Acoustic Emission Parameters Research Articles

An Approach to Evaluate the Fatigue Life of the Material of Liquefied Gases’ Vessels Based on the Time Dependence of Acoustic Emission Parameters: Part 1

In the first part of this article devoted to the assessment of the fatigue life of structural steels at low temperatures, a study was conducted on the effect of pre-cycling in a low-cycle fatigue mode on the time dependences of acoustic emission parameters. Commonly used St-3 steel was tested at −60 °C with varying durabilities, after which it was fractured once during static tests. The multilevel acoustic model used made it possible to estimate the structural parameter at the stage of elastoplastic deformation. The stage of active development of microcracks and their coalescence corresponds to a homogeneous fracture with stable acoustic emission characteristics (signal duration, amplitude variation coefficient, etc.). It was shown that regardless of the maximum voltage (460, 480, and 500 MPa) in the cycle and the operating times of up to 0.3, 0.5, and 0.7, the structural parameter remains within the known limits. The parameters of the Weibull law distribution and the logarithmically normal distribution for the coefficient γ were obtained, theoretical and calculated fatigue curves were plotted, and a method was proposed for evaluating the number of cycles before fracture under irregular loading conditions in the real operation of pressure vessels based on the “rainflow” cycles counting method.

Mechanical properties and piecewise constitutive model of fine sandstone in mining area of western China

Owing to the differences in sedimentary environments in the mining areas of western China, the mechanical properties of rocks in this region are significantly different from those in the central and eastern regions. Therefore, uniaxial cyclic loading-unloading tests were conducted on fine sandstone found in many roof rocks to study the evolution laws of mechanical properties, deformation characteristics, acoustic emission (AE) parameters, and energy under cyclic loading and unloading conditions. The accumulated residual strain, dissipative energy, acoustic emission cumulative ringing counts, and cumulative energy were introduced to characterize the degree of rock damage. Based on this, a piecewise constitutive model was established for fine sandstone. The results indicate that (1) the cumulative ringing counts and cumulative energy of the AE increase in a stepwise manner with an increase in the cyclic loading and unloading times. Still, there is a sudden increase in the plastic failure and post-peak failure stages. (2) The fine sandstone specimens’ input energy, elastic energy, and dissipative energy density increased nonlinearly during the cyclic loading tests. Owing to the closure of the primary pores in the microfracture compaction stage, dominant matrix deformation in the elastic deformation stage, and development and expansion of cracks in the plastic failure stage, with an increase in the cyclic loading and unloading times, the dissipative energy ratio first decreased and then increased. (3) Based on the tangential modulus, residual strain, and Felicity ratio, fine sandstone’s cyclic loading and unloading stress-strain curves were divided into microfracture compaction, elastic deformation, and plastic failure stages. The piecewise constitutive model of fine sandstone constructed with accumulated AE energy was the closest to the real stress-strain curve of the rock, and the deviations of the peak stress and peak strain from the actual situation were 0.52% and 0.84%, respectively. The research results can provide theoretical support for identifying the degree of rock damage in western mining areas and ensure the safe and efficient development of coal resources.

The study of mechanical properties and fracture evolution of coal-rock masses under hard roof-soft floor conditions

