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Ef Find V4 ((TOP)) Crack

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Ef Find V4 Crack

Acoustic emission (AE) is the immediate release of strain energy in the form of an elastic wave during material deformation16,17,18,19,20,21,22,23,24,25,26,27. The AE parameters and AE relationship to the mechanical failure process under compression have been extensively studied by researchers. By utilizing AE techniques, significant development has been made in understanding the progressive failure process of coal and rock. Researchers investigated coal and rock AE behavior under uniaxial compression, characterization of rock crack patterns under different loading rates, and coal AE fractal characteristics28,29,30,31,32,33. Water content has a significant impact on the AE characteristics of coal and rock, resulting in a decrease in elastic energy release and a reduction in the AE signal during loading34. Studies were conducted to investigate the effect of water content on failure patterns and AE features of rock with different water content. For this, uniaxial compression tests and numerical simulations with PFC2D software were used to investigate the evolution of microcracks and failure patterns15,35,36,37,38,39,40,41,42,43. The results indicated that increasing water content within the rock significantly decreased the rock strength, Young's modulus, strain to peak strain ratio in the elastic deformation stage, the maximum energy of a single AE event, and average AE energy. Lin et al. studied the AE parameters during disc cutter-induced rock fragmentation processes under various water conditions44. Read et al. studied the evolution of microcracks in rocks under saturated conditions using AE data45. The results demonstrated that the varying characteristics of crack propagation in rocks could be characterized by the frequency of AE events combined with volume change. The AE parameters and P-wave velocity variations in the porous rock after microcrack closure were studied by Fortin et al.46. Zhou et al. have taken sandstone under different water contents as a research object, and the type I fracture mechanism and AE characteristics were investigated47. The results unveiled that increasing water content decreased fracture toughness (KIc). Zhou et al. also investigated the quasi-static fracture behavior of sandstone containing different water content. For this, notched semi-circular bending (NSCB) tests were conducted, and the cracking process and acoustic emission (AE) signals were recorded simultaneously48. Zhu et al. and Liu et al. have paid their efforts to investigate the frequency features of AE signals during loading on dry and water-saturated rocks. The outcomes revealed that the high-frequency AE signals were pointedly reduced by water content26,49,50. An Analytical damage model based on acoustic emissions was developed by Ali et al. for dry and saturated coal51. Recently, the researchers have introduced the multi-fractal theory to deconstruct the AE signals to reveal better the nonlinear and multi-scale features of the deterioration and fracture process of water-bearing rocks52,53,54. These studies have provided valuable insights into understanding rocks' AE characteristics and damage behaviors under different water conditions. Hitherto, the variation of AE, fractal characteristics, and mechanical parameters of natural and coal with different water content and soaking times is rarely reported.

This paper examines the fractal characteristics of coal with different water contents in order to evaluate the variation in AE characteristics and mechanical parameters during the failure process of the coal. Coal was taken as an example to prepare specimens with varying water contents based on different soaking times. Grassberger Procaccia (GP) algorithms were utilized to find AE fractal characteristics based on the theory of phase space reconstruction. Based on the results of the AE test, comprehensive data could then be synchronized with the results of the mechanical testing in order to determine the type and location of rupture as well as the failure process of the samples. Therefore, this technique can be successfully utilized to predict coal and rock dynamic failure and ensure safety for engineering projects.

Energy dissipation is the most fundamental aspect of rock deformation and failure, reflecting the resulting development of new internal cracks, which weaken and ultimately disappear the strength of the material. According to the viewpoint of energy, the essence of the change of the physical state of matter is the conversion of energy, so rock failure can be regarded as state instability driven by energy.

The findings indicate that AE energy and AE cumulative energy characteristics vary depending on the stage under stress, as well as AE characteristics that vary based on the water content of coal samples. Figure 8. illustrates a good correlation between AE energy and stress at all stages of the process. The loading process has five stages based on stress/strain curve. In order to determine the variation law of AE at different stages, the features of AE are examined. As can be seen in Fig. 8, all samples exhibit almost similar trends in AE under loading conditions.

