Acoustic Emission Count Research Articles

Experimental study on mechanical and fracture characteristics of inclined weak-filled rough joint rock-like specimens

The uniaxial compression experiments with acoustic emission (AE) system are conducted to explore the coupling effect of the JRC and dip angle (α) on the mechanical and fracture characteristics of rock-like specimens containing weak-filled rough joints (WFRJ). With the increasing α, the variation of mechanical properties and the percentage of tensile cracks shows a trend of decreasing and then increasing; the variation of accumulated AE count shows a trend of increasing first, then decreasing, and then increasing again. The increasing JRC weakens the influence of α on mechanical properties but strengthens AE characteristics and the percentage of tensile cracks. The action mechanism of JRC for mechanical properties and AE characteristics is mainly reflected in the waviness and unevenness for α = 27˚ − 60˚ but only in the waviness for α = 0˚ − 90˚. The fracture pattern of WFRJ specimens is strongly related to the waviness of the joint profile. The weak-filled layer is usually broken near the inflection point. Then, the fracture of WFL induces the tensile fracture for α = 0˚ − 45˚ but shear fracture for α = 90˚ in the hard matrix from the inflection point. For α = 67˚, the fracture pattern is almost unaffected by the variation of JRC. Finally, the action mechanism of unevenness and waviness of joint profile on mechanical properties, AE characteristics and fracture pattern is analyzed.

Gas fracturing behavior and breakdown pressure prediction model for granite under different confining pressure and injection rate

Conventional hydraulic fracturing techniques are often found problematic for extracting geothermal energy in hot dry rock (HDR). As an alternative, employing the less viscous gas to replace water as the fracturing fluid showed great potential for more effective fracturing of HDR. In this work, the failure behavior and mechanism of granite during gas fracturing under different confining pressures and gas injection rates are comprehensively examined. It is shown that the breakdown pressure increases with the increase of confining pressure, whereas higher gas injection rate can result in evident decrease of the breakdown pressure. As the confining pressure grows, the acoustic emission (AE) event increases rapidly, with much higher AE counts observed at high gas injection rates than at low injection rates. Comparatively, the AE energy decreases under high confining pressure, due probably to granite transitioning from brittle to ductile. It is interesting that the b-value of AE varies dramatically as the gas injection rate becomes higher with significant fluctuations, indicating the ratio of large fracture and small fracture changes drastically during gas fracturing. In addition, the length of the induced fractures decreases with the increase of confining pressure during gas fracturing, and the length and width of vertical fractures are evidently larger when at high gas injection rate. Last, a novel theoretical predictive model is proposed for estimating breakdown pressure during gas fracturing based on the average tensile stress criteria, which is featured by considering the effect of confining pressure and gas flow behaviors. The theoretical prediction agrees with the experimental results. The present study can provide valuable results for theoretical analysis and engineering applications of gas fracturing in stimulating the HDR reservoirs.

The influence of water on the failure characteristics of sandstone under uniaxial compression conditions by acoustic emission and NMR observation

Revealing the failure characteristics and damage process of rocks soaked in water is particularly critical for preventing and controlling geological disasters. To this end, we employed nuclear magnetic resonance (NMR) and an ultrasonic system to capture the variation in the T2 spectrum, different pore waters and P-wave velocity during soaking (from 0 to 120 h). Furthermore, an acoustic emission (AE)-uniaxial compression test was carried out to analyse the AE characteristics (e.g., hits, absolute energy, and events), and strength of sandstone under different soaking times. The AF (the ratio of AE counts to duration)-RA (the ratio of the AE rise time to amplitude) is used to distinguish the tensile failure shear failure of sandstone during soaking. The results showed that the increase in soaking time decreased the bound water percentage and strength while increasing the P-wave velocity and capillary water percentage. An interesting phenomenon was that the failure mode was transformed due to soaking based on the evolution of the AE response. From the above results and classical water-weakening mechanisms, we suggest that the weakening process after loading can be divided into three stages. In the initially soaking, the reduction in fracture toughness and frictional more serious caused the increase in the occurrence of tensile failure. The quartz hydrolysis and chemical deterioration may aggravate the shear failure and intergranular fracture during long-term soaking. This study clarifies the transformation of weakening mechanisms during soaking and is of great significance for understanding the microscopic damage processes in rock failure.

