Acoustic Emission Parameter Analysis Research Articles

Understanding the evolution mechanism and classification criteria of tensile -shear cracks in rock failure process from acoustic emission (AE) characteristics

This study aimed to investigate the crack evolution and failure mechanism of rocks by conducting uniaxial compression tests (UCT) and Brazilian indirect tension tests (BITT) using four categories of rock specimens. Real-time acoustic emission (AE) signals were detected and recorded during rock failure processes by an AE monitoring system. The study focused on the accumulative AE hits curves, stress–strain curves, and other AE energy characteristics to divide the failure process into five stages, and determine the stress thresholds. Furthermore, the transition trend of rock crack types was analyzed based on RA-AF data. A novel classification criterion of crack types based on clustering and statistical theory was proposed, and the crack boundaries of the four rocks were determined. Additionally, the tension to shear ratios of the rocks in descending order were fine-grained granite, granite, yellow sandstone, and white sandstone. Finally, the crack development trend and failure characteristics of the rock were analyzed using the crack scale and crack dividing line. The study found that the failure mode of the rock could be predicted accurately by AE parameter analysis at Unstable Crack Growth (Stage-IV).

Acoustic and Fracture Energy Correlation in Mode I Fracture with Concrete Damage Plasticity Model and Three-Point Bend Experiment

The progressive damage monitoring and fracture evaluation of plain concrete beam specimens subjected to three-point bending (TPB) are reported in this paper. The objective of the present research is to systematically correlate the fracture behavior of a concrete beam with acoustic emission (AE) data recorded during the present set of experiments. The damage growth and failure mechanisms of the beam specimens are investigated with AE parametric analysis, b-value analysis, and digital image correlation method. Eventually, a reasonable correlation between AE energy and the released fracture energy is established at various stages of loading, which is confirmed with the set of TPB beam experiments and further validated with nonlinear numerical finite-element analysis. The energy released during the cracking processes at different loading stages in the experiment is quantified and systematically analyzed. Acoustic and fracture energies are dependent on active concrete cracking, and their correlation is helpful in determining the extent of damage in the concrete structures. Such a relationship can be used as an operational tool for structural health monitoring of structures as an early warning system supplemented with numerical analysis, a well-calibrated laboratory, and in situ AE experiments.

Nominal tensile strength reduction and its mechanism of ultra-high performance concrete with steel bar reinforcements

A comprehensive experimental study was designed to investigate the tensile response of four series of ultra-high performance concrete (UHPC) with different steel bars. The changing rule of the nominal tensile strength of UHPC in reinforced UHPC with respect to reinforcement details was revealed and the relative mechanism was clarified to both make adequate capacity calculation for structural design and develop the refined material model for finite-element analysis. Polynomial fitting formulas to account for the nominal tensile strength reduction of UHPC with steel bar ratios of 1.57–6.28% were proposed. UHPC nominal elastic tensile strength reduction in reinforced UHPC was found linearly dependent on the restrained degree of steel bars on UHPC shrinkage based on theoretical analysis. The further nominal ultimate tensile strength reduction of strain hardening UHPC in reinforced UHPC was verified to be attributed to the localized damage aggravation around the ribs of the deformed steel bar based on acoustic emission parametric analysis.

Investigation the effect of freeze–thaw cycle on fracture mode classification in concrete based on acoustic emission parameter analysis

This study experimentally investigates the influence of freeze–thaw (F-T) cycles on the mechanical properties and cracking features of concrete. The specimens, which are previously subjected to various numbers of freezing (-20 °C) and thawing (20 °C) cycles, are tested under unixial compression condition. Integrated acoustic emission (AE) and digital image correlation (DIC) techniques are adopted to study the mechanical characteristics of the concrete specimen subject to different F-T cycles under different immersion condition. The micro-structure of the concrete specimen after F-T cycle treatment is observed with T2 distribution by the nuclear magnetic resonance (NMR) technology. The changes in the amount of uniaxial compressive strength (UCS), elastic modulus, and peak strain of the specimens before and after various F-T cycles are comprehensively analyzed. The results show that the brittleness of the specimen decreases and the ductility increases as the number of F-T cycle increases. The peak load and elastic modulus decreased gradually with the increase of F-T cycle times, and the peak strain presented an upward trend. The concrete specimens without F-T cycle action or subjected to less F-T cycles action suddenly lose their bearing capacity and the failure mode is shear slipping. For the specimens undergoing more F-T cycles action, the stress–strain curve exhibits lower slopes during the strain softening stage. Kernel density estimation (KDE) results show that under the lower F-T cycle, the tensile crack has a higher AF (acoustic emission count/duration) value and a lower RA (rise time/amplitude) value, and the obvious shear/mixed cracking phenomenon occurs after the higher F-T treatment. The K-means clustering method has its advantages in the classification of AE cracking events. The b value increased with the increase of F-T cycles, indicating that F-T cycles can caused internal damage of the specimens, and the degree and scale of internal damage increased with the increase of F-T treatment times. Furthermore, the damage degree of the specimens under semi-immersion in water solution is greater than that under complete immersion in water solution.

