Acoustic Emission Intensity Research Articles

Acoustic Emission based determination of delamination initiation in GFRP laminates

Acoustic Emission (AE) has the potential to identify delamination initiation in quasi-static Mode I fracture tests on glass fiber-reinforced polymer-matrix (GFRP) laminates. It had been shown that a combined AE activity and intensity criterion yields critical Mode I delamination resistance values GIC for an AS4/PEEK carbon fiber thermoplastic composite that are comparable to those from data analysis according to ASTM D5528. However, the AE historic index also allows for detection of delamination initiation under Mode I and Mode II loading of carbon fiber reinforced thermoplastics and thermosets. In the present study, these AE criteria for initiation under Mode I, Mode II and Mixed Mode I/II loading of GFRP epoxy laminates are compared with AE signal energy as additional initiation criterion. Initiation under Mode II and Mixed Mode I/II at a ratio of 4:3 was determined with two different set ups for each Mode. For some test configurations noise signals make unambiguous initiation detection difficult. Determination by means of AE energy or AE activity and intensity yields more conservative GC values compared with the AE historic index or criteria defined in the standards, however, with similar repeatability.

Thermal-damage effects on fracturing evolution of granite under compression-shear loading

Shear-dominant instability disasters in high-temperature conditions pose a significant challenge for deep underground rock projects, necessitating a thorough understanding of rock fracture evolution and the impacts of thermal damage. To investigate how thermal damage influences shear fracturing behavior and geophysical responses, experiments were conducted on thermally-treated granite under compression-shear loading conditions. The detailed failure process was meticulously monitored using integrated acoustic emission (AE), electric potential (EP), and digital image correlation (DIC) techniques. The effects of thermal treatment on the patterns of AEs, EPs, and DIC strain fields during the loading process were analyzed. Micromechanics related to the influence of thermal treatment on mechanical properties and geophysical responses were explored using X-ray diffraction (XRD) and scanning electron microscopy (SEM). By defining damage variables through multiparameter analysis using AE, EP, and DIC, the study delved into predictive insights on the failure process of thermal-damage granite. The results indicate that the physical properties of granite undergo significant changes as the treatment temperature increases, with the peak stress gradually decreasing and the plastic characteristics increasing. These changes can be attributed to thermal damage, which causes internal water escape, expansion of mineral particles, and crack formation. Additionally, increasing the thermal treatment temperature leads to a decrease in both AE and EP intensity, as well as a reduction in the occurrence of high-magnitude AE events, while the cumulative AE count rapidly increases during the plastic stage. Notably, thermal treatment induces significant alterations in mineral composition, intergranular fractures, and the formation of a gradual zigzag crack path, resulting in the intensified development of shear microcracks. This observation is supported by the varying of AE characteristic parameter AF/RA and the analysis of DIC displacement decomposition. Moreover, the three defined damage variables exhibit accelerated growth prior to catastrophic failure, with the order of precursor occurrence observed as AE, DIC, and EP. However, the occurrence of the precursor time for these three geophysical signals becomes earlier with increasing treatment temperature. These research findings shed light on the mechanism of compression-shear failure of thermal-damage rock and provide valuable references for predicting precursors of the shear-dominant instability disasters such as induced earthquakes.

Investigating the Mechanical Deterioration Effect of Hard Sandstone Induced by Layer Structure under Uniaxial Compression

The deterioration of the surrounding rock at the tunnel bottom is a damage mechanics issue that occurs under disturbance load. To investigate the anisotropic characteristics of mechanical behavior and the AE response mechanism of layered sandstone, uniaxial compression tests and acoustic emission (AE) monitoring were conducted. The results show that the layer structure causes remarkable anisotropic characteristics in the wave velocities. The strain characteristics and mechanical parameters of layered sandstone exhibit obvious deterioration effects. The local strain and overall strain show a synergistic feature, with the local strain path being more complex and the deformation response being extremely sensitive. The peak stress and elastic modulus both exhibit V-type distribution rules, slowly decreasing first, then rapidly decreasing, and finally increasing rapidly, with the boundary points of the layer angle being 45° and 67.50°. The peak stress and elastic modulus show a nonlinear exponential correlation with the layer angle, and the sandstone belongs to the intermediate anisotropy level. The rupture pattern shows significant anisotropic characteristics, with the failure modes including tension failure, including tension failure I and tension failure Ⅱ, shear failure, and tension–shear composite failure. The fractal dimension shows a negative correlation with the layer deterioration effect. The AE activity exhibits a phased response characteristic to the aging deformation of layer structure. The more obvious the layer deterioration effect is, the longer the AE delay is. The AE intensity of tensile failure sandstone is generally greater than that of oblique shear failure.

