Carregamentos cíclicos são causados de diversas maneiras, como tráfego de veículos, ondas, vento e terremotos. Nos últimos anos, particularmente, tem-se aumentado o número de estudos para este tipo de carregamento devido ao desenvolvimento da engenharia offshore. Além disso, técnicas de melhoramento de solos granulares têm sido empregadas para alterar as características dos solos naturais, com o objetivo de aumentar sua resistência e retardar - ou evitar - a ocorrência de liquefação. Alguns estudos anteriores desenvolveram leis constitutivas completas para areias reforçadas com fibras sob carregamento monotônico, mas não são encontrados na literatura trabalhos sobre a modelagem deste tipo de solos sob carregamentos cíclicos. Sendo assim, essa dissertação desenvolve e valida um novo modelo constitutivo capaz de avaliar o comportamento de solos granulares reforçados com fibras sob carregamento cíclico sob condições não-drenadas. Este modelo é baseado em dois modelos previamente desenvolvidos por Diambra et al. (2013) e Diambra e Ibraim (2014), que utilizam uma técnica de homogeneização para considerar a contribuição da areia e das fibras. O comportamento da areia segue o Modelo Severn-Trent Sand, proposto por Gajo e Muir Wood (1999). Uma vez estruturado o modelo e definido seu procedimento de cálculo, realiza-se uma análise paramétrica, a fim de demonstrar a influência de cada parâmetro das fibras e da areia no comportamento do compósito. Um fator de ajuste para levar em consideração a mudança nas forças interparticulares causada pelas fibras é proposto neste trabalho. Ao final, o modelo é calibrado com resultados experimentais e faz-se uma análise de suas competências e limitações. O processo de calibração mostrou que o modelo é capaz de capturar importantes tendências causadas pela inserção de fibras, como a redução nas deformações axiais e na geração de poropressões, retardando a ocorrência de liquefação. O modelo proposto mostrou-se mais efetivo em reproduzir o comportamento de areias fofas, ou seja, aquelas cujo estado de tensões se encontra acima da linha do estado crítico. / Cyclic loads are induced by several sources, such as traffic, waves, wind and earthquakes. Particularly in the last years, more attention has been given to such loading conditions due to the development of the offshore engineering. Additionally, ground improving techniques have been employed to alter the characteristics of natural soils in order to increase its strength and delay – or avoid – liquefaction. Previous studies have developed complete constitutive laws for fibre-reinforced sands under monotonic loading conditions, but no previous work on modelling granular soils under cyclic loading has been reported. Hence, this research develops and validates a new constitutive modelling which is capable to fully assess the behaviour of fibre-reinforced soils under cyclic loads for undrained conditions. This model is based on two previous models developed by Diambra et al. (2013) and Diambra and Ibraim (2014), which employed a homogenisation technique to scale sand and fibre contribution. The behaviour of the sand follows the Severn-Trent Sand Model proposed by Gajo and Muir Wood (1999). Once the model is structured and its calculation procedure is defined, a parametric analysis is carried out in order to show the influence of each fibre and sand parameter in the composite response. An adjustment factor to account for the change in the interparticle forces caused by the fibres is proposed. Finally, the model is calibrated with experimental results and an analysis of its competences and limitations is performed. The calibration process showed that the model is able to capture important trends caused by the fibre reinforcement, such as a reduction in axial strain and in pore pressure generation, delaying the occurrence of liquefaction. The proposed model was shown to be more effective in reproducing the response of loose sands, i.e. those whose stress states are above the critical state line.

