Acoustic Emission Monitoring Research Articles

The Fracture and Energy of Coal Evolution under Thermo-Mechanical Coupling via a Particle Flow Simulation

The increase in mining depth causes the temperature of the coal seam to rise. Studying the effect of temperature on the mechanical property of coal is very necessary. In this paper, based on the results of conventional triaxial compression experiments of coal samples, the discrete element program PFC2D was used to conduct a conventional triaxial compression simulation of coal samples to obtain microscopic parameters. On this basis, the conventional triaxial simulation study of coal samples at different temperatures was carried out to explore the influence of temperature on the physical and mechanical properties of coal. In this process, acoustic emission and energy monitoring were carried out. The damage and failure process of coal is divided into three stages: the undamaged stage, the stable damage stage, and the rapid damage stage. The relationship between acoustic emission characteristic law, energy transformation law, and crack evolution in the damage and failure process of coal was analyzed in detail. The results indicate that the fracture evolution is affected by the increase in cross-sectional area and the decrease in distance between particles due to the expansion of particles, which corresponds to the phenomenon that the temperature increase produces new microcracks and the particles expand to fill the original cracks in the experiment. This indicates that PFC2D can be used as an effective numerical method for a coal mesoscopic study, and the results of these numerical experiments are also helpful to deepen the understanding of the damage mechanism of coal failure under thermo-mechanical coupling.

Study on the monitoring method of cavity growth in underground coal gasification under laboratory conditions

The growth state of the combustion cavity during underground coal gasification (UCG) affects the gasification process's efficiency, stability, and accurate monitoring and control. In this study, a simulation experiment of UCG was performed, and a horizontal coaxial gasification channel was drilled based on the constructed artificial coal seam. Oxygen and air are used as gasification agents to provide a sufficient temperature field to sustain the gasification process. Moreover, the derivation of the gasification zone during UCG was developed using stratified temperature monitoring and acoustic emission damage monitoring. After the experiment, the artificial coal seam was cut to obtain the profile shape of the combustion cavity, which was compared with the temperature and acoustic emission monitoring results. The results show that the temperature distribution results in the coal seam and the calibration results of the acoustic emission source during horizontal coaxial UCG are consistent with the two-dimensional and three-dimensional combustion cavities reconstructed by the profile. Thus, the effectiveness of stratified temperature monitoring and acoustic emission monitoring to infer the gasification combustion cavity is proven.

Analysis on mechanical properties and mesoscopic acoustic emission characteristics of prefabricated fracture cemented paste backfill under different loading rates.

The mechanical characteristics of cemented paste backfill (CPB) are significantly influenced by the loading rate (LR) and initial defects. Therefore, the CPB with prefabricated fracture (PF, PFCPB) was prepared, and uniaxial compressive strength (UCS) tests considering LR and acoustic emission (AE) monitoring were performed. The particle flow code (PFC2D) was introduced to analyze the mesoscopic crack evolution of the filling body, and the moment tensor theory was used to invert the AE signals characteristics. The results show that as the PF angle increased, the UCS and elastic modulus (EM) of PFCPB decreased and then increased, and the 30° PF was the turning point. The mechanical properties of PFCPB were deteriorated by the presence of PF. Meanwhile, the mechanical properties of PFCPB were positively correlated with the LR. The stress-strain curve of PFCB (excluding 90°) showed bimodal curves. After the UCS test, the macro crack of PFCPB sprouted at the tip of PF or converged toward PF. PFCPB mainly suffered from shear failure, accompanied by a few tensile failures. Numerical simulation results showed that the crack initiation stress of PFCPB was reduced by the PF. The number of cracks first dropped and then gradually increased when the PF angle was enhanced, while gradually increased with the LR increased. Meanwhile, the mesoscopic AE characteristics of CPB were strongly in line with the test results. The AE events of 0 ~ 60° PFCPB experienced two slow rising periods and rapid rising periods. The temporal and spatial distribution characteristics of AE corresponded to the crack evolution trend. The PF was prone to stress concentration, especially at the tip and upper and lower surfaces of 0 ~ 45° PF, resulting in rapid crack initiation and reducing the energy storage limit and mechanical behavior of 0 ~ 45° PFCPB. The increasing LR (within a certain range) was in favor of improving the mechanical behavior of the filling body. The research results can provide a basic reference for the stability evaluation of the filling body with initial defects.

