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Twin-roll casting (TRC) has seen a significant interest in recent years due to its short process, low energy consumption, and low emission. The research included both numerical simulation and experimental validation to examine the flow and temperature field distribution of the TRC process. The "U-shaped" buffer groove is identified as beneficial for maintaining a stable liquid level distribution in the molten pool through comparison with various flow distributor structures. The solidified billet shell dispersion around the casting roller in the molten pool is observed to undergo three distinct stages: stable growth, thickness fluctuation, and rapid growth. The inclusion of two circular side outlets, each with a diameter ranging from 8 to 10 mm, proves advantageous in maintaining a stable distribution of the liquid level within the molten pool. The edge outlet size ranges from 7 to 9 mm, and ensuring a U-shaped buffer groove width/flow distribution of 1.25–1.58 aids in enhancing casting stability. • The "U-shaped" buffer groove is conducive to obtaining a relatively stable molten pool liquid level. • The solidification shell is divided into stable growth, thickness fluctuation, and rapid growth stages. • Two circular side outlets with a diameter of 8–10 mm are beneficial for achieving desired liquid level. • Adjusting the width of the U-shaped buffer groove/flow distribution diameter to 1.25–1.58 can enhance casting stability. [ABSTRACT FROM AUTHOR]

• The high temperature cylindrical surface cooled by spray was simulated. • The axial tilt spray can effectively solve the hot spot problem and improve the cooling uniformity. • The temperature and wall heat flux distribution of the cooling area under different spray angles were analyzed. • Only when the spray boundary is tangent to the cylindrical surface (H = 70 mm), the circumferential tilt Angle has a positive effect on the wall heat flux. In the hot-rolled seamless steel tube production process, quenching is a crucial process. Spray cooling technology can achieve uniform and high-intensity cooling during the steel tube production process, which is expected to improve production efficiency and product quality. This study performs numerical simulations to explore the effects of spray cooling on a heated cylindrical surface. The influence of spray angles is focused on the cooling efficiency, while the spray height is also found to be another crucial variable. In the simulation, the axial tilt angle and circumferential tilt angle vary between 0 ∼ 30°, and the spray height changes within 50 ∼ 150 mm. A high-temperature steel tube is selected as the test object. The detailed analysis is discussed on the variations in the cooling area and wall heat flux under different spray angles. The results indicate that the axial tilt angle can effectively solve the hot spot. With an increase in axial tilt angle and spray height, the cooling area expands gradually. Meanwhile, the wall heat flux decreases with an increasing axial tilt angle and spray height. However, when the spray height is too high (H > 100 mm), the excessive axial tilt angle will have a negative influence on the wetting region. Although the circumferential tilt angle greatly changes the shape of the cooling area, it has a smaller effect on the size. It is worth noting that the spray boundary is just tangent to the cylindrical surface at H = 70 mm. Only at this moment does the circumferential inclined spraying exhibit a positive influence on the wall heat flux. [ABSTRACT FROM AUTHOR]

• The high temperature cylindrical surface cooled by spray was simulated. • The effect of spray spacing on cooling was simulated, and the best spray spacing range was obtained. • The temperature and wall heat flux distribution of the cooling area under different nozzle array forms were analyzed. • It is pointed out that the unidirectional inclined array spray can obtain better cooling effect. Spray cooling technology is an efficient cooling method. In the industrial application, the large area cooling requires the multi-nozzle spray design. Reasonable selection of nozzle array form is particularly important. Therefore, the simulation is applied in this study to investigate the effect of the nozzle array on the spray cooling of steel pipes after the model is validated. A steel pipe with a diameter of 140 mm, a wall thickness of 20 mm, and a length of 350 mm is employed as the research object, with an initial temperature of 900 °C. The scheme has four options for double spray spacing: OLR = −15 %, 0 %, 15 %, and 30 %. The results show that when the OLR grows, the cooling area gradually diminishes, and the cooling area's average temperature and average surface heat flux fall and subsequently increase. When OLR=30 %, the best cooling zone shape is obtained. The minimum average temperature, 231 °C, is obtained when OLR = 15 %. After comprehensive analysis, the ideal spray spacing range is found at OLR = 15 %∼30 %. In order to study the nozzle array, spray nozzles are arranged in multiple rows according to OLR = 30 %. This study also explores the strategies of unidirectional inclined spray and opposite inclined spray for lessening the area of the hot spot. There are six nozzle array arrangements in total. By comparing the results, it is discovered that the inclined spray increases the cooling area and accelerates the surface cooling of steel pipes. Under the side-by-side unidirectional inclined spray and the staggered unidirectional inclined spray, the cooling areas are 146,868 mm2 and 142,914 mm2 respectively, the average temperatures are 157.09 °C and 154.28 °C respectively. It is indicated that side-by-side inclined spray and staggered inclined spray are the better nozzle array arrangement to obtain the maximum cooling efficiency. [ABSTRACT FROM AUTHOR]

