Sandwich structures constitute basic components of advanced supersonic/hypersonic flight and launch vehicles. These advanced flight vehicles operate in hostile environments consisting of high temperature, moisture, and pressure fields. As a result, these structures are exposed to large lateral pressures, large compressive edge loads, and high temperature gradients which can create large stresses and strains within the structure and can produce the instability of the structure. This creates the need for a better understanding of the behavior of these structures under these complex loading conditions. Moreover, a better understanding of the load carrying capacity of sandwich structures constitutes an essential step towards a more rational design and exploitation of these constructions. In order to address these issues, a comprehensive geometrically non-linear theory of doubly curved sandwich structures constructed of anisotropic laminated face sheets with an orthotropic core under various loadings for simply supported edge conditions is developed. The effects of the radii of curvature, initial geometric imperfections, pressure, uniaxial compressive edge loads, biaxial edge loading consisting of compressive/tensile edge loads, and thermal loads will be analyzed. The effect of the structural tailoring of the facesheets upon the load carrying capacity of the structure under these various loading conditions are analyzed. In addition, the movability/immovability of the unloaded edges and the end-shortening are examined. To pursue this study, two different formulations of the theory are developed. One of these formulations is referred to as the mixed formulation, While the second formulation is referred to as the displacement formulation. Several results are presented encompassing buckling, postbuckling, and stress/strain analysis in conjunction with the application of the structural tailoring technique. The great effects of this technique are explored. Moreover, comparisons with the available theoretical and experimental results are presented and good agreements are reported. / Ph. D.

Honeycomb core sandwich structures are an integral part of many of today's aerospace structures. When subjected to high-speed flight, thermal loading can induce significant stresses. The need for thermal properties to perform thermal stress analyses in these structures is the motivation behind this research. The thermal property estimation approach used here involves the minimization of a least-squares function containing both measured and calculated values. In addition, an applied heat flux is necessary at one boundary for the simultaneous estimation of thermal properties. The specific objectives are to develop a thermal model to describe honeycomb core sandwich structures, optimize experimental designs for use in parameter estimation, develop a finite element-based parameter estimation algorithm, and estimate the pertinent thermal properties of the structure. A combined conductive/radiative heat transfer model was used for the analysis of the structure. Due to the composition of the structure, it was determined that a one-dimensional model would be sufficient. This model was used in both parameter estimation and experimental design. Experimental design involves finding input variables for an experiment such that the response of the system contains the highest possible amount of information on the parameters of interest which characterize the response. In this study, the design was performed by using a combination of two methods. The first involved maximizing the temperature derivatives with respect to unknown thermal properties. The second involved a scaled confidence interval approach. The experimental parameters optimized were heating time and total experiment time. A finite element program was used to perform transient temperature calculations because of the flexibility it has to analyze complex structures. Parameters estimated in this study exhibited a great deal of correlation, or interaction. This showed the need for a constrained parameter estimation algorithm. A penalty function method was developed for this purpose. The last part of this study involved the actual estimation of thermal properties. An experimental apparatus was designed and built to record the transient temperature response of the test sample. A four-sheet SPF/DB sandwich was used as the test sample. Thermal properties were estimated using four combinations of sensors and boundary conditions. It was found that in one case parameters could be simultaneously estimated despite the presence of correlation. These estimated parameters were shown to produce reasonably small errors when used in transient temperature calculations. It was also shown that large temperature gradients produce estimates with smaller confidence intervals. The importance of maintaining accurately known boundary conditions was also demonstrated. / Master of Science

