Acoustic Emission Count Research Articles

Study on the Instability Characteristics and Precursor Signals of Rockburst in Ancient Fluvial Sedimentary Microfacies Coal Seam

The complex geological structure of coal seam is one of the important causes of rockburst. This paper investigates the instability process and precursor characteristics of ancient fluvial sedimentary microfacies coal seam. The results show that: (1) When the inclination angle is small，the sample experiences fracture and instability with relatively high peak stress. When the inclination angle is large, the sample experiences slip and instability, and high energy release under low stress condition. (2) Before the instability of the combined samples, the fracture development clearly results in a low total fault area ( A(t) ) value, relatively high and obviously low microseismic activity (S) values alternately appear, and the acoustic emission (AE) count and lack of shock (b) value will fluctuate significantly. (3) The peak strength of the combined samples decreases with increasing the inclination angle. The thickness of the coal is large, the strength of the combined samples is small, and the development and expansion of cracks and energy release are relatively weak. When the loading rate is relatively small, the peak AE count and energy release are relatively large. This research not only enriches the theory of deformation failure and dynamic disaster of complex coal seams, but also provides a reference for early warning of rockburst.

Impact of water saturation on the deterioration of mechanical properties and damage mechanisms of sandstone composites

ABSTRACT In-depth research on the impact of water saturation on the mechanical properties of rocks is crucial for safe and reliable rock engineering. This study conducted uniaxial compression tests and acoustic emission (AE) monitoring on coarse-grained sandstone, fine-grained sandstone, and their composites under natural and saturated water conditions. The results revealed that under saturated water conditions, the mechanical properties of coarse-grained sandstone deteriorated significantly, with peak stress (σ p), crack damage stress (σ cd), crack closure stress (σ cc), and elastic modulus (E 0) decreasing by 35%, 52%, 33%, and 47%, respectively. The deterioration of fine-grained sandstone was less severe, with σ p, σ cd, σ cc, and E 0 reducing by 13%–24%. The mechanical properties of the composites were significantly affected by the water state. Under natural water conditions, the σ p, E 0, and σ cd values of the composites were between those of the individual rock samples. However, when the composites contain saturated samples, the σ p, σ cd, and σ cc of the composites were lower than those of the individual rock samples. Under saturated conditions, the total absorbed energy (U) and stored elastic energy (U e) of the rock samples decreased, AE counts and crack signals reduced.

Effects of Thermal and Stress Cycles on the Mechanical Properties and Acoustic Emission Characteristics of Rocks

Abstract The surrounding rock of underground rock chamber is frequently affected by disturbance load and circulating temperature; it is meaningful to study the mechanical properties of surrounding rock under these conditions for the construction of a safe and effective underground chamber. This study investigates the mechanical properties, failure modes, and acoustic emission (AE) characteristics of basalt and sandstone under various pretreatments, including water saturation pretreatment (rock samples [SR]), rock samples subjected to cyclic temperature pretreatment (SR-CTT), rock samples subjected to cyclic loading pretreatment (SR-CLT), and rock samples subjected to combined cyclic loading and temperature pretreatment (SR-CTT-CLT). A series of uniaxial compression tests (UCTs) were conducted to analyze how these pretreatments affect the mechanical properties of basalt and sandstone. These two kinds of rock exhibit distinct failure modes, SR-CLT and SR-CTT-CLT make the failure of basalt change from inclined shear to X-shaped shear, while SR-CLT makes it turn into splitting. AE data reveal that basalt generally exhibits lower AE counts than sandstone, with the highest counts observed under SR-CTT-CLT. Energy analysis indicates that basalt accumulates more total energy (Et) and elastic energy (Ee) compared to sandstone, with different pretreatments affecting energy dissipation (Ed) and damage severity differently in each rock type. These findings contribute to understanding the complex interactions between pretreatment methods and rock behavior in engineering applications.

