Brainstem derived serotonin (5-HT) normally facilitates spinal motoneuron excitability and inhibits sensory afferent transmission and associated spinal reflexes. Because the 5-HT innervation of the spinal cord is almost exclusively derived from brainstem neurons, spinal cord injury leads to an immediate and dramatic loss of 5-HT and this in turn leads to the simultaneous loss of motoneuron excitability and increase (disinhibition) of sensory afferent transmission. This thesis examined how spinal cord 5-HT receptors adapt over the months after SCI (chronic injury) to compensate for the loss of 5-HT. We showed that after SCI 5-HT2B and 5-HT2C receptors become constitutively active (active in the absence of 5-HT) with chronic injury, and this leads to a recovery of motoneuron excitability and contributes to the recovery of locomotor function. Unfortunately, this also contributes to the development of muscle spasms when combined with the disinhibition of sensory afferent transmission. In contrast, 5-HT1 receptors that modulate sensory afferent transmission do not become constitutively active after chronic SCI, and this contributes to the continued disinhibition of sensory afferent transmission and associated hyperreflexia and muscle spasms after chronic SCI. However, exogenous application of 5-HT1B and 5-HT1F receptor agonists can restore inhibition over sensory afferent transmission and ultimately reduce muscle spasms. In summary, 5-HT2 receptors exhibit a remarkable adaptation to the loss of 5-HT with SCI, whereas 5-HT1 receptors do not. Understanding and promoting this natural plasticity may help in the development of better therapeutic interventions for treating SCI.

No description available.

The contributions of this thesis are an implicit modelling method for the coupled constitutive relations and an energy-based method for material modelling. The two developed methods utilise implicit models to represent material constitutive relations without the requirement of physical parameters. The first method is developed to model coupled constitutive relations using state-space representation with neural networks. State-space representation is employed to express the desired relations in a compact fashion while simultaneously providing the capability of modelling rate- and/or path-dependent behaviour. The employment of neural networks with the generalised state-space representation results in a single implicit model that can be adapted for a broad range of constitutive behaviours. The performance and applicability of the method are highlighted through the applications for various constitutive behaviour of piezoelectric materials, including the effects of hysteresis and cyclic degradation. An energy-based method is subsequently developed for implicit constitutive modelling by utilising the energy principle on a deformed continuum. Two formulations of the proposed method are developed for the modelling of materials with varying nature in directional properties. The first formulation is based on an implicit strain energy density function, represented by a neural network with strain invariants as input, to derive the desired stress-strain relations. The second formulation consists of the derivation of an energy-based performance function for training a neural network that represents the stress-strain relations. The requirement of deriving stress is eliminated in both formulations and this facilitates the use of advanced experimental setup, such as multi-axial load tests or non-standard specimens, to produce the most information for constitutive modelling from a single experiment. A series of numerical studies -- including validation problems and practical cases with actual experimental setup -- have been conducted, the results of which demonstrate the applicability and effectiveness of the proposed method for constitutive modelling on a continuum basis.

This thesis explores the design of flexible pipes, buried in shallow trenches with dry sand backfill. The thesis reports the comprehensive analysis of twenty-two full-scale load tests conducted between 1989 and 1991 on pipe installations, mainly within a laboratory facility, at the University of South Australia. The pipes were highly flexible, spirally-wound, uPVC pipes, ranging in diameter from 300 to 450 mm. Guidelines were required by industry for safe cover heights for these pipes when subjected to construction traffic. The tests were designed by, and conducted under the supervision of, the author, prior to the author undertaking this thesis. As current design approaches for pipes could not anticipate the large loading settlements and hence, soil plasticity, experienced in these tests, finite element analyses were attempted. Extensive investigations of the materials in the installations were undertaken to permit finite element modelling of the buried pipe installations. In particular, a series of large strain triaxial tests were conducted on the sand backfill in the buried pipe installations, to provide an understanding of the sand behaviour in terms of critical state theory. Subsequently a constitutive model for the soil was developed. The soil model was validated before implementation in an element of finite element program, AFENA (Carter and Balaam, 1995). Single element modelling of the triaxial tests proved invaluable in obtaining material constants for the soil model. The new element was applied successfully to the analysis of a side-constrained, plate loading test on the sand. The simulation of the buried pipe tests was shown to require three-dimensional finite element analysis to approach the observed pipe-soil behaviour. Non-compliant side boundary conditions were ultimately adjudged chiefly responsible for the difficulty in matching the experimental data. The value of numerical analyses performed in tandem with physical testing was apparent, albeit in hindsight. The research has identified the prediction of vertical soil pressure above the pipe due to external loading as being the major difficulty for designers. Based on the finite element analyses of the field tests, a preliminary simple expression was developed for estimation of these pressures, which could be used with currently available design approaches to reasonably predict pipe deflections.