Leak State Detection and Size Identification for Fluid Pipelines with a Novel Acoustic Emission Intensity Index and Random Forest.

In this paper, an approach to perform leak state detection and size identification for industrial fluid pipelines with an acoustic emission (AE) activity intensity index curve (AIIC), using b-value and a random forest (RF), is proposed. Initially, the b-value was calculated from pre-processed AE data, which was then utilized to construct AIICs. The AIIC presents a robust description of AE intensity, especially for detecting the leaking state, even with the complication of the multi-source problem of AE events (AEEs), in which there are other sources, rather than just leaking, contributing to the AE activity. In addition, it shows the capability to not just discriminate between normal and leaking states, but also to distinguish different leak sizes. To calculate the probability of a state change from normal condition to leakage, a changepoint detection method, using a Bayesian ensemble, was utilized. After the leak is detected, size identification is performed by feeding the AIIC to the RF. The experimental results were compared with two cutting-edge methods under different scenarios with various pressure levels and leak sizes, and the proposed method outperformed both the earlier algorithms in terms of accuracy.

Strength and acoustic emission weakening mechanism of sandstone after prolonged water immersion: Insights from micro-fracture

In this study, a series of uniaxial compression and acoustic emission (AE) tests were conducted on sandstone specimens immersed in water for different durations (0–720 h). The mechanical properties and AE characteristics of dry, unsaturated, saturated, and long-term saturated sandstone specimens were comparatively analyzed. The experimental results show that with the passage of immersion time, the uniaxial compressive strength and elastic modulus of the specimens both decrease, while the Poisson’s ratio increase; the mechanical parameters change exponentially with the immersion time and are linearly related to the water content; the dominant failure mode evolves from tensile failure to shear failure gradually; and the AE intensity and frequency both weaken. Besides, the mechanism of long-term water–rock interaction was explored. It was proposed that the weakening of rock strength and AE signals is largely attributed to the transformation in the micro-fracture form (from transgranular fracture to intergranular fracture), which is induced by the deterioration of grain cementation. This view is supported by optical microscopic observations on micro-fracture behavior as well as AE response.

Quantitative Visualization Monitoring of Cracks at Shotcrete-Rock Interface Based on Acoustic Emission

To investigate the possibility of quantitative monitoring of the fracture process zone (FPZ) at the shotcrete-rock interface, the acoustic emission (AE) and digital image correlation (DIC) are used to monitor the three-point bending test of shotcrete-rock specimens. Firstly, the AE intensity signal characteristics during damage to the shotcrete-rock interface are analyzed. Then, the spatial b-value of AE is used to visually characterize the shotcrete-rock interface damage, and the interface damage characteristics of two specimens, shotcrete-granite and shotcrete-sandstone, are analyzed using this analysis method. The analysis reveals that not only the AE spatial b-value can determine the location of microdamage within the interface but it can also characterize the degree of damage. Finally, a new parameter, Tb-value, is constructed based on the AE spatial b-value to quantitatively characterize the FPZ, and the newly established characterization method is validated with the FPZ monitored by DIC. The results show that the Tb-value not only locates and visually characterizes the location of the FPZ within the specimen but also enables the quantitative determination of the FPZ. This provides a new idea for localizing and quantitatively monitoring cracks and FPZs inside structures using AE techniques.

