Acoustic Emission Instrument Research Articles

Micro- and macro-scale fracture behaviour of brittle rocks: Comparison between the conventional Brazilian test and the advanced universal snap-back indirect tensile test (AUSBIT)

Post-peak behaviour is crucial for the estimation of rock mass fracturing in cave mining operations where hard rocks can exhibit class-II or snap-back response when subjected to loading. Despite the rapid development of research into class-II rocks under compression, the corresponding behaviour in tensile tests has rarely been investigated, which is critical considering the complexity of rock mass fracturing under various stress states. The post-peak response of brittle rocks involves abrupt micro-fracturing, leading to brittle macro-scale behaviour. Controlling the fracture process using the Advanced Universal Snap-Back Indirect Tensile test (AUSBIT) allowed the acquisition of the complete macro-scale class-II behaviour in the post-peak regime, facilitating the use of advanced techniques for insights into both micro and macro-scale fracture. In this study, the AUSBIT tests with digital image correlation (DIC) and acoustic emission (AE) instrumentation were conducted to analyse the progressive failure in Calca granite and Gosford sandstone specimens. Post-test observations of the fracture surfaces were performed using a scanning electron microscope (SEM). From a macroscale viewpoint, the lateral strain control in AUSBIT enabled controlled cracking with significant lateral strain extension prior to failure accompanied by gradual energy dissipation and higher rates of AE activity as smaller magnitudes of energy are being released by each AE hit or microcrack compared to conventional Brazilian tests. The stable microcrack propagation was also identified from SEM observations with more uniform profiles of microcracks and less debris observed in AUSBIT specimens. These findings were more significant in Calca granite, which verified its extreme class-II behaviour while also demonstrating the efficiency of AUSBIT in controlling the violent failure of high-strength brittle rocks commonly encountered in deep mining projects, leading to the acquisition of more accurate material behaviour in terms of micro and macro-scale post-peak features which was unattainable from conventional indirect tensile tests.

Characterization of Acoustic Emissions from Concrete Based on Energy Activity Coefficient

Single-stage compression loading experiments were carried out on concrete specimens of various strengths to explore the characteristic parameters of the acoustic emission signal and its damage evolution law in the concrete damage process. These specimens were monitored in real time with acoustic emission and DIC instruments during the loading process, and internal pores and slices were scanned with CT scanning instruments after compression. The acoustic emission phenomenon was expressed using the energy activity coefficient, and the law relating to the phenomenon was summarized. The results show that when the peak and mean values in the first adjacent time domain grow rapidly, the specimen produces a large crack and enters the stage of rapid crack development, which can be taken as an indication of the impending damage to the specimen. The energy activity coefficient reflects the damage development intensity as follows: the smaller the energy activity coefficient, the more the cracks developed; the faster the speed, the larger the deformation. With an increase in the load level, the energy activity coefficient gradually tends to stabilize, and the specimen enters the stage of rapid crack development. However, when the energy activity coefficient suddenly increases again, the specimen is destabilized and destroyed. Therefore, the energy activity coefficient responds to the degree of congenital defects in the specimen. As the load increases, the energy activity coefficient is more stable, and the defects are smaller; in contrast, the energy activity coefficient drastically oscillating indicates that the material is very defective.

Research on a Corrosion Detection Method for Oil Tank Bottoms Based on Acoustic Emission Technology.

This paper presents an acoustic emission (AE) detection method for refined oil storage tanks which is aimed towards specialized places such as oil storage tanks with high explosion-proof requirements, such as cave oil tanks and buried oil tanks. The method utilizes an explosion-proof acoustic emission instrument to detect the floor of a refined oil storage tank. By calculating the time difference between the defective acoustic signal and the speed of acoustic wave transmission, a mathematical model is constructed to analyze the detected signals. An independent channel AE detection system is designed, which can store the collected data in a piece of independent explosion-proof equipment, and can analyze and process the data in a safe area after the detection, solving the problems of a short signal acquisition distance and the weak safety protection applied to traditional AE instruments. A location analysis of the AE sources is conducted on the bottom plate of the tank, evaluating its corrosion condition accurately. The consistency between the evaluation and subsequent open-tank tests confirms that using AE technology effectively captures corrosion signals from oil storage tanks' bottoms. The feasibility of carrying out online inspection under the condition of oil storage in vertical steel oil tanks was verified through a comparison with open inspections, which provided a guide for determining the inspection target and opening order of large-scale oil tanks.