• A novel method was proposed to determine the damage zone of structures. • A geometrical feature factor was defined to represent the stress concentration. • LCF and creep weight functions were proposed to assess the effect of stress gradient. • A systematic method was developed to estimate the creep-fatigue life of structures. This paper develops a novel weight function modified critical distance method to estimate the creep-fatigue lives of geometrically complex structures. A geometrical feature factor was defined to account for the geometry-induced stress gradient, namely the average normalized stress gradient. The conventional weight function was modified using this concept to assess the weight effect of stress distribution on the low-cycle fatigue (LCF) and creep lives, respectively. The weight function modified critical distance methods were verified by the LCF and the creep test data of the specimens with significantly different geometries. Furthermore, creep-fatigue experiments on the full-scale hollow and solid turbine blades were conducted. The creep-fatigue lives of turbine blades were predicted using the systematical weight function modified methodology. The predicted results showed a good agreement with the experimental lives. Compared with other models, this novel method achieves a significantly better accuracy in the case of LCF, creep and creep-fatigue life predictions. [ABSTRACT FROM AUTHOR]

Highlights • A new way to research the work safety is proposed. • The work safety success theory based on the deductive method has been put forward. • The theoretical research of work safety has been deepen. • A dynamic work safety entropy model is constructed and applied. • The conclusion of the paper can be a reference of achieving the zero accident. Abstract Traditional work safety theory has played a significant role in promoting the development of work safety in that its application has effectively reduced the number of accidents and saved thousands of lives. However, as time goes on, the "marginal effect" of the theory is becoming more and more obvious. The world's statistics shows that the number of workplace accidents can only be reduced to a certain level, and cannot be reduced to zero. Also, as reflected from the increasing concerns and research of occupational diseases, the number of new occupational disease case has been found increasing year by year. This paper puts forward the work safety success theory based on the deductive method, which is different from the traditional work safety theory based on the inductive method. It deepens the theoretical research of work safety, and is an important improvement to the traditional theory. Combined with the theory of thermodynamics in physics, the entropy theory is used to explain the work safety success theory. And a dynamic work safety entropy model has been constructed and applied in specific cases, which further validates the work safety success theory. At present, technology and equipment have laid a good foundation for data collection and analysis of work safety research, and the work safety success theory provides a new methodology for the whole process work safety research with big data. The conclusion of this paper can provide reference for achieving the goal of zero accident in the workplace. [ABSTRACT FROM AUTHOR]

• The creep-fatigue tests for seven different kinds of geometrical complex specimens were conducted. • Characteristic parameters were proposed to describe the feature of different geometrical complex specimens. • The SVR and ANN machine learning was used to predict the creep-fatigue life of the specimens. This study proposes a supervised machine learning approach to predict the creep-fatigue life of complex geometrical specimens. Seven different specimens were tested under creep-fatigue loading, and finite element analysis and test results showed that the stress distribution and life of the specimens were significantly influenced by the diameters and arrangement of holes. Characteristic parameters were proposed to describe the specimens' features, and support vector regression (SVR) and artificial neural network (ANN) methods were utilized to predict their life. The results indicate that both methods are effective in predicting the life of the specimens, with the ANN showing better performance when input data is limited. This study offers valuable insights into the leading factors behind the failure of complex geometrical specimens under creep-fatigue loading. [ABSTRACT FROM AUTHOR]

Cooling has a great impact on the microstructure of metal materials, which is a crucial manufacturing link in the steel field. However, steel tube often has defects such as bending and cracking during the cooling process. The reasons are differences in cooling strength and uneven cooling. The jet angle is one of the important parameters in the cooling process. Therefore, this study takes the jet angle as the research variable. During the experiment and simulation, the jet angle was changed from 0° to 30°, and other process parameters remained unchanged. The distribution of velocity and temperature of the steel tube are analyzed by simulation method. The results show that the jet angle has a significant effect on the cooling of the seamless steel tube. The asymmetry of velocity is more significant with the increase of jet angle. In addition, the wetting behavior of steel tube surface under different jet angles was observed experimentally. The wetting distance in the downstream direction is greater than that in the countercurrent direction. With cooling time increasing, this difference gradually increases. Moreover, the cooling intensity could be improved to a certain extent by changing the jet angle. [ABSTRACT FROM AUTHOR]

