Acoustic Emission Count Research Articles

Mechanical behavior and fracture mechanism of red-bed mudstone under varied dry-wet cycling and prefabricated fracture planes with different loading angles

Red-bed mudstone is a common geological material in engineering construction in the Southwest region in China. Its initial fractures are easily influenced by the combined effects of dry-wet cycling and external loads. Therefore, evaluating the mechanical behavior and fracture mechanism of red-bed mudstone under dry-wet cycling and prefabricated fracture planes with different loading angles is of great significance. In this study, the cracked straight through Brazilian disc (CSTBD) specimens with a prefabricated crack were tested using an RMT testing machine. Multiple-scale analysis methods, including digital image correlation (DIC) and acoustic emission (AE) techniques, 3D scanning, and scanning electron microscopy (SEM), were employed to comprehensively analyze the mechanical evolution and fracture damage mechanism of mudstone under dry-wet cycling. The experimental results indicate that as the loading angle increases, the peak load gradually increases, while an increase in the number of dry-wet cycles leads to a decrease in peak load. At the same time, dry-wet cycling causes the expansion and connection of initial microcracks, forming weak layers that affect the morphology of the main crack and the distribution of secondary cracks. Furthermore, with an increasing number of dry-wet cycles, the acoustic emission count significantly decreases, and the relative high value range of acoustic emission count expands. 3D scanning results show that an increase in the number of dry-wet cycles results in rougher fracture planes, while an increase in loading angle generally increases the roughness of the fracture planes. Additionally, SEM observations reveal that with an increasing number of dry-wet cycles, intragranular fracture patterns and microcrack distribution become more widespread, and tensile and shear fracture patterns are also observed in the center of the fracture plane. These research findings have important practical value for evaluating the stability and reliability of red-bed mudstone under dry-wet cycling and prefabricated fracture planes with different loading angles.

Evaluating Fractal Damage and Acoustic Emissions of Soft Rock–Coal Combinations in a Deep Mining Area

Weakly cemented soft rock mines in the Ordos Basin are susceptible to mining disasters, including roof collapse and substantial deformation of surrounding rocks, during coal mining operations. Researching the damage characteristics of structures composed of low-strength “soft rock–coal” combinations is crucial for effectively preventing and controlling disasters in deep soft rock mining. To investigate the fractal damage characteristics of soft rock–coal combinations with different height ratios, uniaxial compression tests were conducted on specimens containing soft rock percentages of 20%, 40%, 50%, 60%, and 80%. The results show that the uniaxial compressive strength and modulus of elasticity of the soft rock–coal combinations increased with increasing proportions of soft rock. The soft rock–coal combination was clearly segmented, and the 40%, 50%, and 60% soft rock–coal combinations had good self-similarity. The fractal dimensions were 2.374, 2.508 and 2.586, which are all within the interval [2, 3]. When the percentage of soft rock was 20%, the specimen damage yielded flaky coal bodies with smaller grain size, whereas the coal–rock interface was spalled by small conical rock bodies. As the soft rock proportion increased, the percentage mass of fragments with particle size greater than 20 mm increased from 83.34% to 94.15%. The failure mode in soft rock–coal combinations is primarily attributed to the partial tensile splitting of the coal body. As the proportion of soft rock increased, there was a gradual reduction in the extent of coal body damage. Moreover, the acoustic emission absolute energies and counts decreased as the proportion of soft rock increased. The acoustic emission energy was reduced from 2.46 × 109 attoJ to 3.41 × 108 attoJ, and the acoustic emission counts were reduced from 18,276 to 7852.

