Energetic materials comprise a set of systems of tremendous technological importance. Besides helping shape landscapes to establish communications, they have been used to reach fuel reservoirs, deploy safety bags and prevent heart strokes. Understanding its behavior can help in attaining strategic and tactical superiority, and importantly, preserve lives of people who handle these materials. The large discrepancy in length and time scales at which characteristic processes of energetic materials are of relevance pose a major challenge for current simulation techniques. We present a systematic study of crystalline energetic materials of different sensitivity and analyze their properties at different theoretical levels. Equilibrium structures, vibrational frequencies, conformational rearrangement and mechanical properties can be calculated within the density functional theory and molecular dynamics at finite temperatures. We have found marked differences in the calculated properties in systems with ranging sensitivities. Reactions at elevated temperatures have been studied using ab initio molecular dynamics method for crystals of nitroethane. Furthermore, while presenting the state of the art of energetic materials modeling, the limitations of each methodology are also discussed. Prospective systems and an elasticity driven approach that can be applied to other type of materials is also presented.

Understanding how materials deform and break is a subject of critical importance in industry. At the same time, it requires from the knowledge of the basic processes governing the phenomenon and hence, fundamental physics research is a must. The presence of power law distributions in both temporal and spatial properties and the universality of the behavior seem to suggest that fracture and plasticity could be explained as some type of critical phenomena. This means that there should be some general principles that rule the process and that are more important than a detailed description of the interactions and atomic structure of the media. Hence, simplified theoretical approaches based on fundamental concepts can help to capture the essential ingredients in the system. This Thesis is devoted to the study of the deformation and failure of materials in the presence of disorder with the help of statistical mechanics tools and models.

Two experiments were designed to evaluate human sensitivity to elasticity. Elastic objects bend when a force is applied to them. Observers saw two computer-generated bending rods (defined by the motions of 50 dots) on any given trial and were required to judge which rod was more flexible. Elasticity difference thresholds were calculated for each observer for each of three bending conditions. The rods bent in a plane that was either frontoparallel or oriented 42.5 or 85 degrees from frontoparallel. The results showed that observers could precisely discriminate between bending rods of different elasticities, independent of whether the bendings occurred in the frontoparallel plane or in depth. To rule out the possibility that the ability to judge bending motion was based on the ability to judge 2-dimensional (2-D) speed a second experiment was conducted to obtain difference thresholds for 2-D speed. The observers' speed discrimination thresholds were not positively correlated with their elasticity discrimination thresholds, which suggests that the observers' ability to judge bending motion was not based on their capability to discriminate differences in speed.

Food and nutrition policy analysis, especially for a developing country, requires information on food price and expenditure elasticities. Often, such information is not readily available for different income groups. For Lesotho, this information currently does not exist. This study analyses the food consumption and nutrition of households in Lesotho by estimating a set of food demand parameters. The Lesotho government, in the process of developing the agriculture sector, aims to increase the domestic production of maize through the Food Self Sufficiency Programme (FSSP) by providing incentives to producers. The FSSP has the potential to affect prices at the consumer level for maize and maize meal. Higher maize meal prices could have adverse nutritional impacts especially for low income households. The objective of this study was to analyze the effects of price and income changes on the nutritional well being of consumers with a focus on low income households. A related objective was to provide an estimate of the parameters for food demand. The Almost Ideal Demand System was used to estimate demand parameters for seven food groups: maize, other cereals, meat, milk, eggs, fats and oils, and fruits and vegetables. Data for analysis was obtained from the Household Budget Survey (HBS) that was carried out in 1986/87 by the Bureau of statistics over a twelve month period. Households were divided into low, middle and high income groups. The share equations were first estimated by imposing the homogeneity and symmetry restrictions. The null hypothesis of symmetry given homogeneity is not rejected in any of the income groups. The results of the estimation indicate that total food expenditure elasticity declines as income increases. The sign and magnitude of the elasticities indicates that all food groups are relative necessities. Expenditure elasticity of maize declines from 0.91 for the low income group to 0.47 for the high income households. The estimated demand and expenditure elasticities were used to calculate calorie and protein price and expenditure elasticities for the low income households. (Abstract shortened by UMI.)

