Age determinations from Bykovsky Peninsula, Siberia

A wide variety of environmental records is necessary for analysing and understanding the complex Late Quaternary dynamics of permafrost-dominated Arctic landscapes. A NE Siberian periglacial key region was studied in detail using sediment records, remote sensing data, and terrain modelling, all incorporated in a geographical information system (GIS). The study area consists of the Bykovsky Peninsula and the adjacent Khorogor Valley in the Kharaulakh Ridge situated a few kilometres southeast of the Lena Delta. In this study a comprehensive cryolithological database containing information from 176 sites was compiled. The information from these sites is based on the review of previously published borehole data, outcrop profiles, surface samples, and our own field data. These archives cover depositional records of three periods: from Pliocene to Early Pleistocene, the Late Pleistocene and the Holocene. The main sediment sequences on the Bykovsky Peninsula consist of up to 50 m thick ice-rich permafrost deposits (Ice Complex) that were accumulated during the Late Pleistocene. They were formed as a result of nival processes around extensive snowfields in the Kharaulakh Ridge, slope processes in these mountains (such as in the Khorogor Valley), and alluvial/proluvial sedimentation in a flat accumulation plain dominated by polygonal tundra in the mountain foreland (Bykovsky Peninsula). During the early to middle Holocene warming, a general landscape transformation occurred from an extensive Late Pleistocene accumulation plain to a strongly thermokarst-dominated relief dissected by numerous depressions. Thermokarst subsidence had an enormous influence on the periglacial hydrological patterns, the sediment deposition, and on the composition and distribution of habitats. Climate deterioration, lake drainage, and talik refreezing occurred during the middle to late Holocene. The investigated region was reached by the post-glacial sea level rise during the middle Holocene, triggering thermo-abrasion of ice-rich coasts and the marine inundation of thermokarst depressions.

Consolidation properties and strength characteristics of ODP Site 204-1244 sediments

Eight whole-core samples from Ocean Drilling Program Site 1244, Hydrate Ridge, Cascadia continental margin, were provided to Massachusetts Institute of Technology (Cambridge, Massachusetts, USA) for geotechnical characterization. The samples were collected from depths ranging from 5 to 136 meters below seafloor (mbsf). Seven of the eight whole-core samples were located within the gas hydrate stability zone, whereas the eighth sample was located in the free gas zone. Atterberg limits testing showed that the average liquid limit of the soil is 81% and the average plastic limit is 38%, giving an average plasticity index of 43%. The liquid limit is sensitive to oven drying, shown by a drop in liquid limit to 64% when tests were performed on an oven-dried sample. Loss on ignition averages 5.45 wt%. Constant rate of strain consolidation (CRSC) tests were performed to obtain the compression characteristics of the soil, as well as to determine the stress history of the site. CRSC tests also provided hydraulic conductivity and coefficient of consolidation characteristics for these sediments. The compression ratio (Cc) ranges from 0.340 to 0.704 (average = 0.568). Cc is fairly constant to a depth of 79 mbsf, after which Cc decreases downhole. The recompression ratio (Cr) ranges from 0.035 to 0.064 (average = 0.052). Cr is constant throughout the depth range. In situ hydraulic conductivity varies between 1.5 x 10**-7 and 3 x 10**-8 cm/s and shows no trend with depth. Ko-consolidated undrained compression/extension (CKoUC/E) tests were also performed to determine the peak undrained shear strength, stress-strain curve, and friction angle. The normalized undrained strength ranges from 0.29 to 0.35. The friction angle ranges from 27 to 37. Because of the limited amount of soil, CRSC and CKoUC/E tests were also conducted on resedimented specimens.

Electric current of an electrified jet issuing from a long metallic tube.

This paper presents an analysis of the transport of electric current in a jet of an electrically conducting liquid discharging from a metallic tube into a gas or a vacuum, and subject to an electric field due to a high voltage applied between the tube and a far electrode. The flow, the surface charge and the electric field are computed in the current transfer region of the jet, where conduction current in the liquid becomes surface current due to the convection of electric charge accumulated at its surface. The electric current computed as a function of the flow rate of the liquid injected through the tube increases first as the square root of this flow rate, levels to a nearly constant value when the flow rate is increased and finally sets to a linear increase when the flow rate is further increased. The current increases linearly with the applied voltage at small and moderate values of this variable, and faster than linearly at high voltages. The characteristic length and structure of the current transfer region are determined. Order-of-magnitude estimates for jets which are only weakly stretched by the electric stresses are worked out that qualitatively account for some of the numerical results.

