Cumulative Acoustic Emission Energy Research Articles

Evolution of the damage precursor based on the felicity effect in shale

Damage precursors during hydraulic fracturing in shale gas reservoirs may be better understood if the deformation, failure, and acoustic emission (AE) characteristics under cyclic loading are known. Therefore, the purpose of this paper is to investigate the quantitative damage based on the Felicity effect under constant stress lower limit uniaxial cyclic loading-unloading rates (0.5, 1.0, 1.5, 2.0, and 2.5 kN/s). Variations in the b-value and the spatiotemporal evolution of cumulative AE were also used to observe how shale fractures formed. The findings reveal that during the unloading stage, there are many cumulative AE events when the stress level is low (≤1.50 kN/s) but that this number drops significantly when the stress level increases above (>2.0 kN/s). The AE amplitude, AE counts, and cumulative AE energy of each cycle in a loading-unloading test show an increasing trend, but the rate increases in the last cycle. During the whole process of loading and unloading, the Kaiser effects were present in the 3rd cycle at stress levels (≤1.5 kN/s). Still, the Felicity effect appeared in the 2nd and 1st cycles during 2.0 and 2.5 kN/s cyclic loading. The Kaiser effect occurs in the linear elastic stage, while the Felicity effect occurs in the crack initiation and crack damage stage. Furthermore, the Felicity ratio (FR) variations during shale deformation and failure can be divided into four phases: (Phase I = 1.01 ≥ FR > 0.89), (Phase II = 0.89 ≥ FR > 0.48), (Phase III = 0.48 ≥ FR > 0.23), and (Phase IV = FR ≤ 0.23). The b-value is relatively higher under the loading rate below (≤1.50 kN/s), indicating an increase in the number of small AE events. In contrast, the fact that the b-value is relatively smaller under the loading rate above (>2.0 kN/s) indicates that, the number of large AE events increases the number of cracks and fractures. These findings provide important design references for damaged precursors during hydraulic fracturing in shale gas reservoirs.

Numerical simulations on the structural heterogeneity and fracture bias effect of three-point bending specimen

Granite is a common surrounding rock in tunnels, nuclear waste repositories, and other underground engineering. It is essential to investigate granite’s mechanical behavior and fracture mechanism at the mesoscale. In this paper, the three-point bending model considering granite meso-heterogeneity was constructed, and the three-point bending test and acoustic emission (AE) simulation were carried out under different crack sizes and porosities, different prefabricated fracture offset distances, and bedding conditions. The results show that large-size mineral particles and intrinsic fractures mainly determine the sample’s crack propagation pattern. As the prefabricated crack’s deviation distance increases, the sample’s peak load, fracture energy, number of AE events, and internal damage range all gradually rise. The sample’s crack propagation path has a distinct step shape due to bedding, and tensile fractures primarily occur in the through-layer crack, whereas shear fractures mainly occur at the bedding plane. The specimen’s surface crack’s fractal dimension is positively associated with the cumulative acoustic emission energy during the fracture process. The conclusion of the research can offer a theoretical foundation for analyzing the stability of the surrounding rock.

Enhancing the acoustic emission technique using fuzzy artificial bee colony-based deep learning for characterizing selective laser melted AlSi10Mg specimens

This article presents a classification of Acoustic Emission (AE) signals from AlSi10Mg specimens produced via Selective Laser Melting (SLM). Tensile tests characterized the mechanical properties of specimens printed in different orientations (X, Y, Z, 45°). Initially, a study quantified damage modes based on the stress-strain curve and cumulative AE energy. AE signals for each specimen (X, Y, 45°, Z), across deformation stages (elastic and plastic), and damage modes were analyzed using continuous wavelet transform to extract time-frequency features. A novel convolutional neural network, based on artificial bee colonies and fuzzy C-means, was developed for scalogram classification. Data augmentation with Gaussian white noise enhanced the approach. Cross-validation ensured robustness against overfitting and suboptimal local maxima. Evaluation metrics, including the confusion matrix, precision-recall curve, and F1 score, demonstrated the algorithm's high accuracy of 92.6%, precision-recall curve of 92.5%, and F1 score of 92.5% for AE signals based on printing direction (X, Y, 45°, Z). The study highlighted the potential for improving AE signal classification related to elastic and plastic deformation stages with 100% accuracy. For damage modes, the algorithm achieved a confusion matrix accuracy of 90.6%, a precision-recall curve of 90.4%, and an F1 score of 90.5%. This approach demonstrates high accuracy in classifying AE signals across different printing orientations, deformation stages, and damage modes of AlSi10Mg specimens manufactured through SLM.

