3,166 research outputs found

    Stochastic Simulation of Mudcrack Damage Formation in an Environmental Barrier Coating

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    The FEAMAC/CARES program, which integrates finite element analysis (FEA) with the MAC/GMC (Micromechanics Analysis Code with Generalized Method of Cells) and the CARES/Life (Ceramics Analysis and Reliability Evaluation of Structures / Life Prediction) programs, was used to simulate the formation of mudcracks during the cooling of a multilayered environmental barrier coating (EBC) deposited on a silicon carbide substrate. FEAMAC/CARES combines the MAC/GMC multiscale micromechanics analysis capability (primarily developed for composite materials) with the CARES/Life probabilistic multiaxial failure criteria (developed for brittle ceramic materials) and Abaqus (Dassault Systmes) FEA. In this report, elastic modulus reduction of randomly damaged finite elements was used to represent discrete cracking events. The use of many small-sized low-aspect-ratio elements enabled the formation of crack boundaries, leading to development of mudcrack-patterned damage. Finite element models of a disk-shaped three-dimensional specimen and a twodimensional model of a through-the-thickness cross section subjected to progressive cooling from 1,300 C to an ambient temperature of 23 C were made. Mudcrack damage in the coating resulted from the buildup of residual tensile stresses between the individual material constituents because of thermal expansion mismatches between coating layers and the substrate. A two-parameter Weibull distribution characterized the coating layer stochastic strength response and allowed the effect of the Weibull modulus on the formation of damage and crack segmentation lengths to be studied. The spontaneous initiation of cracking and crack coalescence resulted in progressively smaller mudcrack cells as cooling progressed, consistent with a fractal-behaved fracture pattern. Other failure modes such as delamination, and possibly spallation, could also be reproduced. The physical basis assumed and the heuristic approach employed, which involves a simple stochastic cellular automaton methodology to approximate the crack growth process, are described. The results ultimately show that a selforganizing mudcrack formation can derive from a Weibull distribution that is used to describe the stochastic strength response of the bulk brittle ceramic material layers of an EBC

    Spiral cracks in drying precipitates

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    We investigate the formation of spiral crack patterns during the desiccation of thin layers of precipitates in contact with a substrate. This symmetry-breaking fracturing mode is found to arise naturally not from torsion forces, but from a propagating stress front induced by the fold-up of the fragments. We model their formation mechanism using a coarse-grain model for fragmentation and successfully reproduce the spiral cracks. Fittings of experimental and simulation data show that the spirals are logarithmic, corresponding to constant deviation from a circular crack path. Theoretical aspects of the logarithmic spirals are discussed. In particular we show that this occurs generally when the crack speed is proportional to the propagating speed of stress front.Comment: 4 pages, 5 figures, RevTe

    Numerical modeling in timber engineering – moisture transport and quasi-brittle failure

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    With the rising popularity of timber structures and the increasing complexity of timber engineering projects, the need for numerical simulation tools specific to this building material is gaining rapidly in importance. in particular, moisture transport processes and the quasi-brittle failure behavior, both difficult to describe, present major challenges and are of great relevance in practical construction. For these reasons, this article presents numerical modeling concepts for predicting moisture gradients, estimating effective stiffness and strength, and numerically identifying potential cracking mechanisms in wooden components. These concepts are validated through experimental test programs, and the associated challenges are addressed. selected results ultimately demonstrate the capabilities and relevance of such methods for timber engineering

    Aging concrete structures: a review of mechanics and concepts

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    The safe and cost-efficient management of our built infrastructure is a challenging task considering the expected service life of at least 50 years. In spite of time-dependent changes in material properties, deterioration processes and changing demand by society, the structures need to satisfy many technical requirements related to serviceability, durability, sustainability and bearing capacity. This review paper summarizes the challenges associated with the safe design and maintenance of aging concrete structures and gives an overview of some concepts and approaches that are being developed to address these challenges

