To explore the variation in rock acoustic emission (AE) characteristics with strain rate, uniaxial compression tests at different loading rates and impact loading tests were conducted on granite ...using a MTS322 rock mechanical test system and split Hopkinson pressure bar (SHPB) system, respectively. The effect of the strain rate on the AE characteristic parameters, rock fracture properties, and destruction evolution were systematically analyzed. The results demonstrated that with increasing strain rate, the cumulative AE count decreases as a power function, and the variation between the cumulative AE count and strain rate can be fitted log-linearly with a slope of 0.48. The peak frequencies of the AE signals are mostly distributed in the zones of 0–100 kHz, 175–250 kHz, and 400–550 kHz. The signal proportion in the 0–100 kHz zone gradually increases with strain rate, while the signal proportions in the 175–250 kHz and 400–550 kHz zone exhibit decreasing trends. A transition of a sudden increase in the RA-value and decrease in the AF-value occurs when the stress reaches a certain level, and the stress level corresponding to this transition will increase with strain rate. Meanwhile, the RA–AF distribution is mostly concentrated on the abscissa in the low strain rate tests, but gradually concentrates on the longitudinal axis as the strain rate increases. This indicates that tensile cracking becomes the dominant fracture mode with increasing strain rate. The b-value decreases with increasing strain rate in the uniaxial compression tests; however, the b-value in the impact loading tests is higher than that in the uniaxial compression tests. Furthermore, to distinguish the signals generated by stress wave propagation from the signals generated by rock fracturing in the impact loading tests, a four-parameter k-means algorithm is used to conduct a clustering analysis. The results indicate that the signals can be classified into four clusters: tensile fracturing signals (cluster A), mixed stress wave and shear fracture signals (cluster B), mixed stress wave and tensile fracture signals (cluster C), and stress wave signals (cluster D).
A series of rock tests including Brazilian indirect tension test (BITT), three-point bending test (TPBT), modified shear test (MST) and uniaxial compression test (UCT) were conducted to investigate ...the acoustic emission (AE) characteristics and crack classification during rock fracture. The test results show that the rock fracture process presents an obvious segmented variation feature and has a dramatic increasing period, according to the change trends of AE hits and AE energy characteristic parameters. The AE characteristics are closely related to the types of micro-cracks produced in the rock fracture process. The elastic strain energy released by shear crack is greater than that released by tensile crack. Most of AE signals generated in compression and shear failures that mainly produce shear cracks have low average frequency (AF) values and low peak frequencies (below 100 kHz). On the contrary, most of AE signals generated in bending and tensile failures that mainly produce tensile cracks have low RA (ratio of rise time to amplitude) and high peak frequencies (above 100 kHz). In addition, the dividing lines were defined to distinguish the tensile cracks and shear cracks in the AF-RA scatter plots for different rocks. For instance, the AE signals above dividing line accounted for more than 62%, which indicated that the tensile cracks were dominant in TPBT. However, the AE signals below dividing line accounted for more than 74%, and the shear cracks were dominant in UCT. Therefore, the AE characteristics can be used to determine the fracture modes of rock, then to shed light on the micro-crack properties.
Evaluating the ability of coal seams to form fracture networks by hydraulic fracturing is important for the development of coalbed methane (CBM) reservoirs. In this paper, a new index for evaluating ...coal brittleness was established from the perspective of energy evolution during coal failure. Uniaxial and triaxial compression tests of coal monitored by an acoustic emission (AE) system were carried out and the applicability of the new index and the influence of the confining pressure and cleat orientation on the coal brittleness were analyzed. The pre-peak and post-peak dissipated energies were the essential factors in determining the coal brittleness. The new index can characterize the influence of the external stress and cleat orientation on coal brittleness, and can also comprehensively reflect the mechanical properties of the coal during the pre-peak and post-peak stages. The corresponding AE energy curves can be divided into
Rapid Fracture Type, Stable Fracture Type
and
Plastic Fracture Type
. For the
Rapid Fracture Type
, the accumulation rate of AE energy showed sudden changes when reaching the yield stress and peak strength, which represented high brittleness. The
Plastic Fracture Type
represented low brittleness, and the accumulated AE energy curves were smooth—first concave and then convex. The brittleness index of coal studied in this paper can provide a new method for selecting the optimal CBM reservoir and optimizing the fracturing scheme.