Study on the failure and acoustic emission characteristics of coal under graded cyclic loading and unloading stress paths

To study the influence of cyclic disturbance stress on the mechanical behavior of coal during mining, a gas containing coal fluid-solid coupling servo seepage experimental system was used to conduct experimental research on the acoustic emission (AE) characteristics of gas containing coal under two stress paths of graded cyclic loading and unloading. The AE characteristics of coal damage and failure processes under different cyclic stress paths were analyzed. The research results indicate that: (1) The overall characteristics of AE signals for both graded cyclic loading and unloading paths are basically the same. With increasing of the amount of graded cyclic loading or unloading, the AE count reaches its maximum value when reaching failure, and the cumulative ringing calculation of AE increases exponentially. (2) The AE signals under the graded cyclic loading or unloading path exhibit obvious zoning characteristics. In the low and medium stress regions, the AE signal basically satisfies the Kaiser effect, while reaching the high stress region before failure, the AE signal exhibits a significant Felicity effect. (3) The concentration coefficient of AE and the intensity coefficient of the Kaiser effect have been newly defined. They are used to quantitatively characterize the extent of the Kaiser effect of AEs under graded cyclic stress. It was found that as the variation of graded cyclic stress increases, the concentration coefficient and Kaiser effect intensity coefficient both show a decreasing trend. (4) Combining the AF and RA values of AE, it was found that the coal failure signals of the two stress paths were basically similar, that is, the overall failure was mainly tensile failure, and the signals of cyclic unloading tensile failure were significantly more than those of cyclic loading. The AE signal characteristics studied in this article are of great significance for predicting coal power disasters.

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.

Annealing impact on mechanical performance and failure analysis assisted with acoustic inspection of carbon fiber reinforced poly‐ether‐ketone‐ketone composites under flexural and compressive loads

AbstractThis study investigates the effect of the annealing treatment for carbon fiber reinforced Polyether‐ketone‐ketone (CF/PEKK) composite structures under flexural and compressive loadings through reference, pre‐damaged, and annealed sample sets. Significant recovery of pre‐existing damage is observed after the annealing process, following both flexural and compressive loading. Acoustic emission (AE) inspection is employed to monitor the failure behavior and assess the impact of pre‐damage and annealing on CF/PEKK composite. Initially, AE inspection reveals that the reference CF/PEKK material exhibits a notable fiber‐related failure with 85% of cumulative AE counts under flexural load, whereas matrix‐related failures are more pronounced with 92% cumulative AE counts under compressive load. Pre‐damages in the matrix alter the cumulative count percentages and initiation time that are related to matrix, interface, and fiber‐related failures, under flexural and compressive loadings. After annealing, each cumulative AE count percentages are comparable to reference sample values, due to changes in microstructure and relieving of residual stresses. The annealing effect is further validated through dynamic scanning calorimetry (DSC) analysis results with increased glass transition temperature (Tg) and degree of crystallization (Xc). Overall, these findings indicate that annealing treatment effectively restores structural integrity and improves the mechanical performance of CF/PEKK composites.Highlights Annealing aims for damage recovery in CF/PEKK under flexural and compressive loads. Significant damage recovery in CF/PEKK is seen after annealing. Annealing raises Tg and crystallinity, and enhances CF/PEKK structural integrity.

The Re-Crushing Spatiotemporal Evolution Law of Broken Coal in the Goaf for Sustainable Utilization of Abandoned Mines

The broken coal samples’ (BCS) re-crushing characteristics in the goaf during roof compaction directly affect the mechanics and seepage characteristics of the caving zone. This will further affect the safety of coal mining and the sustainable utilization of abandoned mines. Thus, the experiment of BCS compaction is carried out with the help of an acoustic emission (AE) monitoring system. The Hurst exponent changes of the AE counts at different stages were obtained using the R/S analysis method. The results indicate that the compaction and re-crushing of the BCS at the laboratory scale have long-term memory. When providing sufficient stress, the AE activity of BCS will continue to develop according to the current trend. Based on the AE breakage location technology, the spatial distribution re-crushing characteristics of the BCS are obtained. Re-crushing of the BCS demonstrates uniform breakage in the horizontal direction and layered breakage in the vertical direction. In the horizontal direction, the boundary area first began to break, and the damage gradually spread evenly to the central area. In the vertical direction, the upper layer was the first to be broken, and then the damage began to shift to the middle and lower layers.

