Principal Stress Direction Research Articles

Crack Initiation and Propagation of Embedded Three-Dimensional Parallel Cracks in Transparent Rock-Like Material

Abstract In order to understand the initiation and propagation of three-dimensional (3D) embedded parallel cracks in brittle rock under uniaxial and biaxial compression, a preparation method of a transparent rock-like material was proposed in order to observe the propagation process of cracks in brittle rock. Uniaxial and biaxial compression tests were carried out on the transparent rock-like samples containing two embedded 3D parallel cracks. Four types of crack propagation modes were observed: wing crack, petaloid crack, flat-wing crack, and ring-like crack. It is also found that the direction of lateral pressure relative to the strike of the embedded crack has a significant effect on the propagation direction of new generated cracks: a lateral pressure parallel to the strike of the embedded cracks promotes crack propagation in the direction of the maximum principal stress; on the other hand, a lateral pressure normal to the strike of the embedded cracks promotes cracks propagation around the embedded crack at an angle of 30°–60°, forming ring-like cracks, which is similar to the zonal disintegration phenomenon monitored around deep underground excavations.

Laboratory characterization of induced seismicity

Subsurface fluid injection causes an increase in seismic activity near injection sites. To ensure public safety and improve public acceptance of subsurface fluid injection, it is important to understand the underlying mechanisms giving rise to these behaviors and characterize the factors affecting induced seismicity. The work reported here aims to better understand induced seismicity in faulted/fractured subsurface rock formations. A series of experiments are conducted in core flooding and triaxial configurations on test samples with a preexisting fracture/fault to study frictional slipping at small and large scales respectively. The geometries of both surfaces on either side of the fault are characterized using x-ray CT. The extent of fault gouging in a specimen after slipping is quantified by defining a cumulative gouging parameter using x-ray CT measurements before and after the experiment. Acoustic emission (AE) events emanating from the specimens in both configurations are monitored. The results demonstrate that (1) the orientation of fault relative to the major principal stress direction affects the slip characteristics, (2) frictional slipping at a fault can occur at different length scales, (3) with an increase in the scale of slipping, the spectral ratio of low-frequency bins increases while high-frequency bins reduces, and (4) locations on the fault surface that exhibit a sudden change in surface normal are most susceptible to gouging/damage during frictional slipping.

Orogenic Gold Deposit in Metamorphic Rocks: Minerals and Structural Control at Rarowatu Area, Southeastern Arm of Sulawesi, Indonesia

This research aims to examine the relationship between hydrothermal alteration and mineralization (ore mineralogy) in the study area and geological structures in the deformation mechanism. The hydrothermal alteration was determined based on petrographic analysis, and ore mineralogy which was determined based on the ore microscopic analysis. The deformation mechanism is determined by paleo stress analysis using win-tensor, and the direction of principal stress on joints/veins and faults is calculated by the right-dihedron method. Hydrothermal alteration includes silicification, argillic, propylitic, and phyllic alterations; and ore mineralogy consists of stibnite, cinnabar, pyrite, chalcopyrite, sphalerite, covellite, hematite, and arsenopyrite at metamorphic rocks. The geological structure developed in the study area consists of shear fractures and Wumbubangka right Normal-slip Fault, which is accompanied by Wumbubangka right-slip lag fault and Wumbubangka reverse-slip fault formed in a simple shear mechanism. Crenulation, micro-folds, and porphyroblastic are also recognized in thin sections. The Wumbubangka right normal-slip fault, considered a syn-mineralization structure, formed the transpressional and transtensional geologic structures, forming the quartz vein parallel to and crosscutting the foliation. The correlation between surface geological structures and microstructures indicates that tectonic regimes have controlled the alteration and gold mineralization in the study area.