Meso-macro damage deterioration of weakly cemented red sandstone under the coupling effect of high-humidity and uniaxial loading

The physical and mechanical damage of surrounding rock caused by water–rock interaction and excavation disturbance has an important influence on the stability of the chamber. Water absorption test was conducted for the weakly cemented red sandstone in the mine under high-humidity environment. Five rock samples with different water contents were obtained through different curing times. The microstructure variation characteristics and microfracture extension and penetration mechanism of rock samples with different water contents were analyzed by scanning electron microscopy (SEM). Combined with acoustic emission monitoring, comprehensive damage variables under two contiguous states of rock samples in high-humidity environment, i.e., initial damage induced by water absorption and hydraulic coupling damage, were constructed, and the stress–strain curve variation characteristics, damage evolution law, and macroscopic failure mode of red sandstone under uniaxial compression were analyzed. The results show that the water content of red sandstone in a high-humidity environment increased logarithmically with the water absorption time. The meso-fabric changes caused by water absorption can cause initial damage, resulting in subsequent significant variation characteristics, including mechanical loading stress–strain curve characteristics, energy evolution, and macroscopic failure modes. The comprehensive damage variable evolution law constructed by acoustic emission monitoring information reflected the coupling effect of initial damage and mechanical loading damage.

Experimental Study on the Entropy Change Failure Precursors of Marble under Different Stress Paths

In this paper, the indoor loading and unloading tests and acoustic emission (AE) monitoring tests of marble under different stress paths are carried out. Based on the AE events in the process of rock failure, this paper introduces the concept of information entropy of Shannon, and defines a new cumulative state information entropy which can characterize the degree of disorder and accumulation of rock microcracks. This paper also offers a comparative analysis on the evolution law of rock entropy under different paths and different loading and unloading conditions. The results show that the newly defined cumulative state information entropy can well reflect the order change of the loading and unloading process system, and there are significant differences in entropy evolution under different stress paths, but the entropy generally evolves with the development of the system from “quasi-equilibrium” to “far away from equilibrium”. The higher the unloading level is, the greater the confining pressure is, the stronger the unloading disturbance is, and the entropy value drops sharply at the starting point of unloading. The faster the unloading rate is, the later the rock fracture instability is, and the later the entropy drop point of the system appears. The change rate of entropy value ∣ΔH∣ = 0.1 can be used as the precursory signal threshold of rock confining pressure unloading disturbance damage and peak rupture instability.

The natural frequencies of AISI 316 stainless steel and analytical simulation of a Lamb wave excited by a point source

Monitoring and analysis of acoustic emissions (AEs) generated by material defects is important during nondestructive testing. Here, we characterize the AEs generated during tensile deformation of AISI type 316 stainless steel (SS). The active frequencies of this SS, spanning the 100–200 kHz range, were determined via tensile testing. Fast Fourier Transform (FFT) analysis revealed that the AEs featured four frequency components representative of all signals observed during the strain-hardening stage. For completeness, the propagation of the AE wave, a Lamb wave generated by the point source, was simulated (using the determined frequency components) in a two-dimensional 2 × 80 mm SS plate. The displacement fields that triggered AE of a Lamb wave from a point source were derived by solving the Navier-Lamé equation. This analytical modeling of AE is useful for characterizing emission generated by defects.

Tension-after-impact analysis and damage mechanism evaluation in laminated composites using AE monitoring

The present paper focuses on the detection of dominant damage mechanisms in tensile loading of laminated composite specimens subjected to low-velocity impact (LVI) using acoustic emission (AE) method. Moreover, the effects of various nanoparticles (NPs) on the tension-after-impact (TAI) strength of glass fiber-reinforced polymer (GFRP) composites have been investigated. The hierarchical method was employed to cluster the acoustic emission data and detect the damage mechanisms of the composites. These damage are matrix cracking, fiber–matrix debonding, fiber breakage, and fiber pull-out, the peak frequency ranges of which were obtained to be (100–200), (250–370), (410–530), and (570–650) kHz, respectively. Subsequently, the existence of these mechanisms was verified using Scanning Electron Microscope (SEM) images, and their shares in composite failure under tensile loading were calculated by wavelet transform method. According to the results, the NPs have enhanced the impact strength of the GFRPs by up to 66% reduction in damage index. Furthermore, they have increased TAI strength by up to 39%, claims also supported by the AE analysis.