The flow and temperature field variation of hot rolled seamless steel tube with outer diameter of 140 mm and wall thickness of 10 mm were studied by experiment and numerical simulation. The finite element model of circular jet impinging cooling steel tube was established, and the steel tube was divided into different regions in circumferential direction. It was found that the uniformly arranged multiple jets have non-uniform cooling characteristics under the influence of gravity and hollow characteristic. Under the condition of 12 jets, the average heat transfer coefficient in circumferential direction of −15 ∼ −45° region is 8385 W/(m2K), and the fluctuation of the average heat transfer coefficient in different regions reaches 3222 W/(m2K). According to statistical average cooling rate of the experiment, the average cooling rate increases by 34% when 2 jets are increased to 4 jets. When the number of jets exceeds 8, increasing the number of jets has a limited effect on improving average cooling rate. Through reasonable configuration of jet velocity of different nozzle positions, a way to realize uniform cooling was proposed and preliminarily verified. The improved average heat transfer coefficient fluctuation of circumferential was limited to 1000 W/(m2K), which significantly improved the uniformity of jet cooling. • The multiple jet uniformly arranged in the circumferential direction of steel tube has non-uniform cooling characteristics. • When the number of jets exceeds 8, increasing the number of jets has a limited effect on improving the average cooling rate. • A way to realize uniform cooling is proposed and preliminarily verified, which significantly improves the cooling uniformity. [ABSTRACT FROM AUTHOR]

• A biaxial experimental methodology was developed to apply cyclic tension and high frequency transverse vibration. • Blade-like specimen and additional lumped mass were designed to simulated the vibration mode of structural detail. • High stress ratio related creep damage significantly influenced CTBF damage at high temperature. • LCF, HCF-creep interaction, oxidation should be considered in the structural design. Combined tensile and bending fatigue (CTBF) behavior and failure mechanism of a blade-like specimen, made of a Ni-based alloy DZ125, are experimentally investigated. In order to reveal the geometrical feature and the stress state of the fillet between turbine blade bottom and platform, a blade-like specimen was designed. A novel high temperature experimental methodology was developed to apply the cyclic tensions and high frequency transverse vibration, to the blade-like specimen. CTBF, dwell fatigue (DF) and high stress ratio bending fatigue (BF) tests were performed at 850 °C. Experiment results show that the introduction of high frequency vibration significantly reduced the cyclic life in comparison with DF, while the cyclic tension related dynamic strain aging (DSA) has a positive effect on high cycle fatigue (HCF) life. LCF-related and HCF-related multi-cracks are initiated at the surface due to the effect of bending stress gradient. During CTBF at elevated temperature, LCF, HCF, creep, oxidation, and their interactions contribute to the crack initiation, and linked creep voids, oxidation, and carbide precipitates accelerate the crack growth. [ABSTRACT FROM AUTHOR]

In order to further explore the mechanism of electromagnetic radiation of coal and rock during loading, the change characteristics of electrical parameters including surface potential and charge density of the four types of rocks under dynamic and static conditions were assessed and tested using Atomic Force Microscope (AFM) and Micro-loading device. The results show that under static conditions, the micro-surface potential of the four rocks ranges between −922 mV and 235 mV, and the micro-surface potential has a good correlation with the charge density of the same type rock; under dynamic conditions, the micro-surface potential of the rock increases constantly with the loading, the range of the average value of the electric potential is −0.366∼1.188 V, and the micro-surface potential is positively correlated with the load, which has an obvious force-electric coupling effect. This work reveals the electrical characteristics of rock micro-surface under dynamic and static loading conditions, which may provide important experimental basis for further revealing the microscopic mechanism of electromagnetic radiation and improving theoretical system of electromagnetic radiation. [ABSTRACT FROM AUTHOR]

[Display omitted] • The LTHC fatigue life is considered as the summation of pit growth life and notch (i.e. pit) to fracture life. • The effective strain amplitude is defined by assessing the critical distance of the pit-induced non-uniform stress distribution. • The weight function modified TCD, combined with Walker model provides the best pit-to-fracture life prediction. • The proposed notch fatigue analysis methodology combined with pit evolution model provides the acceptable life prediction. This paper proposes a novel corrosion fatigue life estimation methodology considering the effect of hot corrosion and non-uniform stress field. The period of corrosion pit growing to the critical size was assessed by Faraday law. According to the cyclically plastic material constitutive response of smooth specimen, the finite element analysis was performed by regarding the critical corrosion pit as a micro semi-ellipsoidal notch. A novel definition about the effective strain was proposed considering the effect of non-uniform stress field in the high stress zone. The fatigue life from pit formation to final fracture was estimated by introducing the defined effective strain amplitude into Manson-Coffin-Basquin, Morrow, SWT and Walker models. Compared with the existing classical approaches, such as hot spot-based, continuum damage mechanics-based, strain energy-based models, the proposed approach provided significantly better corrosion fatigue life prediction. Finally, Walker model was suggested as the most suitable one after further discussion about the effect of mean stress. [ABSTRACT FROM AUTHOR]