Influences of clean fracturing fluid viscosity and horizontal in-situ stress difference on hydraulic fracture propagation and morphology in coal seam

The viscosity of fracturing fluid and in-situ stress difference are the two important factors that affect the hydraulic fracturing pressure and propagation morphology. In this study, raw coal was used to prepare coal samples for experiments, and clean fracturing fluid samples were prepared using CTAB surfactant. A series of hydraulic fracturing tests were conducted with an in-house developed triaxial hydraulic fracturing simulator and the fracturing process was monitored with an acoustic emission instrument to analyze the influences of fracturing fluid viscosity and horizontal in-situ stress difference on coal fracture propagation. The results show that the number of branched fractures decreased, the fracture pattern became simpler, the fractures width increased obviously, and the distribution of AE event points was concentrated with the increase of the fracturing fluid viscosity or the horizontal in-situ stress difference. The acoustic emission energy decreases with the increase of fracturing fluid viscosity and increases with the increase of horizontal in situ stress difference. The low viscosity clean fracturing fluid has strong elasticity and is easy to be compressed into the tip of fractures, resulting in complex fractures. The high viscosity clean fracturing fluids are the opposite. Our experimental results provide a reference and scientific basis for the design and optimization of field hydraulic fracturing parameters.

Seismic behavior of polypropylene fiber reinforced rubber concrete-filled circular steel tubular columns

In this study, the seismic behavior of polypropylene fiber reinforced rubber concrete-filled circular steel tubular columns subjected to low cyclic loading was investigated. The effects of varying rubber content, slenderness ratio, axial compression ratio, and steel content on specimen damage were analyzed. Digital Image Correlation (DIC) and Acoustic Emission (AE) instruments were utilized to monitor the full-field strain on the surface of the specimens and the generation and development of internal concrete cracks. A fiber modeling method was developed for calculating the compression bending bearing capacity of the columns. The results indicated a uniform failure pattern, primarily located at the column base. Adding rubber powder decreased the bearing capacity of the specimen, yet enhanced their energy dissipation capacity and ductility. Higher axial compression ratio led to early failure in the steel tube, with the slenderness ratio significantly affecting seismic performance. The calculation methods according to GB 50936–2014 (CECS 254–2012) and DL T5085–1999 showed higher accuracy than other codes, with average values of 0.558 and 0.576, and standard deviations of 0.067 and 0.089, respectively. The variation coefficient for the calculation method in GB 50936–2014(CECS 254–2012) was 12.02%, lower than that of DL T5085–1999. The proposed fiber modeling method demonstrated superior calculation accuracy, with an average value of 0.957, a standard deviation of 0.032, and a variation coefficient of 3.29%.

Acoustic emission source location method for glass fibre reinforced plastics based on virtual loading focusing enhancement technology

ABSTRACT Because of the inhomogeneity and anisotropy of glass fibre reinforced plastics (GFRP), it is always a challenge to detect the real-time dynamic quality during manufacturing and service. In this paper, a virtual loading focusing enhancement technology (VLFET) based on time reversal (TR) theory is proposed to accurately locate the acoustic emission (AE) source in the GFRP. First, the focusing enhancement theoretical model with virtual reverse loading is deduced according to the TR theory. Then, after measuring the optimal detection range and the average acoustic velocity, the sensor array is positioned on the GFRP specimen. The excitation signals applied to simulate the AE source at some positions are produced by using lead breaking, and the detection signals are received by the sensor array. Finally, the signals received by the sensor array are processed according to the theoretical model, and the fluctuation image in the monitoring area is reconstructed with fluctuation amplitude of each pixel to evaluate the AE source position. The experimental results demonstrate that the excitation sources can be accurately evaluated with the VLFET. The location error with the proposed method is less than 3.1%, higher than that from the current AE instrument. The proposed technology can also be applied to the AE evaluation of other anisotropic materials.

