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Evaluation of Compression Strength of Concrete Specimens via Experimental Results and Numerical Simulation

STRENGTH OF MATERIALS 2020年第1期52卷 138-148页

作者： Shirinabadi, R. Moosavi, E. Islamic Azad Univ Dept Petr & Min Engn South Tehran Branch Tehran Iran

The concrete compression strength is an effective characteristics among other properties of practical significance. Although coring testing on such members as columns is not recommended. But sometimes, for determining the concrete strength in the column, this method should be applied to the reinforced concrete one. Coring in a reinforced concrete column creates a cylindrical cavity in it, which has an apparently negative effect on the bearing capacity of the structural member. The effect of different sizes of cavities on uniaxial compression strength of concrete was investigated based on the experiment and numerical simulation (particle flow code). The results of the experiments show that the cavities have a great influence on the uniaxial compression strength. For example, if the cavity volume is calculated about 14% of the sample volume, it can reduce of the uniaxial compression strength down to 58%. If the cavity diameter is 60% of the sample width, the strength can go down to 74%.

关键词： concrete coring uniaxial compression strength particle flow code

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Discrete element modeling on crack evolution behavior of sandstone containing two oval flaws under uniaxial compression

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ARABIAN JOURNAL OF GEOSCIENCES 2020年第11期13卷 1-19页

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

To understand the fracture coalescence in specimens containing oval flaws, PFC2D was used to investigate the sandstone containing two oval flaws with different ligament angles under uniaxial compression. The micro-parameters were first confirmed by intact sandstone, and then, these were adopted to simulate the flawed specimen under uniaxial compression. Numerical results show that the peak strength, the peak strain, elastic modulus, and the ultimate failure modes were approximated well with these obtained by experiment. The failure modes obtained by numerical simulation can be divided into the following four typical categories based on the crack coalescence between the pre-existing flaws: no crack coalescence;indirect crack coalescence outside the bridge area;single crack coalescence inside the bridge area;tensile crack coalescence outside the bridge area. Based on parallel bond force fields and stress variation of measurement circles along the crack trajectory, the crack evolution processes were analyzed in detail and crack types were identified. The mode III can be divided into mode IIIa and mode IIIb. The mode IIIa is the single shear crack coalesced inside the bridge area;the mode IIIb is the single crack that initiated with tensile crack and coalesced with shear crack, and it only occurred as beta = 90 degrees. The mode IV can be divided into mode IVa and mode IVb. The mode IVa is that wing crack initiated at the middle of oval and coalesced at another oval tip, and it occurred as beta = 120 degrees. The mode IVb is that coalesced with anti-wing crack outside the bridge area, and it occurred as beta = 150 degrees. The stress distribution along the ligament has an obvious effect on ligament angle and flaw angle, and crack initiation only affects the stress concentration factor value.

关键词： Sandstone particle flow code Two oval flaws Measurement circle Crack evolution

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Process Simulation and Mesoscopic Analysis of Rockfill Dam Compaction Using Discrete Element Method

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INTERNATIONAL JOURNAL OF GEOMECHANICS 2020年第6期20卷 1-13页

作者： Liu, Donghai Sun, Longfei Ma, Hongyan Cui, Wei Tianjin Univ State Key Lab Hydraul Engn Simulat & Safety Tianjin 300350 Peoples R China Tianjin Univ State Key Lab Hydraul Engn Simulat & Safety Engn Tianjin 300350 Peoples R China

To investigate the mesoscopic mechanism of rockfill dam material compaction, the discrete element method is used to simulate the entire compaction process. An irregular aggregate model is first established using laser scanning technology, and an accurate method for determining the load of a roller is proposed. The entire construction process of the rockfill dam-including the dumping, paving, and compaction phases-is then simulated using a two-dimensional (2D) particle flow code model, and a quantitative relationship between the porosity of the 2D model and the actual porosity is proposed for analyzing the influence of different compaction parameters and the motion law of the aggregate. The results show that the compaction effectiveness of the dam material improves with decreasing roller velocity and lift thickness, as well as with an increase in the excitation force. It is shown that the vertical acceleration of the particles decreases exponentially along the depth. Furthermore, the vertical velocity of the particles decreases with increasing compaction passes at the same depth, while the rate of increase in the vertical displacement of the particles decreases. During the compaction settlement process, the contribution of small particles is more apparent than that of larger particles. The mesoscopic mechanism of rockfill dam material compaction is analyzed, and a basis for reasonably selecting the compaction parameters and gradation is derived. Moreover, this method is less time-consuming and labor-intensive and costs less than conventional compaction experiments.

