Acoustic Emission Results Research Articles

High-temperature mechanical performance and damage behavior of molybdenum tailings modified fly ash-based geopolymers

Molybdenum tailings modified fly ash based geopolymers were an eco-friendly building material with excellent mechanical performance and fracture toughness. In this work, cubic compressive test and acoustic emission technique were used to evaluate the effect patterns of molybdenum tailings contents and thermal temperature conditions on the high-temperature mechanical performance and damage behaviors of molybdenum tailings modified fly ash-based geopolymers. The molybdenum tailings contents were 0 %, 10 %, 20 %, 30 % and 40 %, and the target temperatures were 300 °C, 600 °C and 900 °C, respectively. The results of compressive tests showed that the low contents of molybdenum tailings could limit the growth of macroscopic cracks during the loading process and improve the compressive strength of fly ash-based geopolymers when the heat treatment temperature did not exceed 600 °C. The acoustic emission results showed that 20 % molybdenum tailings contents could improve the structural stability and high-temperature mechanical performance of fly ash-based geopolymers, reduced the growth of microcracks in the specimen during the loading process, and delayed the formation and cracking of macrocracks, but did not change the specimen's compressive damage mode. The 40 % contents of molybdenum tailings changed the compressive damage mode of the fly ash-based geopolymers, delayed the cracking of the matrix structure under the peak loading, and significantly improved its resistance to plastic deformation.

Damage indicators in unidirectional natural fibre composites under fatigue loading

This paper investigates the occurrence of failure under fatigue loading of unidirectional flax fibre composites with different matrices. Various parameters, such as dissipated energy, residual strain, and stiffness, were measured to evaluate fatigue damage and the fatigue life of composites. The results indicate that absolute dissipated energy may not be the most suitable indicator for assessing damage in natural fibre composites when they are subjected to fatigue before reaching failure. The research proposes to rather consider the “loss factor”, defined as the ratio of dissipated energy to total energy stored in each cycle. This indicator appears to provide a more effective means of evaluating fatigue life of the studied composites, especially when compared to the dissipated energy observed during fatigue tests at low stress levels. Fatigue and acoustic emission results indicate that biocomposites with better fibre–matrix adhesion show a delayed damage initiation and propagation, as well as longer fatigue life.

Characterization of compressive fatigue behavior and acoustic emission analysis of Ti6Al4V cellular lattice materials fabricated by laser powder bed fusion

AbstractThis study investigates the mechanical properties and fatigue performance of Ti6Al4V cellular lattice materials (CLMs) featuring five distinct unit cell types (BCC‐Z, BCC, Octet, Truncated cuboctahedron [TCO], and Trabecular) at a relative density of 25%. Compression tests were conducted to assess static properties, including Young's modulus and yield strength. Subsequently, compression–compression fatigue tests (R = 0.1) were performed to evaluate fatigue behavior. Acoustic emission analysis was employed during static and fatigue tests to explore the potential for failure prediction. Results reveal that BCC‐Z and TCO exhibit slightly higher Young's moduli, surpassing 20 GPa, while BCC, Octet, and Trabecular display moduli ranging from 6 to 12 GPa. Regarding normalized fatigue behavior, BCC‐Z demonstrates superior fatigue resistance, followed by TCO. Notably, the acoustic emission parameters significantly correlate with the unit cell type. Lastly, a strong relationship between the initiation of failure and changes in acoustic emission parameters is observed, establishing a meaningful link between the static and fatigue curves and acoustic emission results.

Enhancement of the tensile properties of cement mortar composites with nanoadditives produced by chemical vapor deposition

While graphene effectively enhances the performance of cement-based materials, its current production methods are characterized by environmental unfriendliness and high energy consumption. To investigate a large-scale and eco-friendly graphene preparation approach, Carbon Nano Sheets (CNS) were synthesized via chemical vapor deposition (CVD). The influence of CNS content on the tensile strength of cement mortar was assessed through the Brazilian splitting test. Concurrently, the mechanism of CNS was examined using acoustic emission monitoring and scanning electron microscope. The experimental results revealed that methane can be effectively decomposed into high-quality CNS at 1080 ℃ when using fly ash, silica fume, and sand as substrates. The Brazilian splitting test revealed that CNS effectively enhances the tensile strength of cement mortar, with improvements ranging from 9 % to 58.7 %. Acoustic emission results indicated that the inclusion of CNS reduces the occurrence of micro-fractures during the failure of cement mortar specimens. Furthermore, nano-mechanical testing and microstructural characterization demonstrate that CNS can reduce micro-cracks and pores in the interface transition zone and hydration products, playing a role in dense hydration products. Furthermore, it can decrease the width of the interface transition zone and enhance the micro-mechanical properties of cement pastes. This study offers a novel approach for the eco-friendly production of cement nano additives.