Experimental study on the effect of anchors on the creep mechanical properties and acoustic emission characteristics of fractured sandstone under uniaxial compression

This study conducts laboratory tests to investigate the creep and acoustic emission (AE) of unanchored and anchored fractured sandstone samples under uniaxial compression by taking the Triassic fine sandstone collected from the Xiaolangdi Reservoir area as the research object and ensuring the same testing conditions using an RLJW-2000 rock rheometer and a PCI-Ⅱ AE instrument. Moreover, this study identified the effects of anchors on the creep mechanics and AE characteristics of fractured sandstone samples by analyzing the strain, long-term strength, AE event count, AE rate, AE energy rate, and rising angle – average frequency (RA-AF) characteristics of each sample before and after anchoring. According to the study results: (1) Under the same stress level, the anchored sample's instantaneous strain, creep strain, and total strain are smaller than those of the unanchored sample. Anchors exerted the largest inhibitory effect on creep strain, followed by the total strain, while anchors inhibited instantaneous strain to the smallest extent. (2) Compared to the unanchored sample, the anchored sample experienced a significant increase (as high as 50%) in long-term strength. Anchor reinforcement significantly improves the long-term strength of samples. (3) Under the same stress level, the AE event count and cumulative AE event count of the anchored sample are smaller than those of the unanchored sample. Anchors significantly inhibited microcracks initiation, propagation, overlapping, and penetration in samples. (4) The maximum AE rate of the anchored sample is only 69.30% of that of the unanchored sample, whereas the peak AE energy rate of the anchored sample is only 23.21% of that of the unanchored sample. Anchors reduce the AE event count generated and the AE energy released by samples per unit of time, exerting a significant crack-arrest effect on them. (5) The presence of anchors changes the type and number of microcracks in the samples' creep process. Anchors greatly inhibit the generation of high-RA and low-AF AE signals and generate low-RA and high-AF ones. As a result, more tensile microcracks occur in samples under stress levels close to the failure stress level (especially the last stress level), which inhibit the formation of shear microcracks in samples and causes samples to be more prone to macroscopic tensile-shear failure. (6) The microcrack characteristics of samples characterized by RA-AF values are consistent with the macrocrack characteristics of samples. Therefore, RA-AF values can satisfactorily characterize the microcracks' variation characteristics in the sandstone samples' creep process before and after anchoring.

Acoustic Emission Characteristics of Different Porcine Bones Under Loading

In order to obtain the acoustic emission (AE) characteristics of fresh porcine bones of different parts under loadings, the fresh porcine femur, scapula and vertebra were investigated by using a compression testing machine and the AE instrument, respectively. The results show that the compression curve of the porcine bones has an obvious dependence on the shape of the porcine bones, and cracks can be observed with an increase of loading. Some obvious inflection points can also be found on the compression curve. The AE parameters of porcine bones under loading agrees well with the variation of the inflection points. The porcine femur exhibits the most obvious variation of AE parameters with an increase of loading and has the largest frequency band of 20--140~kHz and a dominant frequency value of 65 kHz. However, the AE energy of the scapula vertebra is weak under loading effects, and the vertebra has the smallest dominant frequency value of 25 kHz.