    Efficient computational mesoscale modeling of concrete under cyclic loading

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    Tesi amb diferents seccions retallades per drets de l'editor.Concrete is a complex material and can be modeled on various spatial and temporal scales. While simulations on coarse scales are practical for engineering applications, a deeper understanding of the material is gained on finer scales. This is at the cost of an increased numerical effort that can be reduced by the three methods developed and used in this work, each corresponding to one publication. The coarse spatial scale is related to fully homogenized models. The material is described in a phenomenological approach and the numerous parameters sometimes lack a physical meaning. Resolving the three-phase mesoscopic structure consisting of aggregates, the mortar matrix and the interfaces between them allow to describe similar effects with simpler models. This work addresses two computational challenges related to mesoscale modeling. First, aggregate particles take up a high volume fraction and an efficient particle-packing algorithm is required to generate non-overlapping, random esostructures. Enforcing an additional distance between the aggregates is essential to obtain undistorted meshes for finite element simulations, but further complicates the packing problem. An event-driven molecular-dynamics algorithm is applied to this problem that, in contrast to traditional methods, allows movement and a dense arrangement of the aggregates. This allows creating concrete mesostructures with realistic aggregate volume fractions. The second challenge concerns stability problems in mesoscale simulations of concrete fracture. The geometric complexity and the combination of three material laws for each of the phases leads to numerical instabilities, even for regularized material models. This requires tiny time steps and numerous iterations per time step when integrated with a classic backward Euler scheme. The implicit–explicit (IMPL-EX) integration extrapolates internal variables that account for the nonlinear behavior. This linearizes the equations, provides additional robustness and a computational speedup. In combination with a novel time step control method, a three-dimensional mesoscale compression test is accelerated by a factor of 40, compared to an adaptive backward Euler algorithm. The life time of concrete under cyclic loads is commonly predicted with empirical Wöhler lines. They relate the number of endured cycles with the applied load amplitude and can be included in constitutive formulations. They can, however, hardly be generalized to geometries and load configurations other than the ones tested. On a finer temporal scale, fatigue failure is modeled by the accumulation of damage within each loading cycle. This resolves the whole process of failure, includes stress redistributions and size effects and can easily be extended to multiphysics phenomena. The third computational challenge solved here is the efficient temporal integration that would not be feasible in a naive cycle-by-cycle integration of thousands or millions of cycles. The cost of evaluating a single cycle is reduced by reformulating the problem in the frequency space. It is sufficient to equilibrate the structure once for each Fourier coefficient which significantly speeds up this evaluation. The accumulated damage of one cycle is integrated in time using an adaptive cycle jump concept. For a two dimensional void test structure, the combination of both techniques leads to a 25 times faster simulation compared to the full integration. These three main contributions decrease the numerical cost of mesoscale simulations, allow larger and more detailed models and are a basis to deepen the understanding of the complex failure patterns in concrete.El hormigón es un material complejo y puede ser modelado en varias escalas espaciales y temporales. Mientras que las simulaciones en escalas gruesas son prácticas para aplicaciones de ingeniería, se obtiene una comprensión más profunda del material en escalas más finas. Esto es a costa de un mayor esfuerzo numérico que puede ser reducido por los tres métodos desarrollados y utilizados en este trabajo, cada uno de los cuales corresponde a una publicación. La escala espacial gruesa está relacionada con modelos totalmente homogeneizados. El material se describe con un enfoque fenomenológico y los numerosos parámetros a veces carecen de significado físico. La resolución de la estructura mesoscópica trifásica formada por los áridos, la matriz de mortero y las interfaces entre ellos permite describir efectos similares con modelos más sencillos. Este trabajo aborda dos retos computacionales relacionados con el modelado a mesoescala. En primer lugar, las partículas agregadas absorben una fracción de gran volumen y se requiere un algoritmo eficiente de empaquetamiento de partículas para generar mesoestructuras aleatorias que no se solapen. Hacer cumplir una distancia adicional entre los agregados es esencial para obtener mallas no distorsionadas para simulaciones de elementos finitos, pero complica aún más el problema de empaquetado. A este problema se le aplica un algoritmo de dinámica molecular impulsado por eventos que, a diferencia de los métodos tradicionales, permite el movimiento y una disposición densa de los agregados. Esto permite crear mesoestructuras de hormigón con fracciones de volumen de agregado realistas. El segundo reto se refiere a los problemas de estabilidad en las simulaciones mesoescalares de fracturas de hormigón. La complejidad geométrica y la combinación de tres leyes materiales para cada una de las fases conduce a inestabilidades numéricas, incluso para modelos materiales regularizados. Esto requiere pequeños pasos de tiempo y numerosas iteraciones por paso de tiempo cuando se integra con un esquema clásico de Euler hacia atrás. La integración implícita- explícita (IMPL-EX) extrapola variables internas que dan cuenta del comportamiento no lineal. Esto linealiza las ecuaciones, proporciona robustez adicional y una aceleración computacional. En combinación con un nuevo método de control de paso en el tiempo, una prueba de compresión tridimensional de mesoescala es acelerada por un factor de 40, en comparación con un algoritmo adaptativo de Euler hacia atrás. La vida útil del hormigón bajo cargas cíclicas se predice comúnmente con las líneas empíricas de Wöhler. Relacionan el número de ciclos soportados con la amplitud de carga aplicada y pueden ser incluidos en formulaciones constitutivas. Sin embargo, difícilmente pueden generalizarse a geometrías y configuraciones de carga distintas a las probadas. En una escala temporal más fina, la falla por fatiga es modelada por la acumulación de daño dentro de cada ciclo de carga. Esto resuelve todo el proceso de fracaso, incluye redistribuciones de estrés y efectos de tamaño, y puede extenderse fácilmente a fenómenos multifísicos. El tercer reto computacional resuelto aquí es la integración temporal eficiente que no sería factible en una integración costosa de miles o millones de ciclos ciclo a ciclo. El costo de evaluar un solo ciclo se reduce reformulando el problema en el espacio de frecuencias. Es suficiente equilibrar la estructura una vez para cada coeficiente de Fourier, lo que acelera significativamente esta evaluación. El daño acumulado de un ciclo se integra en el tiempo utilizando un concepto de salto de ciclo adaptativo. Para una estructura de prueba de vacío bidimensional, la combinación de ambas técnicas conduce a una simulación 25 veces más rápida en comparación con la integración completa.Postprint (published version