Carregamentos cíclicos são causados de diversas maneiras, como tráfego de veículos, ondas, vento e terremotos. Nos últimos anos, particularmente, tem-se aumentado o número de estudos para este tipo de carregamento devido ao desenvolvimento da engenharia offshore. Além disso, técnicas de melhoramento de solos granulares têm sido empregadas para alterar as características dos solos naturais, com o objetivo de aumentar sua resistência e retardar - ou evitar - a ocorrência de liquefação. Alguns estudos anteriores desenvolveram leis constitutivas completas para areias reforçadas com fibras sob carregamento monotônico, mas não são encontrados na literatura trabalhos sobre a modelagem deste tipo de solos sob carregamentos cíclicos. Sendo assim, essa dissertação desenvolve e valida um novo modelo constitutivo capaz de avaliar o comportamento de solos granulares reforçados com fibras sob carregamento cíclico sob condições não-drenadas. Este modelo é baseado em dois modelos previamente desenvolvidos por Diambra et al. (2013) e Diambra e Ibraim (2014), que utilizam uma técnica de homogeneização para considerar a contribuição da areia e das fibras. O comportamento da areia segue o Modelo Severn-Trent Sand, proposto por Gajo e Muir Wood (1999). Uma vez estruturado o modelo e definido seu procedimento de cálculo, realiza-se uma análise paramétrica, a fim de demonstrar a influência de cada parâmetro das fibras e da areia no comportamento do compósito. Um fator de ajuste para levar em consideração a mudança nas forças interparticulares causada pelas fibras é proposto neste trabalho. Ao final, o modelo é calibrado com resultados experimentais e faz-se uma análise de suas competências e limitações. O processo de calibração mostrou que o modelo é capaz de capturar importantes tendências causadas pela inserção de fibras, como a redução nas deformações axiais e na geração de poropressões, retardando a ocorrência de liquefação. O modelo proposto mostrou-se mais efetivo em reproduzir o comportamento de areias fofas, ou seja, aquelas cujo estado de tensões se encontra acima da linha do estado crítico. / Cyclic loads are induced by several sources, such as traffic, waves, wind and earthquakes. Particularly in the last years, more attention has been given to such loading conditions due to the development of the offshore engineering. Additionally, ground improving techniques have been employed to alter the characteristics of natural soils in order to increase its strength and delay – or avoid – liquefaction. Previous studies have developed complete constitutive laws for fibre-reinforced sands under monotonic loading conditions, but no previous work on modelling granular soils under cyclic loading has been reported. Hence, this research develops and validates a new constitutive modelling which is capable to fully assess the behaviour of fibre-reinforced soils under cyclic loads for undrained conditions. This model is based on two previous models developed by Diambra et al. (2013) and Diambra and Ibraim (2014), which employed a homogenisation technique to scale sand and fibre contribution. The behaviour of the sand follows the Severn-Trent Sand Model proposed by Gajo and Muir Wood (1999). Once the model is structured and its calculation procedure is defined, a parametric analysis is carried out in order to show the influence of each fibre and sand parameter in the composite response. An adjustment factor to account for the change in the interparticle forces caused by the fibres is proposed. Finally, the model is calibrated with experimental results and an analysis of its competences and limitations is performed. The calibration process showed that the model is able to capture important trends caused by the fibre reinforcement, such as a reduction in axial strain and in pore pressure generation, delaying the occurrence of liquefaction. The proposed model was shown to be more effective in reproducing the response of loose sands, i.e. those whose stress states are above the critical state line.

Carregamentos cíclicos são causados de diversas maneiras, como tráfego de veículos, ondas, vento e terremotos. Nos últimos anos, particularmente, tem-se aumentado o número de estudos para este tipo de carregamento devido ao desenvolvimento da engenharia offshore. Além disso, técnicas de melhoramento de solos granulares têm sido empregadas para alterar as características dos solos naturais, com o objetivo de aumentar sua resistência e retardar - ou evitar - a ocorrência de liquefação. Alguns estudos anteriores desenvolveram leis constitutivas completas para areias reforçadas com fibras sob carregamento monotônico, mas não são encontrados na literatura trabalhos sobre a modelagem deste tipo de solos sob carregamentos cíclicos. Sendo assim, essa dissertação desenvolve e valida um novo modelo constitutivo capaz de avaliar o comportamento de solos granulares reforçados com fibras sob carregamento cíclico sob condições não-drenadas. Este modelo é baseado em dois modelos previamente desenvolvidos por Diambra et al. (2013) e Diambra e Ibraim (2014), que utilizam uma técnica de homogeneização para considerar a contribuição da areia e das fibras. O comportamento da areia segue o Modelo Severn-Trent Sand, proposto por Gajo e Muir Wood (1999). Uma vez estruturado o modelo e definido seu procedimento de cálculo, realiza-se uma análise paramétrica, a fim de demonstrar a influência de cada parâmetro das fibras e da areia no comportamento do compósito. Um fator de ajuste para levar em consideração a mudança nas forças interparticulares causada pelas fibras é proposto neste trabalho. Ao final, o modelo é calibrado com resultados experimentais e faz-se uma análise de suas competências e limitações. O processo de calibração mostrou que o modelo é capaz de capturar importantes tendências causadas pela inserção de fibras, como a redução nas deformações axiais e na geração de poropressões, retardando a ocorrência de liquefação. O modelo proposto mostrou-se mais efetivo em reproduzir o comportamento de areias fofas, ou seja, aquelas cujo estado de tensões se encontra acima da linha do estado crítico. / Cyclic loads are induced by several sources, such as traffic, waves, wind and earthquakes. Particularly in the last years, more attention has been given to such loading conditions due to the development of the offshore engineering. Additionally, ground improving techniques have been employed to alter the characteristics of natural soils in order to increase its strength and delay – or avoid – liquefaction. Previous studies have developed complete constitutive laws for fibre-reinforced sands under monotonic loading conditions, but no previous work on modelling granular soils under cyclic loading has been reported. Hence, this research develops and validates a new constitutive modelling which is capable to fully assess the behaviour of fibre-reinforced soils under cyclic loads for undrained conditions. This model is based on two previous models developed by Diambra et al. (2013) and Diambra and Ibraim (2014), which employed a homogenisation technique to scale sand and fibre contribution. The behaviour of the sand follows the Severn-Trent Sand Model proposed by Gajo and Muir Wood (1999). Once the model is structured and its calculation procedure is defined, a parametric analysis is carried out in order to show the influence of each fibre and sand parameter in the composite response. An adjustment factor to account for the change in the interparticle forces caused by the fibres is proposed. Finally, the model is calibrated with experimental results and an analysis of its competences and limitations is performed. The calibration process showed that the model is able to capture important trends caused by the fibre reinforcement, such as a reduction in axial strain and in pore pressure generation, delaying the occurrence of liquefaction. The proposed model was shown to be more effective in reproducing the response of loose sands, i.e. those whose stress states are above the critical state line.