In order to understand the behavior of AE parameters under the applied load, the AE characteristics were studied in accordance with the five stages, using D1 samples as an example. The first stage is the compaction stage, where the closing of coal cracks due to stress, resulting in a very limited number of AE events. The second stage is the linear elastic stage, where the coal's distortion enters the elastic deformation stage after it has been compressed. As a result, no new cracks are formed, the number of AE events is significantly reduced, and the AE energy is kept to a minimum. Stage 3 is the stable crack propagation-SCPS-stage, where the stress reached the point of crack initiation, the AE energy increased dramatically, indicating new cracks generated and propagated. Stage 4 is referred to as "crack accelerated propagation stage" where, the cracks continue to expand and consolidate until eventually a fracture network formed, resulted in macro cracks. After the macro cracks were coupled, large quantities of elastic energy were released, which had been stored during the primary phases of the sample compression, leading to very active acoustic emission events. As a result, the acoustic emission energy reached its maximum instantly. As a result of this, there is also a sudden change in the dissipation energy and the cumulative AE energy. Stage 5 is the "post peak, and residual phases", where the stress lowers, and the acoustic emission energy drops dramatically after the coal sample is failed. However, residual stress results in some residual strength in the coal sample, resulting in many secondary fractures that form and expand. There is a slight increase in AE energy rate prior to failure. The cumulative energy and dissipation energy also varied accordingly.

Furthermore, the stress of sample D1 drops to zero, as shown in Fig. 8d, quickly after the peak, although the maximum AE energy rate is substantially lower than that of the other samples. This is because coalescence occurs from the side of the sample, and neither sample has any cracks on the front surface. When AE is not present, the specimen's eventual failure rate is lower than other specimens. The cumulative energy of A1, B1, C1, D1, and E1 are 87,025 mV μs, 89,154 mV μs, 100,020 mV.μs,105,630 mV μs, and 121,005 mV μs, respectively, Indicating that the cumulative energy of AE is also affected by water content and concluding that the greater the water content, the lower the cumulative energy of AE.

Under varying water contents, the dissipation energy and cumulative energy curves demonstrate a consistent trend. Figure 8 shows that the dissipation and AE cumulative energy curves show a linear trend in the compaction and linear elastic stages, and a slight stable growth at the stable crack propagation and crack propagation accelerating stages. However, an abrupt rise occurs when the loading approaches peak, which can be used to predict failure. As shown in Fig. 8a,e, the coal's cumulative energy and dissipation energy curves increased dramatically when it was loaded from a linear elastic state to a stable crack propagating stage. In addition, the dissipation energy value increased with increasing moisture content at peak loading stage, indicating that a significant amount of energy was dissipated in plastic deformation and crack development.

lnC(r) and ln(r) function of water content for specimens subjected to varying stages of loading are plotted in Fig. 11. As illustrated in Table 4, each data point set is matched with a straight line, and the fitting correlation coefficient is calculated as R2. Consequently, D is determined using Eq. (12). Table 4 indicates that Phases 3, 4, and 5 demonstrate R2 values greater than 0.90 in all specimens, with the majority exceeding 0.96. As a result, it can be assumed to be linearly connected to D, implying that the AE energies of all samples in phases 3, 4, and 5 exhibit fractal properties. Changes in D can represent the evolution of cracks in coal, depending on the nonlinear characteristic function of AE energy. As a result, D is a correlation dimension that's useful for predicting rock and coal internal mechanical behaviors and structural degradation.

According to the findings, the presence of water in coal samples reduces the EM and UCS. This study's mechanical behavior can be compared to that of prior investigations57. A major finding of the study is that the UCS of coal decreases as the water concentration increases, as demonstrated by Hongru Li et al. who also examined the effect of water content on the mechanical characteristics of rock specimens during uniaxial compression58. In this study, we investigate the impact of water content on the mechanical properties, the acoustic emission response, and the fractal characteristics of coal samples, and provide practical recommendations as a result. Based on the results, we conclude that the amount of water in a rock has a profound effect on its mechanical behavior and AE signals. As the water content increases, the UCS and EM decrease, whereas peak strain and energy dissipation increase. However, the research was limited to samples with low moisture content and short periods of soaking. In a study conducted by Ali et al., EM and UCS of saturated coal samples gradually decrease with increasing water content4. Laboratory and theoretical analyses have previously confirmed the mechanical properties of coal specimens under uniaxial compression. Researchers may find the results to be useful in solving problems related to underground engineering, such as those associated with water-rich coal environments. 350c69d7ab


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