Experimental and Numerical Studies on Determination of Indirect Tensile Strength of Rocks

Indirect tension tests using Brisbane tuff Brazilian disc specimens under standard Brazilian jaws and various loading arcs were performed. The standard Brazilian indirect tensile tests caused catastrophic, crushing failure of the disc specimens, rather than the expected tensile splitting failure initiated by a central crack. This led to an investigation of the fracturing of Brazilian disc specimens and the existing indirect tensile Brazilian test using steel loading arcs with different angles. The results showed that the ultimate failure load increased with increasing loading arc angles. With no international standard for determining indirect tensile strength of rocks under diametral load, numerical modelling and analytical solutions were undertaken. Numerical simulations using RFPA^2D software were conducted with a heterogeneous material model. The results predicted tensile stress in the discs and visually reproduced the progressive fracture process. It was concluded that standard Brazilian jaws cause catastrophic, crushing failure of the disc specimens instead of producing a central splitting crack. The experimental and numerical results showed that 20° and 30° loading arcs result in diametral splitting fractures starting at the disc centre. Moreover, intrinsic material properties (e.g. fracture toughness) may be used to propose the best loading configuration to determine the indirect tensile strength of rocks. Here, by using numerical outcomes and empirical relationships between fracture toughness and tensile strength, the best loading geometry to obtain the most accurate indirect tensile strength of rocks was the 2α?=?30° loading arc.     

Measurement of Indirect Tensile Strength of Anisotropic Rocks by the Ring Test

Summary This paper presents a new approach, combined with the Boundary Element Method (BEM) analysis and the diametrical compression on a thin disc with a small central hole, referred to as the ring test, for determining the indirect tensile strength of anisotropic rocks. The stress distribution around the hole can be successfully obtained by the proposed single-domain BEM. The complex variable function method was used for conveniently computing the tractions and displacements of a two-dimensional anisotropic body. If we assume that the tensile strength is given by the maximum absolute value of stress in the direction perpendicular to the loaded diameter at the intersection of loaded diameter and the hole, then from the failure load recorded by laboratory testing of ring (disc), the indirect tensile strength of rocks could be obtained. A marble from Hualien (Taiwan) with clearly black-white foliation, which was assumed to be transversely isotropic, was selected to conduct both ring tests and Brazilian tests for evaluating the tensile strength. The variation of the marble tensile strength with the inclination angle of foliation and with the hole size was also investigated. In general, the tensile strength of anisotropic rocks determined by ring test is not a constant, but depends on the elastic properties of rocks, the angle between the planes of rock anisotropy and the loading direction, the diameter of the central hole, and the contact condition of loading.     

Investigating the Effect of Cyclic Loading on the Indirect Tensile Strength of Rocks

This paper presents the results of laboratory experiments during the investigation of the stress–strain characteristics of Brisbane tuff disc specimens under diametral compressive cyclic loading. Two different cyclic loading methods were used: namely, sinusoidal cyclic loading and cyclic loading with increasing mean level. The first method applied the S–N curve approach to the indirect tensile strength (ITS) of rock specimens for the first time in the literature, and the second method investigated the effect of increasing cyclic loading on the ITS of rock specimens. The ITS of Brisbane tuff disc specimens was measured using the Brazilian tensile strength test. The reduction in ITS was found to be 33% with sinusoidal loading tests, whereas increasing cyclic loading caused a maximum reduction of 37%. It is believed that the fracturing under cyclic loading starts at contact points between strong grains and weak matrices, and that contact points at grain boundaries are the regions of stress concentration (i.e., indenters). Transgranular cracks emanate from these regions and intergranular cracks sometimes pass through the contact points. Once cracking begins, there is a steady progression of damage and a general ‘loosening’ of the rock, which is a precursor to the formation of intergranular cracks.     