Prediction of bendig load capacity of timber beams by finite element method simulation of knots and grain deviation

A finite element model was used to simulate timberbeams with defects and predict their maximum load in bending. Taking into account the elastoplastic constitutive law of timber, the prediction of fracture load gives information about the mechanisms of timber failure, particularly with regard to the influence of knots, and their local graindeviation, on the fracture. A finite element model was constructed using the ANSYS element Plane42 in a plane stress 2D-analysis, which equates thickness to the width of the section to create a mesh which is as uniform as possible. Three sub-models reproduced the bending test according to UNE EN 408: i) timber with holes caused by knots; ii) timber with adherent knots which have structural continuity with the rest of the beam material; iii) timber with knots but with only partial contact between knot and beam which was artificially simulated by means of contact springs between the two materials. The model was validated using ten 45 �� 145 �� 3000 mm beams of Pinus sylvestris L. which presented knots and graindeviation. The fracture stress data obtained was compared with the results of numerical simulations, resulting in an adjustment error less of than 9.7%

Acoustic displacement triangle based on the individual element test

A three node, displacement based, acoustic element is developed. In order to avoid spurious rotational modes, a higher order stiffness is introduced. The higher order stiffness is developed from an incompatible strain field which computes element volume changes under nodal rotational displacements fields. The higher order strain satisfies the IET requirements, non affecting convergence. The higher order stiffness is modulated, element by element, with a factor. Thus, the displacement based formulation is capable of placing the spurious rotational modes over the range of physical compressional modes that can be accurately captured by the mesh.

Application of digital image correlation at the microscale in fiber-reinforced composites

Digital image correlation (DIC) is applied to analyzing the deformation mechanisms under transverse compression in a fiber-reinforced composite. To this end, compression tests in a direction perpendicular to the fibers were carried out inside a scanning electron microscope and secondary electron images obtained at different magnifications during the test. Optimum DIC parameters to resolve the displacement and strain field were computed from numerical simulations of a model composite and they were applied to micrographs obtained at different magnifications (250_, 2000_, and 6000_). It is shown that DIC of low-magnification micrographs was able to capture the long range fluctuations in strain due to the presence of matrix-rich and fiber-rich zones, responsible for the onset of damage. At higher magnification, the strain fields obtained with DIC qualitatively reproduce the non-homogeneous deformation pattern due to the presence of stiff fibers dispersed in a compliant matrix and provide accurate results of the average composite strain. However, comparison with finite element simulations revealed that DIC was not able to accurately capture the average strain in each phase.

WYPIWYG Hyperelasticity

In this paper we describe a new promising procedure to model hyperelastic materials from given stress-strain data. The main advantage of the proposed method is that the user does not need to have a relevant knowledge of hyperelasticity, large strains or hyperelastic constitutive modelling. The engineer simply has to prescribe some stress strain experimental data (whether isotropic or anisotropic) in also user prescribed stress and strain measures and the model almost exactly replicates the experimental data. The procedure is based on the piece-wise splines model by Sussman and Bathe and may be easily generalized to transversely isotropic and orthotropic materials. The model is also amenable of efficient finite element implementation. In this paper we briefly describe the general procedure, addressing the advantages and limitations. We give predictions for arbitrary ?experimental data? and also give predictions for actual experiments of the behaviour of living soft tissues. The model may be also implemented in a general purpose finite element program. Since the obtained strain energy functions are analytic piece-wise functions, the constitutive tangent may be readily derived in order to be used for implicit static problems, where the equilibrium iterations must be performed and the material tangent is needed in order to preserve the quadratic rate of convergence of Newton procedures.