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.
Temperature can affect the physical and mechanical properties of rocks. With the aim of better understanding the effect of thermally-induced cracks on rock failure process, we heated granite disks ...(up to 800 °C) to induce different degrees of thermal damage, which were then used for Brazilian tests. The acoustic emission (AE) technique was applied to collect AE signals throughout each test. Specifically, the effect of thermally-induced cracks on AE characteristics (AE spatio-temporal characteristics, damage parameters DAE, amplitude-frequency characteristics, and b-value) was studied. Our results show that the thermally-induced cracks increase with the treated temperature, leading to more AE damage occurring at the early stage of the loading process. The existence of a large amount of thermally-induced cracks causes the scattered AE distribution on the disks treated at 600 °C and 800 °C. The average dominant frequency of AE signals decreases with the treated temperature. The decreasing of high-frequency signals and the increasing of low-frequency signals can be explained by the different mechanisms of thermally-induced cracks and stress-induced cracks. The b-value increases with treated temperature and decreases with the increasing applied stress.
Display omitted
•The water state changes the tunnel fracturing, the acoustic emission parameters and the infrared radiation temperature.•The AE quiet period lasted longer for the dry models, and the quiet period of ...AE event rate and the sharply increasement of AE energy rate can be regarded as the precursor to rockburst on the time-scale.•Before the rockburst occurs in the tunnel, the sudden changes of the temperature of the tunnel walls can be used as the precursor to rockburst on the space-scale.
The brittle failure behaviors of hard rock exposed by tunnelling in a moist environment are of great significance. A serials of laboratory experiments of sandstone tunnel models were conducted to investigate the influence of water contents on rockburst. The rock models contained three different water contents. The rockburst process was monitored by the acoustic emission (AE) monitoring system and infrared thermal imager monitoring system. The rockburst patterns, the AE distribution and average infrared radiation temperature (AIRT) were captured during the compression process. The AE parameters (AE event rate and AE energy rate) and the temperature parameters of the tunnel models with different water content were analyzed. The water softens the rocks and reduced their mechanical parameters (such as the elastic modulus and peak strength), and the tunnel model in the saturated state exhibited a lower dynamic failure rate, leading to quicker static failure. The rockburst of the saturated tunnel model generated more AE events and was more damaged in the early loading period, and then demonstrated a weaker dynamic fracturing. The quiet period of AE event rate and the sharply increasement of AE energy rate can be regarded as the precursor to rockburst on the time-scale. The water also promoted the effects of infrared radiation temperature of the saturated models. The sudden change of the temperature of the tunnel walls can be used as the precursor to rockburst on the space-scale. The knowledge of the influence of the moisture state on the rockburst will improve the design and construction of deep tunnels.
