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Copyright of China Rural Water & Hydropower is the property of China Rural Water & Hydropower Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Eastern coalfield goes into deep mining gradually in China. Fracture development rises greatly in surrounding rocks, increasing the difficulty in ground control under increased mining disturbance. In order to reveal failure mechanism of fractured rocks and improve ground control of deep coal mine, the influence of fracture angle on mechanical behavior, surface strain field and fracture propagation is analyzed by using uniaxial compression test combined with DIC technique. The results show that stress-strain curve of rock specimen with pre-existing fracture presents double-peak shape, induced by shear inter-locking effect between pre-existing and newly-developed fractures. With the growth in fracture angle, elastic modulus and damage degree rise in rock specimen while uniaxial compressive strength (UCS) experiences decreasing and subsequent increasing stages, respectively. Quantitative relationship between the UCS and fracture angle is deduced. Preponderant dip angle is defined for failure of rock with pre-existing fractures and the value is 45° when the fracture belongs to open-type. Deformation localization rock with pre-existing fractures occurs at the fracture tip. The initiation stress reaches 80% of initial yield stress when localization area is dominated by tensile stress, and the percentage decreases to 60% when dominated by shear stress. Propagation path of deformation localization area is consistent with that of surface fracture. Rock deformation stage transits from strain localization to fracture development when strain magnitude reaches 5.0%. Rock specimen fails in tension and shear mixed mode when fracture angle is 60° or 75°. Otherwise, only tension failure is observed in the loading process. It takes longer to form a tensile fracture and horizontal displacement curves of feature points on two sides deviate from each other. It takes shorter to form a shear fracture and vertical displacement curves show deviation trend. Grain based model (GBM) is developed for fractured specimen, which indicates small crack development is dominated by tensile type. Shear crack experiences both rising and subsequent declining stages with the growth in fracture angle. With the DIC technique, failure position, failure time and fracture propagation path can be predicted, which provides valuable precursor for instability prevention of surrounding rock at depth.

In order to study the deformation localization failure characteristics of sandstone under the coupling action of seepage and stress, the triaxial compression tests of sandstone under different drainage conditions were carried out using the visualized triaxial servo control test system combined with three-dimensional digital image correlation (3D-DIC). The rock mechanics, seepage and deformation localization characteristics were analyzed, and the micro-morphology of fracture surface after sandstone failure was analyzed using electron microscope scanning. The results shown that the peak strength and elastic modulus of sandstone under drainage condition were higher than those under undrained condition, the peak strength, elastic modulus and Poisson’s ratio of sandstone increased with the increasing seepage water pressure, the time point of penetration crack and maximum permeability would be advanced. When the seepage water pressure was the same, the localized zone of deformation field cloud map of the sand surface was wider in the undrained condition than that in the drained condition, i.e., the macroscopic crack of rock was more obvious. Water flow under drained condition took away the mineral particles inside the rock and formed holes, the rupture surface of which was smoother than that under undrained condition, whereas flakes cuttings were obviously attached to the surface of the particles under undrained condition. The initiation point of radial deformation localization was consistently higher than that of axial deformation localization for all drainage conditions, with an average increase of 1.23%. The initiation stress levels of the radial and axial deformation localization increased with increasing seepage water pressure, i.e., the initiation time point was earlier. The radial and axial initiation stress level of sandstone under drainage condition were higher than those under undrained condition, with an average increase of 1.85% and 2.21% respectively. When the water pressure was the same, the initiation stress and stress level were more significantly affected by water pressure in the undrained condition than in the drained condition.

Copyright of Coal Science & Technology (0253-2336) is the property of Coal Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Coal Science & Technology (0253-2336) is the property of Coal Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The safety assessment of PBX explosive is inseparable from the characterization of its mechanical response and damage. In order to accurately capture the mechanical behavior of PBX explosive, the numerical characterization method of PBX explosive mechanical behavior is studied based on Karagozian& Case (K&C) model framework. The damage evolution mode in the K&C model is modified, and the pressure-dependent characteristics of PBX explosive modulus and damage evolution mode are considered. Based on a series of published experimental data, the strain rate effect in the K&C model was corrected, the ratio of tension to compression meridian of PBX under different pressures was discussed, the deformation localization in the process of structural damage was dealt with, and the treatment of the tensile failure unit in the calculation was presented. The proposed model can be used to well depict the cratering morphology of explosive specimen in the Steven test, and successfully capture the tensile and shear failure images of explosive specimen. [ABSTRACT FROM AUTHOR]

