Search

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]

In this paper, the effects of materials and confining pressures on the mechanical properties of flawed coarse and fine sandstone is first analyzed. Then, the evolution processes of spatiotemporal acoustic emission (AE) events of coarse and fine sandstone specimens containing two flaws subjected to biaxial compression are investigated to character the damage characteristics and the fracture mechanism. Third, the ultimate failure modes of coarse sandstone specimens are compared with those of the fine sandstone specimens. Finally, the Electron Probe Microscope Analysis (EPEI) technique is conducted to observe the main macrofracture surfaces of coarse and fine sandstone specimens. Besides, the distribution characteristics of two AE indexes are also employed to qualitatively reveal the failure mechanism of tested specimens. The experimental results can provide new insight into the mechanical responses and the cracking behaviors of flawed rocks in complex stress loading conditions. Highlights: The damage characteristics and the fracture mechanism of coarse and fine sandstone are revealed.The distribution characteristics of two AE indexes are employed to qualitatively reveal the failure mechanism.Electron Probe Microscope Analysis technique is conducted to observe the main macrofracture surfaces of sandstone.The ultimate failure modes of coarse and fine sandstone specimens are investigated. [ABSTRACT FROM AUTHOR]

An evidently precursory acoustic emission (AE) time series, characterized by a time‐reversed Omori law, is registered prior to the unconfined catastrophic rupture of brittle sandstone. This provides insights into the physical process controlling catastrophic rupture and underlies the failure forecast. The onset of acceleration of damage is first identified based on the acousto‐optical characteristics. The precursory time series of rise time (RT) rate is used to forecast pseudo‐prospectively the catastrophic rupture in the framework of the time‐reversed Omori law. Investigations on multiple unconfirmed failure cases on the laboratory scale show that forecasts become more precise and less biased as the precursory AE time series proceeds toward rupture. In contrast to retrospective forecast, pseudo‐prospective forecast obtains a higher quality. Moreover, the use of amplitude as a precursor benefits the increase of warning time, and the flaw‐to‐flaw interaction introduces a strong disturbance on the forecast. Plain Language Summary: Catastrophic rupture in brittle rocks has important consequences for natural hazards such as landslides, earthquakes, or cliff collapses. Forecasting the catastrophe time of brittle rocks, albeit challenging even to this day, is much concerned by seismologists, physicists, and engineers. Encouragingly, an evidently precursory acoustic emission (AE) time series is commonly registered in the approach to the catastrophic rupture in the laboratory. Precursory AE time series is well characterized by a time‐reversed Omori law, which underlies the predictability of catastrophic rupture. In the forecast, RT rate, reflecting physically the damage progressions with a constantly intensifying shear mechanism in the approach to the catastrophic rupture, is used as a potential precursor. By using the time‐reversed Omori law, an accurate forecast of catastrophic rupture is achieved as the precursory time series of RT rate proceeds toward rupture. Key Points: The onset of acceleration of damage is identified using the acousto‐optical dataPseudo‐prospective forecast outperforms retrospective forecast in the forecast qualityThe flaw‐to‐flaw interaction introduces a strong disturbance on the forecast [ABSTRACT FROM AUTHOR]

In this paper, uniaxial compression tests are conducted on fissured red sandstone specimens to predict fracture damage (large‐scale events). The acoustic mission (AE) coupled with digital image correlation (DIC) technologies are used to monitor and record the real‐time cracking process of tested specimens. The AE characteristics are analysed during the cracking process. Moreover, three types of b‐value methods based on the AE parameters are adopted to predict the occurrence of large‐scale events (macro‐cracking). The results show that every macro‐cracking leads to a rapid decrease in three types of b value. When the fissured specimens reach to ultimate failure, all three types of b value reach to the minimum. The b value based on the AE parameters can be used as a predictor of large‐scale events during the cracking process of fissured rocks. [ABSTRACT FROM AUTHOR]

