Engineering composites and especially long fibre carbon composites have been in high demand not only in aerospace and automotive applications, but also in high end everyday applications. In aerospace, carbon composites are used predominantly for secondary structures attached by joints or fasteners to various alloys or even different composites, and are exposed to service loads and repetitive impacting. Impact fatigue (IF) is not studied adequately for long cycles and relevant literature is investigating mainly drop weight tests and high speed projectile experiments. The main aim of this research was to investigate the behaviour long fibre CFRP'S exposed to repeated low-velocity, low energy impacts, and to observe the damage effects of this regime on the structural integrity of these materials. Two types of specimen configurations using CFRPS's were used and exposed to loading conditions relevant to the Izod impact fatigue test (IIFT), and the tensile impact fatigue test (TIFT), in order to determine the fatigue behaviour of the specimens for each of these load conditions. For the IIFT, the fatigue life was investigated using IM7/8552 unidirectional specimens and T700/LTM45 cross-ply specimens were utilised for the TIFT. The specimen thicknesses were altered in both cases and parametric studies were carried out, where it was seen that IF results in high level of scatter and the apparent decrease in life was seen at relatively modest levels of maximum force after relatively few cycles. In the case of the IIFT, a durability limit was not apparent which increases the complications when designing against IF. In the case of the TIFT the stiffness deterioration was reflected as an increase of the loading time, in the force vs time graph, over the total fatigue life span. Fatigue crack growth was investigated using fractography and X-ray micro-CT at the micro and macro level. It was seen, that IF had the potential to initiate cracks and to cause their propagation at low levels of loading. For the IIFT, a single crack was growing substantially in the fibre direction and across the sample width causing matrix cracking and probably breaking of some fibres, which acted as impact wave guides since matrix cracks were propagating initially along the length of the fibres. In the case of the TIFT multiple damage modes were presented (matrix cracks, axial splits and delaminations). Their sequence and progression was successfully v captured and contrasted against the number of impacts. Axial splits governed the damage scenario, with delaminations extending between them and the free edges. For the TIFT, IF was studied using the force-life (F-Nf) and energy-life (E-Nf) curves. The tests undertaken showed that when halving the thickness of the laminates the fatigue life presented a 10-fold decrease as well as higher scatter. Finite element modelling was undertaken to validate the experimental data of the TIFT test. Successful simulation of a single impact was carried out using a fully transient 3-D model of the actual experiment configuration which involved geometric non-linearities in addition to the multiple contact conditions. The analysis was undertaken using the Abaqus 6.11 explicit solver. Since the numerical single impact results (force vs time response) was in agreement with the experimental results, the crack modes, experimentally observed, were also incorporated in the model utilising the use of the cohesive zone elements (CZE).

The Gulf of Mexico is a shared resource at risk. It is facing many problems such as exponential population growth, degrading water quality, consequences of oil and gas activities, unsustainable exploitation of resources, and lack of public education and political interest. In response, Mexico and the United States have devised their own set of programs and strategies to best manage their coastal zones. There have been a few official coastal management efforts between the U.S. Gulf states and Mexico. However, the existence of these collaborative programs does not supplant the need for a binational, integrated coastal management effort. Texas and Tamaulipas have participated in these cooperation programs, but direct coastal collaboration between the two states has been limited. Although there is an obvious disparity between the two regarding CZM practices, both states face similar challenges that require a cooperative effort and combined resources. This report explores different alternatives for binational cooperation between Texas and Tamaulipas at the regional level, in the context of the existing CZM frameworks and strategies of each state. / text