A Coupled Tensor–DEM–FEM Model for the Whole Process of Internal Fine-Scale Damage to Surface Damage in Red-Bed Soft Rocks in the Coastal Area of South China

Calculation and characterization of the whole process of internal microscopic damage to surface damage in red-bed soft rock is a theoretical research difficulty and an urgent need for engineering safety protection. However, the current study cannot accurately and directly correlate internal and external damage. Therefore, in this paper, a coupled tensor–DEM–FEM model is proposed to deal with surface damage by indoor triaxial test digital image processing (DIC), internal damage by FJM acoustic emission study, and internal and external damage by moment tensor correlation. The study demonstrates that the whole process damage process of the red-bed soft rock peak front can be divided into six distinct phases, with early damage beginning with the elastic phase; the local strain divergence value begins to spiral out of control during the period of crack acceleration development; the overall acoustic emission intensity distribution is in the range of [−8.5, −6.3] in two dimensions and in the range of [−11, −9] in three dimensions; the R were between −40 and 40, which corresponded to the results of the indoor tests. A model has been developed that allows a direct reflection of the whole damage process. The method can be used to better understand the disaster mechanism and guide engineering practice.

Microcrack monitoring and fracture evolution of polyolefin and steel fibre concrete beams using integrated acoustic emission and digital image correlation techniques

The use of polymer and steel fibres in plain concrete appears to be an excellent solution for limiting crack propagation and improving the post-ductility performance of concrete structures. Based on this premise, this study investigated the fracture evolution of polyolefin fibre-reinforced concrete (PFRC) and steel fibre-reinforced concrete (SFRC) specimens through the integrated application of two diagnostic techniques, acoustic emission (AE) and digital image correlation (DIC), under three-point bending tests. Based on the processing of AE signals, different AE statistical parameters such as the cumulative number of hits, amplitude distribution, and some representative analysis methods including the b-value method, Ib-value method, and AE intensity analysis methods were selected to analyse the early detection of cracking and post-cracking behaviour in PFRC vs SFRC specimens during mechanical degradation. Simultaneously, the DIC technique was used to validate the fracture evolution of the AE results. Furthermore, to verify the reliability of the AE and DIC results, the damage localisation and fracture evolution of the PFRC versus SFRC specimens were confirmed by integrating the AE fracture energies and DIC outcomes. The tests and analysed results showed that the addition of steel fibres to plain concrete significantly improved the ability to restrict crack propagation and provided higher post-cracking resistance compared to PFRC specimens owing to their stronger fibre–matrix bonding, effective fibre bridging, and crack-arresting mechanism. The present study indicates that the combined AE and DIC techniques are highly effective for the early detection of damage and ductility performance in fibre-reinforced concrete structures.

The Influence of Cyclic Load Amplitude on Mechanical Response and Acoustic Emission Characteristics of Granite

Cyclic loading and unloading tests with varying stress amplitudes were carried out to study the evolution trends of the elastic modulus, plastic strain, dissipated energy density, and acoustic emission events for granite under cyclic loading with accidental extreme stress. The results were as follows: (1) The loading deformation modulus before and after extreme stress is different. In addition, at extreme stress levels, the loading deformation modulus of granite specimens decreases by approximately 5~13%, but the unloading deformation modulus does not change significantly. (2) When extreme stress causes rock damage, most of the plastic deformation potential of the rock at this stress level is released in advance. The stress constantly varies in the subsequent low-amplitude cycle, and the plastic strain caused by the extreme stress is partly recovered. (3) As the extreme stress increases, the cumulative dissipated energy density of granite increases significantly. Compared with the control group without a stress extremum, the cumulative dissipated energy density of samples in two groups with stress extrema of 20 kN and 40 kN increased by 48% and 153%, respectively. (4) A significant acoustic emission event occurs only when the rock is subjected to a load exceeding the maximum historical stress for the first time, and the acoustic emission intensity is positively correlated with the difference between this stress and the historical maximum value. (5) Extreme stress values below the crack damage threshold reduce the crack growth potential of the rock in advance, and extreme stress above the crack damage threshold aggravates rock damage.

Study on Synchronous Response Law of Acoustic and Electrical Signals of Outburst Coal Rock under Load and Fracture

A multisignal nanosecond synchronous acquisition system to measure acoustic emission (AE) and electromagnetic radiation (EMR) generated during the process of loading and failure of coal and rock samples is established. The correlation between the energy of the AE and EMR signals and the loading stress of outburst coal-rock samples was studied, and the characteristics of the AE and EMR signals during the process of loading and fracturing the outburst coal and rock samples were analyzed. The results show that (1) before the failure of the outburst coal and rock samples, the fluctuation of the AE and EMR signals is the largest, with the same rising and falling trend, and the intensity is not strictly positively correlated, with the phenomenon of low EMR when the AE intensity is high; (2) the EMR and AE deviation degree and frequency exhibit a good response to coal and rock fracturing. The correlation between EMR and stress drop is stronger than that of AE, and the AE signal is richer than the EMR signal. The results show that it is feasible to develop combined AE and EMR early warning technology to improve the early forecasting accuracy of coal and gas outbursts.

