Le design de pièces de structures aéronautiques en matériau composite requiert un grand nombre de tests et de simulations à différentes échelles. Dans ce contexte industriel, les modèles de comportement sont amenés à prendre une importance de plus en plus prégnante. L'idée développée dans cette thèse est d'améliorer la description de la fissuration transverse à travers le mésomodèle des composites, introduit au LMT dans les années 80. L'objectif est d'être en mesure de prédire précisément par la simulation la rupture d'un stratifié, en conservant des coûts de calculs raisonnables en vue de substituer à tout ou partie des essais mécaniques onéreux. On propose ainsi une nouvelle version de notre modèle qui combine, à la fois la prise en compte de la fissuration transverse localisée sous forme de split, et une description homogénéisée pour les fortes densités de fissures transverses. Afin de concilier ces deux approches un critère de détection ad hoc est proposé, le cas échéant les macrofissures sont introduites dans le modèle par le biais d'éléments cohésifs.Une partie de l'originalité de ce travail est de concevoir et développer des méthodes de calculs, en vue de leur transfert vers l'industrie. Pour assurer la pérennité des développements, et pouvoir traiter des problèmes industriels (grand nombre de degrés de liberté, géométries industrielles) on utilise uniquement des logiciels de calculs commerciaux. `A cette fin, l'ensemble des développement a été implémenté de manière non intrusive sous SAMCEF. Enfin, pour éprouver la nouvelle stratégie proposée, une campagne de validation en partenariat avec AIRBUS GROUP INNOVATION a été menée sur des structures en matériau composites stratifiées formées selon des empilements complexes et présentant des accidents géométriques. Le développement de lois de comportements adoucissantes entraîne des problèmes de dépendance au maillage, la question de la localisation est alors abordée sous un angle pratique. / Structural aerospace parts design in composite material requires a large number of tests and simulations at different scales. In this industrial context, constitutive laws are more and more important.The idea of this thesis is to improve the description of transverse cracking in the composite mesomodel, introduced in the LMT during the 80's. The objective is to be able to predict accurately the failure of a laminated composite structure, at cheap computational cost, in order to replace some of expensive mechanical tests.We propose a new version of our model which combine, isolated transverse cracks description such as splits, and an homogenized representation to model important transverse cracks density. To reconcile these two approcahes an ad hoc criterion is introduced, if necessary macrocracks are set in the model by using cohesive elements. A significant proportion of this works is to design and elaborate methods and computation strategy, in order to transfer from academic world to industry. To maintain computer code, and be able to compute industrial mechanical problems (large number of DOF, complex geometry) only commercial softwares must be used. For this purpose, the whole code is implemented in a non intrusive way on SAMCEF software. Finally, to challenge the new strategy, a campaign of validation, with our industrial partner AIRBUS GROUP INNOVATION was carried about structures in laminated composite material, with complex stackings with stress concentrations. Constitutive laws with softening triggers off mesh dependance issues, then the question of localization is discussed from a practical angle.

Résumé / Abstract

Indiana University-Purdue University Indianapolis (IUPUI) / Objectives: To investigate the effect of bulk-fill resin based composite materials on cuspal deflection in large slot mesio-occlusal-distal cavities (MOD) in premolar teeth. Methodology: Thirty-two sound maxillary premolar teeth with large slot MOD cavities were distributed to four groups (n=8). Three groups were restored with bulk-fill resin composite materials (Tetric EvoCeram, x-tra fil, and Sonic Fill, respectively) in a single increment. The conventional composite group, Filtek Z100, was used to restore the cavities in 2mm increments. Cusp deflection was recorded post irradiation using a Nikon measurescope UM-2 (Nikon, Tokyo, Japan), by measuring the changes in the bucco-palatal width of the premolar teeth at 5 minutes, 24 hours, and 48 hours after completion of the restoration. The cuspal deflection was obtained by recording the difference between the baseline measurements and the other measurements for each tooth. Results: Cuspal deflection was significantly higher in Conventional Composite than in Tetric EvoCeram Bulk Fill (p=0.0031), x-tra Fil Bulk (p=0.0029), and SonicFill Bulk (p=0.0002). There was no significant difference in cuspal deflection for Tetric EvoCeram Bulk, X-tra Fil Bulk, and SonicFill Bulk Composites. Conclusions: All the investigated bulk-fill resin composites exhibited cuspal deflection lower than conventional resin composite. One of the aims of research and studies on the resin composite materials is improving their clinical longevity, and simplifying their use. For that purpose bulk-fill materials are considered promising materials and further clinical studies should be conducted.

The primary objective of this research is to investigate the morphological and mechanical properties of composite materials and porous materials prepared by thermally induced phase separation. High melting crystallizable diluents were mixed with polymers so that the phase separation would be induced by the solidification of the diluents upon cooling. Theoretical phase diagrams were calculated using Flory-Huggins solution thermodynamics which show good agreement with the experimental results. Porous materials were prepared by the extraction of the crystallized diluents after cooling the mixtures (hexamethylbenzene/polyethylene and pyrene/polyethylene). Anisotropic structures show strong dependence on the identity of the diluents and the composition of the mixtures. Anisotropic crystal growth of the diluents was studied in terms of thermodynamics and kinetics using DSC, optical microscopy and SEM. Microstructures of the porous materials were explained in terms of supercooling and dendritic solidification. Dual functionality of the crystallizable diluents for composite materials was evaluated using isotactic polypropylene (iPP) and compatible diluents that crystallize upon cooling. The selected diluents form homogeneous mixtures with iPP at high temperature and lower the viscosity (improved processability), which undergo phase separation upon cooling to form solid particles that function as a toughening agent at room temperature. Tensile properties and morphology of the composites showed that organic crystalline particles have the similar effect as rigid particles to increase toughness; de-wetting between the particle and iPP matrix occurs at the early stage of deformation, followed by unhindered plastic flow that consumes significant amount of fracture energy. The effect of the diluents, however, strongly depends on the identity of the diluents that interact with the iPP during solidification step, which was demonstrated by comparing tetrabromobisphenol-A and phthalic anhydride. A simple method to prepare composite surfaces that can change the wettability in response to the temperature change was proposed and evaluated. Composite surfaces prepared by nanoporous alumina templates filled with polymers showed surface morphology and wettability that depend on temperature. This effect is attributed to the significant difference in thermal conductivity and the thermal expansion coefficient between the alumina and the polymers. The reversibility in thermal response depends on the properties of the polymers.

