Shear Failure Characteristics Research Articles

Shear failure characteristics and stress-strain relationship of Moso bamboo parallel to the fibers

Bamboo is susceptible to longitudinal splitting, which is mainly driven by its shear properties parallel to the fibers. In this work, bowtie tests including 124 Moso bamboo internode and node specimens were conducted to investigate the shear behaviors of Moso bamboo in shear parallel to the fibers. Based on the test results, the difference in shear strength between internode and node specimens using the analysis of variance (ANOVA) was quantitatively evaluated. The distribution of shear strength was fitted using Normal, Log-normal, and Weibull distributions, and the corresponding standard value was calculated using the non-parametric method. The design value was obtained based on reliability analysis. Additionally, prediction formulas for shear strength were fitted using univariate and multiple regression analyses based on outer diameter, wall thickness, and density. Various shear stress-strain constitutive models were established and validated. The research results indicated that there is no statistically significant difference in shear strength between internode and node specimens with the value of 17.17 MPa and 17.48 MPa, respectively. The distribution of shear strength followed the Normal and Log-normal distributions, but not the Weibull distribution. The standard value and design value of shear strength were 14.91 and 9.47 MPa, respectively. The shear strength was found to be negatively correlated with the outer diameter and wall thickness, but positively correlated with the density. The recommended strength prediction formulas and stress-strain constitutive models match well with the test results. This study can serve as a reference for the improvement of bamboo standard and design basis of Moso bamboo.

Numerical simulation of mechanical properties and damage process of carbon nanotube concrete under triaxial compression and splitting conditions

To investigate the mechanical properties and failure processes of carbon nanotube concrete, a three-dimensional meso-scale finite element model of concrete was constructed. Using ABAQUS software, numerical simulations were conducted on concrete samples with different carbon nanotube concentrations under triaxial compression and splitting conditions. The results indicate that under splitting conditions, the failure of the specimens initially occurs along the loading direction from both ends, with few cracks and slow propagation. Subsequently, the cracks rapidly expand, penetrating the entire specimen, leading to the formation of a macro-fracture surface with a crack aligned with the loading direction. This failure mode manifests as a straight crack. At the same loading rate, the tensile strength of the specimens with different carbon nanotube concentrations increases to varying degrees during splitting failure, with a maximum increase of 45.10% to 4.15 MPa. nder triaxial compression conditions, the main failure mode of the specimens exhibits shear failure characteristics. As the confining pressure gradually increases, the failure angle of the specimens enlarges. Under low confining pressure, there are fewer wing-shaped tensile cracks at the shear failure surface of the specimens. As the confining pressure increases, irregular shear failures and multiple shear planes appear in the specimens, resulting in more complex failure modes. For specimens with the same carbon nanotube concentration, the triaxial compressive strength of carbon nanotube concrete specimens increases with increasing confining pressure. The triaxial compressive strengths of the specimens under confining pressures of 5 MPa, 10 MPa, 15 MPa, and 20 MPa are 90.48 MPa, 122.72 MPa, 137.22 MPa, and 172.65 MPa, with strength increases of 35.63%, 51.66%, and 90.81%, respectively.

Mechanical property testing and damage assessment of the regenerated rock mass with very weakly bonded cement

The regenerated rock mass is a bearing structure formed by natural compaction in a hollow area, and the investigation of its optimal consolidation material and consolidation parameters is the key to improving the supporting effect and bearing stability of roadways. The effects of consolidation materials and parameters on the stability of the regenerative rock mass were studied using laboratory tests, numerical simulations, and theoretical analyses. Acoustic emission was used to monitor the variation characteristics of energy and ringing count during the process of rock mass failure, and the bonding interface area of the extremely weak cementation regeneration structure was tested by electron microscope scanning. The results show that there is a quadratic function relationship between the water–cement ratio of different cementing materials and the bond strength of the recycled rock mass; the regenerative rock mass with superfine cement exhibited the highest compressive strength and the largest cumulative energy of acoustic emission. This shows that it has the strongest bearing capacity, the highest elastic performance, the most stable micro-fracture development, and the best cementation effect, followed by ordinary cement, gypsum, and laterite. The scanning test showed that the regenerated structure had more internal pores, a loose structure, and poor cementation. Three-dimensional scanning modeling of four representative broken rock blocks was carried out, and the simulation verified that the regenerated structure had macroscopic “X”-shaped shear failure characteristics. The numerical simulation also verified three forms of rupture in the regenerative structure detected by electron microscopy scanning. Exploring the mechanism of action of the regenerative rock mass in the goaf provides a certain reference value for the stability control of the regenerated rock mass roadway.