Exports are an important source of income for Sweden. They are influenced by macroeconomic factors such as GDP. This paper examines the elasticity of Swedish export to changes in the GDP of Sweden’s 25 most important export partners. The sensitivity to changes in GDP, the elasticity, can be different for different goods. Therefore, we examine export elasticities for five different commodity groups, which include durable as well as non-durable goods. Moreover, special focus is put on the trade relationship between Sweden and Germany in order to see if their long common trade history has any impact on the elasticity of Swedish exports to Germany. The analysis is based on an export demand function that links exports to GDP and geographical distance. We include dummy variables in our regression model to control for EU-membership and common borders.       For Swedish exports to Germany, we find that exports of food and live animals are least elastic, whereas exports of machinery and transport equipment are most elastic. This is coherent with previous empirical findings about demand elasticities of non-durable and durable goods. We find that exports in two out of five commodity groups are unit elastic. This means that when German GDP increases by one percent, Sweden’s export to Germany in these commodity groups also grows by approximately one percent. Thus, Sweden is not able to capture additional profit through over proportional increases in exports to Germany. For Swedish exports to its 25 most important trading partners, on average, we find that exports of manufactured goods as well as machinery and transport equipment are the least elastic exports. This gives them the lowest growth potential.

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New elastographic techniques such as poroelastography and viscoelasticity imaging aim at imaging the temporal mechanical behavior of tissues. These techniques usually involve the use of curve fitting methods as applied to noisy data to estimate new elastographic parameters. As of today, however, image quality performance of these new elastographic imaging techniques is still largely unknown due to a paucity of data and the lack of systematic studies that analyze performance limitations of estimators suitable for these novel applications. Furthermore, current elastographic implementations of poroelasticity and viscoelasticity imaging methods are in general too slow and not optimized for clinical applications. In this paper, we propose a new elastographic time constant (TC) estimator, which is based on the use of the Least Square Error (LSE) curve-fitting method and the Levenberg-Marquardt (LM) optimization rule as applied to noisy elastographic data obtained from a tissue under creep compression. The estimator's performance is analyzed using simulations and quantified in terms of accuracy, precision, sensitivity, signal-to-noise ratio (SNR) and speed. Experiments are performed as a proof of principle of the technical applicability of the new estimator on real experimental data. The results of this study demonstrate that the new elastographic estimator described in this thesis can produce highly accurate, sensitive and precise time constant estimates in real-time and at high SNR. In the future, the use of this estimator could allow real-time imaging of the temporal behavior of complex tissues and provide advances in lymphedema and cancer imaging.

Aerospace structures with large aspect ratio, such as airplane wings, rotorcraft blades, wind turbine blades, and jet engine fan and compressor blades, are particularly susceptible to aeroelastic phenomena. Finite element analysis provides an effective and generalized method to model these structures; however, it is computationally expensive. Fortunately, these structures have a length appreciably larger than the largest cross-sectional diameter. This characteristic is exploitable as these potential aeroelastically unstable structures can be modeled as cantilevered beams, drastically reducing computational time. In this thesis, the nonlinear equations of motion are derived for an inextensional, non-uniform cantilevered beam with a straight elastic axis. Along the elastic axis, the cross-sectional center of mass can be o set in both dimensions, and the principal bending and centroidal axes can each be rotated uniquely. The Galerkin method is used, permitting arbitrary and abrupt variations along the length that require no knowledge of the spatial derivatives of the beam properties. Additionally, these equations consistently retain all third-order nonlinearities that account for flexural-flexural-torsional coupling and extend the validity of the equations for large deformations. Furthermore, linearly independent shape functions are substituted into these equations, providing an efficient method to determine the natural frequencies and mode shapes of the beam and to solve for time-varying deformation. This method is validated using finite element analysis and is extended to swept wings. The importance of retaining cubic terms, in addition to quadratic terms, for nonlinear analysis is demonstrated for several examples. Ultimately, these equations are coupled with a fluid dynamics solver to provide a structurally efficient aeroelastic program.

In the present study, the buckling behavior of beams is analyzed by a plane strain finite element. The displacement-type finite element formulation is based on elasticity and has no any other simplification and assumption except that the beam is of moderate depth. Also all the displacement boundary conditions can be imposed exactly. These are the advantages that beam theories of conventional approach, which simulate beams with neutral plane behaviors, do not have. Therefore the present analyses should be able to obtain buckling load and buckling mode more accurately than conventional method. Numerical values of buckling loads of the present approach will be compared with previously published results of the Euler-Bernoulli beam theory and the Timoshenko beam theory, and further with the high order beam theory to reveal their differences. The effects of the geometry ratio, the distribution of axial loads and the displacement boundary conditions on buckling of beams are also discussed.

Reproduction de : Thèse de docteur-ingénieur : mathématiques : Grenoble 1 : 1956. / Titre provenant de l'écran-titre. Bibliogr. p. 91.