This research work involved the prediction of the fatigue life of an automotive rear suspension twistbeam assembly fitted to a vehicle travelling over a customer correlated durability route. This was achieved by making use of the integrated concepts of scaling and superposition of linear static finite element analysis being driven by experimental load data - the so called 'quasi-static time domain' approach. A study of the free body diagram of the twistbeam resulted in an indeterminate load set of some 24 components, with experimental data indicating that a state of static unbalance existed. Subsequent to developing a matrix-based generalised method ofload cell calibration to confum the foregoing, a modal technique was developed to partition the experimental data into a static load set, causing elastic deformations, and a rigid load set, imparting rigid body accelerations. The semi-independent characteristics of the twistbeam necessitated the coupling of large structural displacements with inertia relief. This required extensive modifications to the current techniques and led to the development and use of a three dimensional functional response matrix in place of the conventional two dimensional one. Recommendations concerning appropriate finite element boundary conditions were also formulated to handle these effects. Finally, the limitations of the uniaxial fatigue model were revealed under the application of a set of tools for analysing the biaxiality and mobility of the maximum absolute principal stress.

No description available.

En situation d’interaction pastille-gaine (IPG), les gaines de crayons combustible en alliage de zirconium sont susceptibles de rompre lors de transitoires de puissance incidentels dans les réacteurs à eau pressurisée, par un mécanisme de corrosion sous contrainte induite par l’iode (CSC-I). Cette étude traite de l’amorçage intergranulaire des fissures de CSC-I dans le Zircaloy-4 recristallisé, en milieu méthanol iodé à température ambiante, en s’intéressant particulièrement aux paramètres mécaniques critiques et à la concentration en iode. Pour cela, une approche mêlant expériences et simulations numériques a été adoptée. Un modèle de comportement mécanique macroscopique de l’alliage, viscoplastique et anisotrope, a été établi et validé sur une large gamme de sollicitations. Par la réalisation de nombreux essais de traction à vitesse de déformation imposée et de fluage en flexion quatre points, nous avons montré l’existence d’une concentration seuil I0 proche de 10-6 g.g-1 nécessaire à l’apparition du dommage de CSC-I, mais également celle d’une concentration de transition I1 proche de 2 10-4 g.g-1 au-delà de laquelle le mécanisme change, menant à un amorçage anticipé des fissures et une sensibilité réduite de l’endommagement aux paramètres mécaniques. L’importance de la concentration sur des paramètres tels que la densité des fissures, leur longueur moyenne et la vitesse de propagation intergranulaire et transgranulaire a été mise en évidence. Les résultats expérimentaux montrent que la déformation plastique macroscopique n’est pas indispensable à l’amorçage de fissures de CSC-I, pour un temps d’essais suffisamment long en présence de contrainte. Son principal effet est de précipiter l’apparition de fissures par la création de sites d’amorçage, par rupture de la couche d’oxyde et accumulation de contrainte intergranulaire. En dessous de I1 la détermination des déformations critiques à l’amorçage montre un fort effet de vitesse. Dans ce domaine, une contrainte seuil de 100 MPa a été déterminée, bien inférieure à la limite élastique. L’utilisation d’éprouvettes entaillées et de la simulation numérique a permis de mettre en évidence un fort effet protecteur de l’augmentation de la biaxialité des contraintes vis-à-vis de l’amorçage, dans le domaine élastique comme dans le domaine plastique. Des éprouvettes préalablement irradiées aux protons à une dose de 2 dpa ont été testées dans les mêmes conditions que les éprouvettes non irradiées. La sensibilité accrue du matériau irradié à la CSC-I a pu être quantifiée et nous avons constaté qu’un effet de concentration et un effet de vitesse de sollicitation subsistent après irradiation. L’irradiation induit une localisation plus importante de la déformation menant à un amorçage prématuré des fissures, mais une sensibilité chimique plus importante, tendance à la piquration et décalage de I1, apparaît comme le principal responsable de la plus forte sensibilité de l’irradié. / During the pellet-cladding interaction, Zirconium-alloy fuel claddings might fail when subjected to incidental power transient in nuclear Pressurized Water Reactors, by Iodine-induced Stress Corrosion Cracking (I-SCC). This study deals with the intergranular initiation of I-SCC cracks in fully recrystallized Zircaloy-4, in methyl alcohol solution of iodine at room temperature, with the focus on critical mechanical parameters and iodine concentration. It was carried out with an approach mixing experiments and numerical simulations. An anisotropic and viscoplastic mechanical behavior model was established and validated over a wide range of loadings. With numerous constant elongation rate tensile tests and four points bending creep tests, the existence of a threshold iodine concentration I0 close to 10-6 g.g-1 was highlighted, necessary to the occurrence of I-SCC damage, along with a transition concentration I1 close to 2 10-4 g.g-1. Above I1 the mechanism changes, leading to a sped up crack initiation and a loss of sensitivity towards mechanical parameters. The importance of concentration on parameters such as crack density, crack average length and intergranular and transgranular crack velocities was evidenced. Experimental results show that plastic strain is not required for I-SCC crack initiation, if the test time is long enough in the presence of stress. Its main influence is to rush the occurrence of cracking by creating initiation sites, by way of breaking the oxide layer and building up intergranular stress. Below I1, the critical strains at initiation show a substantial strain rate sensitivity. In this domain, a threshold stress of 100 MPa was found, well below the yield stress. Thanks to the combined use of notched specimens and numerical simulations, a strong protective effect of an increasing stress biaxiality ratio was found, both in the elastic and plastic domains. Proton-irradiated samples, up to a dose of 2 dpa, were tested in the same conditions as fresh specimens. The higher I-SCC sensitivity of the irradiated material was measured and the effects of concentration and strain rate were found to remain. Irradiation leads to a higher strain localization causing early crack initiation, but the main reason for the higher sensitivity of the irradiated material seems to be a chemical one, with higher pitting occurrence and a shift of I1.