Experimental investigation on fractal mechanism and shear failure characteristics of cement mortar at different curing ages

In this paper, the NMR and direct shear tests were conducted successively on cement mortar specimens with different curing ages (7 days, 14 days and 28 days). The porosity, shear strength, shear stiffness and shear energy of cement mortar are measured and analyzed through interpreting the pore distribution patterns and their corresponding fractal dimensions. The cross-sections of shear surface were characterized into point cloud data via 3D laser scanning and quantified by three dimensionless parameters (the root mean square value of the first derivative, root mean square value and joint roughness coefficient). The grey relation analysis (GRA) method and principal component analysis (PCA) is employed to analyze the relationship between the pore structures (i.e., gel pores, transition pores, capillary pores and pores or cracks) and distribution and the fractal dimensions. The results indicate that the shear peak strength is linearly correlated with the joint roughness coefficient. Compared to the fractal characteristics of pore size (T2 spectra) and pore distribution (MRI), we demonstrated that there is a strong correlation between the peak shear strength and the fractal dimension of shear fracture surface, which suggests that the quantitative characterization of internal pores and shear strength variations in cement mortar specimens can be achieved through the numerical values of fractal dimensions.

Study on the shear mechanical response and failure characteristics of prefabricated double-cabin utility tunnel joints

Study on the shear mechanical response and failure characteristics of prefabricated double-cabin utility tunnel joints

Effect of steel fiber orientation on punching shear resistance of steel fiber reinforced cementitious composites round slabs

The slab-column structure is commonly used in civil engineering. At the slab-column joints, the load from the slab is transmitted to the column. In the extreme state, punching shear failure may occur at the joints, and the failure is brittle without warning. When the punching shear failure occurs at the joint of round slab and column, usually the cracking is a conical shape because the direction of tensile stress caused by punching shear is generally radial. By incorporating steel fibers into cementitious composites round slabs, the steel fibers can bridge the conical surface, and therefore enhance the punching shear resistance of round slabs. If the steel fibers in the round slab are dispersed in the direction conducive to the punching shear resistance, the steel fibers can take full effect of strengthening and toughening, thus improving the punching shear resistance of the slab. So, the purpose of this investigation is to improve the punching shear resistance of steel fiber reinforced cementitious composite slabs by maximizing the reinforcement of steel fibers on the composites. Firstly, a model of punching shear resistance of steel fiber reinforced cementitious composite round slabs was derived based on the Modified Compression Field Theory with consideration of the orientation of steel fibers. According to the model, the slab with radial distribution of steel fibers around the punching shear area shows the highest punching shear resistance. Secondly, the approach of radially aligning steel fibers in the slab around the punching area by magnetic driving is proposed using a self-developed magnetic device. And, radially dispersed steel fiber reinforced cementitious composites (JD-SFRC) round slabs were prepared. Thirdly, the punching shear resistance of JD-SFRC round slabs, as well as unidirectionally aligned (1D), two-dimensionally aligned (2D), and randomly distributed (RD) steel fiber reinforced cementitious composites round slabs were tested. At last, the effect of the orientation of steel fibers on the punching shear resistance and failure characteristics of the round slabs were analyzed by comparing the results from tests and the model. Meanwhile, the effect of the volume fraction of steel fibers on the punching shear resistance of the slab was examined. The results show that, When the volume fraction of steel fiber is 1.0 %, compared with RD-SFRC round slabs, the ultimate shear resistance of 1D-SFRC, 2D-SFRC and JD-SFRC round slabs is increased by 14 %, 15 %, and 24 %, respectively.