• A double-slope method was proposed to calculate the critical distance. • An elastic geometrical feature factor was defined to account for stress concentration. • A stress sensitivity factor was proposed to assess the plasticity effect. • A fatigue life estimation approach was proposed with a good accuracy. This paper proposes a notch fatigue life prediction model to account for the effect of geometry and applied stress. A double-slope method was proposed to calculate the critical distance, which was utilized to determine the damage zone of geometrically complex structures. An elastic geometrical feature factor was defined to characterize the stress concentration. Considering the effect of applied stress on low cycle fatigue (LCF) life, a stress factor was proposed. A fatigue life estimation approach was established by introducing the elastic geometrical feature factor, the stress factor and an improved critical distance method. This model was validated by the experimental results of notched specimens with various geometries and loadings. Compared with the existing models, the proposed model provides a better accuracy. [ABSTRACT FROM AUTHOR]

• Finite element method for calculating material entropy generation based on the viscoplastic UMAT. • An entropy-based crack driving force was proposed within defined fracture process zone (FPZ). • A crack growth criterion based on cumulative entropy generation as crack grows a length of FPZ. • An entropy-based model was proposed to predict fatigue crack growth in GH4169 at high temperature. This paper developed an entropy-based approach to estimate the fatigue crack growth (FCG) behavior of GH4169 at high temperature. Firstly, systematic FCG experiments of GH4169 at 650 °C were conducted to obtain the experimental data. Then, a finite element analysis combined with Chaboche viscoplasticity and node release technology was developed to obtain cyclic stress–strain responses and entropy generation fields. Based on thermodynamic analysis, numerous discrete material elements as sub-systems were divided along the crack growth direction in simplified representative 2D middle section. Fracture process zone (FPZ) was defined equal to the size of discrete material elements (ρ) which can be determined by distribution of normal stress perpendicular to the crack plane. Subsequently, an effective cyclic entropy generation related to ρ was defined as the crack driving force. Effective cumulative entropy generation was calculated when crack increment was ρ and the results indicated that effective cumulative entropy generation float within a certain range. The average of all effective cumulative entropy generation values was defined as crack growth entropy. Finally, an entropy-based FCG rate model was established to estimate the FCG behavior of GH4169, and a good correlation between experimental results and model predictions are achieved for all the high temperature FCG tests. [ABSTRACT FROM AUTHOR]

Highlights • Artificial support and C-shaped bracing structure mainly support unsupported roof. • Middle and rear parts of unsupported roof is the key position of roof falling. • The key time for roof falling is when the shape of roof is square. • The safety control criteria for the unsupported roof were proposed. Abstract Roof-fall accidents, happening near the driving faces of coal roadways, threaten the safety of construction operatives. Taking Jining No. 3 Coal Mine of Yanzhou Coal Mining Group Corp. (Jining City, Shandong Province, China) as the research object, we analysed the roof structure near the driving face of a coal roadway. Meanwhile, the disaster-inducing mechanism was studied through mechanical analysis and numerical simulation. The results demonstrated that the artificial support structure and C-shaped bracing structure co-determine the safety of the roof. With the length of unsupported roof (LUR) increasing, the control effects of the two structures experienced three stages of behaviour (i.e. a superimposed support stage, collaborative support stage, and independent support stage) on the roof. In these, coal at the top corners became looser and the roof span increased. As a result, the safety of the unsupported roof deteriorated, with the rear part being the first to become damaged, thus triggering a roof-fall accident. Based on the above conclusion, the research proposed safety-control criteria for an unsupported roof, and thus provides a critical basis for assessing support strength and ensuring the safety of tunneling construction operations. [ABSTRACT FROM AUTHOR]

• Synchronously monitored the multidirectional crack vibration during rock fracture. • Detecting crack vibration signals in various directions reveals a strong temporal correlation with the load drop. • The fracture-induced crack vibration is a typical low-frequency signal with the main frequency mostly below 25 kHz. • The fracture-induced crack vibration of rock under load is damped vibration with a small and variable damping. Understanding crack vibrations in rock under load is crucial for comprehending and predicting rock failure. With this motive, a novel multidirectional crack vibration (MCV) monitoring experiment is carried out using miniature vibration acceleration sensors with directional sensing. The obtained results show that the crack surface produced by uniaxial compression failure of rock is rough and its direction is variable. Furthermore, the volume distribution of the sample after failure is uneven, and there is a noticeable difference in mass between the broken samples on either side of the crack. Different directions of crack vibration signals were detected during rock failure, all exhibiting a strong temporal correspondence with the stress drop, but the amplitudes and main frequencies of the vibrations differ across directions. The fracture-induced crack vibration is a typical low-frequency signal, with the highest recorded frequency at 55.63 kHz and the lowest being 0.86 kHz, with the main frequency mostly below 25 kHz, which is notably lower than acoustic emission. This difference suggests that fracture-induced crack vibration and acoustic emission are distinct physical quantities. The disparities in rock vibration amplitude and main frequency among different directions, especially those perceived by opposing sensors, indicate that the two sides of the crack cannot be considered as the same vibration system. The analysis shows that the fracture-induced crack vibration of rock under load is damped vibration with a small and variable damping. Besides, due to the difference in vibration parameters on both sides of the crack, their vibration amplitude and frequency are also different. The miniature acceleration sensors used in this work can effectively capture MCV during rock failure and can be employed to further investigate the vibration precursor of rock instability failure, so as to develop appropriate assessment and prediction methods for underground dynamic disasters. [ABSTRACT FROM AUTHOR]