Correlational fractal analysis of the Kaiser effect in rock: implications for salt-cavern engineering under triaxial cyclic loading and unloading

ABSTRACT The Kaiser effect exhibits stress-induced changes in the acoustic properties of rock, so accurately identifying the Kaiser point of rock under complex conditions is essential for understanding the damage and failure mechanisms. In this paper, triaxial loading and unloading tests were conducted on salt rock using MTS815 material testing machine and PCI-II acoustic emission (AE) system. By introducing the Kaiser point calculation approach involving AE signal optimisation, Grassberger-Procaccia (GP) algorithm, K-means clustering, and random sample consensus (RANSAC) algorithm, we achieved enhanced accuracy in analysing the Kaiser effect for salt rock. The cumulative AE energy exhibited a distinct surge point during the cycle number (N) was less than 21, closely aligned with the optimised key AE energy point. For N > 21, the cumulative energy displayed stable changes, rendering conventional method ineffective. By utilising screened AE signals (8796 samples) within the GP algorithm, the surge point of cumulative energy consistently aligned with the sudden drop point of the correlation dimension when N<21, with a maximum error of 6 s. Remarkably, the GP algorithm identified the prominent Kaiser point even when N > 21, suggesting its applicability throughout the loading cycle.

Study on the acoustic-thermal effects and damage models during uniaxial compression failure process of phosphorite with different H/Di

Determining the size of ore pillars is crucial for the stability of mining structures and the recovery rate in mining design. To investigate the damage characteristics of phosphorite under different height-to-diameter ratios (H/Di), uniaxial compression tests were conducted on phosphorite specimens with varying H/Di. The effects of these H/Di on the mechanical properties and acoustic-thermal signals were analyzed, leading to the establishment of a joint acoustic emission-infrared damage model. Results showed: (1) Peak stress and ultimate strain of phosphorite specimens were negatively correlated with the H/Di, with compressive strength decreasing and stabilizing as the H/Di increased. (2) Increasing H/Di led to a transition from tensile to shear crack dominance during specimen failure, with cumulative acoustic emission energy and ringing counts positively correlated with H/Di. (3) Infrared radiation temperature before instability failure showed a cyclic pattern of slow increase, sudden rise, and sudden decrease, reaching a peak at failure. Using these findings, separate damage models for different H/Di were established based on two characteristic signals, correcting for monitoring losses, and finally merged into a joint acoustic-thermal damage model. The resulting model aligned well with experimental data, validating its rationality and significantly improving damage prediction accuracy. Thus, the model holds promising application prospects for calculating recovery rates, designing ore pillar sizes, managing goafs, and maintaining abandoned mine stability in phosphate mining design.

Study of the Internal Rebreaking Characteristics of Crushed Gangue in Mine Goaf during Compression

The deformation and re-crushing characteristics of different lithological caving crushed gangues in mine goaf directly affect the overburden strata movement, which significantly affects the surface subsidence of mining goaf. The effect of particle size on the re-crushing characteristics of different lithological caving crushed gangues in mine goaf is investigated in this study based on an innovative compression–AE (acoustic emission) measuring method. The results showed the following: (1) The compression deformation was divided into three stages: rapid, slow, and stable compaction. With the increase in axial pressure, the large particle skeletons were destroyed, medium particles were displaced and slid, and small particles filled the pores. (2) For singular lithologies, stress was positively correlated with pressure, and porosity was negatively correlated with stress. The composite sample was between the singular gangue samples. (3) The fractal dimension of crushed gangue samples was exponentially related to the proportion of gangue in singular and combined lithologies. (4) The cumulative AE count and energy of the combined lithological gangue samples were between those of the singular samples. The research results provide a theoretical foundation for further research into the characteristics of the overlying strata, surface movement, and safety management of the goaf.