关键词： Rockfill dam Construction quality Compaction mechanism Discrete element method particle flow code

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Meso-fracture mechanism of Longmaxi shale with different crack-depth ratios: Experimental and numerical investigations

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ENGINEERING FRACTURE MECHANICS 2021年 257卷 108025-108025页

作者： Lei, Bo Li, Hongtao Zuo, Jianping Liu, Haiyan Yu, Meilu Wu, Genshui China Univ Min & Technol Beijing Sch Mech & Civil Engn Beijing 100083 Peoples R China China Univ Min & Technol Beijing State Key Lab Coal Resources & Safe Min Beijing 100083 Peoples R China

The mechanical properties and fracture mechanism of shale are vital to the design of hydraulic fracturing schemes. To better understand the formation and evolution of hierarchical crack networks in shales, a scanning electron microscope (SEM) with a loading device was applied to carry out three-point bending experiment on Longmaxi shale with different crack-depth ratios (CDR). The micro-damage and fracture processes of shale were observed in situ. Based on the double-K fracture criterion, the subcritical crack length, and the initial and unstable fracture toughness were calculated theoretically. The results demonstrated that the crack initial load, peak load, and subcritical crack length decreased significantly with the increase of the CDR. Both the initial and unstable fracture toughness were relatively stable when the CDR was between 0.3 and 0.5, and they can be regarded as constants. The meso-failure mechanism of shale was tensile and shear failure, and fracture patterns were inter-granular, trans-granular and coupled fracture. Generally, tensile failure typically results in irregular rough sections, whereas shear failure results in smooth sections with parallel slip lines. The micropores inside and between the crystals can induce the connection and penetration of cracks. The initiation of microcracks is the result of the continuous accumulation and nucleation of multiple micro/nanoscale damage, and the tortuous crack path is caused by the variation in the local stress field at the crack tip. In addition, clump and smooth joint model (SJM) were introduced to simulate the effect of the sedimentary particle and bedding plane on crack propagation based on particle flow code in two dimensions (PFC2D). The simulation results were in good agreement with the experimental observations.

关键词： Keyword Longmaxi shale SEM particle flow code CDR Fracture mechanism

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Calibration of the Microparameters of Rock Specimens by Using Various Machine Learning Algorithms

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INTERNATIONAL JOURNAL OF GEOMECHANICS 2021年第5期21卷

作者： Xu, Chen Liu, Xiaoli Wang, Enzhi Wang, Sijing Tsinghua Univ State Key Lab Hydrosci & Engn Beijing 100084 Peoples R China

High accuracy in the simulation of the discrete-element method (DEM) depends on the proper selection of microparameters. In this study, the range of microparameters was determined through sensitivity analysis. Subsequently, four levels of orthogonal experimental tables were established and 148 sets of data were collected. In addition, five data mining methods, namely, support vector regression (SVR), nearest-neighbor regression (NNR), Bayesian ridge regression (BRR), random forest regression (RFR), and gradient tree boosting regression (GTBR), were used to establish a microparameter prediction model. The results indicate that machine learning methods have significant potential in determining the relationship between macro and microparameters of the DEM model. RFR achieved the best performance among the five models whether the input data were collected from the tests of the Brazilian tensile strength and uniaxial compression or only the uniaxial compression test. In addition, the deviation between the predicted and measured macroparameters was less than 8%. This approach allowed for more accurate modeling of complex structures in a rock under various stress conditions through DEM simulations.

关键词： Discrete-element method particle flow code Sensitivity analysis Data mining Microparameter calibration

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Experimental and numerical investigation on shale fracture behavior with different bedding properties

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ENGINEERING FRACTURE MECHANICS 2021年 247卷 107639-107639页

作者： Lei, Bo Zuo, Jianping Liu, Haiyan Wang, Jintao Xu, Fei Li, Hongtao China Univ Min & Technol Beijing Sch Mech & Civil Engn Beijing 100083 Peoples R China China Univ Min & Technol Beijing State Key Lab Coal Resources & Safe Min Beijing 100083 Peoples R China

Research on crack evolution mechanism and fracture behavior of bedding shale is of great significance for shale gas exploitation. The mechanical properties of shale, especially the strong anisotropy, have great influence on the nucleation and propagation of hydraulic fractures. To investigate the mechanical properties of bedding shale, the micro fracture process of Longmaxi shale under three-point bending load was observed using scanning electron microscope (SEM) equipped with a loading device, and the smooth joint model based on particle flow code (PFC2D) was established to simulate the effect of different bedding properties on shale fracture behavior. The results demonstrated that the mechanical properties of the weak bedding planes have significant effect on the fracture pattern, tensile strength, fracture toughness, crack initiation angle, crack propagation path, and complex fracture network generation. In addition, the evolution characteristics of microcracks were studied by fractal method. The results showed that fractal method can well quantitatively characterize the evolution process of microcracks in bedding shale.

