Thesis (D.M.A.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 124-126). Available also in a digital version from Dissertation Abstracts.

Thesis (D.M.A.)--University of Texas at Austin, 2000. / Vita. Publisher's no.: UMI 9983125. Includes bibliographical references (leaves 124-126). Available also in a digital version from Dissertation Abstracts.

In this work, understanding the microstructural effects on stress state and strain rate dependent plasticity, damage, and failure of aluminum and magnesium alloys were examined. Several experimental techniques were employed to implement the test data into a physics-based internal state variable plasticity-damage model. Effects arising from various strain rates, stress states, and material orientations were quantified and discussed within the framework of linking microstructural features to mechanical properties. The method developed for determining structure-property relations was validated by accurately capturing the effects for a variety of materials and loading conditions. The end result is a methodology capable of predicting the onset of damage and failure for a material loaded under complex dynamic conditions.

Ce travail a pour but l'optimisation du dépouillement de l'essai aux barres de Hopkinson dans les cas où une analyse classique ne peut s'appliquer. L'analyse classique suppose, en effet, l'équilibre des efforts imposés à l'échantillon au cours du èhargement. Le comportement est, dans ce 'cas assimilable à une situation quasi-statique et où les champs mécaniques dans l'échantillon sont considérés comme homogènes, donné sous la forme d'une courbe contrainte - déformation. Dans certains cas cependant, cette hypothèse d'équilibre est illicite: l'échantillon doit être considéré comme une structure et non plus comme un élément de matière, puisque les effets d'inertie y jouent un rôle important. Il s'agit alors de trouver le comportement de l'échantillon à partir des efforts et des vitesses mesurées durant l'essai. Ceci constitue un problème de type inverse pour lequel l'information (sur le comportement) réside dans la donnée de conditions aux limites duales. On propose une résolution du problème inverse général qui consiste à déterminer le comportement d'une structure pour laquelle on se donne le chargement à la fois en effort et en déplacement sur la frontière, comme fonctions du temps. Cette analyse permet, dans ce cadre mal posé (et pour des problèmes voisins comme le problème de Cauchy), de donner une solution au problème d'évolution de la structure. La résolution de ce problème inversé répose sur une notion d'écart au comportement, développée et discutée dans ce travail.

The importance of understanding non-metallic material behaviour at high strain rates is becoming ever more important as new materials are being developed and used in shock loading applications. Applying conventional methods for high strain rate testing to non-metallic materials proved ineffective due to impedance mismatch between the specimen and apparatus and so a new test method was developed. A polymeric Hopkinson bar was developed enabling non-metallic materials, such as polycarbonate and rubber, to be tested at strain rates from 500 s^-1 to 4000 s^-1. Conventional Hopkinson bar analysis is invalid due to the viscoelastic nature of the polymeric bar material. As waves propagate along the bar length, the inherent material behaviour causes the waves to undergo a degree of attenuation and dispersion. Through the use of spectral analysis, a comparison of the dispersive relationships for 6061 T-6 aluminium, extruded acrylic and low density polyethylene is presented. The application of spectral methods to viscoelastic wave analysis was validated by three separate methods. A comparison of predicted and measured waves along the bar length allowed the dispersive relationship to be substantiated. The use of an enhanced laser velocity system further verified the predicted wave magnitude. A comparison of results for polycarbonate and ballistic gelatin to published results showed good agreement.

The importance of understanding non-metallic material behaviour at high strain rates is becoming ever more important as new materials are being developed and used in shock loading applications. Applying conventional methods for high strain rate testing to non-metallic materials proved ineffective due to impedance mismatch between the specimen and apparatus and so a new test method was developed. A polymeric Hopkinson bar was developed enabling non-metallic materials, such as polycarbonate and rubber, to be tested at strain rates from 500 s^-1 to 4000 s^-1. Conventional Hopkinson bar analysis is invalid due to the viscoelastic nature of the polymeric bar material. As waves propagate along the bar length, the inherent material behaviour causes the waves to undergo a degree of attenuation and dispersion. Through the use of spectral analysis, a comparison of the dispersive relationships for 6061 T-6 aluminium, extruded acrylic and low density polyethylene is presented. The application of spectral methods to viscoelastic wave analysis was validated by three separate methods. A comparison of predicted and measured waves along the bar length allowed the dispersive relationship to be substantiated. The use of an enhanced laser velocity system further verified the predicted wave magnitude. A comparison of results for polycarbonate and ballistic gelatin to published results showed good agreement.