Stick-Slip Phenomena and Acoustic Emission in the Hertzian Linear Contact

AE detection and analysis usually requires a specific, costly platform due to its particular burst nature and high-frequency content. This experimental study investigates the relationship between low-demand acoustic emission parameters (AE) and the occurrence of stick–slip (SS) at the Hertzian linear contact. Hence, the correlation of basic AE characteristics (amplitude, energy, and evolution in time) with stick–slip characteristics (static and kinetic friction coefficients, amplitude, energy, and evolution in time) is pursued. Tribological tests were conducted on cylinder–plane specimens under dry friction conditions with different loads at different low driving speeds and Hertzian contact pressures at a constant stiffness. The AE, normal, and friction forces were recorded simultaneously on the experimental stand. At the cylinder–plane interface, the jumps specific to the stick–slip phenomenon (friction coefficient—COF) were followed after a few milliseconds by AE jump peaks. The results of the experiments show that the amplitude and energy generated by AE were sensitive to the occurrence of the stick–slip phenomenon, while the AE and COF energies in the stick and slip phases had the same law of variation based on the driving velocities. The results show that the amplitude and energy of the sampled low-frequency AE signals were enough to detect the friction in SS and demonstrate the potential of AE as a tool for detecting and monitoring the tribological behaviour of SS at the linear Hertzian contact.

Monotonic and cyclic bond responses of steel bar with steel-polypropylene hybrid fiber reinforced recycled aggregate concrete

The bond failure between steel bars and concrete is the main reason for the strength degradation and deformation increase of structures in service. In this study, the bond behavior of steel bars with steel-polypropylene hybrid fiber reinforced recycled aggregate concrete (HFRAC) under monotonic and cyclic loadings was investigated. The acoustic emission (AE) technique was applied to monitor the bond response and the inner damage evolution of specimens during the loading process, and the microstructure of HFRAC was analyzed through the scanning electron microscope (SEM). The hysteresis curves, failure modes, bond strength degradation rate, and energy dissipation capacity, were systematically investigated by considering the effects of water-cement ratio, recycled aggregate replacement ratio, steel fiber volume fraction, and polypropylene fiber volume fraction. Experimental results demonstrated that there was more obvious degradation in the bond strength for the HFRAC specimens under cyclic loading, compared with that under monotonic loading. In the case of cyclic loading, the bond strength was degraded with the increase of the number of cycles, and the degradation rate increased quickly with the increase of the displacement amplitude. Moreover, the analysis of AE parameters measured on steel bar bonded recycled aggregate concrete (RAC) and HFRAC specimens, including the AE hits, AE energy, localization of AE events, and average frequency, was performed. Results showed that the AE technique could effectively monitor the bond damage of HFRAC specimens at different stages. A good correlation between the results of AE parameters analysis and the cracking process of specimens was confirmed. Based on the Weibull cumulative distribution function, the bond-slip models of steel bars with HFRAC under monotonic and cyclic loadings were proposed by considering the coupling effect of hybrid fibers and recycled coarse aggregates.