Research on Damage Mechanism and Mechanical Characteristics of Coal Rock under Water Immersion

This study aims to reveal the impact of immersion duration on the internal structural damage and mechanical property degradation of coal rocks. Coal rocks from the post-mining area of Liangshuijing Coal Mine were selected as the research subject. Physical and mechanical tests were carried out on these with different immersion durations (0 d, 15 d, 30 d, 60 d, 120 d, and 240 d) using scanning electron microscopy (SEM), nuclear magnetic resonance (NMR), acoustic emission (AE), digital image correlation (DIC), and compression testing, further elucidating the damage degradation mechanisms of water-immersed coal rocks. The research demonstrates that changes in the pore structure of coal rocks can be divided into two stages as the soaking time varies: the stage of water swelling (saturation process) and the stage of soaking damage (long-term immersion process). The water swelling stage of coal rock extends from surface drying and contraction to water swelling, and the soaking damage stage of coal rock extends from expansion to soaking damage. During the stage of soaking damage, the water showed dynamic changes from macropores to mesopores to micropores, with a gradual increase in the number of micropores. The AE count and cumulative count of coal rock decrease first and then increase, and the four stages’ acoustic characteristics and macroscopic characterization phenomena appear. The mechanical properties declined. After 240 d of immersion, the uniaxial compressive strength and elastic modulus decreased by 48.93% and 29.53%, respectively, and the plastic characteristics were enhanced. These research results provide a beneficial reference for understanding and predicting the instability and destruction of water-immersed coal rocks.

Influence of freeze–thaw cycles on mechanical properties of moraine soils

The changes of mechanical properties of the soil caused by freeze–thaw cycles pose significant threats to the transportation systems that pass through the distribution area of moraine deposits. In order to study the mechanical mechanism and deformation characteristics of moraine soils, this study carried out a series of uniaxial compression tests, and the effects of different freeze–thaw cycles (0, 1, 3, 6 and 10), rock content (19.8 %, 41.9 %, 53.9 %) as well as water content (6 %, 9 %, 12 %, 15 %, 18 %) were considered. Besides, acoustic emission (AE) was utilized here to analyze the samples deformation response after the freeze–thaw cycle. Research results indicate that the stress–strain curves of the moraine soils exhibited four deformation stages. The AE counts of the samples were closely related to the rock content. After the freeze–thaw cycle, the AE signals of the samples with low rock content were more active in the compression stage and the linear elastic stage. However, there is no significant change in the AE counts of the samples with high rock content. Based on the fracture morphology, the failure modes of moraine soil could be divided into four types: oblique shear failure, splitting failure, bifurcation failure and bulging failure. After undergoing freeze–thaw cycles, the mechanical strength of the moraine soils is continuously reduced, and the reduction is fastest under the optimal water content for the uniaxial compressive strength (UCS). In addition, by orthogonal analysis, it was found that the most critical factors affecting the elastic modulus and UCS were water content and rock content, respectively. Studying the mechanical properties and failure mechanisms of moraine soils with different rock and water content after freeze–thaw cycles can guide the protection measures design for moraine soil slopes under different conditions.

Acoustic-Gas Coupling Response Law in the Whole Process of Coal and Gas Outburst

The intensification of the global energy crisis has led to an increasing demand for coal. China is a major coal-producing country in the world and also the country with the most severe coal and gas outburst disasters. Thus, the coal and gas outburst experiment was conducted, and the following results were obtained: the whole outburst process was divided into three stages, namely the outburst preparation stage, the outburst gestation stage, and the outburst development stage. The gas pressure and acoustic emission signals show significant changes in all three stages, while the variation patterns are different. The gas pressure changes were strongest and the acoustic emission signals were highest during the development stage. Therefore, the outburst development stage was further subdivided into four phases, and the correlation between acoustic emission and gas pressure in each phase was analyzed in detail. Furthermore, the acoustic emission signals in three stages were compared and analyzed. The peak values of acoustic emission count and energy reached 285 times·s−1 and 245 V in the preparation stage and reached 265 times·s−1 and 231 V in the gestation stage, respectively, only 1.66%~1.78% and 2.19%~2.32% of the development stage, namely 15,980 times·s−1 and 10,566 V. Moreover, it was found that the cumulative count and cumulative energy showed a parabolic relationship with the development time of the outburst. Based on the above experimental results, during the production process in coal mines, the dangerous state of outbursts can be monitored through gas pressure changes in the outburst preparation stage and gestation stage. Once in the development stage, more sensitive signals of acoustic emission and their fitting results are used for outburst hazard monitoring and early warning. Monitoring and warning of outbursts of combined gas pressure and acoustic emission signals can effectively improve the safety level of coal mine production.