For a full understanding of the cracking processes in granite, uniaxial compression tests are conducted on prismatic specimens with pre-existing flaws, and digital imaging and AE techniques are applied to monitor real-time rupture behaviors. The digital image correlation (DIC) method is used to detect the process zone nucleation characteristics in granite. A novel approach for representing AE data in terms of the inter-event time (IET) function F (τ) is employed to analyze fracture-related AE event rate characteristics. Based on the coupled analyses of the acousto-optic-mechanical characteristics, the complete cracking processes of granite are classified into six levels, which are distinguishable by five characteristic stresses. Process zone nucleation, which represents the clustering of micro-cracks, is incorporated into the classification for the first time. Process zone nucleation is visualized as a white patch and is either linear or diffuse. By DIC analysis, the maximum principal strain field reliably predicts a linear white patch, and the maximum shear strain field implies a diffuse white patch. The evolutionary rules of the IET function F (τ) strongly support the new classification of cracking levels in granite. For the identification of stress thresholds and cracking levels, a combined acousto-optic-mechanical methodology is established, in which the role of the IET function F (τ) is highlighted. • Acousto-optic-mechanical characteristics are analyzed in a coupled way to study the cracking process of granite. • Process zone nucleation is incorporated into the classification of cracking levels for the first time. • The DIC method is used to evaluate the process zone nucleation characteristics. • The inter-event time function F (τ) is first applied to identify the cracking processes of granite. • The combined acousto-optic-mechanical methodology is established to identify stress thresholds. [ABSTRACT FROM AUTHOR]

Uniaxial compression tests combined with nondestructive testing techniques are performed to explore the roles of non‐isometric flaws in crack developments in brittle rocks. The acoustic emission (AE) rate‐process theory is adopted to analyze fracture‐related AE event rate characteristics. The full‐field optical method is applied to detect cracking modes. Experimental results show that AE activity is quite active when the matrix microcracking is dominant, while after each macrocracking event, AE activity becomes inactive because of the stress release. Multiphysical data for each tested flaw configuration faithfully confirm the rupture progressivity. The larger the flaw length ratio, the lower the peak stress (also peak axial strain and elastic modulus), as well as the more progressive the cracking process. Moreover, ultimate failure is triggered by the shear fracturing from the relatively long flaw. The short flaw is conditionally involved in ultimate failure when the stress buildup effect dominates. Finally, the fracture mechanism of brittle rocks with non‐isometric flaws is revealed. [ABSTRACT FROM AUTHOR]

Abstract The excavation of the Gaoligongshan tunnel encountered a complex geological condition, resulting in a disastrous rockburst. The rockburst during the excavation of the Gaoligongshan tunnel led to casualties and to the instability of supporting structures, which further damaged boring equipment. Therefore, the forecasting of rockburst events is of great importance. In this paper, areas with high or low risk of rockburst are identified using the Gaoligongshan tunnel stress-based rockburst criterion. Then, an acoustic emission (AE) method is adopted to forecast the magnitude and location of areas at high-risk of rockburst. Based on statistical analysis of AE characteristics and rockburst events, an AE-based rockburst criterion is proposed. As a result, in the Gaoligongshan tunnel, the stress-based and AE-based rockburst criteria are helpful in determining construction schemes and selecting support measures. Highlights • Areas with high or low risk of rockburst are identified using the Gaoligongshan tunnel stress-based rockburst criterion. • An acoustic emission (AE) method is adopted to forecast the magnitude and location of areas at high-risk of rockburst. • Based on statistical analysis of AE characteristics and rockburst events, an AE-based rockburst criterion is proposed. [ABSTRACT FROM AUTHOR]