关键词： Longmaxi shale SEM particle flow code Bedding properties Fracture behavior

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A microscopic study of sand arches and sand skeletons under hydrodynamic force based on the CFD-DEM model

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JOURNAL OF NATURAL GAS SCIENCE AND ENGINEERING 2021年 92卷 104017-104017页

作者： Song, Yuqi Ranjith, P. G. Wu, Bailin Song, Zhenlong Monash Univ Dept Civil Engn Deep Earth Energy Lab Bldg 60 Clayton Vic 3800 Australia CSIRO Energy Oil Gas & Fuels Res Program Clayton Vic 3168 Australia Southern Univ Sci & Technol Dept Earth & Space Sci Shenzhen 518055 Guangdong Peoples R China

Sand production is one of the oldest challenges in the oil and gas industry, causing billions of dollars of losses every year. The main aim of the research reported here is to use our validated 3-D numerical model (Song et al., 2020) based on computational fluid dynamics (CFD) coupled with the discrete element method (DEM) to investigate sand arching under hydrodynamic force in oil and gas production wells. The main findings are as follows: The sand arches observed in our 3-D model have complex 3-D backbones. Sand arches with fewer sand grains are more stable and more common than those with more sand grains. Sand arches contain particles that are coarser than the average size of sand in reservoirs. Most associated angles in arches are less than 180 degrees. For a concave arch with an associated angle greater than 180 degrees, a bead hanging from the equator can be compensated by neighboring particles. A concave sand arch exists only when static friction is introduced. The two arch abutments of sand arches bear the maximum contact force. The critical drawdown pressure gradient of the reservoir increases when the frictional coefficient between sand and screen/liner material increases from 0.0 to 0.75. However, simply increasing the frictional coefficient does not enhance the stability of sand arches if the frictional coefficient is greater than 0.75. The sand skeleton generates greater inner contact force when a higher fluid pressure gradient is introduced. If the drag force exceeds the maximum strength of the sand arch, catastrophic sand production occurs without a new sand arch forming. The collapse and reconstruction of sand arches cause the fall and rebound respectively of the mean coordination number. The greater the mass of the sand production, the greater the mean coordination number's maximum retracement. The proposed method is sufficiently robust and efficient for application to the simulation of fluid-particle interactions for a wide variety of problems in g

关键词： Computational fluid dynamics Critical drawdown pressure Discrete element method particle flow code Sand arch Sand production

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Influence of graded coarse aggregate content and specific surface area on the fracture properties of asphalt mixtures based on discrete element simulations and indoor tests

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CONSTRUCTION AND BUILDING MATERIALS 2021年 299卷 123942-123942页

作者： Liang, Hehao Shi, Liwan Wang, Duanyi Xiao, Xin Deng, Kailing Foshan Univ Sch Transportat Civil Engn & Architecture Foshan Land Sea Smart Civil Engn Mat Technol R&D Foshan 528225 Guangdong Peoples R China South China Univ Technol Sch Civil Engn & Transportat Guangzhou 510640 Guangdong Peoples R China

There are some deficiencies in the current understanding of the influence of gradation size on the crack resistance of asphalt mixtures. This study first uses the particle flow code (PFC) to perform virtual semi-circular bending (SCB) tests to explore the effect of gradation changes on the fracture properties of asphalt mixtures and determine the related influence rules. The conclusions are then verified through indoor tests. The results show that for samples with the same nominal maximum particle size and differ-ent gradations, the crack resistance of the asphalt mixtures cannot be improved by simply increasing or decreasing the coarse aggregate content. Changes in both the coarse aggregate content and specific sur-face area have a parabolic correlation with the fracture properties of the asphalt mixtures. Based on these relationships, the reference value ranges of the coarse aggregate content and specific surface area of AC-20 asphalt mixtures for optimal crack resistance can be calculated as 52.02%-54.02% and 5.72-6.06 m(2)/kg, respectively. Calculation methods and suggestions are provided based on the presented research con-clusions for improving the crack resistance of asphalt mixtures by optimising the gradation in pavement engineering and these conclusions have a certain promotion significance. (C) 2021 Elsevier Ltd. All rights reserved.