Acoustic emission parametric analysis to investigate the damage evolution of anchor systems designed by the concrete capacity design method

Acoustic emission parametric analysis to investigate the damage evolution of anchor systems designed by the concrete capacity design method

Experimental investigation on the damage behavior of ultra-high performance concrete subjected to cyclic compression

This study deals with the compressive response and damage evolution of ultra-high performance concrete (UHPC) subjected to uniaxial monotonic and cyclic compressive loadings. A total of 36 prismatic specimens are tested for different fiber aspect ratios and volume fractions. The acoustic emission (AE) technique is applied to monitor the damage progression and unravel the failure mechanism of UHPC. The test results demonstrate that the incorporation of steel fibers improves the mechanical properties of UHPC in terms of peak stress, peak strain and elastic modulus, and in particular the ultimate strain and toughness for cyclic loading scenarios. The cyclic envelope curve is noted to conform with the corresponding monotonic response. The plastic strain accumulation is linearly proportional to the envelope unloading strain. Moreover, significant stiffness degradation with increasing loading cycles is observed during the cyclic loading process, which can be alleviated by inclusion of steel fibers. Results from AE show that after the peak stress, a concentrated AE signals are released accompanied with an abrupt stress drop implying that a significant amount of fiber pullout, sliding and fracturing events are occurring during the loading process that significantly prohibit the damage process. The cumulative AE counts increase with an increase in fiber volume fraction, whilst the mixture with medium fiber aspect ratio of 60 shows the most AE counts. It is also found from the AE parametric analysis that the failure of UHPC with steel fibers is predominately induced by the accumulation of shear cracks, which differs from the failure of UHPC without fiber where tensile cracks are dominant. Finally, an elastoplastic damage model is proposed to describe the uniaxial monotonic and cyclic stress-strain behavior. The prediction yields a close estimation to the experimental results from both current study and literature.

Investigation on the effect of freeze-thaw on fracture mode classification in marble subjected to multi-level cyclic loads

For rock engineering in cold regions, rock is often subjected to coupled fatigue damage caused by freeze-thaw (F-T) and stress disturbance. To investigate the cracking propagation and to quantify the degree of damage and the type of crack classification, multi-level cyclic loading experiments were carried out on marble with F-T treatments of 0, 20, 40 and 60 cycles. Real-time acoustic emission (AE) and post-test computed tomography (CT) scanning technologies were employed to reveal the fracturing evolution and to further classify different crack types to aid in understanding dynamic fracturing. An AE parameter analysis method combined with a b-value analysis is able to identify the cracking event modes and cracking propagation. Results show that the RA (ratio of the rise time to amplitude) and AF (ratio of the AE counts to the duration) pattern is strongly influenced by the previous F-T treatment. The results of the kernel density estimation (KDE) and the k-mean cluster method indicate that a tensile cracking event and shear/mixed cracking event can be classified from an RA and AF data set. The proportion of tensile cracks decreases and shear/mixed cracks increases with the increase of F-T cycles. In addition, b-value analysis reveals the damage propagation at different cyclic levels and a minimum b-value was observed at the last cyclic level. Moreover, post-test CT scanning shows that complex crack network can form for a sample subjected to low freeze-thaw treatment, and the scale of tensile cracks decreases and shear/mixed cracks increases as F-T cycle grows. It is suggested that the deterioration of rock meso-structures impacts the fracturing mode and the associated rock stability in cold regions. The decreasing of the b-value and an increasing of the RA values provide an early warning for rock fracturing and implementation of rock mass stability prediction.

Fracture behavior and acoustic emission characteristics of reinforced concrete under mixed mode I-II load conditions

In this paper, the fracture behavior and acoustic emission (AE) characteristics of reinforced concrete under mixed mode I-II load conditions are studied using three-point bending beams with a straight offset notch. Different offset ratios, different crack-depth ratios, and different sizes of reinforced concrete specimens and a set of concrete specimens as a control group were explored. The critical loads, the crack mouth opening displacements, the final failure angles, and the crack trajectory was recorded to analyze the fracture behavior of specimens under different factors. The 3D crack source locations by AE technique was adopted to locate the damage during the fracture process, which was in a good agreement with the crack propagation path obtained through the fracture test. A criterion, based on the simultaneous appearance of the rise of RA (rise time/peak amplitude), the decline of AF (counts/duration time), and Ib-value (ratio of weak events to strong events), was proposed to identify the three critical conditions in the mixed mode I-II fracture process, which include the initial crack at the notch tip, crack at the bottom of mid-span, and the peak load. Then, the fracture process was divided into four stages: initiation of micro-crack, stable crack propagation in a single path, stable crack propagation in double paths, and unstable crack propagation. The AE hits can accurately distinguish the moments of crack at mid-span and peak load. The modified AE parameters analysis method was utilized to classify the cracking mode at different stages of damage and the results showed that the shear cracks followed the tensile cracks, the proportion of AF to RA for common specimens in this study can be set to 80: 1, while that of small size specimen with a span length of 600 mm can be set to 40: 1. The results can provide a theoretical basis for the establishment of remote monitoring and early warning systems for large structures.

