Experimental Investigation of Mode I Rock Fracture Toughness

Programa Oficial de Doutoramento en Enxeñaría Civil . 5011V01[Abstract] Mode I fracture toughness (KIC) is one of the most important parameters in rock fracture mechanics. KIC is an intrinsic material property that represents the ability of a material containing a pre-existing defect to resist tensile failure. The International Society for Rock Mechanics (ISRM) has proposed four suggested methods for determining KIC. However, these methods present some drawbacks. The main objective of this thesis is to develop a simple testing approach, called pseudo-compact tension (pCT) test, to measure KIC in rocks using cylindrical single edge-notched specimens loaded in pure tension. The study is based on the results of a large set of fracture toughness experiments performed with different rock types, sample sizes, and a range of notch length ratios. The KIC values derived with the pCT test are compared with those obtained with one of the suggested methods of the ISRM, the semi-circular bend (SCB) test. Some selected pCT and SCB tests were also complemented with the concurrent monitoring of the acoustic emission (AE) activity. With the obtained data, we compare and analyse both testing methods, the effects of specimen size, notch length and lithology, the mechanical evolution along the experiments and the significance of different AE parameters.[Resumen] La tenacidad de fractura en modo I (KIC) es uno de los parámetros más importantes en la mecánica de fractura aplicada a rocas. KIC es una propiedad intrínseca del material que representa su capacidad para resistir su rotura a tracción en la presencia de un defecto preexistente. La Sociedad Internacional para la Mecánica de Rocas (ISRM) ha sugerido cuatro métodos para determinar KIC, pero todos presentan algunos inconvenientes. El principal objetivo de esta tesis es desarrollar un método de ensayo simple, llamado ‘pseudo-compact tensión’ (pCT), para determinar KIC en rocas mediante muestras cilíndricas ensayadas bajo condiciones de tracción pura. El estudio se basa en los resultados de un número de ensayos llevados a cabo con distintos tipos de roca, tamaños de probeta y longitudes de entalla. Los valores de KIC obtenidos con el ensayo pCT se comparan con los obtenidos con el ensayo ‘semi-circular bend’ (SCB), uno de los métodos sugeridos por la ISRM. En algunos de estos ensayos también se registró la emisión acústica (AE). Con los datos obtenidos, comparamos y analizamos ambos métodos, los efectos del tamaño de probeta, longitud de entalla y litología, la evolución mecánica y la importancia de diferentes parámetros de AE durante los ensayos.[Resumo] A tenacidade de fractura en modo I (KIC) é un dos parámetros máis importantes na mecánica de fractura aplicada a rochas. KIC é unha propiedade intrínseca do material que representa a súa capacidade para resistir a súa rotura a tracción na presenza dun defecto preexistente. A Sociedade Internacional para a Mecánica de Rochas (ISRM) suxeriu catro métodos para determinar KIC, mais todos presentan algúns inconvenientes. O principal obxectivo desta tese é desenvolver un método de ensaio simple, chamado ‘pseudo-compact tensión’ (pCT), para determinar KIC en rocha mediante mostras cilíndricas ensaiadas baixo condiciones de tracción pura. O estudo baséase nos resultados dun número de ensaios levados a cabo con distintos tipos de rocha, tamaños de probeta e lonxitudes de entalla. Os valores de KIC obtidos co ensaio pCT compáranse cos obtidos co ensaio ‘semi-circular bend’ (SCB), un dos métodos suxeridos pola ISRM. Nalgúns destes ensaios tamén se rexistrou a emisión acústica (AE). Cos datos obtidos, comparamos e analizamos ambos métodos, os efectos do tamaño de probeta, lonxitude de entalla e litoloxía, a evolución mecánica e a importancia de diferentes parámetros de AE durante os ensaio

Size Effect and Other Effects on Mode I Fracture Toughness Using Two Testing Methods

Financiado para publicación en acceso aberto: Universidade da Coruña/CISUG[Abstract] Mode I fracture toughness (KIC) is an intrinsic material property that quantifies its resistance to tensile fracture propagation. The International Society for Rock Mechanics has endorsed four methods to determine the KIC of rock, namely, the short rod, chevron bend, cracked chevron notched Brazilian disc, and semi-circular bend (SCB) methods. In this study, we compare the results of the SCB technique with those of the recently proposed pseudo-compact tension (pCT) test, which has proven to be convenient for the assessment of KIC in both fragile and ductile rocks. We select the SCB as a benchmark method due to its popularity, simplicity, and straightforward testing configuration. We discuss the results of 146 tests performed with different lithologies (Arcera, Pinacas and Corvio sandstones, and Blanco Mera granite), different sample sizes (100, 50 and 38 mm diameter), and a range of notch lengths. We also assess test repeatability and intercomparability of the results obtained using the two techniques. Compared with the SCB test, the pCT test allows for improved control of the specimen behaviour after the peak load, which provides a greater wealth of fracture mechanics information. pCT specimens yield results with higher repeatability than SCB samples. Although we observe that KIC tends to decrease with an increase in the notch length ratio, this effect appears to be non-significant based on statistical assessments. Accordingly, the corresponding mean KIC values are comparable for medium- and large specimens. The influence of specimen size is more pronounced in the SCB tests, while the pCT tests show less dependence for harder lithologies. Therefore, to set up minimum specimen requirements for fracture toughness testing, in addition to geometrical constraints, some key lithology-dependent properties (strength, mineralogy, grain size, etc.) should also be considered. Further methodological considerations related to test execution are also discussed.This work was funded by Repsol S.A. and supported by the Xunta de Galicia, the European Union (European Social Fund - ESF) and the MINECO/AEI/FEDER EU project BIA2017-87066-R. Funding for open access charge: Universidade da Coruña/CISUG