abstract: A method for evaluating the integrity of geosynthetic elements of a waste containment system subject to seismic loading is developed using a large strain finite difference numerical computer program. The method accounts for the effect of interaction between the geosynthetic elements and the overlying waste on seismic response and allows for explicit calculation of forces and strains in the geosynthetic elements. Based upon comparison of numerical results to experimental data, an elastic-perfectly plastic interface model is demonstrated to adequately reproduce the cyclic behavior of typical geomembrane-geotextile and geomembrane-geomembrane interfaces provided the appropriate interface properties are used. New constitutive models are developed for the in-plane cyclic shear behavior of textured geomembrane/geosynthetic clay liner (GMX/GCL) interfaces and GCLs. The GMX/GCL model is an empirical model and the GCL model is a kinematic hardening, isotropic softening multi yield surface plasticity model. Both new models allows for degradation in the cyclic shear resistance from a peak to a large displacement shear strength. The ability of the finite difference model to predict forces and strains in a geosynthetic element modeled as a beam element with zero moment of inertia sandwiched between two interface elements is demonstrated using hypothetical models of a heap leach pad and two typical landfill configurations. The numerical model is then used to conduct back analyses of the performance of two lined municipal solid waste (MSW) landfills subjected to strong ground motions in the Northridge earthquake. The modulus reduction "backbone curve" employed with the Masing criterion and 2% Rayleigh damping to model the cyclic behavior of MSW was established by back-analysis of the response of the Operating Industries Inc. landfill to five different earthquakes, three small magnitude nearby events and two larger magnitude distant events. The numerical back analysis was able to predict the tears observed in the Chiquita Canyon Landfill liner system after the earthquake if strain concentrations due to seams and scratches in the geomembrane are taken into account. The apparent good performance of the Lopez Canyon landfill geomembrane and the observed tension in the overlying geotextile after the Northridge event was also successfully predicted using the numerical model. / Dissertation/Thesis / Ph.D. Civil and Environmental Engineering 2011

[pt] No presente trabalho são introduzidas considerações sobre modelos constitutivos viscoelasticos e suas formulações. É apresentada uma teoria de estruturas uni-dimensionais imersas no espaço tri-dimensional, a partir de uma formulação diferencial, permitindo-se a análise de problemas no contexto de grandes deformações.(caracteristicas não-lineares). Adota-se uma lei constitutiva viscoelastica linear, sob forma diferencial. São resolvidos três problemas planos que caracterizam a aplicação do modelo proposto: 1) Viga reta engastada submetida em sua extremidade livre a uma força não-seguidora compressiva cuja direção coincide com a do eixo longitudinal da viga na configuração indeformada. 2) Viga curve bi-rotulada com ambas as extremidades Fixas carregada uniformemente. 3) Viga reta engastada submetida em sua extremidade livre a uma força cíclica perpendicular ao eixo longitudinal da viga na configuração indeformada. Nos dois primeiros problemas é adotada a hipótese quasi-estatica, conservando a influência da inercia apenas no terceiro problema quando é feito um estudo comparativo entre o modelo elástico e o viscoelástico. São propostos os métodos numéricos de solução para os casos viscoelásticos quasi-estáticos - baseados num acoplamento do Método do Tiro (Runge-Kutta de Quarta ordem / Steffensen); considerando o problema espacial, e de Euler Modificado; na evolução temporal - e para o caso dinâmico, valendo-se da aproximação de diferenças finitas para avaliar as velocidades e acelerações. / [en] In the present work, considerations about viscoelastic cosntitutive models and their formulations are introduced. A theory of unidimensionail structures embeded in the tri-dimensional space is presented in a differential formula, allowing the analysis of problems in the large deformations point of view. A linear viscoelastic consitutive law is adopted in a differential representation. Three plane problems which characterize the application of the proposed model are solved: 1) Straight cantilever subjected, at the free end, to a compressive non follower force. The direction of this force coincides with the longitudinal axis of the beam at the undeformed configuration; 2) Curved bean with fixed supports at the ends, under uniform charge. 3) Straight cantilaver subjected at the free end to a cyclic force, perpendicularly to the longitudinal axis of the beam at the undeformed confiquration. In the two first problems; the quasistatic hypothesis is adopted the inertial influence being considered only at the third problems, where a comparative study between the elastic and the viscoelastic models is carried out. The numerical methods for the solution in the quasistatic viscoelastic cases are proposed - based in a coupling between the shooting method (Runge-Kutta of fourth order/Steffensen) for the spatial problem and the Modified Euler Method for the temporal evolution - as well as for the solution of the dynamical case - employing an approximation by finite differences for the evaluations of the velocities and accelerations.

