Experimental investigation and numerical simulation on the crack initiation and propagation of rock with pre-existing cracks

Rock mass behavior is determined not only by the properties of the rock matrix but also mostly by the pre-existing cracks in the rock mass. Before the overall failure of rock, the crack initiation and propagation around the tip of pre-existing cracks (i.e., pre-crack) will occur and contribute to rock failure. In this paper, the deep granite from a gold mine is taken and made to specimens with the pre-crack of 0.3 mm thickness. Uniaxial compression tests are carried out on the pre-cracked specimens. The acoustic emission (AE) sensors and digital image correlation (DIC) system are employed to record the failure characteristics of the specimens. The extended finite element method (XFEM) with the non-local stress field calculation is used to simulate the crack initiation and propagation of pre-cracks. The crack patterns, opening and shearing displacements of the cracked surface, and the crack length development are obtained from numerical simulations. Finally, the effects of friction of crack surface on the crack pattern and crack propagation are investigated and discussed. It has been found that, for pre-cracked specimens, crack initiation and propagation will occur when the stress is much smaller than the rock compressive strength. And in the range of pre-crack angle 30-60°, the larger the pre-crack angle is, the larger the compressive strength is. The crack patterns from numerical simulations have a good agreement with those from experimented DIC results. Moreover, the order of crack propagation speed is consistent with the order of the compressive strength. The crack pattern and crack propagation are affected by the friction coefficient of the cracked surface

Uniaxial experimental study of the deformation behavior and energy evolution of conjugate jointed rock based on AE and DIC methods

Conjugate joint is one of the most common joint forms in natural rock mass, which is produced by different tectonic movements. To better understand the preexisting flaws, it is necessary to investigate joint development and its effect on the deformation and strength of the rock. In this study, uniaxial compression tests of granite specimens with different conjugate joints distribution were performed using the GAW-2000 compression-testing machine system. The PCI-2 acoustic emission (AE) testing system was used to monitor the acoustic signal characteristics of the jointed specimens during the entire loading process. At the same time, a 3D digital image correlation (DIC) technique was used to study the evolution of stress field before the peak strength at different loading times. Based on the experimental results, the deformation and strength characteristics, AE parameters, damage evolution processes, and energy accumulation and dissipation properties of the conjugate jointed specimens were analyzed. It is considered that these changes were closely related to the angle between the primary and secondary joints. The results show that the AE counts can be used to characterize the damage and failure of the specimen during uniaxial compression. The local stress field evolution process obtained by the DIC can be used to analyze the crack initiation and propagation in the specimen. As the included angle increases from 0° to 90°, the elastic modulus first decreases and then increases, and the accumulative AE counts of the peak first increase and then decrease, while the peak strength does not change distinctly. The cumulative AE counts of the specimen with an included angle of 45° rise in a ladder-like manner, and the granite retains a certain degree of brittle failure characteristics under the axial loading. The total energy, elastic energy, and dissipation energy of the jointed specimens under uniaxial compression failure were significantly reduced. These findings can be regarded as a reference for future studies on the failure mechanism of granite with conjugate joints

Determination method of fatigue strength and precursors of fatigue failure of porous weakly cemented siltstone