Esta dissertação apresenta um projeto de um dispositivo para levantar dados característicos de materiais submetidos às altas taxas de deformação, dispositivo este que leva o nome do seu idealizador o engenheiro Inglês Sir Bertram Hopkinson. Mais especificamente, esta dissertação está inseparavelmente ligada ao desenvolvimento de um embalado para transporte de elementos radioativos como sendo uma das partes do escopo geral, de um projeto da CAPES em convênio com o Centro de Ciência e Tecnologia de Materiais - CCTM do, Instituto de Pesquisas Energéticas e Nucleares IPEN - CNEN/SP, autarquia associada à Universidade de São Paulo. O desenvolvimento do dispositivo faz parte do escopo de obtenção, e levantamento dos dados necessários para o projeto, e a construção do embalado. Esta dissertação versa sobre a concepção mecânica do dispositivo, importante, dividida em duas partes, dimensionamento das barras, que seriam a barra de impacto, a barra de entrada, e a barra de saída, e o dimensionamento do dispositivo de impacto. O dimensionamento das barras envolve conhecimentos do conceito de ondas elásticas em meios sólidos para que o comprimento das barras seja estimado de forma a servir de guia das ondas elásticas, que provocarão a deformação no corpo de prova, e possibilite a boa leitura dessas ondas para análise dos dados. O dispositivo de impacto, este tem que ser robusto o suficiente para produzir a onda de tensão que deforme o corpo de prova, mas não para deformar plasticamente as barras, que terão que continuar durante todo o teste dentro do regime elástico. / This dissertation presents a design of a device to collect characteristic data of materials submitted to the high strain rates, device that takes the name of its idealizer the English engineer Sir Bertram Hopkinson. More specifically, this dissertation is inseparably linked to the development of a package for the transport of radioactive elements as part of the general scope of a CAPES project in partnership with the Materials Science and Technology Center (CCTM), Nuclear and Energy Research Institute IPEN - CNEN / SP, autarchy associated with the University of São Paulo. The development of the device is part of the scope of procurement, and collection of data required for the design, and the construction of the packaging. This dissertation deals with the mechanical design of the device, important, divided into two parts, dimensioning of the bars, which would be the impact bar, the input and output bars and the design of the impact device. The sizing of the bars involves knowledge of the concept of elastic waves in solid media so that the length of the bars is estimated in order to serve as a guide for the elastic waves, which will cause deformation in the test body, and enable a good reading of these waves for analysis of the data. The impact device has to be robust enough to produce the stress wave that deforms the test body but not to deform the bars plastically, which will have to continue throughout the test within the elastic regime.

Esta dissertação apresenta um projeto de um dispositivo para levantar dados característicos de materiais submetidos às altas taxas de deformação, dispositivo este que leva o nome do seu idealizador o engenheiro Inglês Sir Bertram Hopkinson. Mais especificamente, esta dissertação está inseparavelmente ligada ao desenvolvimento de um embalado para transporte de elementos radioativos como sendo uma das partes do escopo geral, de um projeto da CAPES em convênio com o Centro de Ciência e Tecnologia de Materiais - CCTM do, Instituto de Pesquisas Energéticas e Nucleares IPEN - CNEN/SP, autarquia associada à Universidade de São Paulo. O desenvolvimento do dispositivo faz parte do escopo de obtenção, e levantamento dos dados necessários para o projeto, e a construção do embalado. Esta dissertação versa sobre a concepção mecânica do dispositivo, importante, dividida em duas partes, dimensionamento das barras, que seriam a barra de impacto, a barra de entrada, e a barra de saída, e o dimensionamento do dispositivo de impacto. O dimensionamento das barras envolve conhecimentos do conceito de ondas elásticas em meios sólidos para que o comprimento das barras seja estimado de forma a servir de guia das ondas elásticas, que provocarão a deformação no corpo de prova, e possibilite a boa leitura dessas ondas para análise dos dados. O dispositivo de impacto, este tem que ser robusto o suficiente para produzir a onda de tensão que deforme o corpo de prova, mas não para deformar plasticamente as barras, que terão que continuar durante todo o teste dentro do regime elástico. / This dissertation presents a design of a device to collect characteristic data of materials submitted to the high strain rates, device that takes the name of its idealizer the English engineer Sir Bertram Hopkinson. More specifically, this dissertation is inseparably linked to the development of a package for the transport of radioactive elements as part of the general scope of a CAPES project in partnership with the Materials Science and Technology Center (CCTM), Nuclear and Energy Research Institute IPEN - CNEN / SP, autarchy associated with the University of São Paulo. The development of the device is part of the scope of procurement, and collection of data required for the design, and the construction of the packaging. This dissertation deals with the mechanical design of the device, important, divided into two parts, dimensioning of the bars, which would be the impact bar, the input and output bars and the design of the impact device. The sizing of the bars involves knowledge of the concept of elastic waves in solid media so that the length of the bars is estimated in order to serve as a guide for the elastic waves, which will cause deformation in the test body, and enable a good reading of these waves for analysis of the data. The impact device has to be robust enough to produce the stress wave that deforms the test body but not to deform the bars plastically, which will have to continue throughout the test within the elastic regime.