Waveform Complexity and Positioning Analysis of Acoustic Emission Events during the Compression Failure Process of a Rock Burst Prone Sample

The localization results of acoustic emission (AE) events can reflect the location and pattern of burst-prone rock failures. However, event localization heavily depends on the quality of the original waveform of the sensor. Therefore, this study analyzed the AE waveform of a rock sample under compression to evaluate its failure localization and quality. From the research results, it could be seen that the initial failure was relatively calm, with clear take-off points, which can be better used for accurate AE event positioning. However, the later failure was severe, causing the take-off points of most sensors to be very unclear, and positioning methods that rely on take-off points cannot be used for positioning, let alone simply using the positioning results of the built-in software. This research result reminds researchers who use AE signals for event localization to first examine the quality and status of the original waveform, providing a basis for obtaining accurate localization results, in order to further accurately study the subsequent failure patterns. The above facts indicate that the initial failure is small and scattered, while the later failure is large and concentrated, with certain fractal characteristics.

Wavelet entropy-based damage characterization and material phase differentiation in concrete using acoustic emissions

Damage evolution in concrete is a complex phenomenon involving micro-cracks that originate from different material phases (mortar matrix, ITZ and aggregate), and culminate into a major crack. The information about the individual damaged material phases can provide major insights into the global failure behaviour, cracking path, and fracture processes. The evolution of damage, damage mechanisms and the material phase of origin of micro-cracks is highly influenced by the type of concrete (Normal and high strength). In order to investigate the evolution of damage in concrete of varying strength, notched beam specimens with different water-to-cement ratios have been prepared and tested under CMOD control. The evolution and growth of damage have been continuously monitored during testing through acoustic emission techniques. Based on acoustic emission results, it has been demonstrated that the damage progression in concrete can be better described as a function of the wavelet entropy of the AE events occurring during the entire loading duration, and therefore, an analytical model for the damage evolution has been proposed. The study reveals that, under the high-strength category, the level of deterioration reduces as the w/c ratio rises, while in normal-strength concrete, a reverse trend is observed. Furthermore, a novel approach has been proposed that utilizes wavelet entropy and amplitude to differentiate amongst various damaged material phases with the help of an unsupervised clustering algorithm. Normal-strength concrete with a w/c ratio of 0.4 has been used to demonstrate the proposed methodology and has been validated with the help of experiments performed on the mortar and the parent stone of aggregate. A higher occurrence of mode-I cracks compared to mode-II cracks across all material phases has been observed for the specimen. Additionally, ITZ and aggregate exhibit a greater proportion of tensile and shear cracking events than the matrix phase. In the end, a deep learning convolutional neural network (CNN) model is devised utilizing labelled scalogram images of the AE waveforms to discriminate between the various damaged material phases. A 10-fold training cross-validation of the dataset has been carried out to achieve a stable performance. The deep learning model performs well in discriminating the damaged material phases with high training, validation, and testing accuracy.

Acoustic Emission monitoring of OPC mortars at elevated temperatures in fracture tests

The action of moderate and high temperatures produces the degradation of main physical and mechanical properties of Ordinary Portland Cement (OPC) composites. As an example, from an application point of view and at a structural level, the case of concrete radiation shielding containers in nuclear facilities can be mentioned, which can be severely affected by thermal cracking. In this work, the results of Acoustic Emission (AE) monitoring of mortar prismatic specimens under three-point bending tests are analyzed. Samples are firstly submitted to a heating treatment in an electrical furnace, with maximum target temperatures ranging between 100 to 600 ºC. After the heating process, the prisms were cooled down and finally, mechanically tested, i.e. under residual conditions. The AE signals were continuously recorded during each bending test and its characteristics at different loading stages were explored, scrutinizing variations in AE parameters, such as event rise time, amplitude and energy. Additionally, average frequency (AF) and RA value parameters were employed to assess fracture modes aiming to highlight situations where tension or shear/mixed modes predominate through the test. A comparative analysis of the AE characteristics during loading path and progressive damage is presented.

