Cumulative Acoustic Emission Counts Research Articles

Effect of Curing Temperature on Mechanical Performance and Acoustic Emission Properties of Cemented Coal Gangue-Fly Ash Backfill

Cemented coal gangue-fly ash backfill (CGFB), which is a mixture of coal gangue, fly ash, binder and water, is introduced and employed to fill underground mined-out areas of coal mines. Once placed, the CGFB mixtures should have favorable stability, which is closely relevant to the mechanical performance of the CGFBs. The mechanical behavior of CGFB is affected by the environment temperature. In this study, with the help of acoustic emission (AE) and infrared thermal imaging technique, the influence of curing temperature on the mechanical behavior of CGFB specimens is investigated. A uniaxial compressive strength (UCS) testing apparatus coupled with AE monitoring and infrared thermography is equipped to investigate the mechanical (UCS, elastic modulus, stress–strain relation), acoustic (AE cumulative counts and energy), and thermal (average infrared radiation temperature) properties of CGFB samples cured for different ages (1, 3 and 7 days) at various curing temperatures (20, 50, 75 and 90 °C). This study provides an effective coupled monitoring method for evaluating the performance of CGFB structures, and this can contribute to a better understanding of the thermo-mechanical-acoustic behavior of CGFBs, and thus a better design of stable CGFBs.

Mechanical properties and fracture evolution of sandstone specimens containing different inclusions under uniaxial compression

To investigate the effects of inclusions filled in holes, including inclusion strength, stiffness and shape, on the mechanical properties and fracture evolution of rock, a series of uniaxial compression tests were conducted on prismatic sandstone containing a prefabricated hole filled with different types of inclusions using a servo-hydraulic machine. Digital image correlation (DIC) and acoustic emission (AE) techniques were applied jointly to record and analyze the deformation and crack process of sandstone. The results indicate that both the inclusion type and shape are important factors affecting the strength and deformation properties of sandstone. The relatively rigid inclusion not only significantly improves the mechanical properties of pre-holed specimens, but also can provide a better support for structural integrity of rock after failure. Observations show that both DIC and AE techniques are capable of revealing the fracture evolution of specimens, and there is a good consistency between the mechanical behavior and surface strain fields and internal AE signals. Based on the strain localization characteristics, the crack sequences and relative stress levels for each group of specimens are summarized, especially the crack development inside the inclusion is first observed. The crack behavior and crack type around inclusions are greatly influenced by the inclusion type and shape due to dramatic changes in stress distribution around inclusions. After rock failure, the crack extent around inclusions can be characterized by the cumulative AE counts, and the final failure modes of specimens under different filling conditions can be categorized as tensile failure, shear failure and mixed tensile/shear failure.

Quasi-Static Flexural Properties of a Pultruded Glass Fiber/Unsaturated Polyester Square Pipe

This paper explores quasi-static flexural properties and fracture behavior of a pultruded glass fiber/unsaturated polyester square pipe for automotive structural applications. Three-point flexural testing is performed in an Instron Universal Testing Machine with steel jigs supporting the top and bottom surfaces of the pipe. Acoustic emission (AE) measurements are recorded during flexural testing to evaluate initial fracture in the pipe structure. After final fracture, five cross-sections of the pipe are cut at 50-mm intervals along the longitudinal axis, with the first cut located at the mid-span of the pipe. Cross-sections of a pipe from an interrupted test where initial fracture is detected from the AE method are also prepared. Damage locations and behavior on each cross-section are observed. The flexural testing results show that the cumulative AE counts increase rapidly from 2.5 kN, that final failure occurs at a maximum load of approximately 13 kN, and that corresponding initial and final failure occurs in the two corner regions on the compressive side of flexural loading. Failure initiates by stress concentrations due to the upper jig on the top surface during bending. The cross-sectional observations also reveal clear deformation behavior of the pipe where failure is present, marked by inward bending of the top surface and upper corners located on the compressive side, near the jig. The locations of maximum stresses and deformations obtained from finite element analysis of this pipe structure are in very good agreement with the experimental observations.

