Modeling time-dependent mechanical behavior of hard rock considering excavation-induced damage and complex 3D stress states

To investigate the long-term stability of deep rocks,a three-dimensional(3D)time-dependent model that accounts for excavation-induced damage and complex stress state is developed.This model comprises three main components:a 3D viscoplastic isotropic constitutive relation that considers excavation damage and complex stress state,a quantitative relationship between critical irreversible deformation and complex stress state,and evolution characteristics of strength parameters.The proposed model is implemented in a self-developed numerical code,i.e.CASRock.The reliability of the model is validated through experiments.It is indicated that the time-dependent fracturing potential index(xTFPI)at a given time during the attenuation creep stage shows a negative correlation with the extent of excavationinduced damage.The time-dependent fracturing process of rock demonstrates a distinct interval effect of the intermediate principal stress,thereby highlighting the 3D stress-dependent characteristic of the model.Finally,the influence of excavation-induced damage and intermediate principal stress on the time-dependent fracturing characteristics of the surrounding rocks around the tunnel is discussed.

Apparent stress as an indicator of stress meta-instability:The 2021 M_(S)6.4 Yangbi earthquake in Yunnan,China

Investigating spatiotemporal changes in crustal stress associated with major earthquakes has implications for understanding seismogenic processes.However,in individual earthquake cases,the characteristics of the stress after it reaches its maximum value are rarely discussed.In this study,we use the 2021 M_S6.4 Yangbi earthquake in Yunnan,China and events of magnitudes M_L≥3.0 occurred in the surrounding area in the previous 11 years to investigate the spatiotemporal evolution of apparent stress.The results indicate that apparent stress began to increase in January 2015 and reached a maximum in January 2020.Apparent stress then remained at a high level until October 2020,after which it declined considerable.We suggest that the stress was in the accumulation stage from January 2015 to January 2020,and entered the meta-instability stage after October 2020.During the meta-instability stage,the zone of decreasing stress expanded continuously and the apparent stress increased around the Yangbi earthquake source region.These features are generally consistent with the results of laboratory rock stress experiments.We propose that apparent stress can be a good indicator for determining whether the stress at a specific location has entered the meta-instability stage and may become the epicenter of an impending strong earthquake.

Interaction between in situ stress states and tectonic faults:A comment

Understanding the in situ stress state is crucial in many engineering problems and earth science research.The present article presents new insights into the interaction mechanism between the stress state and faults.In situ stresses can be influenced by various factors,one of the most important being the existence of faults.A fault could significantly affect the value and direction of the stress components.Reorientation and magnitude changes in stresses exist adjacent to faults and stress jumps/discontinuities across the fault.By contrast,the change in the stress state may lead to the transformation of faulting type and potential fault reactivation.Qualitative fault reactivation assessment using characteristic parameters under the current stress environment provides a method to assess the slip tendency of faults.The correlation between in situ stresses and fault properties enhances the ability to predict the fault slip tendency via stress measurements,which can be used to further refine the assessment of the fault reactivation risk.In the future,stress measurements at greater depths and long-term continuous real-time stress monitoring near/on key parts of faults will be essential.In addition,much attention needs to be paid to distinguishing the genetic mechanisms of abnormal stress states and the type and scale of stress variations and exploring the mechanisms of pre-faulting anomaly and fault reactivation.

Effects of stress state on texture and microstructure in cold drawing-bulging of CP-Ti sheet

Three different stress states of the combination of tensile（t） stress and compressive（c） stress,t t,t c and t c c,exist in the deformed commercially pure titanium（CP-Ti） sheet during cold drawing-bulging.The textures and microstructures in the different stress state regions were investigated by means of XRD and TEM analysis.Similar development of texture and microstructure is achieved with less thickness strain under multiaxial stresses in drawing-bulging than in cold rolling.The results show that texture and microstructure are much sensitive to multiaxial stresses.Twinning is more easily activated under compressive stress than tensile stress.Prism a slip is heavily affected by tensile stress,resulting in a remarkable change of the intensity of（0°,35°,0°） texture,while pyramidal c＋a slip,forming（20°,35°,30°） texture,weakens with the increase of thickness strain in spite of stress state.

Degradation of P-MOSFETs Under Off-State Stress

The hot carrier effects under off- state stress m ode( Vgs=0 ,Vds<0 ) have been investigated on9nm P- MOSFETs with channel length varying from1.0 2 5 μm to0 .5 2 5 μm.Both on- and off- state currents are discussed. It is found that the off- state leakage current decreases after a higher voltage stressing,which is induced by the charge injection occurred close to the drain junction.However,the leakage current increases after a lower voltage stressing because of the newly generated interface traps.It is also found that the on state saturation current and threshold voltage degrade significantly with the stress tim e,which we believe is due to the charges injected near the gate- drain overlapping region and/ or the stress- induced interface trap generation.The degradation of Idsatcan be ex- pressed as a function of the product of the gate current( Ig) and the num ber of charges injected into the gate oxide ( Qinj) in a simple power law.Finally,a lifetime prediction model based on the degradation of Idsatis proposed.

