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The single-peak overload test based on DIC technology was carried out in this study, and U71MnG steel was used to explore the influence of dislocation motion on crack propagation during overload. The changes in the shape and size of the plastic zone during the overload fatigue cycle are tracked and recorded, and the trends in the stress intensity factors of the Christopher–James–Patterson (CJP) and dislocation correction models are compared. The degree of influence of the dislocation motion on the variation in the stress intensity factors is evaluated, and the variation pattern of the plastic flow factor is derived (the amount of crack tip blunting, ρ ). The results showed that the dislocation correction model increased the accuracy of the solution of the coefficient set, and the predicted size of the plastic zone of the correction model was more consistent with the experimental data. A better match with the crack tip was observed in the experimental plastic zone, the dislocation correction model error with the theoretical plastic zone fluctuates within 10%, whereas the CJP model can reach a maximum of 36.75%, demonstrating the insensitivity of the dislocation correction model. The plastic flow factor ρ follows the same pattern as that of the plastic zone area and stress intensity factor amplitude, ρ increases slowly with the increase of crack length before overload, ρ increases significantly after overload and then decreases sharply, and it recovers to be stable with the disappearance of the overload hysteresis effect of crack propagation.

HighlightsThe radius of the plastic zone and the radial stress is introduces into the stress-intensity ratio.A new rockburst criterion is proposed to predict the rockburst classification.The certain relationship between the rockburst rating and the radius of the plastic zone and radial stress is studied.

Coal mining will lead to rock strata bending deformation and the redistribution of stress field. The change of principal stress magnitude and direction will influence the failure zone distribution of the coal pillar, which determines the stability and safety of roadway near goaf. In this paper, a theoretical model is established to analyze the rotation of principal stress in the coal pillar caused by the bending deformation of key strata due to coal mining. The theoretical principal stress magnitude and direction in the coal pillar are obtained and the variation of principal stress are analyzed with different coal pillar widths and different roof breaking locations. Furthermore, we derived a novel plastic zone distribution in the coal pillar, which considering mining induced principal stress rotation. The plastic zone in the coal pillar presents the trapezoidal shape with a curved waist, which is different from traditional rectangular plastic zone. This also reveals the mechanism of the upper part of the coal pillar is more prone to become instability and present large deformations. Additionally, a numerical model is established based on the geological conditions of Lingdong coal mine, which is used to verify the theoretical model. The numerical simulation of the stress rotation trend is consistent with the theoretical analysis results. The numerical plastic zone in coal pillar presents a curved waist trapezoidal shape, which match well with the onsite deformation of coal pillar. To a certain extent, this verifies the rationality of the theoretical model.

Fault structures near underground coal mine return roadways frequently influence the deformation of surrounding rock, thereby constraining roadway stability. However, when the roof deformation is not clear in the case of a roadway through a normal fault, it will directly affect the establishment of a reasonable control program for the roof. Based on elastic-plasticity theory, this paper proposes a method for calculating the maximum deformation position of the roof plate in a roadway through a normal fault. This method accurately determines the maximum deformation position while considering the variation in widths between the upper and lower roof plates. Numerical simulations revealed the dual characteristics of asymmetric deformation in the roof plate and the asymmetric morphology of the plastic zone during the mining period. Furthermore, it delineated the significant impact of the width of the roof plate of the lower disc on roof deformation. A statistical analysis was conducted on the standard deviation (S) and coefficient of variation (CV) of both the theoretical and numerical simulation results, as well as the error rate between them. The analysis responded well to the reliability of the study results. The proposed reinforcement support program effectively guided the stability control of the roof plate in the roadway through the normal fault at the site. The research findings provide valuable insights for predicting and controlling roof deformation in roadways and tunnels under similar engineering conditions.HighlightsA theoretical calculation method of the maximum deformation location of the roof plate of the roadway through a normal fault is proposed.The asymmetric deformation characteristic of the roof plate of the roadway through the normal fault is revealed.Significant sensitivities of the surface deformation of the roof plate and the area of the plastic zone to the width of the roof plate of the lower disk are obtained.The feasibility of the theoretical predictive analysis of the location of maximum roof deformation in a roadway through a normal fault is determined.