As the depth of coal mining in China continues to increase, the fracturing of coal rock masses has an increasingly complex impact on the surrounding rock roadways. The majority of the mine’s roadways run through coal rock masses with hard roofs and soft bottoms, which typically exhibit complex dynamic behaviour. To further research the mechanical behaviour and fracture evolution of coal rock masses under hard-roof and soft-floor conditions, the study is based on the majority of working faces in a mine, which have hard roofs and soft floors. Uniaxial compression tests were utilized to study the mechanical properties of coal rock masses under hard-roof and soft-floor circumstances, using acoustic emission monitoring, whole-process imaging technologies, and fractal dimension analysis. The experimental results are as follows: The uniaxial compressive strength of the coal rock mass is significantly higher than that of its weakest component. The results of the experiment are as follows: The uniaxial compressive strength of coal rock mass is significantly higher than that of its weakest component. Samples with different soft rock strengths exhibited dissipated energy greater than the accumulated energy before the stress maximum, accompanied by volume expansion and the formation of shear surfaces. Samples with higher soft rock strengths tend to exhibit brittle failure, while weaker samples show stress-softening behaviour. Internal fracture complexity varies amongst samples with varying soft rock strengths. A fractal study of the acoustic emission parameters was carried out utilizing MATLAB programming. The fractal analysis results show that acoustic emission ringing counts and energy time series of coal rock masses under hard-roof and soft-floor settings have good fractal properties. The fractal analysis results show that acoustic emission ringing counts and energy time series of coal rock masses under hard-roof and soft-floor settings have good fractal characteristics. Acoustic emission ringing counts tend to have a larger correlation dimension than acoustic emission energy. However, while the sample is fracturing on a vast scale, the ringing count correlation dimension fluctuates very little. The correlation dimension distribution of samples with lower strength is more concentrated after the stress maximum, implying that the deformation and fracturing of the floor rock in highways under hard-roof and soft-floor circumstances are more complex. Both the correlation dimension D of acoustic emission ringing counts and energy indicate a continuous fall before peak stress, which can be used to anticipate coal rock mass fracture. This study, based on the mechanical behaviour and fracture evolution of coal rock masses under hard-roof and soft-floor conditions, provides a foundation for disaster avoidance by controlling the stability and structural deformation of floor rock in hard-roof and soft-floor highways.

Acoustic emission characteristics and damage constitutive model of fractured sandstone under uniaxial compression

To investigate the statistical laws of acoustic emission energy (AEE) avalanche dynamics of sandstone under varying fracture lengths and dip angles, as well as to determine the relationship between acoustic emission (AE) parameters and damage variables, we studied the mechanical properties and AE characteristics of sandstone with a single fracture subjected to uniaxial compression with the aid of the Shimadzu AG-IS test system and the PCI-2 AE system. The AEE characteristics of fractured sandstone under load were analyzed based on the statistical method of avalanche dynamics, with emphasis on AEE distribution, aftershock sequence, and waiting time distribution. The Weibull distribution function that incorporates a correction coefficient β was employed to optimize the Weibull parameters based on the strain equivalent hypothesis theory, which led to the establishment of a statistical damage constitutive model for fractured rock. The results indicated that the peak strength of rock samples initially decreases and then increases with increasing fracture dip angle, and decreases with increasing fracture length. The AEE of the rock sample followed a power-law distribution, with a power-law index ranging from 1.62 to 1.81, which was independent of fracture length and dip angle. The aftershock sequence also adhered to a power-law distribution, whereas the waiting time exhibited a multi-power-law distribution. The proposed statistical damage constitutive model aligned more closely with the experimental curves compared to previous models that normalizes AEE.

The law of axial compressive damage of wood components with T-shaped and L-shaped cracks

ABSTRACT Wood components are easy to crack, which has great influence on their performance. So it is particularly essential to monitor and clarify the crack propagation law. In this paper, T- and L-shaped cracks were prefabricated on wood components, and acoustic emission (AE) and digital imaging correlation (DIC) technology were combined to monitor and analyse the damage of wood components under axial compression. The effects of different crack types on the damage forms, mechanical properties, damage index and propagation characteristics of acoustic emission signals were studied. It shows that the damage to L-cracked wood components was more serious than that of T-cracked wood and the acoustic emission signal strengthened as the load increased and the degree of rupture intensified. By establishing the corresponding relationship between the initiation and propagation behavior of wood microcracks and the acoustic emission parameters and surface strain, a measurement and evaluation system for in-situ monitoring the crack damage evolution of wood components with parallel and transverse cracks based on acoustic emission technology and digital image correlation method was successfully constructed. The test results provide a reference for further study on the damage mechanism and in-situ monitoring method of crack evolution behavior of wood with cracks.