Aging deterioration of mechanical properties on coal-rock combinations considering hydro-chemical corrosion

Coal-rock masses located in water-rich environments, such as heavy water mines, and water-isolated coal-rock pillars installed underground, are consistently subjected to erosion from acidic groundwater. In this study, we employed various techniques including acoustic emission (AE), digital image correlation (DIC), scanning electron microscopy (SEM), electron dispersive spectroscopy (EDS), and X-ray diffraction (XRD) to investigate the evolution of mechanical properties and AE damage characteristics of coal-rock combinations under water-chemical corrosion. Moreover, we analyzed the microscopic deterioration mechanism. Our results reveal that as the acidity of the solution increases, the cumulative energy and AE count of the specimen decrease to varying degrees. The AE ringing counting process can be categorized into stable, active, and sharp development stages. Moreover, as the pH value of the solution decreases, the AE b-value follows a “V" type variation. The dynamic b-values demonstrate localized sudden drops, indicating the occurrence of large-scale cracks. The internal cracks within the coal-rock combination transform from shear cracks to tensile cracks, corresponding to a transition in failure mode from shear failure to cleavage failure. Furthermore, a detailed analysis of the deterioration mechanism of the coal-rock combination under water-rock interaction was conducted. Under on-site groundwater conditions (pH = 6.24), the overall degradation of uniaxial compressive strength (UCS) and elastic modulus reached 13.37% and 28.73%, respectively. The physical, chemical, and mechanical effects of the water-chemical solution induce microstructural changes within the coal-rock combination. Consequently, the total percentage of macropores (>1 μm) in the sandstone section increased by 8.85%, reaching 35.3%, thereby further deteriorating its mechanical properties. In contrast, the proportion of macropores in the coal section only increased by 0.95%. Therefore, water-rock action has a more severe damaging impact on the sandstone section compared to the coal section of the combination.

Experimental and theoretical analysis of spalling in deep hard rock tunnels with different arch structures

To study the spalling characteristics and induction mechanism of tunnels with different arch structures, a series of true triaxial tests were conducted on three types of cubic granite specimens with a through-going arched hole and analytical stress solutions were obtained for the three arch sections. Test results show that the failure process of specimens includes four periods: a quiet period, a buckling deformation period, a gradual buckling and exfoliation period of rock fragments, and a V-shaped notch formation period. Specimens with different arch structures are listed in a descending order as the three-centered arch with f/B (arch rise/tunnel width) = 1/3 (II), three-centered arch with f/B = 1/4 (III), and semi-circular arch with f/B = 1/2 (I) according to the size of rock fragments. Before the spalling of arched tunnels, the acoustic emission (AE) count is low, while the AE count and cumulative AE count are high during spalling. Stress analysis results show that as the vertical stress rises, the arch feet of tunnels undergo initial failure at first due to stress concentration; spalling cracks appear in both side walls due to compressive stress concentration, which induce spalling failure; tensile cracks initiate in the vault and invert due to tensile stress concentration. The sequence of spalling in tunnels with different arch structures varies due to a change in section structure. Therefore, the quality of section structures can be evaluated according to the stress distribution characteristics of surrounding rocks. The vertical stress inducing the initial failure of tunnel surrounding rocks increases gradually in different arch structures from the II, III, to I; the failure severity of surrounding rocks decreases in different arch structures, also from II, III, to I. In addition, the stress analysis results also verify the accuracy of the test results. In practical engineering applications, to consider both the section utilization ratio and control of spalling in surrounding rocks, selecting tunnel sections with f/B = 1/4 is more economic and safer than that of f/B = 1/3.