Deterioration of mudstone exposed to cyclic hygrothermal conditions based on thermoacoustic emission

The periodic effect of an alternating hygrothermal environment makes the microdamage of rock gradually accumulate, thus leading to significant structural damage. The study of the progressive deterioration of mudstone exposed to cyclic hygrothermal conditions is of guiding significance to the stability of bank slopes. In this paper, purple mudstone samples from the Three Gorges Reservoir area were collected for laboratory cyclic hygrothermal experiments. Thermoacoustic emission tests of mudstone samples with different water absorptions under multiple hygrothermal cycles were carried out, and the influence of water content on the macroscopic surface damage and acoustic emission (AE) characteristics of mudstone was analyzed. The results show that the AE counts of mudstone with various water absorptions during heating can be divided into three typical stages of AE activity: active, intense and quiet. The active period of AE activity corresponds to mudstone surface temperatures of 35–50 ℃, which is considered to be the temperature range most likely to induce damage: fracturing, spalling and disintegration. The gradual increase in water absorption of mudstone leads to a decrease in the cumulative AE count during heating as the number of hygrothermal cycles increases, indicating that more clay minerals in the mudstone absorb water and undergo inhomogeneous deformation during each hygrothermal cycle, so that a large number of microcracks or even a network of cracks have been developed in the process of water absorption. This understanding of the deterioration of mudstone provides evidence for the assessment of mudstone strata stability.

Acoustic emission monitoring and analyses for avalanche release and slab fracturing events observed in Great Himalaya

ABSTRACT The acoustic emission (AE) monitoring of failures, fractures and instabilities evolving within a snowpack may provide vital information about the avalanche formation preceding release processes. In this work, AE activity was collected continuously over five years from the snowpack built on avalanche starting zone in Great Himalaya, and the analyses are presented in the context of an avalanche release and two slab fracturing events detected successfully during this study period. For non-invasive detection of AE activity from snow, a specialised sensor-arrestor array was deployed over the avalanche slope. In our investigations, an abnormal AE activity was observed both during the slab fracturing as well as the avalanche occurrence periods. For AE-based direct assessment of snow instability, a method is proposed to estimate instability index (βin), based on AE count and amplitude values of a hit, and it is applied for analyses of avalanche formation preceding release and slab fracturing cases. The prominent AE attributes including βin-variations are also analysed at appropriate window scales for a smooth representation of the ongoing instability and failure processes underneath the slab. The AE spectral analyses of avalanche release and slab fracturing reveal the dominant frequencies around 45–47 kHz and 34–43 kHz, respectively. A progressively increasing behaviour was observed with an abrupt increase in the relevant AE parameters and also the βin-values, nearly 13 hours before the avalanche release, and it continued until the avalanche released which can be associated to the instability development prior to avalanche release.

Statistical analysis on the fracture behavior of rubberized steel fiber reinforced recycled aggregate concrete based on acoustic emission

Rubberized steel fiber reinforced recycled aggregate concrete (RSRAC) is an environmental-friendly building material produced from waste concrete and tires. However, the evolution of fracture behavior in RSRAC under flexural loads still remains unknown, and the individual as well as the coupling effects of recycled aggregate (RA), crumb rubber, and steel fiber are also unclear. In this paper, four groups of specimens with different contents of RA, crumb rubber, and steel fiber were prepared, and a series of four-point bending tests were conducted. The loading process was monitored by acoustic emission (AE) in real-time, and the fracture behavior was then analyzed through AE signals using multiple statistical methods including Ib-value method, cumulative energy analysis, intensity analysis, K-means clustering and RA-AF analysis. Scanning electron microscope (SEM) was also employed to give explanations in microscale. Results show that the fracture process of RSRAC can be divided into micro-crack stage, main collapse stage, and residual stage. A fluctuation greater than 0.06 in Ib-value could be regarded as a criterion for this division. AE counts, cumulative AE energy as well as cluster analysis are also effective in judging the sub-stages. The dominant failure mode in micro-crack stage and residual stage is tensile crack, and shear cracks mainly occur in the main collapse stage. SEM investigation has revealed that the shear cracks in the main collapse stage mainly come from the shear failure of acicular crystals or micro-chimeras at the interface. For effects of different materials, it is found that rubber can reduce the brittleness and improve the ductility of the concrete, but will lower the strength properties. Steel fiber can increase the brittleness of the fracture behavior and reduce the ductility, but is favorable to strength. While RA can increase the brittleness and is harmful to both ductility and strength. The coexistence of rubber and steel fiber can significantly shorten the residual stage. The work is expected to provide a deeper understanding on the properties of RSRAC.

