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Experiment and discrete Element Modelling on Strength, deformation and Failure Behaviour of Shale Under Brazilian Compression

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ROCK MECHANICS ANd ROCK ENGINEERING 2019年第11期52卷 4339-4359页

作者： Yang, Sheng-Qi Yin, Peng-Fei Huang, Yan-Hua China Univ Min & Technol Sch Mech & Civil Engn State Key Lab Geomech & Deep Underground Engn Xuzhou 221116 Jiangsu Peoples R China

Bedding planes or layers can have a serious effect on the mechanical behaviour of shale rock. Fractures occurring along bedding planes form a fracture network in shale hydraulic fracturing;thus, the tensile strength and fracture mode are important for hydraulic fracturing design. In this research, shale disk specimens are prepared and analysed under Brazilian test conditions. during the test, a 3d digital image correlation system (dIC) is employed to capture surface deformation, and an AE sensor and strain observation system are, respectively, used to record the AE event and central strain of the shale disk during the failure process. A new inherently anisotropic model is established with a banded-particle model and smooth joint model using the particle flow code (PFC2d). The laboratory test result shows that the Brazilian tensile strength (BTS) value decreased gradually along with bedding inclination, which corresponds to a trend of the decrease of the strength over the entire interval, but it is a rather systematic decrease, approximating a linear variation. By considering the specimen after failure, three types of fracture patterns are observed: arc fracture (AF) through outside the central part, central fracture along the loading direction (LA) and mixed fracture patterns of the two. The dIC and central strain observation systems confirm that the split fractures do not always propagate along the diametrical loading direction, which means that the traditional isotropic elastic theory no longer works for layered shale. The micro-level failure behaviour and mechanism are analysed by PFC simulation. The PFC simulation reveals that the rock matrix tensile fracture and shear fracture along bedding plane are the main fracture pattern of shale disk specimens under the Brazilian test. The bedding inclination and interlayer bonding force play a very important role in the anisotropic behaviour of the shale.

关键词： Shale Brazilian test Inherent anisotropy Failure mode 3d digital image correlation system Particle flow code

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Experimental investigation of the strength and failure behavior of layered sandstone under uniaxial compression and Brazilian testing

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ACTA GEOPHYSICA 2018年第4期66卷 585-605页

作者： Yin, Peng-Fei Yang, Sheng-Qi China Univ Min & Technol State Key Lab Geomech & Deep Underground Engn Xuzhou 221116 Jiangsu Peoples R China

As a typical inherently anisotropic rock, layered sandstones can differ from each other in several aspects, including grain size, type of material, type of cementation, and degree of compaction. An experimental study is essential to obtain and convictive evidence to characterize the mechanical behavior of such rock. In this paper, the mechanical behavior of a layered sandstone from Xuzhou, China, is investigated under uniaxial compression and Brazilian test conditions. The loading tests are conducted on 7 sets of bedding inclinations, which are defined as the angle between the bedding plane and horizontal direction. The uniaxial compression strength (UCS) and elastic modulus values show an undulatory variation when the bedding inclination increases. The overall trend of the UCS and elastic modulus values with bedding inclination is decreasing. The BTS value decreases with respect to the bedding inclination and the overall trend of it is approximating a linear variation. The 3d digital high-speed camera images reveal that the failure and fracture of a specimen are related to the surface deformation. Layered sandstone tested under uniaxial compression does not show a typical failure mode, although shear slip along the bedding plane occurs at high bedding inclinations. Strain gauge readings during the Brazilian tests indicate that the normal stress on the bedding plane transforms from compression to tension as the bedding inclination increases. The stress parallel to the bedding plane in a rock material transforms from tension to compression and agrees well with the fracture patterns;"central fractures" occur at bedding inclinations of 0 degrees-75 degrees, "layer activation" occurs at high bedding inclinations of 75 degrees-90 degrees, and a combination of the two occurs at 75 degrees.

关键词： Layered sandstone 3d digital image correlation system Strength anisotropy Fracture pattern Uniaxial compression Brazilian test

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Analytical calculation of stress intensity factor of cracked steel I-beams with experimental analysis and 3d digital image correlation measurements

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ENGINEERING FRACTURE MECHANICS 2011年第18期78卷 3226-3242页

作者： Ghafoori, E. Motavalli, M. Empa Swiss Fed Labs Mat Sci & Technol Struct Engn Res Lab CH-8600 Dubendorf Switzerland

An analytical method to calculate the stress intensity factor for cracked steel I-beams under both bending moment and axial load is presented. The method is based on the approach of crack surface widening energy release rate. The crack surface widening energy release rate is formulated by a G*-integral and elementary strength theory of materials. Comparisons between the analytical results and results available in the literature for specific cases demonstrate the validity of the methodology. Furthermore, the fatigue and fracture behavior of the steel I-beam are experimentally investigated. The fatigue crack growth rate, residual deflection and stiffness reduction of a cracked beam under cyclic loading are studied. A three-dimensional digital image correlation system is used to illustrate the stress evolution pattern and the plasticity zone around the crack tip using image processing technique, thereby providing further verification of the theoretical models. (C) 2011 Elsevier Ltd. All rights reserved.

关键词： Stress intensity factor Fatigue crack growth 3d digital image correlation system Steel I-beam Fracture mechanics

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