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Theses

Compréhension et modélisation des mécanismes et des effets de géométrie dans la partie basse de la transition ductile-fragile

Abstract : The main goal of this thesis is to study the effect of temperature and geometry on the toughness of a ferritic steel in the lower part of the brittle to ductile transition zone. This work proposes to model the failure mechanisms observed in this zone using a local approach to failure. Although, according to experimental evidences, fracture in the transition zone results from a coupling between brittle and ductile fracture, the present thesis is mainly focused on the study of the lower part of the transition zone in which the fracture mechanism is mainly brittle. An experimental study of the material, a ferritic 18MND5 steel, is first proposed. An experimental database, consisting partially of existing tests was carried in order to have, at different temperatures, tests on uncracked specimens (TC and AE) mainly used for the study of elasto-plastic behaviour and tests on cracked specimens (CT and SENT) to study the fracture behaviour. A fractographic SEM study of the fracture surfaces allows, firstly, to describe the fracture mechanisms as a function of geometry and temperature and, secondly, to distinguish purely brittle specimens from specimens with ductile crack advance. Based on the results of the previous observations, plasticity and brittle fracture of the material are modelled. A new methodology for Weibull stress computation is proposed so as to ensure that it is correctly evaluated. Particularly, it is verified that Weibull computation is converged with respect to the mesh size. A procedure is proposed to filter out strong stress fluctuations. Cleavage is described using a modified version of Beremin's model which takes into account the effect of plastic strains on the Weibull stress. This formulation allows fitting, at a given temperature, of a unique set of parameters to model the fracture probabilities on different geometries (CT of different thicknesses and SENT). The fitted parameters are temperature dependent. In addition, the thesis also provided the opportunity to initiate the modeling of ductile failure in the transition. This includes the construction of a relevant experimental database which now covers the entire transition zone and the use of a non-local GTN type model which is fitted on axisymmetric notched bars. This work could constitute a starting point for future modelling of the coupling between ductile and brittle fracture in the transition.
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Submitted on : Monday, April 19, 2021 - 8:38:08 AM
Last modification on : Wednesday, November 17, 2021 - 12:28:39 PM
Long-term archiving on: : Tuesday, July 20, 2021 - 6:15:01 PM

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  • HAL Id : tel-03201543, version 1

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Aboubakr Amzil. Compréhension et modélisation des mécanismes et des effets de géométrie dans la partie basse de la transition ductile-fragile. Mécanique des matériaux [physics.class-ph]. Université Paris sciences et lettres, 2021. Français. ⟨NNT : 2021UPSLM002⟩. ⟨tel-03201543⟩

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