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To study the damage and acoustic emission (AE) energy evolution of basalt under high temperature, a series of tests under uniaxial stress are carried out on basalt heated at 25 °C, 200 °C, 400 °C, 600 °C, 800 °C and 1000 °C. Through the comprehensive analysis of AE parameters such as absolute energy and amplitude, the AE characteristics and damage evolution laws of basalt were explored. The results show that high temperature has a significant effect on the characteristics of rock stress–strain curve. With the increase of temperature, the compaction stage of the curve becomes longer, the plasticity increases, and the peak strength of basalt decreases. The higher the heat treatment temperature, the faster the strength decreases, and the strength decreases by 63.91% at 1000 °C. The AE localization points converge from the dispersed state to the aggregated state. With the increase of temperature, the period of convergent nucleation of the AE localization sites is advanced, high amplitude AE signals shifted from a concentrated distribution near the peak stress to a discrete distribution throughout the entire process. The heat treatment of the rock has an additive effect on the burstiness. 600–800 °C can be used as the threshold temperature of basalt to transform from brittle failure to plastic failure. The high temperature effect is responsible for the transition of the AE cumulative absolute energy at 400–600 °C. The influence of high temperature on damage evolution is discussed. The initial damage increases exponentially after 800 °C. With the increase of temperature, the evolution rate of damage variable becomes faster and gradually changes from linear to nonlinear. [ABSTRACT FROM AUTHOR]

Nuclear power is a high-quality clean energy source, but nuclear waste is generated during operation. The waste continuously releases heat during disposal, increasing the adjoining rock temperature and affecting the safety of the disposal site. Basalt is widely considered a commonly used rock type in the repository. This study of basalt's mechanical characteristics and damage evolution after thermal damage, with its far-reaching engineering value, was conducted by combining experimental work and theory. Uniaxial compression tests were conducted on basalt exposed to 25 °C, 500 °C, 700 °C, 900 °C, and 1100 °C conditions, and acoustic emission (AE) equipment was utilized to observe the acoustic emission phenomenon during deformation. This study was carried out to examine the mechanical characteristics, the sound emission features, the progression of damage laws, and the stress–strain framework of basalt after exposure to different types of thermal harm. As the temperature rises, the rock's maximum strength declines steadily, the peak strain rises in tandem, the rock sample's ductility is augmented, the failure mode changes from shear to tensile failure, and cracks in the failure area are observed. At room temperature, the acoustic emission signal is more vigorous than in the initial stage of rock sample loading due to thermal damage; however, after the linear elastic stage is entered, its activity is lessened. In cases where the rock approaches collapse, there is a significant surge in acoustic emission activity, leading to the peak frequency of acoustic emission ringing. The cumulative ring count of acoustic emission serves as the basis for the definition of the damage variable. At room temperature, the damage evolution of rock samples can be broken down into four distinct stages. This defined damage variable is more reflective of the entire failure process. After exposure to high temperatures, the initial damage of the rock sample becomes more extensive, and the damage variable tends to be stable with strain evolution. The stress–strain constitutive model of basalt deformation is derived based on the crack axial strain law and acoustic emission parameters. A powerful relationship between theoretical and experimental curves is evident. [ABSTRACT FROM AUTHOR]

To explore the mechanical properties and damage characteristics of basalt under high temperature and high pressure, triaxial compression tests are conducted on thermal damage rock samples, and the evolution process from progressive damage to macroscopic failure of rock is tracked and quantified by CT image reconstruction and acoustic emission technology. The results show that: (1) the ability of basalt to resist load and deformation changes from slight enhancement to rapid deterioration with the increase of temperature, and 600 ℃ is its threshold. The failure mode of rock gradually changes from brittleness to plasticity with the increase of temperature, and its post-peak stress gradually presents certain plastic flow characteristics. (2) With the increase of temperature, the initial damage variable D0 of basalt presents a stage feature of changing from low-speed development to rapid growth. When the temperature exceeds 400 ℃, the total damage variable D will have obvious precursor characteristic of small increase before the drastic change, and this characteristic will become more obvious with the increase of temperature. (3) The main mineral types of basalt hardly change with the increasing temperature, while the temperature has a certain influence on the proportion of some components. The proportion of anorthite and enstatite shows the changing trend of first decreasing and then increasing, while the evolution of andesine is opposite, and the proportion of other mineral components is basically unchanged. (4) The main reason for the improvement of mechanical properties of basalt within the temperature threshold is that the expansion of mineral particles and the constraint of confining pressure promote the closure of internal primary cracks and increase the compactness. However, when the heat treatment temperature of basalt exceeds the threshold value, the structural deterioration caused by heat treatment is gradually prominent, and the defects generated play a leading role in the mechanical properties of rock. Highlights: The mechanical properties of thermal damage basalt under triaxial loading were clarified and its temperature threshold was obtained The evolution process from progressive damage to macroscopic failure of basalt was tracked and quantified The microscopic failure mechanism of thermal damage basalt was revealed The number of tensile cracks of saturated coal increased with the cyclic loading frequency due to the prominent lag effect of strain behind stress. [ABSTRACT FROM AUTHOR]