An experimental investigation was undertaken to analyze the behavior of sandwich composite structures exposed to one-sided heating where a debond exists between the unexposed facesheet and core material. Sandwich composites of plain weave E-glass/epoxy facesheets and an end-grain balsa wood core manufactured using the Vacuum Assisted Resin Transfer Molding (VARTM) technique were the only materials analyzed. These were selected due to their current use in naval vessels and the heightened interest in the fire response properties of balsa wood and its utility as a core material. In order to better understand the interfacial behavior, Mode I Double Cantilever Beam (DCB) fracture tests were performed at ambient, 60 C, and 80 C to determine the influence of the decreased Mode I fracture toughness. While ambient testing showed that stable crack growth could be obtained, high temperature tests resulted in considerable damage occurring to the core at the crack-front preventing stable crack growth. This can be attributed to the significant decrease in the balsa core strength and material properties even for small increases in temperature. Additionally, Mode II Cracked Split Beam (CSB) tests were performed at ambient temperature to examine the sliding dominant crack-growth. Again, the occurrence of balsa core damage prevented stable crack-growth and an accurate measurement of Mode II fracture toughness was not obtained. Intermediate-scale compression testing with one-sided heating at two heat flux levels was performed with a custom designed load frame on sandwich composite columns. This enabled the influence of the debond to be measured using a 3D-Digital Image Correlation (DIC) technique spatially linked with a thermographic camera. The DIC allowed for a detailed observation of debond growth and buckling prior to global failure of the test article. A behavior similar to that observed in the Mode I DCB fracture tests occurred: as the interfacial temperature increased, the amount of crack growth decreased. This crack growth was followed by a core failure at the crack-front, triggering a global failure of the test article. This global failure for test articles containing a debond manifested itself primarily as an anti-symmetric post-buckling shape. Test articles with no debond exhibited the typical progression of the out-of-plane displacement profile for a fixed-fixed column. As the out-of-plane displacement increased, core failure ultimately occurred near the gripped region where the zero-slope condition is required, triggering global failure of the no debond test article. These tests highlight that the reduction in strength and material properties of the end-grain balsa wood core significantly outweigh the reduction in interfacial fracture toughness due to the increased temperatures. / Master of Science

Yes / An evaluation of the in vitro cytotoxicity of nine electron-deficient half-sandwich metal complexes towards two colorectal cancer cell lines (HCT116 p53+/+, HCT116 p53-/-) and one normal prostate cell line (PNT2) is presented herein. Three complexes were found to be equally cytotoxic towards both colorectal cancer cell lines, suggesting a p53-independent mechanism of action. These complexes are 12 to 34  more potent than cisplatin against HCT116 p53+/+ and HCT116 p53-/- cells. Furthermore, they were found to exhibit little or no cytotoxicity towards PNT2 normal cells, with selectivity ratios greater than 50. To gain an insight into the potential mechanisms of action of the most active compounds, their effects on the expression levels of a panel of genes were measured using qRT-PCR against treated HCT116 p53+/+ and HCT116 p53-/- cells, and cell cycle analysis was carried out. / The Royal Society grant UF150295, The Academy of Medical Sciences grant SFB003\1170

Yes / The synthesis, characterisation and evaluation of the in vitro cytotoxicity of four indole-based half-sandwich metal complexes towards two ovarian cancer cell lines (A2780 and A2780cisR) and one normal prostate cell line (PNT2) are presented herein. Although capable of inducing catalytic oxidation of NADH and able to reduce NAD+ with high turnover frequencies, in cells and in the presence of sodium formate, these complexes also strongly interact with biomolecules such as glutathione. This work highlights that efficient out-of-cells catalytic activity might lead to higher reactivity towards biomolecules, thus inhibiting the in-cells catalytic processes.