Photoelastic stress analysis of mode I fracture toughness tests using PMMA samples

[Abstract:] Rocks are usually inhomogeneous and anisotropic materials. The presence of foliation planes, grain boundaries or even microcracks may alter the stress distribution. In order to identify whether unusual behaviours in rocks are due to these imperfections or result from other factors (e.g. experimental configuration), the analyses of homogenous and isotropic materials is an useful approach. We have performed a series of mode I fracture toughness (KIC) tests using polymethyl methacrylate (PMMA) samples, which has the advantage of allowing photoelastic stress analysis based on its birefringent nature. Three different testing configurations were considered in the study: S\ emi-circular bend (SCB) test, the pseudo-compact tension (pCT) test, and a new alternative configuration based on the previous two that we have called pseudo-SCB (pSCB) test. To perform the photoelastic analysis, all the experiments were complemented with a specially-designed experimental setup consisting in two orthogonally arranged circular polarizers placed on both sides of the tested specimens. Using a source of white (polychromatic) light on one end it is possible to record the stress distribution using a digital camera aligned with the samples on the other end. As the load increases, a distinct evolving pattern of colour fringes can be visualized in the samples illustrating the spatially distributed stress levels. Based on this analysis we observe in some of the tests performed non-symmetrical stress fields. Although this behaviour could be related with the testing configuration, results suggest that other features, such as the shape of the notch tip, imperfections in sample preparation, or the misalignment of the samples in the testing device may also have an influence in stress distribution.Ministerio de Asuntos Económicos y Transformación Digital, MINECO; BIA2017- 87066-

Estudio sobre la promoción de la marca país y análisis de campañas de promoción de éxito

Ciencias de la Comunicació

Desarrollo de un Programa Informático para la Formulación de Piensos Compuestos

El objetivo principal del presente TFG es desarrollar un programa informático destinado a la formulación de piensos compuestos de animales monogástricos, que sea suficientemente potente y versátil para poder ser utilizado para fines docentes, en pequeñas empresas, ganaderías, etc. La formulación del pienso se realiza en función del coste económico (mínimo coste) a la cual se impondrán restricciones (máximos o mínimos) de ingredientes y nutrientes. La solución del problema se aborda mediante técnicas de programación lineal. El programa presenta una interfaz amigable para la introducción de las necesidades a cubrir de la especie animal en cuestión. También permite realizar un análisis post-óptimo asociado a la solución. Se han utilizado herramientas matemáticas e informáticas aplicadas a la formulación de dietas reales

CO2‐Brine substitution effects on ultrasonic wave propagation through sandstone with oblique fractures

Seismic monitoring of injected CO2 plumes in fractured storage reservoirs relies on accurate knowledge of the physical mechanisms governing elastic wave propagation, as described by appropriate, validated rock physics models. We measured laboratory ultrasonic velocity and attenuation of P and S waves, and electrical resistivity, of a synthetic fractured sandstone with obliquely aligned (penny‐shaped) fractures, undergoing a brine‐CO2 flow‐through test at simulated reservoir pressure and temperature. Our results show systematic differences in the dependence of velocity and attenuation on fluid saturation between imbibition and drainage episodes, which we attribute to uniform and patchy fluid distributions, respectively, and the relative permeability of CO2 and brine in the rock. This behavior is consistent with predictions from a multifluid rock physics model, facilitating the identification of the dispersive mechanisms associated with wave‐induced fluid flow in fractured systems at seismic scales

Hydrostatic, strike-slip and normal stress true triaxial hydrofracturing testing of Blanco Mera Granite: breakdown pressure and tensile strength assessment

