Cumulative Acoustic Emission Counts Research Articles

Annealing impact on mechanical performance and failure analysis assisted with acoustic inspection of carbon fiber reinforced poly‐ether‐ketone‐ketone composites under flexural and compressive loads

AbstractThis study investigates the effect of the annealing treatment for carbon fiber reinforced Polyether‐ketone‐ketone (CF/PEKK) composite structures under flexural and compressive loadings through reference, pre‐damaged, and annealed sample sets. Significant recovery of pre‐existing damage is observed after the annealing process, following both flexural and compressive loading. Acoustic emission (AE) inspection is employed to monitor the failure behavior and assess the impact of pre‐damage and annealing on CF/PEKK composite. Initially, AE inspection reveals that the reference CF/PEKK material exhibits a notable fiber‐related failure with 85% of cumulative AE counts under flexural load, whereas matrix‐related failures are more pronounced with 92% cumulative AE counts under compressive load. Pre‐damages in the matrix alter the cumulative count percentages and initiation time that are related to matrix, interface, and fiber‐related failures, under flexural and compressive loadings. After annealing, each cumulative AE count percentages are comparable to reference sample values, due to changes in microstructure and relieving of residual stresses. The annealing effect is further validated through dynamic scanning calorimetry (DSC) analysis results with increased glass transition temperature (Tg) and degree of crystallization (Xc). Overall, these findings indicate that annealing treatment effectively restores structural integrity and improves the mechanical performance of CF/PEKK composites.Highlights Annealing aims for damage recovery in CF/PEKK under flexural and compressive loads. Significant damage recovery in CF/PEKK is seen after annealing. Annealing raises Tg and crystallinity, and enhances CF/PEKK structural integrity.

Study on the characteristics of acoustic-thermal precursors of destabilization damage in coal-rock combination bodies with different proportions

The instability and failure processes of coal-rock combinations are accompanied by the release of acoustic emission (AE) and infrared radiation (IR) signals. To investigate the characteristics of AE and IR signals during the failure process in coal-rock combinations with different ratios, and to analyze the effectiveness and applicability of the monitoring methods for these two signals in various specimen ratios. In this paper, the uniaxial compression tests were conducted on seven coal-rock combinations with different ratios by using the rock mechanics loading system and the cooperative monitoring platform of infrared and acoustic emission. The results show that (1) the AE counts for failure precursors in coal-rock combinations are positively correlated with the coal-rock ratio, whereas the AE peak counts are negatively correlated. Moreover, as the coal-rock ratio decreases, the slope of the cumulative AE count curve during the peak failure stage approaches 1. (2) During the elastic and yield stages, the maximum infrared radiation temperature (MIRT) curve fluctuates, and just before peak failure, the curve displays a distinctive “V” shape. This “V” shape becomes more pronounced as the coal-rock ratio decreases. (3) In the monitoring of damage precursors of coal-rock combinations, when the ratio of coal to rock is 1:3 or less, IR monitoring is better than AE monitoring. Conversely, when the ratio of coal to rock is greater than 1:3, AE monitoring is more suitable. Consequently, the combined monitoring of AE and IR signals can increase the reliability and precision of early warning signals for coal-rock assemblage damage precursors.

Evolution and Characterization of Uniaxial Compression Damage of Coal Measure Rock

In order to study the evolution law of uniaxial compression damage and its characterization method, three kinds of rocks with different lithology that commonly exist in coal measure rock were taken as the research objects to explore the relationship between their mineral composition, microstructure and mechanical properties, and the feasibility of acoustic emission (AE) characteristic parameters and AE spatial location to characterize the evolution process of uniaxial compression damage of rocks was compared and analyzed. A damage constitutive model based on AE cumulative ringing count was established and verified by numerical simulation. The results show that the differences in the micromorphology and mineral composition of coal measures with different lithology directly affect their physical and mechanical properties and load damage characteristics. AE characteristic ringing count can effectively represent the damage evolution process of rock during loading. Based on rock porosity characteristics and AE cumulative ringing count during loading, a constitutive model considering initial damage and cumulative damage was established. The theoretical analysis results of the damage constitutive model are in good agreement with the results of laboratory experiments and numerical simulation, which can be used to analyze the damage process of coal measure rock under load.