Bifurcation diagrams for polymer blends with diffuse interfaces in confined and adaptive geometries

Dynamics of binary mixtures such as polymer blends, and fluids near the critical point, is described by the model-H, which couples momentum transport and diffusion of the components [1]. We present an extended version of the model-H that allows to study the combined effect of phase separation in a polymer blend and surface structuring of the film itself [2]. We apply it to analyze the stability of vertically stratified base states on extended films of polymer blends and show that convective transport leads to new mechanisms of instability as compared to the simpler diffusive case described by the Cahn- Hilliard model [3, 4]. We carry out this analysis for realistic parameters of polymer blends used in experimental setups such as PS/PVME. However, geometrically more complicated states involving lateral structuring, strong deflections of the free surface, oblique diffuse interfaces, checkerboard modes, or droplets of a component above of the other are possible at critical composition solving the Cahn Hilliard equation in the static limit for rectangular domains [5, 6] or with deformable free surfaces [6]. We extend these results for off-critical compositions, since balanced overall composition in experiments are unusual. In particular, we study steady nonlinear solutions of the Cahn-Hilliard equation for bidimensional layers with fixed geometry and deformable free surface. Furthermore we distinguished the cases with and without energetic bias at the free surface. We present bifurcation diagrams for off-critical films of polymer blends with free surfaces, showing their free energy, and the L2-norms of surface deflection and the concentration field, as a function of lateral domain size and mean composition. Simultaneously, we look at spatial dependent profiles of the height and concentration. To treat the problem of films with arbitrary surface deflections our calculations are based on minimizing the free energy functional at given composition and geometric constraints using a variational approach based on the Cahn-Hilliard equation. The problem is solved numerically using the finite element method (FEM).

Smart aeronautical structures: development and experimental validation of a structural health monitoring system for damage detection

En muchas ��reas de la ingenier��a, la integridad y confiabilidad de las estructuras son aspectos de extrema importancia. Estos son controlados mediante el adecuado conocimiento de danos existentes. T��picamente, alcanzar el nivel de conocimiento necesario que permita caracterizar la integridad estructural implica el uso de t��cnicas de ensayos no destructivos. Estas t��cnicas son a menudo costosas y consumen mucho tiempo. En la actualidad, muchas industrias buscan incrementar la confiabilidad de las estructuras que emplean. Mediante el uso de t��cnicas de ��ltima tecnolog��a es posible monitorizar las estructuras y en algunos casos, es factible detectar da��os incipientes que pueden desencadenar en fallos catastr��ficos. Desafortunadamente, a medida que la complejidad de las estructuras, los componentes y sistemas incrementa, el riesgo de la aparici��n de da��os y fallas tambi��n incrementa. Al mismo tiempo, la detecci��n de dichas fallas y defectos se torna m��s compleja. En a��os recientes, la industria aeroespacial ha realizado grandes esfuerzos para integrar los sensores dentro de las estructuras, adem��s de desarrollar algoritmos que permitan determinar la integridad estructural en tiempo real. Esta filosof��a ha sido llamada ���Structural Health Monitoring��� (o ���Monitorizaci��n de Salud Estructural��� en espa��ol) y este tipo de estructuras han recibido el nombre de ���Smart Structures��� (o ���Estructuras Inteligentes��� en espa��ol). Este nuevo tipo de estructuras integran materiales, sensores, actuadores y algoritmos para detectar, cuantificar y localizar da��os dentro de ellas mismas. Una novedosa metodolog��a para detecci��n de da��os en estructuras se propone en este trabajo. La metodolog��a est�� basada en mediciones de deformaci��n y consiste en desarrollar t��cnicas de reconocimiento de patrones en el campo de deformaciones. Estas ��ltimas, basadas en PCA (An��lisis de Componentes Principales) y otras t��cnicas de reducci��n dimensional. Se propone el uso de Redes de difracci��n de Bragg y medidas distribuidas como sensores de deformaci��n. La metodolog��a se valid�� mediante pruebas a escala de laboratorio y pruebas a escala real con estructuras complejas. Los efectos de las condiciones de carga variables fueron estudiados y diversos experimentos fueron realizados para condiciones de carga est��ticas y din��micas, demostrando que la metodolog��a es robusta ante condiciones de carga desconocidas. ABSTRACT In many engineering fields, the integrity and reliability of the structures are extremely important aspects. They are controlled by the adequate knowledge of existing damages. Typically, achieving the level of knowledge necessary to characterize the structural integrity involves the usage of nondestructive testing techniques. These are often expensive and time consuming. Nowadays, many industries look to increase the reliability of the structures used. By using leading edge techniques it is possible to monitoring these structures and in some cases, detect incipient damage that could trigger catastrophic failures. Unfortunately, as the complexity of the structures, components and systems increases, the risk of damages and failures also increases. At the same time, the detection of such failures and defects becomes more difficult. In recent years, the aerospace industry has done great efforts to integrate the sensors within the structures and, to develop algorithms for determining the structural integrity in real time. The ���philosophy��� has being called ���Structural Health Monitoring��� and these structures have been called ���smart structures���. These new types of structures integrate materials, sensors, actuators and algorithms to detect, quantify and locate damage within itself. A novel methodology for damage detection in structures is proposed. The methodology is based on strain measurements and consists in the development of strain field pattern recognition techniques. The aforementioned are based on PCA (Principal Component Analysis) and other dimensional reduction techniques. The use of fiber Bragg gratings and distributed sensing as strain sensors is proposed. The methodology have been validated by using laboratory scale tests and real scale tests with complex structures. The effects of the variable load conditions were studied and several experiments were performed for static and dynamic load conditions, demonstrating that the methodology is robust under unknown load conditions.