Study on the characteristics of acoustic-thermal precursors of destabilization damage in coal-rock combination bodies with different proportions

The instability and failure processes of coal-rock combinations are accompanied by the release of acoustic emission (AE) and infrared radiation (IR) signals. To investigate the characteristics of AE and IR signals during the failure process in coal-rock combinations with different ratios, and to analyze the effectiveness and applicability of the monitoring methods for these two signals in various specimen ratios. In this paper, the uniaxial compression tests were conducted on seven coal-rock combinations with different ratios by using the rock mechanics loading system and the cooperative monitoring platform of infrared and acoustic emission. The results show that (1) the AE counts for failure precursors in coal-rock combinations are positively correlated with the coal-rock ratio, whereas the AE peak counts are negatively correlated. Moreover, as the coal-rock ratio decreases, the slope of the cumulative AE count curve during the peak failure stage approaches 1. (2) During the elastic and yield stages, the maximum infrared radiation temperature (MIRT) curve fluctuates, and just before peak failure, the curve displays a distinctive “V” shape. This “V” shape becomes more pronounced as the coal-rock ratio decreases. (3) In the monitoring of damage precursors of coal-rock combinations, when the ratio of coal to rock is 1:3 or less, IR monitoring is better than AE monitoring. Conversely, when the ratio of coal to rock is greater than 1:3, AE monitoring is more suitable. Consequently, the combined monitoring of AE and IR signals can increase the reliability and precision of early warning signals for coal-rock assemblage damage precursors.

Effect of Wet‐Dry Cycles on the Loading–Unloading Damage of Granite

ABSTRACTInvestigating the effect of wet‐dry (W‐D) cycling on loading–unloading damage is crucial for ensuring the stability of granite slopes in mountainous areas prone to strong earthquakes. This study conducted a series of W‐D cycling and loading–unloading tests on granite samples, complemented by weight measurements, wave velocity assessments, and nuclear magnetic resonance (NMR) tests on samples subjected to W‐D cycling. The results indicated that W‐D cycling reduced the mechanical properties of granite and increased the irreversible strain, dissipated energy, and number of acoustic emission (AE) counts during loading–unloading. The increase in the number of W‐D cycles led to a significant rise in the number of small‐scale, intergranular, and transgranular microcracks and the gradual formation of middle‐scale and large‐scale microcracks. W‐D cycling weakened the granite because of crystal expansion and contraction as well as mineral dissolution, whereas loading–unloading damaged the granite through inconsistent deformation resulting from the varying hardness of different minerals. This study provides a foundation for understanding the formation mechanism of seismically cracked slopes under the combined effects of rainfall and earthquakes.

Uncovering the Fracturing Mechanism of Granite Under Compressive–Shear Loads for Sustainable Hot Dry Rock Geothermal Exploitation