Detection of acoustic emission from nanofiber nonwovens under tensile strain – An ultrasonic test setup for critical medical device components

In the biomedical field, nanofiber materials are gaining increasing application. For material characterization of nanofiber fabrics, tensile testing and scanning electron microscopy (SEM) are established standards. However, tensile tests provide information about the entire sample without information about single fibers. Conversely, SEM images examine individual fibers, but cover only a small section near the surface of the sample. To gain information on failure at the fiber level under tensile stress, recording of acoustic emission (AE) is a promising method, but challenging due to weak signal intensity. Using AE recording, beneficial findings can be obtained even on "invisible" material failure without affecting tensile tests. In this work, a technology for recording weak ultrasonic AE of tearing nanofiber nonwovens is presented, which uses a highly sensitive sensor. Functional proof of the method using biodegradable PLLA nonwoven fabrics is provided. The potential benefit is demonstrated by unmasking significant AE intensity in an almost imperceptible bend in the stress-strain curve of a nonwoven fabric. AE recording has not yet been performed on standard tensile tests of unembedded nanofiber material intended for safety-related medical applications. The technology has the potential to enrich the spectrum of testing methods, even those not confined to medical field.

Acoustic Emission Waveform Characteristics of Red Sandstone Failure under Uniaxial Compression after Thermal Damage

During the exploitation of deep resources, rocks are often in a high-temperature, high-pressure environment. It is of great significance to study the acoustic emission (AE) characteristics of thermal damaged rock under load to improve the accuracy of monitoring in practical engineering. In this paper, sandstone was heated at different temperatures, before a uniaxial compression test was performed and the AE in the process was monitored. The results show that the strength and AE energy of sandstone decrease gradually with an increase in heating temperature. Through frequency domain analysis of the AE waveform at the time of failure, it was found that the frequency and intensity of AE also showed a downward trend with an increase in temperature. In addition, multifractal theory is introduced to deconstruct the waveform data. The multifractal characteristics of the waveforms decrease with an increase in temperature. It provides new parameters for waveform analysis, which can be combined with frequency analysis as parameters to more accurately identify rock failure in engineering practice. The attenuation of AE of thermally damaged sandstone may be related to an increase in porosity and a decrease in elastic energy release.

Mixed mode fracture of geometrically similar FRUHPC notched beams with the localizing gradient damage model

A localizing gradient damage model is proposed in this paper to capture the mechanisms underlying the fracture of fiber reinforced ultra-high performance concrete (FRUHPC): (i) an initially expanding damage process zone due to the formation of numerous fine cracks, each widening up slowly due to the fiber bridging effect; (ii) a gradual formation of a localized macroscopic crack, when at least one of the fine cracks has opened up sufficiently to trigger a fiber pull-out process. Restricting to tensile-shear dominated failures in monotonic loadings, the simplest form of damage model is adopted, where an isotropic damage variable is driven by a strain-like variable. The performance of the proposed localizing gradient damage model is first demonstrated by comparing the damage evolution process against the corresponding intensity of acoustic emission (AE) events obtained experimentally, for a notched FRUHPC beam in three-point bending. It was shown that the model is able to reproduce the slightly widening fracture process zone (FPZ) during strain hardening, as well as the eventual localized damage process zone during strain softening. Specifically, during strain hardening, the proposed localizing gradient damage model predicts an energy dissipation bandwidth that compares well with the AE energy maps, together with a consistent propagation of crack front. With a correct description of the underlying damage mechanisms, the predictive capability of the proposed model is next demonstrated through the mixed mode fracture of a series of geometrically similar FRUHPC notched beams. Since the specimens are of the same casting batch as those in the three-point bending test, the numerical material parameters calibrated earlier are adopted directly for the mixed mode fracture tests. Without any further undue calibrations, the localizing gradient damage model is shown to predict well the structure responses and size effect of the series of geometrically similar FRUHPC notched beams in mixed mode fracture.

