The behavior of the ballast particles during their interaction with tamping tines in tamping operation is studied by developing a simulation model using the Discrete Element Model (DEM), with the aim of optimizing the railroad tamping operation. A comprehensive literature review is presented showcasing the applicability of DEM techniques in modeling ballast behavior and its feasibility in studying the fundamental mechanisms that influence the outcome of railroad tamping process is analyzed. The analysis shows that DEM is an excellent tool to study tamping operation as its important and unprecedented insights into the process, help not only to optimize the current tamping practices but also in the development of novel methods for achieving sustainable improvements in the track stability after tamping. The simulation model is developed using a commercially available DEM software called PFC3D (Particle Flow Code 3D). A detailed explanation is provided about how to set up the DEM model of railway ballast considering important parameters like selection and calibration of particle shapes, ballast mechanical properties, contact model, and parameters governing the contact force models. Tamping operation is incorporated into the simulation model using a half-track layout with a highly modular code that enables a high degree of adjustability to allow control of all process parameters for achieving optimized output. A parametric study is performed to find the best values of tine motion parameters to optimize the linear tamping efficiency and a performance comparison has been made between linear and elliptical tamping. It is found that squeeze and release velocity of the tines should be lesser for better compaction of the particles and linear tamping is better compared to elliptical tamping. / Master of Science / Railway track stability is the resistance of the tracks to deformation and is affected by the rail traffic, ballast fouling (contamination of ballast) and the changing environmental conditions. The track stability depends on the normal and frictional support provided by the ballast to the sleepers. Non-uniform ballast consolidation below the railway sleeper results in erratic wheel-rail contact forces, low traffic speeds, poor ride quality, and derailments. Thus, tamping is a railway track maintenance method done periodically on the railway tracks to ensure track stability. Tamping process involves compacting the railroad ballast underneath the sleeper. The sleeper is lifted by a desired height and then vibrating tamping tools called tines are inserted into the ballast below the sleeper to fill the void created by lifting of the sleeper and the sleeper is dropped back on to the ballast. So, it is important to understand the ballast mechanics, dynamics and ballast’s behavioral response to the tamping operation. Since, large scale experiments such as this are difficult, this operation has been simulated in a commercially available software called PFC3D using a Discrete Element Model (DEM) to represent the railway ballast. It is shown through a simulation that though spherical particles provide better computational efficiency, they cannot capture the exact ballast behavior like clumps (a collection of spherical pebbles). So using clumps to represent ballast, efforts are made to optimize the linear tamping efficiency. This is done by changing the values of parameters like tine amplitude, tine frequency, insertion velocity and squeeze velocity and finding their optimum values. Linear tamping results are compared with elliptical tamping. Thus, an optimum tamping cycle would help save money spent on the track maintenance activities.

The development of Discrete Element Model (DEM) of railway ballast for the purpose of studying the behavior of ballast particles during tamping is addressed in a simulation study, with the goal of optimizing the railroad tamping operation. A comprehensive literature review of applicability of DEM techniques in modeling the behavior of railway ballast is presented and its feasibility in studying the fundamental mechanisms that influence the outcome of railroad tamping process is analyzed. A Discrete Element Model of railway ballast is also developed and implemented using a commercially available DEM package: PFC3D. Selection and calibration of ballast parameters, such as inter-particle contact force laws, ballast material properties, and selection of particle shape are represented in detail in the model. Finally, a complete tamping simulation model is constructed with high degree of adjustability to allow control of all process parameters for achieving realistic output. The analysis shows that DEM is a highly valuable tool for studying railroad tamping operation. It has the capability to provide crucial and unprecedented insights into the process, facilitating not only the optimization of current tamping practices, but also the development of novel methods for achieving sustainable improvements in track stability after tamping in the future. Different ways of modeling particle shapes have been evaluated and it has been shown that while using spheres to represent irregular ballast particles in DEM provides immense gains in computational efficiency, spheres cannot intently capture all properties of irregularly shaped particles, and therefore should not be used to model railway ballast