Failure Analysis and Ultimate Expansion Mechanical Behavior Analysis of Thin-Walled Solid Expandable Tubular

Summary With the vigorous exploitation of shale gas, the cost of drilling and production of shale gas wells, which can reduce the cost, has also received attention. Equal-diameter solid expandable tubular (SET) technology is one of the best choices at present. For this paper, we carried out a series of expansion experiments for 20G pipe, and the experimental results show that the pipe breaks and fails at 30% expansion ratio. The cracked pipe is analyzed by scanning electron microscopy (SEM), and the method to solve the failure of cracked pipe is proposed. In addition, a 3D dynamic model is established to study the relationship between the inner diameter, wall thickness, expansion ratio, and ultimate expansion ratio of the pipeline. The study results show that (1) SET made of 20G expanded successfully at 25% expansion ratio and failed at 30% expansion ratio, the fracture surface presents 45°, with typical shear failure characteristics; (2) SEM results showed that the pit characteristics were normal, the pipe was cracked under high stress, and the 20G pipe could not meet the expansion ratio of 30%; and (3) the maximum stress of 20G pipe during expansion is proportional to the expansion ratio. It is recommended that the SET expansion rate of 20G material should not exceed 26%. The research results will provide ideas and methods for the design of SET.

Study on the double-sided shear test and three-dimensional numerical simulation of anchor cable with C-shaped tube

Using an indoor pipe-cable-rod double-sided shear test system, tests for double-sided shear are carried out on three types of prestressed anchor cables and anchor cables with C-shaped tubes that eliminate the joint surface's resistance to movement. The shear failure characteristics and shear strength contribution of the anchor cable with C-shaped tube were compared and analyzed. For the anchor cable with C-shaped tube, the C-shaped tubes' geometrical specifications were altered and the effects of the C-shaped tubes' thickness and diameter on the anchor cables with C-shaped tubes' shear strength were investigated. The research results showed that the thickness of the C-shaped tube had a substantial impact on improving the ultimate shear capacity of the anchor cable with C-shaped tube. With the increase in the thickness of the C-shaped tube, the increment of the ultimate shear capacity showed an exponential growth trend. However, under the same thickness, the ultimate shear capacity had a negative correlation with the diameter of the C-shaped tube. In addition, the anchor cable with C-shaped tube was the subject of numerical simulation research using the three-dimensional numerical simulation program ABAQUS. The stress distribution and the development trend with increasing shear load were obtained. The simulation findings revealed that the numerical model and the outcomes of the laboratory tests were in good agreement, which might serve as a guide for future research.

Mechanical properties and energy damage evolution mechanism of fiber-reinforced cemented sulfur tailings backfill under uniaxial compression.

This paper studies mechanical properties and energy damage evolution of fiber-reinforced cemented sulfur tailings (CSTB) backfill. The effects of fiber length and fiber content on the stress, toughness and failure properties of the CSTB were systematically revealed. In addition, the energy index evolution law was studied, and the energy damage evolution mechanism of CSTB was revealed. The results show that the deformation failure of fiber-reinforced CSTB mainly goes through four stages: initial crack compaction, linear elastic deformation, yield failure and post-peak failure. The peak stress and residual stress of the CSTB firstly increase and then decrease with the increase of fiber content and the addition of fiber can promote the change from brittle failure to ductile failure of the CSTB. Adding appropriate amount of fiber can improve the toughness of CSTB, and the influence degree of fiber length on the toughness index of CSTB is 6mm>12mm>3mm. The total strain energy increases linearly along the variation of fiber content, while the elastic strain energy and dissipated energy increase exponentially at the peak stress point. In the process of CSTB deformation and failure, "gentle-linear growth-slow growth-rapid decline" is for elastic strain energy, while "gentle-slow growth-rapid growth-linear growth" is for dissipation energy. The damage and failure of CSTB mainly experienced four stages: initial damage, slow growth of damage, accelerated damage and damage failure, and the damage evolution curve also showed the changing characteristics of "gentle-slow growth-rapid growth-linear growth". The CSTB without added fiber showed obvious "Y-type" and "linear-type" shear failure characteristics and the phenomenon of shear cracks penetrating the backfill appeared. No big shear crack occur when it is damaged, showing that the fiber addition restrain the crack growth and improve the overall crack resistance of the CSTB. Hydration products are obviously distributed on the surface of the fiber, which indicates that the fiber will be evenly dispersed in the CSTB and form a certain bonding force with the cement-tailings matrix, thus improving the overall mechanical properties of the CSTB.