Acoustic Emission Monitoring of Grinding-Polishing of Extra-Low Dispersion Lens

Acoustic Emission Monitoring of Grinding-Polishing of Extra-Low Dispersion Lens

Loading rate dependence of staged damage behaviors of granite under uniaxial compression: Insights from acoustic emission characteristics

A series of laboratory-based uniaxial compression tests combined with acoustic emission (AE) monitoring was conducted to investigate the loading rate dependence of AE characteristics and the damage process of granite. Experimental results showed that the mechanisms of formation and evolution of cracks within the rock during loading can be characterized by AE parameters and real-time spatial AE locations. With increasing loading rate, the cumulative AE count decreased as a negative power function while the cumulative AE energy decreased almost linearly. Decreasing b-values indicate additional large-magnitude AE events and more concentrated distribution modes of fractures under high-speed loading conditions. Moreover, with increasing loading rate, the dominant failure mechanism was observed to transition from shear to tensile. A transition from tensile to shear crack modes during the damage process of each sample was noted to correspond with a decrease in b-values and the onset of crack nucleation, which may serve as a diagnostic precursor to rock failure. Crack nucleation patterns also exhibited rate dependence. Under low-speed loading conditions, nucleation was initiated at the unstable crack growth stage and was characterized by interactions between linked tensile microcrack arrays to form a shear macrorupture zone, whereas crack nucleation advanced to the stable crack growth stage under high-speed loading conditions, resulting in vertical splitting.

The Experimental Investigation on Mechanics and Damage Characteristics of the Aeolian Sand Paste-like Backfill Materials Based on Acoustic Emission.

With the wide application of the filling mining method, it is necessary to consider the influence of rock activity on the filling body, reflected in the laboratory, that is, the influence of loading rate. Therefore, to explore the response characteristics of loading rate on the mechanical and damage characteristics of aeolian sand paste filling body, DNS100 electronic universal testing machine and DS5-16B acoustic emission (AE) monitoring system were used to monitor the stress–strain changes and AE characteristic parameters changes of aeolian sand paste filling body during uniaxial compression, and the theoretical model of filling sample damage considering loading rate was established based on AE parameters. The experimental results show that: (1) With the increase in loading rate, the uniaxial compressive strength and elastic modulus of aeolian sand paste-like materials (ASPM) specimens are significantly improved. ASPM specimens have ductile failure characteristics, and the failure mode is unidirectional shear failure → tensile failure → bidirectional shear failure. (2) When the loading rate is low, the AE event points of ASPM specimens are more dispersed, and the large energy points are less. At high loading rates, the AE large energy events are more concentrated in the upper part, and the lower part is more distributed. (3) The proportion of the initial active stage is negatively correlated with the loading rate, and the proportion of the active stage is positively correlated with the loading rate. The total number of AE cumulative ringing decreases with the increase in loading rate. (4) Taking time as an intermediate variable, the coupling relationship between ASPM strain considering loading rate and the AE cumulative ringing count is constructed, and the damage and stress coupling model of ASPM specimen considering loading rate is further deduced. Comparing the theoretical model with the experimental results shows that the model can effectively reflect the damage evolution process of ASPM specimens during loading, especially at high loading rates. The research results have significant reference value for subsequent strength design of filling material, selection of laboratory loading rate and quality monitoring, and early warning of filling body in goaf.

Analysis of Acoustic Emission Characteristics and Failure Mode of Deep Surrounding Rock of Sanshandao Gold Mine.