Uniaxial compressive damage evolution and constitutive modeling of fissure-like rocks under different loading rates based on acoustic emission

In natural environments, most rocks possess internal fissures and are often exposed to diverse external loads arising from engineering activities and ground stress, among other factors. This study aims to explore the influence of different loading rates on the mechanical properties and acoustic emission (AE) characteristics of fissured rocks and to develop an intrinsic damage model. To achieve this, prefabricated fissured rock specimens that mimic natural rocks were prepared. Uniaxial compression tests along with AE monitoring were carried out at various loading rates. Subsequently, a loss—damage constitutive model was developed, which describes the deformation and damage process of rocks based on the characterization of AE energy. The results are as follows: (1) Prefabricated fissured rock samples have lower strength compared to non-fissured rock samples due to the existence of prefabricated fissures. As the loading rate increases, both the peak strength and the elastic modulus increase. (2) Stress thresholds are significantly affected by the loading rate, showing a positive correlation. Prefabricated fissures reduce these thresholds, thus accelerating the damage process. (3) There is a strong correlation between AE characteristics and stress—strain curves. AE parameters, namely the number of rings, energy, and cumulative energy, go through three stages: calm development, active development, and surge development. The number of AE rings, energy, and cumulative energy are positively correlated with the loading rate, while the cumulative ringing counts decrease. (4) A damage constitutive model constructed with AE energy and the Logistic function can accurately represent the specimen’s response to different loading rates. This model closely matches the actual stress—strain curve, indicating improved accuracy and applicability.

Damage Evolution Characteristics of Steel-Fiber-Reinforced Cellular Concrete Based on Acoustic Emission

In order to investigate the steel fiber parameters on the damage characteristics and crack evolution of cellular concrete materials, uniaxial compression–acoustic emission combined tests were carried out on steel-fiber-reinforced cellular concrete (SFRCC) with different steel fiber contents (0%, 0.5%, 1%, 1.5%, and 2%) and different porosities (10% and 20%). The material damage evolution characteristics were analyzed by acoustic emission parameters and IB values, and the crack types were identified using Gaussian mixture clustering method (GMM) pairs. The results show the following: the inclusion of steel fibers increased the compressive strength of cellular concrete by 19.8~46.3% at 10% porosity, and by 37.1~102.2% at 20% porosity; the addition of steel fibers significantly increased the density and intensity of the acoustic emission signals; the decreasing tendency of the IB value at the peak stress slowed down with the increase in the amount of steel fibers, and the steel fibers could effectively inhibit the crack development; crack classification results show that the proportion of shear cracks in all stages of cellular concrete increased significantly after the addition of steel fibers.

Experimental investigation of the precursor characteristics and early warning of coal burst based on quantitative analysis of acoustic emission signals

Coal burst is one of the most frequent and destructive dynamic disasters encountered during underground mining engineering. However, the understanding of quantitative precursor characteristics of coal burst is still in its infancy, rendering it difficult to provide effective early warning of disaster. In this study, to quantitatively study precursor characteristics and warning signs of coal burst, the coal burst experiments were carried out on coal-rock combination with a crack. The acoustic emission (AE) technique was employed to quantitatively analyse the precursor information during coal burst process. Testing results indicated that coal burst process is classified into three stages based on evolution in AE energy, i.e., early incubation stage, late incubation stage and occurrence stage. The first significant increase in AE energy could be identified as the beginning of the late incubation stage of coal burst, accompanying by the phenomenon of macro-failure initiation. AE signals during the whole process could be classified as five types according to their dominant frequency and amplitude characteristics, i.e., HF-HA, LF-HA, EHF-LA, HF-LA and LF-LA respectively. The dramatic increase in number proportion of HF-HA and LF-HA signals is highly correlated with occurrence of coal burst. In addition, a comprehensive classification criterion for the coal burst prediction was proposed under a quantitative analysis of three AE parameters, i.e., first energy index (FEI), coal burst risk indicator based on AE energy (CRIE) and frequency spectrum (CRIF−A). The findings in this study could facilitate accurate coal burst prediction.