Shear-dominated hazards, such as induced earthquakes, pose an escalating threat to the sustainability and safety of the geothermal exploitation. Variations in fault orientations and compression–shear stress ratios exert a profound influence on the failure processes underlying these disasters. To better understand these effects on the shear failure mechanisms of hot dry rocks, mode-II fracturing tests on granites were conducted at varying loading angles (specifically, 55°, 60°, 65°, and 70°). These tests were accompanied by a comprehensive analysis of the mechanical properties, energy dissipation behavior, acoustic emission (AE) responses, and digital image correlation (DIC)-extracted displacement fields. The tensile–shear properties of stress-induced microcracks were discerned via AE characteristic parameter analysis and DIC displacement decomposition, and the mode-II fracture energy release rate was quantitatively characterized. The results reveal that with increasing compression–shear loading angles, the mechanical properties of granites are weakened, and the elastic strain energy at peak stress gradually decreases, while the slip-related dissipated energy increases. Throughout the fracturing process, the AE count progressively climbs and reaches a peak near catastrophic failure, with an upsurge in low-frequency and high-amplitude AE events. Microcrack distribution concentrates aggregation along the shear plane, reflecting the emergent displacement discontinuities evident in DIC contours. Both the AE characteristic parameter analysis and DIC displacement decomposition demonstrate that shear-sliding constitutes the paramount mechanism, and the fraction of shear-oriented microcracks and the ratio of tangential versus normal displacement escalate with increases in shear stress. This analysis is supported by the heightened propensity for transgranular microcracking events observed through scanning electron microscopy. As the shear-to-compression stress increases, the energy concentration along the shear band intensifies, with the gradient of the fitting line between cumulative AE energy and slip displacement steepening, indicative of a heightened mode-II energy release rate. These results contribute to a deeper understanding of the mode-II fracture mechanism of rocks, thereby providing a foundational basis for early warnings of shear-dominant geomechanical disasters, and improving the safety and sustainability of subsurface rock engineering.

Using acoustic emission to understand the early-age reaction and cracking evolution of alkali-activated slag-brick powder paste

Real-time monitoring and accurately understanding the continuous occurring psychical-chemical characterization during the hardening process of alkali-activated materials present a formidable challenge. In this study, the early-age reaction process of alkali-activated slag-waste brick powder paste (ASWP) was in-situ and continuous monitored by using acoustic emission (AE) system and temperature. A novel machine learning method was employed to categorize the evolution of AE signals into three distinct stages: reaction acceleration, reaction stabilization and volume shrinkage stage. During reaction acceleration stage (0–2h), the waste brick powder (WBP) increased AE counts rate (4000–7300 counts/h), with RT values exceeding 200μs, indicating the settlement of WBP. Additionally, WBP reduced the high r-value AE activity in volume shrinkage stage, effectively mitigated the drastic volume shrinkage, and delayed its occurrence time by approximately 4 h. Semi-quantitative analyses of FTIR and TG-DTG indicate that WBP reduced gel phase formation in the early-age stage, decreasing the relative areas in Q2 site by 16% before 36 h. However, WBP positively impacted ASWP hydration at long ages, increasing Q2 site d areas by 60%–100%. AE technique offers a new perspective for in-depth understanding of the reaction mechanism and microstructural development of alkali-activated materials in situ and continuously.

Co-effects of parallel flaws and interface characteristics on the fracture behavior of rock-concrete composite specimens

The fracture behaviour of rock-concrete composite (RCC) containing interfaces and flaws is important for assessing the stability and safety of concrete systems constructed on rock foundations. Uniaxial compression experiments were performed on RCC specimens, and acoustic emission (AE) and digital image correlation (DIC) techniques were adopted to capture the characteristics of AE and crack extension process. The impacts of interface dip angle, concrete strength, and flaw geometry on the coalescence behavior of RCC specimens were analyzed. The findings indicate that four kinds of crack coalescence patterns were recognized: indirect coalescence, left-tip and right-tip cracks connected, coalescence of same-side tip cracks, and no connection. The peak strength displays a reducing trend with increasing the interface dip angle, while the peak strength presents an increasing trend with increasing concrete strength. The overlapping parallel distribution of composite specimens presents a higher peak strength. The crack coalescence mode of overlapping parallel distribution changes from the coalescence of same-side tip cracks to the no-connection mode when interface dip angle is larger. It is worth noting that cracks tend to appear first in the concrete when the strength is relatively low. The AE counts tended to increase as the interface dip angle increased, with the earliest rise in AE counts in the left-stepping distribution and longer duration in the overlapping parallel distribution. Additionally, the energy characteristics of composite specimens correlate closely with the interface dip angle and flaw geometry.