In mining engineering, crack distribution has a considerable influence on the mechanical behavior and stability of the surrounding rock mass. Using the granite of the Sanshandao gold mine as experimental samples, the deformation and failure of fractured rock were analyzed based on a rock uniaxial compression test with acoustic emission monitoring. We analyzed the characteristics of different stages of rock sample deformation, and evaluated the failure mode of seven types of rock samples. The results show that the cracks had a considerable impact on rock sample strength and mechanical behavior, and the strength of intact rock was the highest, while that of the sample with parallel double cracks was the lowest. The acoustic emission parameters, AF, RA, and lg(AF/RA), have different change trends in different stages of rock deformation and failure. Based on these change trends, the failure modes of rock samples with different crack distributions were identified. Additionally, for the rock samples with seven types of crack distribution, a sudden or progressive failure caused by the b-value curves was observed. The research findings provide a database for deep surrounding rock stability in the study area and provide suggestions for failure prediction.

Acoustic Emission Monitoring of Burn Damage Occurring Under Different Grinding Conditions Using the Low-Order Frequency Moments of a Spectrogram

Acoustic Emission Monitoring of Burn Damage Occurring Under Different Grinding Conditions Using the Low-Order Frequency Moments of a Spectrogram

Experimental research of fracture damage behavior of loess with different prefabricated cracks

Fracture damage in loess poses a significant threat to the safety of loess-based infrastructure in the China Loess Region. To investigate the fracture damage behavior of loess, three-point bending tests were performed on semi-circular loess specimens containing cracks prefabricated at four angles relative to the loading direction. High-speed photography and acoustic emission monitoring were used to record the fracture damage process. Mechanical strength, crack evolution and acoustics emission signals were investigated in relation to prefabricated crack angles. Based on the results, a fracture damage mechanism for loess was obtained. This paper provides an in-depth understanding of the fracture damage behavior of loess, and a meaningful reference for the prevention and control of geo-crises caused by cracking.

(Digital Presentation) Lifetime Prediction of SiC Power Module By Using Time-Series Analysis of Acoustic Emission during Power Cycling Test

Acoustic emission (AE) based condition monitoring is a popular method to inspect a material health condition in many areas[1]. It monitors irreversible structure changes of material through radiation of acoustic waves. Typically, the structure change generates a typical spectrum of acoustic waves starting at 1 kHz, and falling off at several MHz, which can be detected and analyzed via an AE system. Meanwhile, AE signals convey massive information such as rise time, AE energy, AE peak frequency, which are expressive of the failure process. AE based condition monitoring technology exhibits excellent capability of detecting, positioning, and characterizing damage and has been wildly used in monitoring of inner structure changes in bridges, pressure containers, and pipeline systems. In the last decade, the AE technology has been applied to condition monitoring of power modules and shows a promising application scenario in the power electronics field, which help us acquire accurate failure information and making precise perdition of a failure in the power module[2].In this work, we firstly combined the AE monitoring and a deep learning model - Gated Recurrent Unit (GRU) to monitor and predict the failure of power module as shown in Figure 1[3]. The proposed method integrally analyzes and summarizes the patterns from the time-series data obtained during power cycle tests of power modules and predicts the failures. Specifically, the time-series data consisting of junction temperature, electric power, thermal resistance, and several AE parameters were obtained from AE sensor data and power cycling test. The proposed method (a) automatically extracts common or different time-series patterns among devices and estimates device states, and (b) effectively predicts failures by switching prediction models according to changes in device states.By time-series analysis of AE sensor data, it was confirmed that the AE parameters obtained from the AE sensor data capture the progression and signs of failures. In addition, by verifying the effectiveness of the time-series pattern detection, it was confirmed that the proposed method can accurately extract the timeseries patterns during normal conditions, the time-series patterns that show signs of failure, and the time-series patterns after failure. Furthermore, by verifying the prediction accuracy and learning efficiency, it was showed that the proposed method is capable of faster learning while retaining the same or better performance compared to existing methods. Reference [1] A. Nair and C. Cai, "Acoustic emission monitoring of bridges: Review and case studies," Eng Struct, vol. 32, no. 6, pp. 1704-1714, 2010.[2] S. Müller, C. Drechsler, U. Heinkel, and C. Herold, "Acoustic emission for state-of-health determination in power modules," in 2016 13th International Multi-Conference on Systems, Signals & Devices (SSD), 2016: IEEE, pp. 468-471.[3] R. Dey and F. M. Salem, "Gate-variants of gated recurrent unit (GRU) neural networks," in 2017 IEEE 60th international midwest symposium on circuits and systems (MWSCAS), 2017: IEEE, pp. 1597-1600.