Si, pour les matériaux composites élaborés, de nombreuses études expérimentales ainsi que des modèles locaux de comportement ont été développés, la maîtrise du comportement des interfaces entre couches de surface ou d’assise de chaussées est actuellement un réel verrou scientifique. La méthode de dimensionnement française actuelle ne prend en compte, aux interfaces, que des conditions conventionnelles de collage ou de glissement parfait. Afin d’appréhender le comportement local de l’interphase\interface, les outils de la photomécanique apparaissent incontournables. La présente thèse propose une modélisation de l’interface rugueuse et endommageable par un modèle de zone cohésive en mode mixte. Ce manuscrit de thèse comporte trois chapitres. D’abord, le chapitre I présente un état de l’art sur les interfaces dans les matériaux et les structures et en particulier dans les couches de chaussées. Ensuite dans le chapitre II, une identification expérimentale des paramètres mécaniques et géométriques du modèle d’interface est proposée à travers des essais de traction et de cisaillement et des mesures de la texture (PMT et projections de franges). Les résultats obtenus (adhésion, rugosité,…) seront les paramètres d’entrée d’un modèle d’endommagement d’interface. Enfin, le chapitre III aborde la modélisation des interfaces entre couches de chaussées sous l’angle des modèles de zones cohésives avec la prise en compte de la rugosité géométrique. A l’issu de cette étude, une loi est proposée permettant de prendre en compte l’effet de la rugosité à une échelle locale dans une interface lisse équivalente à l’échelle globale. / Interface between bituminous layers is an important parameter for the pavement computational design.New pathologies in pavement structure require today rational methods taking into account theinterfaces behavior. Due to these concerns, the current study is based on a damage cohesive zonemodel (CZM) in mixed mode of the rough interfaces. The model was initially proposed by Allix-Ladevèze. This thesis presents a comprehensive interface modeling including delay effect, based ondamage energy release rate. The process of the present study is presented in three chapters. The firstchapter present the stat of art of interfaces. The second devoted to identify the parameters of theinterface model and material properties through advanced optical method as Digital Image Correlation(DIC) and (H-DIC). In the third chapter, a study of the influence of the elastic normal and tangentialstiffness and coupling parameters in the mixed mode on the debonding interfacial energy is presented.An analytical model provides relations between the interfaces stiffness, the coupling parameter of theCZM and the interfacial roughness. Then, a parametric numerical analysis is conducted to study theroughness effect on the interface constitutive law. Results show clearly the roughness influence in thiskind of structures. The damage behaviours predicted by the proposed model for pure mode I, puremode II and for mixed mode with taking into account of roughness are found in good agreement withexperimental results.

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L’accroissement des besoins en énergie électrique pour les systèmes embarqués et leur augmentation de puissance nécessitent de concevoir des systèmes d’électronique de puissance toujours plus performants. Une solution d’avenir concerne la mise en œuvre de composants à base de diamant qui permettent l’augmentation conséquente des tensions et courants mis en jeux, mais aussi de la température maximale de jonction admissible. Le cadre de ces travaux est celui du projet de recherche Diamonix 2, qui concerne l’étude et l’élaboration d’un composant diamant fonctionnant à haute température. L’objectif du travail doctoral présenté ici est l’étude du packaging haute température de ce type de composant diamant. Plusieurs choix de matériaux et de techniques aptes à l’élaboration d’un assemblage de puce diamant sur un substrat métallisé ont été effectués. La caractérisation microstructurale et mécanique de trois types de jonctions ont été réalisées (refusion d’un alliage AuGe, frittage de nano pâtes d’argent et diffusion en phase solide d’indium dans des couches d’argent). Des essais mécaniques de cisaillement de divers assemblages ont permis d’évaluer le comportement thermomécanique des jonctions et des interfaces. Les essais de cisaillement ont servi à l’identification inverse des paramètres interfaciaux d’un modèle de zones cohésives, pour différents types d’interfaces. Des modèles éléments finis d’assemblage, incluant le comportement viscoplastique des jonctions et des lois d’endommagent des interfaces, ont servi à simuler le comportement thermomécanique du packaging d’un composant diamant. / The increase of electric power demand for embedded systems requires more efficient power electronics modules. A solution to reach this goal relates to the use of diamond-based components that allow high voltage, current density and the maximum allowable junction temperature. The framework is the same as that of the Diamonix 2 research project, which involves the elaboration and the study of a diamond-based die dedicated to high temperature environment. The purpose of the present work is to optimize and simulate the thermomechanical behavior of high temperature diamond die packaging. To reach this goal, the choice of materials that allow high temperature assemblies of diamond die/ceramic substrate was done (AuGe solder alloy, sintering of nano-silver paste, transient liquid phase bounding of indium in silver layers). Microstructural and mechanical characterization of the attachment and the diamond die/junction was realized. Nanoindentation and shear tests are performed for the mechanical characterization. Shear tests results carried out on the two assemblies have been used to identify the interfacial parameters of the bilinear cohesive zone model (CZM) for the diamond die/junction and ceramic substrate/junction interfaces. Finite element modelling of the diamond component packaging including viscoplastic behavior of the junctions and damage law of the interfaces of assemblies were built.