Experimental study on instability failure of rock mass with rough joints under one-side constraint compression

The development of rough joints parallel to the free surface in the surrounding rock will lead to instability and failure of the surrounding rock, and the occurrence of rockburst, plate cracking, and other disasters. This paper used a custom-made mold, 3D-printing technology, acoustic emission (AE) equipment, and digital image correlation (DIC) to systematically analyze the fracture characteristics and instability process of rock sample with different roughness joints under one-side constraint compression. The results showed that, in the same sample group, the peak stress gradually increases with the increase in the distance between the joint and the free surface, and that the peak stress and strain at the same distance of the joint will decrease first and then increase as the roughness increases. The energy storage limit of the sample with joints 10 mm away from the free surface was greater than that for 5 mm, the energy dissipation ratio before the peak strength of specimens decreases as the roughness increases. The smooth-jointed specimens have higher resistance to deformation under one-side constraint compression, and the failure pattern of that specimen can be tensile type. However, as roughness increases, shear cracks gradually appear along the joint convex position, and the damage degree will be increased. The closer the joint is to the free surface, the more broken the specimen will be. The AE events within the same sample group increased with the increase in the distance between the joint and the free surface, and the cumulative AE counts increased with the increase in roughness when the joint space position was the same.

Examination of the Effects of Different Frequencies on Rock Fracturing via Laboratory-Scale Variable Amplitude Fatigue Loading Experiments

In this study, the characteristics of the fracture evolution of argillaceous shale under increasing-amplitude loading were investigated. The GCTS RTR-2000 test system and in-situ acoustic emission (AE) monitoring were employed to execute the tests. The following results were observed. (1) The strength, deformation, and fatigue life increased with the frequency, and the morphology of the hysteresis curve changed regularly with time. (2) The cumulative damage of the rock at the location in which the stress amplitude suddenly increased exceeded that at the fatigue loading stage. The AE count and AE energy were affected by the loading frequency. (3) The secant modulus exhibited different values for different loading frequencies; the smaller the loading frequency, the fewer loading stages the samples experienced, and the faster the secant modulus decreased. The change in Poisson’s ratio over the entire process was composed of a steady growth stage and a rapid growth stage. (4) The rock exhibited two stages of damage evolution, with rapid damage accumulation occurring at the beginning of the loading and relatively smooth damage occurring thereafter. This study developed a cumulative fatigue damage model that can adequately fit the accumulated damage during the fracturing process. The experiments revealed that variable amplitude fatigue loading at different frequencies significantly influences the damage deterioration and the failure law of the rock. The results are expected to improve the understanding of the frequency effect on the fracture behavior and help predict the lifespan of rock structures. This is of great significance to the promotion of slope management, landslide disaster prevention, and mine reuse at the West Open-pit Mine.

Study on Crack Propagation of Rock Bridge in Rock-like Material with Fractures under Compression Loading with Sudden Change Rate

