An embedded cohesive crack model for finite element analysis of quasi-brittle materials

This paper presents a numerical implementation of the cohesive crack model for the anal-ysis of quasibrittle materials based on the strong discontinuity approach in the framework of the finite element method. A simple central force model is used for the stress versus crack opening curve. The additional degrees of freedom defining the crack opening are determined at the crack level, thus avoiding the need for performing a static condensation at the element level. The need for a tracking algorithm is avoided by using a consistent pro-cedure for the selection of the separated nodes. Such a model is then implemented into a commercial program by means of a user subroutine, consequently being contrasted with the experimental results. The model takes into account the anisotropy of the material. Numerical simulations of well-known experiments are presented to show the ability of the proposed model to simulate the fracture of quasibrittle materials such as mortar, concrete and masonry.

A new UTD for the electromagnetic plane wave scattering by a canonical circular cylinder with an impedance boundary condition

A UTD solution is developed for describing the scattering by a circular cylinder with an impedance boundary condition (IBC), when it is illuminated by an obliquely incident electromagnetic (EM) plane wave. The solution to this canonical problem will be crucial for the construction of a more general UTD solution valid for an arbitrary smooth convex surface with an IBC, when it is illuminated by an arbitrary EM ray optical field. The canonical solution is uniformly valid across the surface shadow boundary that is tangent to the surface at grazing incidence. This canonical solution contains cross polarized terms in the scattered fields, which arise from a coupling of the TEz and TMz waves at the impedance boundary on the cylinder. Here, z is the cylinder axis. Numerical results show very good accuracy for the simpler and efficient UTD solution, when compared to exact but very slowly convergent eigenfunction solution.

Development of One Meter Long Double-Sided CeO2 Buffered Ni-5at.W Templates by Reel-to-Reel Chemical Solution Deposition Route

High performance long-length coated conductors fabricated using various techniques have attracted a lot of interest recently. In this work, a reel-to-reel design for depositing double-sided coatings on long-length flexible metallic tapes via a chemical solution method is proposed and realized. The major achievement of the design is to combine the dip coating and drying processes in order to overcome the technical difficulties of dealing with the wet films on both sides of the tape. We report the successful application of the design to fabricate a one-meter-long double side coated CeO2/Ni-5at%W template. The CeO2 films on both sides exhibit a dense, crack-free morphology, and a high fraction of cube texture on the surface. Homogeneity studies on global texture over the length also reveal that the average full width at half maximum values of the in-plane and out-of-plane orientation on the CeO2 layer are 7.2 ° and 5.8° with standard deviation of 0.26° and 0.34°, respectively, being indicative of the high quality epitaxial growth of the films prepared in the continuous manner. An all chemical solution derived YBCOLow-TFA/Ce0.9La0.1O2 /Gd2Zr2O7/CeO2 structure is obtained on a short sample, demonstrating the possibility of producing long-length texture templates for coated conductors by this low cost deposition route.

The Cohesive Crack Model Applied To Notched PMMA Specimens Obeying a Non Linear Behaviour Under Torsion Loading

This article presents a new material model developed with the aim of analyzing failure of blunt notched components made of nonlinear brittle materials. The model, which combines the cohesive crack model with Hencky's theory of total deformations, is used to simulate an experimental benchmark carried out previously by the authors. Such combination is achieved through the embedded crack approach concept. In spite of the unavailability of precise material data, the numerical predictions obtained show good agreement with the experimental results.