The problem examined is an infinite anisotropic layer with a through crack at arbitrary orientation, subjected to uniform in-plane remote loading. The purpose of this study is to gain a better understanding of several theoretical models for predicting the direction of crack propagation and the level of load causing crack extension, and to present a new model for predicting the critical load. The discussed models are particularly examined in detail with regard to the physical parameters affecting the results. Comparison is made with available experimental results. It is shown that the normal stress ratio theory provides good agreement with experimental crack growth direction, independent of physical parameters, and that the newly proposed traction ratio theory predicts well the critical load causing crack extension. / M.S.

The effect of particle size matching and mismatching on the processability, and the mechanical and physical properties of particulate reinforced composites is investigated in this study. These composites were made from dry powder-powder blends. Polymer and reinforcement materials were chosen, characterized and molded into composite plaques. For the same particle volume fraction (400/0), stiffness was found to increase, in general, as particle size decreased. The intimacy of mixing, stiffness and strength improvements depended upon the reinforcement type. These results were compared with predictions from simple micromechanics models to gain a better understanding of their physical behavior. / Master of Science

This study investigates the effects of typical construction details on the strength of steel deck reinforced concrete composite slabs. Past research on composite slabs has been centered primarily around single span, single panel width slabs with unrestrained ends. The test specimens in this study are more representative of actual slab construction. The effects of multiple spans, multiple panels, end restraint from pour stops, and deck anchorage from shear studs and welds are investigated. The results of this experimental study are analyzed using methods given in the Steel Deck Institute Composite Deck Design Handbook. The models were found to conservatively predict the strength of the composite slabs. Recommended modifications to the calculation methods are given. / Master of Science

This study investigates the further development of the commercially available re-entrant steel deck profile. The effects of various embossments and continuous construction are investigated through three Series of composite slab load tests. The test specimens in this study were constructed to simulate actual field construction of composite slabs as part of reinforced concrete structures. The results of this experimental study are analyzed using methods given in the ASCE Standard for the Structural Design of Composite Slabs. Recommended design procedures for the improved re-entrant profile are given and various future profile modifications are suggested. / Master of Science

The objective of the research reported herein is to present a comprehensive evaluation of all available experimental data from push-out tests utilizing the ELCO Grade 8 standoff screws. The goal is to develop a strength prediction equation and determine reliability parameters compatible with the Load Factor Resistance Design (LRFD) procedure that would allow the use of this shear connector in design of composite floor systems. The study considers results from push-out tests using this type of screw reported by Hankins et al. (1994), Alander et al. (1998), Webler et al. (2000), and Mujagic et al. (2000). Further, this study identifies the limitations in earlier approaches aimed at predicting the strength of standoff screws. An improved strength prediction model is developed that considers all applicable limit states and determines maximum strength of a connector. A reliability study was also conducted to derive strength reduction factors to be used in design. Parameters considered in the model include deck type and geometry, screw height, concrete compressive strength, top chord angle yield strength, and stand-off screw rupture strength. Results from strength prediction model were compared with results from composite joist tests. / Master of Science

Damage states in laminated composites were studied by considering the model problem of a laminated beam subjected to three-point bending. A combination of experimental and theoretical research techniques was used to correlate the experimental results with the analytical stress distributions. The analytical solution procedure was based on the stress formulation approach of the mathematical theory of elasticity. The solution procedure is capable of calculating the ply-level stresses and beam displacements for any laminated beam of finite length using the generalized plane deformation or plane stress state assumption. The beam lamination can be any arbitrary combination of monoclinic, orthotropic, transversely-isotropic, and isotropic layers. Prior to conducting the experimental phase of the study, the results from preliminary analyses were examined. Significant effects in the ply-level stress distributions were seen depending on the fiber orientation, aspect ratio, and whether or not a grouped or interspersed stacking sequence was used. The experimental investigation was conducted to determine the different damage modes in laminated three-point bend specimens. The test matrix consisted of three-point bend specimens of 0° unidirectional, cross-ply, and quasi-isotropic stacking sequences. The dependence of the damage initiation loads and ultimate failure loads were studied, and their relation to damage susceptibility and damage tolerance of the beam configuration was discussed. Damage modes were identified by visual inspection of the damaged specimens using an optical microscope. The four fundamental damage mechanisms identified were delaminations, matrix cracking, fiber breakage, and crushing. The correlation study between the experimental results and the analytical results was performed for the midspan deflection, indentation, damage modes, and damage susceptibility. The correlation was primarily based on the distributions of the in-plane component of shear stress, t_xz. The exceptions were for the case of a very small aspect ratio (less than 1.0) where the crushing mode of damage was predicted based on the maximum contact pressure, and for very large aspect ratios (greater than 12.0) where a maximum tensile bending stress criterion was used for predicting the damage initiation loads. / Ph. D.