abstract: Shock loading is a complex phenomenon that can lead to failure mechanisms such as strain localization, void nucleation and growth, and eventually spall fracture. Studying incipient stages of spall damage is of paramount importance to accurately determine initiation sites in the material microstructure where damage will nucleate and grow and to formulate continuum models that account for the variability of the damage process due to microstructural heterogeneity. The length scale of damage with respect to that of the surrounding microstructure has proven to be a key aspect in determining sites of failure initiation. Correlations have been found between the damage sites and the surrounding microstructure to determine the preferred sites of spall damage, since it tends to localize at and around the regions of intrinsic defects such as grain boundaries and triple points. However, considerable amount of work still has to be done in this regard to determine the physics driving the damage at these intrinsic weak sites in the microstructure. The main focus of this research work is to understand the physical mechanisms behind the damage localization at these preferred sites. A crystal plasticity constitutive model is implemented with different damage criteria to study the effects of stress concentration and strain localization at the grain boundaries. A cohesive zone modeling technique is used to include the intrinsic strength of the grain boundaries in the simulations. The constitutive model is verified using single elements tests, calibrated using single crystal impact experiments and validated using bicrystal and multicrystal impact experiments. The results indicate that strain localization is the predominant driving force for damage initiation and evolution. The microstructural effects on theses damage sites are studied to attribute the extent of damage to microstructural features such as grain orientation, misorientation, Taylor factor and the grain boundary planes. The finite element simulations show good correlation with the experimental results and can be used as the preliminary step in developing accurate probabilistic models for damage nucleation. / Dissertation/Thesis / Ph.D. Mechanical Engineering 2013

Modelos constitutivos são relações matemáticas entre grandezas físicas que buscam descrever o comportamento dos materiais quando submetidos a ações externas. A hipoplasticidade é um modelo constitutivo desenvolvido para solos a partir de uma modificação da equação hipoelástica. Este modelo tem como principais características a existência de uma única equação constitutiva e o seu caráter não linear, o que lhe confere a propriedade de introduzir deformações irreversíveis desde o início das ações externas. Neste trabalho são estudados dois novos modelos desenvolvidos com o objetivo de melhorar as previsões para carregamentos cíclicos. O primeiro, denominado hipoplasticidade estendida, é caracterizado pela introdução de superfícies de memória e uma nova equação constitutiva específica para o recarregamento. O segundo modelo, a hipoplasticidade cíclica, é uma modificação deste último onde são introduzidos fatores capazes de modificar as superfícies de memória. Os novos modelos são primeiramente aplicados em situações teóricas para verificar sua aplicabilidade. Posteriormente, utilizando dados experimentais, é feita sua calibração e aplicação para então compararem-se as previsões teóricas com os resultados experimentais. Verifica-se que os novos modelos contemplam avanços significativos na previsão do comportamento dos solos sob carregamentos cíclicos. Para permitir um número maior de simulações foi desenvolvida uma planilha eletrônica com a capacidade de representar quantos ciclos sejam desejados, efetuar a alteração dos parâmetros do solo durante a calibração do modelo de maneira fácil e rápida, assim como visualizar para cada um dos intervalos se foi utilizada a equação geral ou a específica do recarregamento. / Constitutive models are mathematical relationships between physical quantities that approximate the behavior of materials when subjected to external actions. Hypoplasticity is a constitutive model developed for soils from a modification of the hypoelastic equation. The main features of this model are the existence of a unique constitutive equation and its nonlinear character, which gives it the property of introducing irreversible deformation from the beginning of external actions. In this work two new models developed in order to improve the predictions of cyclic loading are studied. The first one, which is called extended hypoplacity, has as its main feature the addition of a memory surface and the introduction of a new equation specific for reloading. The second model, cyclic hypoplasticity, which is a modification of this last one, is characterized by the introduction of factors that are capable of modifing the memory surfaces. The new models are first checked in theoretical situations to verify their applicability. Subsequently, using experimental data, the models are calibrated, applied, and then compared to experimental results. The new models include significant advances in predicting soil behavior under cyclic loading. To allow a larger number of simulations, a spreadsheet was developed with the following abilities: simulate as many cycles as are desired; easy to change soil\'s parameters during the calibration phase; and display for each of the intervals which of the equations was used.