Experimental and numerical study of cracking of concrete due to reinforcement corrosion

La corrosión del acero es una de las patologías más importantes que afectan a las estructuras de hormigón armado que están expuestas a ambientes marinos o al ataque de sales fundentes. Cuando se produce corrosión, se genera una capa de óxido alrededor de la superficie de las armaduras, que ocupa un volumen mayor que el acero inicial; como consecuencia, el óxido ejerce presiones internas en el hormigón circundante, que lleva a la fisuración y, ocasionalmente, al desprendimiento del recubrimiento de hormigón. Durante los últimos años, numerosos estudios han contribuido a ampliar el conocimiento sobre el proceso de fisuración; sin embargo, aún existen muchas incertidumbres respecto al comportamiento mecánico de la capa de óxido, que es fundamental para predecir la fisuración. Por ello, en esta tesis se ha desarrollado y aplicado una metodología, para mejorar el conocimiento respecto al comportamiento del sistema acero-óxido-hormigón, combinando experimentos y simulaciones numéricas. Se han realizado ensayos de corrosión acelerada en condiciones de laboratorio, utilizando la técnica de corriente impresa. Con el objetivo de obtener información cercana a la capa de acero, como muestras se seleccionaron prismas de hormigón con un tubo de acero liso como armadura, que se diseñaron para conseguir la formación de una única fisura principal en el recubrimiento. Durante los ensayos, las muestras se equiparon con instrumentos especialmente diseñados para medir la variación de diámetro y volumen interior de los tubos, y se midió la apertura de la fisura principal utilizando un extensómetro comercial, adaptado a la geometría de las muestras. Las condiciones de contorno se diseñaron cuidadosamente para que los campos de corriente y deformación fuesen planos durante los ensayos, resultando en corrosión uniforme a lo largo del tubo, para poder reproducir los ensayos en simulaciones numéricas. Se ensayaron series con varias densidades de corriente y varias profundidades de corrosión. De manera complementaria, el comportamiento en fractura del hormigón se caracterizó en ensayos independientes, y se midió la pérdida gravimétrica de los tubos siguiendo procedimientos estándar. En todos los ensayos, la fisura principal creció muy despacio durante las primeras micras de profundidad de corrosión, pero después de una cierta profundidad crítica, la fisura se desarrolló completamente, con un aumento rápido de su apertura; la densidad de corriente influye en la profundidad de corrosión crítica. Las variaciones de diámetro interior y de volumen interior de los tubos mostraron tendencias diferentes entre sí, lo que indica que la deformación del tubo no fue uniforme. Después de la corrosión acelerada, las muestras se cortaron en rebanadas, que se utilizaron en ensayos post-corrosión. El patrón de fisuración se estudió a lo largo del tubo, en rebanadas que se impregnaron en vacío con resina y fluoresceína para mejorar la visibilidad de las fisuras bajo luz ultravioleta, y se estudió la presencia de óxido dentro de las grietas. En todas las muestras, se formó una fisura principal en el recubrimiento, infiltrada con óxido, y varias fisuras secundarias finas alrededor del tubo; el número de fisuras varió con la profundidad de corrosión de las muestras. Para muestras con la misma corrosión, el número de fisuras y su posición fue diferente entre muestras y entre secciones de una misma muestra, debido a la heterogeneidad del hormigón. Finalmente, se investigó la adherencia entre el acero y el hormigón, utilizando un dispositivo diseñado para empujar el tubo en el hormigón. Las curvas de tensión frente a desplazamiento del tubo presentaron un pico marcado, seguido de un descenso constante; la profundidad de corrosión y la apertura de fisura de las muestras influyeron notablemente en la tensión residual del ensayo. Para simular la fisuración del hormigón causada por la corrosión de las armaduras, se programó un modelo numérico. Éste combina elementos finitos con fisura embebida adaptable que reproducen la fractura del hormigón conforme al modelo de fisura cohesiva estándar, y elementos de interfaz llamados elementos junta expansiva, que se programaron específicamente para reproducir la expansión volumétrica del óxido y que incorporan su comportamiento mecánico. En el elemento junta expansiva se implementó un fenómeno de despegue, concretamente de deslizamiento y separación, que resultó fundamental para obtener localización de fisuras adecuada, y que se consiguió con una fuerte reducción de la rigidez tangencial y la rigidez en tracción del óxido. Con este modelo, se realizaron simulaciones de los ensayos, utilizando modelos bidimensionales de las muestras con elementos finitos. Como datos para el comportamiento en fractura del hormigón, se utilizaron las propiedades determinadas en experimentos. Para el óxido, inicialmente se supuso un comportamiento fluido, con deslizamiento y separación casi perfectos. Después, se realizó un ajuste de los parámetros del elemento junta expansiva para reproducir los resultados experimentales. Se observó que variaciones en la rigidez normal del óxido apenas afectaban a los resultados, y que los demás parámetros apenas afectaban a la apertura de fisura; sin embargo, la deformación del tubo resultó ser muy sensible a variaciones en los parámetros del óxido, debido a la flexibilidad de la pared de los tubos, lo que resultó fundamental para determinar indirectamente los valores de los parámetros constitutivos del óxido. Finalmente, se realizaron simulaciones definitivas de los ensayos. El modelo reprodujo la profundidad de corrosión crítica y el comportamiento final de las curvas experimentales; se comprobó que la variación de diámetro interior de los tubos está fuertemente