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Yes / An established statistical mechanical theory of amorphous polymer deformation has been incorporated as a plastic mechanism into a constitutive model and applied to a range of polymer mechanical deformations. The temperature and rate dependence of the tensile yield of PVC, as reported in early studies, has been modeled to high levels of accuracy. Tensile experiments on PET reported here are analyzed similarly and good accuracy is also achieved. The frequently observed increase in the gradient of the plot of yield stress against logarithm of strain rate is an inherent feature of the constitutive model. The form of temperature dependence of the yield that is predicted by the model is found to give an accurate representation. The constitutive model is developed in two-dimensional form and implemented as a user-defined subroutine in the finite element package ABAQUS. This analysis is applied to the tensile experiments on PET, in some of which strain is localized in the form of shear bands and necks. These deformations are modeled with partial success, though adiabatic heating of the instability causes inaccuracies for this isothermal implementation of the model. The plastic mechanism has advantages over the Eyring process, is equally tractable,and presents no particular difficulties in implementation with finite elements. / F. Boutenel acknowledges an Erasmus Programme Scholarship

Yes / In constitutive models of polymers, there has been a long history of the use of strain-rate dependent viscous processes, such as the Eyring and Argon models. These are combined with elastic elements to generate viscoplastic models that exhibit typical phenomena such as rate dependent yield, creep and stress relaxation. The Eyring process is one of the most frequently used such mechanisms. It has two significant drawbacks: it implies a temperature dependence of mechanical behaviour that is in an opposite sense to that observed; and it predicts a strain rate dependence of yield stress that is less complex than that observed, leading to the requirement for two or more Eyring processes. In recent years, new ideas for amorphous polymers have been developed that lead to an alternative plastic mechanism that addresses these concerns. In this paper a constitutive model that incorporates this mechanism is developed, and its effectiveness in modelling macroscopic mechanical behaviour of polymers is explored with respect to published data.

São propostas modificações em um modelo hipoplástico buscando representar o comportamento mecânico de argilas sobreadensadas. São introduzidos no modelo a razão de sobreadensamento, o intercepto de coesão e índices que representam características do trecho sobreadensado, resultando em parâmetros com claro sentido físico e de fácil determinação. A equação constitutiva é então calibrada com parâmetros de solos sobreadensados e a capacidade dos modelos de representar o comportamento de dois solos em particular é verificada, confrontando as previsões teóricas com resultados experimentais em diversas situações de carregamento e para uma larga faixa de razões de sobreadensamento. Verifica-se que as modificações introduzidas no modelo hipoplástico contemplam avanços significativos na representação dos efeitos do sobreadensamento, como a curvatura da envoltória de resistência no trecho sobreadensado, o aumento do módulo de deformabilidade com o grau de sobreadensamento, diferente rigidez no carregamento e no recarregamento, o aumento das tensões desviadoras de ruptura em solos sobreadensados, a diminuição da tendência à contração volumétrica com o aumento da razão de sobreadensamento, chegando à expansão volumétrica, e, nas solicitações não-drenadas, pressão neutra negativa quando o solo encontra-se fortemente sobreadensado. / Modifications in a hypoplastic model are proposed intending to represent the mechanical behavior of overconsolidated clays. The overconsolidation ratio, the cohesion intercept and indices that represent overconsolidation characteristics are introduced in the model, resulting in parameters with a clear physical meaning and of easy to determine. The constitutive equation is then calibrated with parameters of overconsolidated soils and the capacity of the models of representing the behavior of two soils in particular is checked, confronting the theoretical predictions with experimental results in several loading situations and in a wide overconsolidation ratio range. It is verified that the modifications introduced into hypoplastic model contemplates significant advances in the representation of the effects of overconsolidation, like the curvature of the strength envelope in the overconsolidation region, the increase of the deformability modulus with the overconsolitadion ratio, different stiffness in loading and reloading, the increase of the deviator stress at failure in overconsolidated soils, presenting peak deviator stress, a decrease of the tendency to volumetric contraction with the increase of the overconsolidation ratio, including to the volumetric expansion, and, in undrained tests, negative pore pressure when the soil is heavily overconsolidated.

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.

São propostas modificações em um modelo hipoplástico buscando representar o comportamento mecânico de argilas sobreadensadas. São introduzidos no modelo a razão de sobreadensamento, o intercepto de coesão e índices que representam características do trecho sobreadensado, resultando em parâmetros com claro sentido físico e de fácil determinação. A equação constitutiva é então calibrada com parâmetros de solos sobreadensados e a capacidade dos modelos de representar o comportamento de dois solos em particular é verificada, confrontando as previsões teóricas com resultados experimentais em diversas situações de carregamento e para uma larga faixa de razões de sobreadensamento. Verifica-se que as modificações introduzidas no modelo hipoplástico contemplam avanços significativos na representação dos efeitos do sobreadensamento, como a curvatura da envoltória de resistência no trecho sobreadensado, o aumento do módulo de deformabilidade com o grau de sobreadensamento, diferente rigidez no carregamento e no recarregamento, o aumento das tensões desviadoras de ruptura em solos sobreadensados, a diminuição da tendência à contração volumétrica com o aumento da razão de sobreadensamento, chegando à expansão volumétrica, e, nas solicitações não-drenadas, pressão neutra negativa quando o solo encontra-se fortemente sobreadensado. / Modifications in a hypoplastic model are proposed intending to represent the mechanical behavior of overconsolidated clays. The overconsolidation ratio, the cohesion intercept and indices that represent overconsolidation characteristics are introduced in the model, resulting in parameters with a clear physical meaning and of easy to determine. The constitutive equation is then calibrated with parameters of overconsolidated soils and the capacity of the models of representing the behavior of two soils in particular is checked, confronting the theoretical predictions with experimental results in several loading situations and in a wide overconsolidation ratio range. It is verified that the modifications introduced into hypoplastic model contemplates significant advances in the representation of the effects of overconsolidation, like the curvature of the strength envelope in the overconsolidation region, the increase of the deformability modulus with the overconsolitadion ratio, different stiffness in loading and reloading, the increase of the deviator stress at failure in overconsolidated soils, presenting peak deviator stress, a decrease of the tendency to volumetric contraction with the increase of the overconsolidation ratio, including to the volumetric expansion, and, in undrained tests, negative pore pressure when the soil is heavily overconsolidated.

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.

Silicon in single crystal form has been the material of choice for the first demonstration of the MIT microengine project. However, because it has a relatively low melting temperature, silicon is not an ideal material for the intended operational environment of high temperature and stress. In addition, preliminary work indicates that single crystal silicon has a tendency to undergo localized deformation by slip band formation. Thus it is critical to obtain a better understanding of the mechanical behavior of this material at elevated temperatures in order to properly exploit its capabilities as a structural material. Creep tests in simple compression with n-type single crystal silicon, with low initial dislocation density, were conducted over a temperature range of 900 K to 1200 K and a stress range of 10 MPa to 120 MPa. The compression specimens were machined such that the multi-slip <100> or <111> orientations were coincident with the compression axis. The creep tests reveal that response can be delineated into two broad regimes: (a) in the first regime rapid dislocation multiplication is responsible for accelerating creep rates, and (b) in the second regime an increasing resistance to dislocation motion is responsible for the decelerating creep rates, as is typically observed for creep in metals. An isotropic elasto-viscoplastic constitutive model that accounts for these two mechanisms has been developed in support of the design of the high temperature turbine structure of the MIT microengine. / Singapore-MIT Alliance (SMA)

No description available.

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.