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The stability of coal walls (pillars) can be seriously undermined by diverse in-situ dynamic disturbances. Based on a 3D particle model, this work strives to numerically replicate the major mechanical responses and acoustic emission (AE) behaviors of coal samples under multi-stage compressive cyclic loading with different loading and unloading rates, which is termed differential cyclic loading (DCL). A Weibull-distribution-based model with heterogeneous bond strengths is constructed by both considering the stress–strain relations and AE parameters. Six previously loaded samples were respectively grouped to indicate two DCL regimes, the damage mechanisms for the two groups are explicitly characterized via the time-stress-dependent variation of bond size multiplier, and it is found the two regimes correlate with distinct damage patterns, which involves the competition between stiffness hardening and softening. The numerical b-value is calculated based on the magnitudes of AE energy, the results show that both stress level and bond radius multiplier can impact the numerical b-value. The proposed numerical model succeeds in replicating the stress–strain relations of lab data as well as the elastic-after effect in DCL tests. The effect of damping on energy dissipation and phase shift in numerical model is summarized. Highlights: Fatigue behaviors of coal are reproduced by particle model under DCL Micro seismic monitoring is implemented based on heterogeneous particle model The distinction incurred by DCL is reproduced and revealed [ABSTRACT FROM AUTHOR]

This work aims to reveal the fracture the damage evolution characteristics of fissure‐holes containing rock under multistage constant‐amplitude variable‐frequency cyclic loads. Testing results show that the peak strength and fatigue lifetime both increase as the fissure angle increases from 10° to 70°. However, the volumetric deformation, AE activities, and crack network scale get to a maximum for a sample having a 50° fissure angle. More small‐sized cracks were formed for rock having a high fissure angle, and the proportion of low‐frequency signal is relatively high. A damage evolution model was proposed based on AE rate, and an inverted "S"‐shaped curve reflects the damage propagation of the experimental data. Additionally, the highlighted internal crack network reveals the crack coalescence and hole spalling behaviors; three types of rock bridge coalescence modes of single tensile coalescence, double‐tensile coalescence, and double shear coalescence were identified. Highlights: Failure of pre‐flawed granite under multi‐level variable‐frequency (MLVF) cyclic loads was discussed.Fissure angle influences the rock deformation, lifetime, AE activities, and the final crack pattern.A damage evolution model was proposed by AE rate to describe three‐phase damage propagation.The crack coalescence behaviors were visualized and three coalescence modes were identified. [ABSTRACT FROM AUTHOR]

Chen et al. [[6]] developed a method to obtain the size distribution characteristics of the real source from the apparent amplitude in doubly truncated distribution using rock acoustic emission (AE) tests. 10.3390/math10193490 9 Qiu J., Feng F. Effect of Different Tunnel Distribution on Dynamic Behavior and Damage Characteristics of Non-Adjacent Tunnel Triggered by Blasting Disturbance. With the increasing requirements for energy, resources and space, numerous rock engineering projects (e.g., mining, tunnelling, underground storage, geothermal energy, petroleum, water conservancy and hydropower) are more often being constructed and operated in large-scale environments with complex geology. [Extracted from the article]

Rock joints have obvious acoustic emission (AE) Kaiser effect and Felicity effect under multilevel cyclic shear conditions. The TFD-20H/50J rock shear apparatus was used to carry out cyclic loading and unloading joint shear tests, and the acoustic emission parameters and frequency spectrum characteristics of the whole shearing process were analyzed. The results show that, under the cyclic loading, the shear stress-displacement curve forms several cyclic hysteresis loops, and the number of loops increases with the increase of normal stress. With the cycles increase, the shear damage gradually increases, and the Felicity ratio gradually decreases. The Felicity ratio at the final shear failure moment is about 0.94~0.99. The ratio of the RA value (rise time/amplitude) and the average frequency value (RA-AF) is used to classify the cracking mode of the joint sample. There are two AE crack signal types (tensile type and shear type) during shear damage. The peak frequency is displayed as high, medium, and low three frequency bands, which are distributed in the range of 0~35 kHz, 35~122 kHz, and 122~300 kHz, respectively. Both low-frequency and high-frequency signals account for less than 10%, and medium-frequency signals account for more than 90%. The research of the AE monitoring signals of multilevel shear behaviors can help understand the shear-friction mechanisms of rock joints. [ABSTRACT FROM AUTHOR]