Effect of cooling methods on mechanical behaviors and thermal damage distributions of granite: Experiments and simulations

This work investigated the fundamental reason for the deterioration of mechanical properties of granite after two different heat treatments (slow heating followed by water or air cooling). The responses of mechanical and acoustic properties of the granite after two different heat treatments were obtained by a servo testing machine, a Digital Image Correlation technology (DIC), and an acoustic emission (AE) system. Meanwhile, the heat transfer laws and thermal damage distributions of granite under different cooling methods were discussed by using COMSOL software. The results show that the tensile strength of the water-cooled specimen is lower than that of the air-cooled. The AE counts of the water-cooled specimen are more intensive than that of the air-cooled during the compaction stage. With increasing initial temperature, the accumulated AE counts of the air-cooled specimen increase, but the accumulated AE counts of the water-cooled specimen first increase and then decrease. Meanwhile, the cooling methods have a great influence on the distribution of tensile stresses and thermal damage of the specimens. The distribution of tensile stress and thermal damage of air-cooled specimens is random, which is mainly controlled by the heterogeneity of the thermal expansion coefficient. However, the tensile stresses and thermal damage of water-cooled specimens are distributed on the surface of the specimen, which is mainly controlled by the temperature gradient. Meanwhile, water cooling has a more severe thermal damage on specimens than air cooling. The difference in the convective heat transfer coefficient between rocks and cooling mediums determine heat transfer process. Results indicate that the selection of work fluid with a higher convective heat transfer coefficient is of great significance for improving the efficiency of geothermal energy extraction.

Experimental characterization of impact damage in foam-core sandwich structures using acoustic emission, optical scanning and X-ray computed tomography techniques

Sandwich structures used in transportation and energy industries are commonly subjected to low-velocity impact in the course of manufacturing, service and maintenance, which can lead to the premature degradation of structural stiffness and strength. A single non-destructive evaluation technique is often difficult to provide a comprehensive assessment on impact damage from online to offline and from visible to invisible. In the present study, acoustic emission (AE), three-dimensional (3D) optical scanning and X-ray computed tomography (CT) techniques are combined to provide a systematic approach for damage characterization of thick foam-core sandwich panels under low-velocity impact. In the tests, the mechanical responses such as force-time and force-displacement curves are obtained from the testing system. The AE technique is adopted to monitor damage initiation and evolution in situ; optical scanning and X-ray computed tomography techniques are used for off-line damage identification and quantification. The results show that the AE signal waveform stream, cumulative counts and cumulative energy correlate well with the mechanical responses of the sandwich panel under the low-velocity impact, enabling capture of valuable information for online damage monitoring. Unlike general cases, the AE signals generated from low-velocity impact are found to be presented in strong bursts of nearly coincident events in the short duration. Therefore, the distributions of AE amplitude and peak frequency between adjacent AE counts are extracted to study the different damage modes and damage mechanisms. The X-ray computed tomography images enable to precisely visualize the fracture behaviors of the PET foam-core inside the sandwich panel in a nondestructive fashion, which evidently supports the AE and optical scanning results. The study is anticipated to demonstrate effectiveness and capability of the proposed experimental methodology for damage detection and characterization on sandwiched composite structures subjected to impact loading.

Analysis of failure characteristics and mechanical mechanisms of sandstone during spherical indenter intrusion based on acoustic emission experiment

This paper aims to investigate the effects of steel ball diameter on failure characteristics and mechanical mechanisms of sandstone during the spherical indenter intrusion process. By using equipment such as acoustic emission (AE), the process of spherical indenter intrusion into sandstone was monitored and recorded for analysis of the fracture characteristics, load-depth relationship, and specific energy of sandstone under spherical indenters with different diameters. The results showed that as the diameter of the spherical indenter increased, the peak load of the sandstone increased continuously, and the failure mode transitioned from “jump-type” to plastic, with the acoustic emission count exhibiting a staged variation characteristic. During this process, the accumulated acoustic emission count, intrusion coefficient, and specific energy of intrusion all exhibited a “parabolic” law of first decreasing and then increasing, all of which reached minimum values when the ratio of the diameter of indenter to sandstone was close to 1.0. This indicated that as the diameter of the indenter approaches that of the sandstone, the intrusion process becomes more facile and efficient. Finally, the study revealed the formation mechanism of dense core and crack inside the sandstone during intrusion, and discussed the law of the crushing rate in ball milling from the perspective of compression. The conclusions contribute to revealing the fragmentation mechanics of rocks in the ball milling process and aid in determining the ball loading parameters in the milling process based on rock properties, aiming to achieve energy-saving and cost reduction.