Experimental investigation on the deterioration process and spatial variation of corrosion damage of RC segmental specimens under sustained load

• Early deterioration damage of a bearing segment is investigated via AE technique. • Effect of sustained load on spatial variability of steel corrosion is studied. • Spatial correlation between steel corrosion and surface corrosion cracks is studied. For the reinforced concrete (RC) shield tunnel in coastal regions, premature corrosion damages are always investigated after construction because of the combined effects of sustained earth and water pressures and marine chloride attacks. To evaluate the performance of these deteriorated shield tunnels, the effect of sustained load on structural performance degradation must be considered. In this paper, an experiment was conducted using the corrosion-accelerated specimens of a tunnel segment. The deteriorating damage processes of the RC segmental specimens were illustrated via acoustic emission (AE) with an emphasis on the influence of sustained load. Based on AE intensity analysis, intensive cracking damages for a bearing segment were identified when the average steel cross-section area losses increased from 0.5% to 2%. Meanwhile, the spatial variations of corrosion damage were discussed incorporating the impact of tensile load. The effect of tensile load on spatial correlation of distribution between steel cross-section area loss and surface corrosion cracks was experimentally verified. The experimental outcomings can help in modelling a random field-based probabilistic model associated with non-uniform steel corrosion in the bearing segmental linings.

Evolution characteristics of precursor information of coal and gas outburst in deep rock cross-cut coal uncovering

As mines become deeper, the potential for coal and gas outbursts in deep rock cross-cut coal uncovering is enhanced. The outburst precursors are unclear, which restricts the effectiveness and reliability of warning systems. To reveal the evolution characteristics of coal and gas outburst precursor information in deep rock cross-cut coal uncovering, briquette specimens are constructed and experiments are conducted using a self-developed true triaxial outburst test system. Using acoustic emission monitoring technology, the dynamic failure of coal is monitored, and variations in the root mean square (RMS) of the acoustic emissions allow the effective cracking time and effective cracking gas pressure to be defined. These characteristics are obviously different in deep and shallow coal. The characteristic parameters of gas outburst exhibit stepwise variations at different depths. The RMS and cumulative RMS have stepped failure characteristics with respect to changes in gas pressure. The characteristic parameters of coal failure are negatively correlated with the average in-situ stress and effective stress, but positively correlated with the lateral pressure coefficient of in-situ stress and the critical gas pressure. The transition characteristics are highly sensitive in all cases. The critical depth between deep and shallow coal and gas outbursts is 1700 m. The expansion multiple of acoustic emission intensity from the microfracture stage to the sharp-fracture stage of coal is defined as the outburst risk index, N1. For depths of 1100–1700 m, N1 ≥ 7 denotes a higher risk of outburst, whereas at depths of 1700–2500 m, N1 ≥ 3 indicates enhanced risk.

Effect of grouting material strength on bond strength of sleeve and acoustic emission characterization of bond failure damage process

The grouted sleeve connection is widely used in fabricated structures, and the bonding performance is a vital indicator to evaluate its property. Total of 24 grouted sleeves specimens were designed to study the effect of grouting material strength on bond strength of two forms of sleeves under monotonic tensile loading. The qualitative and quantitative evaluation of the bond failure damage process of the grouted sleeves was carried out relying on acoustic emission (AE) technology. Results demonstrated that the relative displacement between the rebar and grouting material was generated after the hardening stage of the tensile process. More importantly, the relationship between grouting material strength and bonding strength was revealed. Compared with Carbon steel sleeves, ductile iron sleeves exhibited better bonding properties in the tensile process. Moreover, a designing method was proposed to prevent bond failure of the ductile iron sleeves and carbon steel sleeves. The trends of AE ringing count and energy with the variation of load could better characterize the bonding failure damage process, which corresponded well to the five stages of the tensile process. The rapid increase of the ringing count and energy level could be a precursor to the occurrence of bond failure of the grouted sleeve. The safety zone to avoid the bond failure of the grouted sleeves was established based on the AE intensity analysis. The emergence of the AE signal corresponding to the bond failure should be vigilant in engineering practice.