Strainburst often occurs in the loading process of tangential stress concentration from surrounding rocks after excavation and unloading of deep rock mass. The concentrated tangential stress is relatively larger on the tunnel walls, which decreases towards the interior of surrounding rocks with a certain gradient. To explore the fracture evolution characteristics of surrounding rocks under different tangential gradient stresses and understand various phenomena in the strainburst process, a large‐scale true‐triaxial gradient and hydraulic‐pneumatic composite loading rockburst test device was adopted to carry out the strainburst tests under four different gradient stresses. Based on the macroscopic failure phenomena, distribution of rockburst debris, macro‐micro morphology of failure cross section, and monitored data of acoustic emission (AE), the fracture evolution of rockburst specimens was analyzed under different stress gradient loadings. The results indicate that the stress gradient has an obvious influence on the fracture characteristics, resulting in different rockburst phenomena. With the increase of stress gradient, the failure stress of rockburst decreases, while the dynamic failure characteristics of rockburst trends to be significant. The increase of stress gradient contributes to higher fragmentation degree of debris, more uniform distribution and better continuity. The total number of cracks in the model decreases with the rise of the stress gradient, and the proportion of shear failure increases, which is the main reason for the enhancement of the rockburst intensity. [ABSTRACT FROM AUTHOR]

Understanding the effects of thermal fatigue damage on the failure mechanisms of rocks is a key concern in underground engineering. The effects of high temperature on the physical–mechanical behaviors and the failure mechanism of basalt under uniaxial compression are investigated with a combination of acoustic emission (AE), computed tomography (CT), and scanning electron microscope (SEM). The high temperature heavily affects the physical–mechanical properties of basalt but has no effect on the mineral compositions. The evolution characteristics of inter‐event time function F(τ) and cumulative AE energy can be employed to characterize the fracture process of thermally damaged basalt. The damage mechanisms of thermal fatigue are attributed to the occurrence of intergranular cracks, intragranular cracks, and transgranular cracks and irregular holes within basalt. The failure mechanisms of basalt change from shear fracture to mixed tensile–shear fracture and finally to tensile fracture based on the statistical characteristics of low and high dominant frequencies. Highlights: The thermal fatigue effects on the physical–mechanical behaviors of basalt are studied.The relationship between failure process and AE characteristics is constructed.The damage mechanisms of thermal fatigue for basalt are revealed.The failure mechanisms of basalt under uniaxial compression are revealed. [ABSTRACT FROM AUTHOR]

To study the influence of fracture dip angle on the mechanical properties and fracture evolution mechanism of granite under triaxial stress state, MTS 815 mechanics test system was used to conduct triaxial tests on granite with different fracture dip angles, and PCI–II acoustic emission (AE) system was used to monitor the whole process information. The results show that the brittle characteristics of fractured samples with 30°, 45° and 60° dip angles are obviously weakened, while the plastic characteristics are enhanced. The fractures destroy the structural integrity of rock, resulting in the reduction of rock resistance to load and deformation. With the increase in fracture dip angles from 0° to 90°, the compressive strength and elastic modulus of rock samples show a nearly "U"-shaped changing trend of decreasing first and then increasing, and the deterioration ratio coefficients are 7.8 ~ 43.3% and 7.5 ~ 66.9%, respectively. Due to the difference of fracture dip angles, the failure of granite sample shows two modes: "through-cutting fracture surface failure" and "shear failure along fracture surface." It mainly depends on the angle between fracture surface and maximum principal stress. The fracture dip angle affects the variation law of AE signals during the fracture process. Especially for the fractured rock samples with 30°, 45° and 60° dip angles, the AE ring count and energy show obvious "migration" phenomenon, and the signal concentration distribution area is widened and moved backward. With the increase in dip angle, the active degree of rock fracture decreases at first and then increases. [ABSTRACT FROM AUTHOR]