Characterization of damage on precast pre-stressed concrete composite slabs under static loading based on acoustic emission parameters

Precast pre-stressed concrete composite slabs have been widely used in recent years owing to excellent mechanical properties and cost savings. The low accuracy of visual observation and the complexity of actual measurement make the determination of the precise information of structural safety difficult. This article presents an acoustic emission study on the precast pre-stressed concrete composite slab under static loading. First, the correlation between acoustic emission hits (or energy) and crack width (or deflection) was obtained and compared in detail. Second, the stiffness of this composite slab was predicted using the acoustic emission amplitude. Third, a novel analysis method for damage fracture classification is proposed, which is based on the traditional parameter and fuzzy C-means clustering. This method can be used to determine visual crack emerging, rebar yielding, and eventual failure mode by an acoustic emission parametric analysis without involving visual inspection or mechanical measurement. It can be effectively used for fracture characterization and health monitoring of composite slabs.

A new method for determining the crack classification criterion in acoustic emission parameter analysis

At a microscopic scale, the failure of brittle materials results from crack initiation, propagation and coalescence. Acoustic emission (AE) technique, especially parameter analysis, has been widely applied to investigate cracking process and mechanism in civil engineering. However, crack classification in AE parameter analysis mostly derives from the empirical relation between the RA value and the average frequency, and the crack classification criterion, i.e., the optimal transition line between shear and tensile cracks in the parameter analysis has not been determined yet. Based on statistical analysis of dominant frequency characteristics of AE signals, a new method is proposed for determining the optimal transition line for crack classification in AE parameter analysis. Spectrum analyses of AE waveform data in the representative specimens are carried out to acquire the dominant frequency of AE waveforms. Proportions of waveforms distributed in low and high dominant frequency bands (L-type and H-type waveforms) are determined. The ratios of tensile and shear cracks, viewed as measurements, are determined by the statistical analysis of dominant frequency characteristics of AE waveforms. For a series of different transition line, the predicted ratios of tensile and shear cracks in AE parameter analysis are determined. The optimal transition line is determined to be the one corresponding to the least square difference between predicted data and measurements. The determined optimal transition line can be directly applied for crack classification in AE parameter analysis in the subsequent experiments of this brittle material. The reliability of the proposed method were validated by laboratory tests of rock subjected to compression. It can be found that the optimal transition line in the parameter analysis is approximately from 1:100 to 1:500 for brittle rock under compression. The findings in this study contributes to the enhancement of the accuracy and efficiency of AE source mechanism and damage process analysis.

Evaluation of fracture mode classification in flawed red sandstone under uniaxial compression

To study the classification of fracture modes of rocks during the cracking process, uniaxial compression tests were conducted on intact and flawed red sandstone specimens. Meanwhile, both acoustic emission (AE) and digital image correlation (DIC) technologies were adopted to monitor and record the real-time cracking process of the specimens tested. In this study, the interevent time function F(τ) (AE events rate) was utilized to distinguish the transition from microcracking to macrocracking for the tested specimens. An AE parameter analysis method based on two indices of RA (rise time/amplitude) and AF (AE counts/duration) values was performed to classify the different cracking modes during the loading process. The classification results of the cracking modes coincide with the cracking type of macrocracks captured by the photographic system. Moreover, the adequacy of the crack classification was also evaluated by the kernel density estimation (KDE) function, a nonparametric density estimation method. KDE is used as a parametric model to overcome the randomness found in the data set generated by AE testing and can well identify and visualize the high concentration regions of RA and AF values. A continuous increase of RA values (more than 400 ms/v) can serve as an early warning for the ultimate failure of red sandstone. The results of this present investigation can be applied in the health monitoring of rock engineering.