Damage Evolution Characteristics of Rock Salt under Different Stress Conditions

Understanding the damage evolution characteristics of rock material is essential to the long‐term stability and safety analysis of the underground facility. In this study, a series of cyclic loading tests under tensile or compressive stresses are conducted to investigate the damage evolution, deformation, peak strength, and failure pattern of rock salt. A special attention is paid on the microcracking process by using a 3D acoustic emission (AE) test system. The laboratory tests show that the damage degree of rock salt under compression is the highest, followed by the damage in the direct tensile test. The lowest value of damage is determined by using the Brazilian test. The damage degrees where the damage rate starts to decrease are about 0.83 in the direct tensile test, about 0.75 in the Brazilian test, and about 0.91 in the compression test. The failure mode of rock salt changes from the tensile mode in the uniaxial compression test to the compression‐shear mode in the confined compression test at low confinement. But from the confining pressure of 15 MPa, the rock salt displays great plastic dilatant distortion and without appreciable macroscopic fractures. Accordingly, with increasing confining pressure, the positions where the rapid increase in cumulative AE counts occurs and where the AE event with high energy appears are changed, from the beginning of the test at low confinement to the postpeak stage of the test at high confinement.

Investigation of the acoustic emission characteristics during deformation and failure of gas-bearing coal-rock combined bodies

With increasing mining depth, coal-gas compound dynamic disasters have become an important factor restricting mining safety. In the present study, conventional triaxial compression (CTC) tests were conducted on the gas-bearing coal, gas bearing coal-mudstone combination and gas bearing coal-sandstone combination using the RLW-500G triaxial experimental system. The gas bearing coal-sandstone combined samples were subjected to unloading tests, including unloading confining pressure (UCP) under constant axial tests and UCP-reloading axial stress (UCP-RAS) tests. In addition, the acoustic emission (AE) signals and permeabilities were measured simultaneously during the mechanical process. The experimental results indicate that the deformation of the coal-rock body is stronger under unloading conditions than in the CTC tests. Moreover, the damage of the coal-rock combination body is more severe in the UCP-RAS tests than in the UCP tests. Under lower confining pressure, the AE cumulative counts and the energy are higher for the gas-bearing coal-rock body than the gas bearing coal body. As the confining pressure increases, the AE cumulative counts and the energy are lower for the gas-bearing coal-rock body than for the gas bearing coal body. The AE cumulative counts and the energy of the three specimens under different stress paths decrease with the increase in the confining pressure or with the decrease in the gas pressure. The AE cumulative counts and energy of the gas-bearing coal-rock body are highest for the UCP-RAS test, followed by the UCP test and the CTC test. This study provides some references for understanding the mechanisms of coal-gas compound dynamic disasters and the basis for an early warning system.

Peak frequency analysis via wavelet transform for impact damage mechanisms in woven composites

Impact damages have been a major concern in the design of laminated composite structures since the damages mainly occur within the materials in a very short time. In this article, impact induced damage mechanisms of woven composites were investigated by means of novel SHPB-AE coupled tests. A step-consisting new methodology was developed; first, damage mechanisms of woven composites were determined by peak frequency analysis via wavelet transform (WT); second, the extraction of a range of acoustic emission (AE) parameters was achieved from AE energy, duration, and amplitude on the basis of peak frequency analysis; finally, the quantification of damage mechanisms was accomplished through a normalized cumulative AE count. In order to confirm the features of damage, the microscopic characterization of damage mechanisms was performed for the materials. As a result, the determination of impact damage mechanisms in tested composite materials was possible by peak frequency analysis via WT along with waveform and intensity examinations. In addition, the mechanisms could be identified by confirming the progress-based distribution and band formation of peak frequencies. AE parameter ranges were also extracted in terms of amplitude, duration, and energy according to the damage mechanisms. The damage process of the woven composites under impact loading began in the matrix and immediately caused fiber breakage to occur. Subsequently, multiple damage mechanisms were shown, such as fiber-matrix debonding, fiber pull-out, severe deformation and rupture in Kevlar fiber tips. The proposed methodology for the novel SHPB-AE coupled test of woven composites therefore showed an effective way to figure out in situ information on the damage under impact loading.