Yes / The synthesis, characterisation, and evaluation of the in vitro cytotoxicity of five maleonitriledithiolate-based ruthenium metal complexes bearing various phosphine ligands towards two ovarian cancer cell lines (A2780 and A2780cisR), one non-small-cell lung cancer cell line (H460) and one normal prostate cell line (PNT2) are presented herein. These 18-electron complexes were designed with four water-soluble phosphine ligands to increase the water-solubility character of the corresponding electron-deficient ruthenium complex which showed great in vitro promises, and triphenylphosphine for comparison. The complexes with triphenylphosphine-3,3',3''trisulfonic acid and triphenylphosphine present similar cytotoxicity compared to the 16-electron precursor, with equal cytotoxicity to both A2780 and A2780cisR. Hints at the mechanism of action suggest an apoptotic pathway based on ROS production. No toxicity was observed in preliminary in vivo pilot studies for these two complexes in subcutaneous A2780 and A2780cisR xenograft models, with some evidence of tumour growth delay. / The support of the Royal Society (University Research Fellowship No. URF150295, and RGF\EA\201001), the Academy of Medical Sciences/ The Wellcome Trust/ The Government Department of Business, Energy and Industrial/ The British Heart Foundation Springboard Award (SBF003\1170), and the CNRS is acknowledged. LRP is supported by a PhD studentship funded by the University of Bradford.

Lightweight sandwich composite structures exhibit high strength and bending stiffness and have been used in various load-bearing aerospace structures. A primary weakness of these structures is the low interfacial strength that can result in the disbonding between the facesheet and the core. Through-thickness reinforcement techniques, such as stitching, can be utilized to improve the interfacial strength of sandwich composites. This study focuses on fabrication and structural testing of stitched sandwich composite main landing gear (MLG) strut doors. The MLG strut doors are constructed from carbon fiber non-crimp fabric facesheets and closed-cell foam core that are stitched using a modified lock stitching technique. To assess the effects of stitching, two doors were fabricated, one with and one without through-the-foam stitching. A vacuum-assisted resin transfer molding process was used to infuse the doors. The strut doors were subjected to quasi-static loading, and the mechanical responses of the stitched and unstitched strut doors are presented.

Dans ce travail de thèse, une méthode d’identification inverse des propriétés mécaniques du matériau viscoélastique (module de cisaillement et facteur de perte) fonctionnalisé dans une structure sandwich à trois couches symétriques est proposée. L’objectif de ce travail est de pouvoir identifier les propriétés mécaniques in situ. A travers un modèle éléments finis basé sur le modèle Zig-Zag de Rao, notre modèle assure la détermination des paramètres modaux de la poutre sandwich. L'approche inverse consiste en une procédure itérative qui détermine les formes de mode étant donné les paramètres matériaux puis calcule les propriétés viscoélastiques à partir des modes en utilisant un quotient de Rayleigh jusqu'à ce que la convergence sur les propriétés du matériau soit satisfaite. Les paramètres d’entrée du modèle inverse sont les fréquences de résonance et facteurs de perte de la poutre sandwich obtenues expérimentalement. En conséquence, la dépendance en fréquence des propriétés du matériau viscoélastique de la poutre sandwich est déterminée par une démarche automatisée. La méthode a été comparée avec succès aux formules de Ross-Kerwin-Ungar ; à une approche d'optimisation standard et à la littérature. A partir des résultats, nous avons pu déduire les lois de comportement du cœur viscoélastique suivant des modèles rhéologique tels que le modèle de Maxwell généralisé, d’ADF, de GHM et du Zéner fractionnaire. Ce dispositif expérimental couplé à la méthode d’indentification a permis l’investigation des paramètres modaux de la poutre à différentes températures pour étudier l’effet de la température sur les lois rhéologiques. Pour étudier la robustesse de notre méthode, nous avons procédé des essais de reproductibilité, de reproductibilité sur une population d’échantillon. L’efficacité de notre méthode étant été prouvée, une étude de sensibilité a été menée sur les caractéristiques géométriques de notre structure et les paramètres d’entrée. Les résultats obtenus montrent le fort impact de certains paramètres sur l’identification / In this work, a inverse identification method of the mechanical properties of the viscoelastic material (shear modulus and loss factor) functionalized in a sandwich structure with three symmetrical layers is proposed. The objective of this work is to be able to identify the mechanical properties in situ. Through a finite element model based on Rao's Zig-Zag model, our model ensures the modal parameter determination of the sandwich beam. The inverse approach consists of an iterative procedure that determines the mode shapes given the material parameters and then calculates the viscoelastic properties from the modes using a Rayleigh quotient until convergence on the properties of the material is satisfied. The input parameters of the inverse model are the resonance frequencies and loss factors of the sandwich beam obtained experimentally. As a result, the frequency dependence of the viscoelastic properties of the sandwich beam is determined by an automated way. The method has been successfully compared to Ross-Kerwin-Ungar formulas; a standard optimization approach and the literature. From the results, we have been able to deduce the constitutive laws of the viscoelastic heart according to rheological models such as the generalized Maxwell model, ADF, GHM and fractional Zener. This experimental device coupled to the method of identification allowed the investigation of modal parameters of the beam at different temperatures to study the effect of the temperature on the rheological laws. To study the robustness of our method, we carried out tests repeatability, reproducibility on a sample population. Since the effectiveness of our method has been proven, a sensitivity study has been carried out on the geometrical characteristics of our structure and the input parameters. The results obtained show the strong impact of certain parameters on identification