Study on the Weibull distribution function-based stochastic damage evolution law for uniaxial compression in high-performance concrete with full aeolian sand

The issues of environmental degradation due to natural sand overextraction are becoming increasingly severe. In desert regions, utilizing aeolian sand as a substitute for natural sand in construction projects facilitates the resource economization and the effective management of desertification. The concrete industry is expected to significantly contribute to this objective, as high-performance concrete with full aeolian sand (FA-HPC) has demonstrated excellent mechanical properties. However, current researches have yet to establish a rational damage evolution law for uniaxial compression in FA-HPC from the perspective of damage characteristics. In this study, uniaxial compression tests were conducted on FA-HPC with different mixture proportions. Simultaneously, the stress-strain behavior was examined, and acoustic emission (AE) signals were collected and localized. Based on the Weibull distribution function-based stochastic damage evolution law for uniaxial compression, regression analyses of the stress-strain and AE cumulative ringing count for FA-HPC were performed. Damage evolution model for effective stress and damage variable were established and validated with experimental data to confirm their applicability. The scale and shape parameters of the model were predicted using the mixture proportions as independent variables. The results show that the rationality of the damage evolution law assumption was validated through AE source localization. The goodness of fit values for damage variable and effective stress equation range from 0.87 to 0.98, with the ascending segment and the vicinity of the peak point closely matching the experimental data, indicating that the Weibull distribution function-based stochastic damage model for uniaxial compression is applicable for FA-HPC. The predicted values of the damage evolution model parameters based on the mixture proportions align well with the measured values, achieving a goodness of fit of 0.99, thereby guiding the study of FA-HPC's compressive characteristics prior to the formation of primary cracks. Notably, there remains a need to propose corrective equations to enhance the correlation between regression curves and experimental data in the late failure stages of FA-HPC in future research, achieving precise predictions by considering the interactions of multiple factors.

Damage evolution behavior of tunnel lining concrete with non‐through crack under single‐sided sulfate attack

AbstractIn order to study the dynamic damage evolution law of tunnel‐lining concrete with non‐through crack under single‐side sulfate attack, a 420‐day sulfate attack concrete test was conducted and the uniaxial compression and acoustic emission (AE) tests were carried out on each corrosion time. The test results showed that the compressive strength of each group can be divided into rising stage and falling stage, and the existence of non‐through crack reduced the compressive strength of concrete specimens and accelerated the corrosion of concrete specimens by sulfate. AE ringing count can be divided into compaction stage, elastic stage, yield stage, and post‐peak stage; the variation patterns of AE varied at different stages. Sulfate attack caused the yield point of concrete to advance. With the increase of corrosion time, microcracks increased gradually, cement mortar was consumed continuously, concrete spalling occurred, the proportion of AE cumulative ringing count Stage II decreased gradually, and the proportion of Stage III increased. Non‐through crack changed the energy storage of Stages II and III and reduced the AE activity of concrete after corrosion. The deeper the crack was, the smaller the average cumulative ringing count was and the lower the AE activity was. AE cumulative ringing counts can be used to characterize the damage evolution of corroded concrete.

Fracture evolution characteristics of asphalt concrete using digital image correlation and acoustic emission techniques

The cracking damage of asphalt concrete significantly affects the service life of pavement. The objective of this research is to investigate fracture mechanisms of asphalt concrete by employing a synergistic approach combining digital image correlation (DIC) and acoustic emission (AE) techniques under three-point bending tests. Firstly, the crack tip opening displacement (CTOD) and the fracture process zone (FPZ) derived from DIC were utilized to classify fracture stages and describe the entire crack propagation process. Meanwhile, AE-related parameters were applied to detect the internal damage and fracture characteristics. Subsequently, the results from DIC and AE were utilized for comparative analysis to evaluate the cracking resistance of different materials. Finally, the fractal dimension and AE b-value were used to distinguish the characteristics of the critical damage conditions. The results indicated that both techniques are effective in classifying the fracture stage of asphalt concrete. DIC can visualize and analyze crack propagation processes and paths on concrete surfaces, especially at the stage of macrocrack initiation and propagation. AE allows for continuous monitoring and highly sensitive detection of early-stage damage inside concrete. The distinct advantages of DIC and AE result in different fracture stage identification results at the early stage, but consistent after the appearance of macrocracks. Based on the relationship between CTOD and AE cumulative count, a simple approach is proposed to directly evaluate the cracking resistance of different asphalt concretes. The formation of macrocracks in asphalt concrete can be reflected by a shift where the correlation dimension drops steeply to the minimum value, and a minimum inflection point in the AE b-value curve acts as a precursor of complete fracture. The research offers an in-depth insight of both surface deformation and internal damage, thus enhancing the understanding of crack growth behavior and fracture process in asphalt concrete.