Effects of water content on fracture and mechanical behavior of sandstone with a low clay mineral content

This study investigated the effect of water content on quasi-static fracture behavior of sandstone. Notched semi-circular bending (NSCB) tests were conducted on a total number of 20 sandstone specimens with different water contents (0, 1.0, 2.0 and 3.5%) to determine their fracture toughness. During the NSCB tests, the cracking process and acoustic emission (AE) signals were recorded continuously with the aid of a charged couple discharge (CCD) camera and an AE system, and the crack propagation velocity was also measured accurately via a crack propagation gauge (CPG). Test results demonstrated that both the fracture toughness and crack propagation velocity observably decreased with the increase of water content, the variation trend of which could be described by exponential equations. The cumulative AE counts of wet specimens in the NSCB tests were much less than those of dry ones, which indicated that the sandstone specimens underwent more ductile failure and released less elastic energy due to water-softening effects.

Effects of Thermal Damage on Strain Burst Mechanism for Brittle Rocks Under True-Triaxial Loading Conditions

Strain burst is a common problem encountered in brittle rocks in deep, high-stress mining applications. Limited research focuses on the effects of temperature on the strain burst mechanism and the kinetic energies of rocks. This study aims to investigate the effects of thermal damage on the strain burst characteristics of brittle rocks under true-triaxial loading-unloading conditions using the acoustic emission (AE) and kinetic energy analyses. The time-domain and frequency-domain responses related to strain burst were studied, and the damage evolution was quantified by b-values, cumulative AE energy and events rates. The ejection velocities of the rock fragments from the free face of the granite specimens were used to calculate kinetic energies. The experimental results showed that thermal damage resulted in a delay in bursting but increased the bursting rate at ~ 95% of normalised stress level. This is believed to be due to the micro-cracks induced by temperature exposure, and thus the accumulated AE energy (also supported by cumulative AE counts) at the initial loading stage was reduced, causing a delay in bursting. The strain burst stress, initial rock fragment ejection velocity, and kinetic energy decreased from room temperature (25 °C) to 100 °C, whereas they resulted in a gradual rise from 100 to 150 °C demonstrating more intense strain burst behaviour.

Scale Effect on the Anisotropy of Acoustic Emission in Coal

Acoustic emission (AE) in coal is anisotropic. In this paper, we investigate the microstructure‐related scale effect on the anisotropic AE feature in coal at unconfined loading process. A series of coal specimens were processed with diameters of 25 mm, 38 mm, 50 mm, and 75 mm (height to diameter ratio of 2) and anisotropic angles of 0°, 15°, 30°, 45°, 60°, and 90°. The cumulative AE counts and energy dissipation increase with the specimen size, while the energy dissipation per AE count behaves in the opposite way. This may result from the increasing amount of both preexisting discontinuities and cracks (volume/number) needed for specimen failure and the lower energy dissipation AE counts generated by them. The effect of microstructures on the anisotropies of AE weakens with the increasing specimen size. The TRFD and its anisotropy reduce as the specimen size increases, and the reduction of fractal dimension is most pronounced at the anisotropic angle of 45°. The correlation between TRFD and cumulative AE energy in the specimens with different sizes are separately consistent with the negative exponential equation proposed by Xie and Pariseau. With the specimen size gain, the reduction of the TRFD weakens with the increasing amount of cumulative absolute AE energy.