Over the past decades simulation techniques, and in particular finite element method, have been used successfully to predict the response of systems across a whole range of industries including aerospace, automotive, chemical processes, geotechnical engineering and many others. In these numerical analyses, the behaviour of the actual material is approximated with that of an idealised material that deforms in accordance with some constitutive relationships. Therefore, the choice of an appropriate constitutive model that adequately describes the behaviour of the material plays an important role in the accuracy and reliability of the numerical predictions. During the past decades several constitutive models have been developed for various materials. In recent years, by rapid and effective developments in computational software and hardware, alternative computer aided pattern recognition techniques have been introduced to constitutive modelling of materials. The main idea behind pattern recognition systems such as neural network, fuzzy logic or genetic programming is that they learn adaptively from experience and extract various discriminants, each appropriate for its purpose. In this thesis a novel approach is presented and employed to develop constitutive models for materials in general and soils in particular based on evolutionary polynomial regression (EPR). EPR is a hybrid data mining technique that searches for symbolic structures (representing the behaviour of a system) using genetic algorithm and estimates the constant values by the least squares method. Stress-strain data from experiments are employed to train and develop EPR-based material models. The developed models are compared with some of the existing conventional constitutive material models and its advantages are highlighted. It is also shown that the developed EPR-based material models can be incorporated in finite element (FE) analysis. Different examples are used to verify the developed EPR-based FE model. The results of the EPR-FEM are compared with those of a standard FEM where conventional constitutive models are used to model the material behaviour. These results show that EPR-FEM can be successfully employed to analyse different structural and geotechnical engineering problems.

In this licentiate thesis the work done in the TURBO POWER project Influence of high temperature hold times on the fatigue life of nickel-based superalloys will be presented. The overall objective of this project is to develop and evaluate tools for designing against fatigue in gas turbine applications, with special focus on the nickel-based superalloy Inconel 718. Firstly, the constitutive behaviour of the material has been been studied, where focus has been placed on trying to describe the mean stress relaxation and initial softening of the material under intermediate temperatures. Secondly, the fatigue crack propagation behaviour under high temperature hold times has been studied. Focus has here been placed on investigating the main fatigue crack propagation phenomena with the aim of setting up a basis for fatigue crack propagation modelling. This thesis is divided into two parts. The first part describes the general framework, including basic constitutive and fatigue crack propagation behaviour as well as a theoretical background for the constitutive modelling of mean stress relaxation. This framework is then used in the second part, which consists of the four included papers.