Acoustic Emission Analysis of the Cracking Behavior in ECC-LWSCC Composites

Acoustic emission (AE) analysis was utilized to assess the cracking behavior of six lightweight self-consolidating concrete (LWSCC)–engineering cementitious composite (ECC) beams under flexural loading. Two control beams were fully cast with ECC containing either polyvinyl alcohol (PVA) fibers or steel fibers (SF). The remaining four beams were ECC-LWSCC composite beams, with the ECC layer containing PVA fibers or SF placed on either the tension or compression side. The results showed that the control beams had the highest ultimate load capacity, followed by beams repaired in tension, and then beams repaired in compression. PVA fibers exhibited higher performance compared to steel fibers at the first crack load, while steel fibers enhanced the beam’s performance at the ultimate load stage. During the flexural testing, AE parameters such as the number of hits, signal amplitude, and cumulative signal strength (CSS) were collected until failure. The analysis of these AE parameters was effective in detecting the first crack and evaluating cracking propagation in all beams. Changing the type of fibers (PVA and SF) in the ECC layer showed a significant effect on AE parameters. Moreover, adding a new ECC layer to an existing LWSCC layer resulted in variations in the signal amplitude. Finally, the flexural failure mode was confirmed with the aid of the rise time/maximum amplitude vs. average frequency analysis.

Detection of Stress Corrosion Cracking in SS304 under Calcium Silicate Thermal Insulation Using Acoustic Emission Technique

Despite its remarkable structural strength, austenitic stainless steels such as SS304 are susceptible to stress corrosion cracking (SCC) under thermal insulation due to the synergism of cyclic loading and chloride-containing environments. This study aims to detect SCC for insulated SS304 samples using the acoustic emission technique (AET) as monitoring tool to characterize the various stages of SCC damage mechanism. As-received (AR) and sensitized (SEN) SS304 samples were prepared as per ASTM G30 and experimented with soluble chloride drip test under calcium silicate thermal insulation following ASTM C692. Both AR and SEN samples revealed SCC behavior correlated with measured acoustic signals. AR samples exhibited a single crack with low energetic signals, whereas SEN showed multiple cracks with higher energetic signal events. The difference can be attributed to microstructure change by heat sensitization and the enhancement of the corrosion environment under calcium silicate insulation when wetted by salt solution. The acoustic emission (AE) parameters were characterized and correlated with different stages of SCC phenomena, showing that AET is useful for predicting SCC to avoid catastrophic failures in industries.

Influence of water saturation on mechanical characteristics and fracture evolution of coal rock assemblage with rough interfaces

To comprehensively investigate the influence of water content on the mechanical and crack propagation characteristics of coal rock assemblage (CRA) with a rough interface, uniaxial compression tests were conducted on specimens with varying water content. Nuclear magnetic resonance (NMR) and acoustic emission (AE) techniques were employed to monitor the water content and AE signals throughout the experiment. The physical and mechanical properties, as well as the extent of crack development and acoustic emission (AE) parameters, were comprehensively investigated under conditions of water erosion. The results demonstrate that a rough interface contributes to an enhancement in the compressive strength of the composite material. Moreover, the moisture content exerts a significant influence on various aspects of the composite specimen, including its compressive strength, time b value, crack development, and crack propagation. With the increase in water content, the initial single slope shear failure of the composite specimen gradually transitions into a multi-section shear failure mechanism. Under the influence of water-rock interaction, sandstone within the formation undergoes a metamorphosis from a densely cemented structure to an irregular honeycomb-like configuration. This transformative process engenders novel porosity and fractures, ultimately compromising the rock’s mechanical strength. The analysis focuses on the relationship between the AE parameter b and uniaxial stress and water content, with emphasis on its relevance to damage theory. A damage model based on water immersion rate was established to elucidate the correlation between damage variables and water content. This was achieved by considering the characteristics of water-rock coupling AE and constructing a structural model of the water absorption process in different pore throats, thereby providing valuable insights for stability design and evaluation of roadway rock masses.