Fractal characteristics and acoustic emission of anisotropic shale in Brazilian tests

The strata of shale contain structural weak planes such as laminations and joints, which do not have the same mechanical properties as intact rock masses. Tensile strength is a critical parameter that determines the capacity of a rock and its resistance to deformation and failure. Focusing on tensile strength and acoustic emission (AE), the characteristics of shale were investigated at various orientations of the laminations with respect to the loading direction. By coupling Brazilian test and the AE technique, the mechanical properties and damage patterns of shale can be explored. The shale exhibits clear laminations and contains a high proportion of brittle minerals by XRD and SEM analysis. The stress–time curve of the Brazilian test can be divided into three stages with distinct brittleness characteristics, and the tensile strength exhibited undulatory trend as bedding angles increases. At low bedding angles, the compaction of fissures and pores within the shale is not significant, and the cumulative AE count–time curves exhibited a flat-to-sharply rising trend. By contrast, the curves showed a gradually increasing “stepped” tendency at high bedding angles. Analysis of the AE time sequence based on fractal theory reveals that fractal dimension values fluctuate with increase of the stress, signifying the initiation of complex microcracks within the shale. The fractal dimension values sharply dropped when approaching the limit of tensile strength, signifying the occurrence of major cracks. The sudden drop of AE time sequence correlation dimension values can serve asan early warning for the coming failures. The research findings could be instrumental in the monitoring of rock mass instability, microcrack mechanisms, and earthquake sequences.

Acoustic emission studies for characterization of fatigue crack growth in 316LN stainless steel and welds

This paper focuses on the study of the stages IIa and IIb in Paris region of fatigue crack growth (FCG) for base metal and weld specimens of 316LN stainless steel (SS) by using acoustic emission (AE) technique. Meanwhile, the fatigue properties and AE characteristics were analyzed based on the K-means clustering method and fractographic observations. The results show that 316 SS weld exhibits a higher resistance to crack growth than that of base metal due to the deflection of crack path under mixed loading mode. The transition from stage IIa to IIb in the Paris region can be identified by the change of slope in AE cumulative count and cumulative energy vs. ΔK, which otherwise is not feasible from da/dN vs. ΔK plots. The AE signal characteristics of different source mechanisms such as ductile crack growth, plastic deformation within cyclic plastic zone and shear crack growth are significantly different in the two sub-stages during fatigue. The results suggest that AE technique is sensitive to the identification of the presences of stage IIa, stage IIb in the Paris region and the shear crack growth during FCG.

Acoustic Emission Parameters of Three Gorges Sandstone during Shear Failure

In this paper, an experimental investigation of sandstone samples from the Three Gorges during shear failure was conducted using acoustic emission (AE) and direct shear tests. The AE count rate, cumulative AE count, AE energy, and amplitude of the sandstone samples were determined. Then, the relationships among the AE signals and shearing behaviors of the samples were analyzed in order to detect micro-crack initiation and propagation and reflect shear failure. The results indicated that both the shear strength and displacement exhibited a logarithmic relationship with the displacement rate at peak levels of stress. In addition, the various characteristics of the AE signals were apparent in various situations. The AE signals corresponded with the shear stress under different displacement rates. As the displacement rate increased, the amount of accumulative damage to each specimen decreased, while the AE energy peaked earlier and more significantly. The cumulative AE count primarily increased during the post-peak period. Furthermore, the AE count rate and amplitude exhibited two peaks during the peak shear stress period due to crack coalescence and rock bridge breakage. These isolated cracks later formed larger fractures and eventually caused ruptures.

An experimental investigation on acoustic emission characteristics of sandstone rockburst with different moisture contents

Rockburst occurred frequently during deep mining in China. The mechanism of rockburst is very complicated and related to many factors. In order to investigate the influence of moisture contents of rockmass on rockburst, we conducted a series of laboratory rockburst experiments of sandstone under three different moisture contents by the Modified True-Triaxial Apparatus (MTTA), in which the acoustic emission (AE) system was employed to monitor the internal damage of rock mass. A high-speed video camera was utilized to record the detail of rockburst. Based on the experimental results, the AE characteristics, such as AE count, AE energy, and AE frequency, were analyzed. The rockburst process, type, and indensity under different moisture contents were discussed. The research results show that with the increase of moisture contents, rock strength was soften, the elastic and the cumulative damage of the rock were reduced, resulting in a gradual decrease in AE cumulative counts and cumulative energy over the course of rockburst. This study provides an experimental basis and reference for better understanding to the rockburst mechanism and control.