Monitoring the lack of grease condition of rolling bearing using acoustic emission

Rolling element bearings are the vital parts of machines, and their condition is often critical to the operation or process. Lubricant such as grease can present a film between the bearing surfaces and minimises the friction and wear. Lack of lubricant may lead to ineffective performance or malfunction of the bearing. Therefore, in order to avoid unexpected breakdowns, reliable lubrication monitoring techniques are demanded. Acoustic emission (AE) technology can detect the friction between moving parts in the machines. The object of this paper is to evaluate the grease amount in the rolling element bearing with AE signals. Four parameters of AE are studied, including event count rate, ring count per event, energy rate, and RMS. The first three parameters are derived from AE parameter analysis, and RMS is calculated directly on the continuously sampled signal. Eight amounts of the grease in the same bearing are tested. Experiments on grease consumption with running time are also carried out. According to the results, RMS and energy rate can be used to estimate the remaining amount of the lubricant. The method is also verified by field tests on articulated industrial robots.

Monitoring the lack of grease condition of rolling bearing using acoustic emission

Rolling element bearings are the vital parts of machines, and their condition is often critical to the operation or process. Lubricant such as grease can present a film between the bearing surfaces and minimises the friction and wear. Lack of lubricant may lead to ineffective performance or malfunction of the bearing. Therefore, in order to avoid unexpected breakdowns, reliable lubrication monitoring techniques are demanded. Acoustic emission (AE) technology can detect the friction between moving parts in the machines. The object of this paper is to evaluate the grease amount in the rolling element bearing with AE signals. Four parameters of AE are studied, including event count rate, ring count per event, energy rate, and RMS. The first three parameters are derived from AE parameter analysis, and RMS is calculated directly on the continuously sampled signal. Eight amounts of the grease in the same bearing are tested. Experiments on grease consumption with running time are also carried out. According to the results, RMS and energy rate can be used to estimate the remaining amount of the lubricant. The method is also verified by field tests on articulated industrial robots.

Cracking Mode Analysis of Crack Initiation in Rocks under Uniaxial Compression

Crack initiation is related to the behavior of the preexisting microcracks within a rock specimen, which suggests the specimen starts to fail. The determination of crack initiation stress is important for identifying the elastic stage and related mechanical parameters. Uniaxial compression tests with acoustic emission monitoring were performed to study crack initiation for tight sandstone, loose sandstone, and granite. The evolution of the cracking mode, i.e., the statistics of the cracking mode under compression, was obtained through modified acoustic emission parameter analysis. Based on the logarithm of the acoustic emission parameter (LAEP), a cracking mode analysis (CMA) method is proposed and used to determine the crack initiation stress. Results from the tests indicate that the crack initiation stress between the same rock specimens obtained by CMA is very close. The mean ratio of crack initiation stress to compression strength is 0.45, 0.34, and 0.35 for tight sandstone, loose sandstone, and granite, respectively. According to the results of CMA, crack volumetric strain (CVS) method, and lateral strain response (LSR) method, there is no big difference among those methods in tight sandstone and loose sandstone. In granite, the results obtained by CMA are close to those obtained by CVS, but smaller than those obtained by LSR. The CMA interprets the initiation of cracks from the fracture behavior of microcracks and is an objective method to determine the initiation stress.

Experimental Investigation on Damage Behavior of Polypropylene Fiber Reinforced Concrete under Compression

This paper presents an experimental investigation on the stress–strain behavior and the damage mechanism of polypropylene fiber reinforced concrete (PFRC) under monotonic and cyclic compression. Fifty-four specimens for different fiber volume fractions and aspect ratios were tested. Acoustic emission (AE) technique was used to monitor the damage progression. The damage mechanism of concrete was analyzed based on the AE parametric analysis. The results show that the incorporation of polypropylene fiber (PF) has a positive effect on the monotonic and cyclic behaviors of concrete, especially for the post-cracking branch. The toughness and ultimate strain are enhanced and the performance degradation in terms of elastic stiffness and strength is alleviated by the addition of PF. However, PF has little influences on the plastic strain, and the damage process of concrete is mainly driven by the envelope strain. The effect of fiber volume fraction on the cyclic behavior of concrete shows more pronounced than that of aspect ratio. In addition, it is found from AE results that the damage, closely related to AE events, has a quick evolution just after the peak stress, with the AE hits having a concentrated release. The total amount of AE hits increases with increasing fiber volume fraction due to fiber pullout and sliding, while the concrete with fiber aspect ratio of 280 reaches the largest amount. Meanwhile, as substantiated by AE, the failure of PFRC shows an obvious shear mode, with shear cracks dominating the damage progression. Finally, a damage elasto-plastic model is developed to predict the monotonic and cyclic responses of PFRC and the prediction yields a fairly close estimation with experimental results.