In order to study the influence of sudden change of loading rate on crack propagation and failure mode of rock bridge of fractured rock mass, the specimens used in this paper were rock-like materials containing two prefabricated fractures. The mechanical properties and the failure mode of the specimens under different constant loading rates and sudden changes in loading rate were tested. The photographic monitoring and acoustic emission were carried out at the same time. The results show that: (1) In the process of sudden change loading, the specimen shows the characteristics of approximate elastic deformation, and the stress-strain curve after sudden loading is similar to the corresponding stress-strain curve at the corresponding constant loading rate. (2) Combined with acoustic emission detection, it is found that when the loading rate is not much different before the mutation, the loading rates is the same after the mutation and the stress-strain curve of the specimen is similar. In the low-speed loading stage, the acoustic emission count is generally low, while in the high-speed loading stage, the acoustic emission count is generally high. The sudden change from low-speed loading to high-speed loading easily induces stress drop, resulting in crack generation and increase of acoustic emission count. (3) The rate of the specimen during the crack development period plays a decisive role in the failure mode of the specimen. Before the sudden change of loading rate, the low-speed loading within a certain range has little effect on the specimen. When the high-speed loading is carried out when the stress is low in the early stage of the mutation, the acoustic emission count of the specimen is high, which will cause some damage inside the specimen. As a result, even if the rate is the same after the mutation, the final peak stress and failure mode may be different.

Acoustic Emission Characterization Analysis of Quasi-Static and Fatigue Compression Properties of Aluminum Foam

In order to explore the evolution of physical and mechanical properties and acoustic emission (AE) characteristics of aluminum foam under fatigue and quasi-static compression from a microscopic point of view, the AE monitoring technology was used to analyze the deformation, hardening, and energy absorption characteristics of open-cell aluminum foam under quasi-static compression at different rates (2, 10 and 50 mm/min) and fatigue loading tests with different peak stress ratios k (k = maximum stress/yield stress) by means of MTS fatigue testing machine and CCD camera. The results indicated that under different compression rates, the AE ring down count had the same trend as the engineering stress–strain response of the specimens, the AE ring down count rate at the plastic deformation stage showed the same performance as the work hardening rate, and the AE energy absorption efficiency corresponded well to the experimental results. The specimen entered the densification stage with the stability of AE count and the decrease in energy absorption efficiency. During the fatigue tests of different k values, the change trend of strain was consistent with the response of acoustic emission characteristic parameters, and the fatigue compression damage caused by the deformation process of the specimen can be monitored by the change in AE characteristics. The AE characteristics can dynamically monitor the compression process and provide a new research method and idea for the study of mechanical properties of aluminum foam.

Analysis of damage characteristics of steel fiber-reinforced concrete based on acoustic emission

Steel fiber-reinforced concrete (SFRC) has been widely used as a building material worldwide. The macroscopic destruction of this material is a manifestation of the internal microdamage evolution. Hence, investigating the internal damage evolution process in concrete for monitoring in-service concrete has considerable significance in terms of safety. This study investigates the damage characteristics of SFRC with different steel fiber contents (0, 15, 30, 45, and 60 kg/m3) using Brazilian disk splitting tests and acoustic emission (AE) techniques. The changing trend of the characteristic parameters (counts and energy, respectively) of AE signals throughout the failure process of concrete is analyzed. Then, the moment tensor method is used to determine the temporal and spatial evolution of microcracks in SFRC. Research results show that the AE characteristic parameters are closely related to the internal damage of concrete specimens, and show different characteristics of AE signals in different failure stages. When the matrix cracks, the AE counts become denser, the values increase sharply, and the energy is closely related to fiber content (the lower the fiber content, the higher the energy value, and the minimum AE energy is 1.80e5 aJ (in SFRC60)). According to the moment tensor method, the cumulative microcracks are first appear to near the central axis and then dispersed to the surrounding area in the SFRC with the effect of steel fiber, effectively weakening the local damage. Based on the R-value method, the proportion analysis results of microcracks show that tensile damage is the primary destruction mechanism throughout the failure process of the Brazilian disk. The minimum proportion for tensile microcracks is 56.67%.

Experimental Study on Cyclic Hydraulic Fracturing of Tight Sandstone under In-Situ Stress