Durant le vol d'un aéronef, la collision avec un oiseau est un risque important que les autorités de certification imposent de prendre en compte. Dans le cas du choc sur pointe avant, la protection du fond pressurisé est assurée par un bouclier. La compréhension du comportement d'une telle structure sandwich sous impact est essentielle pour permettre l'amélioration des boucliers existants. Ces travaux ont pour buts de comprendre l'influence des différents paramètres de conception du bouclier sur son comportement et sur la protection de la cible, et de mettre en place une méthodologie pour réaliser une telle étude. Pour cela, un modèle éléments finis générique est créé pour être utilisé dans l'étude paramétrique. Une méthode de mesure de la déformée est proposée pour permettre la comparaison rapide d'un grand nombre de cas et la compréhension du comportement de chaque bouclier. Elle s'appuie sur la décomposition de la déformée en trois modes : Indentation, Flexion et Écrasement. Une étude de criblage est ensuite réalisée pour classer les paramètres de définition par ordre d'influence. L'étude paramétrique est réalisée sur les six paramètres les plus influents. Un plan d'expérience de type carré Latin est choisi et sept grandeurs différentes sont suivies. Le cadre des processus gaussiens est utilisé pour créer des modèles réduits, qui sont utilisés pour étudier l'évolution du comportement du bouclier sur l'ensemble du domaine à l'aide d'analyses de sensibilité. Les effets de chaque paramètre sont identifiés et expliqués. Enfin, une méthode pour l'utilisation de ces modèles réduits dans le cadre d'optimisations est proposée. / During an aircraft flight, the possible collision with a bird is a major threat, and the certification authorities require to take ît into account. In the case of a nose strike, the pressurized bulkhead is protected by a shield. Understanding the behaviour under impact of such a sandwich structure is essential. This work has two main goals: understanding the design parameters influence on the shield behaviour, and propose a methodology to conduct this study. Firstly, a generic finite element model is created to be used in a parametric study. A tool to measure the shield deformation is proposed to make it possible to easily compare the behaviour of different shields and to help understanding the behaviour of a shield. This tool is based on the projection ofthe shield deformation on a basis comprising three modes: Indentation, Bendîng and Crushing. A screening study is then conducted to rank the design parameters with respect to their influence. A parametric study is then conducted on the six first parameters. A Latin hyper-square is used for the design of experiment and seven different quantifies are studied. The Gaussian processes framework is used to create surrogates models. Global sensitivity analyses are then conducted to study the variation of the shield behaviour in the whole design space. The effects of each parameterare measured and explained. Finally, a method to minimize the shield mass, using the surrogate models to enforce minimal target protection criteria, is presented.