Research on mechanical properties and acoustic emission characteristics of sandstone under splitting loading after temperature treatment

In underground engineering, tunnels are important passages for underground wastewater, personnel and vehicles. As the surrounding rock usually experiences the splitting failure, the mechanical properties of the rock subjected to splitting loading are important for surrounding rock stability after a fire. In this study, Brazilian splitting tests were carried out on sandstone after different temperatures (25 °C to 1000 °C), and the acoustic emission (AE) signals of the rock splitting failure process were obtained. Based on the test, the thermal damage rock splitting strength and AE parameters were analyzed, and the effect mechanism of temperature on rock tensile strength was elucidated. The results show that (1) the rock tensile strength fluctuates in the range of 25–400 °C, and the deterioration effect on the tensile strength is significant when the tensile strength exceeds 400 °C. (2) The AE energy follows a power-law distribution, in which the power exponent decreases when it exceeds 400 °C, the main frequency distribution band widens, and the number of AE signals with low energy and high main frequency increases. (3) Mineral decomposition and thermal fracture degrade rock mechanical properties and fracture acoustic signals, resulting in a temperature dependence of rock tensile strength and AE parameters. The research results provide a reference for fracture analysis and stability monitoring of tunnel surrounding rock splitting failure after a fire.

Assessment of foundation damage during earthquakes using acoustic emission monitoring: An experimental study on the shaking table test

Assessing structural foundation damage following an earthquake is critical for safety evaluation. However, assessing the damage to pile foundations with traditional visual inspections and non-destructive testing methods is challenging. This study evaluated the use of acoustic emission (AE) monitoring for damage detection and location in foundation structures during earthquake simulations by conducting shaking table experiments. Scaled models of foundation structures with and without surrounding soil were used in the experiments, and the performance of the AE monitoring system was evaluated by comparing the AE parameters via visual inspection. The experimental results showed that the AE monitoring system could effectively predict the initiation of cracks in foundation structures that experienced an earthquake. In addition, an appropriate filtering criterion for the shaking table experiments was established based on the AE characteristics of the foundation structures during the earthquake simulation, thereby improving the performance of the AE monitoring system for damage location. Consequently, this study contributed to a better understanding of the applicability of AE monitoring systems to foundation structures during earthquakes.

Strength and damage constitutive model of backfill body after high temperature treatment

Constructing mine heat storage reservoirs with water-blocking capabilities using functional filling technology is an innovative approach for underground high-temperature heat storage. However, high-temperature environments can easily cause thermal damage to the backfill body, posing significant risks to the safety of the heat storage reservoir. In this study, we conducted mechanical and microstructural tests on heated backfill body samples to observe changes in their mechanical properties and microstructure at varying temperatures. The test results show that as the temperature goes from room temperature (25 °C) to 350 °C, the backfill body’s compressive strength goes down a lot, with peak stress dropping from about 7 MPa to 3 MPa. At the same time, its ductility and ability to deform go up. Higher temperatures intensify shear failure, leading to the extension of cracks along the shear direction and subsequent surface spalling. The proportion of shear failure increased from 27.86 % to 68.21 %. The microstructure of ettringite, calcium silicate hydrate, and calcium hydroxide breaks down at different temperatures, which makes the pores bigger and the structure less rigid. This study developed a thermal damage constitutive model that incorporates acoustic emission parameters. We can use this model to evaluate and predict the deformation and strength characteristics of backfill after exposure to high-temperature treatment.