Sandstone oil–gas reservoirs in the Junggar Basin, China have great development potential. However, their ultra-deep formation depth leads to high crustal stress and high breakdown pressure. Therefore, in this research, we studied the cyclic hydraulic fracturing of tight sandstone with different combinations of “high-pressure duration + low-pressure duration” under high-stress conditions. Through laboratory experiments, the pump pressure curves, hydraulic fracture morphology, acoustic emission counts, and peak frequency of the samples were obtained. The results showed that: (1) Compared with conventional hydraulic fracturing, the breakdown pressure of cyclic hydraulic fracturing was reduced by more than 30%, the minimum threshold of cyclic pump pressure required for sample breakdown was between 60%Pb and 70%Pb, and cyclic hydraulic fracturing more easily formed complex and diverse hydraulic fractures. (2) In cyclic hydraulic fracturing, under the same upper limit of cyclic pump pressure, the shorter the high-pressure duration, the fewer the cycles required for sample breakdown. (3) Under the same “high-pressure duration + low-pressure duration” condition, the lower the upper limit of the cyclic pump pressure, and the greater the number of cycles required for sample breakdown. (4) The AE cumulative counts curves fluctuated greatly during cyclic hydraulic fracturing, rising in an obvious step-wise manner and the AE peak frequency was banded and mainly divided into three parts: low frequency, medium frequency, and high frequency.

Prediction of Coal Dilatancy Point Using Acoustic Emission Characteristics: Insight Experimental and Artificial Intelligence Approaches

This research offers a combination of experimental and artificial approaches to estimate the dilatancy point under different coal conditions and develop an early warning system. The effect of water content on dilatancy point was investigated under uniaxial loading in three distinct states of coal: dry, natural, and water-saturated. Results showed that the stiffness-stress curve of coal in different states was affected differently at various stages of the process. Crack closure stages and the propagation of unstable cracks were accelerated by water. However, the water slowed the elastic deformation and the propagation of stable cracks. The peak strength, dilatancy stress, elastic modulus, and peak stress of natural and water-saturated coal were less than those of dry. An index that determines the dilatancy point was derived from the absolute strain energy rate. It was discovered that the crack initiation point and dilatancy point decreased with the increase in acoustic emission (AE) count. AE counts were utilized in artificial neural networks, random forest, and k-nearest neighbor approaches for predicting the dilatancy point. A comparison of the evaluation index revealed that artificial neural networks prediction was superior to others. The findings of this study may be valuable for predicting early failures in rock engineering.

Study on the Variation Laws and Fractal Characteristics of Acoustic Emission during Coal Spontaneous Combustion

Acoustic emission (AE) technology has the advantage of online localization to study the change laws of AE in the process of coal spontaneous combustion and to reveal the generation mechanisms of AE signal during the process of heating and rupture of coal body from a microscopic perspective. This paper first constructs a large-scale coal spontaneous combustion AE test system and conducts experimental tests on the AE signal in the process of coal spontaneous combustion. The results show that with the increase of temperature in the process of coal spontaneous combustion, the AE signal shows a trend of increasing fluctuations. Low-temperature nitrogen adsorption experiments studied the pore structure of coal spontaneous combustion, and the results showed a correspondence between the development of pores and the temperature of coal spontaneous combustion. Further, through the analysis of the evolution of the pore structure of coal through Fourier transform and fractal theory, it is found that the high-frequency main frequency AE signal and average frequency are continuously enhanced with the increase of temperature. The fractal dimension of the pore structure and the fractal dimension of the AE count of the coal body first rise and then decline. The mechanism of coal spontaneous combustion AE of coal is revealed, and the pore development caused by thermal stress when coal heats up is the main source of AE signal generation. The research in this paper is of great significance for applying AE technology to detect the position of coal spontaneous combustion.

Flexural behavior of seawater-mixed steel fiber reinforced concrete exposed to simulated marine environments

The application of seawater-mixed concrete can effectively reduce the use of the freshwater for concrete production. Moreover, the steel fibers added into concrete matrix can improve its mechanical properties. This paper investigated the flexural behavior of seawater-mixed steel fiber reinforced concrete (SFRC) exposed to simulated marine environments for 1 year using four-point bending tests. The influences of the type of mixing water, the type of exposure environment and the degree of pre-cracking damage on the flexural performance of seawater-mixed SFRC were mainly studied. Meanwhile, the acoustic emission (AE) and digital image correlation (DIC) techniques were utilized to monitor the characteristics of the damage in concrete and the evolution of the cracks on the concrete surface. Results showed that there were many rust spots on the surface of seawater-mixed SFRC specimens after 1 year of chloride attack, especially for cracked specimens, and the moderate steel fiber corrosion was only found near the cracks of the cracked SFRC specimens with an average crack width of 0.46 mm. The seawater-mixed SFRC specimens and freshwater-mixed SFRC specimens exposed to 7% NaCl solution for 1 year showed slight decreases in their flexural strength and toughness compared with unexposed specimens, respectively. However, the seawater-mixed SFRC exposed to freshwater environments for 1 year under wet-dry cycle treatments and the cracked seawater-mixed SFRC with an average crack width of 0.46 mm exposed to chloride environments for 1 year showed obvious deterioration, respectively. It was noted from AE and DIC results that the AE counts of different types of concrete specimens indicated easily distinguishable change characteristics and the maximum strain at the main crack of cracked seawater-mixed SFRC was larger due to serious damage after exposure for 1 year. The results of AE and DIC had a good correspondence with the variation trend of load–deflection curves.