The thesis presents the development and validation of novel computational technology for modelling and analysis of damage and failure in quasi-brittle materials. The technology is demonstrated mostly on concrete, which is the most widely used quasi-brittle material exhibiting non-linear behaviour. Original algorithms and procedures for generating two-dimensional (2D) and three-dimensional (3D) heterogeneous material samples are developed, in which the mesoscale features of concrete, such as shape, size, volume fraction and spatial distribution of inclusions and pores/voids are randomised. Firstly, zero-thickness cohesive interface elements with softening traction-separation relations are pre-inserted within solid element meshes to simulate complex crack initiation and propagation. Monte Carlo simulations (MCS) of 2D and 3D uniaxial tension tests are carried out to investigate the effects of key mesoscale features on the fracture patterns and load-carrying capacities. Size effect in 2D concrete is then investigated by finite element analyses of meso-structural models of specimens with increasing sizes. Secondly, a 3D meso-structural damage-plasticity model for damage and failure analysis of concrete is developed and applied in tension and compression. A new scheme for identifying interfacial transition zones (ITZs) in concrete is presented, whereby ITZs are modelled by very thin layers of solid finite elements with damage-plasticity constitutive relations. Finally, a new coupled method named non-matching scaled boundary finite element-finite element coupled method is proposed to simulate crack propagation problems based on the linear elastic fracture mechanics. It combines the advantage of the scaled boundary finite element method in modelling crack propagation and also preserves the flexibility of the finite element method in re-meshing. The efficiency and effectiveness of the developed computational technology is demonstrated by simulations of crack initiation and propagation problems.

FRP laminates and fabrics, used as an externally bonded reinforcement (EBR) to strengthen or repair concrete members, have proven to be an economical retrofitting method. However, when used to strengthen long-span members or members with limited access, the labor and equipment demands may negate the benefits of using continuous EBR FRP. Recently, CFRP rod panels (CRPs) have been developed and deployed to overcome the aforementioned limitations. Each CRP is made of several small diameter CFRP rods placed at discrete spacing. To fulfill the strengthening length, CRP’s are spliced together and made continuous by means of overlaps (or finger joints). In this doctoral dissertation, the effectiveness of spliced CRPs as flexural strengthening reinforcement for RC members was investigated by experimental, analytical and numerical methods. The experimental research includes laboratory tests on (1) RC beams under four-point bending and (2) double-lap shear concrete specimens. The first set of tests examines the behavior of concrete members strengthened with spliced CRPs. Several beams were fabricated and tested, including: (a) unstrengthened, (b) strengthened with spliced CRPs, (c) strengthened with full-length CRPs, and (d) strengthened with full-length and spliced CFRP laminates. The double-lap shear tests serve to characterize the development length and bond strength of two commonly used CRPs. Several small-scale CRPs, with variable bond lengths, were tested to arrive to an accurate estimation of development length and bond strength. Several other specimens were additionally tested to preliminarily examine the effects of bond width and rod spacing. A 3D nonlinear finite element simulation was utilized to further study the response of CRP strengthened RC beams, by extracting essential data, that couldn’t be measured in the experimental tests. Additionally, analytical tools were added to investigate the behavior of tested bond and beam specimens. The first tool complements the double-lap shear tests, and provides mathematical terms for important characteristics of the CRP/concrete bond interface. The second tool investigates concrete cover separation failure, which was observed in the beam testing, for RC beams strengthened with full-length and spliced CRPs.

L'objectif de cette thèse est de progresser dans la compréhension de l'influence de la microstructure sur l'anisotropie plastique et la ténacité de deux nuances d'alliage Al-Cu-Li 2198 sous forme de tôle. L'épaisseur de tôles est 2 mm et 6 mm. Deux traitements thermiques (T351 et T851) ont été étudiés pour chaque nuance. Différentes techniques de caractérisation multi-échelles telles que la microscopie optique, la microscopie électronique en transmission ou encore la tomographie à rayons X ont été utilisées pour identifier les microstructures des matériaux et les micro-mécanismes d'endommagement. L'anisotropie plastique et l'effet d'épaisseur sur la plasticité ont été étudiés via des essais de traction sur les éprouvettes lisses et entaillées selon différentes directions. Les résultats montrent que le comportement plastique est anisotrope dans le plan de tôle. Le comportement en déchirure ductile a été examiné en utilisant des éprouvettes de petite taille de type Kahn ainsi que des plaques larges de type M(T). L'anisotropie de ténacité a été étudiée sur les éprouvettes chargées selon différentes configurations. La fractographie par microscope électronique à balayage (MEB) et la tomographie synchrotron aux rayons X ont clarifié le rôle des structures granulaires et des traitements thermiques sur les mécanismes de la rupture inter-granulaire et trans-granulaire. La croissance de cavités reste limitée dans la zone de propagation de fissure. En fin, la simulation de la déchirure ductile par élément finis est basée sur l'approche locale de la rupture en utilisant un modèle de zone cohésive (CZM). Les paramètres cohésifs ont été ajustés sur les éprouvettes Kahn. Les paramètres identifiés ont été employés pour prédire la déchirure ductile des essais M(T). Les résultats montrent que la simulation des essais M(T) est plus sensible aux valeurs des paramètres ajustés que la simulation des essais Kahn. L'effet d'épaisseur a été évalué à l'aide de la technique de relâchement des nœuds en analysant la variation de la contrainte et de la déformation dans la direction de l'épaisseur.