This thesis presents results of novel methodologies applied to oriented systems. Both pure liquid crystalline materials as well as molecules oriented in liquid crystalline matrices have been studied. In particular this thesis presents investigations related to various aspects of NMR in liquid crystalline media, such as, assignment of resonances and the study of director dynamics of spinning liquid crystals in different phases and with different symmetry. Simplified methods for structure determination of solutes dissolved in liquid crystal solvents have been proposed. Diffusion ordered spectroscopy has been used to study a mixture of liquid crystals of opposite diamagnetic susceptibility at its coexistent phase. The methods presented represent novel techniques to characterize the liquid crystalline phase. NMR spectroscopy which has become a method of choice for understanding ordering mechanisms of mesogens requires a robust method for obtaining assignments of the NMR spectra of various nuclei that are found in the mesogens [1, 2]. It turns out that the spectra in the isotropic phase and in the nematic phase of a liquid crystal molecule are very different due to the presence of chemical shift anisotropy in the mesophase spectrum. There are a host of methodologies available for assigning spectra in the isotropic phase [3]. These methods however fail, when applied to the spectrum of the molecules in the mesophase due to the dominating role of strong anisotropic interactions, such as homonuclear couplings among protons. Problems arising while assigning spectral lines of liquid crystals in their nematic phase have been dealt with in chapter 2. To circumvent these problems, a property of the liquid crystal molecules under off-magic angle sample spinning can be utilized. It has been shown by Courtieu et al. [4] that the director/symmetry axis of a Δχ + ve liquid crystal aligns along the spinning axis for θ between 0 ° and θm, where θ is the angle between the spinning axis and the magnetic field and θm = 54.7° is the magic angle. It may be noted that the spectrum of θ = 0° spinning angle corresponds to the normal static spectrum, while the spectrum of θ = θm corresponds to the isotropic spectrum. In an earlier study, Teearr et al. [5] had recorded the 13C liquid crystal spectra as a function of very closely spaced θ values from 90° all the way up to 0°. From these plots of chemical shift versus the angle of spinning, it is possible to follow the trajectory of each 13C line from its position from θ = θm to θ = 0° and then match the spectrum in the isotropic phase (equivalently the magic angle sample spinning spectrum of the nematic phase) to the spectrum of the static sample in the nematic phase. However this method requires recording spectra at closely spaced angle intervals, so that one can unambiguously follow the trajectory of each of the lines without missing out any crossover of trajectories. However, this operation is time consuming. In this thesis we propose an alternate method, where we utilize the fact that the above trajectory has a very distinct relationship to the isotropic and anisotropic chemical shift and the problem of assignment does not require a continuous variation of angles, but just a few selected experiments should enable the assignment of the spectrum in the anisotropic phase. Thus the method of assignment has been made simpler and faster. It is shown that in addition to the assigned isotropic spectrum, only one other Off-magic angle spinning spectrum whose spinning angle θ is accurately known is necessary to obtain the complete assignment of the static spectrum. This procedure is non-trivial due to possibilities of errors in assignments arising out of inaccuracies in the knowledge of chemical shifts and the spinning angle. A computational procedure is proposed to take into account deviations arising out of non-ideal experimental conditions. A discussion regarding the details of the procedure and also situations where there can be ambiguities and how they can be resolved has been elaborated. The developed method has been demonstrated on a well known thermotropic liquid crystalline system, N-(4-ethoxybenzylidene)-4-n-butlyaniline [EBBA]. Since assignment of resonances in the nematic phase is a primary requirement for any further analysis regarding the ordering and deeper understanding of the role of various substituents in the mesogens we believe our novel prescription will be of immense use and utility. The third chapter presents the study of director dynamics in a lyotropic liquid crystal composed of Potassium laurate, 1-Decanol and D2O [6] under variable angle sample spinning using 2H NMR spectrum of D2O. A very interesting interplay of the magnetic orienting torque due to interaction of the liquid crystal director with the magnetic field and viscous torque arising from the viscosity of the sample on the director comes to fore. The relative magnitude of these torques has a direct bearing on the spectral pattern and line shapes observed, providing valuable insights into magnetohydrodynamics of the spinning liquid crystals. This study leads to even more interesting behavior for liquid crystals which deviate from uniaxial symmetry. This competition between magnetic and viscous torques has been quantitatively visualized by simulation of the 2H spectrum. It has been possible to visualize the observed spread