Assessment of mode I/III fracture toughness of bi-material rock-like ENDB and ENDC specimens

Mode I/III fractures at the bi-material interface are common and present unique challenges in civil and geotechnical engineering. However, there are few studies on the mechanical characteristics of bi-material rock-like specimens under mixed-mode I/III loading. This study used bi-material rock-like Edge-notched disc bend (ENDB) specimens and Edge-notched diametrically compressed (ENDC) specimens to investigate their mechanical and fracture characteristics under pure mode I, III, and mixed mode I/III loading. The results showed that the fracture toughness KI > KIII, KI and KIII decreased overall with increasing loading angle(β, α), the KI and KIII of the ENDB specimens are generally lower than that of ENDC specimens. The fracture energy (Wi) of ENDB specimens is gradually increased with the loading mode changed from pure mode I to pure mode III, while the ENDC specimens have a trend of first decreasing and then increasing. Besides, the ENDB specimens exhibit significant torsion fracture, and the torsional angle(θ) gradually increased with loading angle β, in which low-strength material has larger θ than high-strength material. In contrast, ENDC specimens fractured along the interface in pure mode I and mixed mode I/III loading, while fractures along the loading point to the centre of the specimen under pure mode III loading. Moreover, the acoustic emission results show that tearing cracks are more prominent in ENDC specimens than in ENDB specimens. Generally, ENDB specimens show more pronounced tensile/torsional fracture characteristics than ENDC specimens, it’s recommended to be adopted in mode I/III fracture toughness testing for bi-material rock-like materials.

The influence of thickness and recycled milled glass fiber fillers on the delamination resistance of polymer composite angle brackets

AbstractComposite structures with L, C, and T shape geometries are commonly used in the design of aerospace structural components. These complex structures are prone to delamination failure at the curved region. This study investigates the effect of thickness and recycled milled glass fiber fillers to improve the strength properties and delamination resistance of glass/epoxy composite angle brackets. Different thicknesses of 3.5 mm (16 layers), 5 mm (24 layers), and 6.5 mm (32 layers) specimens were subjected to four‐point bending test with acoustic emission (AE) monitoring for evaluating damage resistance. Mechanical results reveal that the specimen with a higher thickness of 6.5 mm has better damage tolerance, followed by 3.5 and 5 mm angle brackets. Moreover, the location of AE signals and % of AE hits noticed the initiation and propagation of delamination in 5 mm thickness specimen at the earlier stage of loading. However, the curved beam strength and interlaminar tensile strength (ILTS) of 5 mm thickness specimen were improved by 31.02% and 19.2% respectively, through the incorporation of 2 wt% of fillers. Further, the interfacial adhesion between the fillers and epoxy matrix was studied using Fourier‐transform infrared analysis to confirm the improvement of delamination resistance in 5 mm thickness specimen. Finally, the mechanical and AE results were well correlated with field emission scanning electron microscope images to characterize the damage mechanisms in composite angle brackets.Highlights Effect of thickness on the delamination resistance of composites was studied. Interlaminar properties of composite angle brackets were assessed. Acoustic emission parameters were used to study the damage mechanisms. Recycled milled glass fillers were employed to improve delamination resistance. Mechanical and acoustic emission results were correlated with field emission scanning electron microscope images.

Influence of Spherical Caverns on the Failure Characteristics of Neighboring Tunnels under True Triaxial Conditions: Insights from an Experimental Test and Discrete Element Simulation

Caverns are generally formed by a combination of regional geological action and groundwater, and their improper treatment will inevitably lead to dangerous conditions in underground works. To detect the specific failure mechanism of tunnel-surrounding rock induced by invisible caverns, a true triaxial compression test is conducted, accompanied by acoustic emission technology and an internal borehole camera, for monitoring the acoustic response and visible secondary cracks, and a corresponding DEM simulation is carried out to reveal the meso-mechanism. The results indicate the following: (1) The invisible cavern demonstrates a negative influence on the stability of the tunnel and leads to a 25.82% reduction in the peak z-axis load of the specimens. (2) The acoustic emission results show that the relatively severe dominant failures mainly occur near the peak stress in all types of specimens, and the speed and intensity of the cavern-existing specimen is significantly greater than that of the cavern-free specimen. (3) The cavity-free tunnel shows mirror-symmetric splitting failure on the left and right sidewalls, while the secondary cracks appear earlier and show asymmetrical distribution in the cavern-existing specimen, and the volume of broken rock blocks near the free surface is larger. (4) The cavern directly changes the failure process of the tunnel-surrounding rock (intermediate rock failure occurs earlier than splitting failure), the distribution of principal stress, and the corresponding mechanism of secondary failures. (5) Application of the displacement and velocity trend fields helped to reveal accurate failure procedures in the true triaxial test.