• The investigations of the occurrence of tension and shear mechanisms in rock fracturing process. • The development of the integrated acousto-optic-mechanics multi-physics field characterization method for a crack. • Using the AF-RA-based Kernel density median criterion and the optical angular bisector criterion to distinct tension- vs shear-induced cracks. Rock macrofracturing often involves the tension-shear coexistence mechanism on the microscopic scale. This paper addresses the occurrence of tension and shear mechanisms in rock macrofracturing process based on the self-developed integrated acoustic-optic-mechanics (AOM) multi-physics field monitoring technique. With this advanced technique, three-point bending (3-p-b) tests with conventional loading and unconventional loading, as a control of the conditions that produce the different mechanisms, are first carried out on sandstone to establish AOM criteria for crack nature classifications. By using the AF-RA-based Kernel density median criterion and the optical angular bisector criterion, a coupled analysis of acoustical-optical data for the distinction of tension- vs shear-induced cracks in two different flaw configurations under uniaxial compression has been available. Results show that the crack initiation mode in tension-type rock bridge tests is dominated by the tension mechanism, while the ultimate failure pattern is shear-dominated. Comparably, failure of shear-type rock bridge is faithfully dominated by the shear mechanism. The paper, therefore, makes two contributions—a better understanding of rock fracturing process and providing a method of establishing the simple and intuitive criteria to justify the crack natures. [ABSTRACT FROM AUTHOR]

Forecasting the timing of an earthquake is both fascinating and challenging. In this paper, four efforts were made to prospectively forecast the laboratory earthquake timing of granite samples. First, a weighted linear least squares and the 1st-order polynomial model (WLLS-1OPM) with two indexes of span and acceleration was proposed. In WLLS-1OPM, the forecast model of span and acceleration indexes was established based on the stick-slip history of simulated faults through back-propagation neural network (BPNN), and the onset point of damage acceleration was prospectively determined by WLLS-1OPM. Second, on the basis of the prospective definition of the damage acceleration onset point, a prospective forecast model was proposed, and the precursory acoustic emission (AE) time series, power law exponent and catastrophic rupture time of the proposed prospective forecast model were all determined a priori and prospectively. Third, the sliding instability time was forecasted respectively based on the proposed prospective forecast model and the retrospective forecast model, and the results showed that the proposed prospective forecast model has higher accuracy. Finally, based on the proposed prospective forecast model, the most effective AE parameter for forecasting sliding instability time was determined to be AE event, followed by amplitude, ringing count and rise time (RT). This study is helpful for forecasting the instability time of shallow-focus tectonic earthquakes. [ABSTRACT FROM AUTHOR]

The precursory response rate with the relationship of the time-reversed Omori law can characterize the damage acceleration behavior in the approach to the local instability of rocks. In the present study, the precursory acoustic emission (AE) response rate obtained by the local spatial AE events is employed to forecast the time-of-instability of sandstone under biaxial and true triaxial compression. The phase of damage acceleration of rocks is first identified retrospectively based on the mechanical properties and the spatio-temporal AE evolution characteristics. The precursory AE event rate charactering the power-law acceleration behavior in the vicinity of instability point is qualitatively recalculated by the spatial AE events in the damage localization bands determined by the AE localization technique. The precursory time series of spatial AE event rate are adopted to pseudo-prospectively predict the time-of-instability according to the relationship of the time-reversed Omori law. The accurate forecast, being both more precise and less biased, is achieved as the fitted data subsets continuously update towards the instability point. The forecast accuracy based on the spatial AE response rate is higher than that based on the temporal AE response rate. Furthermore, the forecast accuracy obtained by using the pseudo-prospective way is higher than that obtained by using the retrospective way. [ABSTRACT FROM AUTHOR]

