L'objectif de ce travail est d'étudier l'influence de l'état de dispersion de la silice sur les propriétés viscoélastiques et mécaniques des élastomères chargés silice pour l'application pneumatique. En effet, il est montré qu'une grande partie de la résistance au roulement dépend de la dissipation d'énergie de l'élastomère chargé. De plus, les propriétés mécaniques pourraient intervenir dans les propriétés ultimes telles la propagation de fissure en fatigue et l'usure : l'étude de ces différentes propriétés est donc primordiale. La première partie s'attèle donc à l'identification des leviers pouvant moduler la dispersion de la silice dans les élastomères. La dispersion pouvant être vue comme la compétition entre les forces de cohésion des charges et les forces appliquées au système pour les rompre, ces paramètres ont été étudiés de manière systématique. Enfin, l'influence des interactions silice-matrice est abordée (modification de l'état de surface de la silice, nature de l'élastomère) : le nombre d'interactions silice-élastomère par chaîne de polymère pourrait intervenir dans les mécanismes de dispersion. Dans la seconde partie, l'influence de ces différents états de dispersion sur les propriétés viscoélastiques et mécaniques est discutée. L'amélioration de la dispersion à iso-formulation permet de diminuer le module élastique linéaire dans le domaine linéaire et entraîne une augmentation des modules aux grandes déformations en traction. Concernant l'étude des propriétés ultimes, notre dispositif expérimental n'a pas permis de mettre en évidence, sur les formulations testées, d'influence de la dispersion sur la dynamique de propagation de fissure en fatigue. En revanche, l'amélioration de la dispersion entraîne une amélioration de la résistance à l'usure et ce malgré une diminution de la dureté / Filled elastomers are used in tread tires. It has been demonstrated that most of rolling resistance of tires is due to filled elastomer energy dissipation. In that way, understanding viscoelastic properties of these materials is a key point. Then, filled elastomer behavior at high deformations may be involved in ultimate properties of tire application such as fatigue crack propagation and wear. The aim of this work is to study the impact of silica dispersion on viscoelastic and mechanical properties of filled elastomers. First, levers impacting silica dispersion are evaluated. Dispersion of fillers can be considered as a competition between fillers cohesion forces and applied forces to the system to break them, these parameters have been studied methodically. Finally, the impact of silica-matrix interactions is studied, changing silica surface treatments or elastomer natures. The quantity of interactions possible per polymer chain between the silica and the elastomer may play a role in silica dispersion. Secondly, the impact of silica dispersion on viscoelastic and mechanical properties is discussed. It is shown than increasing silica dispersion leads to a decrease of linear elastic modulus and an increase of reinforcement in tensile at high deformations. Finally, regarding ultimate properties, our experimental device on the selected formulations has not shown any impact of silica dispersion on fatigue crack propagation. Nevertheless, we observe a better wear resistance with increasing dispersion, despite the lower materials hardness

This Ph.D. thesis deals with methodology for determination of residual fatigue lifetime of railway axles based on damage tolerance approach. This approach accepts an existence of potential defect, which could lead to fatigue failure of whole axle. The behavior of crack in railway axle is described by approaches of linear elastic fracture mechanics. There are plenty of factors, which more or less influence determined residual fatigue lifetime. The aim of this thesis is to quantify effects of these factors. The first part of Ph.D. thesis represents overview of studied problems relating to fatigue damage of railway axles. This part is focused on parameters, which influence fatigue crack growth in railway axle materials. The second part of thesis shows procedure for determination of residual fatigue lifetime, which was developed at the Institute of Physics of Materials of the Academy of Sciences of the Czech Republic. The main aim of this thesis was to improve current procedure for more precise estimation of residual fatigue lifetime. Significant part of this work is determination of significance of studied factors, which influencing calculated residual fatigue lifetime of railway axles (e.g. effect of threshold value, load spectrum, retardation effects, residual stress, axle geometry, material of axle etc.). The procedures described and results obtained can be also used for determination of residual fatigue lifetime of general mechanical component (not only railway axles). Therefore, results obtained in this Ph.D. thesis can be used e.g. for assessment of regular inspection intervals of cyclically loaded general mechanical parts.

This Ph.D. thesis deals with methodology for determination of residual fatigue lifetime of railway axles based on damage tolerance approach. This approach accepts an existence of potential defect, which could lead to fatigue failure of whole axle. The behavior of crack in railway axle is described by approaches of linear elastic fracture mechanics. There are plenty of factors, which more or less influence determined residual fatigue lifetime. The aim of this thesis is to quantify effects of these factors. The first part of Ph.D. thesis represents overview of studied problems relating to fatigue damage of railway axles. This part is focused on parameters, which influence fatigue crack growth in railway axle materials. The second part of thesis shows procedure for determination of residual fatigue lifetime, which was developed at the Institute of Physics of Materials of the Academy of Sciences of the Czech Republic. The main aim of this thesis was to improve current procedure for more precise estimation of residual fatigue lifetime. Significant part of this work is determination of significance of studied factors, which influencing calculated residual fatigue lifetime of railway axles (e.g. effect of threshold value, load spectrum, retardation effects, residual stress, axle geometry, material of axle etc.). The procedures described and results obtained can be also used for determination of residual fatigue lifetime of general mechanical component (not only railway axles). Therefore, results obtained in this Ph.D. thesis can be used e.g. for assessment of regular inspection intervals of cyclically loaded general mechanical parts.