• MSC model is proposed to replicate multi-level cyclic loading results. • Damage variable is stress-time dependent in numerical model. • Damage variable is characterized by reduction of bond diameter. • AE characteristics during cyclic loading are simulated based on PFC3D. Under the framework of the particle flow code (PFC), a three-dimensional multi-level stress corrosion model (MSC) is proposed to reproduce the mechanical behaviour of brittle geo-materials and concrete exposed to multi-level cyclic compressive loading. The damage induced by the variable cyclic load levels during consecutive loading stages in numerical simulations is quantitatively characterized by time-stress-dependent reducing of the bond diameter. The change of maximum or minimum load levels correspond to different evolution characteristics of the bond diameter. The simulation results with the adoption of the MSC model shows good consistency with laboratory testing. The MSC model is proved to replicate the mechanical characteristics of concrete samples subjected to cyclic loading with both, variable maximum and minimum load levels. A three-dimensional acoustic emission (AE) monitoring system is implemented into PFC3D. The cumulative AE energy collected during the simulation is designated as the released bond strain energy at bond breakage. The cumulative AE energy in the simulation shows good agreement with laboratory test results. [ABSTRACT FROM AUTHOR]

Based on dissipated energy approach (DEA), the energy dissipation characteristics of concrete samples subjected to uniaxial cyclic loading have been investigated. The effect of different cyclic load levels on energy dissipation is quantitatively analyzed by different damage indicators during cyclic fatigue tests. It is concluded that the cumulative speed of energy dissipation and increasing growth-rate of damage indicators in Continuum Damage Theory (CDT) follow an exponential function in relation to the maximum cyclic load level and follow a logarithmic function in relation to the minimum cyclic load level. The loading strategy has an effect on total energy dissipated during fatigue tests, in other words: Energy dissipation and damage evolution are stress path dependent. [ABSTRACT FROM AUTHOR]

This article aims to numerically investigate the mechanical behaviour of concrete Brazilian Disk (BD) samples when exposed to multi-level cyclic Brazilian loading. A numerical 3D particle-based model was successfully established to simulate the dynamic stress-controlled cyclic testing. A generalized model is proposed based on the time-dependent evolution of bond diameter. This model shows great effectiveness in terms of reproducing the stress-strain curves, axial strain evolution and displacement fields as observed during lab testing. An acoustic emission (AE) tracking algorithm was implemented in the model to characterize and visualize the released AE energy connected with micro fracturing. The AE energy is found to be stress-level dependent which is consistent with lab testing. The failure pattern in numerical simulation is also in line with lab results. The distinctions between cyclic compressive and cyclic Brazilian tests are discussed and summarized. [ABSTRACT FROM AUTHOR]

• AE behavior and P-wave speed of concrete are revealed in cyclic Brazilian test. • Deformation behavior of concrete is revealed in cyclic Brazilian test. • DIC measurement is performed in cyclic Brazilian test. This work aims to investigate the fatigue behaviour of concrete subjected to indirect cyclic tensile loading. A series of laboratory tests were performed based on Brazilian disk-shaped (BD) concrete samples. Two schemes of cyclic loading tests were designed by considering complex variations of maximum and minimum stresses. The effect of loading parameters, such as loading method, maximum and minimum stress, on sample deformation, acoustic emission (AE) characteristics, P-wave speed evolution and failure patterns were analysed. The features of AE behaviour and P-wave speed were compared between monotonic loading and indirect cyclic loading. Insight on the specific fatigue behaviour was obtained from AE hit counts, AE energy and P-wave speed degradation. The failure patterns of BD concrete samples are governed by the loading method as well as the stress paths. The stress paths have a direct and pronounced impact on the distribution of main and sub cracks on the surface of BD sample at failure. [ABSTRACT FROM AUTHOR]