A recycled crushed rock sand mortar based on Talbot grading theory: Correlation of pore structure and mechanical properties

The use of crushed rock sand (CRS) as a substitute for natural sand is a necessary and feasible approach to address depletion of natural sand resources. This study investigated the effect of CRS aggregate gradation, based on Talbot grading theory, on pore structure and mechanical properties of mortar. 10 CRS mortars with different Talbot gradations were prepared, and quantitative relationships between Talbot gradation, pore characteristics (size distribution, fractal dimensions, and pore connectivity), acoustic emission (AE) characteristics, and compressive mechanical properties were analyzed using nuclear magnetic resonance (NMR) and AE techniques. The optimal Talbot index value was determined to be 0.4, based on the observed changes in pore structure and mechanical properties. Furthermore, the influence of Talbot index on internal damage process of CRS mortar was analyzed in conjunction with AE characteristics, and a prediction model for mortar strength was established based on porosity and AE cumulative counts. The correlation between pore structure, AE characteristics and mechanical properties of CRS mortar under Talbot grading was systematically investigated, which provides theoretical support for optimal design of CRS mortar materials. The application of Talbot grading theory in design of mortar is explored to provide a scientific basis for improvement of mortar engineering performance.

Effects of foliation angle on mechanical characteristics of carbonaceous slate using uniaxial compression tests

Carbonaceous slate is one kind of metamorphic rocks with developed foliation, which is frequently encountered during tunnel construction in Western China. The foliation plays a crucial role in the stability of tunnels. For this, we conducted uniaxial compression tests, acoustic emission (AE) monitoring and scanning electron microscope (SEM) tests on carbonaceous slate. The results show that the strength, failure mode, and AE characteristics exhibit marked anisotropy with the angle between the axial and the foliation (β). As β increases, the ultrasonic wave velocity decreases monotonically, whereas the uniaxial compressive strength (UCS) displays a distinctive U-shaped trend. The elastic modulus initially decreases and then increases. The cumulative AE counts curve and energy curve show a stepped growth when β ≤ 45°. The AE events are active during the crack compaction phase and remain calm during the linear elastic deformation phase when β > 45°. Upon failure, the energy release accounts for the highest proportion (67%) when β = 45°, while the proportions in other cases are less than 37%. The maximum percentage (31%) of shear cracks is reported when β = 60°, which is six times greater than that at β = 0°. Moreover, Kernel density estimation analysis reveals that the high concentration area with low AF (AE counts / duration time) and high RA (rise time / amplitude) increases initially, and then decreases when β > 60°. In addition, nine types of cracks and seven modes of failure were identified. The foliation angle has a pronounced impact on shear failure modes in comparison with tensile failure modes. The supports could suffer larger deformation when β ≥ 60° compared to other cases. The failure behaviors correspond well with field observations.

Effect of cyclic loading level on mechanical response and microcracking behavior of saturated sandstones: Correlation with water weakening phenomenon