Financiado para publicación en acceso aberto: Universidade da Coruña/CISUG[Abstract:] We have designed and built a versatile testing device to perform hydraulic fracturing experiments under true triaxial conditions. The device, based on a stiff biaxial frame that can be installed in a servocontrolled press, can accommodate cube rock samples of up to 150 mm-edge. Using a low-permeability rock known as Blanco Mera granite, we have performed a series of tests across a range of confining pressures including hydrostatic, normal, and strike-slip regimes. We have verified the applicability of two simple fracture mechanics-based models for the interpretation of experimental results, and we have determined the value of tensile strength of the rock from the injection curves recorded. The orientation of the hydraulically-triggered fractures with respect to the applied stress has also been analyzed. Although the models proposed by Rummel and Abou-Sayed provided reasonably satisfactory results, especially for hydrostatic and strike-slip tests, the presence of heterogeneities and defects in the rock matrix may have a strong influence on the fracture behavior and, therefore, affect the interpretation of hydrofracturing tests.This work was funded by REPSOL S.A. and the support of the Spanish Ministry of Science and Innovation (PID2021-126419NB-I00). A. Muñoz-Ibáñez also acknowledges the Margarita Salas grant from the Spanish Ministry of Universities, funded by the European Union – Next Generation EU. Funding for open access charge: Universidade da Coruña/CISUG

Hydrofracturing tests on granite samples using a true triaxial device equipped with acoustic emission sensors

[Abstract:] We present a series of tests performed on granite samples using a true triaxial device designed and built at the Rock Mechanics Laboratory (University of A Coruña). The experiments were performed using cubic rock samples of 150 mm-edge, which were loaded to different stress conditions (σh < σh < σy < 45 MPa) on each loading axis. The device is based on a stiff steel frame that can be coupled to a servo-hydraulic testing machine that provides de vertical stress (av), while two high-pressure pumps are used to deliver the lateral stress (σh and ah). An additional high-pressure pump is used to inject the fluid (mineral oil) into the rock sample at a low constant-flow rate. The aluminium loading platens, which are bevelled at the edges to avoid interaction among adjacent faces, have holes and grooves to introduce acoustic emission sensors that allow the location of fracture propagation. The specimens were drilled using a 6 mm drill bit until reaching the geometrical centre. Then, a 1/8" (~3.18 mm) stainless steel tube is glued to the samples with epoxy. Strain measurements during the experiments were conducted using four strain gages attached to the orthogonal faces of the specimens. The system was further equipped with three LVDTs to account for the bulk displacement on each axis. Our results suggest a linear relationship between the breakdown pressure and the confining stress under hydrostatic conditions but no clear correlation in non-hydrostatic stress regime.Ministerio de Asuntos Económicos y Transformación Digital, MINECO; BIA2017- 87066-

Transport properties of saline CO2 storage reservoirs with unconnected fractures from brine-CO2 flow-through tests

CO2 storage in fractured reservoirs may lead to fast CO2 flow through interconnected fracture networks; but the role of isolated fractures on brine-CO2 multiphase flow systems remains unclear. We present the results of a brine-CO2 flow-through experiment in which we assess the change in transport properties of a synthetic sandstone plug (a surrogate of a saline siliciclastic CO2 reservoir) containing non-connected fractures aligned 45° from its axis. The test was performed at 40 MPa of constant hydrostatic confining pressure and ~11 MPa of pore pressure, at room temperature (~19.5 °C), using pure liquid-CO2 and 35 g L−1 NaCl salt solution. The injected CO2-brine volume fraction was increased from 0 to 1 in 0.2 units-steps (drainage). Upon achievement of the maximum CO2 saturation (SCO2 ~0.6), the plug was flushed-back with the original brine (imbibition). During the test, we monitored simultaneously pore pressure, temperature, axial and radial strains, and bulk electrical resistivity. The fractured sample showed lower values of cross- and end-points in the relative permeability curves to CO2 compared to non-fractured samples, from comparable experiments performed at similar pressure and brine salinity conditions, but different temperature. Our results suggest that a non-connected fracture network affects the mobility of the individual phases, favouring the trapping of CO2 in the porous medium and improving the storage efficiency of the reservoir. These evidences show the need of a better understanding of fracture connectivity prior to discard fractured reservoirs as unsuitable geological formations for CO2 storage

¿Existieron vicios del razonamiento práctico en cuanto a la precipitación e inconsideración en el caso de la sentencia 01294 de 2018 del Consejo de Estado, analizado desde la prudencia política y jurídica?

13 p.A continuación, se planteará el caso de la ex representante a la cámara y senadora electa por el partido conservador colombiano Aida Merlano Rebolledo, quien perdió la investidura de su cargo en el año 2018 como consecuencia de la violación de los topes máximos de financiación de su campaña electoral, y se realizará un análisis detallado de los vicios del razonamiento prudencial de precipitación e inconsideración que se manifestaron tanto en su actuar como en el procedimiento judicial que se le realizó respectivamente. De esta manera, (y con los conceptos de los vicios del razonamiento ya entendidos) se aplicará lo establecido por prudencia jurídica y política en el caso en concreto para poder plantear una conclusión interpretativa objetiva sobre el actuar ético- moral de dicha mujer, y el por qué se expone que no fue el correcto, lo cual se espera de un jurista en el cual, los ciudadanos depositan su confianza para la buena administración de su país.Introducción Prudencia jurídica Prudencia política Resumen sentencia Vicios del razonamiento Conclusión Referencia