In-situ acoustic emission investigation of asphalt fracture process and damage quantification for different asphalt mixtures

Abstract The identification of cracking damage and the dynamic diagnosis of damage evolution are of great importance to prolong the service life of asphalt mixture materials. Acoustic emission (AE) can identify the formation and propagation of cracks by detecting the released energy of the damage; therefore, it provides a viable real-time technique to estimate the damage state in the context of structural health monitoring. In this study, the crack formation and propagation of three different asphalt mixtures were investigated using in-situ AE monitoring and microscopy imaging. A three-point bending test was performed using specimens fabricated from three different types of asphalt mixtures. The variation of AE parameters such as the cumulative AE energy and the AE count was obtained and correlated with the crack sizes to characterize the damage process of the three mixtures. Based on the AE parameters, the whole damage process can be divided into three distinct phases, namely, the elastic deformation, damage accumulation, and crack propagation. The AE parameters in the three different mixtures show unique features, and they can be used to capture the transition between phases and identify the damage state. A universal power law model is proposed to correlate the AE energy and the crack density for different types of asphalt mixtures, providing a viable means for damage quantification and predictive maintenance.

Time-dependent damage characteristics of shale induced by fluid–shale interaction: a lab-scale investigation

The shale’s multi-scale mechanical behaviors were investigated to understand the time-dependent damage characteristics induced by shale–fluid interaction. The test results indicate that, as fluid–shale interaction proceeds, the mechanical strength of shale has undergone a weakened to an enhanced process as interaction proceeds, and the size distribution of fragments tends to be more uniform, leading to a positive correlation between the mechanical strength and fractal dimension. Within 2 weeks of fluid–shale interaction show a decreasing trend for the fractal dimension of fragments in post-failure and cause less than 2 mm for the dominant size of fragments. The dominant size increases to greater than 30 mm when the fluid–shale interaction is over 2 weeks. Finally, the correlation dimension associated with ring counts of acoustic emission (AE) at each loading stage is determined in terms of the G-P algorithm to predict the damage degree of shale.

Experimental study on effect of cyclic gas pressure on mechanical and acoustic emission characteristics of salt rock

The utilization of renewable resources like wind and solar energy has led the Chinese government to prioritize the development of energy storage systems, garnering significant attention for salt cavern compressed air energy storage (CAES) technology. The stability of the surrounding rock in salt caverns is critical for ensuring sustained operation. By conducting mechanical tests on salt rock under fixed axial and confining pressures with cyclic gas pressure, the mechanical behavior and acoustic emission characteristics of salt rock under the influence of gas pressure were analyzed. The mechanical and acoustic emission characteristics of salt rock were analyzed. The results indicated that the triaxial compressive strength of salt rock negatively correlated with constant gas pressure. Both radial and axial strains demonstrated distinct phases: deceleration, steady-state, and acceleration. The steady-state creep rate of axial strain and the lower limit of cyclic gas pressure can be fitted in the form of an exponential function. The number of cyclic air pressure cycles decreased as the lower limit of cyclic air pressure increased. Acoustic emission count was used to define the damage variables, clarifying the formation and accumulation of salt rock damage in each stage, distinguishing the growth rates of damage during loading and unloading. It was observed that the higher the lower limit of cyclic air pressure, the quicker the damage accumulated with cycling. During the test, the absolute energy of the acoustic emissions from the salt rock conformed to a damped power-law distribution. The power-law distribution index γ of the absolute energy of acoustic emissions shifted with the number of cyclic air pressure cycles, and the sudden decrease of the distribution index γ plays a certain early warning role for the failure of salt rock.