Atomic force microscopy (AFM) serves as a prominent micro- and nano-scale observation method in various scientific domains encompassing biology, materials science, physics, and chemistry. Its application in the characterization of coal and rock, has gained significant attention in recent years, resulting in numerous valuable research findings. These findings offer fresh insights and novel perspectives for related research endeavors. This paper presents a systematic review of the progress made in utilizing AFM to measure coal and rock. It summarizes the key achievements and the significance of the current research in the areas of coal and rock pore structure, micro-surface mechanics, and electricity. Furthermore, based on the existing progress and challenges, this review also proposes potential avenues for future research directions of this measurement method. The application of AFM not only introduces a new approach for investigating the pore structure of coal and rock but also offers experimental support and innovative conceptual frameworks for exploring the mechanical mechanisms behind coal and rock fracture, the microscopic processes of gas adsorption, outbursts, and the generation of electromagnetic radiation induced by coal and rock fractures. [ABSTRACT FROM AUTHOR]

This study investigates the creep deformation, damage, and rupture behaviors of 2D woven SiO 2 /SiO 2 composites via experimental and numerical methods. In situ monotonic tensile tests and creep tests were conducted at 900 °C using a self-designed experimental system and digital image correlation. The tested specimens were characterized by X-ray computed tomography and scanning electron microscopy to conduct quantitative analyses and fracture observations. The obtained creep strain–time curves consist of primary and secondary stages, similar to the creep strain–time curves of most ceramic matrix composites. The matrix at the intersection of fiber bundles cracked under tensile loading. During subsequent creep loading, the propagation of matrix cracks, interfacial debonding, and fiber breakage in longitudinal fiber bundles were observed. At the mesoscale, the creep rupture entails a mechanism analogous to that observed in the monotonic tensile tests. Overall, the SiO 2 /SiO 2 composites employed in this study exhibit excellent potential for long-term operation under mechanical loads at high temperatures. Next, a micromechanics-based creep model was proposed to simulate the creep behavior of the composites. In this model, the primary creep law and rule of mixtures were combined to describe the stress redistribution of various constituents and predict the deformation of the composites. In addition, the rupture life was predicted based on the global load-sharing model, two-parameter Weibull model, and shear lag model. The degradation of the matrix modulus and fiber strength was also considered to improve the accuracy of the simulation. The predicted results were in good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

SiO 2f /SiO 2 composites are among the most ideal high-temperature wave-transparent materials used in hypersonic vehicles. The purpose of the study is the thorough experimental investigation of the tensile behavior of a 2D twill woven SiO 2f /SiO 2 composite, and the establishment of an accurate and efficient simulation method for such composites. The digital image correlation (DIC) method was utilized to capture local deformation data during tensile tests. Meanwhile, a progressive failure analysis (PFA) model employing the exponential damage evolution law was subsequently developed with UMAT in ABAQUS. Simulations of the mechanical properties and strain distributions show good consistency with experimental results. The results at room temperature and 900 °C demonstrate that the strain distributions exhibit obvious periodic patterns related to the woven structure. In addition, band-shaped strain concentrations can be observed at the intersection zones between adjacent longitudinal and transverse fiber bundles. These zones are regarded as critical regions. This was validated by the damage evolution observed in the simulations. Owing to the grain coarsening of quartz fibers and the embrittlement of different constituents at 900 °C, notable degradation of the mechanical properties and brittle fracture characteristics were observed. [ABSTRACT FROM AUTHOR]

The average jet flow rate, vapor volume fraction inside the nozzle, and the distribution of pressure, shear force, turbulence intensity, temperature, and Nusselt number on the heat transfer surface were simulated when water jet impinged on a 900 °C steel plate by a straight cone nozzle and an angular nozzle under the condition of 0.7 MPa inlet pressure. Results indicate that the strong cavitation effect of the angular nozzle makes its jet impingement heat transfer intensity superior to the straight cone nozzle. Although the average jet flow rate of the angular nozzle is 3.33% lower than that of the straight cone nozzle, its shear force, turbulence intensity and Nusselt number at the stagnation point are 18.43%, 20.43%, and 18.81% higher than those of the straight cone nozzle. Experiments also show that the jet impingement heat transfer intensity of the angular nozzle is better than that of the straight cone nozzle. Its maximum cooling rate at the stagnation point increased by 13.16%, and the time to reach the maximum cooling rate decreased by 60%, compared with the straight cone nozzle. It is hoped that the results can help improve the performance of online cooling equipment for hot-rolled steel. [ABSTRACT FROM AUTHOR]