Multiscale structural characterizations of anisotropic natural granite residual soil

While the anisotropy of sedimentary soil—particularly the underlying role of soil structure—is well understood, similar knowledge about granite residual soil formed by weathering is scarce. In particular, the evolution of soil structure during the hollow cylinder torsional shear tests (among the most appropriate for studying soil strength anisotropy) remains largely unknown. This study systematically investigates the multiscale structural properties of granite residual soil using stereo microscopy, scanning electron microscopy, energy-dispersive spectrometry, and computed tomography. Furthermore, the structural evolution during hollow cylinder torsional shear tests is traced. Results indicate the strength anisotropy of residual soil is associated with horizontal fissures which cause low shear strength when the soil is sheared with a principal stress direction of 45°. The cementation via Fe is characterized by a uniform distribution. Different evolutionary patterns are observed for soils within the shear band, inside the necking area, and in other regions. It is not always the case that particles are rearranged with their long axis perpendicular to the major principal stress. This study improves the understanding of natural residual soil structure and provides some insights into its anisotropic behavior.

Numerical Study on the Hydraulic Fracturing Pattern in the Hard Roof in Response to Mining-Induced Stress

A hard roof can cause serious issues corelated with the stability of the panel including large deformation of the roadway in the gob, rock burst, coal and gas outbursts, etc. Currently, hydraulic fracturing has ever-increasingly been used to help control these above-mentioned issues in many engineering cases. This paper presents a series of numerical simulations for hydraulic fracturing with a recently developed model to examine the weakening effect of this pre-conditioning measure on a hard roof. The results show that large deflections in the principal stress direction occur above the coal seam after mining, which can be progressively enhanced as the working face continuously advances. This further could significantly affect the hydraulic fracturing pattern and result in arc-shaped fracture propagation, especially for the hydraulic fracture in stress-descending areas. The obtained results suggest that the hydraulic fracturing operation in a hard roof is preferable in the areas close to the middle of the gob where the created fractures would be deflected more. At last, sensitivity analysis shows that geological conditions have great influence on the hydraulic fracturing pattern. Among the factors analyzed in this paper, the difference between the maximum and minimum stress has the largest influence and should be fully considered. This study could provide a theoretical basis for the practical hydraulic fracturing operation in a hard roof.

The Spatio-Temporal Evolution of Rock Failure Due to Blasting under High Stress

The research aims to investigate the failure characteristics of rock caused by blasting under high stress, explore the energy generation and its transfer and release in rock under the effects of blasting. Physical experiments and numerical simulations were performed. The results showed that, as with AE time-series data, the attenuation time of the AE activity increases with the number of blasting events, thereby decreasing the overall stability of the samples. In terms of AE spatial evolution, different initial stress fields play a role in directional guidance in initiation, propagation, and coalescence of blasting-induced cracks. The direction of propagation of microcracks is consistent with the direction of the maximum principal stress. The blasting-induced disturbance in a high-stress state accelerates the extension and propagation of microcracks and is accompanied by the occurrence of numerous high-energy AE events. Numerical simulation showed that the maximum principal stress exhibits a guiding effect on the propagation of blasting-induced cracks and the pattern development of the damage zone, which is consistent with the conclusion of physical experiments. The research provides a theoretical guidance for designing and optimising the blasting parameters of deep rock.

Confinement enhanced damage-plasticity model for concrete

The purpose of this paper is to develop a confinement enhanced damage-plasticity model for concrete that can characterize the improvement of strength and ductility under high confining stress. The influence mechanisms of confinement in damage and plasticity sub-space are discussed separately. In the damage subspace, a confinement influence function considering the difference between uniaxial and multiaxial damage evolution is suggested based on the strain energy equivalence confinement. A confinement enhanced damage evolution model is established, which can capture the strength increase under high confining stress. In the plasticity subspace, a confinement enhanced phenomenological plasticity model, which includes equation of state of the spherical space, is provided to reproduce the high ductility at the principal stress direction under lateral confining stress. The simulated strength and stress–strain curves under active confinement agree well with the experimental results. Furthermore, the predicted three-dimensional ultimate stress points are consistent with the classic strength surface, validating good performance of the proposed model.