A flexible polyurethane elastomer (PUE) is studied, and the improved impact-resistant performance is revealed. Compressive stress–strain curves over a wide loading rate range were derived. Under static loading, the rubbery-like characteristics are demonstrated, which are flexible and hyperelastic, to process a large strain of about 60% followed by full recovery upon unloading. Under high-rate loadingcompared with the mechanical data of polyurethane elastomer (PUE) and polyurea (PUA) materials in the literature. Orderly parallel deformation bands were formed from carrying a large strain. The fibrils were found between deformation bands for enhancing the yield/plateau stress. A considerable plastic zone ahead of propagating crack with numerous crazes and microcracks was produced for realizing the dynamic strain energy absorption. This work presents a scientific innovation for developing outstanding impact-resistant polyurethane elastomers for transparent protection engineering.

Coalbursts are one of the most serious disasters in coal mines. Exploring the critical conditions of coalbursts from a theoretical perspective is the basis for realizing early prediction and quantitative prevention of coalburst. Based on the principle of virtual displacement and disturbance response instability theory, we proposed the energy extreme point method for identifying the instability of coal–rock systems and determined the critical conditions (the length of the critical plastic zone and critical load) for coalburst. Based on actual engineering field data, the critical values of the above two indexes were calculated for 15 coalburst mines in China. Finally, we analyzed the effects of coal bursting liability, roadway section size, roof stiffness, coal–rock interlayer property, and support measures on the length of the critical plastic zone and critical load. The results suggest that the calculated length of the critical plastic zone and critical load can be used as early indicators of coalburst. In addition, the length of the critical plastic zone can be used to calculate the elastic deformation energy, allowing the magnitude of the coalburst to be predicted. The critical load can be used to obtain the critical mining depth and safety factor to facilitate mine safety evaluation. The length of the critical plastic zone and critical load obviously decrease with increasing coal bursting liability. As the width-to-height ratio of the roadway increases, the length of the critical plastic zone decreases, while the critical load increases. Increasing roof stiffness leads to increases in both the length of the critical plastic zone and critical load. Increasing the friction coefficient between the coal and rock layers causes the critical plastic zone to decrease and the critical load to increase. Finally, support measures can obviously increase the critical load.HighlightsThe energy extreme point method for identifying instability in coal-rock systems was proposed.An analytical model of coalburst in a rectangular roadway considering the mechanical behavior of the coal-rock interface was established.Critical conditions of 15 coalburst mines in China was calculated based on the energy extreme point method.Influencing factors of critical conditions of coalburst were analyzed.

Load spectrum enhancement is a pivotal accelerated fatigue testing methodology employed to substantially reduce test duration and associated costs. This technique operates by strategically elevating load amplitudes while ensuring the preservation of the original failure mechanism. In this study, a novel fatigue life prediction model for variable amplitude loading is developed by integrating the theories of Equivalent Initial Flaw Size (EIFS) and the Equivalent Plastic Zone (EPZ). This integrated approach explicitly accounts for both the small crack effect and load interaction effects, which are critical yet often oversimplified aspects of fatigue damage accumulation. The model is subsequently applied to quantitatively establish the relationship between the Load Enhancement Factor (LEF) and the test time or compression ratio. Finally, fatigue tests on typical 2A14 aluminum alloy structures under variable amplitude loading are conducted to validate the proposed model. The results demonstrate a significant life reduction with increasing LEF, achieving a remarkable test time reduction of over 50% at an LEF of 1.2. All experimental data fall within a scatter band of three, relative to the model prediction. Additionally, the predicted mean compression ratio exhibits approximate agreement with the experimental data, with errors within an acceptable range. This work provides a physically grounded and practically validated framework for implementing efficient and reliable load spectrum enhancement.