Mechanical properties and damage characterization of cracked granite after cyclic temperature action

Due to the unique geographical environment of the plateau, large-scale damage and destruction of fractured surrounding rock often occur during geotechnical engineering construction as a result of high-temperature cycles. Therefore, this study aims to investigate the mechanical properties and damage characteristics of fractured granite under the influence of cyclic temperature, uniaxial compression tests were conducted on granite specimens with pre-existing fractures at cyclic temperatures of 30 °C, 50 °C, 70 °C, 100 °C, and 130 °C. The study integrated analyses of characteristic stress, acoustic emission parameters, damage variables, fractal dimensions, and SEM to explore the mechanical properties and damage features of granite. The results indicated that at a 45° fracture inclination and a temperature of 70 °C, granite exhibited a distinct turning point in mechanical properties and damage characteristics. At the same cyclic temperature, granite with a 45° pre-existing fracture showed significant decreases in peak stress, elastic modulus, and σci/σm ratios, with the AE b-value drop point noticeably earlier, and both cumulative AE ring count and total energy reduced. The damage variable quickly reached its maximum, and the development of internal microcracks in the specimen is highly orderly. At the same fracture inclination, peak stress, elastic modulus, and σci/σm slightly increased at 70 °C, while AE ring counts and total energy were lower, indicating the degree of internal thermal damage in the specimen has significantly decreased, and the development of internal microcracks in the specimen has shown a marked reduction in orderliness. These findings provide theoretical insight into the meso-damage and failure mechanisms of granite influenced by different cyclic temperatures and fracture inclinations.

Study on the Optimization of b-Value for Analyzing Weld Defects in the Primary System.

This study presents a method to add a crack analysis algorithm to the Acoustic Leak Monitoring System (ALMS) to detect and evaluate the crack growth process in the primary system piping of nuclear power plants. To achieve this, a fracture test was conducted by applying stepwise loading to welded specimens that simulate the cold leg section, and acoustic emission (AE) signals were measured in relation to the increase in strain using an AE testing system. The experimental results indicated that the stability and instability of cracks could be assessed through the Kaiser effect and the Felicity effect when detecting crack growth using AE signals. Additionally, by utilizing both root mean square (RMS) and amplitude parameters simultaneously to calculate the b-value, it was confirmed that the RMS-based b-value minimizes the effects of AE signal attenuation and allows for a more stable assessment of crack progression. This demonstrates that the RMS, which reflects signal energy, is effective for real-time monitoring of the crack growth state. Finally, the results of this study suggest the potential for real-time crack monitoring using AE data in piping systems of critical structures, such as nuclear power plants; by adding a simple AE analysis method to the ALMS system, a practical approach has been derived that enhances the safety of the structure and allows for quantitative assessment of crack progression. Future research is expected to further refine the AE parameters and algorithms, leading to the advancement of safety monitoring systems in various industrial settings.

Study of bond-slip performance of steel tube with UHPC based on acoustic emission parameter analysis

Conventional methods for assessing bond performance are hindered by challenges in measuring strain and a lack of sufficient sensitivity to internal structural damage. This paper examines the bond behavior between steel and ultra-high-performance concrete (UHPC) based on acoustic emission (AE). The tests included a total of 2 concrete-filled steel tube (CFST) specimens and 4 concrete-encased CFST specimens. The bonding test was analyzed using AE parameters in combination with the damage process, damage morphology, and loading curve of the test specimens. Furthermore, damage correspondence was explored for the AE parameter indexes and the bond damage indexes of the conventional method. The findings reveal that the AE peak frequencies associated with the bond failure between steel tube and concrete span 30–100 kHz, 110–130 kHz, and 220–260 kHz. The AE event rate, cumulative absolute energy, Ib-curve, and percentage of tensile cracks can be used as judgmental and early warning features for interface damage. The damage process at the interface and the concrete crack development can be characterized by the severity index HI and the severity value Sr. The measurement curves obtained from various AE sensor positions exhibit remarkable similarity. A correlation can be discerned between the severity index (Sr) of the AE and the bond damage degree (Dt) ascertained through conventional methods, effectively mirroring the structural damage progression.

Correlation between proprioception, functionality, patient-reported knee condition and joint acoustic emissions.