Development of an FBG–MEMS-Based 3-D Principal Stress Monitoring Device in Soil

The stress state is the basis for evaluating soil deformation and strength. To study the 3-D stress state of the soil, a 3-D stress device based on fiber Bragg grating (FBG) and a microelectromechanical system (MEMS) was designed to measure the 3-D stress state, principal stress, and direction of principal stress. First, the FBG 1-D earth pressure cell is developed and calibrated. Based on the calculation relationship between the normal stress and the 3-D stress state, and considering the device rotation, the 3-D stress device was developed and verified in the large-scale triaxial test. The results show that the range of the 1-D earth pressure cell is 1 MPa, and the linearity is within 2.5% FS. The device can effectively measure soil’s 3-D stress, principal stress, and direction. The measured results are consistent with the reference and theoretical results. The device provides a new method for effectively measuring 3-D stress in soil.

Shear wave splitting analysis of local earthquakes from dense arrays in Shimian, Sichuan

The Shimian area of Sichuan sits at the junction of the Bayan Har block, Sichuan-Yunnan rhombic block, and Yangtze block, where several faults intersect. This region features intense tectonic activity and frequent earthquakes. In this study, we used local seismic waveform data recorded using dense arrays deployed in the Shimian area to obtain the shear wave splitting parameters at 55 seismic stations and thereby determine the crustal anisotropic characteristics of the region. We then analyzed the crustal stress pattern and tectonic setting and explored their relationship in the study area. Although some stations returned a polarization direction of NNW-SSE, a dominant polarization direction of NW-SE was obtained for the fast shear wave at most seismic stations in the study area. The polarization directions of the fast shear wave were highly consistent throughout the study area. This orientation was in accordance with the direction of the regional principal compressive stress and parallel to the trend of the Xianshuihe and Daliangshan faults. The distribution of crustal anisotropy in this area was affected by the regional tectonic stress field and the fault structures. The mean delay time between fast and slow shear waves was 3.83 ms/km, slightly greater than the values obtained in other regions of Sichuan. This indicates that the crustal media in our study area had a high anisotropic strength and also reveals the influence of tectonic complexity resulting from the intersection of multiple faults on the strength of seismic anisotropy.

Experiment study on seismic behavior of squat UHPC shear walls subjected to tension-shear combined cyclic load

Five squat UHPC shear walls with high axial tension ratio ranging from 0.5 to 0.6 were tested under tension-shear combined cyclic load to replicate seismic performance of bottom shear walls in high-rise buildings. All specimens exhibited shear-sliding failure mode characterized by multiple shear-related cracks distributed in a dense array and full-length localization of horizontal crack along the end of anchorage rebars. The seismic behaviors were comprehensively evaluated by hysteretic responses, energy dissipation capacity, strength and stiffness degradation. Additionally, detailed analysis on the strain development of vertical reinforcement and web horizontal rebars were carried out to study their shear resistance contribution. It was found that, with the assistance of steel fibers providing an efficient shear resistance consistent with the varying direction of principal tensile stress, the shear capacity of UHPC shear wall was slightly improved even when axial tension ratio increased to 0.6. The ultimate drift capacity of UHPC specimens reached 3.3% drift with an excellent strength retention ability of more than 500kN in this test. Besides, the accumulated energy consumption of UHPC specimen after crack localization improved by 38% compared with the normal concrete shear wall. Results of strain analysis demonstrated that vertical reinforcement provided an effective clamping force in the shear-sliding resistance. A modified design formula considering efficient tension resistance offered by UHPC was proposed with adequate safety concerns and accuracy.