L’objectif de la thèse est la caractérisation expérimentale du frottement entre l’acier et un matériau fragile. Les pressions et les vitesses de glissement qu’on cherche à atteindre sont respectivement de l’ordre de 10 à 100 MPa et l’ordre de 10 à 100 m/s. Les tribomètres classiques ne peuvent pas être utilisés car les pressions qu’on cherche à atteindre sont suffisamment élevées pour mener le matériau fragile à rupture. Pour pallier cette difficulté, le matériau doit être confiné. Un échantillon cylindrique du matériau est alors inséré dans un tube en acier qui fait à la fois office de confinement et de surface de frottement. Avec cette configuration, comme nous ne pouvons pas effectuer de mesures directes au niveau de l’interface, les paramètres de frottement sont identifiés à partir de mesures indirectes et de modèles analytique et numérique. Deux types de dispositifs ont été conçus pour effectuer à la fois des essais d’orientation en quasistatique et des essais sur barres de Hopkinson. Les essais quasi-statiques permettent une identification fiable du frottement et montrent que des pressions de 100 MPa peuvent être obtenues avec notre configuration sans dégrader le matériau fragile. En revanche, les essais sur barres de Hopkinson ne donnent pas satisfaction. Un dispositif spécifiquement adapté à la dynamique rapide a alors été conçu. Il permet d’identifier le frottement sous des pressions de 100 MPa et des vitesses de10 m/s. / The aim of the thesis is the experimental characterisation of the friction between steel and a brittle material. The desired pressures and the desired sliding velocities are respectively of the order of 10-100 MPa and 10-100 m/s. Usual tribometers cannot be used because the desired pressures are high enough to fracture the brittle material. The material has to be confined to overcome this difficulty. A cylindrical sample of the material is therefore inserted into a steel tube which acts both as a confinement and a sliding surface. Such a configuration does not enable to carry on direct measurements on the interface, the friction parameters are thus identified from indirect measurements and from analytical and numerical models. Two types of set-up have been designed to carry on both quasi-static tests and tests on split Hopkinson pressure bars. Quasi-static tests enable a reliable identification of friction and show that the desired pressures can be reached with our configuration whilst retaining the brittle material integrity. Unfortunately, the results obtained with split Hopkinson pressure bars are not satisfactory. A set-up specifically adapted to dynamic situations has thus been designed. It enables identification of friction under pressure of 100 MPa and velocities of 10 m/s.

In this work, three aluminum sheet alloys, AA5754, AA5182 and AA6111, which are prime candidates for replacing mild steel in automobile structures, are tested in tension at quasi-static and high strain rates. <br /><br /> In order to characterize the constitutive response of AA5754, AA5182 and AA6111 at high strain rates, tensile experiments were carried out at strain rates between 600 s^-1 and 1500 s^-1, and at temperatures between ambient and 300??C, using a tensile split Hopkinson bar (TSHB) apparatus. As part of this research, the apparatus was modified in order to provide an improved means of gripping the sheet specimens. Quasi-static experiments also were conducted using an Instron machine. <br /><br /> The experimental data was fit to the Johnson-Cook and Zerilli-Armstrong constitutive models for all three alloys. The resulting fits were evaluated by numerically simulating the tensile experiments conducted using a finite element approach.