Abstract Tensile strength of brittle materials is usually obtained through Brazilian tests. It is accepted that failure is initiated at the centre of the sample and that it propagates through the material, creating a tensile failure plane along the vertical diameter or at the majority of it. Then, the tensile stress developed at the centre of the disc is considered as the tensile strength of the material tested. However, the stress state along the vertical diameter is always biaxial, even in the centre of the sample. This implies that the strength measured using such technique is not the uniaxial tensile strength. In this article, the expressions of the stress state supported by a tubular sample subjected to a novel device to determine the tensile strength of brittle materials are described. Besides, it is noticed that the failure plane contains points with the maximum uniaxial tensile strengths so the testing method is adequate to determine the uniaxial tensile strength of brittle materials.
Abstract The precise determination of uniaxial compressive strength (UCS) is crucial for assessing the load-bearing capacity of geological materials. To ensure the reliability and reproducibility of results, various standards and recommendations have been established. Although indirect measurement techniques exist, the uniaxial compression test (UCT) remains the benchmark for precise UCS characterization. However, discrepancies between the theoretical assumptions underlying the test and the observed outcomes—such as variations in the accepted failure patterns and sensitivity to specimen and platen properties—highlight the need for deeper analysis. In this context, numerous standards recognize the major influence of the specimen’s length-to-diameter ratio ( $$\:L/D$$ ), yet they recommend different $$\:L/D$$ values. To elucidate its relevance in the stress field generated within the specimen, this study investigates the stress distribution within cylindrical rock specimens across a range of L/D ratios (0.1, 1.0, 3.0, and 5.0), using a validated modelling framework supported by previously obtained experimental results. Findings reveal that the use of standardized steel platens slightly alters the stress ratio at the failure initiation point, deviating the stress state from the desired uniaxial compression condition. To address this, a method for identifying an optimal L/D ratio is proposed, enabling failure to initiate under conditions that closely approximate true uniaxial compression. Additionally, it is demonstrated that the deviations produced by the standardized platens can be mitigated by employing platens with stiffness comparable to that of the tested specimen.
Muchos de los fallos que ocurren en materiales empleados en Ingenieria (como hormigon y acero), estan gobernados por la resistencia a traccion. Sin embargo, esto difiere en macizos rocosos, donde los mecanismos de rotura mas frecuentes responden a campos de tensiones que predominantemente actuan en la region compresiva. Este aspecto, anadido a la baja resistencia a traccion de las rocas, ha priorizado el estudio del comportamiento rocoso en el dominio compresivo frente al de traccion. Para calcular la resistencia a traccion en rocas, destacan fundamentalmente metodos de determinacion indirectos, debido a la dificultad de reproducir condiciones de traccion uniaxial en laboratorio. Dentro de estos metodos el mas utilizado es el ensayo Brasileno, aunque existe un gran debate en la conveniencia de emplear este metodo de ensayo ya que el valor de resistencia a traccion obtenido con el difiere del calculado con metodos directos. En este sentido, muchas investigaciones afirman que subestima el verdadero valor de resistencia a traccion, lo que se traduciria en que el ensayo Brasileno es conservador puesto que, al minimizar la resistencia a traccion, opera en el lado de la seguridad. Esto es favorable desde el punto de vista, por ejemplo, de un sostenimiento en una obra civil. Sin embargo, si se pretende aprovechar la debilidad presente en diferentes rocas con vistas a su explotacion como es el caso de las rocas ornamentales, se requiere un conocimiento preciso de los parametros que controlan la rotura para optimizar el proceso.En este trabajo se describe un dispositivo y procedimiento de ensayo de caracterizacion denominado “Ensayo de traccion mediante cuna” desarrollado por el Grupo de Ingenieria del Terreno de la Universidad de Oviedo. El dispositivo, de sencilla aplicacion, ha demostrado estar mas proximo a la verdadera resistencia a traccion utilizando un material de resistencia a traccion uniaxial conocida. La fiabilidad del metodo ha sido comprobada en material rocoso, mediante su comparacion con datos procedentes de estudios previos.
In the last decades, rigorous research has been carried out with the end of understanding the gas dynamic phenomenon and although different preventive techniques have been employed, even today there are numerous accidents even with the loss of life. This work analyses an alternative and innovative method of fracturing and degassing coal, by generating CO2 with a pyrotechnic device called PYROC (Pyrotechnic Break Cartridges). Medium-scale tests of generation of CO2 into coal samples are carried out and their effect is analysed comparing the initial and final permeabilities of the coal samples once the generation of CO2 has finished. These permeabilities are calculated by injecting methane. Besides, the influence of different parameters as the length of the boreholes, the pressure of the gas or the initial permeability of the coal have been analysed with a numerical simulation of one face of one of the sublevels of a mine. The results show that the method increases the safety in mining operations because it fractures and degasses the coal, increases the permeability of the coal in the borehole of injection from 9.5 mD to 31 mD, decreases the methane gas pressure below pre-detonation levels for 1 min, achieves decompressed lengths between 8 and 10 m ahead of the face with pressures of injection of 50 MPa, relaxes the total length of the borehole for initial coal permeability values equal to or greater than 0.002 mD, and allows to work with low permeable coals with high induced stresses and high methane concentrations.
In this work, the sup port of two general galleries located in poor quality rock mass and subjected to the influence of high thickness coal layer exploitations is designed and optimized.The process is carried out in four phases:A first preliminary support is defined employing different geomechanical classifications and applying the New Austrian Tunnelling Method (NATM) using bolts and shotcrete.An instrumentation campaign is carried out with the goal of analysing the behaviour of the support.The study noticed the failure of the support due to the time of placement of the different elements.A back-analysis using the Flac and Phases software has allowed the evaluation of the properties of the rock mass and the support, the study of the influence of the time of placement on the component elements (bolts and shotcrete), and the redefinition of that support.Subsequently, a new support is designed and optimized through numerical modeling after the start of mining without experience in these sizes of sublevel caving that caused the failure of the previously designed support.The new support is formed by yieldable steel arches that are more suitable to withstand the stresses generated by nearby mining work.