Experimental study on evaluating fracture processes of different rocks using multiple physical parameters

To improve the accuracy of rock fracture evaluation, three types of rocks with different lithologies, namely, granite, coal, and sandstone, were processed into cuboid samples with unilateral notches for three-point bending tests. Real-time data were obtained via load, resistance, and acoustic emission (AE) monitoring. Various fracture patterns of the fractured rocks were collected, and the correlations between multiple physical parameters were determined. The results indicated that granite had the highest fracture toughness and initial fracture energy required for macroscopic fracturing, followed by sandstone, which had the lowest fracture toughness and initial fracture energy. The primary crack participated in the fracture process of coal and produced the most complex crack morphology, exhibiting the characteristics of “Progressive fracture” different from the “Instantaneous fracture” of granite and sandstone. The crack length reached 90.5 mm, which was much longer than that of sandstone. The temporal characteristics obtained from multiple physical quantities were synchronized with the fracture behavior. The resistivity increased gradually with the fracture, and the resistivity fluctuation caused by crack propagation increased. The resistivity change rate of granite was the highest when macrofractures occurred, whereas that of coal after complete fracture was the highest. Rock fracture produced a large number of AE events with small amplitude and low peak frequency, concentrated in the 100 ± 50 kHz band. The AE count corresponded to the crack propagation process, and the AE location indicated the crack complexity. The peak change rate of the resistivity was used to evaluate the fracture performance and exhibited a significant linear relationship with the peak load, fracture toughness, and initial fracture energy. The relationship between the accumulated AE energy and the fracture length was a quadratic function, which was used to evaluate the fracture complexity. The multiple physical quantity monitoring method is promising for predicting the fracture behavior of rocks.

Experimental and numerical study on the mechanical behavior of layered rock anchored by CRLD anchor cable

The study of the mechanical behavior of layered anchored rock is essential for addressing the stability control challenges of surrounding rock of the roadway in layered rock masses. This paper focuses on a novel constant resistance large deformation (CRLD) anchor cable and conducts a comparative study on the uniaxial compressive mechanical behavior of unanchored rock, conventional high-strength (HS) anchor cable, and CRLD anchor cable anchored rock under different bedding dip angles α (30°, 45°, 60°, 75°, and 90°). The strength characteristics, failure characteristics, and acoustic emission characteristics of various specimen types were analyzed with emphasis. The results indicate that with the increase of bedding dip angle, the elastic modulus, peak strength, and residual strength of all specimen types exhibit a trend of decreasing first and then increasing. Compared to unanchored and HS anchored rock, the values of various mechanical parameters for CRLD anchored rock show varying degrees of improvement. The failure modes of various specimen types under different bedding dip angles can be classified into five categories based on the characteristics of the crack distribution. Through comparison, it was found that the CRLD anchor cable can effectively restrict the occurrence of bedding shear failure within the anchored zone in layered rocks with smaller dip angles (30°, 45°). It also transforms the failure mode of layered rocks with larger dip angles (60°, 75°) from a single mode of bedding shear failure to a mixed mode of bedding-bedrock failure. However, under a bedding dip angle of 90°, the two types of anchored rocks exhibit similar failure forms. Comparing the acoustic emission characteristics of the two types of anchored rocks, it is observed that under different bedding dip angles, CRLD anchored rocks exhibit a more uniform distribution of acoustic emission counts, a smoother energy release process, and significantly lower peak count and cumulative energy values. Finally, a numerical model for layered CRLD anchored rocks was constructed based on the discrete element method. The reliability of the experimental results from the physical model in this study was verified by statistically analyzing the quantity evolution process and orientation distribution of microcracks in the numerical model.