Continuum models of the mechanical behavior of rolled and die-cast magnesium alloys

En los ��ltimos a��os ha habido una fuerte tendencia a disminuir las emisiones de CO2 y su negativo impacto medioambiental. En la industria del transporte, reducir el peso de los veh��culos aparece como la mejor opci��n para alcanzar este objetivo. Las aleaciones de Mg constituyen un material con gran potencial para el ahorro de peso. Durante la ��ltima d��cada se han realizado muchos esfuerzos encaminados a entender los mecanismos de deformaci��n que gobiernan la plasticidad de estos materiales y as��, las aleaciones de Mg de colada inyectadas a alta presi��n y forjadas son todav��a objeto de intensas campa��as de investigaci��n. Es ahora necesario desarrollar modelos que contemplen la complejidad inherente de los procesos de deformaci��n de ��stos. Esta tesis doctoral constituye un intento de entender mejor la relaci��n entre la microestructura y el comportamiento mec��nico de aleaciones de Mg, y dar�� como resultado modelos de policristales capaces de predecir propiedades macro- y microsc��picas. La deformaci��n pl��stica de las aleaciones de Mg est�� gobernada por una combinaci��n de mecanismos de deformaci��n caracter��sticos de la estructura cristalina hexagonal, que incluye el deslizamiento cristalogr��fico en planos basales, prism��ticos y piramidales, as�� como el maclado. Las aleaciones de Mg de forja presentan texturas fuertes y por tanto los mecanismos de deformaci��n activos dependen de la orientaci��n de la carga aplicada. En este trabajo se ha desarrollado un modelo de plasticidad cristalina por elementos finitos con el objetivo de entender el comportamiento macro- y micromec��nico de la aleaci��n de Mg laminada AZ31 (Mg-3wt.%Al-1wt.%Zn). Este modelo, que incorpora el maclado y tiene en cuenta el endurecimiento por deformaci��n debido a las interacciones dislocaci��n-dislocaci��n, dislocaci��n-macla y macla-macla, predice exitosamente las actividades de los distintos mecanismos de deformaci��n y la evoluci��n de la textura con la deformaci��n. Adem��s, se ha llevado a cabo un estudio que combina difracci��n de electrones retrodispersados en tres dimensiones y modelizaci��n para investigar el efecto de los l��mites de grano en la propagaci��n del maclado en el mismo material. Ambos, experimentos y simulaciones, confirman que el ��ngulo de desorientaci��n tiene una influencia decisiva en la propagaci��n del maclado. Se ha observado que los efectos no-Schmid, esto es, eventos de deformaci��n pl��stica que no cumplen la ley de Schmid con respecto a la carga aplicada, no tienen lugar en la vecindad de los l��mites de baja desorientaci��n y se hacen m��s frecuentes a medida que la desorientaci��n aumenta. Esta investigaci��n tambi��n prueba que la morfolog��a de las maclas est�� altamente influenciada por su factor de Schmid. Es conocido que los procesos de colada suelen dar lugar a la formaci��n de microestructuras con una microporosidad elevada, lo cu��l afecta negativamente a sus propiedades mec��nicas. La aplicaci��n de presi��n hidrost��tica despu��s de la colada puede reducir la porosidad y mejorar las propiedades aunque es poco conocido su efecto en el tama��o y morfolog��a de los poros. En este trabajo se ha utilizado un enfoque mixto experimentalcomputacional, basado en tomograf��a de rayos X, an��lisis de imagen y an��lisis por elementos finitos, para la determinaci��n de la distribuci��n tridimensional (3D) de la porosidad y de la evoluci��n de ��sta con la presi��n hidrost��tica en la aleaci��n de Mg AZ91 (Mg- 9wt.%Al-1wt.