Acoustic emission characteristics and stress release rate of coal samples in different dynamic destruction time

Considering the importance of the prediction of rock burst disasters, and in order to grasp the law of acoustic emission (AE) of coal samples in different dynamic destruction time, the SH-II AE monitoring system was adopted to monitor the failure process of coal samples. The study of the change rule of the AE numbers, energy, ‘b’ value and spectrum in the micro crack propagation process of the coal samples shows that as dynamic damage time went by, AE presented high-energy counts and the accumulated counts increased during the compression phase. The AE energy and cumulative counts increased during the elastic stage. The AE blank area increased gradually and the blank lines were more and more obvious in the molding stage. The AE counts and energy showed a trend of decrease in the residual damage phase. AE ‘b’ values gradually became sparse, and the large scale cracks percentage compared with micro cracks decreased and the degree of damage decreased. The AE frequency spectrum peak went from the residual damage phase to the molding phase, and finally it was nearly stable, besides the bandwidth of the main frequency is gradually narrowed. Also, the frequency peak changed from single peak frequency to bi-peak frequency and to the single peak frequency. Uniaxial compressive strength is more sensitive than the elastic modulus to dynamic damage time.

High strain-rate failure in carbon/Kevlar hybrid woven composites via a novel SHPB-AE coupled test

Abstract The high strain-rate induced failure characteristics in a carbon/Kevlar hybrid composite subjected to high strain-rate compressive loading were studied using a novel SHPB-AE coupled test. The tests were performed based on a split-Hopkinson pressure bar (SHPB) apparatus and an acoustic emission (AE) technique, respectively. Within the strain-rate range of 1002–1941 s −1 , a high strain-rate compressive test was conducted on the cylindrical carbon/Kevlar hybrid composite specimens located between the incident and transmitted bars. A wide bandwidth type AE sensor was connected to the specimen with a fine copper waveguide to monitor the AE signals in real time during the test. Specific types of failure mechanisms were observed with optical microscopy and scanning electron microscopy. First, AE characteristics originating from the specimens were investigated profoundly to distinguish the AE signals from diverse damage or failure sources. AE signals were then analyzed in terms of the AE amplitude, the AE count, the slope of cumulative AE count and the peak frequency. Finally, signals were classified into four types based on the waveform and corresponding peak frequency by a short time Fourier transforms (STFT). As a result, under high strain-rate compressive loading, the strain level at the damage initiation was shortened with increasing strain-rate. The failure process of the carbon/Kevlar hybrid woven composite showed initial matrix fracture and then brittle carbon fiber breakage. Subsequently, multiple failure mechanisms appeared, such as fiber-matrix debonding, fiber pull-out, excessive deformation and breakage in the Kevlar fiber tips including splitting and fibrillation. The application of the novel SHPB-AE coupled test to the carbon/Kevlar hybrid composite was discussed in depth for characterizing the failure process, and it was an effective and relevant methodology to grasp in situ information on the failures under high strain-rate compressive loading.

Evaluation of Failure Modes of Pure Resin and Single Layer of Adhesively Bonded Lap Joints Using Acoustic Emission Data

The purpose of this present study is to monitor the failure modes of pure resin and single layer of adhesively bonded lap joints using acoustic emission (AE) technique under tensile loading. Parametric analysis is performed using AE count rate, cumulative counts, time, frequency, amplitude and duration on the AE data obtained during the tensile test of adhesively bonded lap joints. After preliminary investigations in the parametric analysis, it was seen that AE amplitude parameter changes with the different AE events, thus failure modes were characterized using frequency analysis. Fast fourier transform (FFT) analysis has been proposed to identify the importance of peak frequency content of each failure mode corresponding to the AE hits using frequency FFT analysis. Short time fast fourier transform resulting frequency is correlated with FFT analysis of AE data, to find the peak frequency ranges for each of the failure modes. Scanning electron microscope as complementary, post-test inspection method is used to find microscopic evidence for the assumed assignment of failure modes.