Damage deformation properties and acoustic emission characteristics of hard-brittle rock under constant amplitude cyclic loading

In order to study the deformation and damage characteristics of the limestone specimens with high strength and brittleness under constant amplitude cyclic loading, the deformation and the acoustic emission (AE) characteristics were analysed, and the relationship between them was sought. The damage variables under different amplitude cyclic loading were defined by AE counts. The results showed that the radial deformation of the limestone specimens was more sensitive and unstable than the axial deformation. The concept of apparent residual strain was proposed to describe the specimen deformation characteristics, and it resulted that the radial apparent residual strain produced at higher stress state would recover at lower stress state. The limestone specimens showed obvious Kaiser effect and Felicity effect under cyclic loading. When the upper limit of the cyclic loading was close to the peak stress of the specimen, the AE counts generated in unloading sections were almost the same as that in the loading sections. The damage was increased as the amplitude and the stress level increased and the unloading process at higher stress level would also lead to the aggravation of damages. Specimens would absorb more energy under cyclic loading than under uniaxial loading. Reasonable driving parameters should be controlled in underground excavation practice, to ensure that the stress level of surrounding rock mass in a periodic stress state is located before peak stress and such that to limit the occurrence of rock burst to a certain extent.

Mechanical behavior and energy characteristics of red sandstone with different seawater immersion heights under biaxial compression

Rock structures engineered during undersea mining are typically subjected to varying water distributions due to the motion of seawater, which considerably influences their stability. Hence, it is essential to understand the influence of seawater distribution on the mechanical behavior and energy characteristics of rocks. In this study, biaxial compression tests were conducted on red sandstone at various seawater immersion heights, and acoustic emission signals during compression were monitored. The results illustrate that the mechanical properties of the red sandstone deteriorate significantly upon seawater immersion. With an increase in seawater immersion height, the peak strength and elastic modulus of the rock specimens decreased exponentially. When the seawater immersion height was varied from 0 to 1/4 H under lateral stresses of 5, 10, 15, and 20 MPa, the peak strength decreased by 18.94%, 20.29%, 17.47%, and 14.87%, respectively, and the elastic modulus decreased by 4.6%, 8.1%, 11.7%, and 10.9%, respectively. Brittleness also decreased gradually. During compression, the acoustic emission (AE) and accumulated AE counts exhibited a stationary phenomenon, first increasing slowly and then suddenly. However, the AE counts decrease with increasing seawater immersion height. Meanwhile, with increasing seawater immersion height, the proportion of tensile cracks gradually increased and that of shear cracks gradually decreased. As the seawater immersion height increased, both the peak total input strain energy and peak total elastic strain energy decreased, whereas the peak total dissipated strain energy exhibited the opposite trend.

Effects of foliation angle on mechanical characteristics of carbonaceous slate using uniaxial compression tests

Carbonaceous slate is one kind of metamorphic rocks with developed foliation, which is frequently encountered during tunnel construction in Western China. The foliation plays a crucial role in the stability of tunnels. For this, we conducted uniaxial compression tests, acoustic emission (AE) monitoring and scanning electron microscope (SEM) tests on carbonaceous slate. The results show that the strength, failure mode, and AE characteristics exhibit marked anisotropy with the angle between the axial and the foliation (β). As β increases, the ultrasonic wave velocity decreases monotonically, whereas the uniaxial compressive strength (UCS) displays a distinctive U-shaped trend. The elastic modulus initially decreases and then increases. The cumulative AE counts curve and energy curve show a stepped growth when β ≤ 45°. The AE events are active during the crack compaction phase and remain calm during the linear elastic deformation phase when β > 45°. Upon failure, the energy release accounts for the highest proportion (67%) when β = 45°, while the proportions in other cases are less than 37%. The maximum percentage (31%) of shear cracks is reported when β = 60°, which is six times greater than that at β = 0°. Moreover, Kernel density estimation analysis reveals that the high concentration area with low AF (AE counts / duration time) and high RA (rise time / amplitude) increases initially, and then decreases when β > 60°. In addition, nine types of cracks and seven modes of failure were identified. The foliation angle has a pronounced impact on shear failure modes in comparison with tensile failure modes. The supports could suffer larger deformation when β ≥ 60° compared to other cases. The failure behaviors correspond well with field observations.