%Zn) colada por inyecci��n a alta presi��n. La distribuci��n real de los poros en 3D obtenida por tomograf��a se utiliz�� como input para las simulaciones por elementos finitos. Los resultados revelan que la aplicaci��n de presi��n tiene una influencia significativa tanto en el cambio de volumen como en el cambio de forma de los poros que han sido cuantificados con precisi��n. Se ha observado que la reducci��n del tama��o de ��stos est�� ��ntimamente ligada con su volumen inicial. En conclusi��n, el modelo de plasticidad cristalina propuesto en este trabajo describe con ��xito los mecanismos intr��nsecos de la deformaci��n de las aleaciones de Mg a escalas meso- y microsc��pica. M��s especificamente, es capaz de capturar las activadades del deslizamiento cristalogr��fico y maclado, sus interacciones, as�� como los efectos en la porosidad derivados de los procesos de colada. ---ABSTRACT--- The last few years have seen a growing effort to reduce CO2 emissions and their negative environmental impact. In the transport industry more specifically, vehicle weight reduction appears as the most straightforward option to achieve this objective. To this end, Mg alloys constitute a significant weight saving material alternative. Many efforts have been devoted over the last decade to understand the main mechanisms governing the plasticity of these materials and, despite being already widely used, high pressure die-casting and wrought Mg alloys are still the subject of intense research campaigns. Developing models that can contemplate the complexity inherent to the deformation of Mg alloys is now timely. This PhD thesis constitutes an attempt to better understand the relationship between the microstructure and the mechanical behavior of Mg alloys, as it will result in the design of polycrystalline models that successfully predict macro- and microscopic properties. Plastic deformation of Mg alloys is driven by a combination of deformation mechanisms specific to their hexagonal crystal structure, namely, basal, prismatic and pyramidal dislocation slip as well as twinning. Wrought Mg alloys present strong textures and thus specific deformation mechanisms are preferentially activated depending on the orientation of the applied load. In this work a crystal plasticity finite element model has been developed in order to understand the macro- and micromechanical behavior of a rolled Mg AZ31 alloy (Mg-3wt.%Al-1wt.%Zn). The model includes twinning and accounts for slip-slip, slip-twin and twin-twin hardening interactions. Upon calibration and validation against experiments, the model successfully predicts the activity of the various deformation mechanisms and the evolution of the texture at different deformation stages. Furthermore, a combined three-dimensional electron backscatter diffraction and modeling approach has been adopted to investigate the effect of grain boundaries on twin propagation in the same material. Both experiments and simulations confirm that the misorientation angle has a critical influence on twin propagation. Non-Schmid effects, i.e. plastic deformation events that do not comply with the Schmid law with respect to the applied stress, are absent in the vicinity of low misorientation boundaries and become more abundant as misorientation angle increases. This research also proves that twin morphology is highly influenced by the Schmid factor. Finally, casting processes usually lead to the formation of significant amounts of gas and shrinkage microporosity, which adversely affect the mechanical properties. The application of hydrostatic pressure after casting can reduce the porosity and improve the properties but little is known about the effects on the casting���s pores size and morphology. In this work, an experimental-computational approach based on X-ray computed tomography, image analysis and finite element analysis is utilized for the determination of the 3D porosity distribution and its evolution with hydrostatic pressure in a high pressure diecast Mg AZ91 alloy (Mg-9wt.%Al-1wt.%Zn). The real 3D pore distribution obtained by tomography is used as input for the finite element simulations using an isotropic hardening law. The model is calibrated and validated against experimental stress-strain curves. The results reveal that the pressure treatment has a significant influence both on the volume and shape changes of individuals pores, which have been precisely quantified, and which are found to be related to the initial pore volume. In conclusion, the crystal plasticity model proposed in this work successfully describes the intrinsic deformation mechanisms of Mg alloys both at the mesoscale and the microscale. More specifically, it can capture slip and twin activities, their interactions, as well as the potential porosity effects arising from casting processes.