A multi-fracture balanced extension control method for horizontal wells in anisotropic shale

Shale has anisotropy in elastic parameters and permeability coefficient in parallel and perpendicular directions to the bedding plane. In the study of fracture propagation during hydraulic fracturing, it is necessary to consider the influence of shale anisotropy. Through the development of fluid pipe element, fluid pipe connector element, cohesive damage element, and rock matrix element with pore pressure, a fluid–structure coupling numerical model with comprehensive consideration of dynamic flow distribution, dynamic fracture propagation and extension, and stress interference are established. The anisotropic characteristic parameters of shale reservoirs are given to study the balanced propagation control method of multi-fracture in horizontal shale Wells. The results show that under anisotropic conditions, hydraulic fractures are easy to expand in the direction parallel to the bedding plane but not in the direction perpendicular to the bedding plane. Finally, It forms a long extension in the length direction and a short extension in the height direction. The stress interference along the direction of the minimum horizontal principal stress is gradually intense with the increase of the degree of anisotropy. During multi-fracture propagation in horizontal shale wells, the stress interference generated in the direction of minimum horizontal principal stress can be balanced by effectively controlling the perforation friction so as to promote the balanced extension of multiple fractures. At the same time, with the increase of injection flow and the decrease of the distance between clusters, the stress interference degree gradually becomes serious. Therefore, it is necessary to strictly design how to control the perforation friction in the multi-fracture fracturing process of shale horizontal Wells with large flow rates and small cluster distances. The research conclusions provide a theoretical reference for the efficient development of shale petroleum resources.

Failure Mechanism and Stability Control of Soft Roof in Advance Support Section of Mining Face

There is a great risk of roof falls in the advance support section of the mining face (ASSoMF), and it is difficult to control the roof. Based on the soft roof of Lijiahao coal mine, this paper studies the stress distribution of the ASSoMF and the space-time evolution of the surrounding rock plastic zone by using theoretical analysis and numerical simulation, and reveals its failure mechanism. Based on the control effect of support resistance on plastic zone, it is proposed that the advance support should mainly adapt to the roadway deformation. Advance equipment without repeated support for mechanized movement has been developed, and the support timing analysis and strength check have been carried out. Results show that the roadway at ASSoMF is in a non-uniform stress field, the confining pressure ratio reaches 1.5~7, and the surrounding rock forms asymmetric failure; the principal stress direction deflects, the angle between it and the vertical direction is about 10°~25°, and the plastic zone of the surrounding rock also rotates to the roadway roof. The proposed equipment can adapt to the characteristics of an unsymmetrical large deformation of a soft roof, and can effectively bear the roof load and maintain the stability of the roadway.

Well trajectory optimization of ultradeep and high-pressure drilling engineering based on high in situ stress as the main control factor: A case study from the Ordovician carbonate reservoir in the Shunbei area of the Tarim Basin

With increasing oil and gas exploration and development, well trajectory optimization has gradually become the focus of the oil and gas industry. Considering the wellbore instability in the Shunbei area of the Tarim Basin, the well trajectory was scientifically optimized under the guidance of rock mechanics, drilling engineering, and mathematical methods, combined with actual geologic data, and with in situ stress as the main controlling factor. In this paper, the stress state of the wellbore is analyzed by linear elastic theory to establish the stress distribution model of the wellbore. The safety window model of wellbore stability is established using different rock failure criteria to calculate the collapse pressure and fracture pressure of the formation. Based on this, the safe mud density window is defined to achieve wellbore trajectory optimization. Finally, the influence factors of wellbore stability are discussed, and the applicability of different rock failure criteria is evaluated. The results indicate that under the normal faulting stress regime condition in the study area, the direction of horizontal minimum principal stress is the best drilling direction, where the borehole inclination angle of [Formula: see text] > 50° is the optimal well trajectory. The wellbore stabilities of high-angle deviated wells and horizontal wells are better than those of low-angle deviated wells and vertical wells. The calculation results of the Mogi-Coulomb criterion can describe the conditions of the in situ stress field more accurately. The safe windows for different well trajectories are obtained directly by the numerical method, which is very practical for optimizing well trajectories and improving wellbore stability.

Load-Oriented Nonplanar Additive Manufacturing Method for Optimized Continuous Carbon Fiber Parts.