Rocks frequently endure water environments and diverse cyclic loading from natural and anthropogenic sources across various damage stages. Understanding the mechanical and fatigue behavior of saturated rocks under different cyclic loading levels is crucial for the intelligent design and long-term stability of the slope. Here, we report several monotonic and cyclic compression tests conducted on red and cyan sandstones under dry and water-saturated conditions, coupled with acoustic emission (AE) monitoring. We analyzed key mechanical parameters such as peak stress and peak strain, as well as AE parameters including count and energy, and dominant frequency in AE spectra. Our findings reveal that elevated cyclic loading levels significantly reduce peak stress and increase peak strain in both dry and water-saturated rocks, with more pronounced effects observed under water-saturated condition. The cumulative AE count correlates positively with increasing cyclic loading levels but decreases due to water saturation. Under cyclic loading with varying levels, dry rocks often experience shear cracking and exhibit AE waveform with high dominant frequency, while saturated rocks tend toward tensile cracking and AE waveform with low dominant frequency. Higher cyclic loading intensifies the occurrence of tensile cracking and AE waveform with low dominant frequency, which are constrained during the elastic deformation stage but become more pronounced beyond the crack initiation threshold. Moreover, cyclic loading enhances the water weakening effects in saturated rocks, closely associated with ongoing crack propagation within the rocks.

Thermal-damage effects on fracturing evolution of granite under compression-shear loading

Shear-dominant instability disasters in high-temperature conditions pose a significant challenge for deep underground rock projects, necessitating a thorough understanding of rock fracture evolution and the impacts of thermal damage. To investigate how thermal damage influences shear fracturing behavior and geophysical responses, experiments were conducted on thermally-treated granite under compression-shear loading conditions. The detailed failure process was meticulously monitored using integrated acoustic emission (AE), electric potential (EP), and digital image correlation (DIC) techniques. The effects of thermal treatment on the patterns of AEs, EPs, and DIC strain fields during the loading process were analyzed. Micromechanics related to the influence of thermal treatment on mechanical properties and geophysical responses were explored using X-ray diffraction (XRD) and scanning electron microscopy (SEM). By defining damage variables through multiparameter analysis using AE, EP, and DIC, the study delved into predictive insights on the failure process of thermal-damage granite. The results indicate that the physical properties of granite undergo significant changes as the treatment temperature increases, with the peak stress gradually decreasing and the plastic characteristics increasing. These changes can be attributed to thermal damage, which causes internal water escape, expansion of mineral particles, and crack formation. Additionally, increasing the thermal treatment temperature leads to a decrease in both AE and EP intensity, as well as a reduction in the occurrence of high-magnitude AE events, while the cumulative AE count rapidly increases during the plastic stage. Notably, thermal treatment induces significant alterations in mineral composition, intergranular fractures, and the formation of a gradual zigzag crack path, resulting in the intensified development of shear microcracks. This observation is supported by the varying of AE characteristic parameter AF/RA and the analysis of DIC displacement decomposition. Moreover, the three defined damage variables exhibit accelerated growth prior to catastrophic failure, with the order of precursor occurrence observed as AE, DIC, and EP. However, the occurrence of the precursor time for these three geophysical signals becomes earlier with increasing treatment temperature. These research findings shed light on the mechanism of compression-shear failure of thermal-damage rock and provide valuable references for predicting precursors of the shear-dominant instability disasters such as induced earthquakes.

Effects of temperature on fracture and damage characteristics of deep granite

Investigating the fracture- and failure-related behaviors of rock that is subjected to temperature treatment is important for handling warm rock reservoirs during deep mining of hot dry rock and processing high-level radioactive waste. In this study, we use the semi-circular bending test in combination with acoustic emission (AE) monitoring technology to examine the characteristics of fracture and damage in granite treated at different temperatures and under different fracture modes at a depth of 750 m in the Daliuhang Gold Mine in China. The results showed that the peak load and fracture toughness of granite decreased to varying extents when it was treated at increasingly higher temperatures. The high temperature substantially reduced the bonding capacity of the particles of rock, and led to the formation of a large number of microcracks that dislodged the particles of rock along the edges of the samples. The changes in the AE counts during the different loading phases can be categorized into stabilization, increase, sudden increase, and decay stages. The damage-related variable based on the cumulative AE count revealed that samples treated at and below a temperature of 300 °C were mainly damaged in the late period of loading and exhibited brittle failure. Damage began to accumulate as early as in the middle period of loading in samples treated at temperatures greater than or equal to 600 °C. The results of this study provide a useful reference for mining deeply buried granite under different temperature gradients and fracture modes.

Failure Mechanism and Control Mechanism of Intermittent Jointed Rock Bridge Based on Acoustic Emission (AE) and Digital Image Correlation (DIC).