Compressive Fracture Behavior and Acoustic Emission Characteristics of Sandstone under Constant Crack Water Pressure

Abstract Engineering rock containing flaws or defects under a large water source is frequently subject to the couple influence of constant crack water pressure and geostress. To investigate the fracture behavior of precracked rock under hydromechanical coupling with constant crack water pressure, compression tests were conducted on red sandstone specimens containing a single crack of different angles using a device to realize the constant crack water pressure during loading, and the failure process of rock specimens was monitored by acoustic emission (AE) technique. The results show that the presence of constant crack water pressure has a significant promotion effect on the development of shear wing cracks, and the promotion effect is influenced by the prefabricated crack angle and water pressure. As the constant crack water pressure increases, the failure mode of the 0° precrack specimen changes from “X”- shear failure to the single oblique shear failure along the shear wing crack direction, the main failure crack of the inclined precracked specimens (precrack angles of 15°, 45°, and 60°) changes from a small acute angle with the prefabricated crack to a direction along the shear wing crack, and irregular cracks occur at the chipped prefabricated crack in the 90° precracked specimen. With an increase in the constant crack water pressure, the average energy for a single hit, cumulative AE energy, and cumulative AE hits decrease, and the proportion of the tensile cracks increases and that of the shear cracks decreases.

Rock bolts under cyclic loading: Mechanical performance and damage assessment by acoustic emissions

Rock bolt support is one of the key methods to maintain the stability of underground space. The durability of the rock bolt can be weakened by cyclic loading, resulting in premature failure of the underground structures. A comprehensive understanding of the performance of the bolting system under cyclic loading is essential to assess the safety of the bolting system and underground structures. In this context, this study investigated the mechanical performance of rock bolts installed in simulated soft and medium hard rock and subjected to cyclic loading. The Acoustic Emission (AE) technique was used to monitor internal damage to the bolt system under different loading conditions. A cumulative AE energy slope method has been proposed to determine the onset of the macro crack formation stage. The rock bolt demonstrated an extended fatigue life in soft rock, but its condition worsened when implanted in medium hard rock. The cyclic loading resulted in a slight improvement in the load bearing capacity of the soft rock bolt system and a significant deterioration in the medium hard rock bolt counterpart. The main fatigue failure modes of the rock bolt systems were splitting and shear failure of the surrounding rock, and debonding of the interface. Increasing the amplitude of the cyclic loading reduced the macro crack initiation cycle for both rock bolt systems, whereas increasing the frequency had different effects. The damage variable increased with frequency and amplitude for both rock bolt systems. The toughness of the soft rock bolt degraded during cyclic loading, while the high frequency cyclic loading did not modify the toughness of the medium hard rock bolt.

Failure and Acoustic Emissions of Coal–Rock Combinations with Different Dip Angles in the Shaqu No. 1 Coal Mine

The force and deformation characteristics of the rock layer on the top and bottom of the coal seam change significantly when the dip angle changes. The mechanical properties and damage characteristics of differently inclined coal–rock assemblages were investigated, and the results were combined with acoustic emission information, including acoustic emission ringdown counts, to quantify the damage of inclined coal–rocks under compression. The experimental results showed that the stress‒strain curves of the inclined coal–rock assemblages had four main stages, with approximately similar curves in the early stage and deformation in the later stage. The damage gradually changed from shear damage to interfacial slip damage, and the damage area gradually transitioned to the structural surface from coal body components. The cumulative acoustic emission energy tended to decrease with increasing inclination angle, and the peak acoustic emission energy gradually decreased. When the inclination angle was less than 30°, the cumulative energy of acoustic emissions increased slowly, then decreased, and it finally decreased significantly between 30° and 45°; from 0° → 15° → 30° → 45°, the energy change rates were +3.0%, −25.1%, and −78.2%, respectively. For coal–rock assemblages with different interfacial angles, the sliding damage instability caused by the coal–rock interface increased with increasing interfacial angle within the assemblage. The results of this study provide a deeper understanding of the mechanical properties of coal–rock assemblages with different inclinations and the characteristics of fissure extension. The fractal dimension based on particle number decreased with increasing loading rate, and the larger the loading rate was, the smaller the fractal dimension. In addition, the current findings provide a reliable foundation for further understanding the mechanisms of disasters caused by coal–rock disturbances, such as excavation of inclined roadways and extraction of gas, as well as supporting the development of methods for monitoring, early warning, and prevention and control of these types of disasters.