Comparison of Acoustic Emission Data Acquired During Tensile Deformation of Maraging Steel M250 Welded Specimens

Safety and reliability are primary concerns in launch vehicle performance due to the involved costs and risk. Pressure vessels are one of the significant subsystems of launch vehicles. In order to have minimal weight, high strength material viz. maraging steel M250 grade is used in realizing the pressure vessel casing hardware. Despite the best efforts in design methodology, quality evaluation in production and effective structural integrity assessment is still a farfetched goal. The evolution of such a system requires, first, identification of an appropriate technique and next its adoption to meet the challenges posed by advanced materials like maraging steels. In fact, a quick survey of the available Non-Destructive Evaluation (NDE) techniques suggests Acoustic Emission (AE) as an effective structural integrity assessment tool capable of identifying any impending failure or degradation at an earlier stage. Experience shows that the longitudinal welds in the pressure vessels are quite vulnerable to failure due to the fact that they experience the maximum stress (i.e. hoop stress). Loading welded tensile samples are quite synonymous to the hoop stress experienced by longitudinal welds. An attempt is made to compare the Acoustic Emission data acquired during tensile deformation of maraging steel welded specimens. A total of 16 welded specimen’s with known defects were studied for their tensile behaviour is in connection with Acoustic Emission data. The lowest failure load was 70.5 kN and the highest being 84.8 kN. AE activity graphs viz. cumulative AE activity, hit rate, energy rate, count rate, AE amplitude history, AE count history, AE energy history, amplitude-count correlation and hit amplitude distribution have been investigated and salient features with respect to the data have been critically studied and relevant correlations are arrived at.

Acoustic Emission Characterization and Cracking Characteristics of Reinforced Concrete Pipe

Acoustic emission (AE) and digital image correlation (DIC) technology were used in this study to monitor the behavior of reinforced concrete pipe (RCP) in the optimized three-edge bearing test (TEBT). At the same time, the AE source location of different pipe sections was simulated. The results show that the whole compressive loading process for RCP could be divided into 3 to 4 stages. Both the AE counts and AE energy could well characterize the cracking of concrete, and AE energy was more sensitive to the breaking of steel bar. The amplitude and duration of AE signal produced by the breaking of steel bar were larger than those produced by the cracking of concrete. The AE frequency for concrete cracking was relatively scattered. The AE frequency for steel bar breaking was small and concentrated, and this frequency distribution showed an exponential decreasing trend. The results of optimized AE source location showed that the generation and development of cracks occur in the initial loading stage. The results of multi-index analysis showed that the cracks in the top (loading position) and the bottom (bearing position) of the RCP show that the cracks extend from the inner wall to the outer wall, while the cracks on the left and right sides perpendicular to the loading direction show that the cracks extend from the outer wall to the inner wall.

Effects of roughness on shear properties and acoustic emission characteristics of bonded Rock-Concrete interfaces

The shear properties of bonded rock-concrete interfaces play a crucial role in the analysis of engineering structures stability. In this paper, laboratory direct shear tests and acoustic emission (AE) tests are conducted on standard cylinder specimens with varying levels of rock surface roughness. The aim is to gain a better understanding of shear behavior and failure mechanisms of the bonded interface between rock and concrete. The results show that increasing of roughness enhances the brittleness and peak dilatancy characteristics of the specimens. However, the residual shear strength exhibits the opposite trend. In addition, the distribution of residual concrete on the rock surface changes gradually from localized spots to larger areas. The AE test results demonstrate the occurrence of peak AE peak energy and count generation during shear failure. The evolution of the AE-b value indicates that the macro-fracture surface of the bonded surface is the result of the repeated conversion between large and small-scale micro-cracks. Moreover, the fluctuation value of the AE-b value is not influenced by the roughness of the rock interface. Hence, in this experiment, the 90% fluctuation mean value is selected as the primary parameter for rupture warning. Furthermore, The distribution of RA-AF values reveals the generation of numerous tension-shear composite crack types during the fracture process. The number of shear cracks in the failure stage exhibits an increasing trend with increasing roughness.