An inverse optimization strategy to determine single crystal mechanical behavior from polycrystal tests: Application to AZ31 Mg alloy

An inverse optimization strategy was developed to determine the single crystal properties from experimental results of the mechanical behavior of polycrystals. The polycrystal behavior was obtained by means of the finite element simulation of a representative volume element of the microstructure in which the dominant slip and twinning systems were included in the constitutive equation of each grain. The inverse problem was solved by means of the Levenberg-Marquardt method, which provided an excellent fit to the experimental results. The iterative optimization process followed a hierarchical scheme in which simple representative volume elements were initially used, followed by more realistic ones to reach the final optimum solution, leading to important reductions in computer time. The new strategy was applied to identify the initial and saturation critical resolved shear stresses and the hardening modulus of the active slip systems and extension twinning in a textured AZ31 Mg alloy. The results were in general agreement with the data in the literature but also showed some differences. They were partially explained because of the higher accuracy of the new optimization strategy but it was also shown that the number of independent experimental stress-strain curves used as input is critical to reach an accurate solution to the inverse optimization problem. It was concluded that at least three independent stress-strain curves are necessary to determine the single crystal behavior from polycrystal tests in the case of highly textured Mg alloys.

Neurite, a finite difference large scale parallel program for the simulation of the electrical signal propagation in neurites under mechanical loading

With the growing body of research on traumatic brain injury and spinal cord injury, computational neuroscience has recently focused its modeling efforts on neuronal functional deficits following mechanical loading. However, in most of these efforts, cell damage is generally only characterized by purely mechanistic criteria, function of quantities such as stress, strain or their corresponding rates. The modeling of functional deficits in neurites as a consequence of macroscopic mechanical insults has been rarely explored. In particular, a quantitative mechanically based model of electrophysiological impairment in neuronal cells has only very recently been proposed (Jerusalem et al., 2013). In this paper, we present the implementation details of Neurite: the finite difference parallel program used in this reference. Following the application of a macroscopic strain at a given strain rate produced by a mechanical insult, Neurite is able to simulate the resulting neuronal electrical signal propagation, and thus the corresponding functional deficits. The simulation of the coupled mechanical and electrophysiological behaviors requires computational expensive calculations that increase in complexity as the network of the simulated cells grows. The solvers implemented in Neurite-explicit and implicit-were therefore parallelized using graphics processing units in order to reduce the burden of the simulation costs of large scale scenarios. Cable Theory and Hodgkin-Huxley models were implemented to account for the electrophysiological passive and active regions of a neurite, respectively, whereas a coupled mechanical model accounting for the neurite mechanical behavior within its surrounding medium was adopted as a link between lectrophysiology and mechanics (Jerusalem et al., 2013). This paper provides the details of the parallel implementation of Neurite, along with three different application examples: a long myelinated axon, a segmented dendritic tree, and a damaged axon. The capabilities of the program to deal with large scale scenarios, segmented neuronal structures, and functional deficits under mechanical loading are specifically highlighted.

Efficient PGD-based dynamic calculation of non-linear soil behavior

Non-linear behavior of soils during a seismic event has a predominant role in current site response analysis. Soil response analysis consistently indicates that the stress-strain relationship of soils is nonlinear and shows hysteresis. When focusing in forced response simulations, time integrations based on modal analysis are widely considered, however parametric analysis, non-linear behavior and complex damping functions make difficult the online use of standard discretization strategies, e.g. those based on the use of finite element. In this paper we propose a new harmonic analysis formulation, able to address forced response simulation of soils exhibiting their characteristic nonlinear behavior. The solution can be evaluated in real-time from the offline construction of a parametric solution of the associated linearized problem within the Proper Generalized Decomposition framework.