• Tensile behavior of notched specimen is modelled using a viscoplastic model. • A high temperature DIC-based test system were constructed to measure the strain. • Simulated strains agree well with the DIC-based measurements. • The strain mutation in high strain zone of notched specimen nearly remains constant. This paper aims to validate a viscoplastic constitutive model and investigate the local and the full-field strain of notched specimen. A high temperature digital image correlation (DIC)-based test system was constructed to measure the full-field surface deformation. Surface strain fields of notched DZ125 specimens were measured under monotonic tensile loadings at 20 °C, 650 °C, 760 °C, and 850 °C. Taking the effect of anisotropy, applied stress, and temperature into account, numerical simulations were conducted using a developed viscoplastic constitutive model. DIC-based measurements agreed well with the simulated strains, with the minimum correlation coefficient of 95.47%. Both the numerical and experimental results show that the area of notch plasticity increases with temperature and applied stress, and the high strain zone nearly remains constant under various tensile loadings. For a given notch geometry, the strain mutation in high strain zone nearly remains constant no matter how the temperature and the applied stress change. [ABSTRACT FROM AUTHOR]

Highlights • Full-waveform data acquisition was conducted for AE, EME, and load. • Compared with AE, EME has better consistency with stress drop. • Rock has greater correlations between AE, EME, and stress drop than coal. • AE and EME have the same low-frequency components of 1–17 kHz. Abstract Rockburst is a global problem in underground mining industry, threatening mining safety. Acoustic emission (AE) and electromagnetic emission (EME) are nondestructive and real-time geophysical monitoring methods with great significance for the accurate monitoring and early warning of rockburst hazard. To further evaluate the correlations between AE, EME, and stress drop quantitatively and provide an experimental basis for the in-site monitoring of rockburst hazard, an AE, EME, and load synchronous test system was established and used to obtain the full-waveform data of AE, EME, and load in the failure process of burst-prone coal and rock samples under uniaxial compression. The correlations between AE, EME, and stress drop and the spectral characteristics of AE and EME were studied. The results show that the number of AE events is greater than that of EME events. EME is generally accompanied by the stress drop and high-intensity AE and has better correlation with stress drop than AE. Rock samples have greater correlations among AE, EME, and stress drop than coal samples. For rock samples, the correlation coefficients between AE and stress drop, EME and stress drop, and AE and EME are 0.782, 0.889, and 0.697, respectively, while for coal samples, they are 0.529, 0.889, and 0.538, respectively. The frequency spectra of AE and EME have a high correlation, and they have the same low-frequency components of 1–17 kHz, which can be used as the frequency band of local monitoring in working face scale. [ABSTRACT FROM AUTHOR]

Rock burst is one of the most severe dynamic hazards in the underground mining industry, and it is essential to establish a reliable method to achieve accurate prevention and control of it. This paper proposes a comprehensive spatial and temporal rockburst warning method based on the Mann-Kendall trend test (MKT) and spatially smoothed seismicity model, which can evaluate rockburst risk in real-time and quantitatively. The method uses real-time values of rockburst warning indicators as input, uses MKT to determine the temporal trend of each indicator and takes whether the trend conforms to the rockburst precursor law as the criterion for triggering the alarm. Then combined with the confusion matrix to evaluate the warning effectiveness of each indicator and optimize the indicators, and then obtain the real-time temporal rockburst hazard index Q of the mining area through data fusion. Finally, the spatial rockburst hazard index S contour map is drawn using the spatially smoothed seismicity model to identify local rockburst hazard zones and assist mine personnel in taking preventive and control measures of rockburst in time. The results show that the proposed model considers the precursor information of rockburst evolution from "time-space-strength" multi-dimensionally. The warning effectiveness of Q reaches 0.431, which is higher than any single indicator. The local rockburst hazard zone shown in the contour map of S is consistent with the actual location of strong tremors and rockburst events. The model has been successfully applied in the Wudong coal mine in Xinjiang, China, which can assist mine personnel in making efficient and accurate rockburst prevention decisions and effectively reduce their prevention costs. [ABSTRACT FROM AUTHOR]