Deep foundation pit excavation is an important way to develop underground space in congested urban areas. Rock bridges prevent the interconnection of joints and control the deformation and failure of the rock mass caused by excavation for foundation pits. However, few studies have considered the acoustic properties and strain field evolution of rock bridges. To investigate the control mechanisms of rock bridges in intermittent joints, jointed specimens with varying rock bridge length and angle were prepared and subjected to laboratory uniaxial compression tests, employing acoustic emission (AE) and digital image correlation (DIC) techniques. The results indicated a linear and positive correlation between uniaxial compressive strength and length, and a non-linear and negative correlation with angle. Moreover, AE counts and cumulative AE counts increased with loading, suggesting surges due to the propagation and coalescence of wing and macroscopic cracks. Analysis of RA-AF values revealed that shear microcracks dominated the failure, with the ratio of shear microcracks increasing as length decreased and angle increased. Notably, angle exerted a more significant impact on the damage form. As length diminished, the failure plane's transition across the rock bridge shifted from a complex coalescence of shear cracks to a direct merger of only coplanar shear cracks, reducing the number of tensile cracks required for failure initiation. The larger the angle, the higher the degree of coalescence of the rock bridge and, consequently, the fewer tensile cracks required for failure. The decrease of length and the increase of angle make rock mass more fragile. The more inclined the failure mode is to shear failure, the smaller the damage required for failure, and the more prone the areas is to rock mass disaster. These findings can provide theoretical guidance for the deformation and control of deep foundation pits.

Mechanical properties and acoustic emission characteristics of mixed granite after different numbers of freeze‒thaw cycles

The mechanical properties of rocks in cold regions undergo significant changes as a result of decades of freeze‒thaw cycles with seasonal variations, which can lead to a series of geological disasters, such as collapse. This study investigates the evolution of the mechanical characteristics and internal progressive damage characteristics of mixed granite under freeze‒thaw cycling and axial loading. By measuring the mass, wave velocity, and uniaxial compressive strength of rock samples and combining these metrics with acoustic emission (AE) characteristics, the physical and mechanical properties and microfracture development of mixed granite after different numbers of freeze‒thaw cycles were investigated. The results indicate that as the number of freeze‒thaw cycles increases, the longitudinal wave velocity, uniaxial compressive strength, and elastic modulus of the mixed granite decrease nonlinearly, while the peak strain gradually increases. Combined with the stress‒strain curve, the AE characteristics can be divided into four stages. As the number of freeze‒thaw cycles increases, the AE cumulative count decreases, and the AE counts of the four stages are different. The low-frequency-high-amplitude signals first increases and then tends to stabilize, and they only appeared in the third and fourth stages. At the same time, the proportion of the low-frequency ratio gradually increases, and the proportion of the high-frequency ratio decreases. In addition, based on the rise time/amplitude (RA) and average frequency (AF) characteristics and failure modes, it was found that the internal crack types of mixed granite transition from shear cracks to tensile cracks, among which tensile cracks play a crucial role in rock failure.

Fracture response of thermal-treated granite under mode III loading: Through the edge-notched diametrically compressed disk specimen

The deep rock mass is subject to a high degree of coupling of geothermal and complex loading, and complex mechanical and hydraulic loading induces out-of-plane progression of primary fractures, culminating in the formation of pure mode III fractures. To study the impact of high temperatures on the fracture behavior of pure mode III, the edge-notched diametrically compressed (ENDC) granite discs were heat treated at different temperatures (25, 200, 400, 500, and 600 °C). Subsequently, comparative tests were conducted on both pure mode III and pure mode I fracture, using the digital image correlation (DIC) method and the acoustic emission (AE) technique. The results demonstrate that as the temperature increases, the mass and P-wave velocity decrease. Furthermore, under pure mode III loading, the ENDC sample exhibits higher peak loads and fracture toughness (KIIIc) compared to mode I (KIc), which shows a decrease of 60.72 % in KIIIc and 73.75 % in KIc after thermal treatment at 600 °C compared to the ambient conditions. Simultaneously, the elevated temperature augmentes the cumulative AE counts during loading. The variations in horizontal displacement elevation deviation (HDED) on either side of the deformation concentration zone indicate that the samples are more resistant to crack extension under pure mode III loading. In addition, a positive linear correlation is noted between KIIIc and KIc of ENDC disc samples. The relationship between rebound values and fracture toughness of ENDC discs was discussed, and discovered that the KIIIc and KIc values are quadratic with the rebound values tested by the complete disc. The discussed results may serve as an initial prediction of fracture toughness values.