Critical slowing down precursor information for the acoustic emission response characteristics of defective tuffs

Fracture characteristics of rock and rational structural design are necessary to ensure the stability of underground rock engineering. Early warning of rock damage is a hot and challenging issue for research in geotechnical engineering. In this paper, uniaxial compression tests and acoustic emission (AE) tests were conducted on different defective specimens. The AE characteristics during rock damage were statistically analyzed based on the critical slowing theory. The results showed that the stress fluctuation in the stress–strain curve appeared earlier with the increase of initial damage. The failure pattern shifted overall from tensile to shear cracks. With the load increased, the AE amplitude changed from low amplitude and low density to high amplitude and high-density characteristics. The AE RA/AF was more sensitive to tiny cracks and had better immunity to attenuated AE signals. Both cumulative AE energy and RA/AF showed a precursor characteristic of a sudden increase in slope before rock damage. The AE b-value experienced a period of rising, fluctuation, and decline, with a sudden drop before the large-scale crack extension, showing a decreasing trend overall. The magnitude of fluctuations and density of distribution of the critical slowing down parameters (variance and autocorrelation coefficients) reflected the severity of specimen failure. The early warning characteristic of the variance was more significant, while the autocorrelation coefficient had an apparent disorder, which interfered with the judgment of the system phase transition. The average warning time of different AE parameters' “critical precursor point” was greater than 42 s, and the stress level was lower than 91.50%. The warning time sequence of different parameters in descending order was as follows: variance of AE parameters > autocorrelation coefficient of AE parameters > traditional AE parameters. The overall warning effect of the critical slowing down phenomena was better than that of classic AE parameters.

Mechanical and Acoustic Emission Characteristics of Grouted Reinforcement in Fissure-Containing Rock-like Specimens

In order to study the reinforcing effect of the different grouting materials applied in fissure rock engineering, ultra-fine cement slurry, cement–silicate slurry, and MARITHAN® were used to carry out grouting tests on specimens of fissure-containing rocks. With the help of a uniaxial compression acoustic emission test system, the mechanical characteristics of the grouted specimens were obtained, and the damage process of the specimens was revealed by using the acoustic emission signals. The tested results showed the following: the residual strength of the grouted specimens using the three grouting materials increased by 16.931%, 13.075%, and 39.998%, respectively; the ultra-fine cement grouting and cement–silicate grouting specimens showed damage patterns of shear-slip damage along the original rupture surface; the specimens of MARITHAN® grouting cracked from a position near the end of the specimen with no damage to the grouted body portion; the cumulative acoustic emission energy curves of the grouted specimens showed obvious stage characteristics; and the acoustic emission energy distribution characteristics of the grouted specimens differed depending on the grouting materials.

Effect of water on the damage and energy dissipation feature of coal under uniaxial cyclic loading–unloading condition

AbstractTo gain insight into the reduction mechanism of water on the dynamic destabilization failure character of coal under mining disturbance conditions, the effect of water on the damage and energy dissipation feature of coal was investigated via cyclic uniaxial compressive loading–unloading test, acoustic emission (AE) monitoring technology, and theoretical analysis. It manifests that the softening effect of water on the mechanical properties of coal is still remarkable, lower uniaxial compressive strength and elastic modulus, while greater axial strain and higher Poisson's ratio are observed under the cyclic loading–unloading condition. Water‐saturated specimens dissipate more energy during the cyclic loading–unloading process, which reduces the proportion of input energy accumulated in the coal mass, thus lowering the burst proneness of coal. A greater amount of damage generates in air‐dried specimens than that in water‐saturated specimens at the first cyclic loading–unloading cycle, while lower damage increments are observed in the following cycles. This may be the reason for the minimal energy dissipation of air‐dried specimens in the following cycles. The elastic modulus behaves logarithmically versus the increasing cyclic loading–unloading cycle counts. The energy dissipated due to the damage evolution and plastic deformation of coal specimens positively correlates with the energy released by AE activities, the AE energy in air‐dried specimens has a greater concentration around the peak axial stress and a stronger correlation with the dissipated energy. The cumulative AE energy and energy dissipation density are linearly correlated during the cyclic loading–unloading process, which indicates a possibility in estimating the damage evolution and the dynamic failure risk of coal via cumulative AE energy, under the cyclic loading–unloading condition.