Multi-instrumented analysis of fatigue behavior and damage mechanisms in jute fiber-reinforced polyester composites

The present work deals with the fatigue behavior of a jute fiber-reinforced polyester composites. Two stacking sequences were considered [0]8 (JP_0) and [+45/−45]2S (JP_45). The fatigue lifetimes diagrams were determined. They exhibit a fatigue limit lower than 20 MPa for JP_0 composites and lower than 15 MPa for JP_45, whereas the mean ultimate tensile stress σR is 42.9 MPa for JP_0 and 31.1 MPa for JP_45, and the lifetimes lower than 500000 cycles for JP_0 and 250000 cycles for JP_45. An exhaustive analysis of the fatigue damage is carried out by combining four approaches: measurement of the mechanical parameters, microscopic observations, X-ray tomography, acoustic emission (AE) monitoring. X-ray tomography revealed the presence of multiple fiber/matrix debondings within transverse yarns for the composites [0]8 and yarn/matrix debondings for the composites [+45/−45]2S. Density of matrix cracks is always more important (about 80%) for JP_45 than for JP_0 at the same relative stress level/σR, and decrease of a 3 times factor for JP_0 (5 for JP_45) when the relative stress increases from 50% to 80%. Two types of behaviors are observed: one for low stresses and another for high stresses. Damage evolution is followed by AE by using a supervised classification with 3 classes of events: matrix cracking, fiber/matrix debonding and fiber breakage. The global activities are at the most of 600000 signals, and decreases 10 times from 50% σR to 80% σR. The results of AE highlighted in addition the greater number of matrix cracks for low stresses in agreement with X-ray tomography in the same proportions. High stresses are characterized by a larger number of fiber breaks (4 times for both composites) and fibre/matrix debondings (2 times) for JP_0, unless for fiber/matrix debonding in JP_45 which decreases 3 times. The multi-instrumented damage approach allows to make a precise and quantitative analysis of the fatigue damage mechanisms. The results highlight that for the weak stress levels matrix cracking is predominating whereas for the high stress levels the fiber breakages are significant. A scenario of fatigue damage is proposed: for low stresses the behavior is controlled by the matrix and for high stresses the behavior is controlled by the fibers.

An effective method for laboratory acoustic emission detection and location using template matching

In this paper, a template matching and location method, which has been rapidly adopted in microseismic research in recent years, is applied to laboratory acoustic emission (AE) monitoring. First, we used traditional methods to detect P-wave first motions and locate AE hypocenters in three dimensions. In addition, we selected events located with sufficient accuracy (normally corresponding AE events of relatively larger energy, showing clear P-wave first motion and a higher signal-to-noise ratio in most channels) as template events. Then, the template events were used to scan and match other poorly located events in triggered event records or weak events in continuous records. Through cross-correlation of the multi-channel waveforms between the template and the event to be detected, the weak signal was detected and located using a grid-searching algorithm (with the grid centered at the template hypocenter). In order to examine the performance of the approach, we calibrated the proposed method using experimental data of different rocks and different types of experiments. The results show that the proposed method can significantly improve the detection capability and location accuracy, and can be applied to various laboratory and in situ experiments, which use multi-channel AE monitoring with waveforms recorded in either triggering or continuous mode.

Acoustic emission monitoring of wood materials and timber structures: A critical review

• Different NDE methods applied to wood and timber structures were explained. • Concept of AE and its experimentation was introduced. • Impact of wood characteristics on AE wave (velocity, attenuation) was discussed. • Applications of AE in monitoring of wood materials were critically reviewed. • Opportunities, challenges, and outline of future research were discussed. The growing interest in timber construction and using more wood for civil engineering applications has given highlighted importance of developing non-destructive evaluation (NDE) methods for structural health monitoring and quality control of wooden construction. This study, critically reviews the acoustic emission (AE) method and its applications in the wood and timber industry. Various other NDE methods for wood monitoring such as infrared spectroscopy, stress wave, guided wave propagation, X-ray computed tomography and thermography are also included. The concept and experimentation of AE are explained, and the impact of wood properties on AE signal velocity and energy attenuation is discussed. The state-of-the-art AE monitoring of wood and timber structures is organized into six applications: (1) wood machining monitoring; (2) wood drying; (3) wood fracture; (4) timber structural health monitoring; (5) termite infestation monitoring; and (6) quality control. For each application, the opportunities that the AE method offers for in-situ monitoring or smart assessment of wood-based materials are discussed, and the challenges and direction for future research are critically outlined. Overall, compared with structural health monitoring of other materials, less attention has been paid to data-driven methods and machine learning applied to AE monitoring of wood and timber. In addition, most studies have focused on extracting simple time-domain features, whereas there is a gap in using sophisticated signal processing and feature engineering techniques. Future research should explore the sensor fusion for monitoring full-scale timber buildings and structures and focus on applying AE to large-size structures containing defects. Moreover, the effectiveness of AE methods used for wood composites and mass timber structures should be further studied.