Study on Damage Characteristics of Deep Coal Based on Loading Rate Effect

In this paper, the uniaxial compression tests of coal under different loading rates are carried out by using the MTS rock mechanics loading system and the DS5 acoustic emission instrument, and the mechanical parameters and damage characteristics of coal samples under five loading rates are studied. The conclusions are as follows: The uniaxial compressive strength and the elastic modulus of the sample increase with the increase in the loading rate, whereas the strain at the peak point of stress decreases with the increase in the loading rate; at a low loading rate, the AE ring count of the coal samples is widely distributed, but the maximum AE ring count value is small. At a high loading rate, the AE ring count of the coal samples decreases, whereas the maximum AE ring count value increases significantly. With the increase in the loading rate, the maximum AE ring count and cumulative AE ring count of the sample increase. According to the damage variable curves of the coal samples under different loading rates, the damage evolution process of the samples before the stress peak is roughly divided into three stages: initial damage stage, stable damage development stage and rapid damage development stage. It is found that when the loading rate is low, the deformation time of the coal sample is relatively sufficient, whereas when the loading rate is high, the micro cracks have no time to develop and expand, the sample is destroyed instantly and the destruction of the coal sample is sudden. Therefore, with the increase in the loading rate, the damage variable value at the peak stress of the coal sample generally shows an increasing trend.

A crucial phase in plants - it's a gas, gas, gas!

A crucial phase in plants - it's a gas, gas, gas!

Advances on the acoustic emission testing for monitoring of granular soils

The application of the acoustic emission (AE) method for granular media has been considered to be challenging and remains limited. Nevertheless, recent successful explorations from different aspects have greatly encouraged people's attempts in this filed. Typical applications concerning geotechnical element testing, the soil-structure systems, as well as the AE source location suggest that the recent studies aim to obtain direct insights into the mechanical behaviors of the granular soils based on AE features, and emphasize on the quantitative relations between them. Future development of the AE method demands further advances in the following aspects: higher performance of AE instrumentation technology enabling high signal-to-noise ratio; full waveform and broadband signal data acquisition; new insights into the source mechanisms of AE by incorporating more comprehensive and in-depth signal analysis and emerging new technologies; higher levels of correlation study between AE signals and the associated physical processes.

Acoustic emission wave propagation in honeycomb sandwich panel structures

This paper studies acoustic emission (AE) wave propagation in a glass fibre aluminium honeycomb sandwich panel (HSP). Of particular novelty is the quantification of the through-thickness propagation of AE from one surface of the HSP to the other, which is a real-world monitoring consideration for applications where AE instrumentation is only be permissible on one surface, i.e. aerofoils. Complexity was introduced to the specimens in stages to enable a thorough understanding; first investigating propagation in a large glass fibre laminated plate (GFLP) alone; then in bespoke sandwich specimens with a limited number of honeycomb cells; and, finally, in a large HSP. The results of this paper demonstrate that, whilst some energy is transmitted through the honeycomb core, AE propagating in an HSP becomes bound in the outer plates. Despite this, propagation in these outer plates differs to that in an equivalent plate of the same material in isolation due to the complex interactions with the structural elements of the HSP. Further, propagation of AE transmitted from one surface to the other was quantified for the first time, giving insight into expected attenuation rates and characteristics for practical applications.

Creep properties and resistivity-ultrasonic-AE responses of cemented gangue backfill column under high-stress area

To investigate the creep and instability properties of a cemented gangue backfill column under a high-stress area, the uniaxial compression creep tests were conducted by single-step and multi-step loading of prismatic samples made of cemented gangue backfill material (CGBM) under the high stress-strength ratio. The creep damage was monitored using an electrical resistivity device, ultrasonic testing device, and acoustic emission (AE) instrument. The results showed that the CGBM sample has a creep hardening property. The creep failure strength (CFS) is slightly larger than the uniaxial compressive strength (UCS), ranging in ratio from 108.9% to 116.5%. The instantaneous strain, creep strain, and creep rate increase with increasing stress-strength ratio in the single-step loading creep tests. The instantaneous strain and creep strain decrease first and then increase during the multi-step loading creep process. The axial creep strain of the CGBM column can be expressed by the viscoelastic-plastic creep model. Creep instability is caused by the accumulation of strain energy under multi-step loading and the continuous lateral expansion at the unconstrained middle position during the creep process. The creep stability of a CGBM column in a high-stress area can be monitored based on the variation of electrical resistivity, ultrasonic pulse velocity (UPV), and AE signals.