Shear bearing capacity calculation of low-rise SFRC shear wall with CFST columns based on simplified softened strut and tie model

In the paper, an innovative shear wall, which is referred to as steel fiber reinforced concrete (SFRC) shear wall with concrete filled steel tube (CFST) columns, is introduced, based on the high bearing capacity and large stiffness of concrete filled steel tube column (CFSTC) and good cracking resistance and strong toughness of steel fiber reinforced concrete. The loading mechanism of steel fiber reinforced concrete shear wall (SFRCSW) with concrete filled steel tube columns is analyzed, and shear bearing capacity calculation method. A simplified model of the steel fiber reinforced concrete shear wall web softening tension bar is proposed, concrete and distributed web reinforcement to the shear bearing capacity of steel fiber reinforced concrete shear wall web is identified. Furthermore, a new algorithm to obtain the shear bearing capacity of steel fiber reinforced concrete shear wall with concrete filled steel tube columns is established, and then it is validated by using the test results of steel fiber reinforced concrete shear wall with concrete filled steel tube columns under low-cycle repeated loading. The results showed that all tested shear wall specimens exhibited obvious shear failure characteristics and a typical diagonal cracking pattern after test. The steel fibers obviously improved the crack forms of the steel fiber reinforced concrete shear wall web and the seismic behavior of steel fiber reinforced concrete shear wall with concrete filled steel tube columns. In addition, the proposed calculation method is scientific and accurate to analyze and predict the shear bearing capacity of low-rise steel fiber reinforced concrete shear wall with concrete filled steel tube columns.

Research on Meso‐Damage Experiment and Simulation in the Anchorage Bonding Interface Between Rock Bolt and Rock Mass

Anchorage failure occurs mostly at the anchorage interface, and the mechanical mechanism of the anchorage interface is the key point and difficulty in the analysis and research of anchorage. In order to study the mechanics and failure mechanism of the anchorage interface, the shear failure characteristics of the anchorage interface and the deformation rule of the anchored rock under different rib spacings were analyzed through the indoor plane anchorage pull‐out test. Reasonable rib spacing can significantly improve the shear strength of the anchored solid. Finite element software ABAQUS was used to create an axisymmetric plane model of rock bolt pull‐out under different rib conditions to simulate the whole process of rock bolt pull‐out and anchorage interface failure. The results show that during the pull‐out process of anchorage rock mass, stress concentration zones occur at rib corners, and peak shear stress moves to the depths of the anchorage section. As the load increases, cracks emerge from the top of the rib and the edge of the rib angle of the rock bolt and propagate to the deep anchorage along the edge of the costal angle, resulting in damage at the rock mass interface. Load–displacement curves of different transverse rib shapes, heights, widths, and spacings indicate that rib changes affect the anchor‐bearing limit. The anchoring failure mode changes from rib wear and spreading slip, rib shear convex body propagation to interfacial crack propagation, and rib causes varying degrees of damage to the surrounding rock mass. Therefore, reasonable rib parameter improvements can enhance anchoring force performance.