A complete understanding of the rock fracture-induced electromagnetic radiation (EMR) mechanism and the coupling relationship between EMR and micro-vibration (MV) is highly challenging in rock engineering. For this purpose, a pioneer study is conducted that uses the synchronous monitoring experiments of EMR and MV during rock failure under uniaxial compression by the three-axis electromagnetic antenna and droplet-like MV acceleration sensors. The time-frequency evolution characteristics and correlation of EMR and MV during the failure process of rock under load were obtained, and the EMR-MV coupling effect was analyzed. The obtained results show dominant synchronicity of EMR and MV during the rock's failure process. It is inferred that both EMR and MV are generated with fracture (stress drop), and the signal intensity is proportional to the stress drop; the correlation coefficient between EMR and MV peak value is >0.85. During the rock's failure process, the peak vibration acceleration can reach 105 m/s2, and the attenuation speed of the MV signals was relatively faster than the EMR. However, the dominant frequencies of the two are both concentrated in the low-frequency band (< 30 kHz). The dominant frequency band of EMR is generally less than 20 kHz, while MV showed a slightly wider band, reaching 30 kHz. Besides, almost every fracture generated similar or the same main frequency (with a difference of 1–2 kHz) EMR and MV signals, indicating a noticeable correlation in the frequency domain. The analysis shows that the vibration during the rock's fracture can be regarded as small damping vibration to a certain extent, thus theoretically explaining its faster attenuation. The high time-frequency correlation between EMR and MV indicates that the EMR induced by rock fracture seems to be resulted by the vibrating charged crack clusters. In addition, the maximum EMR peak value mostly appears along the Z -axis of the EMR antenna, which further verifies the rationality of the above hypothesis. In brief, the charge accompanying rock fracture is the physical basis of EMR, and the vibration during the fracture process constitutes the power source of EMR. The research results further clarify the mechanism of EMR induced by rock fracture and lay a theoretical foundation for constructing the EMR location method for rock fracture sources. • Electromagnetic radiation (EMR) and micro-vibration (MV) monitored synchronically during rock fracture. • The EMR and MV during rock fracture showed a high time-frequency correlation. • Verified the hypothesis that rock fracture EMR is generated by vibrating charged crack. [ABSTRACT FROM AUTHOR]

• In-situ investigated rock's micro-surface under load by AFM coupled with micro-loading device. • The local fluctuation of the rock's micro-surface changes in nanometer level with the load strength. • The rock's micro-surface elastic modulus showed a complex local change with load strength. • The complexity of the crack path and the numerous crack branches of rock fracture was explained. Understanding the micro and nano-scale surface deformation and mechanical properties of rock under load possess enormous significance for underground engineering research and provides strong basis for comprehending rock failure as well. According to the documented research, the in-situ observation of micro-surface under dynamic load has not been yet reported. With this motive, a novel approach is presented in this work that takes advantages of combined Atomic Force Microscope (AFM) and customized micro-loading device for in-situ characterization of the the micro-surface topography and elastic modulus of rocks under different load strength. The in-situ comparison of micro-surface properties is carried out using interactive tools of AFM image post-processing software including crop and split, and section analysis. Taking the typical observation results of igneous rock and shale as an example, the results show that the micro-surface topography undulations of rock can reach up to tens of nanometers. Although no apparent fracture occurred, the local fluctuation of the rock samples' micro-surface changed with the increasing load strength at nanometer level, and in the measurement area of 5 × 5 μm2, the topographies of different pixel points are various, showing significant local difference. With the increasing load strength, the elastic modulus distribution of the rock's micro-surface shows heterogenity, and the local elastic modulus also increases or decreases, while the variation difference at different pixel points reaches hundreds of MPa. Additionally, the macroscopic fracture characteristics of rock under load, especially the non-uniform crack propagation and its mechanism are discussed. The analysis indicates that the heterogeneous distribution of the rock's micro-surface elastic modulus and its local complex change with load are accounting for the complexity of the macroscopic crack path and the numerous crack branches. This study provides a novel insight into micro-surface behavior in rock and presents a new method for the investigation of the fracture mechanism of rock. [ABSTRACT FROM AUTHOR]

The development of reliable early rockburst warning models for underground mines is a challenging task considering the complex nature of rockburst phenomena. In this paper, a real-time, integrated multi-system, early rockburst warning model is developed to achieve the quantitative prediction of the rockburst probabilities at a specific time. The model input parameters are data derived from microseismic (MS) and acoustic emission (AE) monitoring systems. The early warning model is based on fuzzy comprehensive evaluation method (FCEM), confusion matrix and maximum membership degree principle. These tools integrate the advantages of both multi-system and multi-parameter into one unified model. During the early warning procedure, six early warning indices representing temporal, spatial and energy parameters are rationalized. Next, the weight of each index is determined from confusion matrix leading to a score value. Real-time early warning results can then be determined from the maximum membership degree principle. The results show that the proposed integrated MS-AE early warning model has the advantage of combining multiple systems and indices, and it has higher prediction accuracy than individual systems. The early warning model is successfully demonstrated with a case study from the Wudong Coal Mine in China. The proposed model improves the rockburst early warning efficiency and provides a solid foundation for ensuring the safety of personnel and equipment in underground mining. [ABSTRACT FROM AUTHOR]