in the director distribution arising out of viscous torque in terms of the energetics of the system under fast spinning. If the magnetic torque dominates over the viscous torque, then the equilibrium corresponds to the director orientation of δ = 0° where the energy is at its minimum. However the viscous and magnetic torques can become comparable as it may happen if the spinning angle is close to the magic angle or when the Δχ of the system is small. In those circumstances additional energy from the viscous torque causes the distribution of the director orientation to spread further away from δ = 0° for a positive Δχ liquid crystal. The trigonometric factor [P2(cosθ)∗P2(cosδ)] being proportional to the total energy of the system has been plotted against the spinning angle. The spectrum of the biaxial phase [7] as a function of the spinning angle shows more interesting director distribution. Here the patterns of the director distribution are observed on either side of the magic angle due to the presence of more than one director. The patterns observed also have information about the symmetry of the phase. This work provides insights into magnetohydrodynamics of spinning liquid crystals and can also be of relevance to samples of biological interest such as bicelles with protein oriented in them [8]. The fourth chapter deals with a novel characterization method relevant for the biaxial phase [9]. As an off shoot of the previous chapter, it effectively overcomes the disadvantages of the previous experimental methods which require simulation and line shape fitting to extract useful parameters. The chapter also presents the measurement of geometrical parameters of oriented solutes in phases exhibiting biaxial symmetry. The measured parameters show the effect of the onset of biaxiality as significant deviation in the value of the measured parameter. The utility of liquid crystalline media as solvents in high resolution NMR spectroscopy has been very rewarding since the pioneering work of Saupe and Englert [6]. The intramolecular interactions within solutes are only partially averaged. As a result one obtains a liquid like spectrum while at the same time very useful anisotropic interactions such as dipolar couplings, chemical shift anisotropies, quadrupolar couplings and anisotropic part spin-spin J couplings are extracted [10]. NMR spectra of molecules dissolved in thermotropic liquid crystals have long been used to obtain structural and orientational information. As the same time the complexity of the spectrum increases with the increase in the number of spins and the reduction in symmetry of the molecule, which can make the spectral analysis forbidding. Generally proton spectra have been used to obtain the geometry of the proton skeleton of the molecule and the information that includes dilute X nuclei such as 13C and 15N are available only from satellites which are buried in the intense proton spectrum. Different inequivalent dilute spins coupled to protons form different coupled spin systems in their natural abundance and appear as satellites in the proton spectra. Identification of transitions belonging to each of the spin system is essential to determine heteronuclear dipolar couplings, which is a formidable task. The fifth chapter deals with development of the techniques to obtain the complete structure of the dissolved molecules including nuclei other than protons in their natural abundance. The use of inverse experiments has been elaborated to overcome the problems of sensitivity and complexity for solute molecules having larger number of spins. In the present study using HSQC and HMQC experiments, we have selectively detected spectra of each inequivalent rare spin coupled to protons in pyrazine, pyrimidine and pyridazine dissolved in thermotropic Phase 4 and Phase 5 liquid crystal solvents. This way we could obtain enhancement in the intensity of satellites signals without the interference from the signals connected to the major isotopomers. Besides, we could resolve a complex spectrum into its sub-spectra corresponding to individual 13C and 15N isotopomers. This separation of the spectra corresponding to individual sub-spin systems makes analysis easy and helps analyze larger systems with higher number of spins and lower symmetry. Besides 1H-1H dipolar couplings, 13C-1H and 15N-1H dipolar couplings have been determined in natural abundance, thereby giving the complete dipolar coupling network between all the spins in the molecule. In this treatment pyrazine, pyrimidine and pyridazine have been used as examples of methodology developed. It is expected that the method will be of wider use for several other similar systems. Chapter six describes the diffusion ordered spectroscopic investigation [11] of a phase arising out of mixing together two liquid crystals having opposite signs of diamagnetic susceptibility anisotropy [12]. Towards this end we have used CH3CN as a probe molecule. The spectrum of CH3CN has with it the information about the parallel or perpendicular orientation of the phase. Such a mixture of liquid crystals have shown interesting behavior at the critical temperature where the two phases seem to coexist. It has been an interesting question to understand what exactly happens for the molecular orientation when the macroscopic anisotropy Δχ vanishes. Earlier Jokisaari et al. [13] have varied the