particles. Inter-particle and wall-particle contact forces are calculated using Hertzian contact mechanics for determining ballast dynamics during tamping. The results indicate that the model is able to accurately predict properties of granular assemblies of the railway ballast in different test cases. The developed model for simulating tamping operation on a half-track layout is expected to be extended in future studies for evaluating rail track settlement and stability, optimization of tamping process, and performance of different ballast gradations. / MS / Development of a virtual simulation model for the stone bed which forms the foundation of traditional rail track structures is discussed in this study for the purpose of improving a conventional railway maintenance practice called tamping. The stone bed, called ballast, is flexible and is susceptible to undesirable deformation due to the forces from train traffic on the rail tracks over their service time. Therefore, periodic restoration of track structure is performed by tamping to maintain the operational quality of the rail tracks and reduce the risk of train accidents. This simulation model is intended to accelerate the scientific development of the current tamping practices by providing unprecedented insight into the behavior of small stones which form the bulk of the ballast and obviating the requirement for costly physical experimentation. The nuances of the mechanical behavior of ballast have been examined by a comprehensive literature review and the selection of a modeling technique called Discrete Element Modeling (DEM) has been justified for modeling of ballast owing to its suitability in capturing intricate dynamics of ballast stones. The virtual simulation model which is developed as results of this work has been found to be extremely efficient in realistically predicting the outcome of tamping process for any set of conditions of interest. This implies that quality of the rail tracks after tamping can be studied for a variety of different test cases and most optimized set of tamping parameters which results in maximum track quality can be analyzed. However, it was observed that the accuracy of the results obtained from the simulation model is dependent on the level of detail which is used to input properties of the ballast into the model. Low level of detail results in less accurate results whereas a high level of detail takes an unreasonably long time to solve. Therefore, a compromise has to be made between accuracy and solution time while programming the simulation model, and additional work is required in the future to improve the solution speed of the model.

Der Vortrag beschreibt die zugrunde liegende Theorie und die Softwarefunktionalität des in PTC Creo Simulate 3.0 eingeführten Kontaktmodells mit endlicher Reibung und vergleicht dieses mit den bis Creo Simulate 2.0 exklusiv verwendeten Kontaktmodellen (ideal reibungsfrei und unendlich reibungsbehaftet). An zwei Modellbeispielen (ein von zwei Bremsbacken geklemmtes Bremsschwert und ein verschraubtes Schwungrad) wird versucht, die Funktionsweise des neuen Modells zu demonstrieren. Wegen aktueller Qualitätsprobleme der Software wird die Brauchbarkeit der Kontaktmodelle für den Anwender bewertet (Stand Creo 3.0 M080 / Creo 2.0 M200) und umfangreiches Feedback an den Softwarehersteller PTC gegeben. / The presentation describes the underlying theory and software functionality of the finite friction contact model introduced with PTC Creo Simulate 3.0. It is being compared with the friction-free and infinite friction contact model used exclusively until Creo Simulate 2.0. It is being tried to demonstrate the mode of operation of the new model with help of two examples: A brake sword clamped by two brake pads and a bolted flywheel. Because of actual software quality problems, the usability of the contact model for the user is being rated (status Creo 3.0 M080 / Creo 2.0 M200). Furthermore, comprehensive feedback is given to the software developer PTC.

Der Vortrag beschreibt die zugrunde liegende Theorie und die Softwarefunktionalität des in PTC Creo Simulate 3.0 eingeführten Kontaktmodells mit endlicher Reibung und vergleicht dieses mit den bis Creo Simulate 2.0 exklusiv verwendeten Kontaktmodellen (ideal reibungsfrei und unendlich reibungsbehaftet). An zwei Modellbeispielen (ein von zwei Bremsbacken geklemmtes Bremsschwert und ein verschraubtes Schwungrad) wird versucht, die Funktionsweise des neuen Modells zu demonstrieren. Wegen aktueller Qualitätsprobleme der Software wird die Brauchbarkeit der Kontaktmodelle für den Anwender bewertet (Stand Creo 3.0 M080 / Creo 2.0 M200) und umfangreiches Feedback an den Softwarehersteller PTC gegeben. / The presentation describes the underlying theory and software functionality of the finite friction contact model introduced with PTC Creo Simulate 3.0. It is being compared with the friction-free and infinite friction contact model used exclusively until Creo Simulate 2.0. It is being tried to demonstrate the mode of operation of the new model with help of two examples: A brake sword clamped by two brake pads and a bolted flywheel. Because of actual software quality problems, the usability of the contact model for the user is being rated (status Creo 3.0 M080 / Creo 2.0 M200). Furthermore, comprehensive feedback is given to the software developer PTC.