Study on the settlement and delamination mechanism and acoustic emission characteristics of mixed aggregate cemented backfill

To study the effects of waste rock (WR) contents on the delamination and mechanical characteristics of the cemented backfill, the backfill specimens with 40%-80% WR content were prepared based on the stacking density of mixed aggregate. The characteristics of the WR distribution inside the cemented backfill with various WR contents were examined by scanning the sections images of the specimens. The strength and damage properties of the cemented backfill were investigated by uniaxial compression acoustic emission (AE) tests, and the strength and damage evolution mechanisms were explained by microstructure testing experiments. The results show that the viscosity and yield stress of the filling slurry gradually decreased with the contents of WR increased, and the WR settlement and delamination more obvious. The uniaxial compressive strength (UCS) has the same trend as the stacking density of the mixed aggregate, showing the trend of “increased first and then decreased” with the content of WR increased. When the content of WR was 60%, it had the highest UCS. As the content of WR increased, the fracture area at the interface of WR and cementitious matrix gradually increased, and the extrusion between coarse aggregates became more obvious, leading to more complex crack extension and higher AE cumulative energy. The main products of hydration in the cemented backfill were calcium silicate hydrated (C-S-H gels), ettringite (AFt), and calcium hydroxide (Ca(OH)2). The specimens with higher content of hydration products have better cementation among aggregate particles, denser microstructure, and higher compressive strength of cemented backfill.

Effect of notch location on flexural and fracture behaviors of UHPGC notched beam based on four-point bending test and acoustic emission monitoring

Ultra-high performance glass sand concrete (UHPGC) is a new composite material with superior mechanical properties and durability made by replacing quartz sand in traditional ultra-high performance concrete (UHPC) with environmentally friendly and economical glass sand recycled from waste glass. This study investigated the flexural and fracture characteristics of UHPGC notched beams with prefabricated notches at three different locations (mid-span, 100 mm off mid-span and 145 mm off mid-span) using four-point bending tests. Meanwhile, the damage evolution processes of notched beams under flexural load were monitored in real time and characterized by acoustic emission (AE) technology. Fracture modes were automatically classified based on Gaussian mixture model (GMM) and support vector machine (SVM) of rise angle (RA) and average frequency (AF). Outlier analysis of b-value were applied to provide a warning prior to complete failure of UHPGC notched specimen. The results revealed that notched position presents significant effects on flexural strength, flexural toughness and fracture energy, and exhibit considerable influence on AE parameters and cracking patterns. The farther the notch is away from the mid-span, the greater the flexural strength, flexural toughness and fracture energy exhibit due to the weakened stress concentration. As it takes longer to gather enough strain energy at the tip of the notch to crack, AE monitoring shows that higher amplitude and hysteretic nature of AE parameters such as accumulated energy and information entropy, etc., can be recorded as notch position away from the mid-span. The farther the notch is away from mid-span, the more shear cracks predominate at the later stages of damage. Cumulative AE energy and fracture energy throughout the damage process are found to exhibit a favorable exponential relationship. In addition, the b-value outlier analysis can provide failure warning for UHPGC notched beams. This study is of great significance in exploring the fracture behavior of UHPGC and providing reference for engineering applications related to UHPGC failure.

Effect of notch-to-depth ratio on flexural and fracture behaviors of UHPGC notched beams based on four-point bending test and acoustic emission

An ecological ultra-high-performance glass sand concrete (UHPGC) was prepared by partially replacing quartz sand in traditional ultra-high-performance concrete (UHPC) with glass sand recycled from waste glass. The four-point bending tests of UHPGC notched beams with different notch-depth ratios (0.2, 0.3, 0.4, and 0.5) were carried out, while acoustic emission (AE) monitoring was employed at the whole fracture process. Flexural toughness and residual flexural tensile strength were used to evaluate the flexural properties. Meanwhile, fracture energy and fracture toughness were used to evaluate the fracture properties. The results reveal that notch-depth ratio exhibits a significant effect on the flexural and fracture performances of UHPGC notched beams. The residual flexural tensile strength, flexural toughness, fracture energy and fracture toughness decrease with the increase of notch depth. Moreover, AE energy parameters can favorably characterize the differences of energy release in the fracture processes of UHPGC beams with different notch depths, and favorable correlations are observed between cumulative AE energy and fracture energy. Besides, AE wavelet entropy are constructed based on wavelet packet transform (WPT) of AE waveforms, and the evolution model of wavelet entropy is established. By comparing with the evolution process of AE energy information entropy, it is proved that the maximum point of wavelet entropy can be used as the key point to predict the peak load.