temperature very finely taking due precautions to maintain homogeneity and stability of temperature to the tune of ±0.05K across the sample volume. Their observation of a powder pattern exactly in the critical temperature was interpreted as arising out of a distribution of directors equally oriented in all directions. In our experiments we have measured the diffusion coefficient of the probe molecule i.e. acetonitrile as we change the temperature of the system through the critical temperature. At the critical temperature we have a situation of being able to measure the parallel and perpendicular orientational diffusion coefficients simultaneously. The measurements show that the parallel component of the diffusion coefficient has reduced and the perpendicular component has increased in comparison to the trend in the immediate neighboring temperatures, thereby indicating that at the exact critical condition the liquid crystal mixture consists of an isotropic distribution of molecules. As a check to rule out any exchange of molecules in different domains of parallel and perpendicular orientations an EXSY experiment was conducted with a mixing time which was same as that of the diffusion delay in the DOSY experiment. The EXSY spectrum showed no exchange cross peaks between the two orientations, this confirms that the anisotropy of the diffusion vanishes at the critical temperature. Nematic liquid crystals exhibit a rich variety of phases and properties. NMR is a very powerful tool to study the various phases at the microscopic and molecular level. It has also turned out that some of these properties can be usefully utilized for investigation of both small and large molecules by NMR. Thus this thesis has attempted to expand several of the techniques already available for various applications and extend the utility of NMR for the study of partially ordered systems.

Cette étude porte sur la détermination d’un critère de rupture des gaines de combustible de Zircaloy-4 détendu hydruré contenant un blister d’hydrures, en conditions accidentelles représentatives d’un accident d’injection de réactivité. Deux plages de comportement différentes en fonction de la température ont clairement été mises en évidence grâce à l’étude bibliographique, aux différentes campagnes d’essais mécaniques et aux analyses des faciès de rupture des éprouvettes rompues : une rupture de type fragile pour la gaine à 25°C et une rupture ductile à 350°C.A 25°C, la rupture fragile a été traitée par une analyse globale en mécanique élasto-plastique de la rupture. A partir des essais mécaniques effectués à 25°C sur les gaines contenant des blisters, des simulations numériques par éléments finis ont été réalisées avec le code CAST3M. Des calculs d’intégrales-J en pointe de fissure ont alors permis d’identifier un critère de rupture en ténacité moyenne de 13,8 +/- 3,1 MPa.m1/2.A 350°C, une campagne d’essais biaxés de type pression interne couplée à la traction axiale a été réalisée sur des tronçons de Zircaloy-4 contenant des blisters, à des biaxialités des contraintes représentatives du RIA. Il a été montré que la rupture de la gaine, avec et sans blister, avait lieu de façon ductile, que la déformation diamétrale à rupture de la gaine diminuait lorsque la profondeur de blister augmentait, et que la biaxialité des contraintes n’avait pas d’effet sur la rupture des gaines contenant un blister suffisamment profond.Un modèle d’endommagement ductile couplé à la plasticité, basé sur un formalisme de type GTN, a été utilisé. Afin d’améliorer la description de l’endommagement des gaines de Zircaloy-4, une nouvelle source de germination de porosités liée au paramètre de Lode a été intégrée dans le modèle. L’évaluation de la triaxialité des contraintes et du paramètre de Lode dans les simulations numériques de la rupture ductile des gaines à 350°C a notamment permis de comprendre certaines tendances expérimentales. / This study deals with the determination of a fracture criterion for hydrided, cold worked and stress relieved Zircaloy-4 fuel cladding tubes with hydride blister, during a reactivity initiated accident. Two types of fracture profiles were identified, depending on the temperature, thanks to a bibliographical study, mechanical tests and fracture profiles analysis : brittle fracture at 25°C, and ductile fracture at 350°C.At 25°C, brittle fracture was studied by a global analysis in elasto-plastic fracture mechanic. Numerical simulations were performed by a finite element method with the CAST3M code, based on mechanical tests on fuel cladding tubes with blisters. Crack tip J-integral calculations were carried out to identify a mean fracture toughness of 13,8 +/- 3,1 MPa.m1/2.At 350°C, internal pressure combined to axial tensile tests were performed on Zircaloy-4 fuel cladding tubes with hydride blisters, at stress biaxialities corresponding to those of a RIA. It was observed a ductile fracture for tubes with and without blister. It was shown that hoop strain at failure decreases when blister thickness increases, and that stress biaxiality has no effect on cladding tubes bearing a thick blister. A ductile fracture model based on the GTN model was employed and a nucleation of voids due to shear stress was introduced, based on the Lode parameter. Stress triaxiality and Lode parameter were assessed in numerical simulations to understand some experimental observations.