Evaluation of experimental techniques for performance estimation of post-tensioned concrete beams

Structural health monitoring (SHM) techniques have become a significant subject in structural evaluation due to the development of non-destructive methods used to monitor and assess the integrity of structures. This paper aims to investigate the effect of damage on the flexural performance indicators and dynamic characteristics of post-tensioned concrete (PC) beams, using both static and vibration tests. The beams were tested at several load levels to reproduce different load stages of a PC member: pre-cracking, during and after the formation of initial cracks, and finally, during and after the formation of larger significant cracks. The performance of the elements is evaluated based on static data, energy calculations, and the three criteria of ACI 437 recommendations. Data from the vibration tests were used to identify the changes in modal parameters after different load levels. Acoustic emission data analysis and results were used to classify the level of damage. Experimental bending stiffness and crack length determination via visual inspection was used as control parameters to assess the deterioration levels of the beams. The outcomes of this study show the relationship level among the different experimental responses obtained: flexural performances, the presence of damage in the beams, and natural frequencies. By considering the relationships of the SHM techniques and their individual findings, the evaluation of performance indicators or damage classification in rectangular concrete prestressed beams reveals that the main limitation is the required knowledge about the previous response of the system.

Progressive Failure Characteristics and Failure Symptoms of Straight-Walled Arched Sandstone Tunnels

In order to investigate the development process of crack formation in shallow-buried sandstone tunnel, biaxial compression tests were conducted on a similar model of the real straight-walled arched sandstone tunnel. The results indicate that the initial crack appeared at the arch line on both sides of the tunnel and propagated downwards, eventually leading to spalling of the rock mass on the surface of the tunnel, forming a V-shaped groove. Additionally, slab cracks were observed in the straight wall on the right side of the tunnel, which were approximately parallel to the vertical load. The failure characteristics of the tunnel were closely related to the fractal dimension of the crack geometry distribution. During the tunnel compaction and elastic deformation stage, the fractal dimension of the cracks in the tunnel surface increased linearly, while during the crack propagation stage, the fractal dimension increased gradually, with a sudden increase occurring just before the rock mass reached its peak load. The acoustic emission results revealed that AE ringing counts and amplitude were inactive during the first 4239 seconds of the test. And they only increased during the crack propagation stage. The continuous decrease of the b-value and the sudden increase of the fractal dimension of cracks can serve as a reliable precursor of tunnel failure.

Acoustic emission applied to mode I fatigue damage monitoring of adhesively bonded joints

The use of adhesively bonded joints has increased considerably due to their lightweight, relevant strength-weight ratio and possibility to join multi-materials. Nevertheless, there are still some challenges in the application of this kind of joints in primary structures, such as guaranteeing their reliability during the components’ useful life. Structural health monitoring methods are suggested to ensure in-service safety and reliability of adhesive joints. The acoustic emission appears promising because it can detect the elastic waves produced within the material when it is under damage or straining. This research focuses on mode I fatigue damage monitoring metallic double cantilever beam adhesively bonded joints using the acoustic emission method. Digital image correlation and visual evaluation were applied during fatigue interruptions to track the crack-tip position within the adhesive and correlate them with the acoustic emission outcomes. The acoustic emission method is susceptible and different kinds of waves (background, friction and damage) can be easily assessed during the tests, producing an immense amount of data. So, unsupervised artificial neural networks for patterning recognition were proposed. Self-organising maps and k-means algorithms were used for data clustering and then classified regarding their sources. Finally, the acoustic emission results, digital image correlation and visual evaluations were compared.

Evaluation of Rock Burst Propensity and Rock Burst Mechanism in Deep Phosphate Mines: A Case Study of Sujiapo Phosphate Mine, Hubei Province, China