Fatigue in concrete is a complex phenomenon involving formation of microcracks, their coalescence into major crack and simultaneous formation of the fracture process zone ahead of the crack tip. Complex phenomena are best dealt through an energy approach and hence it is reasonable to use the theory of thermodynamics. Fracture mechanics and damage mechanics are two theories that are based on physically sound principles and are used to describe failure processes in materials. The former deals with the study of macroscopic cracks, whereas the latter defines the state of microcracking. In this study, the concepts from these theories are utilized to improve our understanding and modeling of fatigue process in concrete. In this thesis, a closed form expression for the thermodynamic function entropy is proposed and examined for its size independency and its use as a material property to characterize failure of concrete under fatigue. In the thermodynamic formalism, dissipative phenomena are described by a dissipation potential or its dual, from which evolution laws for internal variables could be defined. In this work, closed form expressions for dual of dissipation potential are derived using concepts of dimensional analysis and self-similarity within the framework of fracture mechanics and damage mechanics. Consequently, a fatigue crack propagation law and a fatigue damage evolution law are proposed respectively. A method is proposed in this study to correlate fracture mechanics and damage mechanics theories by equating the potentials obtained in each theory. Through this equivalence, a crack could be transformed into an equivalent damage zone and vice versa. Also, damage state corresponding to a given crack in a member can be quantified in terms of a damage index. An analytical way of computing size independent S-N curves is proposed, using a nonlocal damage theory by including aggregate size and specimen size in the formulation. It is realized from this study that fracture mechanics and damage mechanics theories should be used in a unified manner in order to accurately model the process of fatigue in concrete. Furthermore, based on the models developed in this study, several damage indicators for fatigue of concrete are proposed. The advantages and limitations of each of these indices are presented such that, the relevant damage index could be used, based on available parameters. Additionally, deterministic sensitivity studies are carried out to determine the most important parameters influencing fatigue life of a concrete member.

Les outillages de mise en forme à chaud sont soumis à des sollicitations thermomécaniques transitoires. Ils sont donc confrontés à la fissuration par fatigue. Le but de ce travail est d'étudier le comportement et la durée de vie en fatigue des aciers d'outillage, notamment leurs résistances à la propagation de fissure. Dans un premier temps, les chargements thermiques imposés aux outillages de fonderie sous pression de magnésium et d'emboutissage à chaud ont été évalués. Ensuite, le comportement et la durée de vie de l'acier à haute conductivité thermique HDC1 ont été étudiés et comparés à l'acier AISI H11 (acier de référence) en fonction de la température. L'acier HDC1 présente un adoucissement cyclique stable à 20°C et 300°C. Par contre, l'intensité d'adoucissement est plus importante à hautes températures. La durée de vie a été étudiée en utilisant les lois de Manson-Coffin et de Basquin. A hautes températures, l'oxydation devient un mécanisme d'endommagement primordial pour l'acier HDC1 et provoque des durées de vie plus courtes que celles observées sur l'acier AISI H11. La résistance à la propagation de fissure de fatigue a été déterminée dans des aciers à la température ambiante par le biais de critères : l'amplitude de facteur d'intensité de contrainte (∆K) et l'amplitude d'ouverture en pointe de fissure (∆CTOD). La méthode de corrélation d'images a permis de mesurer (∆COD) et d'évaluer (∆CTOD). L'ensemble de ces résultats a permis de mettre en évidence l'effet de fermeture de fissure et le comportement plastique en pointe de fissure. ∆CTOD présente un bon critère pour rationaliser la propagation de fissure dans les aciers étudiés.Enfin, la simulation numérique de la propagation de fissure en fatigue a été menée dans l'acier AISI H11 à 600°C par la méthode de relâchement des nœuds en éléments finis. L'effet des modèles de comportement monotone (élastoplastique) et cyclique (élastoviscoplastique) a été étudié sur le calcul de l'ouverture de fissure et la plasticité en pointe de fissure. L'influence du modèle de comportement est faible sur le calcul de l'amplitude d'ouverture de fissure ∆COD, du fait d'une plasticité confinée en pointe de fissure. Alors que, le modèle de comportement cyclique est mieux adapté pour décrire la plasticité en pointe de fissure. / Hot metal forming tools are subjected to cyclic thermomechanical loading and damage by complex fatigue/wear/oxidation interactions. Thermal solicitations were measured on high pressure die casting and hot stamping processes. Based on thermal measurements, the isothermal fatigue behaviour and lifetime of a new high conductivity steel HDC1 were investigated at different temperatures and strain amplitude then compared to AISI H11 steel. As AISI H11, continue cyclic softening was observed in HDC1 at all temperatures. The Manson-Coffin and Basquin laws were used for life prediction models under different temperatures. It was observed that the fatigue/oxidation interaction was a principal damage mechanism of the HDC1 steel at high temperature. Fatigue crack propagation in steels was investigated at room temperature in SENT specimens. A digital image correlation technique was used to evaluate crack opening (∆COD) and crack tip opening displacement (∆CTOD) ranges. Crack growth rate were investigated using ∆K (Paris law) and ∆CTOD criteria. It was observed that the cyclic crack tip plasticity control the crack propagation resistance. Crack closure could be evaluated by ∆CTOD.Finite element method by debond technique was used to model the crack propagation of AISI H11 at 600°C using both monotonic elasto-plastic (EP) and cyclic elasto-viscoplastic (EVP) constitutive laws materials. The comparison of ∆COD calculated and measured had shown that monotonic EP and cyclic EVP had no significant effect on the ∆COD, mainly due to the small-scale yielding conditions. It is however observed that the cyclic constitutive law was the best suitable model for the crack tip plasticity effect.