On the Measurement of the Tensile Strength of Soft Rocks

Summary. This paper reports on a comparative study of various types of experimental tests for measuring the tensile strength of rocks and rock-like materials. A critical assessment is presented of some widely used laboratory techniques on the basis of experimental data from the literature and from the laboratory investigation performed in this study.Tests were carried out using a triaxial apparatus recently set up at Milan University of Technology. This was designed to reduce random misalignments between specimen and loading frame, which are typical of conventional triaxial cells with external tie bars. The apparatus was then modified to perform various types of tests for determining the tensile strength of materials. An artificial building stone and a natural calcarenite of the Gravina di Puglia geological formation, sampled at Montescaglioso (Matera-Italy) were tested in this research. The experimental investigation included various types of tests, namely the uni-axial, the Brazilian, the ring, the three and four points bending and the Luong test. Specimens of both materials were also compressed to failure in unconfined conditions and loaded cyclically in unconfined tension and compression, Young’s moduli being measured by means of local instrumentation.     

轴向压裂法测定冻土抗拉强度初步研究

通过试验，讨论温度、机速对轴向压裂法测定结果的影响及其规律，确定该法作为冻土抗拉强度间接测定方法的可行性。试验结果表明，测定值稳定，与温度和机速对应性良好，服从一定函数关系，通过修正，该法测定冻土抗拉强度是可行的，对现场原状冻土抗强度测定更具一定意义。     

岩石三轴强度试验

通过自制岩石三轴压力室和配备液压泵与现有万能材料试验机组成岩石三轴试验系统，进行岩石三轴强度试验研究，并与其它方法比对，结果相近，满足了科研和生产的需要。     

脆性岩石单轴压缩变形强度的试验

主要讨论在单轴压缩条件下脆性岩石破坏的全过程,分析与破裂机制有关的问题及试验中主要影响因素,使试验结果为工程评价提供准确、可靠的保证。     

Water Saturation Effects on the Brazilian Tensile Strength of Gypsum and Assessment of Cracking Processes Using High-Speed Video

This study investigates the water weakening effect on the tensile strength, as well as the fracturing behavior, of an artificially molded Hydrocal B-11 gypsum rock. Brazilian disc tests, with the aid of a high-speed video system to monitor and record the cracking processes, are conducted on dry and wet specimens to determine their tensile strengths. The dry specimens are oven-dried, while the wet specimens are prepared by soaking in water for 1, 3, and 10 weeks to achieve different levels of water content. The test results show that the tensile strength drops to nearly half of its dry value after being soaked in water for only 1 week. The tensile strength reduces only slightly further after the specimens have been immersed in water for 3 and 10 weeks. An analysis of the recorded high-speed footage shows that the primary crack initiates at the center as observed from the surface for the majority of the tested specimens. Most importantly, the cracking processes of dry and wet specimens are distinctly different with regard to the speed of crack propagation and the number of cracks developed.     

Numerical Studies on Chip Formation in Drag-Pick Cutting of Rocks

A two dimensional non-linear finite element simulation model has been developed using a mathematical model for progressive rock failure for understanding the mode and sequence of rock failure under a drag pick cutter. Rock cutting simulation has also been done using linear elastic modeling using local stability factor contouring. It has been observed from the simulation results that during negative rake angle cutting the chipping occurs by shear failure of the elements. Whereas, in positive rake angle cutting, some elements were observed to fail in shear and some under tension. The predicted peak cutting force using the developed models was found to be up to 25% higher than the experimental values. The effect of input parameters such as rake angle, flank wear, depth of cut and rock properties on the predicted peak cutting force has been studied, verified from earlier experimental studies and compared with some earlier proposed theories on rock cutting. The elastic stress analysis model based on the stability factor contouring method has also been found to be an effective tool to bracket the expected peak cutting force for a given operational and rock parameters but failed to simulate the effect of pick geometry (rake angle) correctly. The non-linear simulation model using progressive rock element failure is superior to elastic linear stress analysis model by simulating the correct trends for all the rock and machining parameters.     

Prediction of Uniaxial Compressive Strength,Tensile Strength and Porosity of Sedimentary Rocks Using Sound Level Produced During Rotary Drilling

The main purpose of the study is to develop a general prediction model and to investigate the relationships between sound level produced during drilling and physical properties such as uniaxial compressive strength, tensile strength and percentage porosity of sedimentary rocks. The results were evaluated using the multiple regression analysis taking into account the interaction effects of various predictor variables. Predictor variables selected for the multiple regression model are drill bit diameter, drill bit speed, penetration rate and equivalent sound level produced during rotary drilling (L _eq). The constructed models were checked using various prediction performance indices. Consequently, it is possible to say that the constructed models can be used for practical purposes.