In order to study the fatigue behavior of porous weakly cemented siltstone, a quantitative calculation method of characteristic stress was established based on uniaxial step loading and unloading tests of siltstone. Then uniaxial cyclic loading and unloading tests of constant amplitude were carried out to study the volumetric strain, bulk compliance, and acoustic emission evolution of siltstone where the upper limit stress was in different stress ranges. The new method separates volumetric strain into the linear elastic strain of solid-phase dominated by rock skeleton, the non-linear elastic strain of gas-phase dominated by void, and the irreversible plastic strain. The peak points of gas-phase and plastic volumetric strain are taken as crack closure stress and crack initiation stress, besides the points of gas-phase volumetric strain and total volumetric strain changes from positive to negative are taken as crack damage stress and absolute dilatation stress, solving the problem that the calculation of characteristic stress is affected by subjective factors. The crack damage stress and absolute dilatancy stress determined by the new method can be regarded as the threshold stress of low-cycle and high-cycle fatigue failure. When the upper limit stress is lower than the crack damage stress, the continuous friction and slippage between the skeleton particles and the microstructure rearrangement under cyclic loading and unloading result in the specimen being gradually compressed. As the times of cyclic loading-unloading increase, the residual volumetric strain, bulk compliance, and cumulative acoustic emission count tend to be stable. When the upper limit stress is higher than the crack damage stress, due to the generation of a large number of cracks, the residual volumetric strain and bulk compliance continue to decrease while the cumulative acoustic emission count continues to increase, resulting in a fatigue failure of the specimen. The change of bulk compliance from positive to negative can be regarded as the early precursor of fatigue failure. The number of acoustic emission events NAE and k are approximately in a logarithmic-linear relationship. On this basis, it is proposed slope \begin{document}${b_k}$\end{document} to describe the fracture pattern. The sudden increase of \begin{document}${b_k}$\end{document} can be regarded as the early precursor and critical precursor of fatigue failure

Sulfate diffusion in coal pillar : experimental data and prediction model

The stability of coal pillar dams is crucial for the long-term service of underground reservoirs storing water or heat. Chemical damage of coal dams induced by ions-attacking in coal is one of the main reasons for the premature failure of coal dams. However, the diffusion process of harmful ions in coal is far from clear, limiting the reliability and durability of coal dam designs. This paper investigates sulfate diffusion in coal pillar through experimental and analytical methods. Coal specimens are prepared and exposed to sulfate solutions with different concentrations. The sulfate concentrations at different locations and time are measured. Based on experimental data and Fick's law, the time-dependent surface concentration of sulfate and diffusion coefficient are determined and formulated. Further, an analytical model for predicting sulfate diffusion in coal pillar is developed by considering dual time-dependent characteristics and Laplace transformations. Through comparisons with experimental data, the accuracy of the analytical model for predicting sulfate diffusion is verified. Further, sulfate diffusions in coal dams for different concentrations of sulfate in mine water are investigated. It has been found that the sulfate concentration of exposure surface and diffusion coefficient in coal are both time-dependent and increase with time. Conventional Fick's law is not able to predict the sulfate diffusion in coal pillar due to the dual time-dependent characteristics. The sulfate attacking makes the coal dam a typical heterogeneous gradient structure. For sulfate concentrations 0.01–0.20 mol/L in mine water, it takes almost 1.5 and 4 years for sulfate ions to diffuse 9.46 and 18.92 m, respectively. The experimental data and developed model provide a practical method for predicting sulfate diffusion in coal pillar, which helps the service life design of coal dams

Meso-scale mixed-mode fracture modelling of reinforced concrete structures subjected to non-uniform corrosion

Corrosion-induced concrete cracking is a significant problem affecting the durability of reinforced concrete structures. Considerable research has been carried out in addressing this problem but few have considered the cracking process of concrete as a mixed-mode fracture and the concrete as a multi-phase material. This paper develops a meso-scale mixed-mode fracture model for the cracking of concrete structures under non-uniform corrosion of reinforcement. Concrete is treated as a three-phase heterogeneous material, consisting of aggregates, mortar and interfaces. An example is worked out to demonstrate the application of the derived model and is then partially verified against previously published experimental results. In agreement with experimental results, the new model reproduces the observation that microcracks tend to form first at the interfaces before they connect to generate a discrete crack. Toughening mechanisms, e.g., microcrack shielding, crack deflection, aggregate bridging and crack overlap, have been captured in the model. Further, effects of aggregate randomness on the crack width development of concrete structures, differences between uniform and non-uniform corrosion and a comprehensive parametric study have been investigated and presented

Experimental investigation of failure mechanisms of granites with prefabricated cracks induced by cyclic-impact disturbances