To further reveal the strain localization processes of coal specimens in uniaxial compression, the authors record the images of a surface of each specimen by use of a camera. The digital image correlation method is used to obtain strain fields. The authors arrange monitored lines orthogonal each other according to positions of clear strain localization bands. The relation between the variation coefficient of the maximum shear strain and the longitudinal stress is established. The study found that ① the subset size and spacing of monitored points do not have an influence on the developing tendency of the relation above;② the distribution of the maximum shear strain in the normal direction of the strain localization band is not strictly symmetric, which is related to heterogeneities of coal, and the maximum shear strain in the tangential direction is not uniform .reflecting the strain localization band propagation;③ with an increase of the longitudinal strain, the thicknesses of strain localization bands become narrow and the profile of the maximum shear strain in the normal direction becomes steep; ④ the rapid increase of the variation coefficient is due to the fact that unclear strain localization bands are converted into clear bands, which corresponds to the ratio of the longitudinal stress to the uniaxial compressive strength of 0.56. [ABSTRACT FROM AUTHOR]

An elastic-plastic model for transversely isotropic geostructures is developed based on the Cosserat continuum. In the model, the elastic constitutive relationship is described by 5 deformation parameters and the Drucker-Prager yield criterion is extended by introducing fabric tensor and loading direction in the plastic stage. The iterative format for return mapping algorithms and the tangent modulus matrix are formulated for the proposed model. The numerical simulation is implemented based on the user subroutine (UEL) of ABAQUS, and a plane strain 8-noded reduced integrated element is used. The influence of the material principal direction and the anisotropic degree on the strain localization and the bearing capacity of the structure are analyzed. Numerical results show the validity and performance of the proposed model in simulating the strain localization behavior of transversely isotropic geostructures. Furthermore, the mesh dependency accompanied with strain localization is effectively solved. [ABSTRACT FROM AUTHOR]

Two modified softening models are developed based on the Volokh softening model to consider the deformation and failure of granular material which is governed by shear deformation and failure. Different treatments to the volume strain energy and the shear strain energy are used in the two modified models， and a concept of the softening factor to indicate the degree of softening is proposed. Based on user-defined material (UMAT) of ABAQUS， the simulation is implemented. Numerical examples illustrated the capability and performance of the presented models in modeling strain localization and strain softening of granular materials. [ABSTRACT FROM AUTHOR]

The tensile failure procedures of remoulded mudstone samples are experimentally studied using a designed deformation field inspection system. The samples are pulled to failure with the speed of 0.06 mm/min in the process of Brazilian split load. The mudstone is modified by Portland cement in order to enhance its tensile strength and the failure procedure of modified sample under tensile stress is researched. The fracture mechanism of mudstone before and after modification under tensile stress is investigated. The results show: (1) the tensile strength increases linearly with the Portland concrete content of modified samples after curing for 24 h at room temperature; (2) the deformation field localizes on the surface of unmodified samples at the early stage of loading, when the load is 58.2% of peak load, localization band occurs by means of merging deformation regions, and it forms macro crack that fails the sample; (3) the quantity and size of pores between aggregates decrease after modification with Portland cement, and the tensile strength of mudstone increases obviously because of the cementation of silicate; (4) with the increase of Portland concrete content, the position in which localization band occurs by means of merging deformation regions moves towards the load peak; (5) because of more elastic strain energy stored in modified samples, the macro crack extends rapidly when the load exceeds the peak load. [ABSTRACT FROM AUTHOR]

In order to study the relationship between surface potential distribution and destruction localization of the coal sample, the distribution law of strain field and surface potential of coal samples were tested under load. The relationship between surface potential field and strain localization & fracture of coal samples was studied and analyzed. Results show that the coal samples show obvious strain localization characteristics in deformation and failure process, the strain localization concentrated area is the severe damage and fracture part of coal samples, and the strain localization zones is usually the fracture location of coal samples. The distribution character of surface potential field of coal samples during deformation and fracture is similar to the strain field, the potential field will be centralized in the damage localization area, the connection between the points of high potential and low potential reflects the rupture section of coal samples. [ABSTRACT FROM AUTHOR]

Based on the finite element procedure of proposed pressure-dependent elastoplastic Cosserat continuum model, progressive failure phenomena occurring in typical geotechnical engineering problems such as the retaining wall and the excavation slope, characterized by strain localization due to the material dilatancy (non-associated plasticity) or strain softening, are numerically simulated. The non-associated perfect elastoplastic behavior is particularly considered for the retaining wall and the strain softening behavior for the excavation material. Numerical results validate the inability of classical continuum model in simulating the whole failure progress, while the capability and performance of Cosserat continuum model in keeping the well-posedness of the boundary value problems characterized by the strain localization with strain softening behavior or non-associated perfect elastoplastic behavior incorporated and in simulating the entire progressive failure process occurring in geotechnical engineering structure. [ABSTRACT FROM AUTHOR]