A quantitative study on the characteristics of desiccation cracks and their effect on the shear failure characteristics of granite red soil

A quantitative study on the characteristics of desiccation cracks and their effect on the shear failure characteristics of granite red soil

Study on Filling Material Strength and Dam Failure Characteristics of Loess Dam

In the Loess Plateau region, loess, as a widely distributed building material, is often used as a filling material for dams. When the water level reaches a certain height, the body of a dam is prone to shear failure due to the penetration of water. The change in the shear performance of local loess filler can affect the overall strength of loess dams. Therefore, the filler of a loess dam is selected to study the shear performance. The progressive failure process of a loess dam is simulated. The shear failure characteristics of loess filler under the influence of water content, confining pressure, and dry density were explored. The characteristics of the shear failure of a loess dam were analyzed. The remolded loess is prone to shear expansion failure under low confining pressure, low water content, and high dry density, and is prone to shear shrinkage failure under high confining pressure, high water content, and low dry density. When the water content is constant, the cohesion increases with the increase in dry density. When the dry density is constant, the internal friction angle generally increases with the increase in water content. However, when the dry density is high, the permeability of the remolded loess is weakened, resulting in uneven water distribution under a low water content, which affects the test results. The failure process of the loess dam is a progressive shear failure, which is affected by the water level and water pressure, and is destroyed under the action of pore water pressure and water body lateral pressure.

ABAQUS Numerical Simulation Study on the Shear Instability of a Wellbore Induced by a Slip of the Natural Gas Hydrate Layer

To study the shear deformation and failure characteristics of a wellbore and the interaction mechanism with its surrounding rocks induced by a layer slip during natural gas hydrates (NGHs) extraction, this paper conducted a numerical simulation study of wellbore shear induced by a layer slip using ABAQUS software and carried out a laboratory experiment of wellbore shear to verify the accuracy of the numerical model. The results show that the shear force–displacement curves obtained from the laboratory experiments and numerical simulations are consistent with five stages, including the compaction stage, linear stage, plastic stage, strain-softening stage and residual stage. The wellbore shows a “Z”-shaped deformation characteristic after its shear breakage. The shear force of the wellbore is maximum at the shear surface, and it is distributed in an approximate “M” shape along the shear surface. The axial force of the wellbore is small and uniformly distributed in the initial stage of the shear. The wellbore bending moment is minimum at the shear surface, with a value of 0, and it is distributed in a skew–symmetric wave shape along the shear surface. During the shearing, the evolution of the wellbore axial force and shear force can be classified into the distribution pattern along the radial direction on the shear surface and the pattern along the axial direction. The combination of the wellbore axial force and shear force causes the tensile–shear compound failure of the wellbore. During shearing, the wellbore and rock body gradually enter the plastic state with the increase in the shear displacement. When the entire cross-section of the wellbore is in the plastic state, a “necking” phenomenon of the wellbore begins to appear. During the shearing, the frictional dissipation energy and plastic dissipation energy increase constantly. In addition, the elastic strain energy increases to a peak and then decreases to a certain value, which remains unchanged along with the work conducted by the shear force.

Experimental Study of the Shear Characteristics of Fault Filled with Different Types of Gouge in Underground Gas Storage

In the current international situation, energy storage is an important means for countries to stabilize their energy supply, of which underground storage of natural gas is an important part. Depleted gas reservoir type underground gas storage (UGS) has become the key type of gas storage to be built by virtue of safety and environmental protection and low cost. The multi-cycle high injection and production rate of natural gas in the depleted gas reservoir type UGS will cause the in-situ stress disturbance. The slip risk of fault in the geological system increases greatly compared with that before the construction of the storage engineering, which becomes a great threat to the sealing of the gas storage. Reasonable injection and production strategy depend on the reliable assessment of the shear behavior of the fault belt, which can guarantee the sealing characteristics of the UGS geological system and the efficient operation of the UGS. Therefore, the shear behavior of the fault is studied by carrying out experiments, which can provide important parameters for the evaluation of fault stability. However, there is a large gap between the rock samples used in the previous experimental study and the natural faults, and it is difficult to reflect the shear failure characteristics of natural faults. In this paper, similar fault models based on high-precision three-dimensional scanners and engraving machines, filled with three types of fault gouge, are prepared for a batch of representative direct shear tests. The results show that the peak shear strength of the fault rocks with a shear surface is higher than that of the fault rocks with a tensile surface. Compared with the clay mineral content, the roughness of the fault surface is much more significant for the shear strength of the fault rock. For the fault rocks with similar fault surface morphology, the higher the clay content in the fault gouge, the greater the shear strength of the fault rocks. For the fault rocks with different fault surface morphology and the same fault gouge, the cohesion and internal friction angle of the tensile type is generally smaller than that of the shear type.

