Search

To investigate the dynamic mechanical response and damage evolution behavior of ice-rich frozen clay, split Hopkinson pressure bar (SHPB) tests were performed on frozen clay specimens with initial moisture contents of 20%–1,000% under different temperatures, strain rates, and stress states. The stress–strain curves, dynamic strength, peak strain, absorbed energy density, failure mode, and failure progress were studied. The experimental results revealed the following: (1) in the radial-free state, the stress–strain curve of frozen clay with initial moisture contents ranging from 20% to 85% and 1,000% could be divided into three stages: elasticity, plasticity, and failure. In addition, a double peak phenomenon occurs in the stress–strain curves within the initial moisture content range of 120%–480%. (2) In the radial-free state, as the initial moisture content increased, the dynamic strength first increased to a maximum value, then decreased to a minimum value less than the dynamic strength of ice, and eventually increased marginally to the dynamic strength of ice. However, the variation in dynamic peak strain with initial moisture content followed a decrease–increase–decrease three-stage pattern. (3) In the passive confining pressure state, the initial moisture content of frozen soil determined its sensitivity to the confining pressure. (4) The high-speed camera test results indicated that the failure of the ice-rich frozen clay was mainly caused by tensile cracks. The degree of failure of the frozen clay specimens became more evident as the moisture content and strain rate increased. In the passive confining pressure state, the ice-rich frozen clay specimens remained intact except for a small amount of edge peeling. [ABSTRACT FROM AUTHOR]

In the context of repositories for nuclear waste, understanding the behavior of gas migration through clayey rocks with inherent anisotropy is crucial for assessing the safety of geological disposal facilities. The primary mechanism for gas breakthrough is the opening of micro-fractures due to high gas pressure. This occurs at gas pressures lower than the combined strength of the rock and its minimum principal stress under external loading conditions. To investigate the mechanism of microscale mode-I ruptures, it is essential to incorporate a multiscale approach that includes subcritical microcracks in the modeling framework. In this contribution, we derive the model from microstructures that contain periodically distributed microcracks within a porous material. The damage evolution law is coupled with the macroscopic poroelastic system by employing the asymptotic homogenization method and considering the inherent hydro-mechanical (HM) anisotropy at the microscale. The resulting permeability change induced by fracture opening is implicitly integrated into the gas flow equation. Verification examples are presented to validate the developed model step by step. An analysis of local macroscopic response is undertaken to underscore the influence of factors such as strain rate, initial damage, and applied stress, on the gas migration process. Numerical examples of direct tension tests are used to demonstrate the model's efficacy in describing localized failure characteristics. Finally, the simulation results for preferential gas flow reveal the robustness of the two-scale model in explicitly depicting gas-induced fracturing in anisotropic clayey rocks. The model successfully captures the common behaviors observed in laboratory experiments, such as a sudden drop in gas injection pressure, rapid build-up of downstream gas pressure, and steady-state gas flow following gas breakthrough.

Shale oil and gas economic exploitation depends closely on effective hydraulic fracturing stimulations. However, due to the reservoir heterogeneity and in-situ stress variation, the hydraulic fracturing through multiple perforations in shale is confronted with imbalanced fracture growth. To investigate, we reported a new laboratory study that simulates hydraulic fracturing through multiple perforations in shale blocks. The effect of horizontal in-situ stress difference on simultaneous and asynchronous fracture growth from spiral perforations was analyzed. The results show that the in-situ stress difference significantly influences hydraulic fracture initiation and growth, fluid pressure behavior, and fracture orientation. Under a high value of horizontal in-situ stress difference, parallel transverse fractures are initiated simultaneously and growing independently along the direction of σH. Higher injection pressure is required to promote hydraulic fracture propagation with the decrease of horizontal in-situ stress difference. The induced fractures grow asynchronously from two adjacent perforations and later merge into a large tortuous fracture. Besides, the azimuth angle of hydraulic fracture deflects from the direction of σH, and the inclination angle gradually decreases. When the value of σh is close to σH, an inclined longitudinal fracture and multiple mini-transverse fractures are created from spiral perforations, resulting in a prolonged fracturing process and a significant near-wellbore fracture complexity. Highlights: A novel test configuration was developed to investigate multiple hydraulic fractures initiation and propagation from spiral perforations in shale Hydraulic fracture geometry, orientation, perforation breakdown, and fluid pressure behavior showed a clear dependence on stress difference The simultaneous and asynchronous fractures growth from multiple perforations was characterized based on the acoustic emission source location [ABSTRACT FROM AUTHOR]