Ajuste del modelo de Drucker Prager extendido para la simulaci��n de la respuesta mec��nica de un vertisol mediante el m��todo de elementos finitos

La mecanizaci��n de las labores del suelo es la causa, por su consumo energ��tico e impacto directo sobre el medio ambiente, que m��s afecta a la degradaci��n y p��rdida de productividad de los suelos. Entre los factores de disminuci��n de la productividad se deben considerar la compactaci��n, la erosi��n, el encostramiento y la p��rdida de estructura. Todo esto obliga a cuidar el manejo agr��cola de los suelos tratando de mejorar las condiciones del suelo y elevar sus rendimientos sin comprometer aspectos econ��micos, ecol��gicos y ambientales. En el presente trabajo se adecuan los par��metros constitutivos del modelo de Drucker Prager Extendido (DPE) que definen la fricci��n y la dilatancia del suelo en la fase de deformaci��n pl��stica, para minimizar los errores en las predicciones durante la simulaci��n de la respuesta mec��nica de un Vertisol mediante el M��todo de Elementos Finitos. Para lo cual inicialmente se analizaron las bases te��ricas que soportan este modelo, se determinaron las propiedades y par��metros f��sico-mec��nicos del suelo requeridos como datos de entrada por el modelo, se determin�� la exactitud de este modelo en las predicciones de la respuesta mec��nica del suelo, se estimaron mediante el m��todo de aproximaci��n de funciones de Levenberg-Marquardt los par��metros constitutivos que definen la trayectoria de la curva esfuerzo-deformaci��n pl��stica. Finalmente se comprob�� la exactitud de las predicciones a partir de las adecuaciones realizadas al modelo. Los resultados permitieron determinar las propiedades y par��metros del suelo, requeridos como datos de entrada por el modelo, mostrando que su magnitud est�� en funci��n su estado de humedad y densidad, adem��s se obtuvieron los modelos emp��ricos de estas relaciones exhibiendo un R2>94%. Se definieron las variables que provocan las inexactitudes del modelo constitutivo (��ngulo de fricci��n y dilatancia), mostrando que las mismas est��n relacionadas con la etapa de falla y deformaci��n pl��stica. Finalmente se estimaron los valores ��ptimos de estos ��ngulos, disminuyendo los errores en las predicciones del modelo DPE por debajo del 4,35% haci��ndelo adecuado para la simulaci��n de la respuesta mec��nica del suelo investigado. ABSTRACT The mechanization using farming techniques is one of the main factors that affects the most the soil, causing its degradation and loss of productivity, because of its energy consumption and direct impact on the environment. Compaction, erosion, crusting and loss of structure should be considered among the factors that decrease productivity. All this forces the necessity to take care of the agricultural-land management trying to improve soil conditions and increase yields without compromising economic, ecological and environmental aspects. The present study was aimed to adjust the parameters of the Drucker-Prager Extended Model (DPE), defining friction and dilation of soil in plastic deformation phase, in order to minimize the error of prediction when simulating the mechanical response of a Vertisol through the fine element method. First of all the theoretic fundamentals that withstand the model were analyzed. The properties and physical-mechanical parameters of the soil needed as input data to initialize the model, were established. And the precision of the predictions for the mechanical response of the soil was assessed. Then the constitutive parameters which define the path of the plastic stress-strain curve were estimated through Levenberg-Marquardt method of function approximations. Lastly the accuracy of the predictions from the adequacies made to the model was tested. The results permitted to determine those properties and parameters of the soil, needed in order to initialize the model. It showed that their magnitude is in function of density and humidity. Moreover, the empirical models from these relations were obtained: R2>94%. The variables producing inaccuracies in the constitutive model (angle of repose and dilation) were defined, and there was showed that they are linked with the plastic deformation and rupture point. Finally the optimal values of these angles were established, obtaining thereafter error values for the DPE model under 4, 35%, and making it suitable for the simulation of the mechanical response of the soil under study.