The process of the additive manufacturing (AM) of carbon-fiber-reinforced polymer (CFRP) parts based on the process of fused deposition modeling (FDM) has seen considerable research in recent years, which amplifies the importance of adapted slicing and pathplanning methods. In particular, load-oriented techniques are of high interest when employing carbon fiber materials, as classical methods, such as tape-laying and laminating, struggle with highly curved and complex geometries and require the costly production of molds. While there have been some promising propositions in this field, most have restricted themselves to a planar slicing approach, which severely limits the ability to place the fibers along stress paths. In this paper, a nonplanar slicing approach is presented that utilizes principal stress directions to construct optimized nonplanar constituting layers on which pathplanning can be carried out. These layers are oriented such that the effect of the weak interlayer adhesion is minimized. Support material is adaptively generated to enable the use of arbitrary part geometry. Furthermore, a continuous pathplanning method and post-processor are applied to yield manufacturing instructions. The approach is verified for its viability of application through experimental investigation on a multi-axis robotic 3D printer. This constitutes an important step in allowing the fabrication of CFRP parts to further utilize the possibilities of additive manufacturing.

Fracture Parameters Optimization and Field Application in Low-Permeability Sandstone Reservoirs under Fracturing Flooding Conditions

To solve engineering problems in the production process after fracturing and flooding of low-permeability sandstone reservoirs, such as rapid water-cut rise and low water flooding efficiency, a method for optimizing the fracture parameters of low-permeability sandstone reservoirs under fracturing flooding conditions was proposed. A rock property test experiment was first carried out, the fracturing coefficient was defined, and an evaluation method for the brittleness index of low-permeability sandstone was established to optimize the perforation location of the fracturing reservoir. A productivity numerical model for the two-phase flow of oil–water in matrix–fracture media was established to optimize the fracture morphology under fracturing flooding conditions. The results showed that the quartz content, Young’s modulus, and peak stress mainly affected the fracturing coefficient of rock and are the key indicators for evaluating the brittleness of low-permeability sandstone reservoirs. For production wells in the direction of minimum horizontal principal stress, the swept area of water flooding should be expanded, fracture length should be optimized to 90 m, and fracture conductivity should be 20 D·cm. For fracturing production wells in the direction of maximum horizontal principal stress, the advancing speed of the water injection front should be slowed down to reduce the risk of water channeling in injection-production wells. The optimized fracture length was 80 m, and the fracture conductivity was 25 D·cm. The application of these findings can markedly improve oil production and provide a reference for optimizing the fracture parameters of low-permeability sandstone reservoirs under fracturing flooding conditions.

Molecular simulation of the plastic deformation and crack formation in single grit grinding of 4H-SiC single crystal

Silicon carbide (SiC) is a promising semiconductor material for high-performance power electronics devices, but difficult to machine. The development of cost-effective machining technology for SiC wafers requires a complete understanding of the interaction between diamond grit and workpiece material. This work systematically investigated the deformation and crack formation of monocrystalline 4H-SiC involved in single grit grinding using molecular dynamics (MD) simulation. The mechanism for crack initiation and propagation in monocrystalline 4H-SiC was revealed for the first time. Our simulation results showed that the subsurface damage at the earlier stage of plastic deformation was caused by amorphization and dislocation initiation. With the progress of penetration, slip bands were formed. The extension of slip bands in the later stage of plastic deformation was the cause of crack initiation. Stress analysis demonstrated that the tensile stresses parallel to the scratch direction and perpendicular to the scratch plane were the driving force for the crack initiation ahead of a diamond abrasive grit, while the tensile stress perpendicular to the scratch plane was responsible for the crack initiation underneath the grit. The propagation of the two cracks was driven by the maximum principal stress at the crack tip, and the propagation direction depends on the direction of the maximum principal stress.