Effect of initial minimum principal stress and unloading rate on the spalling and rockburst of marble: a true triaxial experiment investigation

Hard rock often performs as brittle failures, such as cracking, spalling, and rockburst, induced by excavation in deep underground engineering. To understand the effect of the minimum principal stress and unloading rate on the spalling and rockburst of marble, unloading experiments were carried out by using true triaxial equipment combined with a high-speed camera, acoustic emission instrumentation, and scanning electron microscope. The experimental results showed that the failure process of marble specimens was more stable and less inclined to exhibit dynamic ejection with a low strain energy release per unit time under a low unloading rate. At the same time, the initial minimum principal stress controlled the marble’s failure mode by changing its mode from predominantly shear failure to shear-tension failure as the minimum principal stress decreased, similar to the spalling behaviors of surrounding rock observed in the underground tunnel. What’s more, the dynamic rockburst of the marble specimen was simulated under the condition with a high unloading rate, high initial minimum principal, and free face on the specimen. Based on these experimental investigations, a flowchart was also presented to estimate the possible failure mode of marble specimens under different initial minimum principal stress and unloading rate conditions, which would be conducive to the disaster prevention of hard rock in deep underground engineering.

Comparison and Analysis of Oil Tank Test Results by PAC and Vallen Acoustic Emission Instrumentations

In this article, two acoustic emission instruments, PAC and Vallen acoustic emission instruments, are applied to carry on the test of crude oil storage tanks. Four experiments contents have carried out such as active defect test, corrosion test, vibration test, leak test. And compare and analyze the acoustic signal characteristics of the above four detection contents of two acoustic emission instruments. The differences and advantages and disadvantages of the four detected acoustic signals of the two acoustic emission detectors are summarized. The research results provide certain basic test support for the improvement and development of acoustic emission testing equipments and supporting software.

Research on acoustic emission characteristics of marble damaged by pre-peak unloading

Excavation of the tunnel in high geo-stress environment induces strong stress redistribution, resulting in unloading damage of surrounding rock. The damaged surrounding rock is the main load bearing body for the tunnel in the running period. Therefore, it is necessary to study the acoustic emission (AE) characteristics during the reloading of damaged rock. The marble was chosen to manufacture specimens in this paper, and we designed pre-peak unloading tests. Confining pressure of pre-peak unloading test is 15MPa and three types of unloading point (70MPa, 80MPa and 90MPa). The laboratory tests are made for studying AE characteristics of damaged marble samples by using MTS816 and acoustic emission instrument AE21C. The test results showed that as the unloading point increases, the peak strength of the rock decreases with increased corresponding peak strain. Rock failure mode gradually changed from brittle failure to quasi-brittle failure; Accumulated AE count is negatively correlated with the unloading point, and the AE quiet period is positively correlated with the unloading point. The evolution of b-value is divided into three phases: slow increase, sharp increase and rapid decrease. The findings of this research can provide the significance for selecting mechanical parameters of the surrounding rock and optimizing support scheme.

Development and Application of a Hydraulic Loading Test System That Provides Continuous and Stable Hydraulic Pressure

To accurately determine the stress characteristics of underground rock masses subjected to hydraulic pressure and reveal the crack-initiation, crack-propagation, and macromechanical characteristics of fractured rock masses under long-term hydraulic pressure, a hydraulic loading system that can deliver continuous and stable hydraulic pressure was developed. The system is composed of a device that supplies a stable hydraulic pressure, flowmeter, pressure gauge, acoustic emission instrument, loading device, and control system. The main innovation of the test system lies in the stabilization of the hydraulic pressure, which distinguishes the new system from traditional hydraulic loading devices and provides several advantages. First, in the new system, air pressure serves as the driving force, and water and gas coexist in the same cavity. Given the high compressibility of gas, the hydraulic pressure can still reach the relative stability during crack initiation and propagation in the specimen. Second, the device is capable of long-term operation with low energy consumption. In contrast to conventional hydraulic loading systems based on servo control, the hydraulic pressure of the new device can be maintained for a long time with little energy consumption. Finally, the device is simple to operate, low cost, convenient to assemble, and can be used in single-, double-, and triple-axis tests. The accuracy and reliability of the test system were verified through its application in the uniaxial compression test of a fractured body under the action of stable hydraulic pressure. The test system provides the foundation for the study of the physical mechanisms of deep underground rock masses subjected to long-term, stable seepage pressure.