Determining the elastic-plastic properties of metallic materials through instrumented indentation

En los ��ltimos a��os, y asociado al desarrollo de la tecnolog��a MEMS, la t��cnica de indentaci��n instrumentada se ha convertido en un m��todo de ensayo no destructivo ampliamente utilizado para hallar las caracter��sticas el��stico-pl��sticas de recubrimientos y capas delgadas, desde la escala macrosc��pica a la microsc��pica. Sin embargo, debido al complejo mecanismo de contacto debajo de la indentaci��n, es urgente proponer un m��todo m��s simple y conveniente para obtener unos resultados comparables con otras mediciones tradicionales. En este estudio, el objetivo es mejorar el procedimiento anal��tico para extraer las propiedades el��stico-pl��sticas del material mediante la t��cnica de indentaci��n instrumentada. La primera parte se centra en la metodolog��a llevada a cabo para medir las propiedades el��sticas de los materiales el��sticos, present��ndose una nueva metodolog��a de indentaci��n, basada en la evoluci��n de la rigidez de contacto y en la curva fuerza-desplazamiento del ensayo de indentaci��n. El m��todo propuesto permite discriminar los valores de indentaci��n experimental que pudieran estar afectados por el redondeo de la punta del indentador. Adem��s, esta t��cnica parece ser robusta y permite obtener valores fiables del modulo el��stico. La segunda parte se centra en el proceso anal��tico para determinar la curva tensi��n-deformaci��n a partir del ensayo de indentaci��n, empleando un indentador esf��rico. Para poder asemejar la curva tension-deformaci��n de indentaci��n con la que se obtendr��a de un ensayo de tracci��n, Tabor determin�� emp��ricamente un factor de constricci��n de la tensi��n (���) y un factor de constricci��n de la deformaci��n (���). Sin embargo, la elecci��n del valor de ������y ��� necesitan una derivaci��n anal��tica. Se describi�� anal��ticamente una nueva visi��n de la relaci��n entre los factores de constricci��n de tensi��n y la deformaci��n basado en la deducci��n de la ecuaci��n de Tabor. Un modelo de elementos finitos y un dise��o experimental se realizan para evaluar estos factores de constricci��n. A partir de los resultados obtenidos, las curvas tension-deformaci��n extraidas de los ensayos de indentaci��n esf��rica, afectadas por los correspondientes factores de constricci��n de tension y deformaci��n, se ajustaron a la curva nominal tensi��n-deformaci��n obtenida de ensayos de tracci��n convencionales. En la ��ltima parte, se estudian las propiedades del revestimiento de cermet Inconel 625-Cr3C2 que es depositado en el medio de una aleaci��n de acero mediante un l��ser. Las propiedades mec��nicas de la matriz de cermet son estudiadas mediante la t��cnica de indentaci��n instrumentada, haciendo uso de las metodolog��as propuestas en el presente trabajo. In recent years, along with the development of MEMS technology, instrumented indentation, as one type of a non-destructive measurement technique, is widely used to characterize the elastic and plastic properties of metallic materials from the macro to the micro scale. However, due to the complex contact mechanisms under the indentation tip, it is necessary to propose a more convenient and simple method of instrumented indention to obtain comparable results from other conventional measurements. In this study, the aim is to improve the analytical procedure for extracting the elastic plastic properties of metallic materials by instrumented indentation. The first part focuses on the methodology for measuring the elastic properties of metallic materials. An alternative instrumented indentation methodology is presented. Based on the evolution of the contact stiffness and indentation load versus the depth of penetration, the possibility of obtaining the actual elastic modulus of an elastic-plastic bulk material through instrumented sharp indentation tests has been explored. The proposed methodology allows correcting the effect of the rounding of the indenter tip on the experimental indentation data. Additionally, this technique does not seem too sensitive to the pile-up phenomenon and allows obtaining convincing values of the elastic modulus. In the second part, an analytical procedure is proposed to determine the representative stress-strain curve from the spherical indentation. Tabor has determined the stress constraint factor (stress CF), ������and strain constraint factor (strain CF), ������empirically but the choice of a value for ������and ������is debatable and lacks analytical derivation. A new insight into the relationship between stress and strain constraint factors is analytically described based on the formulation of Tabor���s equation. Finite element model and experimental tests have been carried out to evaluate these constraint factors. From the results, representative stress-strain curves using the proposed strain constraint factor fit better with the nominal stress-strain curve than those using Tabor���s constraint factors. In the last part, the mechanical properties of an Inconel 625-Cr3C2 cermet coating which is deposited onto a medium alloy steel by laser cladding has been studied. The elastic and plastic mechanical properties of the cermet matrix are studied using depth-sensing indentation (DSI) on the micro scale.