Ce travail de thèse porte sur l'étude de l'influence des micro-perforations sur la réponse vibratoire d'une structure cellulaire de type panneau sandwich NIDA (nid d'abeille). Les réflecteurs d'antenne de satellites placés sur les satellites de télécommunication, comme beaucoup d'autres éléments, sont fabriqués avec ce type de matériaux. Lors du décollage du lanceur pour la mise en orbite du satellite, les sollicitations mécaniques appliquées au système sont de nature acoustique et solidienne. La sollicitation acoustique liée au champ acoustique diffus et de très fort niveau présent dans la coiffe du lanceur est la plus importante. Elle joue un rôle important dans le dimensionnement et la conception du réflecteur d'antenne. L'enjeu de la thèse est d'évaluer le potentiel d'un traitement de ce panneau par micro-perforations pour en réduire les vibrations. L'effet des micro-perforations sur la réponse vibratoire du réflecteur d'antenne est double. D'une part, le chargement acoustique que constitue la pression excitatrice est réduit par un mécanisme d'absorption du à la présence des micro-perforations, couplées aux cavités formées par les cellules NIDA du matériau. Cet effet, connu dans la littérature est décrit notamment par le modèle d'impédance acoustique de D.-Y. Maa, couplé à un modèle d'impédance de la cavité NIDA et prenant en compte les rayonnements interne et externe à la micro-perforation. D'autre part, un effet, de nature vibro-acoustique est induit par le couplage entre les vibrations du panneau et les mouvements acoustiques dans les micro-perforations. La modélisation de cet effet, mal décrit dans la littérature constitue un élément original du travail : un modèle discret construit à partir de l'impédance acoustique d'un orifice permet le calcul d'une force d'amortissement élémentaire, puis, après homogénéisation, à une estimation de l'amortissement modal du panneau micro-perforé. Les modélisations proposées pour la réduction de chargement acoustique et de l'amortissement ajouté par micro-perforation montrent que la réponse vibratoire du panneau est faiblement réduite dans la plage de fréquence d'intérêt, ce que confirment plusieurs tests expérimentaux : comparaison de réponse de panneau micro-perforé ou non en chambre réverbérante et en chambre à bruit. La modification de chargement acoustique apportée par la micro-perforation des deux faces du panneau sandwich NIDA est modélisée dans le dernier chapitre et donne lieu à une augmentation de l'effet dans la gamme de fréquence visée. / This thesis work is about the study of the microperforations influence on the vibratory response of a cellular structure as a honeycomb sandwich panel. Satellites' antenna's reflectors placed on telecommunication satellites, as many satellites' elements, are manufactured in this kind of materials. During the launcher take-off for putting satellite into orbit, the mechanical stresses applied to the system are acoustical and vibration borne stress. The acoustic stress, linked to the high level diffuse acoustic field inside the launcher fairing is the most important. It plays a part in the antenna's reflector size and conception. The issue of the thesis is to evaluate the potential of a treatment using microperforations on this panel in order to reduce its vibration. The microperforations effect on the vibration response of the antenna's reflector is double. On one hand, the acoustic loading applied by the exciter pressure is reduced by an absorption mechanism due to the presence of microperforations, coupled to cavities formed by honeycomb cells. This effect, well known in the litterature, is for instance described by the acoustic impedance model developped by D.-Y. Maa, coupled to an impedance model of honeycomb cavity and taking into account the inner and outer radiations of the microperforation. On the other hand, a vibro-acoustical effect is induced by the coupling between panel vibrations and acoustic movements inside microperforations. The modelling of this effect, not well described in the litterature, constitutes an original element of the thesis work: a discrete model constructed using the acoustic impedance of an orifice, allows the computation of an elementary damping force and then leads, after an homogenisation, to an estimation of the modal damping of the microperforated panel. Both modellings proposed for the acoustic loading reduction and the damping added by microperforations, show that the panel vibration response is weakly reduced in the frequency band of interest, which confirms experimental tests like: response comparison of non microperforated and microperforated panels placed in reverberant room and noise chamber. The acoustic loading modification induced by the microperforation of both sides of the honeycomb sandwich panel is modelling in the thesis last chapter and allows an increase of the effect on the frequency band aimed.