2D discrete element simulation on the marine natural gas hydrate reservoir stimulation by splitting grouting

Reservoir stimulation offers the possibility to solve the problems of low single well production, small mining range, and short stabilized production in NGH field trials. Compared with traditional hydraulic fracturing, split grouting technology has the advantages of both enhancing the gas-water seepage capacity of the reservoir and maintaining the stability of the reservoir. In this study, a 2D discrete element method was used to establish a fine-scale model for NGH reservoir and numerical tests were conducted on split grouting. The pattern, direction and range of the grouting expansion were analyzed under different conditions of hydrate saturation, injection pressure, grout viscosity, injection well size, in-situ stress, and the number of injection wells. The results show that the grouting process can be roughly divided into two stages: squeeze grouting and split grouting. Squeeze grouting occurs in the early stages, and grout expansion is limited in the wellbore area. Subsequently, split grouting occurs always along the direction of the maximum principal stress when injection pressure is high enough, creating dendritic grout veins inside the reservoir and enhancing the connectivity inside the reservoir. Larger grouting range is achieved under lower in-situ stress, lower hydrate saturation or with lower viscous grout. The split grouting capacity can be enhanced with the increase of wellbore size or multiple wells. The results could help to deeply understand the grouting pattern and veins morphologies in natural gas hydrate reservoirs during split grouting and construct potential reservoir stimulation strategies.

Influence of 3D printing on compressor impeller fatigue crack propagation life

Taking a compressor impeller with a pressure ratio less than 3 as the research object, the influence of 3D printing on the fatigue crack propagation life of the compressor impeller is analyzed under the combined action of 3D printing residual stress and external load. The impeller is printed by selective laser melting (SLM) technology. Finite Element Method (FEM) is used to simulate the 3D printing process of the impeller, and the residual stress, residual deformation and other mechanical quantities are comprehensively analyzed. The numerical simulation results are in good agreement with the crack location in 3D printing experiment. The heat treatment and numerical simulation of the printed impeller significantly reduce the residual stress. According to the experimental results and the magnitude and direction of the first principal stress, the position and orientation of the macroscopic crack are determined. The FEM of linear elastic fracture mechanics (LEFM) and the Forman-Newman-de Koning (FNK) model are used to give the propagation law and fatigue crack propagation life of the cracked compressor impeller under different working conditions. The results show that the crack initiation position is affected by the maximum residual stress of the printed impeller after heat treatment. When only the external load is applied, the stress intensity factor (SIF) of the initial crack front varies with the rotational speed as a quadratic power. When the external load and 3D printing residual stress act at the same time, the law will no longer be satisfied. The general trend of the crack propagation path is along the radial direction. The speed has a linear influence on the critical crack size and a nonlinear influence on the fatigue crack propagation life. The research work in this paper can provide a basis for damage tolerance analysis of 3D printing compressor impeller, and also provide a reference for maintenance work.

Behavior of Saturated Remolded Loess Subjected to Coupled Change of the Magnitude and Direction of Principal Stress

Behavior of Saturated Remolded Loess Subjected to Coupled Change of the Magnitude and Direction of Principal Stress

A testing system to understand rock fracturing processes induced by different dynamic disturbances under true triaxial compression

In this context, a testing system to understand rock fracturing processes induced by different dynamic disturbances under true triaxial compression was developed. The system is mainly composed of a static loading subsystem, a dynamic loading subsystem, a specimen box subsystem, and a data measurement subsystem. The static loading subsystem uses low stiffness loss frame structure technology, which greatly improves the frame stiffness in the three principal stress directions (up to 20 GN/m) and ensures the demand of the disturbance experiment in both the prepeak and postpeak stages. The disturbance loads with frequency of 0–20 Hz and stress level of 0–30 MPa were applied using large flow parallel oil source technology characterized with high heat dissipation efficiency. For the disturbance loads with frequency of 100–500 Hz and stress level of 0–30 MPa, they were realized by using high-frequency and centimeter-per-second-scale low-speed disturbance rod technology. Three rigid self-stabilizing specimen boxes were utilized to provide support for the specimen and deformation sensors, ensuring the stability and accuracy of the data obtained. To verify the performance of the true triaxial test system, disturbance experiments were conducted on granite specimens. The results show that the experimental device satisfies the requirements of original design, with an excellent repeatability and reliable testing results.