Geomechanical model test for failure and stability analysis of high arch dam based on acoustic emission technique

The internal damage of the model during the geomechanical model test is difficult to be detected. The failure and instability process of the arch dam are always accompanied by a large number of acoustic emission (AE) signals, which is related to the number of cracks in the model. AE technique was applied in the geomechanical model test of Mengdigou arch dam to analyze its failure process and overall stability. Noise filtering was conducted based on spectrum analysis of AE signal waveform. Parameter analysis was performed after noise filtering. The overall stability of the dam was evaluated by three characteristic overload safety factors: the crack initiation factor K1, nonlinear deformation factor K2 and ultimate load factor K3, which were studied by AE parameter analysis. The failure process of Mengdigou arch dam was analyzed by the statistical analysis of AE hits and the wave velocity variation before and after test. AE analysis results were compared with the displacement and video monitoring results at the same time to verify the correctness of the evaluation method based on AE technique. The results showed that the failure process of the dam and foundation was well described by the AE technique, and the three safety factors of Mengdigou arch dam were defined more clearly. And as for Mengdigou arch dam, the fracture occurred at the dam heel area in the early stage of loading and the abutment of right bank was better than the left. The foundation damage was not serious after test and the overall instability was determined by instability of dam. Based on AE technique, the three overload safety factors was determined more clearly: K1 = 1.5–2.0, K2 = 4.5–5.0, and K3 = 11–11.5, which provided an important reference for the Mengdigou arch dam construction.

AE monitoring of reinforced concrete squat wall subjected to cyclic loading with information entropy-based analysis

Reinforced concrete (RC) squat walls have been widely used to resist horizontal force given its high lateral stiffness and strength. In this research, the crack development process of squat wall under non-parallel cyclic loading was monitored using an acoustic emission (AE) technique. In addition, a novel AE signal analysis method based on information entropy was studied. The results show that the AE technique is effective in monitoring the crack process of the squat wall. A strong correlation between the result of basic AE parameters analysis and real cracking state is also determined. Furthermore, accumulated AE energy shows a good correlation with the accumulated hysteretic energy and the damage assessment based on RA-AF analysis indicates the shear cracks are the dominant crack mode. Information entropy-based analysis was conducted, and its feasibility in monitoring the development of main cracks was validated. A comparison between information entropy-based analysis and b-value analysis was conducted finally, which shows that the information entropy value is more sensitive and specific than Ib value.

Experimental investigation on the stress-strain behavior of steel fiber reinforced concrete subjected to uniaxial cyclic compression

The behavior of steel fiber reinforced concrete (SFRC) under cyclic loading plays an important role in prediction of SFRC structural response. This paper deals with the stress-strain behavior and damage evolution of SFRC under uniaxial cyclic compression. A total of 36 specimens are tested for different fiber volume fractions and aspect ratios. Acoustic emission (AE) technique is used to characterize the damage progression and reveal the failure mechanism of SFRC during the whole loading process. The results show that slight strength degradation for SFRC specimens under cyclic loading is observed in comparison with monotonic loading cases. The increase in volume fraction of steel fiber can lead to a remarkable decrease in the plastic strain accumulation as well as an increase in the elastic stiffness ratio, while the effect of fiber aspect ratio is undiscernable. In addition, it is indicated from the AE parameters analysis that the total AE activities of SFRC specimens are higher than that of plain concrete specimen and become greater with an increase in the fiber volume fraction, while the opposite is true for increasing the fiber aspect ratio. Meanwhile, as substantiated by AE, the failure of SFRC mainly demonstrates a shear cracking mode that is induced by fiber pull-out and fiber sliding events, the amount of which is proportional to both fiber volume fraction and aspect ratio. Finally, based on the test results of stiffness degradation, an analytical formulation for the damage evolution law of SFRC is developed and the prediction yields a close estimation of SFRC damage progression.