abstract: Polymer matrix composites (PMCs) are attractive structural materials due to their high stiffness to low weight ratio. However, unidirectional PMCs have low shear strength and failure can occur along kink bands that develop on compression due to plastic microbuckling that carry strains large enough to induce nonlinear matrix deformation. Reviewing the literature, a large fraction of the existing work is for uniaxial compression, and the effects of stress gradients, such as those present during bending, have not been as well explored, and these effects are bound to make difference in terms of kink band nucleation and growth. Furthermore, reports on experimental measurements of strain fields leading to and developing inside these bands in the presence of stress gradients are also scarce and need to be addressed to gain a full understanding of their behavior when UDCs are used under bending and other spatially complex stress states. In a light to bridge the aforementioned gaps, the primary focus of this work is to understand mechanisms for kink band evolution under an influence of stress-gradients induced during bending. Digital image correlation (DIC) is used to measure strains inside and around the kink bands during 3-point bending of samples with 0°/90° stacking made of Ultra-High Molecular Weight Polyethylene Fibers. Measurements indicate bands nucleate at the compression side and propagate into the sample carrying a mixture of large shear and normal strains (~33%), while also decreasing its bending stiffness. Failure was produced by a combination of plastic microbuckling and axial splitting. The microstructure of the kink bands was studied and used in a microstructurally explicit finite element model (FEM) to analyze stresses and strains at ply level in the samples during kink band evolution, using cohesive zone elements to represent the interfaces between plies. Cohesive element properties were deduced by a combination of delamination, fracture and three-point bending tests used to calibrate the FEMs. Modeling results show that the band morphology is sensitive to the shear and opening properties of the interfaces between the plies. / Dissertation/Thesis / Doctoral Dissertation Mechanical Engineering 2016

The problem of a thin elastomeric layer confined between two stiff adherends arises in numerous applications such as microelectronics, bio-inspired adhesion and the manufacture of soft biomedical products. A common requirement is that the debonding of the elastomeric layer from the adherends be controlled to avoid undesirable failure modes. This level of control may necessitate understanding the collective role of the interfacial adhesion, material properties, part geometries, and loading conditions on the debonding. Analytical and numerical approaches using the finite element method and a cohesive zone model (CZM) for the interfacial debonding are used in this dissertation to delineate the role of the afore-mentioned parameters on the initiation and propagation of debonding for both rigid and non-rigid adherends. Extensively studied in the dissertation is the debonding of a semi-infinite relatively stiffer adherend from an elastomer layer with its other surface firmly bonded to a rigid base. The adherend is pulled upwards by applying normal displacements either on its entire unbonded surface or on the edge of its part overhanging from the elastomer layer. The adherend and the elastomeric layer materials are assumed to be linear elastic, homogeneous and isotropic and the elastomer is assumed to be incompressible. Viscoelasticity of the elastomer is considered in the first part of the work. Plane strain deformations of the system with a bilinear traction-separation (TS) relation in the CZM are analyzed. Two non-dimensional numbers, one related to the layer confinement and the other to the interfacial TS parameters, are found to determine if debonding initiates at interior points in addition to at corner points on the adherend/elastomer interface, and if adhesion-induced instability is exhibited. This work is extended to axisymmetric problems in which debonding can take place at both interfaces. Motivated by an industrial demolding problem, numerical experiments are conducted to derive insights into preferential debonding at one of the two interfaces, including for curved adherends. Results reported herein should help engineers design an elastomer layer sandwiched between two adherends for achieving desired failure characteristics. / Ph. D.