Earthquake prediction refers to predicting the magnitude, location, and time of earthquakes and is challenging. Four attempts to predict the magnitude, location, and time of the laboratory earthquake of granite were made. First, the stick-slip failure process of granite samples was studied by a homemade test setup, and the whole process was monitored in real-time by strip strain gauges, rosette strain gauges, and acoustic emission (AE) transducers. Second, six parameters related to the stress drop were selected: dry density, Young's modulus, loading rate, static friction coefficient of the interface, normal stress, and shear stress. The prediction model of stress drop was established based on a backpropagation neural network (BPNN) considering the above six parameters. The stress drop in this paper is positively correlated with the released energy (magnitude) when the simulated fault slips suddenly. Third, the simulated fault interface was scanned by a high-precision 3D topography scanner before the stick-slip tests, and then the 3D topography scanning image was quantitatively measured based on digital image processing (DIP). According to the measurement results, the relationship between the nucleation position and the microstructure of the simulated fault interface was discussed. Fourth, the characteristics of AE parameters (absolute energy, ringing count rate, amplitude, b 1 value) in the stick-slip process were studied. This paper aims to determine the precursory factors of sudden slip. This study helps explain the mechanism of shallow-focus earthquakes. • The prediction model of stress drop is established based on BPNN. • The microstructure characteristics of nucleation position are studied by DIP technology. • The characteristics of AE parameters in stick-slip process are studied. [ABSTRACT FROM AUTHOR]

This paper experimentally evaluates and quantitatively compares stress-induced damage progression and acoustic emission (AE) in granite and sandstone based on continuous acousto-optic-mechanical (AOM) observations. Experimental results reveal that the step-rise characteristics of the AE event rate preceding the entire macrofracturing process and the apparent white patching phenomena are the most significant acousto-optical evidences for process zone nucleation in granite but not in sandstone. During the unstable crack growth stage in granite, the robustly high AE event rate level has been mechanically correlated with the reactivation and intensification of fracture process zones (FPZs) by the stress build-up, and the relatively low AE event rate level has been revealed as a result of the progression of FPZs into macrocracks. It is the first time that the process zone nucleation in granite has been mechanistically related to the clustering of three types of grain-scale microcracks, i.e., grain boundary cracks, intragranular cracks and transgranular cracks, due to the grain interlocking effect. Comparatively, the clustering of microcracks is insignificant in the brittle failure of sandstone. Mechanistic correlations among the AOM characteristics in the two rock types are also investigated in detail. [ABSTRACT FROM AUTHOR]

• True triaxial compression tests are conducted on coarse and fine sandstone. • The coupled effects of the microstructure and the pre-existing flaws are revealed. • Mechanisms of micro-shear cracking and micro-tensile cracking are revealed. • The micro-shear cracking is seldom generated in fine sandstone. Failure characteristics of flawed rocks subjected to true triaxial stress are not studied, and the cracking mechanism is not fully understood. Especially, the damage and failure mechanisms incorporating the effect of material properties such as grain diameter are seldom revealed. In this study, true triaxial compression tests are conducted on flaws-contained specimens of coarse and fine sandstone, to explore the coupled effects of the microstructure and the pre-existing flaws on cracking characteristics and further to reveal the cracking mechanism. Acoustic emission (AE) data and the mechanical data are the main experimental output. It is found that cracking characteristics are collectively affected by the microstructure features and the geometrical features of specimen-sized pre-existing flaws. When the confining pressure is relatively large, the mechanical interaction between coarse grains inhibits the specimen's rupture and contributes to the increase of the strength of coarse sandstone. Due to the complex interaction between grain-scale microcracks, the micro-shear cracking mechanism and the micro-tensile cracking mechanism are presented in the physical process of damage progression in coarse sandstone. However, the microcrack clustering phenomenon in fine sandstone is insignificant, and the interaction between grain-scale microcracks is weak. Accordingly, the micro-shear cracking is seldom generated in fine sandstone. [ABSTRACT FROM AUTHOR]