    Use of a quartz crystal microbalance to investigate the mechanical stability of silica xerogel membranes for volatile iodine capture

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    There have been numerous studies on silica-based sorbents for the capture of radioiodine off-gassed during the processing of legacy waste or the reprocessing of nuclear waste. While studies investigate their iodine capture efficiency, only limited information about their mechanical applicability is found in the literature. This study investigates mechanical stability of silica-based adsorbents under the gas flow conditions to evaluate and quantify the effects of flow on the structural integrity of the tested materials. Silica-based xerogels, prepared through sol-gel processing using only tetraethoxysilane (TEOS), TEOS with oil as porogen, TEOS with bismuth nanoparticles, and TEOS with both. The technique subjected the adsorbents to a gas stream and used adhesive-coated quartz crystal, connected to a quartz crystal microbalance (QCM), to collect the particles lost by the adsorbents under flow. Collection of particles change the crystal’s resonance, and the change is recorded on the QCM. The QCM data is analyzed to quantify the mass lost by the adsorbents. Adsorbents that did not contain bismuth failed under the flow as the QCM registered mass gains on the crystals, while the adsorbents with bismuth showed no failure. The results were supported by optical imaging that showed cracks on the failed surfaces. The technique used in this study showed encouraging results that validate the conceptualization of the technique as well as provide feedback for improvements to fine-tune technique for continued testing of new and existing materials
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