In underground rock engineering, flaws can significantly affect both the physical properties and mechanical properties of the rock. The dynamic failure mechanism of sandstone with double pre-existing flaws is systematically analyzed through the parallel bond model in particle flow code. First, the model of the intact sandstone sample is established to verify the rationality of the parameters. Simulations of dynamic compression are then performed on sandstone that contains double pre-existing flaws under different impact velocities and confining pressures. The research results show that sandstone samples with various flaw lengths, rock bridge lengths, rock bridge angles and flaw angles exhibit different failure modes, strength reduction and AE evolution characteristics. Furthermore, by analyzing microforce fields, the cracking type and the crack propagation mechanism of sandstone samples are revealed. The rock bridge angle has a significant impact on the propagation of microcracks and the dynamic failure of the rock mass; while the rock bridge length between the two flaws has little effect on this. [ABSTRACT FROM AUTHOR]

Rock failure generally leads to serious consequences, and it is significant to obtain the precursor information prior to failure using associated techniques. Thus, it is essential to acquire and probe the relevant precursor information. In this study, true triaxial tests are performed on red sandstone specimens under varying intermediate principal stress conditions. The thermal infrared image evolution and the temperature-induced change characteristics of rock failure are also analyzed using infrared thermal imaging technology. In addition, with the assistance of a high-speed photography technique, these characteristics during the true triaxial compression and unloading processes are systematically investigated to determine how the intermediate principal impacts on thermal image, temperature, and fracture propagation. Finally, the evolution mechanism of the specimens is summarized, and a non-contact thermal infrared rock failure precursor indicator is proposed, which can give significant advance notice of rock collapse before the abnormal temperature change. The results show that there exist thermal infrared temperature precursors, thermal image precursors, and rapid development of rock macroscopic cracks before rock failure. Abnormal thermal images are prior to the abnormal temperature changes. As the intermediate principal stress increases, thermal abnormalities will change accordingly. Both temperature changes and thermal image anomalous patches can be utilized as precursor information of rock collapse, and the mechanism and specific information of thermal infrared failure precursors can be preliminarily determined in time and space. Our results can function as a significant frame of reference for the analysis and prevention of rock failure due to sudden instability. [ABSTRACT FROM AUTHOR]

In the present study, a new local grid reconstruction algorithm for automatic fracture modelling with complex morphologies evolved in rock crack propagation is developed. The algorithm is inspired by the Coulomb force equilibrium of idealized electrons in a constrained area, which leads to the relative uniform distribution of these electrons. As a physically inspired computing algorithm, nodes in the re-meshing area are equivalent to idealized electrons with different charges. This meta-heuristic algorithm includes several efficient implementation aspects, such as the treatment of initial seed, re-meshing area confirmation, Coulomb force calculation and mesh improvement. The algorithm can efficiently transform a structured background mesh with complex boundaries into a high-quality conforming mesh with low aspect ratio and high stretch value triangular elements. Combined with a discrete crack method, the proposed algorithm has been implemented in cracking simulation. Distinguishing with the previous methods, the meshing quality, efficiency, computing speed and calculating accuracy are greatly improved. Next, based on verification and validation of the methodology, the proposed algorithm is applied to the cracking simulations of compression induced wing cracks and mixed-mode cracks that evolve in three-point bending tests. The ability of the method for the simulation of cracking process is fully demonstrated. Highlights: An automatic local re-meshing algorithm for crack simulation inspired from the Coulomb force equilibrium of idealised electrons in constrained area is developed. Meshing quality, efficiency, computing speed, convergence rate and calculating accuracy are greatly improved. The ability of the algorithm for the cracking process simulation is well demonstrated. [ABSTRACT FROM AUTHOR]

To investigate the dynamic mechanical properties and fracturing behavior of the red sandrock-polypropylene fiber reinforced concrete (R-PPFRC) composite, R-PPFRC composite specimens with different Polypropylene fiber (PPF) dosage were fabricated. Conducted quasi-static compression experiments and dynamic experiments by using Split Hopkinson pressure bar (SHPB) apparatus and testing machine. The fracture characteristics and process of composite specimens were analyzed with the assistance of ultra-high-velocity camera photography. It can be concluded that the dynamic increase factor and the dynamic compressive strength of composite specimens R-PPFRC increase with the increase of load level, indicating a remarkably strain rate strengthening effect. The optimal PPF content is determined finally. The dynamic impact toughness, ultimate toughness, and dissipation energy of R-PPFRC are significantly affected by the load level and the PPF content. The fracturing process of the composite specimen is divided into four phases, crack initiation, crack expansion, crack cluster and ultimate failure. After going through these phases, the ultimate failure pattern is mainly shear and split mixed failure. [ABSTRACT FROM AUTHOR]