Predicting the failure of different rocks subjected to freeze-thaw weathering using critical slowing down theory on acoustic emission characteristics

Freeze-thaw weathering effectively deteriorates the physical and mechanical properties of rocks and seriously threatens the stability of engineering projects in cold regions. This study investigates the influence of freeze-thaw action on mechanical behaviors and failure prediction of different rocks i.e., sandstone, marble, and granite. The rock samples were subjected to a quasistatic compressive test, and a real-time acoustic emission (AE) monitoring technique was used to reveal the damage characteristics of rocks treated with 0, and 25 freeze-thaw cycles. The autocorrelation coefficient (AC) and variance of the AE count were then calculated by applying the Critical Slowing Down (CSD) theory, and the precursory features of rocks failure under the action of freeze-thaw were examined. The results unveiled that after the freeze-thaw action, the uniaxial compressive strength (UCS) of sandstone, marble, and granite was reduced by 16.5%, 8.36%, and 7.75% respectively. The AE count and the accumulative count correspond effectively to the failure of different rocks under compressive loading. Even though the samples have shown the same variation for AE count and accumulative count but those subjected to freeze-thaw weathering have shown an earlier rise in both AE counts and accumulative counts. It was noted that the AE signals had a CSD phenomenon. The sudden and significant increase in AC and variance curves of AE count, RA, and AF parameters can be used to predict rock failure. Furthermore, the precursory characteristics of different rocks treated with freeze-thaw weathering were compared. The ratios of the early warning to the main fracture and complete failure in terms of time are the highest for sandstone followed by granite, and marble. Moreover, the precursory points of treated samples were earlier than those of untreated samples. It was concluded from the results that AE counts' early warning precursory characteristics are more noticeable than other AE parameters, especially in the variance curves. Hence, the results of this study will provide significant importance to predicting rock failure in cold regions.

Damage evolution characteristics and deterioration mechanism of mechanical properties of sandstone subjected to drying-wetting cycles

The dams formed by rock pillars around underground reservoirs are frequently subjected to drying-wetting cycles, so that rock pillars may be deformed, broken and develop a large number of fractures. This further causes a massive loss of water and heavily affects the stability of the dam formed by rock pillars. To explore the damage evolution characteristics and the deterioration mechanism of mechanical properties of sandstone under drying-wetting cycles, mechanical tests were carried out on sandstone samples subjected to different numbers of drying-wetting cycles. Acoustic emission technology was used to real-timely monitor the failure process of sandstone under loading. At the same time, damaged rock samples were observed under a scanning electron microscope. On this basis, the development characteristics of fractures in the sandstone were analyzed, damage evolution of sandstone in different stress loading stages was expounded. Research results show that the more the drying-wetting cycles are, the more obvious the compaction stage and the more significant the deterioration of the peak strength and elastic modulus, accompanied by the gradual decrease of the strain energy and cumulative acoustic emission count. According to scanning electron microscope observations, fractures inside the sandstone increase in quantity and begin to coalesce after 10 drying-wetting cycles, so that loose particles and fracture degree gradually rise, and accumulation of damage causes mechanical properties of sandstone to gradually deteriorate. The research results provide a solid theoretical basis for stability evaluation and protection of dams formed by rock pillars around underground reservoirs in goaf.