Rockburst is one of the major problems in rock underground engineering and mechanics. To make a risk assessment of rockburst and improve the mine safety index, some evaluation systems for rockburst propensity have been proposed and applied, and progress has been made in the study of high-value mines and single evaluation systems, but these evaluation systems are still immature for deep low and medium grade phosphate mines with more complex influencing factors. Therefore, to solve this problem, this study takes the phosphate mining area of the Sujiapo phosphate mine as the main research object, and combines the characteristics of the deep rockburst of the phosphate mine and its physical and mechanical properties; the evaluation system combines fuzzy mathematical method and multiple evaluation methods to determine the propensity of rockburst of deep mining of Sujiapo phosphate mine. The JSM-500LV SEM and acoustic emission results are used to analyze the microscopic mechanism of rockburst from the morphology and composition of the rocks, and the rationality of the evaluation system is further demonstrated. The study shows that dolomite and phosphorite in the study area are moderate rockburst, and shale has a weak rockburst tendency; because the roof dolomite and phosphorite crystallization degree are higher, the elastic modulus is large. Under the action of external load, the roof dolomite stored more elastic strain energy. Deformation damage consumes less energy. The energy released to the outside world after the damage is large, more likely to occur brittle damage, and the rockburst tendency is high. Therefore, the evaluation system combining multiple evaluation methods can comprehensively determine rockburst in phosphate mines.

Failure modes and slabbing mechanisms of hard rock with different height-to-width ratios under uniaxial compression

To determine the relationship between slabbing failure and the specimen height-to-width (H/W) ratio and to analyze the conditions, characteristics, and mechanism of slabbing failure in the laboratory, uniaxial compression tests were conducted using six groups of granite specimens. The entire failure process was recorded using strain gauges and high-speed cameras. The initiation and propagation of fractures in specimens were identified by analyzing the monitoring results of stress, strain, and acoustic emission. The experimental results show that changes in the specimen H/W ratio can transform the macro failure mode. When the H/W ratio is reduced to 0.5, the macro failure mode is dominated by slabbing. Low load-bearing ability is observed in specimens with slabbing failure, and the slabbing fractures are approximately parallel to the loading direction. Moreover, the fracture propagation characteristics and acoustic emission signals of slabbing failure specimens show typical tensile failure characteristics, indicating that slabbing failure is essentially a special tensile failure.

Clustering Characterization of Acoustic Emission Signals Belonging to Twinning and Dislocation Slip during Plastic Deformation of Polycrystalline Sn.

Results of acoustic emission (AE) measurements, carried out during plastic deformation of polycrystalline Sn samples, are analyzed by the adaptive sequential k-means method. The acoustic avalanches, originating from different sources, are separated on the basis of their spectral properties, that is, sorted into clusters, presented both on the so-called feature space (energy-median frequency plot) and on the power spectral density (PSD) curves. We found that one cluster in every measurement belongs to background vibrations, while the remaining ones are clearly attributed to twinning as well as dislocation slips at −30 °C and 25 °C, respectively. Interestingly, fingerprints of the well-known “ringing” of AE signals are present in different weights on the PSD curves. The energy and size distributions of the avalanches, corresponding to twinning and dislocation slips, show a bit different power-law exponents from those obtained earlier by fitting all AE signals without cluster separation. The maximum-likelihood estimation of the avalanche energy () and size () exponents provide (at −30 °C) and (at 25 °C), as well as (at −30 °C) and (at 25 °C). The clustering analysis provides not only a manner to eliminate the background noise, but the characteristic avalanche shapes are also different for the two mechanisms, as it is visible on the PSD curves. Thus, we have illustrated that this clustering analysis is very useful in discriminating between different AE sources and can provide more realistic estimates, for example, for the characteristic exponents as compared to the classical hit-based approach where the exponents reflect an average value, containing hits from the low-frequency mechanical vibrations of the test machine, too.

Characterization of Biocomposites and Glass Fiber Epoxy Composites Based on Acoustic Emission Signals, Deep Feature Extraction, and Machine Learning.

This study presents the results of acoustic emission (AE) measurements and characterization in the loading of biocomposites at room and low temperatures that can be observed in the aviation industry. The fiber optic sensors (FOS) that can outperform electrical sensors in challenging operational environments were used. Standard features were extracted from AE measurements, and a convolutional autoencoder (CAE) was applied to extract deep features from AE signals. Different machine learning methods including discriminant analysis (DA), neural networks (NN), and extreme learning machines (ELM) were used for the construction of classifiers. The analysis is focused on the classification of extracted AE features to classify the source material, to evaluate the predictive importance of extracted features, and to evaluate the ability of used FOS for the evaluation of material behavior under challenging low-temperature environments. The results show the robustness of different CAE configurations for deep feature extraction. The combination of classic and deep features always significantly improves classification accuracy. The best classification accuracy (80.9%) was achieved with a neural network model and generally, more complex nonlinear models (NN, ELM) outperform simple models (DA). In all the considered models, the selected combined features always contain both classic and deep features.