Ultrasonic tomography, which is widely used in the study of the fracturing process of rocks, often exhibits low resolution due to insufficient ray coverage, particularly while evaluating the three-dimensional (3D) fractures. To resolve this issue, we adopted sparseness regularisation in tomography to reconstruct the ultrasonic velocity of rocks. Both numerical and laboratory experiments demonstrate that tomography with sparseness regularisation generates velocity images with clear fracture morphology than that with Tikhonov regularisation. Dynamic monitoring of the fracturing process of a granite slab with two-dimensional (2D) velocity images can reveal the accurate development of the fracturing process. The experiment on the internal structure of tight sandstone after hydraulic fracturing reveals demarcated low-velocity regions in the 3D ultrasonic velocity images of tomography with sparseness regularisation. These low-velocity regions correspond to the positions of the fractures when compared to the X-ray scanning images. Thus, tomography with sparseness regularisation can improve the resolution of ultrasonic velocity images, which can be used to accurately describe the fracture development and strain localisation during rock deformation. [ABSTRACT FROM AUTHOR]

Solid–liquid composites (SLCs) with novel thermal/electronic/mechanical properties imparted by programmable and functional liquid inclusions have attracted considerable research interest in recent years, and are widely used in smart electronics and soft robotics. The feasible application of SLCs requires that they exhibit excellent static physical properties as well as dynamic impact resistance to satisfy complex service conditions, such as drops and impacts. This paper examined the impact resistance of SLCs fabricated by using microfluidic 3D printing. The results of dynamic split-Hopkinson pressure bar (SHPB) tests showed that the performance of the fabricated SLCs improved in terms of energy dissipation and impact resistance compared with pristine materials. In case of dynamic impact in the strain rates ranging from 100 to 400 s - 1 , the SLC specimen deformed without fracture, and its energy dissipation was dominated by the viscosity of the liquid inclusions. For dynamic impact in the strain rates ranging from 500 to 800 s - 1 , the SLC specimen fractured and its energy dissipation was determined by the volume fraction of the liquid inclusions. Thus, the energy dissipation of the SLCs could be tuned by regulating the viscosity and volume fraction of the liquid inclusions to satisfy the requirements of protection against different strain rates. Furthermore, the process of fracture of the SLCs under the dynamic SHPB tests was recorded and analyzed by using a high-speed camera. The results showed that distributed liquid inclusions changed the paths of crack propagation to enhance energy dissipation in the SLCs. This study experimentally verified the enhancement in the energy dissipation of SLCs, and provided design strategies for developing multifunctional SLCs with high impact resistance. [ABSTRACT FROM AUTHOR]

The distribution characteristics and evolution law of rock mass fissures induced by mining are a key scientific issue in the study of deep rock mechanics. In this study, a series of uniaxial compression experiments was conducted on rock-like specimens containing double V-shaped prefabricated fissures at dip angles of α = β = 45°, α = 45°

Thermal shock (TS) is known as the process where fractures are generated when rocks go through sudden temperature changes. In the field of deep rock engineering, the rock mass can be subjected to the TS process in various circumstances. To study the influence of TS on the mechanical behaviors of rock, sandstone specimens are heated at different high temperatures and three cooling methods (stove cooling, air cooling, and freezer cooling) are adopted to provide different cooling rates. The coupled dynamic and static loading tests are performed on the heated sandstone through a modified split Hopkinson pressure bar (SHPB) system. The influence of heating level and cooling rate on the dynamic compressive strength, energy dissipations, and fracturing characteristics is investigated based on the experimental data. The development of the microcracks of the sandstone specimens after the experiment is analyzed utilizing a scanning electron microscope (SEM). The extent of the development of the microcracks serves to explain the variation pattern of the mechanical responses and energy dissipations of the specimens obtained from the loading test. The findings of this study are valuable for practices in rock engineering involving high temperature and fast cooling.

