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Understanding the dynamic failure mechanism of rocks subjected to chemical corrosion is very important. However, the previous works mainly focused on static damage rather than dynamic pulverization with chemical corrosion. In this study, Split Hopkinson Pressure Bar (SHPB) impact tests are performed to study the dynamic mechanical properties and energy evolution of granite after chemical corrosion. The XRD and SEM analyses are employed to reveal the chemical reaction processes and corrosion mechanisms. Moreover, the fractal dimension is applied to determine the failure pattern of granite. The experimental results show that the acidic environment causes muscovite, albite, and microcline to generate SiO2 (quartz), and the newly generated SiO2 has a weak pressure-bearing capacity. As corrosion progressed, micro-particles accumulate on the surface and cause stress concentrations, which implies a decrease in dynamic properties. Chemical corrosion causes internal microcracks to propagate, and more energy is absorbed when they coalesce under the dynamic impact. The chemical solution enters the inside of specimens through the surface cracks for further reaction, which causes the failure pattern to change from splitting failure to crushing failure. The findings can fill in the gap in the chemical corrosion mechanism of granite subjected to dynamic impact. Highlights: Split Hopkinson Pressure Bar tests are performed to study the dynamic mechanical properties of granite after chemical corrosion. The XRD and SEM analyses are employed to reveal the chemical reaction processes and corrosion mechanisms. The acidic environment causes muscovite, albite, and microcline to generate SiO2, and the generated SiO2 has a weak load-bearing capacity. Chemical corrosion causes microcracks to propagate, and more energy is absorbed when they coalesce under the dynamic impact. [ABSTRACT FROM AUTHOR]

Layered composite rocks (LCRs) are often encountered in underground excavation in which extensive deformation, buckling and local collapse near the layer interface frequently occur. However, there is limited advance knowledge of the failure mechanisms of LCRs to interpret the field observations due to the relative scarcity of polyaxial compression testing. To that end, this study conducts the true-triaxial experiments with the acoustic emission (AE) monitoring on the LCR specimens, with the emphases being on the effects of mechanical contrast (MC) between the layers on the fracture behaviors under true-triaxial stresses. First, the strength of LCRs is controlled by the weaker layer. Second, at the unstable fracturing phase, high AE event rate is faithfully recorded but intermittently interrupted by macrofracturing in granite but not in sandstone. Third, main failure planes of LCRs appear sub-orthogonally to σ3 (minimum principal stress), taken the form of delamination fracture and exhibited the mode of σ3-transverse-symmetric fracturing. As the distance from the unloaded surface increases, the fracturing transitions from σ3-transverse-symmetric spalling to σ3-transverse-symmetric shearing. Depending on the MC, both the fracture propagation through the interface and the fracture containment within sandstone are registered. Moreover, the interface effect is numerically revealed through a 3D finite-element modeling. This study gets insights into the failure mechanisms of LCRs with a changing mechanical property in individual layers under true-triaxial stresses. Highlights: True-triaxial experiment investigations of layered composite rocks in a brittle-brittle sequence. Quantitative interpretations of acousto-mechanical properties and fracturing behaviors in layered composite rocks. The fracture propagation through the interface is registered, as well as the fracture containment within the sandstone. The delamination fracture along σ3-unloading direction dominates the feature of σ3-transverse-symmetric fracturing. The interface effect is revealed by a 3D finite-element modelling. [ABSTRACT FROM AUTHOR]

True triaxial experiment with a synchronous acoustic emission (AE) monitoring system is a popular measure for the assessment of strength and cracking behaviors of brittle flawed rocks under anisotropic stress circumstances. This measure was employed to investigate the laboratory-scale brittle failure of flawed sandstone containing two different en echelon flaw configurations. Quantitative interpretations and comparisons of acousto-mechanical properties and cracking behaviors in intact and flawed sandstone have become available. Observations first revealed that en echelon flaws considerably weakened rock strength and deformability, and strongly affected the damage and failure processes. Second, intact specimen showed a quasi-2D spallation failure, while the failure of flawed specimens was dominated by the induced cracks initiated from en echelon flaws. Third, the two fundamental failure types of rock bridge including tensile-dominated failure and shear-dominated failure were registered. Whatever the failure type, micro-shear failure mechanism co-existed with micro-tensile failure mechanism. Moreover, AE amplitude distributions at the stable and unstable cracking phases follow a power law. The current findings provide insights into the brittle failure mechanisms of flawed rocks and have significant implications at scales relevant to seismicity and engineering applications. Highlights: True triaxial experiment investigations of brittle failure in flawed sandstone Quantitative interpretations and comparisons of acousto-mechanical properties and cracking behaviors in intact and flawed sandstone Quasi-2D spallation failure occurs in intact specimen, while the shear cracks initiated from the flaw tips dominate the failure in flawed specimens Co-existences of micro-shear failure mechanism with micro-tensile failure mechanism at the triggering processes of rock fracture AE amplitude distributions at the stable and unstable cracking stages follow a power law [ABSTRACT FROM AUTHOR]