Recent advances in computational mechanics have opened the potential of carrying out the analysis and design of concrete structures in a realistic manner with the use of nonlinear concrete models. This encourages the development of more capable and realistic constitutive models, based on a rigorous approach, for the analysis and design of concrete structures. This research focuses on the development of a thermodynamic approach to constitutive modelling of concrete, with emphasis on the rigour and consistency both in the formulation of constitutive models, and in the identification of model parameters based on experimental tests. The key feature of the thermodynamic framework used in this study is that all behaviour of the defined model can be derived from two specified energy potentials. In addition, the derivation of a constitutive model within this framework merely follows procedures established beforehand. The proposed constitutive model here is based on continuum damage mechanics, in combination with plasticity theory, hence enabling the macroscopic material behaviour observed in experiments to be appropriately modelled. Damage-induced softening is the cause of many problems in numerical failure simulations based on conventional continuum mechanics. The resolution of these problems requires an appropriate special treatment for the constitutive modelling which, in this study, is based on nonlocal theory, and realized through the nonlocality of energy terms in the damage loading functions. For practical applications in structural analysis, the model requires a minimum number of parameters, which can be identified from experimental tests. All the above features of the model have been incorporated in a unified and consistent thermodynamic approach, which also distinguish the approach from existing ones. Numerical implementation and application are important parts of the study. A suitable implicit scheme is adapted here for the integration of the nonlocal rate constitutive equations. For the solution of systems of nonlinear algebraic equations in finite element analysis, the arc-length method in combination with local constraint equations employing dominant displacements is implemented, and proves its reliability in this study. Application of the proposed constitutive models in the analysis and design of concrete structures is straightforward, with several numerical examples showing the practical aspects of the proposed modelling.

Le traitement des sols à la chaux est une méthode couramment utilisée pour améliorer les propriétés mécaniques de sols aux performances insuffisantes. Cependant, ces améliorations mécaniques ne sont pas prises en compte dans les calculs de dimensionnement. Cette thèse propose une méthodologie pour pallier à ce problème. Un programme expérimental approfondi est réalisé afin de décrire avec précision le comportement mécanique d’un sol traité et les processus associés à la structuration introduite par le traitement. La composition chimique, et notamment la quantité de composés cimentaires hydratés, est déterminée par analyses thermogravimétriques et thermodifférentielles. Un couplage non-linéaire entre la quantité d’hydrates et la limite élastique se basant sur ces résultats expérimentaux est proposé. À partir de ces résultats, une nouvelle formulation a été développée afin de modéliser la dégradation de la structure en plasticité, et a servi au développement d’une nouvelle loi de comportement élasto-plastique basée sur le modèle de Cam Clay Modifié. Cette dernière a montré reproduire correctement les principaux traits de comportement spécifiques aux sols traités. Il est démontré que ce modèle est également adapté aux sols naturellement structurés. Afin de prendre en compte les effets du traitement et la présence de structure dans le dimensionnement, un programme basé sur la méthode des éléments finis, comprenant le pré- et le post-processing de la géométrie et des résultats du problème, a été développé. Une validation rigoureuse a confirmé l'implémentation correcte de la méthode et son potentiel pour l’optimisation du dimensionnement des ouvrages / The effects of lime treatment on the mechanical properties of soils are usually not accounted for in the design of geotechnical structures. As a result the potential of lime treatment has not been fully exploited. In this thesis, a comprehensive experimental program has been carried out to identity the key features of the mechanical behaviour of structured materials. The chemical modifications arising from lime treatment were quantified using thermal analysis methods. From these results a non-linear chemo-mechanical coupling was established between the concentration of cementitious compounds and the yield stress. Using these results, a new formulation to model the degradation of the structure at yield has been developed and implemented in a constitutive model for structured materials. This new model, developed in the framework of the Modified Cam Clay model, requires a limited number of additional parameters that all have a physical meaning and can all be determined from a single isotropic compression test. The model has proven to be successful in reproducing the key features of structured materials and for the modelling of the mechanical behaviour of lime treated specimens under various stress paths. Due to similarities in behaviour, it is shown that the formulation is also suitable for naturally structured soils. To account for a structured material in the design of geotechnical structures, a fully functional finite element program for elasto-plastic problems was developed including the pre- and post-processing of the results. A thorough validation has confirmed the good implementation of the finite element method and its suitability for the modelling of complex geometries involving structured materials