Energy principles, which can favorably explain the complete rock failure process, are one of the most reliable analysis approaches in rock mechanics and engineering. In this study, a strain energy approach under true triaxial compression (TTC) is proposed. On this basis, the energy evolution characteristics and variations of different failure behavior types (Class I, Class II and ductile failure) under TTC are analyzed. The variations of the strain energy characteristics of Beishan granite with σ2 and σ3 under TTC are studied. The results indicate that the total strain energy U and the elastic strain energy U e of Beishan granite increase with the increasing σ2 or σ3. The dissipated strain energy U d rapidly increases when the value of ε1/ε1peak is approximately 0.6–0.8. The influence of σ3 on the rock failure mode and energy evolution characteristics is greater than that of σ2. In highly brittle rocks, the tensile cracking of the rock microstructure is dominant, and the rock has a high strain energy storage capacity and a low strain energy dissipation capacity. The cumulative acoustic emission (AE) count rate curve shows the same trend as the total dissipated strain energy U d curve. The research results show that the proposed strain energy analysis method for TTC can explain the macroscopic failure behaviors, microscopic failure mechanism and AE characteristics of Beishan granite under TTC, thereby providing new ideas and methods for investigating the behaviors of deep underground hard rock. [ABSTRACT FROM AUTHOR]

The study of rock fracturing is a fundamental research topic in rock mechanics and engineering. In this paper, based on complete stress–strain curves obtained under TTC and the corresponding AE and strain energy results, the mechanical properties and fracturing process of Beishan granite are studied. A representative test result (σ 2 = 75 MPa, σ 3 = 20 MPa) is selected to analyse and illustrate the three-dimensional fracturing process of the Beishan granite. The results show that the variations in the characteristics of the brittle fracture behaviour, AE and strain energy of Beishan granite change with σ 2 or σ 3 following certain relationships, and the mechanisms driving these various relationships are very different. The AE and strain energy characteristics of Beishan granite are also quite different among the five fracturing evolution stages. The variations in the increments of AE count, AE event and total elastic strain energy per unit time (ΔS c / Δt , ΔS e / Δt and ΔU e / Δt) are studied at different times and under different stresses during rock failure. The rock fracturing evolution can be illustrated through the variation in these parameters, providing deep insight. This research provides a perspective for studying deep underground fracture-forming processes, such as earthquakes, tunnel sudden failure, spalling, splitting and rockburst. [ABSTRACT FROM AUTHOR]

Dynamic impact tests were carried out on red sandstone specimens with single bonded planar joint at various angles based on a split Hopkinson pressure bar (SHPB), to investigate the stress wave propagation and fracturing evolution of the specimens comprising two blocks. The results indicate that the bonded joint will reduce the dynamic strength from 3.10% to 32.13% compared with the intact specimen. The larger the joint angle is, the more significant the wave attenuation is. The variation trend of energy absorption rate is similar to the tendency of the dynamic strength, showing a positive correlation between the energy absorption rate and the dynamic strength. Tensile cracks dominate the specimen failure behavior during the dynamic fracturing process by observing the images captured by a high speed camera, leading to the slabbing failure with different degrees of slippage along the joint surface. In addition, it is found that the variation tendency of dynamic strength based on the experimental results is consistent with the theoretical analysis. [ABSTRACT FROM AUTHOR]

The purpose of this ISRM Suggested Method is to describe a methodology for in situ acoustic emission monitoring of the rock mass fracturing processes occurring as a result of excavations for tunnels, large caverns in the fields of civil, rock slopes and mining engineering, etc. In this Suggested Method, the equipment that is required for an acoustic emission monitoring system is described; the procedures are outlined and illustrated, together with the methods for data acquisition and processing for improving the monitoring results. There is an explanation of the methods for presenting and interpreting the results, and recommendations are supported by several examples. [ABSTRACT FROM AUTHOR]