关键词： Asphalt mixtures Discrete element method Crack resistance Semi-circular bending test particle flow code

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A new approach for quantifying the two-dimensional joint roughness coefficient (JRC) of rock joints

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ENVIRONMENTAL EARTH SCIENCES 2021年第15期80卷 484-484页

作者： Yuan Wei Liu Sifan Tan Hanhua Niu Jiandong Jiang Zhiling Li Xilai Peng Shu Xue Yanyu Wang Wei Sun Xiaoyun Shijiazhuang Tiedao Univ Sch Civil Engn Shijiazhuang 050043 Hebei Peoples R China Hebei Technol & Innovat Ctr Safe & Efficient Min Shijiazhuang 050043 Hebei Peoples R China Guizhou Prov Qual & Safety Traff Engn Monitoring Guiyang 550016 Peoples R China Cent South Univ Sch Civil Engn Changsha 410083 Peoples R China Hunan Zhongda Design Inst Co Ltd Changsha 410083 Peoples R China China Railway 18th Bur Grp Co Ltd Tianjin 300222 Peoples R China China Railway 24th Bur Grp Co Ltd Shanghai 200433 Peoples R China

The joint roughness coefficient (JRC) is the key factor in predicting the peak shear strength of a rock joint. In this paper, we present a new approach for quantitatively calculating the JRC of two-dimensional rock joints. First, the two dimensionless indexes of cumulative relative relief amplitude (CRRA) and weighted average gradient (WAG) are defined to represent the morphology characteristics of a joint profile. Second, the ten standard roughness profiles suggested by Barton and Choubey are digitized, and the values of CRRA and WAG for each profile are calculated. Based on the JRC, CRRA and WAG results of the standard joint profiles, an equation for calculating the JRC, which takes the CRRA and WAG as inputs, is proposed by utilizing the multiple nonlinear fitting method. Finally, the validity of the proposed method is investigated by performing laboratory and numerical direct shear tests of 12 joint profiles with different JRCs. The results show that the established equation can accurately quantify the JRC of a two-dimensional joint. However, the accuracy of the JRC greatly depends on the sampling interval along the joint profile. The recommended length of the sampling interval is no greater than (1/120) L.

关键词： Joint roughness coefficient Peak shear strength Direct shear test Rock joint particle flow code

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A statistical index indicating the degree and mechanical effects of grain size heterogeneity in rocks

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ENGINEERING GEOLOGY 2021年 293卷

作者： Han, Zhenhua Zhang, Luqing Azzam, Rafig Zhou, Jian Wang, Song Chinese Acad Sci Inst Geol & Geophys Key Lab Shale Gas & Geoengn Beijing Peoples R China Chinese Acad Sci Innovat Acad Earth Sci Beijing Peoples R China Rhein Westfal TH Aachen Aachen Germany Beijing Univ Technol Minist Educ Key Lab Urban Secur & Disaster Engn Beijing Peoples R China

Mineral grain size heterogeneity is one of the fundamental reasons leading to differences in rock mechanical behaviours. Several indices have been proposed to quantify grain size heterogeneity, however, the applicability of the existing indices is limited due to various preconditions. An improved grain size heterogeneity index H-e was proposed in this study. By comparing with the previous index, the improved index was proved to be more applicable for analysing rock mechanical properties. Through a series of grain-based model (GBM), the grain size heterogeneity effect, material heterogeneity effect and mineral content effect were further discussed. For a simple grain size heterogeneity effect, the crack initiation stress, damage stress, peak stress and crack initiation stress ratio of rock decreased linearly as the improved index H-e increased. After accounting for the material heterogeneity effect, the above characteristic stresses and elastic modulus decreased. A significant material heterogeneity effect and mineral content effect will impact the linear correlation between the rock macromechanical parameters and the H-e index. When the axial stress was low, the grain size heterogeneity had little effect on the number of microcracks, but this heterogeneity became the main factor affecting the evolution of microcracks as the axial stress increased. The number of intragrain tensile cracks increases and the number of intergrain shear cracks decreases with increasing H-e, when the axial stress reached the rock peak strength. Besides, the combined effects of material heterogeneity and mineral content may control the type and number of microcracks, further weakening the effect of grain size heterogeneity.

关键词： Grain size heterogeneity Heterogeneity index Grain-based modelling particle flow code Alxa granite

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