[NotiAMCA] Ph.D. topic on solid mechanics

[martin.idiart en ing.unlp.edu.ar]

Micromechanical modeling of the viscoplastic behavior of a porous  
polycrystal: application to an irradiated austenitic stainless steel


1. Background and Objectives

This thesis, situated within the research conducted at the IRSN, the  
French Institute for Nuclear Safety, concerns the study of the aging  
of core internals in pressurized water reactors (PWR). The core  
internals are pieces mainly made of austenitic stainless steels,  
located inside the vessel and contribute significantly to the safety  
of PWR. The term "aging" relates to all the mechanisms which alter, in  
time, the initial characteristics of these materials. New models for  
the behavior laws are needed in order to characterize the mechanical  
properties of these materials over extended periods of time (over 40  
years of service).

The microstructure of the materials used for the internals changes  
considerably during their use in the reactor and becomes relatively  
complex (presence of defects, dislocation loops, interaction between  
dislocations). In addition, cavities can appear within the  
polycrystalline structure of these materials during the irradiation  
process. The presence of irradiation defects and their interactions  
first lead to a hardening of these steels. It seems that this  
phenomenon tends to saturate for a relatively low radiation dose  
(compared to the doses reached for internals during 40 years). For  
higher doses, the hardening of the material quickly becomes negative  
which corresponds to an overall softening of the material.

Several constitutive laws have been proposed in the literature both at  
the grain scale (for a single crystal), and at the scale of the  
polycrystal, for which the specific models are based on non-linear  
homogenization methods. However, these models are not appropriate to  
describe the behavior of viscoplastic porous materials, where strong  
heterogeneities are expected around the cavities. In addition, the  
softening phenomenon observed for the high doses is difficult to model  
with current homogenization methods for viscoplastic polycrystals.

The thesis aims to study the effect of the presence of intragranular  
cavities on the behavior of viscoplastic polycrystals and in  
particular for a highly irradiated austenitic stainless steel. It must  
propose new models based on new homogenization methods adapted to this  
problem. The thesis must provide a tool able to give answers to  
important questions from the safety point of view, concerning the  
effect of a given porosity (due to the presence of intragranular  
cavities) on the overall viscoplastic behavior.

The chosen models will be implemented in the CRAFT program, developed  
at CNRS-LMA (Marseille, France) which allows to calculate the  
mechanical response of a heterogeneous material (2D or 3D periodic  
cell) using Fast Fourier Transforms (FFT) numerical simulations.

2. Scientific Challenges

Behavior laws known for a single crystal involve a large number of  
internal variables (dislocation density, density of dislocation loops)  
that evolve during loading, leading to various critical shear stresses  
on slip planes. The presence of cavities influences the evolution of  
the internal variables and should induce strong heterogeneities of  
deformation in the grains of the polycrystal, which should lead to  
highly heterogeneous grains. These strong heterogeneities in the  
behavior of the grains are difficult to take into account by the  
homogenization methods (methods that are means fields theories).

The second scientific challenge is about modeling the softening  
observed in the highly irradiated steels. In general, the  
homogenization methods for viscoplastic polycrystals are written using  
imposed effective stresses, making immediate use difficult in order to  
describe softening. The model to be developed should be written using  
imposed deformation.

3. Tentative program

There are distinct parts relative to the modeling and the numerical  
simulations. These parts can not be independently treated. First,  
perform numerical simulations with the FFT method (CRAFT software) in  
order to get significant results concerning the effect of the  
intragranular porosity (shape and distribution) on the overall  
behavior of a CFC polycrystal. Then, a polycrystalline homogenization  
model will be extended considering specific laws for the  
microstructure evolution (due to the loading). All this work will be  
performed with fixed flow stresses in the sliding planes. In the last  
step of the study, it is planned to take into account the evolution of  
these flow stresses as a function of the internal parameters.

4. Applicants

Applicants:
must hold a five-year degree in mechanical engineering or equivalent.
should have good knowledge on mechanics of materials, numerical  
analysis, and mathematics.
should have strong scientific programming skills.

5. Employment conditions

The thesis will be performed:
for 18 months, at the Laboratoire de Mécanique et d'Acoustique  
(CNRS-LMA), Marseille (France).
for 18 months, at the Universidad Nacional de La Plata, La Plata (Argentina).

A grant form the AMIDEX program of the Aix Marseille Université is  
expected (but not guaranteed to date).

Commencement of thesis work: October 2015.

6. Supervisors

UNLP supervisor: Martin IDIART
AMU supervisor: Mihail GARAJEU
IRSN: Pierre-Guy VINCENT


7. Additional information

For application, please send an email to pierre-guy.vincent en irsn.fr