Effect of height-diameter ratio on the mechanical characteristics of shale with different bedding orientations

Geological exploration cores obtained from shale gas wells several kilometers deep often show different height-diameter ratios (H/D) because of complex geological conditions (core disking or developed fractures), which makes further standard specimen preparation for mechanical evaluation of reservoirs difficult. In multi-cluster hydraulic fracturing, shale reservoirs between planes of hydraulic fractures with different lengths could be simplified to have different H/D ratios. Discovering the effect of H/D on the mechanical characteristics of shale specimens with different bedding orientations will support mechanical evaluation tests of reservoirs based on disked geological cores and help to optimize multi-cluster fracturing programs. In this study, we performed uniaxial compression tests and acoustic emission (AE) monitoring on cylindrical Longmaxi shale specimens under five bedding orientations and four H/D ratios. The experimental results showed that both the H/D-dependent mechanical properties and AE parameters demonstrated significant anisotropy. Increasing H/D did not change the uniaxial compressive strength (UCS) evolution versus bedding orientation, demonstrating a V-shaped relationship, but enhanced the curve shape. The stress level of crack damage for the specimens significantly increased with increasing H/D, excluding the specimens with a bedding orientation of 0°. With increasing H/D, the cumulative AE counts of the specimens with each bedding orientation tended to exhibit a stepped jump against the loading time. The proportion of low-average-frequency AE signals (below 100 kHz) in specimens with bedding orientations of 45° and 60° increased to over 70% by increasing H/D, but it only increased to 40% in specimens with bedding orientations of 0°, 30°, and 90°. Finally, an empirical model that can reveal the effect of H/D on anisotropic UCS of shale reservoir was proposed, the anisotropic proportion of tensile and shear failure cracks in specimens under four H/D ratios was classified based on the AE data, and the effect of H/D on the anisotropic crack growth of specimens was discussed.

Experimental Research on Anisotropy Characteristics of Shale under Triaxial Incremental Cyclic Loading and Unloading

Shale is a common rock type that is associated with underground engineering projects, and several important factors, such as bedding structure, confining pressure, and the loading and unloading path, significantly influence the anisotropy of shale. Triaxial monotonic loading tests and triaxial incremental cyclic loading and unloading tests of shale under three kinds of confining pressures and five types of bedding inclination angles (θ) were thus performed to investigate the anisotropy of shale in terms of mechanical behavior, acoustic emission (AE), and energy evolution, and reveal the mechanism by which shale anisotropy is weakened. The results show that (1) the compressive strength and elastic modulus of shale decrease and then increase as the θ increases, and that both σ3 and incremental cyclic loading and unloading reduce the anisotropy in terms of the compressive strength and elastic modulus of shale, with the ratio of plastic strain to total strain reaching its maximum at a θ of 60° during each loading and unloading cycle. (2) The failure modes of shale with θ of 0°, 30°, and 90° under triaxial monotonic loading are similar to the counterparts under triaxial incremental cyclic loading and unloading, while the failure modes of shale with θ of 45° and 60° differ significantly under the two loading conditions, and interestingly, the degree to which the bedding plane participates in shale crack evolution under incremental cyclic loading and unloading is considerably lower than that under triaxial monotonic loading. (3) The cumulative AE count and AE b-value of shale first decrease and then increase as the θ increases, while the Felicity ratio decreases as the number of cycles increases. (4) As the θ increases, the total energy density U0 and the parameter m, which reflects the accumulation rate of elastic energy, first decrease and then increase, with both reaching a minimum at a θ of 60°. (5) The mode by which cyclic loading and unloading leads to failure in shale with a θ of 60° is similar to that at a θ of 0° and is the main mechanism by which shale anisotropy weakening occurs as a result of cyclic loading and unloading. The results provide experimental support and a theoretical basis for safer and more efficient underground engineering projects that involve shale.