Collapse columns are geological features formed by chimney caving of karst voids. Collapse columns can cut through the rock mass and introduce vast geological complexity in coalfields. When a collapse column becomes accessible to a confined aquifer, water inrush disasters can be triggered. In northern China, inrushes related to collapse columns occur in regions where extensive Ordovician limestone underlies the coal measures. This limestone is strongly karstified and holds large quantities of water. The piezometric surface can be considerably higher than the coal extraction level. Numerical simulation was used to explore how the water inrush pathway forms, based on a demonstration model. First, the evolution of the stress field, damage field, and flow volume within the coal floor was analysed. Then factors such as the column height, mechanical strength of the coal floor, and hydraulic pressure in the aquifer were considered. In the demonstration models, if the height of the collapse column exceeded 35 m and the initial thickness of the aquiclude was less than 15 m, an inrush definitely occurred, even when the strength of the floor strata was relatively high. Finally, a practical case of water inrush in the Fangezhuang coal mine related to a collapse column was numerically investigated. Modelling of the fracturing process provided insight into evolution of the fracture zone and inrush processes that cannot be observed in the field and are difficult to evaluate using static stress analysis. [ABSTRACT FROM AUTHOR]

The slurry type shield method is an important tunnelling method, especially for underwater tunnels in soft soil. If the slurry pressure is too high, it may lead to slurry fracturing of the stratum in front of the excavation face and cause water inrush accidents. Controlling the face stability of shallow shield tunnels is difficult owing to the inadequate understanding of the slurry induced fracturing mechanism. Most importantly, understanding the details of the slurry fracturing propagation process is not enough, as the process is especially complicated. However, the propagation process cannot be directly observed in the field or in small-scale laboratory tests because it occurs in bodies of soil. Thus far, an effective approach to investigate the slurry fracturing mechanism in the surrounding stratum of a slurry shield tunnel has not been reported. In this paper, a numerical simulation method for evaluating tunnelling-induced slurry fracturing is presented. The developed numerical model considered the couplings of stress distribution, fluid flows, and fracturing. The fracture initiation, branching, and propagation in the stratum could be reproduced numerically. A series of simulations for different ratios of cover depth to tunnel diameter were carried out to investigate the propagation process of slurry fracturing. The effects of slurry pressure at the tunnelling face and the cover depth above the tunnel were investigated. The results of the fracturing process induced by slurry, such as the fracture propagation velocity, slurry pressure distribution along the fracture, fracture initiation pressure, fracture extending pressure, and stratum deformations, were analyzed. [ABSTRACT FROM AUTHOR]

Damage distribution and evolution have a significant effect on borehole stress concentrations. To model the complex fracturing process and inelastic deformation in the development of the borehole breakout, we implement a continuum damage mechanics (CDM) concept that takes tensile and compressive failure mechanisms into account. The proposed approach explicitly models the dissipative behavior of the material due to cracking and its evolution, which leads to an inhomogeneous redistribution of material properties and stresses in the vicinity of the borehole wall. We apply a constitutive plastic model for Berea sandstone and compare our numerical results to laboratory experiments performed on Tablerock sandstone. We are able to reproduce several characteristics of the failure process during the breakout development as observed in experimental tests, e.g. localized crack distribution in the vicinity of the borehole wall, damage evolution, which exhibits a widening process in the beginning followed by subsequent growth in depth, and shear fracturing-dominated breakout growth in sandstone. A comparison of our results with laboratory experiments performed on a range of stress conditions shows a good agreement of the size of borehole breakouts. The importance of the constitutive damage law in defining the failure mechanisms of the damaging processes is discussed. We show that the depth and the width of breakouts are not independent of each other and no single linear relation can be found between the size of breakouts and the magnitude of the applied stress. Consequently, only one far field principal stress component can be estimated from breakout geometry, if the other two principal stresses are known and sufficient data on the plastic parameters are available. [ABSTRACT FROM AUTHOR]

To capture the hydraulic fractures in heterogeneous and layered rocks, a numerical code that can consider the coupled effects of fluid flow, damage, and stress field in rocks is presented. Based on the characteristics of a typical thin and inter-bedded sedimentary reservoir, China, a series of simulations on the hydraulic fracturing are performed. In the simulations, three points, i.e., (1) confining stresses, representing the effect of in situ stresses, (2) strength of the interfaces, and (3) material properties of the layers on either side of the interface, are crucial in fracturing across interfaces between two adjacent rock layers. Numerical results show that the hydrofracture propagation within a layered sequence of sedimentary rocks is controlled by changing in situ stresses, interface properties, and lithologies. The path of the hydraulic fracture is characterized by numerous deflections, branchings, and terminations. Four types of potential interaction, i.e., penetration, arrest, T-shaped branching, and offset, between a hydrofracture and an interface within the layered rocks are formed. Discontinuous composite fracture segments resulting from out-of-plane growth of fractures provide a less permeable path for fluids, gas, and oil than a continuous planar composite fracture, which are one of the sources of the high treating pressures and reduced fracture volume. [ABSTRACT FROM AUTHOR]