Thermally Activated Crack Growth and Fracture Toughness Evaluation of Pipeline Steels Using Acoustic Emission

The article presents an approach to assessing the fracture toughness of structural alloys based on thermally activated crack growth and recording acoustic emission signals. The kinetic and structural features of the stable growth of the initiated crack are estimated using a multilevel acoustic emission model based on the time dependence of the logarithm of the cumulative acoustic emission count. The article provides an evaluation of the stable kinetic constants included in the equation of the thermal fluctuation steps of a crack according to literature sources and using the acoustic emission method. It is shown that parameters such as activation energy, activation area before the crack tip, and the rate of non-activation crack growth are stable and show a satisfactory correspondence between the reference literature and real experiments. The approach does not require a set of laboratory experiments to determine the empirical constants of traditional crack growth rate equations, and it also differs in that it takes into account the unique features of the destruction of a particular specimen or technological equipment and allows for a non-destructive assessment of fracture toughness. The values obtained are conservative. The concentration criterion of destruction requires further investigation.

Strain rate effect on the fracture evolution of sandstones under quasi-static loading conditions: Insights from acoustic emission characteristics

To investigate the correlation between sandstone fracture evolution characteristics and strain rate, an acoustic emission (AE) monitoring test was conducted under uniaxial compression. The influence of strain rate on the nucleation mechanism of sandstone micro-fracture agglomeration and the evolution characteristics of multi-type fracture sources were analyzed using AE localization and moment tensor theory. The results indicate that the peak strength, elastic modulus, and peak strain of sandstone specimens exhibit an exponential relationship within the set strain rate range. As the strain rate increases, the three normalized stress thresholds show a slow decreasing trend, with the normalized closure stress threshold decreasing significantly. The AE monitoring results reveal consistent curve evolution trends for the AE count, cumulative count, energy, and cumulative energy of sandstone samples under different strain rates, with a decrease in the cumulative AE count. The proportion of multiple types of fracture sources in sandstone samples varies in different deformation stages. When the sample reaches the stable crack propagation stage, the proportion of shear fracture sources tends to decrease, while the proportion of tensile and compressive fracture sources tends to increase with an increasing loading rate. The relationship between the AE moment magnitude and frequency follows the normal distribution law. As the strain rate increases, the AE b value gradually decreases, and the proportion of large-scale fracture events in the sample increases. The strain rate can increase the overall strength of the fracture source inside the sample.

Mechanical properties and damage evolution characteristics based on the acoustic emission of gangue and high-water-content materials based cemented paste backfill

The long-term, continuous, high-intensity mining of coal resources has caused surface settlement, collapse of surface buildings, and stacking of gangue on ground. Backfill mining with high-water-content materials, as one of the core techniques in green coal mining, can control strata movement and surface settlement, while it fails to dispose of coal gangue. To dispose of coal gangue at large scale, single factor alternative tests were designed based on backfill mining with high-water-content materials and high-water-content materials were used as the binder and gangue as the aggregates. Furthermore, the mechanical properties and damage evolution of gangue and high-water-content materials based cemented paste backfill (GHW-CPB) under three main controlling factors (gangue particle size, gangue content, and water-cement ratio) were studied. The following conclusions were drawn: 1) GHW-CPB shows ductile failure under uniaxial compression and the material has high residual strength, exhibiting good stability after reaching the peak bearing capacity. 2) Relation equations of the three main controlling factors with the uniaxial compressive strength (UCS) and elastic modulus of GHW-CPB are obtained through regression, thus revealing the evolution of the mechanical strength of GHW-CPB. 3) Acoustic emission (AE) and failure characteristics of GHW-CPB in three periods in the uniaxial compression process are determined by using the monitoring systems for crack development and propagation. In addition, the damage equation of GHW-CPB is established based on the cumulative AE count, thus further revealing the damage evolution of GHW-CPB under uniaxial compression. These results provide a theoretical basis for the implementation of backfill mining with GHW-CPB.

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.