Submitted by Aldo Theodoro Gaiotto Junior (engenhotto@hotmail.com) on 2018-09-03T14:22:00Z No. of bitstreams: 1 DISSERTAÇÃO ALDO GAIOTTO.pdf: 6437864 bytes, checksum: 350d39f461d7eb07f22b0d4b350f05b5 (MD5) / Approved for entry into archive by Lucilene Cordeiro da Silva Messias null (lubiblio@bauru.unesp.br) on 2018-09-04T12:15:18Z (GMT) No. of bitstreams: 1 gaiottojunior_at_me_bauru.pdf: 5958184 bytes, checksum: cf475013b5ae09ce289691e5d961d76d (MD5) / Made available in DSpace on 2018-09-04T12:15:18Z (GMT). No. of bitstreams: 1 gaiottojunior_at_me_bauru.pdf: 5958184 bytes, checksum: cf475013b5ae09ce289691e5d961d76d (MD5) Previous issue date: 2018-07-30 / O estudo acerca dos efeitos provocados pelos processos de exploração de fontes de hidrocarbonetos em rochas reservatório está cada vez mais presente nos processos investigativos que visam mitigar e remediar os impactos ambientais originários desta prática, de modo que se possa explorá-los por mais tempo e com menos danos ao meio ambiente. Visando o desenvolvimento de novas técnicas que possibilitem compreender melhor estes efeitos, o presente trabalho estuda as potencialidades de um novo modelo constitutivo, baseado no modelo de fechamento de juntas proposto por Barton e na teoria de dano contínuo, denominado “Modelo de Contato Progressivo”. Desenvolvido para simular o processo de fechamento mecânico das fraturas existentes em rochas em uma condição de contato entre interfaces, o modelo possui uma formulação adaptada à situação pretendida. O processo de fechamento das fraturas ocorre de acordo com a variação da poropressão da rocha. À medida que as paredes das fraturas se aproximam, a área de contato entre elas aumenta, causando um aumento na rigidez compressiva da fratura, segundo a relação hiperbólica dada por Barton-Bandis entre a componente normal da tensão e o deslocamento normal relativo. Por meio de uma abordagem 2D em elementos finitos é analisado o comportamento da poropressão em fraturas submetidas a tensões de compressão. Para representar explicitamente a fratura são utilizados elementos finitos com elevada razão de aspecto, estrategicamente posicionados. São realizados testes computacionais via códigos de programação próprios para validar o modelo. Os testes mostram que os elementos de interface são apropriados para representar as fraturas naturais e que o modelo de contato progressivo é capaz de reproduzir o efeito de fechamento de fraturas. O modelo pode ser empregado em reservatórios naturalmente fraturados para relacionar a tensão normal e a deformação no plano da fratura, além de fornecer valores de permeabilidade equivalente. / The study of the effects caused by the processes of hydrocarbon sources exploration in reservoir rocks is becoming widely present in investigative processes aimed at mitigating and remedying the environmental impacts originating from this practice, so that it can be explored for more time and with less damage to the environment. Aiming the development of new techniques to better understand these effects, the present work studies the potentialities of a new constitutive model, based on Barton's closure model and the continuous damage theory, called "Progressive Contact Model". Developed to simulate the process of fractures mechanical closure in rocks with contact between interfaces condition, the model has a formulation adapted to the intended situation. The fractures closure process occurs according to rock porepressure variation. As the walls of the fractures approach each other, the contact area between the walls increases, causing a growth in the compression stiffness of the fracture, according to the hyperbolic relationship given by Barton-Bandis between the normal component of the stress and the relative normal displacement. Using a 2D approach in finite elements, the porepressure in fractures submitted to compression is analyzed. To represent explicitly the fracture, finite elements with a high aspect ratio, strategically positioned, are used. Computational tests via own programming codes are performed to validate the model. The tests show that the interface elements are appropriate to represent the natural fractures and the progressive contact model is able to reproduce the fracture closure effect. The model can be used in naturally fractured reservoirs to relate normal stress and deformation to the fracture plane, besides to provide equivalent permeability values.