The purpose of this ISRM Suggested Method is to describe a methodology for in situ microseismic monitoring of the rock mass fracturing processes occurring as a result of excavations for rock slopes, tunnels, or large caverns in the fields of civil, hydraulic, or mining engineering. In this Suggested Method, the equipment that is required for a microseismic monitoring system is described; the procedures are outlined and illustrated, together with the methods for data acquisition and processing for improving the monitoring results. There is an explanation of the methods for presenting and interpreting the results, and recommendations are supported by several examples. [ABSTRACT FROM AUTHOR]

Masonry arch structures, and, more generally, vaulted structures, are traditionally assessed using a well-established approach, such as linear elasticity or limit analysis, whereby system behavior at the intermediate stage-which occurs when the material's tensile strength has been exceeded but the collapse mechanism has not yet formed-is disregarded. A more accurate interpretation requires a thorough analysis that can take into account the intermediate cracking stage and uses a constitutive law providing a closer approximation to the actual behavior of the material. In this paper, an evolutionary fracturing process analysis for the stability assessment of masonry arches is presented. This method makes it possible to capture the damaging process that takes place when the conditions evaluated by means of linear elastic analysis no longer apply and before the conditions assessed through limit analysis set in. Furthermore, the way the thrust line is affected by the opening of cracks and the redistribution of internal stresses can be checked numerically. Finally, by applying this evolutionary method, a numerical calculation of the arch of the Mosca Bridge over the Dora River in Turin, Italy, is described, and the results are compared with those obtained by Castigliano's iterative analysis. [ABSTRACT FROM AUTHOR]

Abstract: We have shown that the symmetry of fracturing process in a macro-scale can be quantified by using the concept of symmetropy (an entropy-like measure of symmetry). Here we extend this approach to examining the symmetries in a range from small (partial) scales to larger (whole) scales: this approach is called partial symmetropy (PS). To check its applicability, we consider one illustrative example, the temporal change of the spatial patterns of acoustic-emission events in a well-documented rock fracture experiment. Our results are summarized as follows: (i) the PS enables us to distinguish the nucleation phases from the other phases such as the pre-nucleation and the propagation phases; (ii) the variation of the PS shows that the fracturing process is associated with a type of phase transitions from the subcritical state to the critical state; (iii) the scale dependence of the PS reveals the presence of the sandwich structure that consists of order and non-order in the evolution of the fracturing pattern; (iv) within the framework of non-extensive Tsallis entropy we develop the PS concept, and show that the degree of non-extensivity on a large scale increases immediately after the nucleation. The results shown in the present paper are not obtained by taking the traditional fractal approach, nor using only a simply symmetropy. We therefore propose that the PS is a useful tool to providing a strategy for describing qualitatively the phase changes in the observed fracturing process. [Copyright &y& Elsevier]

Thermal shock (TS) is known as the process where fractures are generated when rocks go through sudden temperature changes. In the field of deep rock engineering, the rock mass can be subjected to the TS process in various circumstances. To study the influence of TS on the mechanical behaviors of rock, sandstone specimens are heated at different high temperatures and three cooling methods (stove cooling, air cooling, and freezer cooling) are adopted to provide different cooling rates. The coupled dynamic and static loading tests are performed on the heated sandstone through a modified split Hopkinson pressure bar (SHPB) system. The influence of heating level and cooling rate on the dynamic compressive strength, energy dissipations, and fracturing characteristics is investigated based on the experimental data. The development of the microcracks of the sandstone specimens after the experiment is analyzed utilizing a scanning electron microscope (SEM). The extent of the development of the microcracks serves to explain the variation pattern of the mechanical responses and energy dissipations of the specimens obtained from the loading test. The findings of this study are valuable for practices in rock engineering involving high temperature and fast cooling. [ABSTRACT FROM AUTHOR]

The behavior of a granite subject to a triaxial compression test ranging from the prefailure stage to the postfailure stage was studied using a fluorescent technique from the geological point of view. Microscopic observations of the specimens at different stages showed changes in the failure process. The start of formation of new microcracks paralleled the compression direction through their propagation until the onset of faulting and ended with the failure of shear zones after the strength failure point. Pores chiefly identified in the feldspar increased in length and width in the early stages, but not in number. It seems that the effect of pore spaces did not have any effect of failure. The microcracks generated on angular edges of quartz or feldspar grains and around biotite grains with increasing compression force. The phenomenon appearing on the crystal boundaries among biotite and quartz or feldspar agrees with the result calculated based on the theory on stress fields with ellipsoidal inhomogeneity suggested by Eshelby. [Copyright &y& Elsevier]