Precursors of Cyclic Loading and Unloading Sandstone Failure Based on “Acoustic-Thermal” Loading–Unloading Response Ratio

Coal mining often causes periodic disruption in the rock mass around the stope. The study of the deformation and failure characteristics of cyclic loading and unloading sandstone is very critical for gaining a thorough understanding of the mechanisms of rock damage, degradation, and failure. This kind of investigation is very helpful in determining the precursors of rock failure and the instability of engineering structures. In this research study, the properties of acoustic emission and infrared radiation of cyclic loading and unloading sandstone are explored using a cyclic loading and unloading sandstone experiment. Based on acoustic emission and infrared radiation, the loading–unloading response ratio of rock is established. It is found that the response variables of sandstone during the loading stage based on acoustic emission (AE) counts and the loading–unloading response ratio based on average infrared radiation temperature (AIRT) both rise suddenly in the last cycle, which may be a precursor of “acoustic-thermal” approaching rock failure. On this basis, the quantitative analysis index of infrared radiation of differential infrared energy change rate (DIECR) is proposed, that is, the change of square of ΔAIRT in unit time, and based on AE counts and DIECR, the loading–unloading response ratio of “acoustic-thermal” is defined. It is found that the “acoustic-thermal” loading–unloading response ratio suddenly increases during the penultimate cycle of loading and unloading. This feature can be taken as the initial precursor of rock failure. Together with the “acoustic-thermal” imminent failure precursor of rock, it constitutes the “initial precursor-imminent failure precursor” combined with the internal fracture and surface infrared radiation temperature field during the cyclic loading and unloading process of rock, realizing the hierarchical monitoring and early warning of cyclic loading and unloading rock failure. The research results lay a theoretical and practical foundation for using infrared radiation to monitor engineering disasters caused by rock fracture and failure in mining engineering.

Numerical study of mechanical properties and microcrack evolution of double-layer composite rock specimens with fissures under uniaxial compression

Layered and heterogenous fissured rock masses are ubiquitous in nature. Fissures exert significant influence as release surfaces and nucleation sites for failure in layered rock masses. However, key mechanisms controlling microcrack initiation and extension in layered and heterogenous rock masses remain unclear. We complete a suite of numerical (granular mechanics) and complementary physical compression experiments on layered rocks containing fissures in different modalities where the overlying layer is both stronger and stiffer. We complete this to understand the impact of local specimen and system stiffness on ultimate strength and modes of failure (stability). We systematically examine the influence of fissure angles, lengths and positions on the mechanical response, including observations of microcrack evolution and tendency for instability (i.e. rock bursting), constrained by acoustic emission (AE). Peak strength and overall elastic moduli vary in a predictable and systematic manner. The strengths and stiffnesses of the double-layer composite specimens fall between those of the homogeneous weak and strong single layer specimens. The AE counts, micro crack growth and failure characteristics of the double-layer composite specimens are closely related to the fissure angles, lengths and positions. Whether the weak rock or the strong rock contains fissures, the rock burst tendency of the double-layer composite specimens decreases in varying degrees–defining the potential of pre-blasting/fracturing to soften the mass. Fissure angles and lengths do not have to be too large. Additionally, failure in the double-layer composite specimens results not only from crack propagation in the weak layer, but also failure of the entire specimen when the length of the fissure in the strong layer is sufficiently large. These observations provide a reference for stability evaluation and disaster mitigation of layered rock masses in engineering.

Study on Acoustic Emission Characteristics of Fatigue Damage of A7N01 Aluminum Alloy for High-Speed Trains.

Online monitoring of the fatigue damage process of A7N01 aluminum alloy base metal and weld seam was conducted based on acoustic emission (AE) and digital microscopic imaging technology. The AE signals were recorded during the fatigue tests and analyzed using the AE characteristic parameter method. Fatigue fracture was observed using scanning electron microscopy (SEM) to analyze the source mechanism of AE. The AE results show that the AE count and rise time can effectively predict the initiation of fatigue microcracks in A7N01 aluminum alloy. The digital image monitoring results of a notch tip verified the prediction of fatigue microcracks using the AE characteristic parameters. In addition, the AE characteristics of the A7N01 aluminum alloy under different fatigue parameters were studied, and the relationships between the AE characteristic values of the base metal and weld seam and the crack propagation rate were calculated using the seven-point recurrence polynomial method. These provide a basis for predicting the remaining fatigue damage in the A7N01 aluminum alloy. The present work indicates that AE technology can be used to monitor the fatigue damage evolution of welded aluminum alloy structures.