influenciada por su posición relativa respecto a la fisura principal, en concordancia con los resultados experimentales. De la comparación de los resultados experimentales y numéricos, se pudo extraer información sobre las propiedades del óxido que de otra manera no habría podido obtenerse. Corrosion of steel is one of the main pathologies affecting reinforced concrete structures exposed to marine environments or to molten salt. When corrosion occurs, an oxide layer develops around the reinforcement surface, which occupies a greater volume than the initial steel; thus, it induces internal pressure on the surrounding concrete that leads to cracking and, eventually, to full-spalling of the concrete cover. During the last years much effort has been devoted to understand the process of cracking; however, there is still a lack of knowledge regarding the mechanical behavior of the oxide layer, which is essential in the prediction of cracking. Thus, a methodology has been developed and applied in this thesis to gain further understanding of the behavior of the steel-oxide-concrete system, combining experiments and numerical simulations. Accelerated corrosion tests were carried out in laboratory conditions, using the impressed current technique. To get experimental information close to the oxide layer, concrete prisms with a smooth steel tube as reinforcement were selected as specimens, which were designed to get a single main crack across the cover. During the tests, the specimens were equipped with instruments that were specially designed to measure the variation of inner diameter and volume of the tubes, and the width of the main crack was recorded using a commercial extensometer that was adapted to the geometry of the specimens. The boundary conditions were carefully designed so that plane current and strain fields were expected during the tests, resulting in nearly uniform corrosion along the length of the tube, so that the tests could be reproduced in numerical simulations. Series of tests were carried out with various current densities and corrosion depths. Complementarily, the fracture behavior of concrete was characterized in independent tests, and the gravimetric loss of the steel tubes was determined by standard means. In all the tests, the main crack grew very slowly during the first microns of corrosion depth, but after a critical corrosion depth it fully developed and opened faster; the current density influenced the critical corrosion depth. The variation of inner diameter and inner volume of the tubes had different trends, which indicates that the deformation of the tube was not uniform. After accelerated corrosion, the specimens were cut into slices, which were used in post-corrosion tests. The pattern of cracking along the reinforcement was investigated in slices that were impregnated under vacuum with resin containing fluorescein to enhance the visibility of cracks under ultraviolet lightening and a study was carried out to assess the presence of oxide into the cracks. In all the specimens, a main crack developed through the concrete cover, which was infiltrated with oxide, and several thin secondary cracks around the reinforcement; the number of cracks diminished with the corrosion depth of the specimen. For specimens with the same corrosion, the number of cracks and their position varied from one specimen to another and between cross-sections of a given specimen, due to the heterogeneity of concrete. Finally, the bond between the steel and the concrete was investigated, using a device designed to push the tubes of steel in the concrete. The curves of stress versus displacement of the tube presented a marked peak, followed by a steady descent, with notably influence of the corrosion depth and the crack width on the residual stress. To simulate cracking of concrete due to corrosion of the reinforcement, a numerical model was implemented. It combines finite elements with an embedded adaptable crack that reproduces cracking of concrete according to the basic cohesive model, and interface elements so-called expansive joint elements, which were specially designed to reproduce the volumetric expansion of oxide and incorporate its mechanical behavior. In the expansive joint element, a debonding effect was implemented consisting of sliding and separation, which was proved to be essential to achieve proper localization of cracks, and was achieved by strongly reducing the shear and the tensile stiffnesses of the oxide. With that model, simulations of the accelerated corrosion tests were carried out on 2- dimensional finite element models of the specimens. For the fracture behavior of concrete, the properties experimentally determined were used as input. For the oxide, initially a fluidlike behavior was assumed with nearly perfect sliding and separation; then the parameters of the expansive joint element were modified to fit the experimental results. Changes in the bulk modulus of the oxide barely affected the results and changes in the remaining parameters had a moderate effect on the predicted crack width; however, the deformation of the tube was very sensitive to variations in the parameters of oxide, due to the flexibility of the tube wall, which was crucial for indirect determination of the constitutive parameters of oxide. Finally, definitive simulations of the tests were carried out. The model reproduced the critical corrosion depth and the final behavior of the experimental curves; it was assessed that the variation of inner diameter of the tubes is highly influenced by its relative position with respect to the main crack, in accordance with the experimental observations. From the comparison of the experimental and numerical results, some properties of the mechanical behavior of the oxide were disclosed that otherwise could not have been measured.