Strength Damage and Acoustic Emission Characteristics of Water-Bearing Coal Pillar Dam Samples from Shangwan Mine, China

Long-term erosion and repeated scouring of water significantly affect the technical properties of coals, which are the essential elements that must be considered in evaluating an underground reservoir coal column dam’s standing sustainability. In the paper, the coal pillar dam body of the 22 layers of coal in the Shangwan Coal Mine is studied (22 represents No. 2 coal seam), and the water content of this coal pillar dam body is simplified into two types of different water content and dry–wet cycle. Through acoustic emission detection technology and energy dissipation analysis method, the internal failure mechanism of coal water action is analyzed. This study revealed three findings. (1) The crest pressure, strain, and resilient modulus in the coal sample were inversely related to the water content along with the dry–wet cycle number, while the drying–wetting cycle process had a certain time effect on the failure to the sample. (2) As the moisture content and the dry–wet cycle times incremented, three features were shown: first, the breakage pattern is the mainly stretching fracture for the coal specimen; second, the number and absolute value of acoustic emission count peaks decrease; third, the RA-AF probability density plot (RA is the ratio of AE Risetime and Amplitude, and AF is the ratio of AE Count and Duration) corresponds more closely to the large-scale destruction characteristics for the coal samples. (3) A higher quantity of wet and dry cycles results in a smoother energy dissipation curve in the compacted and flexible phases of the crack, indicating that this energy is released earlier. The research results can be applied to the long-term sustainability assessment of the dams of coal columns for underground reservoirs and can also serve as valuable content to the excogitation of water-bearing coal column dams under similar engineering conditions.

Multi-dimensional non-uniform deformation and failure of siltstone determined using acoustic, 3D-digital image correlation, and computed tomography

Uniaxial compression tests monitored using linear variable differential transformers (LVDTs), digital image correlation (DIC), computed tomography (CT), acoustic emission (AE), and ultrasonic velocity were utilized to study the multi-dimensional non-uniform deformation and failure characteristics of siltstone. Under one-dimensional (1D) conditions, the characteristic stress of the siltstone was determined by the AE counts, ultrasonic velocity, and volumetric strain measured by the extensometer. Under two-dimensional (2D) conditions, zonal disintegration was observed on the apparent fracture of the siltstone specimen. The crack nucleation process, stress release characteristics, and crack type were analyzed in the “broken area,” “penetrating crack area,” and “non-penetrating crack area” under different characteristic stress states. Subsequently, the evolution of the non-uniformity coefficient of the 2D strain field with the 1D characteristic stress was established. Under three-dimensional (3D) conditions, the fracture size of the siltstone was divided according to the equivalent length of fracture volume, and the spatial distributions of macro-, meso-, and microfractures were analyzed. The siltstone 3D zonal disintegration phenomenon was further quantitatively described by calculating the fractal dimensions of fractures in different radius ranges. Finally, the AE source localization method was improved by modifying the wave velocity. The transformation of the primary and secondary bearing areas in different stress stages was revealed based on the spatio-temporal evolution of the AE sources with different energy levels.

Multiscale study of the effect of fly ash geopolymer on the fatigue cracking of cement stabilized macadam

Understanding of the correlation between the micro-composition and macro-mechanics of cement stabilized macadam (CSM) is crucial to an optimized design of cement-treated base (CTB) with improved crack resistance. In this study, acoustic emission (AE) technique was used to monitor the crack propagation of plain cement stabilized macadam (P-CSM) and fly ash geopolymer cement stabilized macadam (FAG-CSM) under cyclic loading. Experimental tests and characterizations, including such as nanoindentation, Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS), were further performed. The test results show that FAG-CSM has better fatigue performance than that of P-CSM; the AE counts, risetime/AE amplitude values (RA) and b-value can be used to assess the damage degree, cracking mode as well as the bulk modulus of cement composites. Interfacial transition zone (ITZ) is the weakest modulus area in CSM and the incorporation of fly FAG can effectively increase the indentation modulus of ITZ. The volcanic ash reaction of fly ash leads to the hydration of cement to produce more hydration products, which reduce the voidance within ITZ and thus improve the bonding between the aggregate and mortar, which supports the findings of the macro-mechanical analysis and AE monitoring of CSM.