Heart disease is the leading cause of death in the world. Therefore, numerous studies are undertaken to identify indicators which can be applied to discover cardiac dysfunctions at an early age. Among others, the fluid dynamics of the blood flow (hemodymanics) is considered to contain relevant information related to abnormal performance of the heart.This thesis presents a robust framework for numerical simulation of the fluid dynamics of the blood flow in the left ventricle of a human heart and the fluid-structure interaction of the blood and the aortic leaflets.We first describe a patient-specific model for simulating the intraventricular blood flow. The motion of the endocardial wall is extracted from data acquired with medical imaging and we use the incompressible Navier-Stokes equations to model the hemodynamics within the chamber. We set boundary conditions to model the opening and closing of the mitral and aortic valves respectively, and we apply a stabilized Arbitrary Lagrangian-Eulerian (ALE) space-time finite element method to simulate the blood flow. Even though it is difficult to collect in-vivo data for validation, the available data and results from other simulation models indicate that our approach possesses the potential and capability to provide relevant information about the intraventricular blood flow.To further demonstrate the robustness and clinical feasibility of our model, a semi-automatic pathway from 4D cardiac ultrasound imaging to patient-specific simulation of the blood flow in the left ventricle is developed. The outcome is promising and further simulations and analysis of large data sets are planned.In order to enhance our solver by introducing additional features, the fluid solver is extended by embedding different geometrical prototypes of both a native and a mechanical aortic valve in the outflow area of the left ventricle.Both, the contact as well as the fluid-structure interaction, are modeled as a unified continuum problem using conservation laws for mass and momentum. To use this ansatz for simulating the valvular dynamics is unique and has the expedient properties that the whole problem can be described with partial different equations and the same numerical methods for discretization are applicable.All algorithms are implemented in the high performance computing branch of Unicorn, which is part of the open source software framework FEniCS-HPC. The strong advantage of implementing the solvers in an open source software is the accessibility and reproducibility of the results which enhance the prospects of developing a method with clinical relevance. / <p>QC 20171006</p>

La manipulation dextre est un sujet important dans la recherche en robotique et dans lequel peu de travaux ont abordé la manipulation d'objets déformables. De nouvelles applications en chirurgie, en industrie agroalimentaire ou encore dans les services à la personne nécessitent la maîtrise de la saisie et la manipulation d'objets déformables. Cette thèse s’intéresse à la manipulation d’objets déformables par des préhenseurs mécaniques anthropomorphiques tels que des mains articulées à plusieurs doigts. Cette tâche requière une grande expertise en modélisation mécanique et en commande : modélisation des interactions, perception tactile et par vision, contrôle des mouvements des doigts en position et en force pour assurer la stabilité de la saisie. Les travaux présentés dans cette thèse se focalisent sur la modélisation de la saisie d'objets déformables. Pour cela, nous avons utilisé une discrétisation par des systèmes masses-ressorts non-linéaires pour modéliser des corps déformables en grands déplacements et déformations tout en ayant un coût calculatoire faible. Afin de prédire les forces d’interactions entre main robotique et objet déformable, nous avons proposé une approche originale basée sur un modèle rhéologique visco-élasto-plastique pour évaluer les forces tangentielles de contact et décrire la transition entre les modes d’adhérence et de glissement. Les forces de contact sont évaluées aux points nodaux en fonction des mouvements relatifs entre les bouts des doigts et les facettes du maillage de la surface de l’objet manipulé. Une autre contribution de cette thèse consiste à utiliser de cette modélisation dans la planification des tâches de manipulation d’objets déformables 3D. Cette planification consiste à déterminer la configuration optimale de la main pour la saisie de l’objet ainsi que les trajectoires à suivre et les efforts à appliquer par les doigts pour contrôler la déformation de l’objet tout en assurant la stabilité de l’opération. La validation expérimentale de ces travaux a été réalisée sur deux plateformes robotiques : une main Barrett embarquée sur un bras manipulateur Adept S1700D et une main Shadow embarquée sur un bras manipulateur Kuka LWR4+. / Dexterous manipulation is an important issue in robotics research in which few works have tackled deformable object manipulation. New applications in surgery, food industry or in service robotics require mastering the grasping and manipulation of deformable objects. This thesis focuses on deformable object manipulation by anthropomorphic mechanical graspers such as multi-fingered articulated hands. This task requires a great expertise in mechanical modeling and control: interaction modeling, tactile and vision perception, force / position control of finger movements to ensure stable grasping. The work presented in this thesis focuses on modeling the grasping of deformable objects. To this end, we used a discretization by non-linear mass-spring systems to model deformable bodies in large displacements and deformations while having a low computational cost. To predict the interaction forces between robot hand and deformable object, we proposed an original approach based on a visco-elasto-plastic rheological model to evaluate tangential contact forces and describe the transition between the sticking and slipping modes. The contact forces are evaluated at nodes as function of the relative movements between the fingertips and the surface mesh facets of the manipulated object. Another contribution of this thesis is the use of this model in the planning of 3D deformable object manipulation tasks. This planning consists in determining the optimal configuration of the hand for grasping the objects as well as the paths to track and the efforts to be applied by the fingers to control the deformation of the object while ensuring the stability of the operation. The experimental validation of this work has been carried out on two robotic platforms: a Barrett hand embedded on a Adept S1700D ® manipulator and a Shadow hand embedded on a Kuka LWR4+® manipulator.