Plano Horizontal Plano. Del plano horizontal como límite entre lo estereotómico y lo tectónico = Flat Horizontal Plane. On Horizon

Plano Horizontal Plano. Del plano horizontal como límite entre lo estereotómico y lo tectónico

Flat Horizontal Plane - On Horizon

The Flat Horizontal Plane, the platform, is more than just one of the most basic mechanisms of Architecture. In this essay, I would like to move towards understanding this Flat Horizontal Plane not only as the primary mechanism of Architecture, but also, when it is erected, as the spatial limit between the stereotomic and the tectonic.

A discrete spectral model for intermediate crack debonding in FRP-strengthened RC beams

One of the common failure modes of reinforced concrete (RC) beams strengthened in flexure with a bonded fibre-reinforced polymer (FRP) is intermediate crack (IC) debonding, which is originated at a critical section in the vicinity of flexural cracks and propagates to a plate end. Despite considerable research over the last years, few reliable and simplified IC debonding strength models have been developed. This paper firstly presents a one-dimensional model based on the discrete crack approach for concrete and the spectral element method for the numerical simulation of the IC debonding process. The progressive formation of flexural cracks and subsequent concrete-FRP interfacial debonding is formulated by the introduction of a new element able to represent both phenomena simultaneously without perturbing the numerical procedure. Furthermore, with the proposed model, high frequency dynamic response for these kinds of structures can also be obtained in a very simple and non-expensive way, which makes this procedure very useful as a tool for diagnoses and detection of debonding in its initial stage by monitoring the change in local dynamic characteristics.

Light polarization sensitive photodetectors with m- and r-plane homoepitaxial ZnO/ZnMgO quantum wells

Homoepitaxial ZnO/(Zn,Mg)O multiple quantum wells (MQWs) grown with m- and r-plane orientations are used to demonstrate Schottky photodiodes sensitive to the polarization state of light. In both orientations, the spectral photoresponse of the MQW photodiodes shows a sharp excitonic absorption edge at 3.48 eV with a very low Urbach tail, allowing the observation of the absorption from the A, B and C excitonic transitions. The absorption edge energy is shifted by ∼30 and ∼15 meV for the m- and r-plane MQW photodiodes, respectively, in full agreement with the calculated polarization of the A, B, and C excitonic transitions. The best figures of merit are obtained for the m-plane photodiodes, which present a quantum efficiency of ∼11%, and a specific detectivity D* of ∼6.4 × 1010 cm Hz1/2/W. In these photodiodes, the absorption polarization sensitivity contrast between the two orthogonal in-plane axes yields a maximum value of (R⊥/R||)max ∼ 9.9 with a narrow bandwidth of ∼33 meV.