Discrimination of Different AE and EMR Signals during Excavation of Coal Roadway Based on Wavelet Transform

During the process of coal road excavation, various interference signals, induced by environmental noise, drilling, and scraper loader, will affect the risk assessment of coal and gas outburst using acoustic emission (AE) and electromagnetic radiation (EMR) monitoring technology. To distinguish between different interference signals and danger signals, discrete wavelet transform (DWT) was used to decompose and reconstruct signals, and continuous wavelet transform (CWT) was used to obtain the time-frequency plane. The research results show that: (1) interference signals generally exhibit fluctuating changes within small ranges; in comparison, the intensity of AE and EMR signals caused by coal and rock fracture is found to continuously rise for a long period (longer than 2 h). (2) Different interference signals and danger signals differ significantly in their time-frequency plane. (3) Through decomposition and reconstruction of original signal, obvious precursor information can be found in the time-frequency plane of reconstructed signals.

Parameter inversion and microscopic damage research on discrete element model of cement-stabilized steel slag based on 3D scanning technology

The macro- and micro-physical properties of cement-stabilized steel slag (CSS) base materials in a highway project were studied. A discrete element model of CSS with a real steel slag shape was constructed using Particle Flow Code 3D and 3D scanning technology. The sensitivity between the macro- and micro-parameters of the sample was explored, and a nonlinear regression equation was established to analyze the relationship between these parameters. Uniaxial compression simulation tests were conducted on CSS with steel slag contents of 0%, 10%, 30%, and 50%. By combining contact calculation, crack location, energy tracking, acoustic emission (AE) monitoring, and other program systems, the macro- and micro-mechanical properties and micro-crack evolution law of the samples in the failure process were analyzed in terms of strength, energy, and fracture damage. The damage mechanism of CSS was also revealed. Results showed that with the increase in steel slag content, the elastic modulus and peak stress of the samples increased, the Poisson’s ratio decreased, and the post-peak stress curve steepened, indicating obvious brittle failure characteristics. With the increase in steel slag content, the crack initiation stress, thickness of the fracture surface, and number of internal micro-cracks in CSS increased exponentially. In the uniaxial compression test, AE intensity underwent five stages, in which the peak moment of AE intensity exhibited hysteresis compared with the moment of the peak stress. Absorption and release phenomena of strain energy were observed in the process of specimen failure. When the steel slag content increased, the total strain energy absorbed by the specimen increased. When the absorbed energy exceeded the bond strength, the bond ruptured with the release of energy. The main crack of the sample penetrated and stretched to the direction of strain energy release to form a macroscopic fracture surface.

Experimental Investigation on Acoustic Emission Characteristics of Reinforced Concrete Beam Strengthened with CFRP

Reinforced concrete (RC) structures are generally subjected to increasing loads, age-related deterioration and design changes that affect the integrity of structures. Therefore, this paper presents an experimental investigation of a carbon fibre reinforced polymer (CFRP) reinforced RC beam under three-point loading. Acoustic emission (AE) was used to monitor the progression of damage. The loading, AE intensity and AE signal strength were analysed and discussed. The analyses were performed using the AE signal collected from CH6 and CH7. The result is that crack nucleation produces high signal intensity compared to other crack modes. Moreover, the intensity plots fit the identified crack modes when the beams were reinforced with CFRP. Therefore, AE can be used to evaluate the performance of the CFRP reinforced RC beam.

Influence of Freeze-Thaw Cycles on Acoustic Emission Characteristics of Granite Samples under Triaxial Compression

Understanding the acoustic emission (AE) characteristics of rocks that have undergone freeze-thaw cycling is of great significance for the use of AE technology to monitor the stability of rock masses in cold regions. A series of freeze-thaw cycling experiments and triaxial compression AE tests of granite samples were performed. The results show that, with an increasing number of freeze-thaw cycles, the P-wave velocity and peak AE intensity of granite show a substantial downward trend. The AE ringing counts during triaxial compression can be divided into three stages: abrupt period, calm period, and failure period. The overall change of the characteristic AE signal of granite samples that underwent different freeze-thaw cycles is the same. The AE signal during the destruction of granite occurs in clear dual dominant frequency bands. The peak frequency increases with increasing load time, and this trend becomes less clear as the number of freeze-thaw cycles increases. Overall, the peak frequency distribution tends to change from high to low with an increasing number of freeze-thaw cycles. The results provide basic data for rock mass stability monitoring and prediction, which is of great significance for engineering construction and management in cold regions.