Investigation of buckling of laminated composites repaired by resin injection using acoustic emission and cohesive zone simulation method

Due to their low weight ratio, excellent damping behavior, and high strength characteristics, composite materials are widely used in different industries, including aerospace. Despite the excellent mechanical behavior, most of these materials suffer from damages such as delamination when undergoing low-energy impacts. In the present study, repairing the composite structures using the resin injection into the damaged zone is analyzed. In some samples, initial delamination is included during the manufacturing process using the Teflon film, while the other samples are left to be intact. Furthermore, the delamination inside the composite sample has been simulated using the cohesive zone method (CZM) using ABAQUS software. The samples with delamination were repaired with the resin injection method. Finally, all samples underwent the buckling test, and the buckling process of the samples was monitored using the acoustic emission method. The amounts of damage mechanisms of delamination growth, fiber breakage, and matrix cracking were calculated using the wavelet method. The results show a good agreement between the experimental results with both the finite element method (FEM) and the acoustic emission results. Besides, the restoration percentages of compressive strength in samples repaired with stacking sequence [0]8, [90]8, and [0/90]8 were 88%, 87%, and 91.7%, respectively.

Numerical Simulation Analysis of Mechanical Properties on Rock Brittle–Ductility Transformation Under Different Loading Rates

At present, a large number of physical tests and numerical simulations have been carried out to study the effect of confining pressure on rock deformation mechanism, and some achievements have been achieved; however, the mechanism of rock deformation in actual mine engineering needs to be further studied, for example, rock-burst is actually a unilateral unloading process of rock mass, and this process can not be completed by physical test. RFPA3D was used to simulate the brittle–ductility transformation mechanical properties of rock under different confining pressures in this paper. The damage constitutive equation of rock was derived from continuum damage mechanics; the damage coefficients of different rocks were determined based on the numerical results of stress acoustic emission, so the correctness of rock damage constitutive equation was verified. According to the derived brittle–ductility damage equation and the fitting results of ductility cumulative damage data, it was found that the development trend of rock brittleness stage was almost the same, and the extended separation occurred after entering ductility stage. The larger the Poisson’s ratio was, the longer the ductility stage was. The smaller the Poisson’s ratio was, the shorter the ductility stage was, but the larger the bearing capacity was. At the late loading stage, the ductility cumulative damage of rock showed a linear upward trend, the bearing capacity sharply decreased, the rock stability failure occurred, and the ductility damage coefficient increased gradually. The study on the brittle–ductile mechanical properties of rocks can help to deep mine’s rock-burst prediction and prevention and has significant engineering significance.

Characterization of anisotropic mode II fracture behaviors of a typical layered rock combining AE and DIC techniques

Shear-sliding mode fracture, i.e., mode II fracture, is a very common rupture mode of layered rocks. To achieve a better understanding of the anisotropy in the real mode II fracture, which propagates in a self-similar manner, a series of direct shear tests were conducted on single-notched specimens made of shale with three bedding angles, i.e., β = 0°, 45° and 90°. The ranking of the anisotropic mode II fracture toughness (KIIc) with different bedding angles is KIIc,45° >KIIc,90° >KIIc,0°, which is different from that of the anisotropic mode II fracture energy release rate (GIIc), i.e., GIIc,90° >GIIc,45° >GIIc,0°, suggesting that, unlike isotropic rocks, the crack propagation resistance of a layered rock cannot be completely characterized by its fracture toughness. From the acoustic emission monitoring results, it is found that the fracture process initiates and accelerates from the notch front once the applied mode II stress intensity factor (SIF) reaches its first and second thresholds, KII, FPI (approximately 50%∼70% of the fracture toughness) and KII, FPA (approximately 85%∼95% of the fracture toughness), respectively; generally, both KII, FPI and KII, FPA show noticeable anisotropies. The results of the b value indicate that more small-scale events occur in the specimen with β = 0°, and large-scale events account for a greater proportion in the specimen with β = 45°. A dominant high strain zone along the prefabricated crack direction and a secondary high strain zone along the bedding planes were discovered. Combining the acoustic emission and digital image correlation results, three typical fracture mechanisms were observed: (1) shear fracture along bedding planes when shearing along beddings; (2) shear fracture in matrix and along bedding planes for β = 45°; and (3) shear fracture in matrix and tensile fracture of bedding planes when shearing perpendicular to the beddings.