<p dir="ltr">Defects in composite manufacturing often lead to compromised structural integrity and reduced performance of the final product. A robust constitutive modeling framework is needed to efficiently and accurately predict the deformation responses of dry fabrics and pre-impregnated fibers, paving the way for defect simulation. This thesis presents a comprehensive study on the development and application of a novel constitutive model of fabric preforms and pre-impregnated fibers during composite manufacturing processes.</p><p dir="ltr">This work proposes an integrated constitutive study for textile fabrics in the aspects of mesoscale tow and macroscale fabric behavior. First, a textile architecture-based discrete modeling approach was developed to predict and visualize fiber tow and fabric deformation. The fabrics consist of interlacing virtual fiber tows which are represented by Timoshenko beams joined by translational and rotational springs and rotary dashpots, which are used to capture the energy dissipation during in-plane shear deformation. Second, an anisotropic hyper-viscoelastic model was developed using the strain energy density function of a homogenized unit cell to predict the fabric deformation as a continuous field. A Maxwell model consisting of one Maxwell element and an additional spring is used to consider the nonequilibrium stresses generated during in-plane shear, transverse shear, and through-thickness compaction deformations. Both approaches were experimentally characterized and applied to a hemisphere draping model in the commercial Finite Element Analysis (FEA) software, Abaqus, to demonstrate the predictive capabilities.</p><p dir="ltr">Then, the robust hyper-viscoelastic model is extended to predict prepreg compaction and bending behavior. In the compaction aspect, a coupling term of energy that captures the effect of squeezing flow and a highly nonlinear transverse compression energy are proposed to predict the compaction response of prepreg with liquid and rubbery resin. The viscoelastic parameters were characterized by a Computational Fluid Dynamics (CFD) model for liquid resin and a discrete micromechanics model for rubbery resin. The method was applied to stepwise compaction simulation at different temperatures in Abaqus and compared to experiments for validation. In the bending aspect, the effective shear modulus is expressed as a function of the second-order gradient of deformation. Modeling parameters were characterized by an analytical model that captures the underlying fiber and matrix deformation mechanism. Parametric study was conducted to illustrate the influence of each parameter and the capability to enhance the accuracy of bending prediction.</p>

Thesis (PhD)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: See full text for abstract / AFRIKAANSE OPSOMMING: Sien volteks vir opsomming

Given the promise of the Inductive Power Transfer (IPT) technology for eRoad applications, the potential challenges for a successful integration of dynamic IPT technology into the physical road structure are explored extensively in this research work. The Finite Element Method (FEM) is selected for studying the structural performance of an eRoad under operational conditions. In this, an energy-based finite strain constitutive model for asphalt materials is developed and calibrated, to enable the detailed investigation of the structural response and optimization of the considered eRoad. In the context of enabling both dynamic charging and autonomous driving for future electric vehicles, the influences to the pavement (rutting) performance by the changed vehicle behaviour are investigated as well. Moreover, to study the effect on the IPT system by the integration, the potential power loss caused within eRoad pavement materials is further examined by a combined analytic and experimental analysis. The direct research goal of this Thesis is therefore to enhance the possibility of a sustainable implementation of the eRoad solutions into the real society. At the same time, it aims to demonstrate that the road structure itself is an important part of smart infrastructure systems that can either become a bottleneck or a vessel of opportunities, supporting the successful integration of these complex systems. / Givet de förutsättningar som induktiv energiöverföring (IPT Inductive Power Transfer) har för eRoad applikationerna, utforskas möjligheterna för en framgångsrik integration av dynamisk IPT i den fysiska vägkonstruktionen på en djupgående nivå i detta forskningsarbete. Speciellt har finita elementmetoden använts för att studera det strukturella beteendet hos en e-väg under driftsmässiga förhållanden. Inom detta har en energibaserad konstitutiv model för stora töjningar utvecklats och kalibrerats för att möjliggöra detaljerade undersökningar av strukturell respons och optimering av de föreslagna e-vägarna. I samband med att möjliggöra både dynamisk laddning och autonom körning för framtida elektriska fordon, har beläggningars (spårbildnings)egenskaper studerats utifrån de laddande fordonen beteende. Dessutom för att studera effekten av IPT-systemet har den potentiella energiförlusten inom e-vägars beläggningsmaterial undersökts genom en kombinerad analytisk och experimentell undersökning. Som sådant är det direkta forskningsmålet med denna avhandling att utöka möjligheterna för en hållbar implementering av eRoad systemet inom det verkliga samhället. Samtidigt är målet att visa att vägkonstruktionen i sig själv är en viktig del av det smarta infrastruktursystemet som antingen kan bli en flaskhals eller en bärare av möjligheter, stödjande en framgångsrik implementering av dessa komplexa system. / <p>QC 20161108</p>