Yield criteria and strength conditions considering comprehensive mechanical and acoustic emission characteristics of tension–torsion composite deformation of nylon materials

Acoustic velocity measurement and the acoustic emission tests of tension–torsion composite failure process for nylon 6 and nylon 66 standard specimens under different loading rates are carried out by using acoustic emission instrument and electronic tension–torsion tester respectively. The acoustic velocity, stress–strain curve and acoustic emission energy curve of each specimen are obtained; According to the theories of the plane stress state and strength criterion, the principal stress of point on the surface of specimen at failure under the action of combined tension–torsion load is developed. The Mises yield condition is suitable for judging the yield criterion of nylon at tension–torsion failure; Based on the least square method, fitting models of the second invariant of the stress deviator are established with the density, acoustic velocity, tensile loading rate and torsional loading rate of nylon material. The results show that: For nylon 6 material at the low tension–torsion rate, with the increase of strain the secondary yield phenomenon occurs after the initial yield and finally necking occurs until fracture; With the increase of loading rate, the tension strength and normal strain of nylon 66 decreases;According to the regression model, the second invariant value of the stress deviator can be predicted well.

Acoustic Emission Behavior of Tropical Residual Soil

emission technique has extensively been used for a wide variety of engineering materials, but less attention has yet been paid on the application of this non-destructive technique to investigate the fundamental behaviors of geomaterial. In the present study, acoustic emission method was adopted in conjunction with the conventional stress-strain-time measurement to investigate the mechanical behaviors of a selected tropical residual soil. A systematic acoustic emission instrumentation setup, which was devised in a monotonic triaxial shear apparatus, was evidenced to provide reasonable experimental results. From the isotropic consolidation results, it was realized that the Kaiser’s effect was observable and the pre-stressed level as induced by compaction could be determined through acoustic emission. In undrained shearing, the acoustic emission response was found to be corresponding with the axial strain measurement. Initial soil yielding, which was mobilized at small strain range, was also able to be determined. The acoustic emission response of the studied tropical residual soil also showed good similarity with the reported soils constituting considerable fines content.

Geological Core Ground Reorientation Technology Application on In Situ Stress Measurement of an Over‐Kilometer‐Deep Shaft

As mining progresses to depth, engineering activities face the extreme challenge of high in situ stress. To efficiently measure the deep in situ stress before engineering excavation, an innovative deep in situ stress measurement method capable of the geological core ground reorientation technology and acoustic emission (AE) technology was proposed. With this method, nonorientation geological cores collected from the thousand‐meter‐deep borehole were reoriented based on the spatial spherical geometry model and borehole bending measurement principle. The distribution of deep in situ stress of an over‐kilometer‐deep shaft in the Xiangxi gold mine was investigated with real‐time synchronized MTS 815 material testing machine and PCI‐II AE instrument. The results show that the in situ stress changes from being dominated by horizontal stress to being dominated by vertical stress with depth. The horizontal maximum principal stress and vertical stress gradually increase with depth and reach a high‐stress level (greater than 25 MPa) at a depth of 1000 m. The direction of the maximum principal stress is near the north. Meanwhile, to analyze the accuracy of the measured in situ stress comparatively, the stress relief measurements were performed at a depth of 655–958 m in the mine, using the Swedish LUT rock triaxial in situ stress measurement system. The distribution of deep in situ stress obtained by the stress relief method agrees well with that by the AE method, which proves the reliability of the AE in situ stress testing method based on the geological core ground reorientation technology.