In case of earthquakes and crustal movement, the concentration of impounding load over a large region of crust can cause disturbances to the stratum. In order to quantitatively investigate crack initiation, propagation and coalescence processes of jointed stratum based on thermal variations caused by concentrated load, a series of indentation tests were performed on jointed granite specimens, failure process was respectively recorded by synchronized digital image correlation system and infrared camera. Then, strain and infrared radiation temperature of mixed shear-tensile and tensile conical crack were analyzed. In the initiation process, experimental results indicate that the abnormities in the temperature concentration factors are caused by the frictional-thermal effect for mixed mode crack and the thermoelastic effect for tensile mode crack. Subsequently, in the propagation process, these two cracking types followed newly proposed criteria, namely, the maximum temperature gradient criterion for mixed mode crack and the minimum temperature gradient criterion for tensile mode crack. In addition, the intensity of temperature concentrations in crack initiation stage and coalescence stage are more pronounced than that of crack propagation stage. The new findings from the infrared radiation temperature distributions provide some fundamental references for geophysical prospecting in jointed rock mass. [ABSTRACT FROM AUTHOR]

To investigate the influence of weakly filled joint angles on the dynamic mechanical properties and fracture evolution mechanism of sandstone, dynamic uniaxial compression tests were performed on sandstone specimens with joint angles of 0°–90° under strain rates of 35–125 s−1 and an interval of 15° using a modified split Hopkinson pressure bar (SHPB) apparatus. The dynamic stress–strain relationship, dynamic strength, dynamic peak strain, energy characteristic, failure mode, typical failure progress, crack coalescence category, fracture micro-mechanism, and theoretical analysis were examined in this research. The test results indicated that both the dynamic compressive strength and peak strain of jointed sandstone first decreased and then increased with the increase in the joint angle. To maximise the energy reflection coefficient and minimise the absorbed energy density, a critical joint angle α (approximately 45°), was employed. The specimens with joint angles of 0°, 15°, and 30° exhibited considerable plastic deformation capacity, whereas the brittle failure feature of specimens with joint angles of 60°, 75°, and 90° was more typical than that of intact specimens. The stress bimodality phenomena of the specimen with a joint angle of 45° were more distinct than those of the specimen with a joint angle of 60°. Four types of crack coalescence behaviours were identified during the dynamic fracturing processes of the jointed specimens using a high-speed camera; subsequently, these were compared with the crack coalescence categories of jointed specimens under static loading. Moreover, the micro-fracture morphology well-revealed the macroscopic fracture behaviours of jointed sandstone specimens. It was further confirmed that the variable tendency of the dynamic compressive strength of jointed specimens based on experimental results agreed with the theoretical analysis. [ABSTRACT FROM AUTHOR]

• Existing brace fracture models fail to predict the fracture process observed from laboratory data. • Fracturing process concept logically explains brace fracture phenomenon. • Proposed fracture initiation model predicts brace fracture in a consistently reliable manner. Reliably predicting brace fracture is crucial in seismic evaluation of post-fracture performance of concentrically braced frames. This paper evaluates existing fracture models, proposes a new fracture initiation model based on a large amount of experimental data, and compares all models with the test results. The existing ductility-related empirical models and strain-related fiber models are evaluated by experimental tests. A comprehensive database of previously tested square hollow structural sections from different research programs is established to conduct a regression analysis leading to development of a fracture initiation model. The study concludes that the existing fracture models are unable to predict fracture events observed during physical tests. Furthermore, the existing strain-related fracture models, accessible to all users in OpenSees , fail to simulate the cyclic response of braces beyond initial inelastic deformation. The fracture initiation model proposed by this study depends on the reliable measure of ductility, and it has consistently predicted fracture initiations observed during the tests. The fracturing process concept, along with the proposed fracture initiation model, is able to simulate the cyclic response of the braces, whether they are tested as single brace members or in actual braced frames, throughout inelastic deformation, fracture initiation, fracture propagation, and terminal fracture stages, providing a much-improved tool for numerical simulation of the post-fracture response of concentrically braced frames. [ABSTRACT FROM AUTHOR]