Experimental study on strength and failure characteristics of sandstone rock mass with complex cataclastic structure using 3D printing models

A cataclastic rock mass is a poor type of engineering geological rock mass. The determination of the shear failure characteristics and shear strengths of cataclastic rock masses can provide key basis for the design and construction of infrastructure. Physical model samples of a sandstone cataclastic rock mass were first produced by a combination of three-dimensional (3D) printing technology and manual pouring. Shear tests were conducted with respect to the shear stresses parallel to the trace line plane and perpendicular to the trace line plane of the cataclastic rock mass model. Based on an extensive analysis of the shear failure characteristic, shear stress evolution characteristic curve and shear strength. When the shear stress was parallel to the trace line plane, and when the rock block that was cut and confined by the trace line exhibited a significant tip, the end stress concentration effect of the cataclastic rock mass was more significant during the shear process with the anisotropy of the rock block increased. In addition, the shapes of the rock blocks that were confined and cut by the joints were the main influencing factors of the strength of the cataclastic rock mass. When the shear stress was perpendicular to the trace line plane, the structure of the rock wall was the main influencing factor of the deformation and failure process of the shear failure plane and the shear strength. The physical and mechanical properties of the shear failure plane of the cataclastic rock mass were found to be closely related to the joint–rock wall system characteristics of the cataclastic rock mass. Therefore, when determining the shear strength of cataclastic rock mass, the shape and combination form of the rock block, shear direction, and structural failure characteristics of the rock wall should be comprehensively considered during the shear process.

Behavior and modeling of large-scale concrete-filled FRP tubes with longitudinal steel rebars under lateral impact loading

Fiber-reinforced polymer (FRP) has been utilized extensively as a confining material for concrete in civil engineering. Concrete-filled FRP tubes (CFFTs), consisting of a FRP tube, a concrete core and optional longitudinal rebars, are particularly attractive to be used as bridge piers in harsh environments. Most studies on CFFTs have been focused on their behaviors under static loading conditions. To extend existing studies, three identical large-scale CFFTs with longitudinal steel rebars (SR-CFFTs) were investigated experimentally, two of which were tested under lateral impact loading and one was under lateral static loading. To compare the confining behavior between FRP tube and steel stirrups, three identical large-scale reinforced concrete (RC) columns with approximately the same hoop confinement stiffness as SR-CFFTs (i.e., steel stirrups for RC columns, FRP tube for SR-CFFTs) were tested as companion specimens. The test results indicated that, (1) under lateral static loading, the RC column failed with a shear-failure manner, while the SR-CFFT specimen failed with combined bending and shear failure characteristics; (2) under lateral impact loading, the shear-dominant failure mechanism was evident for RC columns, while the failure mechanism of SR-CFFTs was flexural-dominant owing to the effective hoop confinement provided by FRP; (3) under the same impacting velocity, the SR-CFFT specimen had much less damage and much less lateral displacement than the corresponding RC column. The established FE models could provide reasonably accurate predictions for the dynamic behaviors, the impact force–time history curves and the column deformations for both RC columns and SR-CFFTs. Parameter studies were conducted to analyze the effects of the concrete strength, the longitudinal steel ratio, the impact velocity and the impact mass.