Understanding the 3D spatial cracking behavior plays significant role in evaluating the mechanical properties of rocks. This work studied the spatial propagations of 3D cracks and its effects on the mechanical properties during the failure process of sandstone under uniaxial compression using X-ray CT imaging and 3D reconstruction techniques. Results show that five cracking stages can be identified including the initial flaw compaction stage, linear elastic deformation stage, microcrack propagation and coalescence to macrocrack formation stage, stable propagation stage of macrocracks and the sample failure stage. As cracking direction increases from 0° to 90°, peak stress decreases, microcrack initiation stress increases for sample S1 and decreases for samples S2–S3, which is opposite to macrocrack initiation stress of cracks. However, these variations of microcrack initiation stress, macrocrack initiation stress and peak stress are opposite as cracking direction increases from 180° to 270°. The proposed relation between the stress and normalized damage volume is effective to evaluate the stress during the failure process of sandstone. Highlights: Digital framework is proposed to characterize real-time 3D spatial crack characteristics. 3D cracking behavior is studied by digital damage ratio and normalized damage volume. A stress-damage volume relation is established to evaluate the stress during the failure process of rocks. Effects of spatial cracking direction on the failure process and mechanical properties are analyzed. [ABSTRACT FROM AUTHOR]

Multiphase heterogeneity significantly affects mechanical and micro-cracking properties of geomaterials. Heterogeneity representations of geomaterials, however, can bring out fatal errors for cracking behavior investigations due to inaccurate characterizations of multiphase matrixes. This work aims to deeply characterize microscopic heterogeneity in rocks represented by digital multiphase matrixes, and to study their effects on cracking properties of geomaterials. X-ray μ-Computed Tomography (μCT) imaging under the same physical field of view is employed to obtain multiphase microstructures of sandstone with different resolutions. Digital colour difference segmentation (DCDS) and the coupling multiple point statistics-marching cube (MPS-MC) algorithms are applied to accurately characterize three-dimensional (3D) multiphase heterogeneity of rocks. Digital simulations coupled with uniaxial compression are conducted to study multiphase heterogeneity effects on micromechanics and cracking behaviors. Results show that with increasing resolution and voxel scale, heterogeneity index increases. Peak stress and crack ratio increase with increasing heterogeneity index, and the relative errors between experimental and numerical stresses are all less than 5% when the voxel is larger than 3003. Micro-pores extremely improve damage of multiphase microstructures, and the transgranular cracks initiate first, and then intergranular cracks initiate due to large heterogeneity of microscopic elements between micro-pores and microscopic interfacial transition zones (ITZs) and similar strength heterogeneity of microscopic elements in micro-grains. The proposed digital-analysis framework is effective to evaluate multiphase heterogeneity effects on mechanical and cracking responses in various engineering projects. [ABSTRACT FROM AUTHOR]

Water existing in micro-pores has tremendous influences on the mechanical and damage characteristics of rocks. This study aims to investigate the micro-pore water effects on the mechanical and damage characteristics of sandstone subjected to different loading configurations, in which four groups of sandstone specimens with different water contents were applied in the conventional uniaxial compression, cyclic loading and unloading experiments. X-ray CT imaging and 3D reconstruction techniques are applied to capture the cracking behaviors under uniaxial compression and cyclic loading–unloading. The digital microstructural variables are defined to describe the characteristics of microstructures and evolution of micro-pore water in rocks. Numerical simulation is applied to study the micro-pore effects on the mechanical properties of rocks. Results show that the specimens are mainly failed by the mixture modes of primary shear cracks and secondary tensile cracks. With increasing mass water content, the uniaxial compressive strength (UCS) decreases and the pore size first decreases, then increases, and finally decreases due to the infiltration of water phase in microstructures. The water content has a negative effect on the mechanical properties, in which the digital porosity and volumetric water content and the micro-pore water saturation have a negative effect on the UCS. In addition, the damage becomes easier near the pores with larger sizes. [ABSTRACT FROM AUTHOR]

