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Influence des mécanismes de plasticité sur la transition ductile fragile des aciers faiblement alliés. Etude de l'irradiation sur le comportement.

Abstract : The comprehension and modelling of brittle fracture mechanisms in pressure vessel steels is crucial, in particular to assess the integrity of pressure vessel reactors under accidental conditions. The local approach to fracture is one of the most used methods to predict the ductile / brittle transition in low alloyed steels [BER83], [BRO78], [CUR80]; [ZHA99] . In these models, the dispersion of fracture stresses only result from particles size distribution [HAH84], [BER83], [YAN03], [YAN04], [TAN03], [TAN05], [LEE02], [HAU05] , whereas mechanical heterogeneities are not taken into account. Several studies have shown that the initiation of brittle fracture could involve multiple microstructural crack initiators : lath packets boundaries, carbides or manganese sulfides.
Nevertheless, below a behaviour transition temperature Ta (about 25°C), plasticity mechanisms become strongly dependent on temperature. At low temperature, plastic deformation is controlled by the mobility of screw dislocations, whereas, at high temperature plastic deformation hardening result of the interactions between the mobile dislocations with the forest dislocations [KOCKS75], [KUBLV78].
The temperature dependence of plasticity mechanisms has been suggested to be one of the possible cause to explain that the local approach to fracture is not able to describe the whole ductile / brittle transition domain. Therefore, the description of the local stress heterogeneities can be considered as a major contribution to the improvement of local approach to fracture [MAT06].
In past years, crystal plasticity models have already been successfully developed to describe the effects of microstructural heterogeneities on mechanical fields and texture [PEI83], [BEADMK95], [TABFR98] [BARDJ01], [BARFC01], [HOC01], [BHAEDKD01] [ERIR04]. Such crystal plasticity models present great interest for the local approach of a fracture. As a matter of fact, crystal models take into account the effect of microstructural heterogeneities on the local mechanical fields, are able to describe plasticity mechanisms at the scale of a lath packet and their evolution along strain path.
The proposed model is able to describe continuously the evolution of plasticity mechanisms with temperature. The model leans on a viscoplastic constitutive law on each slip systems and depends on dislocation densities as internal variables. The evolution of local behaviour with temperature take into account :
- Thermally activated flow rule based on the double kink mechanism, which commands the motion of screw dislocations at low temperature. [LOU79]
- Hardening law based on the competition between the lattice friction and interaction with forest dislocations. [RAU93]
- Dislocation densities evolution law : competition between production and annihilation of dislocations.[ESTM84]

The interest of such a crystal plasticity model for local fracture is to include microstructural effects, temperature/strain rate evolutions and triaxiality ratio in criterion of rupture, thanks to the computation of the heterogeneities of the local stress field at the scale of a lath packet.

The following work focuses on modelling of stress heterogeneities under highly triaxial loadings in a A508 Cl3 (16MND5) low alloyed steel presenting a bainitic microstructure.
Identification of the model parameters is based on mechanical tests in a wide temperature range (-196°C,25°C) through inverse method and on microstructural observations (crystalline orientations and lath morphologies) by Electron Back Scattering Diffraction (EBSD) et dislocation density measurement with MET.
Simulations are conducted on two different polycrystalline aggregates obtained from EBSD mappings and constituted by three layers of elements. The aggregates are submitted to plane strain boundary conditions with different triaxiality ratio. Local stress fields are obtained for different temperature and triaxiality ratio.
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Maximilien Libert. Influence des mécanismes de plasticité sur la transition ductile fragile des aciers faiblement alliés. Etude de l'irradiation sur le comportement.. Sciences de l'ingénieur [physics]. Ecole Centrale Paris, 2007. Français. ⟨tel-00270833⟩

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