La thèse concerne la prévision du comportement dynamique non linéaire d’équipements optique. Les travaux de recherche menés se concentrent sur la simulation des phénomènes vibratoires en jeu, afin de prévoir la réponse harmonique de l’équipement. Ces travaux ouvrent ainsi la voie à des préconisations d’évolutions dans la conception mécanique du projecteur pour augmenter sa durabilité et le confort de vision. En effet les essais pratiqués montrent que de forts niveaux de vibrations endommagent les composants du projecteur et détériorent la stabilité du faisceau d’éclairage. Afin d’éviter de telles nuisances, la conception du projecteur doit être adaptée grâce à un modèle mécanique qui intègre des comportements non linéaires causés essentiellement par les liaisons pour prévoir le mieux possible les niveaux de vibrations du projecteur embarqué. L’état de l’art est réalisé sur les comportements dynamiques non linéaires, les modèles et méthodes de résolution associés, puis les estimateurs existants de quantification des non linéarités. Les essais d’analyse modale réalisés mettent en évidence la présence de phénomènes non linéaires dus à de multiples causes (jeux-butées, frottements, stick-slip, …) localisées dans les liaisons réflecteur-boitier. Les caractérisations expérimentales menées sur les liaisons, délivrent des boucles efforts-déflexion qui montrent différents types de comportement non linéaire, aident au choix des modèles les plus pertinents et au calage de leurs paramètres. Les limites de validité des modèles de calculs linéaires sont déterminées par des critères formulés empiriquement. Les modèles non linéaires de liaison sélectionnés sont intégrés dans un modèle réduit à un puis à deux degrés de liberté d’un projecteur. La réprésentativité du modèle est évaluée sur la base des analyses modales mesurées du projecteur. Les équations décrivent le comportement dynamique de projecteur et les non linéarités sous l’hypothèse de régime stationnaire. La méthode de balance harmonique associée à une technique de continuation par longueur d’arc résout rapidement les équations et détermine avec précision les réponses dynamiques établies. L’étude est complétée par l’analyse de stabilité selon la théorie de Floquet qui met en évidence la présence des branches de solutions stables ou instables. Enfin des réponses harmoniques sont calculées avec un modèle aux éléments finis du projecteur complet. Les calculs sont basés sur l’identification des modes qui reposent sur la répartition des masses dans la structure, la nature des liaisons. Des études d’influence sont réalisées. Les paramètres étudiés sont les raideurs et précontraintes de contact, le coefficient de frottement, l’amortissement introduit. Leurs impacts sur les niveaux des vibrations sont quantifiés ce qui amène au recalage du modèle éléments finis pour améliorer les résultats modaux du projecteur automobile et sa réponse harmonique forcée. / The thesis deals with the prediction of nonlinear dynamic behavior of automotive headlamps. The attention is focused on building models to estimate the vibration behavior of lighting system to enhance its durability and comfort of vision. Vibration tests show that high levels of vibration damage projector components and degrade the stability of the illuminating beam. To avoid these issus, headlamps design must be adapted to include nonlinear phenomena provided from the joints connecting the reflector and housing subsets. The state of the art is performed on the non-linear dynamic behavior, models and methods and existing estimators quantifying nonlinearities. The modal tests performed demonstrate the presence of non-linear phenomena (clearance, friction, stick-slip, …) located in reflector-housing joints. Experimental investigations carried out on joints show different types of nonlinear behavior and help to identify the most important contact parameters (stiffness and damping). The limits of validity of the linear models are determined by empirically formulated criteria. Selected nonlinear models are integrated in a 1D-model reduced to one then two degrees of freedom of a projector. The representativeness of the model is evaluated basing of modal measurement of headlamp. The Harmonic Balance Method was used to calculate the periodic response. The algorithm calculates also the stability of the periodic solutions found, using Floquet theory, and follows stable or instable branches versus varying system parameters via the arc-length continuation technique. Finally, harmonic responses are predicted with a finite element model of the entire headlamp. The calculations are based on the identification of modes that are based on the weight distribution in the structures and joints proprities. Sensibility studies are carried out on stiffness and preloaded contact, coefficient of friction and damping. Impacts on the vibration levels were quantified that leads to update the finite element model and improve modal and harmonic results of headlamp.