Effect of interfacial angle on the mechanical behaviour and acoustic emission characteristics of coal–rock composite specimens

This paper studies the mechanical and acoustic emission (AE) evolution characteristics of coal–rock composite specimens with different interfacial angles. Uniaxial compression tests with various interfacial angle are conducted, with acoustic emissions features being monitored. The test results show that both the peak stress and elastic modulus decrease linearly as the interfacial angle increases, whereas the peak strain does not exhibit a clear trend. Meanwhile, the failure mode changes from matrix splitting to interface slip failure. The AE evolution characteristics of a coal–rock composite specimen can be categorized into three stages: quiet period, active period and remission period. With an increase in interfacial angle, the duration of the quiet period gradually decreased. With the increase of stress, the RA value first stabilizes and then rises, while the AF value has a trend of periodic fluctuation and continuous decline. The decrease of AF and increase of RA can be used as precursors of instability in coal–rock composite specimens. The failure mechanism of the coal–rock composite structure is analyzed based on the mechanical model containing the structural plane, and the theoretical analysis result is consistent with the experimental result. The paper also discussed the field AE monitoring applications, and the results in this study can provide a reference for the study of rib failures when multi-layers are presented in the roadway.

A study on moment tensor inversion of acoustic emission response on damaging localization of gas-bearing coal under load

During the deformation and fracture process, the acoustic emission (AE) signals can be produced for the of coal, rock and other solid materials, which revealing the damage localization evolution process. The effect of gas adsorption and pressure can change mechanical properties of coal mass and affect its damage development. Based on this, the experimental system for gas-bearing coal loading and AE monitoring was constructed, to analyze AE response characteristics under the joint action of loading stress and gas pressure on coal specimen. Afterwards, the damage localization evolution process of coal mass was studied with the moment tensor inversion method. Results showed that temporal response of AE signals was closely related to the damage degree and loading level of coal specimen, which could reveal its local severe damage and final failure characteristics. The spatial distribution and spread trend of AE fracture events inside coal specimen could be calculated through the moment tensor inversion method. It was basically consistent with the results of crack expansion on the specimen surface. The zones, where fracture events occurred intensively, gathered and spread in a continuous trend, were conductive to forming the macrocrack belt macroscopically. It could be regarded as the hazard zone with dynamic failure occurrence. Moreover, when the coal specimen faced the critical failure, the precursor characteristics of AE response appeared with the shear fracture events dominated markedly. The study results provide a new research idea for revealing the damaging localization evolution process under the coupling effect of stress and gas and lay the application foundation.

Development of a multi-modal sensor network to detect and monitor knee joint condition

Osteoarthritis is a major cause of mobility problems in older people and is a particular problem in former sportspeople. The objective of this study was to develop and characterise a new system for the detection, monitoring and analysis of acoustic emissions from knee joints. 15 adult volunteers participated in the study. The participants performed six sets of three sit-stand-sit cycles. Reflective markers were placed at specific body landmarks recorded by 3D cameras. The exercise was performed with one foot on a force platform. A sensitive condenser microphone with a wide frequency response was connected to a dedicated acoustic analysis unit. Preliminary results provide clear acoustic signals showing a distinctive sequence of impulse-decay forms occurring naturally during each sit-stand-sit cycle. There are distinct differences between the acoustic signals emitted from younger healthy knees and those from aged knees. This work demonstrates the potential for this system to be used as an indication of the state of health of a human knee during movement.