Influence of the fracture process zone (FPZ) poses a challenge to accurately determine the FPZ and traction-free crack during the fracture process in a rock structure. In this study, an engineering ...approach based on concepts of the cohesive zone model, is proposed to identify them in a sandstone beam under three-point bending. Two types of specimens were tested, including center notch and smooth boundary beams. Acoustic emission (AE) and digital image correlation (DIC) were used to monitor the local material behaviors, which involves two crucial parameters: AE energy and DIC opening displacement pattern. (i) Three levels of AE energy have been classified to “filter” the FPZ from AE events cluster. Based on AE energy levels, three groups of AE events can be obtained. For Level 1, AE events involves AE energy that is 1 or 2 order larger than that within Level 2 and 3, such that they occupy about 95% of total AE energy release. Thus, Level 1 is considered to represent the group of large AE energy. For Level 2 and 3, although many AE events are detected, these events have little influence on the fracture process. The FPZ consists of only AE events within Level 1. (ii) Four types of DIC opening displacement patterns have been observed on the specimen surface to identify the FPZ. Every displacement pattern represents a feature that is related to specific loadings in the cohesive zone model. Depending on the distribution of opening displacements and displacement gradients, displacement pattern can be decided and the individual pattern has its own special physical correlation: pattern (I) is related to elastic tension of the material; pattern (II) suggests the possible existence of the FPZ; pattern (III) indicates the possible onset of a traction-free crack; pattern (IV) confirms material softening within the FPZ and material unloading outside the FPZ. These patterns together with different reference loads are used to identify the FPZ and traction-free crack. With the help of two parameters, the precise kinematics of the FPZ and traction-free crack in the specimen are determined, and the fracture properties such as the FPZ length and critical opening displacement are also reported. Finally, experimental results provide insights to understand opening-mode fracture and related structural response, i.e., possible formation of the traction-free crack at peak under three-point bending, reasonable assumption of elastic material behavior before the softening, and the dynamic effect of fracture propagation on laboratory determination of fracture properties.
At a microscopic scale, the failure of brittle materials results from crack initiation, propagation and coalescence. Acoustic emission (AE) technique, especially parameter analysis, has been widely ...applied to investigate cracking process and mechanism in civil engineering. However, crack classification in AE parameter analysis mostly derives from the empirical relation between the RA value and the average frequency, and the crack classification criterion, i.e., the optimal transition line between shear and tensile cracks in the parameter analysis has not been determined yet. Based on statistical analysis of dominant frequency characteristics of AE signals, a new method is proposed for determining the optimal transition line for crack classification in AE parameter analysis. Spectrum analyses of AE waveform data in the representative specimens are carried out to acquire the dominant frequency of AE waveforms. Proportions of waveforms distributed in low and high dominant frequency bands (L-type and H-type waveforms) are determined. The ratios of tensile and shear cracks, viewed as measurements, are determined by the statistical analysis of dominant frequency characteristics of AE waveforms. For a series of different transition line, the predicted ratios of tensile and shear cracks in AE parameter analysis are determined. The optimal transition line is determined to be the one corresponding to the least square difference between predicted data and measurements. The determined optimal transition line can be directly applied for crack classification in AE parameter analysis in the subsequent experiments of this brittle material. The reliability of the proposed method were validated by laboratory tests of rock subjected to compression. It can be found that the optimal transition line in the parameter analysis is approximately from 1:100 to 1:500 for brittle rock under compression. The findings in this study contributes to the enhancement of the accuracy and efficiency of AE source mechanism and damage process analysis.
Increased water content in rocks can result in a decline in their strength and an increase in deformation, which can lead to geological disasters such as landslides and tunnel collapses. Hence, rock ...characteristics with respect to water content need to be investigated for the purposes of conducting risk assessments and implementing preventive measures. In this study, the influence of the water content on rock failure patterns and acoustic emission (AE) characteristics was investigated. Uniaxial compression tests and numerical simulations with the PFC2D software were used to study the evolution process of microcracks and failure patterns in red sandstones with different water contents. The increases in the water content reduced the rock strength, Young's modulus, ratio of strain to peak strain in the elastic deformation stage, maximum energy of a single AE event, and average AE energy. The red sandstone specimens in the saturated state formed a shear macroscopic fracture surface, whereas those in the natural state formed a failure pattern in which shear failures and splitting failures coexisted. The red sandstone failure pattern in the dry state remained between these two conditions. The average frequency centroid of AEs revealed the precursory characteristics for critical failures of red sandstone with different water contents. The findings of this study are useful for future risk assessment and disaster prevention studies.