Engineering rock mass is normally subject to cyclic−dynamic disturbances from excavation, blasting, drilling, and earthquakes. Natural fractures in rock masses can be reactivated and propagated under dynamic and static loadings, which affects the stability of rock mass engineering. However, fractured rock mass failure induced by cyclic-impact disturbances is far from clear, especially considering varying angles between the rock mass and the direction of impact loadings. This work investigated rock deformation and failure characteristics through cyclic impact tests on granite samples with cracks of different angles. A Hopkinson bar was employed for uniaxial cyclic impact tests on granite samples with the crack inclination angles of 0−90°. The magnetic resonance imaging technique was used to determine rocks’ porosity after cyclic impacts. The stress−strain curves, porosity, strength, deformation modulus, failure modes, and energy density of samples were obtained and discussed. Results showed that the crack inclination angles significantly affected the damage evolution and crack morphology of rocks. Under the constant cyclic impact, the dynamic deformation modulus and dynamic strength of rock samples first increased and then decreased with the increase in crack inclination angle. The failures of granite samples for inclination angles of 0 and 90° were dominated by tensile cracking, while those for the inclination angles of 30−60° were dominated by shear cracking. The energy density per unit time gradually decreased with the increase in impact cycles. The results can provide references for the stability analysis and cyclic-impact-induced failure prediction of fractured rock masses

Erratum: Clinical and biological significance of de novo CD5+ diffuse large B-cell lymphoma in Western countries

CD5 is a pan-T-cell surface marker and is rarely expressed in diffuse large B-cell lymphoma (DLBCL). Large-scale studies of de novo CD5+ DLBCL are lacking in Western countries. In this study by the DLBCL Rituximab-CHOP Consortium, CD5 was expressed in 5.5% of 879 DLBCL patients from Western countries. CD5+ DLBCL was associated with higher frequencies of >1 ECOG performance status, bone marrow involvement, central nervous system relapse, activated B-cell–like subtype, Bcl-2 overexpression, and STAT3 and NF-κB activation, whereas rarely expressed single-stranded DNA-binding protein 2 (SSBP2), CD30 or had MYC mutations. With standard R-CHOP chemotherapy, CD5+ DLBCL patients had significantly worse overall survival (median, 25.3 months vs. not reached, P< .0001) and progression-free survival (median, 21.3 vs. 85.8 months, P< .0001) than CD5− DLBCL patients, which was independent of Bcl-2, STAT3, NF-κB and the International Prognostic Index. Interestingly, SSBP2 expression abolished the prognostic significance of CD5 expression, suggesting a tumor-suppressor role of SSBP2 for CD5 signaling. Gene-expression profiling demonstrated that B-cell receptor signaling dysfunction and microenvironment alterations are the important mechanisms underlying the clinical impact of CD5 expression. This study shows the distinctive clinical and biological features of CD5+ DLBCL patients in Western countries and underscores important pathways with therapeutic implications

Prediction and prevention of rockburst in metal mines – A case study of Sanshandao gold mine

Rockburst is a kind of artificial earthquake induced by human activities, such as mining excavations. The mechanism of rockburst induced by mining disturbance is revealed in terms of energy in this context. For understanding the rockburst mechanism, two necessary conditions for the occurrence of rockburst are presented: (1) the rock mass has the capability to store huge amount of energy and possesses a strong bumping-prone characteristic when damaged; and (2) the geological conditions in the mining area have favorable geo-stress environments that can form high-stress concentration area and accumulate huge energy. These two conditions are also the basic criteria for prediction of rockburst. In view of energy analysis, it is observed that artificial and natural earthquakes have similar regularities in many aspects, such as the relationship between the energy value and burst magnitude. By using the relationship between energy and magnitude of natural earthquake, rockburst is predicted by disturbance energy analysis. A practical example is illustrated using the above-mentioned theorem and technique to predict rockburst in a gold mine in China. Finally, the prevention and control techniques of rockburst are also provided based on the knowledge of the rockburst mechanism