• Acoustic emission (AE) and photographic technologies are employed. • The dominant frequency and amplitude characteristics of rocks are investigated. • Four modes of AE signals are found during the cracking process of flawed rocks. • The fracture mechanism of flawed rocks is discussed. To study the time-varying dominant frequency distribution characteristics during the cracking process of intact and flawed rocks, uniaxial compression tests are carried out on three types of red sandstone specimens. Both acoustic emission (AE) and photographic technologies are performed to monitor the real-time cracking process of tested specimens. The relationship between the AE rate curve, the axial stress-time curve and the cracking process is first analyzed, which can be employed to evaluate the microscopic and macroscopic deformation characteristics of rocks. The time-varying dominant frequency and amplitude distribution characteristics in different cracking stages of tested rocks are investigated. Four modes of AE signals, i.e., AE signals with low-frequency and high-amplitude (LF-HA), AE signals with low-frequency and low-amplitude (LF-LA), AE signals with intermediate-frequency and low-amplitude (IF-LA) and AE signals with high-frequency and low-amplitude (HF-LA), are found. The coupling application of four modes of AE signals extracted from unstable deformation stages can be viewed as early warning for unstable failure of rocks. The fracture mechanism of intact and flawed rocks is discussed based on the statistical characteristics of dominant frequency during the cracking process. Micro-shear cracking mainly dominants the ultimate failure of all of tested rocks. [ABSTRACT FROM AUTHOR]

• The real-time fracture process of fissured red sandstone specimen is monitored. • Two indices of RA and AF are performed to classify the different fracture modes. • The adequacy of the fracture mode classification is evaluated by Kernel Density Estimation (KDE). • KDE can identify and visualise the high concentration regions of RA and AF values. • RA values can serve as an early warning for ultimate failure of rocks. To study the classification of fracture modes of rocks during the cracking process, uniaxial compression tests were conducted on intact and flawed red sandstone specimens. Meanwhile, both acoustic emission (AE) and digital image correlation (DIC) technologies were adopted to monitor and record the real-time cracking process of the specimens tested. In this study, the interevent time function F (τ) (AE events rate) was utilized to distinguish the transition from microcracking to macrocracking for the tested specimens. An AE parameter analysis method based on two indices of RA (rise time/amplitude) and AF (AE counts/duration) values was performed to classify the different cracking modes during the loading process. The classification results of the cracking modes coincide with the cracking type of macrocracks captured by the photographic system. Moreover, the adequacy of the crack classification was also evaluated by the kernel density estimation (KDE) function, a nonparametric density estimation method. KDE is used as a parametric model to overcome the randomness found in the data set generated by AE testing and can well identify and visualize the high concentration regions of RA and AF values. A continuous increase of RA values (more than 400 ms/v) can serve as an early warning for the ultimate failure of red sandstone. The results of this present investigation can be applied in the health monitoring of rock engineering. [ABSTRACT FROM AUTHOR]

• Quasi-brittle failure of flawed granite is evaluated in real time by integrated acousto-optic-mechanics (AOM) multi-physics field characterization methods. • Procedural sample-sized localization instabilities are forecasted and evaluated using the time‐reversed Omori law. • The physics relating to the failure predictability of flawed granite is revealed. Forecasting quasibrittle failure in geomaterials is of paramount importance in the physics of fractures but rarely succeeds. To clarify the physics relating to the failure predictability in a quasibrittle regime, a combined AE (acoustic emission)-DIC (digital image correlation) technique is used in uniaxial compression experiments of flawed granite to trace its damage conditions in real time. The mechanistic correlations of the AE rate-process with the internal FPZ (fracture process zone) development are available. It is shown that the complete record of the AE activities does not meet a deterministic scaling law (e.g., typical time‐reversed Omori law), indicating a challenge in predicting ultimate failure via AE. Subsets of a power law acceleration registered much earlier than ultimate failure imply that the procedural sample-sized localization instability is predictable but still depends on the FPZ dynamic evolutions. More importantly, we find that the coexistence of FPZs and the intermittency of crack growth are the two main reasons for the difficulty of forecasting failure in flawed granite. This study sheds light on upcoming investigations in which the roles of FPZs, especially their spatiotemporal patterns, should be highlighted. [ABSTRACT FROM AUTHOR]