Study of Acoustic Emission and Mechanical Characteristics of Coal Samples under Different Loading Rates

To study the effect of loading rate on mechanical properties and acoustic emission characteristics of coal samples, collected from Sanjiaohe Colliery, the uniaxial compression tests are carried out under various levels of loading rates, including 0.001 mm/s, 0.002 mm/s, and 0.005 mm/s, respectively, using AE-win E1.86 acoustic emission instrument and RMT-150C rock mechanics test system. The results indicate that the loading rate has a strong impact on peak stress and peak strain of coal samples, but the effect of loading rate on elasticity modulus of coal samples is relatively small. When the loading rate increases from 0.001 mm/s to 0.002 mm/s, the peak stress increases from 22.67 MPa to 24.99 MPa, the incremental percentage is 10.23%, and under the same condition the peak strain increases from 0.006191 to 0.007411 and the incremental percentage is 19.71%. Similarly, when the loading rate increases from 0.002 mm/s to 0.005 mm/s, the peak stress increases from 24.99 MPa to 28.01 MPa, the incremental percentage is 12.08%, the peak strain increases from 0.007411 to 0.008203, and the incremental percentage is 10.69%. The relationship between acoustic emission and loading rate presents a positive correlation, and the negative correlation relation has been determined between acoustic emission cumulative counts and loading rate during the rupture process of coal samples.

Acoustic Emission of Hydrogen Bubbles on the Counter Electrode during Pitting Corrosion of 304 Stainless Steel

An acoustical investigation into the hydrogen bubbles on the counter electrode during pitting corrosion of 304 stainless steel has been carried out with the potentiodynamic method. After reaching the pitting potential, there was a short time delay before an Acoustic Emission (AE) signal was detected. Given this, the obtained cumulative AE counts were divided into three distinct stages showing their own unique behavior in terms of rise time and duration. The bubble evolution characteristic in each different stage was supposed to be responsible for the feature of AE signal. In the AE Stage III, the cumulative counts of the AE signals provided empirical correlations with both the total number and the total volume of corrosion pits, thus demonstrating the potential of utilizing the AE signal of hydrogen bubble on the counter electrode as a nonintrusive monitoring tool of pitting corrosion. [doi:10.2320/matertrans.M2014373]

Low temperature tensile deformation and acoustic emission signal characteristics of AISI 304LN stainless steel

This investigation examines low temperature tensile deformation behavior of AISI 304LN stainless steel along with synergistic analysis of acoustic emission signals. The tensile tests are done at a range of temperatures starting from 283K till 223K. The fracture surfaces of the broken specimens are investigated using scanning electron microscope. The amount of deformation induced martensite is measured using a feritscope. The obtained results reveal that with decrease in test temperature, both strength and ductility increase. The increase in strength and ductility with decreasing temperature is explained in terms of void morphologies and formation of deformation induced martensite. The rapid increment in strength and ductility at 223K is associated with the burst of martensitic transformation at that temperature; which has been clarified from acoustic emission signals. An additional initiative has been taken to model the evolution of martensite formation from the observed cumulative emission counts using a non linear logarithmic functional form. The fitted curves from the recorded acoustic emission cumulative count data are found to be better correlated compared to earlier obtained results. However, at 223K normal non-linear logarithmic fit is not found suitable due to presence of burst type signals at intervals, therefore; piecewise logarithmic function to model acoustic emission bursts is proposed.

A damage-mechanism-based creep model considering temperature effect in granite

Based on the experimental studies of creep behavior of granite at different temperatures, a damage-mechanism-based creep model is proposed. In the creep tests, the significant thermal effect on the time-dependent behavior of granite is observed. The recorded acoustic emission (AE) events indicated that the overall variation of cumulative AE count during the creep test obeys an exponential function, and the induced microcracks are intensively accumulated with the appearance of the accelerated creep. Based on this understanding, by incorporating the damage evolution process, a creep model was formulated to describe the time-dependent deformation of granite at different temperatures. Finally, a preliminary validation of the proposed model is performed.