The size of plastic zone is an important parameter for determining the surrounding rock stability. In practical engineering, the initial rock environment is dominated by a nonuniform stress field, especially in areas with high tectonic stresses. To explore the plastic zone distribution of the tunnel surrounding rock under a nonuniform stress field, a new analytical solution for the plastic zone radius is proposed, and the reliability of the analytical results is verified with FLAC3D. Moreover, the influence of in situ stress and rock mass parameters on the plastic zone distribution is explored. Finally, the presented solution is verified with several existing engineering cases. The results show that under a nonuniform stress field, the cohesion, friction angle, and vertical stress greatly affect the plastic zone size; the lateral pressure coefficient dominates the plastic zone shape. The average error in determining the size of the plastic zone for the new analytical method is 14.6%, which is 36.1% and 49.9% more accurate than the point criterion method and the stress construction method, respectively. This work could provide a useful reference for predicting the plastic zone distribution and guiding tunnel engineering support with nonuniform stress fields.

To explore the problems of coal wall spalling and roof fall of a working face passing through a fault, the II 1044 working face with a large dip angle in the Taoyuan Coal Mine is taken as the engineering research background. FLAC3D is used to simulate the working face advancing towards the fault. The vertical stress distribution, plastic zone evolution, and fault slip displacement characteristics of the stope are studied when the working face is advanced to different positions, and safety measures are formulated accordingly. The research shows that the vertical stress and the stress concentration area in the lower part of the working face with a large dip angle are larger than those in the upper part of the working face. As the working face advances towards the fault, the slip amount of the silt rock is the largest, and the slip amount of the immediate roof is the smallest. When the working face is 40 ∼ 50 m away from the fault, the influence of mining on the fault slip characteristics changes from weak to strong. Therefore, it is decided to apply grouting 40 m from the fault in the working face, reducing the probability of coal wall spalling. Related monitoring data show that the support pressure of the working face is stable between 23 MPa and 35 MPa after grouting, which is far less than the support pressure without grouting at approximately 45 MPa. Therefore, the pressure grouting method is effective in this working face.

Based on the characteristics of real stress distribution in coal mining, the evolution law of plastic zone shape and spatial position were investigated. The stress model of roadway with non-orthogonal state of in-situ stress and mining-induced stress was constructed. The boundary equation of the roadway plastic zone under in-situ stress deflection was deduced. Additionally, the evolution mechanism of the roadway plastic zone was analysed. We found that the shape and spatial position of the roadway plastic zone are jointly determined by in-situ stress deflection angle α, mining-induced stress concentration coefficient K, and equivalent lateral pressure coefficient η′ when the mechanical parameters (rock cohesion c; internal friction angle φ) were fixed. Specifically, the plastic zone experiences the cyclic evolution phenomenon, where the plastic zone shape varies from petal–ellipse–circle–ellipse–petal shapes in morphology. In this study, we expound the general evolution law of roadway plastic zone under different geological, working conditions and mechanical parameters. The main controlling factors and mechanism of the shape and spatial position evolution of the plastic zone are revealed. A numerical model was established to study the evolution law of plastic zone during the change of corresponding parameters. The results show that the traditional method of predicting the shape of plastic zone by lateral pressure coefficient is inaccurate. The plastic zone distribution calculated using this method was compared with the roadway deformation measured in 1305 bottom drainage roadway of Hudi mine, and the comparison results were in good accordance. The results of this study have certain universality, and enrich the understanding of the roadway plastic zone. It can provide theoretical reference for roadway excavation and support design under different geological and in-situ stress conditions.HighlightsThe non-orthogonal failure behavior of the roadway with the change of complicated stress state is revealed.Plastic zone experiences the cyclic evolution phenomenon, where the plastic zone shape varies from petal-ellipse-circle-ellipse-petal shapes in morphology.Main controlling factors and mechanism of the shape and spatial position evolution of the plastic zone are summarized.The unsymmetrical failure mechanism of roadway under non-orthogonal stress state is revealed.