Non-invasive assessment of joint status using acoustic emissions (AE) is a growing research area that has the potential to translate into clinical practice. The purpose of this study is to investigate the correlation of the knee's AE with measures of proprioception, self-assessment, and performance, as it can be hypothesised that, AE parameters will correlate with joint function metrics due to AE being recorded during interaction of the articular surfaces. Threshold to detect passive motion (TTDPM), Knee Osteoarthritis Outcome Scores (KOOS) and 5 times sit-to-stand test (5STS) were collected from 51 participant. Knee AE were recorded during cycling with 30 and 60 rpm cadences using two sensors in different frequency ranges and three modes of AE event detection. Weak (0.297, p = 0.048) to moderate (0.475, p = 0.001) Spearman's correlations were observed between longer 5STS time and AE parameters (i.e. higher median absolute energy, signal strength, longer AE event rise time and duration). Similarly, AE parameters shown correlation with lower KOOS, especially in the "Function in Sports and Recreation" and "Activities of Daily Living" subscales with correlation coefficients for higher median amplitude up to 0.441, p = 0.001 and 0.403, p = 0.004, respectively. The correlation with the TTDPM was not detected for most of the AE parameters. Additionally, a lower frequency sensor and 60 rpm cadence AE recordings showed higher correlations. Considering that this study included subjects from the general population and the number of participants with KOOS <70 was relatively small, higher correlations might be expected for clinically confirmed OA cases. Additionally, different ICCs might be expected for alternative signal parameters and proprioception assessment methods. Overall, the study confirms that AE monitoring offers an additional modality of joint assessment that reflects interaction between cartilage surfaces and can complement orthopaedic diagnostics, especially in the context of remote monitoring, drug testing, and rehabilitation.

Study on the brittle failure of flawed concrete-sandstone based on entropy theory and AE crack classification

Interfacial instability in fractured concrete–sandstone composite is frequently encountered in practical engineering applications. This study investigated the impact of various fracture angles (0°, 30°, 45°, 60°, and 90°) on the fracture characteristics of concrete–sandstone composites under uniaxial compression. The real-time study of composite damage evolution during compression was conducted via the acoustic emission (AE) technique. By investigating the intrinsic relationship between entropy theory and AE parameters, the time-dependent evolution laws of AE counting entropy, AE energy entropy, and AE amplitude entropy are revealed. The frequency of the change in the AE entropy significantly increased prior to sample failure, serving as a reliable precursor signal for the failure of composites with varying fracture angles. Additionally, we have enhanced the crack classification methodology by incorporating elements from both the conventional RA/AF method and the dominant frequency approach. The cracks in the concrete–sandstone composite were classified via the Gaussian mixture model (GMM) clustering method. The proportions of tensile cracks with different crack angles were 80.4%, 18.8%, 68.9%, 65.2% and 71.8%, respectively. The experimental results show that the results of GMM clustering are in good agreement with the crack types of the samples, which indicates that the GMM clustering method has certain advantages and accuracy in the classification of crack types in composite materials.

Investigation of scale effects of rock bridges based on Multi-Physical field monitoring

The presence of rock bridges is crucial for ensuring the stability of rock slopes. However, evaluating the mechanical properties of rock bridges solely based on the joint persistence coefficient (K) raises doubts due to potential variations in the scales of rock bridges within a rock mass with a constant K value. In this study, direct shear tests with different scales were carried out on sandstone specimens. Acoustic Emission (AE) and Digital Image Correlation (DIC) techniques were utilized to monitor the damage procession of the specimens. The evolution process of the specimen was divided into stages and three threshold points were determined based on AE parameters. As the scale decreased, the Joint Roughness Coefficient (JRC) increased, the tensile failure increased, and the shear failure decreased. As the normal stress increased, the JRC with the same scale decreased, the tensile failure decreased, and the shear failure increased. The change of fracture mechanism was the primary reason for the strength deterioration with decreasing scales. The decrease of the scale was not conducive to the failure prediction. By improving the rock bridge failure potential (RBP) index, the proposed RBPn index could appropriately characterize scale effects under different normal stresses.