Study on Shear Behavior and Failure Characteristics of Bolted Anisotropic Rock Joints

Anisotropic discontinuity exists widely in rock masses of mines, tunnels, slopes, water conservancy and hydropower projects. The shear characteristics of bolted anisotropic rock joints are extremely important for the stability design of engineering rock mass. However, few scholars have studied the bolted anisotropic rock joint. The different rock properties on both sides of the rock joint, especially the different rock strengths, will greatly affect the deformation characteristics and failure mode of the rock mass. Based on this, a series of shear tests were carried out on the bolted anisotropic rock joint under different normal stresses, and the characteristics of shear stress-shear displacement curve, shear strength, failure characteristics of the rock joint and deformation characteristics of the bolt are discussed. λ is defined as the strength ratio of upper and lower rock on the structural surface. The results show that the effect of λ on the shear stress-shear displacement curve is not obvious at the pre-fracture stage. The shear stress-shear displacement curve at the pre-breaking stage of the bolt presents a softening stage when the normal stress is equal to 0.5 MPa, tends to be horizontal when the normal stress is equal to 1 MPa and presents a hardening stage when the normal stress is greater than 1 MPa. After the bolt is broken, the shear stress-shear displacement curve presents a stepped-down descent. With the increase in λ, the breaking shear stress of the bolt increases. Elliptic failure occurs on the surface of the bolted anisotropic rock joint, and the length of the major axis of the ellipse decreases with the increase in λ value and normal stress. The bolts with different λ values of anisotropic rock joint show "Z-shaped" tensile bending deformation characteristics after shear fracture, and the horizontal and vertical components of the bolt deformation decrease with the increase in λ value and normal stress. The fracture shear displacement of the bolt increases with the increase in normal stress and decreases with the increase in λ value. The research results are helpful to further understand the shear mechanical characteristics and differences of bolted rock joints and provide a reference for solving the engineering problems of the composite layered rock mass.

Analysis of Bottomhole Rock Stress in Deep-Well Drilling Considering Thermal-Hydro-Mechanical Coupling

Drilling is a key step in the exploitation of deep oil and gas resources. In order to clarify the stress state of the rocks and the mechanism of rock breakage in deep-well drilling, a thermal-hydro-mechanical coupling model for deep-well drilling was established, and the effects of drilling on the temperature, pressure, and stress in the formation were studied. Furthermore, the effects of the formation parameters and wellbore parameters on the bottomhole stress were analyzed. The results revealed that after the formation was drilled, the temperatures in different horizontal in situ stress directions were not significantly different, but the difference in the pore pressure between the maximum and minimum horizontal stress directions was large. The average effective stress at the bottom of the hole was the smallest, and in some areas, it was tensile stress. For deep-well drilling, as the formation pressure increased, the in situ stress increased, and the permeability decreased, leading to greater average effective stress of the bottomhole rock. As a result, it was harder to break the rock, and the drilling efficiency decreased. Reducing the wellbore pressure and wellbore temperature is conducive to forming tensile stress near the borehole axis in the bottomhole, causing tensile damage. The average effective stress of the formation near the shoulder of the drill bit was compressive stress, and it is advisable to take advantage of the rock shear failure characteristics to improve the drilling efficiency in this area. The results of this study can help us to understand the stress state of the bottomhole rocks and the mechanism of rock breakage and can provide a reference for the optimization of drilling tools and drilling parameters in deep-well drilling.

Experimental Study on the Dynamic Tensile Characteristics of Coal Samples.

As coal mines continue deep mining, the frequency of coal and rock dynamic disasters has also gradually increased. In this paper, dynamic tensile strength deformation, energy evolution, and crack development under an impact test were studied on Brazilian coal samples, using a split Hopkinson pressure bar (SHPB) test device. A high-speed camera was adopted to capture the failure process of the coal specimens. The research results demonstrate that when the impact velocity is greater than 4.75 m/s, the dynamic tensile strength of the vertical bedding direction is higher than that of the parallel bedding direction of the coal samples. With the increase in the impact velocity, the dynamic strain and ultimate strain rate of two types of coal samples are increased, and the average value of the first and second dynamic deformation moduli of coal samples shows decreasing characteristics. As the incident energy increases, the sum of reflected and transmitted energy increases, and the absorbed energy also increases in the two types of coal samples. The two types of Brazilian disc coal samples mainly showed tensile and shear failure characteristics. The dynamic tensile deformation characteristics of the two types of coal specimens are less affected by the impact angles. However, the crack propagation of coal samples was mainly influenced by the impact angles. The test results can be used for the prediction of coal and rock outburst in deep underground coal excavation.