Precursor Signal Identification and Acoustic Emission Characteristics of Coal Fracture Process Subjected to Uniaxial Loading

In deep underground mine engineering, the critical warning signals before the sudden failure of coal are crucial to predict coal or rock dynamic catastrophes and to help the coal industry grow sustainably. Therefore, with the objective of accurately identifying the precursor signals of coal fracture, a uniaxial compression test was adopted. Tests were performed on multiple sets of raw coal samples, and acoustic emission (AE) technology was used to capture the deformation and destruction courses of the coal samples. Furthermore, the signal intensity of AE energy was discussed. Based on the critical slowing down theory, the AE energy sequence was processed. The results indicate that there are significant discrepancies in the strength of coal affected by initial pore fissures. During the whole loading process, the AE energy signals showed obvious stage characteristics, and there was a high risk of rapid coal energy storage during the unstable rupture development (URD) stage, which predicted the imminent destruction of the coal. The variance mutation point that was not affected by the lag step selection was easier to identify than that of the autocorrelation coefficient, and the precursor points were all in the URD stage, which is more accurate than using the AE cumulative energy curve slope.

Deformation behaviour, crack initiation and crack damage of weathered composite sandstone-shale by using the ultrasonic wave and the acoustic emission under uniaxial compressive stress

Anticipating the progressive damage and strength of an anisotropic and interbedded rock mass is challenging due to the overwhelming variables which need to be considered. Therefore, the strength and progressive damage of each rock material is not a realistic model which can be used to define the deformation of the interbedded anisotropic rock mass. In this research, weathered (sandstone-shale-sandstone) composite samples under unconfined compressive stresses were concurrently assessed using acoustic emission (AE) and ultrasonic pulses. Shale interlayer significantly impacts the composite's behaviour, according to the mechanical properties and stress-strain relationships of weathered sandstone, shale, and composite. The deformation behaviour was derived following (velocity, amplitude and energy) responses of both the ultrasonic and AE waves. The intensity of AE energy reflects the best interaction with the progressive deformation of the rocks under uniaxial stress. Although the ultrasonic velocity was only sensitive toward the macro-deformation or specimen failure, the ultrasonic parameters (i.e., amplitude and energy) reflected a substantial alteration, specifically during the plastic deformation. The cumulative AE energy tangent (CSEET) method exhibited clear uncertainties in defining the CI and CD of composite specimens compared to sandstone and shale. The impact of AE energy attenuation due to the joint between shale and sandstone in the composite specimen was highlighted. For instance, the AE energy induced by CI and CD was about 6.0 E+09 mV and 2.4 E+11 mV for sandstone, 2.0 E+08 mV and 2.5 E+8 mV for shale, 1.6 E+08 mV and 2.2 E+8 mV for composite. The crack initiation (CI) (i.e., represents the plastic threshold of rock where the micro-cracks took place), and the crack damage (CD) (i.e., represents the stage where unstable macro-cracks begin to grow), which was indicated by the ultrasonic parameters, were addressed as alternative novel approaches for determining the plastic deformation of composite specimens. The CI commenced at 43%–80%, 53%–78%, and 35%–55% of the peak stress, whereas the CD occurred at 84%–97%, 78%–95%, and 75%–94% of the peak stress for the weathered sandstone, weathered shale, and composite, respectively. This research may help evaluate the deformation behaviour of the interbedded rock masses using the ultrasonic approach, presuming a sufficient material strength differential between the interbedded rocks.