In this paper, the creep behavior of sandstone under the different confining pressures is investigated. The nuclear magnetic resonance (NMR) measurements are carried out to detect the micro-pore characteristics of sandstones before and after compression tests and creep tests. The variations of the parameters, including transverse relaxation time (T2) spectra distribution, percentage of the pore distribution and the changes of porosity are analyzed quantificationally. The experimental results show that creep behavior can be regarded as a process of gradual accumulation of damage to rocks. The confining pressure can limit the increase in porosity during the creep stage, and the effect of confining pressure on the incremental value of porosity is more obvious under high-stress conditions than under low-stress conditions. Moreover, the relationship between the creep stain and the incremental value of porosity is established to reveal the relationship between micro-crack expansion of rocks and macro-creep characteristics of sandstone. It is found that, as the loading ratio or confining pressure increases, the variation of the creep strain is similar to the variation of the incremental value of porosity. The larger the confining pressure, the smaller the effects of creep strain on the incremental value of porosity. However, the greater the loading ratio is, the faster the incremental value of porosity increases. The present study can provide a theoretical basis for the deep mining of mineral resources and the structural stability analysis of deep-buried underground engineering. [ABSTRACT FROM AUTHOR]

Brittleness has significant effects on the fracturing behaviors of preflawed rock-like materials. In this study, uniaxial compressive experiments are conducted on rock-like specimens containing three flaws, and a three-dimensional digital image correlation technique is applied to capture the real-time displacement and strain fields on the rock-like specimens' surface. The present experimental study focuses on the effects of the brittleness of rock-like materials on crack initiation, propagation, and coalescence behaviors in three flaw-contained specimens. The experimental results show that the crack initiation mode transforms from the tensile mode to the shear mode with a decrease in the brittleness index of rock-like materials. Meanwhile, crack coalescence modes transform from the tensile crack coalescence mode to the mixed tension-shear coalescence mode, then to the shear coalescence mode as the brittleness index of rock-like materials decreases. Moreover, the ratio of the crack initiation stress to the peak stress increases, while the ratio of the crack coalescence stress to the peak stress decreases as the brittleness index decreases. This paper also addresses the influences of the rock bridge angle on the mechanical and cracking behaviors of three flaw-contained specimens. For specimens with the lower brittleness index, the ratio of the crack initiation stress to the peak stress decreases, while the ratio of the crack coalescence stress to the peak stress increases as the rock bridge angle increases. However, for specimens with the higher brittleness index, the ratio of the crack initiation stress to the peak stress and the ratio of the crack coalescence stress to the peak stress decrease with increasing the rock bridge angle. The experimental results provide a better understanding of the sensitivity of mechanical and cracking behaviors of three flaw-contained specimens to the brittleness of rock-like materials under uniaxial compression. [ABSTRACT FROM AUTHOR]

In this paper, a 3D numerical reconstruction method is proposed based on improved simulated annealing algorithms and limited morphological information from experiments. Firstly, X-ray micro-CT experimental tests are applied to obtain the CT images and the mechanical parameters of Xingluokeng sandstone. Then, the voxel block exchanging algorithm is introduced to accelerate the preliminary reconstruction stage of the porous sandstone model. Two-point probability control function, lineal-path control function and fractal control function are employed to characterize complex pore morphologies. The system updating algorithm is improved to avoid wasting time on the unnecessary and unsuccessful position exchange. Finally, to verify the proposed method, the topological and mechanical properties of the reconstructed porous sandstone model are analyzed by a distance-ordered homotopic thinning algorithm as well as the numerical compression. Moreover, the numerical results of the reconstructed model are compared with the experimental data. It is found that the proposed method in this paper has a good ability to predict the properties of the rocks for engineering purposes. [ABSTRACT FROM AUTHOR]

The crack initiation, growth, wrapping and coalescence of two 3D pre-existing cross-embedded flaws in PMMA specimens under uniaxial compression are investigated. The stress-strain curves of PMMA specimens with 3D cross-embedded flaws are obtained. The tested PMMA specimens exhibit dominant elastic deformation and eventual brittle failure. The experimental results show that four modes of crack initiation and five modes of crack coalescence are observed. The initiations of oblique secondary crack and anti-wing crack in 3D cracking behaviors are first reported as well as the coalescence of anti-wing cracks. Moreover, two types of crack wrapping are found. Substantial wrapping of petal cracks, which includes open and closed modes of wrapping, appears to be the major difference between 2D and 3D cracking behaviors of pre-existing flaws, which are also first reported. Petal crack wraps symmetrically from either the propagated wing cracks or the coalesced wing cracks. Besides, only limited growth of petal cracks is observed, and ultimate failure of specimens is induced by the further growth of the propagated wing crack. The fracture mechanism of the tested PMMA specimens is finally revealed. In addition, the initiation stress and the peak stress versus the geometry of two 3D pre-existing cross-embedded flaws are also investigated in detail. [ABSTRACT FROM AUTHOR]