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.

Study on Acoustic Emission Characteristics and Damage Evolution Law of Shale under Uniaxial Compression

Investigating the correlation between acoustic emission (AE) parameters and damage mechanisms in rock mechanics can help understand rock damage evolution under loading and provide a theoretical basis for engineering support and safety detection. Therefore, this paper presents experimental works on the correlation between AE and failure mechanisms of rock mass under uniaxial compression stress, with the aim of capturing the damage evolution leading to a new damage constitutive model. The experimental results indicate that the uniaxial compression process of shale can be divided into four stages according to AE characteristics. AE signals are minimal during the crack compaction and elastic stages. The crack initiation strength σci, which is approximately 55% of the uniaxial compressive strength, is identified when the cumulative AE counts and damage factor begin to increase slowly. When axial stress reaches the damage strength σcd, which is approximately 80% of the uniaxial compressive strength, a significant number of AE signals are generated. AE phenomena can be observed during the unstable crack development and post-crack stages. Considering the initial damage to the rock, the damage factor D initially decreases and then increases with increasing cumulative ring-down counts rather than exhibiting a monotonic increase. The damage factor D is proportional to the cumulative AE counts N in the stage before rock failure.

Damage evolution and failure characteristics of a cemented gangue backfill considering direct shear and dynamic tests

Backfill mining is an effective technique that can prevent and control the impact of dynamic disasters which can occur in deep underground coal mines. This technique allows to reduce the surface subsidence and achieve a greener mining. The backfill shear mechanical properties under mining disturbances, are of great significance for ensuring the surrounding rock stability in deep mining. Therefore, studying the shear mechanical properties of the cemented gangue backfill body (CGBB) under coupled dynamic and static loads actions in deep coal mining is of great importance. In this paper, shear mechanical tests and the acoustic emission (AE) technology were used to study the shear mechanical response characteristics of CGBB under low-frequency dynamic normal load conditions. The results show that a dynamic normal load with an amplitude of 5 kN improves the shear CGBB strength, while a dynamic load with an amplitude greater than 10 kN will cause a shear strength deterioration, and the degree of deterioration is positively correlated with the load amplitude. Under low-frequency dynamic normal load conditions, the shear CGBB stress-strain curve exhibits classical characteristics. AE indicate that the main CGBB damage changes from a nonlinear deformation to an elastic stage under low-frequency dynamic normal load conditions. During the shear failure process, the cumulative AE counts are positively correlated with the load amplitude. The crack type identification based on RA-AF shows that the low-frequency dynamic normal load affects the CGBB crack type. Under constant normal load (CNL) conditions, tensile or mixed cracks account for 63.95% and are the main fracture mode. Conversely, under low-frequency dynamic normal load conditions, shear cracks account for more than 60% and are the main failure mode. This research provides a reference for the CGBB stability analysis under complex stress conditions for the case of a deep underground coal mining.

Experimental study on the strain energy evolution mechanism of thick and hard sandstone roof in Xinjiang Mining Area

For understanding the mechanical performance and strain energy evolution mechanism of thick hard roof sandstone samples, a sequence of uniaxial compression trials with acoustic emission (AE) monitoring were carried out. The results indicate: (1) The stress-strain curve of the thick hard roof sandstone specimens exhibits distinct stage characteristics. Based on the evolvement of instantaneous axial stiffness, it is separated into Fracture Closure Phase, Elastic Deformation Phase, Steady Fracture Expansion Phase, Unsteady Fracture Expansion Phase, and Post-Peak Phase. (2) The AE energy and cumulative count curves of the thick hard roof sandstone specimens also exhibit significant stage characteristics and can be mutually corroborated with the stage division of the stress-strain curve. (3) Based on the energy conservation principle, the evolution of strain energy density in the thick hard roof sandstone specimens under uniaxial compression loading was analyzed, and plastic strain energy increment was employed to study the stage characteristics of strain energy dissipation. (4) A damage constitutive model for the thick hard roof sandstone specimens was constructed, considering the characteristics of strain energy dissipation. This model effectively describes the stress-strain relationship among the samples, which undergo strain hardening, strain softening, and sudden destruction.