• A three-axis antenna is made to measure the near-field generated by rock fracture. • The three-axis antenna is relatively advantageous in response to lower frequency band. • Signals of same fracture received at the same location vary in different directions. In general, the generation of electromagnetic radiation initiates with the evolution of the crack in a rockmass. Theoretical observations depict that the low-frequency electromagnetic field excited by a dipole on cracks are directional in the near-field range. Therefore, in this paper, to achieve the measurement of low-frequency near-field, an electromagnetic antenna capabale of simultaneously measuring in directions of three axes is designed. After calibration of the standard alternating field, the antenna is used to monitor electromagnetic signals generated during the uniaxial compression and the Brazilian test. For the same fracture, signals differ in directions at the same position, which are actually components in three coordinate directions. And the three-axis antenna is relatively advantageous in response to low-frequency signals compared with the wide-band loop antenna. The outcomes of this research can provide theoretical as well as a practical basis for characterizing electromagnetic radiations associated with rock fractures. [ABSTRACT FROM AUTHOR]

• Micro-surface potential of coals (R o from 0.90% to 2.47%) was measured by AFM at five scanning scales. • Characteristics of coal micro-surface potential especially its scale effect was analyzed. • The influence factors of coal micro-surface potential were discussed. • The contribution of coal micro-surface potential to electromagnetic radiation was expounded. Micro-surface potential of different coals (Reflectance range; R o = 0.90% ~ 2.47%) was measured by atomic force microscopy (AFM) at five scanning scales, attempting to gain clearer insights into the electrical characteristics of coal micro-surface. Results show that coal micro-surface potential is affected by metamorphism and has significant scale effect. Generally, the distribution of coal micro-surface potential is heterogeneous (showing Gaussian distribution) and the potential is mostly positive, which can reach more than 800 mV. The potential difference of various points measured in certain scanning area can exceed 100 mV. The mean potential is more likely to decrease with increasing scanning scale, while the variance increases obviously. Analysis indicates that the scale effect is mainly due to the heterogeneity of coal and the metamorphism can affect surface potential by changing functional groups, distribution of surface microstructures, micro-morphologies and internal connectivity. Moreover, the contribution of coal micro-surface potential to electromagnetic radiation (EMR) produced by coal fracture has been discussed, revealing that the existence of coal micro-surface potential especially its scale effect provides a physical basis for the generation of EMR. [ABSTRACT FROM AUTHOR]

• Three rock burst mechanism in SIETCS and the factors influencing them are proposed. • Associated mechanical models and elastic deformation energy functions are developed. • Static and dynamic stresses are analyzed via theoretical analysis and MS monitoring. • A load-reduction and prying-reduction rock burst-prevention strategy is put forward. Rock bursts frequently occur in steeply inclined and extremely thick coal seams (SIETCS), posing severe challenges to safe mining. To reduce the risk of rock burst in SIETCS, this study investigated the mechanisms of the rock bursts occurrence in SIETCS and formulated the principles for its prevention. To this end, field investigation, geophysical monitoring, theoretical analyses, and numerical simulation were employed. Mechanical models have been developed for a "steeply inclined suspended roof structure" and a "steeply inclined suspended rock pillar structure", which are relevant to the rock burst mechanisms. The elastic deformation energy distribution functions for both models have been obtained, and the factors influencing the elastic deformation energy have been analyzed. The sources of microseismic (MS) events associated with rock bursts monitored in the typical SIETCS (with a dip angle of 87°) are mainly concentrated in the roof and interlayer rock pillar, making up 17.0% and 60% of the events recorded, respectively. The elastic deformation energy of the roof and rock pillar is mainly influenced by the dip angle of the coal seam, the lateral pressure coefficient, and the supporting force coefficient. The peak stress of the coal body at the compressive and prying area is 1.7 times of the horizontal tectonic stress. The minimum normal and tangential dynamic load stresses generated by the recorded rock bursts are 84.5 MPa and 48.6 MPa, respectively; such stress levels exert strong destructive forces when superimposed with static stress. The analytical results of the failure law of rock burst, MS monitoring, mechanical model, numerical simulation, and elastic deformation energy function of the typical SIETCS identify the main causes of rock burst as the high static stress of a coal body under the coupled action of compressive and prying effects of roof and rock pillar and the dynamic stress caused by breakage of the roof and rock pillar. The damage models and the damage process by which a rock burst is induced have been constructed. Three mechanisms by which a rock burst can occur in SIETCS are proposed. Finally, prevention principles of load-reduction and prying-reduction for rock burst in SIETCS have been developed. [ABSTRACT FROM AUTHOR]