Il est de plus en plus admis que l’effet d’échelle doit être pris en compte dans la conception des structures de Génie Civil. Pour le béton, ce problème est complexe car celui-ci ne possède pas d’adoucissement plastique, et sa rupture est due à la fissuration caractérisée par une grande zone de microfissuration (fracture process zone) qui dépend de la taille du granulat max d .Cette fissuration passe par un adoucissement sous la forme de microfissures et de glissement interparticules. Expérimentalement, l’effet d’échelle sur le béton est très souvent étudié à l’aide des corps d’épreuves homothétiques entaillés où l’on cherche à relier la résistance nominale ( oN )estimée à partir de la charge de rupture en flexion à une dimension caractéristique D. Ceci conduit à une diminution du ratio dmax/D avec l’augmentation de la taille de la structure. Parmi les objectifs de cette thèse est d’étudier expérimentalement l’impact de l’hétérogénéité ( dmax/D)supposé comme facteur fondamental de l’effet d’échelle. Trois coupures granulaires ont été testées sur trois tailles de poutres différentes en suivant le processus de fissuration par émission acoustique et la technique de corrélation d’images. Celles-ci permettent de suivre l’ouverture des fissures et identifient assez clairement la FPZ. Les résultats mettent en évidence une grande influence de la taille du granulat sur le comportement à la rupture du béton. Il existe une relation directe entre les paramètres de l’effet d’échelle obtenus par la loi de Bazant et la taille du granulat( dmax ). Le traitement des résultats d’une même taille avec différents granulométries dans le même diagramme conduit à la même loi d’effet d’échelle structurelle classique avec une valeur de transition identique. La modélisation du comportement mécanique est effectuée par la méthode d’éléments discrets (DEM). Le modèle de contact linéaire ne s’avère pas adéquat pour le mortier et le béton où le rapport compression / traction est très élevé. De ce fait, Il a été modifié pour prendre en compte la contribution des moments inter-granulaires. Les paramètres micromécaniques sont déterminés par des essais classiques avec une analyse inverse en utilisant l’algorithme de Levenberg-Marquardt. Les résultats montrent que cette approche est capable de reproduire le comportement à la fissuration locale du béton et de reproduire l’effet d’échelle et celui des granulats. Ensuite, un modèle d'adoucissement est développé afin de mieux reproduire la réponse post pic et le processus de fissuration. / It is now commonly understood that in the design of civil engineering structures, size effect must be taken into consideration. For concrete, this problem is complex because it does not exhibit plastic softening. The failure of concrete is generally preceded by propagation of cracks, characterized by alarge microcracking zone (fracture process zone or FPZ) which is proportional to the maximum aggregate size ( dmax ). This fracture process is accompanied by strain-softening in the form of microcracking and fractional slip.Experimentally, size effect in concrete is commonly studied by using geometrically similar notched beams where thenominal strength ( oN ) obtained from the bending failure loadis related to the characteristic dimension (D). This leads to adecrease in the ratio of dmax/D with an increase in the size of the structure. One of the objective of this thesis is to study experimentally the effect of heterogeneity ( dmax/D) size. This ratio is recognized as a fundamental factor causing the size effect. Three aggregate grading segments were tested on three different sizes of beams and the cracking process was investigated by acoustic emission and the image correlation technique. These methods make it possible to trace the crack.openings and identify distinctively the FPZ. The results demonstrate a significant influence of the aggregate size on the fracture behaviour of concrete. There is a direct relationship between the size effect parameters obtained by Bazant's law and maximum aggregate size ( dmax ). The results obtained from the specimen having the same size but made of concretes with different aggregate sizes produced the same classical size effect with identical transitional between LEFM and strength based laws. The mechanical behaviour is modelled by the Discrete Element Method (DEM). However, the linear contact model inserted in DEM is not suitable to satisfy the materials like mortar and concrete with high unconfined compressive strength to tensile strength ratio. As a result, the model is modified to take into account the contribution of interparticle moments. The micromechanical parameters are determined by conventional tests with inverse analysis using the Levenberg-Marquardt algorithm. The results showed that this approach is able to reproduce the local cracking behaviour of concrete as well as classical size effect and aggregate size effect. Then, a softening model is developed to better reproduce the post-peak response and the cracking process.