Este trabalho trata da formulação de leis constitutivas para meios elásticos, que uma vez danificados passam a apresentar diferentes comportamentos em tração e em compressão e certo grau de anisotropia. Inicialmente é revista uma formulação para meios elásticos anisótropos e bimodulares, sendo a mesma, então, estendida para incorporar os casos de meios elásticos com anisotropia e bimodularidade induzidas pelo dano. Seguindo os conceitos da mecânica do dano contínuo e a extensão mencionada da formulação, propõe-se um modelo constitutivo para o concreto assumindo por hipótese fundamental, a equivalência de energia entre meio danificado real e meio contínuo equivalente. Tal hipótese garante a simetria do tensor constitutivo e a sua consistência termodinâmica. De acordo com o modelo proposto, o material é considerado como um meio elástico inicialmente isótropo que passa a apresentar anisotropia induzida pela evolução do dano. Além disso, a danificação pode também induzir uma resposta bimodular no material, isto é, respostas elásticas diferentes para estados de tensão de tração ou de compressão predominantes. Nesse sentido, dois tensores de dano governando as rigidezes em regimes predominantes de tração ou de compressão são introduzidos. Sugere-se então, um critério afim de caracterizar os estados dominantes. As deformações permanentes induzidas pelo dano são, de uma forma geral, desconsideradas. No entanto, propõe-se uma versão unidimensional do modelo que permite a sua consideração. Por outro lado, os critérios para a ativação inicial dos processos de danificação e de sua posterior evolução são escritos em termos de densidade de energia de deformação. Os parâmetros do modelo podem ser identificados mediante experimentos, onde estados de tensão uniaxial e biaxial são induzidos. Também propõem-se leis de evolução de dano com base nos resultados experimentais. A boa coerência do modelo é ilustrada comparando-se uma série de respostas experimentais e numéricas no concreto relativas a estados de tensão uni, bi e triaxiais. Por fim, o modelo é empregado em análises unidimensionais e planas de vigas e pórtico em concreto armado com o objetivo de mostrar a sua potencialidade. / This thesis deals with the formulation of constitutive laws for elastic media that start to present different behaviours in tension and compression and some anisotropy degree when damaged. Initially a formulation for bimodular and anisotropic elastic media is reviewed; then it is extended to take into account the bimodularity and anisotropy induced by damage. Following the concepts of the continuum damage mechanics and the formulation extension, a constitutive model is proposed here by exploring the fundamental hypothesis of energy equivalence between real and continuous medium. Such hypothesis guarantees symmetry to the constitutive tensor and its thermodynamic consistency. According to the proposed modeling, the material is assumed as an initial elastic isotropic medium presenting anisotropy induced by damage evolution. Moreover, damage can also induce a bimodular response in the material, i.e., distinct elastic responses whether traction or compression stress states prevail. To take into account the bimodularity conveniently, two damage tensors governing the rigidity in traction or compression regimes are introduced. A criterion is proposed in order to characterize the dominant states. In general, the permanent strains induced by damage are disregarded. However, a one-dimensional version of the model that allows considering permanent strains is proposed. On the other hand, damage criteria indicating the initial and further evolution of damage are expressed in terms of strain energy densities. The model parameters can be identified from experiments where one-axial and two-axial stress states are induced. Damage evolution laws are also proposed on the basis of experimental results. The good performance of the model is illustrated by comparing a variety of numerical and experimental responses, from one to three-axial stress states. Finally, the model is used in one-dimensional and plane analysis of reinforced concrete beams and frame in order to show its potentiality.

Esta pesquisa apresenta uma ampliação da investigação da capacidade de um modelo constitutivo hipoplástico de representar o comportamento tensão-deformação dos solos. Os modelos hipoplásticos apresentam características que os toram mais adequados, em muitas situações, a fornecer respostas mais realistas que os modelos baseados na elastoplasticidade clássica, que são os mais utilizados na prática da engenharia atual. O trabalho então consistiu da simulação, através de um equacionamento hipoplástico, de ensaios triaxiais drenados realizados em solos e trajetórias de tensão aos quais o modelo em estudo ainda não havia sido aplicado, tais como em corpos de prova adensados anisotropicamente e em trajetórias com decréscimos da tensão desviadora, bem como da comparação entre a resposta do modelo e os respectivos resultados desses ensaios. Em termos qualitativos, os resultados obtidos nesse confronto são bons em todas as situações estudadas. Quantitativamente, são bons na maioria das situações e razoavelmente bons em outras. / This research presents an extension of the investigation of the capacity of a hypoplastic constitutive model to represent the stress-strain behavior of soils. Hipoplastic models have characteristics that make them more suitable, in many situations, to provide more realistic answers than models based on classical elastoplasticity, which are most often used in engineering practice today. The work then consisted in the simulation, through a hypoplastic equation, of drained triaxial tests performed on soils and stress paths to which the model had not been applied yet, such as anisotropically consolidated specimens and paths with decreasing deviatoric stress as well as in the comparison between the response of the model and the results of those tests. The results obtained in this comparison are qualitatively good in all the situations studied. Quantitatively, they are good in most situations and reasonably in the others.