Laboratory study on the damage characteristics of shale under different liquid nitrogen injection rates

Liquid nitrogen (LN2) is an environmentally sustainable fracturing fluid that is used in oil and gas field. LN2 flow rate has a significant impact on the cryogenic damage effect. A sequence of laboratory experiments was carried out to explore the cryogenic damage characteristics of shale under different LN2 flow rates (0.05 L/min-0.20 L/min) in this paper. LN2 was injected into a shale borehole for 20 min. The infrared radiation temperature (IRT) system and the acoustic emission (AE) system were applied to monitor the magnitude of the cryogenic damage in real time. After LN2 pre-conditioning, the shale samples were broken in uniaxial compression condition or fractured in high temperature (100 °C) and triaxial stress (12 MPa) condition. The differences in microscopic characteristics of shale with and without LN2 pre-treatment were observed by the scanning electron microscope (SEM). The experimental results show that a low temperature region around the borehole grew with increasing LN2 flow rates. There were many new microcracks generation between the laminations during LN2 injection. The cumulative AE energy increased by at least 15 times when the LN2 flow rates increased from 0.05 L/min to 0.20 L/min. The uniaxial compression strength (UCS) of shale decreased by 22.33% and the elastic modulus decreased by 24.43%. High LN2 flow rate is favorable for in-situ cryogenic damage, but the economic efficiency decreased. The reduction in average breakdown pressure (BP) increased and then decreased with increasing LN2 flow rate. When the IRT of shale didn't change, the cryogenic damage would not increase any longer. It is necessary to co-ordinate the LN2 flow rate and the LN2 injection time for better cryogenic damage effect. The study provides evidence of the advantages of LN2 assisted fracturing and valuable perspectives on the characteristics of cryogenic damage at different LN2 flow rates.

Experimental investigation on the influence on mechanical properties and acoustic emission characteristics of granite after heating and water-cooling cycles

This study explores explore the failure characteristics of high temperature granite under different numbers of heating–cooling cycles. Combining a series of laboratory uniaxial compression tests with acoustic emission (AE) monitoring, it reveals the AE characteristics and damage behavior of high temperature granite in different damage stages. Increasing the number of heating–cooling cycles exponentially raised the mass-loss and volume-increase rates of the granite and exponentially decreased the P-wave velocity. These results indicate that multiple heating–cooling cycles irreversibly damaged the high temperature rock mass. Moreover, the variations in both the P-wave velocity and the stress threshold at each stage gradually plateaued after three thermal cycles. Under uniaxial compression, the evolution mechanism of microcracks in the rock was successfully described by the AE characteristic parameters and real-time spatial AE position. The cumulative AE counts and AE energy rates were consistent in different damage stages of the rock specimen. Both quantities began increasing after entering the unstable crack growth stage. As the number of heating–cooling cycles increased, the main fracture mechanism of rock rupture transitioned from mixed mode to shear mode, as evidenced by the distributions of rise angle and average frequency. Meanwhile, the gradually increasing b-value indicated that small-scale fracture events gradually dominated the rock damage. The present results can assist the design of deep geothermal-resource mining schemes and safe mining constructions.

Influence of the combined action of water and axial pressure on the microscopic damage and mechanical properties of limestone

As one of the main methods, hydraulic fracturing method is widely used in underground energy exploitation. Groundwater not only restricts the energy exploitation efficiency, but seriously affects the safety and stability of underground energy storage structure. To study the influence of groundwater and geostress on hydraulic fracture propagation and rock failure mechanism, the water injection tests were performed on limestone under different constant axial loading (0, 1, 2 and 3 MPa). After water injection, the evolution law of the micropore structure in water-injection samples were studied through SEM and mercury-injection tests; Moreover, the mechanical properties of the water-injection samples were studied through laboratory loading crushing tests. During loading process, the acoustic emission (AE) and infrared radiation temperature (IRT) parameters were collected synchronously and nondestructively. The results indicated that the pore size and porosity of limestone considerably increased with increasing axial pressure, and obvious stage characteristics of the stress-strain curve, cumulative AE energy and IRT difference were determined. Furthermore, the strength, cumulative AE energy and cumulative IRT difference decreased continuously with increasing axial pressure. Compared to limestone subjected to only water injection, the cumulative AE energy of the specimens subjected to high-pressure water under 1–3 MPa axial pressure decreased by 11.31%, 24.71% and 25.56%, and the IRT difference decreased by 10.71%, 19.64% and 26.79%, respectively. In addition, the extreme values of both cumulative AE energy and IRT occurred earlier than the peak stress, which can be interpreted as a sign of structural failure in limestone.