Contact mechanical study of layered structures is useful to various fields of engineering, such as - mechanical engineering, civil engineering, materials engineering and biomechanics. Thin hard film coating on a compliant substrate used in cutting tool industry is an example of a layered structure. The protective coating saves the substrate from fracture and wear. However, due to film material brittleness, fracture in the films is of concern. We have developed an analytical model for a film-substrate bilayer system under normal contact loading, which helps us to obtain the stress distribution in the film and fracture behaviour. Our contact model is based on Hankel’s Transform technique, where we assume a Hertzian pressure boundary condition. At each depth of penetration of the indenter in the film-substrate system, we estimate effective modulus of the system based on Gao’s approach. We have validated our analysis by surface strain measurements and photoelastic stress study in the film on a substrate. Experimental observations from literatures show the dependence of different fracture modes in a thin hard film with columnar structure on film thickness and substrate plasticity. We perform fracture analysis, a parametric study of the fracture modes in the film under contact loading. When the film thickness is small and the substrate is relatively hard (e.g. tool steel), the film and the substrate deform conformally under contact loading and the columns of TiN slide against each other into the substrate. On the other hand, when the film is thicker and the substrate is soft (e.g. mild steel or aluminium), the strain mismatch between the film and substrate acts as an added traction at the interface and drives cracks, such as radial tensile stress driven bending cracks that start from the interface at the center of indentation; maximum shear stress driven inclined shear crack that starts inside the film and propagate at an angle to the indentation axis and tensile stress driven edge crack that starts from the free surface outside the contact. We can draw a fracture map based on these calculations which provides a guide to select film thickness depending on the substrate hardness, so that the benign mode of damage, i.e., columnar shear occurs in the film. Apart from generating the fracture map, we can obtain rationale for different fracture phenomenon in the film by studying the indentation stress field. Principal tensile stresses, responsible for driving edge cracks from the free surface outside the contact, become compressive as one approaches the substrate if the substrate is compliant. The cracks therefore do not penetrate deep into the film rather curve away from the axis of indentation. At the transition zone from one mode of damage to other in the fracture map, different modes of fracture may co-exist. The whole column may not shear, rather the shear can start from somewhere in the middle of the film, where the shear stress is maximum and it can end without reaching the interface. The indentation energy is then dissipated in other forms of damage. The contact analysis is further applied to TiN /AlTiN multilayered films having similar elastic properties. Experimental observations suggest that with decreasing layer thickness the fracture resistance of the multilayers increase and some plastic yielding occurs at the top layers of the film. However no substantial change in strain capacity (Hardness/ Young’s Modulus) of the film is observed. Hence we attribute the increase of fracture resistance of multilayers to film plasticity and mimic it by reducing the modulus of the film. The analysis validates the propensity of edge cracking and transgranular cracking as they decrease with increasing number of layers in a multilayer. We next extend our bilayer analysis to a more general trilayer problem where the moduli of the layers vary by several orders. The test system here is a mica-glue-glass system which is used in surface force apparatus experiments. Gao’s trilayer analysis is used to fit the experimental data obtained from surface force apparatus experiments, where a glass sphere indents the trilayer. The parallel spring model used in Gao’s approximation is found to be inadequate to rationalize the experimental data. We have modified Gao’s formulations by reducing the problem to a bilayer problem where the layers are the first layer (in contact) and an equivalent layer which has properties determined by a rule of mixture of the properties of all the layers excluding the top layer set out as a set of springs in series. The modified formulations give a better fit to the experimental data and it is validated from nanoindentation experiments on the same system. The formulation is used to obtain the compression of the glue, which contributes significantly to the deformation of the trilayer system in the SFA experiments. Thus, the analysis can be used to deconvolute the influence of glue in the actual mechanical response of the system in an SFA experiment, which has so far been neglected.