P'3], but the management of multiple non-linearities and the need to use digital artefacts to ensure convergence in this highly coupled multi-physical context encourages us to explore new avenues. We will first develop a "simple" thermomechanical model, i.e. with a fixed configuration (without evolution) of damage, in order to identify the reduction in properties. We choose development in an internally created code so as not to be limited by constraints in the choice of resolution methods often imposed in commercial codes. Then, an evolution of this first model will be made to take into account the kinetics of "chemical" degradation due to the increase in temperature. The study of the characteristic times of kinetics and mechanics will make it possible to couple the two aspects in a single model. Initially, the kinetic model implemented will be simple (few reactions) to facilitate coupling.
In order to better take into account the degradation of the material, the addition of continuous damage and phase fields will be a strong asset of this study. The phase field method will allow efficient management of surfaces and interfaces by scalar fields (here, the transition between the composite transformed into carbon and the still healthy composite). For this modeling, two quantities will be necessary:
A characteristic distance from the damage: we will use the image analysis and covariance 3D tools developed internally [P’4] so as to define the characteristic length of the material studied. In the anisotropic framework of composite materials, this quantity will be tensorial.
Weighting functions: these functions will allow us to obtain the evolution of the mechanical and thermal properties (and to compare it with the kinetics of chemical degradation) so as to take into account the reduction of the latter during loading. These functions will depend on the damage rate d. Classically, they take the form of a power function (d) = (1-d) n. We will use experimental data relating the heat flux to the thickness of coal available at Institut P ’as well as simulations from full-field tomographic images using an FFT solver in order to validate these weighting functions.
The goal is to be able to provide a predictive tool capable of simulating the thermomechanical behavior of any structures subjected simultaneously to mechanical loading and to a heat flow, where thermal degradation, mechanical damage and heat transfers are coupled, while limiting times Calculation. The development of a finite element including the phase field as an additional degree of freedom will be the last part of the thesis work. This implementation will be done in the Foxtrot finite element code internal to the laboratory. It can be based on the developments made during a recent thesis [P'5] for the development of a finite element adapted to this study and on the experience of the research unit in the calculation of structures subjected to fire. [P'6].
On the digital side, the doctoral student will benefit from mechanical skills from researchers from the Damage and Durability team and from thermal decomposition and combustion, from researchers from the Heterogeneous Combustion team:
- Support from two teacher-researchers and a research engineer from the team, specialists in numerical methods and in finite element simulation.
- Support from three teacher-researchers in numerical modeling of thermal decomposition and combustion.
[P'1] THY Quach, A. Benelfellah, B. Batiot, D. Halm, T. Rogaume, J. Luche, D. Bertheau, Determination of the tensile residual properties of a wound carbon / epoxy composite first exposed to fire, Journal of Composite Materials, 51 (1), p.17-29, 2017
[P'2] A. Benelfellah, D. Halm, D. Bertheau, P. Boulet, Z. Acem, D. Brissinger, T. Rogaume, Effect of a coupled thermomechanical loading on the residual mechanical strength and on the surface temperature of wound carbon / epoxy composite, Journal of Composite Materials, 51 (22), p.3137-3147, 2017
[P’3] C. Mercadé, Modeling of the degradation of a carbon-epoxy composite material subjected to a coupled thermo-mechanical stress. Application to type IV hydrogen tanks. Doctoral thesis, ISAE-ENSMA, 2017
[P’4] A. Nait-ali, O. Kane-Dialo, S. Castagnet, Catching the time evolution of microstructure morphology from dynamic covariograms. Mécanique Accounts, 343 (4), p301-306, 2015.
[P’5] S Ben, Elhaj Salah, Non-local and stochastic modeling of materials with high property gradient by asymptotic development. Doctoral thesis, ISAE-ENSMA, 2019.
[P'6] D. Halm, F. Fouillen, E. Lainé, M. Gueguen, D. Bertheau, T. Van Eekelen, Composite pressure vessels for hydrogen storage in fire conditions: Fire tests and burst simulation, International Journal of Hydrogen Energy, 42 (31), p.20056-20070, 2017
EURAXESS offer ID: 495549
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