Soutenance de Thèse Lukasz FARBANIEC

Université Paris 13, Sorbonne Paris Cité, Institut Galilée
Laboratoire des Sciences des Procédés et des Matériaux
Soutenance de Thèse
Lukasz FARBANIEC
Spécialité : Mécanique option Matériaux
DEFORMATION MECHANISMS AND FRACTURE STRENGTH OF POLYCRYSTALLINE ULTRAFINE-GRAINED
MATERIALS : EXPERIMENTAL AND NUMERICAL INVESTIGATIONS
mardi 6 novembre 2012 à 14 h
Salle de Conférence du Bât. L1
The objective of this thesis is to provide insights of two fundamental topics regarding the deformation and failure
mechanisms of ultrafine face-centered cubic metals. The first topic considers the mechanisms of plastic deformation as a
desired grain fragmentation process in order to achieve a significant refinement of structural elements. For this purpose,
the so-called Dynamic Plastic Deformation (DPD) technique is used to cause the grain refinement of polycrystalline nickel.
The second topic deals with the effect of the stress state on ductile toughness along with identification of the mechanisms
leading to tensile failure. To this end, a small-scale experimental and numerical modeling techniques are developed and
implemented to predict fracture behavior of polycrystalline nickel and steel.
The DPD method has been recently developed and applied for the high stacking fault energy materials. This study
is devoted to the intermediate stacking fault energy nickel. It is shown that the increase in strength induced by plastic
strain appear as a result of various grain subdivision mechanisms. The strain, strain rate, deformation temperature and
stacking fault energy play a key role in this process. The obtained microstructure is investigated by electron
backscattering diffraction, transmission electron microscopy, x-ray diffraction and x-ray line profile analysis. The influence
of the deformation microstructure on mechanical properties is investigated by means of uniaxial quasi-static compression
tests at room temperature. The results of the microstructural characterization reveal the presence of the lamellar structure
composed mainly of small disoriented crystallites due to dislocation accumulation into sub-grain boundaries. The texture
after deformation exhibits preferential orientations including a strong fiber texture. Compression tests show an increased
yield strength compared to the initial state and dependence of strain hardening on the orientation of the compression axis
relative to the impact direction. Subsequently, a fast Fourier transform numerical procedure is applied to
the microstructure after deformation. The results show that the fiber texture together with the lamellar structure strongly
affect the mechanical behavior depending on the direction of the strain.
The main purpose of the second topic is to develop an effective procedure for fracture toughness estimation
in situations where the available stock of a material is insufficient to generate conventional fracture toughness specimens.
For this purpose, tensile tests on smooth and notched round bar specimens are performed to estimate the fracture strain
in a wide range of stress triaxiality. The capability of the local approach to fracture methodology to reproduce and predict
physical failure behavior is examined. It is shown that stress triaxiality played a major role on damage evolution as
demonstrated by the progressive reduction of material ductility under increasing triaxial state of stress. Consequently,
the obtained results are used to formulate a criterion applied to the crack tip situation. The numerical results are compared
with the experimental data to validate the procedure and estimate fracture toughness of the investigated material.
Composition du jury :
J. BESSON - HDR, École des Mines de Paris - Rapporteur
R. BRENNER - HDR, Institut d'Alembert - Examinateur
A. ABDUL-LATIF - Professeur, Université Paris 8 - Examinateur
J. LI - Professeur, Université Paris 13 - Directeur de thèse
Contact : [email protected]
K. WIERZBANOWSKI - Professeur, AGH, Pologne - Rapporteur
M. GASPERINI - Professeur, Université Paris 13 - Examinateur
H. COUQUE - HDR, Nexter Munitions, Bourges - Directeur de thèse
G. DIRRAS - Professeur, Université Paris 13 - Directeur de thèse