Tectonic Stress State Changes Before and After the Wenchuan M_s 8.0 Earthquake in the Eastern Margin of the Tibetan Plateau

Crustal tectonic activities are essentially the consequences of the accumulation and release of in situ stress. Therefore, studying the stress state near active faults is important for understanding crustal dynamics and earthquake occurrences. In this paper, using in situ stress measurement results obtained by hydraulic fracturing in the vicinity of the Longmenshan fault zone before and after the Wenchuan Ms 8.0 earthquake and finite element modeling, the variation of stress state before and after the Wenchuan M. 8.0 earthquake is investigated. The results show that the shear stress, which is proportional to the difference between principal stresses, increases with depth and distance from the active fault in the calm period or after the earthquakes, and tends to approach to the regional stress level outside the zone influenced by the fault. This distribution appears to gradually reverse with time and the change of fault properties such as frictional strength. With an increase in friction coefficient, low stress areas are reduced and areas with increased stress accumulation are more obvious near the fault. In sections of the fault with high frictional strengths, in situ stress clearly increases in the fault. Stress accumulates more rapidly in the fault zone relative to the surrounding areas, eventually leading to a stress field that peaks at the fault zone. Such a reversal in the stress field between the fault zone and surrounding areas in the magnitude of the stress field is a potential indicator for the occurrence of strong earthquakes.

Effect of varying normal stress on stability and dynamic motion of a spring-slider system with rate- and state-dependent friction

Incorporating rate and state friction laws, stability of linearly stable （i.e., with stiffness greater than the critical value） spring-slider systems subjected to triggering perturbations was analyzed under variable normal stress condition, and comparison was made between our results and that of fixed normal stress cases revealed in previous studies. For systems associated with the slip law, the critical mag- nitude of rate steps for triggering unstable slips are found to have a similar pattern to the fixed normal stress case, and the critical velocity steps scale with a/（b - a） when k = kcr for both cases. The rate-step boundaries for the variable normal stress cases are revealed to be lower than the fixed normal stress case by 7 %-16 % for a relatively large ct = 0.56 with （b - a）/a ranging from 0.25 to 1, indicating easier triggering under the variable normal stress condition with rate steps. The difference between fixed and variable normal stress cases decreases when the α value is smaller. In the same slip- law-type systems, critical displacements to trigger instability are revealed to be little affected by the variable normal stress condition. When k 〉 kcr（V,）, a spring-slider system with the slowness law is much more stable than with the slip law,suggesting that the slowness law fits experimental data better when a single state variable is adopted. In stick-slip motions, the variable normal stress case has larger stress drops than the constant normal stress case. The variable normal stress has little effect on the range of slip velocity in systems associated with the slowness law, whereas systems associated with the slip law have a slowest slip velocity immensely smaller than the fixed normal stress case, by ~ 10 orders of magnitude.

Effect of stress state on deformation and fracture of nanocrystalline copper:Molecular dynamics simulation

Deformation in a microcomponent is often constrained by surrounding joined material making the component under mixed loading and multiple stress states. In this study, molecular dynamics （MD） simulation are conducted to probe the effect of stress states on the deformation and fracture of nanocrystalline Cu. Tensile strain is applied on a Cu single crystal, bicrystal and polycrystal respectively, under two different tension boundary conditions. Simulations are first conducted on the bicrystal and polycrystal models without lattice imperfection. The results reveal that, compared with the performance of simulation models under free boundary condition, the transverse stress caused by the constrained boundary condition leads to a much higher tensile stress and can severely limit the plastic deformation, which in return promotes cleavage fracture in the model. Simulations are then performed on Cu single crystal and polycrystal with an initial crack. Under constrained boundary condition, the crack tip propagates rapidly in the single crystal in a cleavage manner while the crack becomes blunting and extends along the grain boundaries in the polycrystal. Under free boundary condition, massive dislocation activities dominate the deformation mechanisms and the crack plays a little role in both single crystals and polycrystals.

2-D elastic FEM simulation on stress state in the deep part of a subducted slab

Based upon some simplified numerical models, a 2-D plain strain elastic FEM program is compiled to study the distributions of the stress fields produced by the volume change of the phase transformation from olivine to spinel, by the volume change from temperature variation, and by density difference and boundary action in a piece of subducted slab located in transition zone of the mantle. Thermal stress could explain the fault plane solutions of deep focus earthquakes, but could not explain the distribution of deep seismicity. When large extent metastable olivine is included, the stress field produced by the density difference contradicts with the results of fault plane solutions and with the distribution of deep seismicity. Although the stress produced by volume change of the phase transformation from olivine to spinel dominates the stress state, its main direction is different from the observed results. We conclude that the deep seismicity could not be simply explained by elastic simulation.

Inversion of breakout data from inclined boreholes for stress state of the upper crust in Jizhong depression

The forward and inverse problems of studying crustal stress state from breakout data of inclined boreholes are concisely stated. direction of the maximum horizontal principal stress (compressive) and relative magnitudes of the horizontal stresses to the vertical stress in the upper crust in two regions of the Jizhong depression, the North of China, are obtained by analyzing the breakout data of 6 inclined wells. To get stable results in the analysis wesearched for the unknown parameters both forwardly and inversely. The results show that the maximum azimuths of the horizontal Principal compressive stresses in the central and southern part of the Jizhong depressionare N86°E and N77°E, respeCtively, while the relative magnitudes of the three principal stresses in the uppercrust (about 1000-4000 m) of the depression are variable. In the centra; part of the Jizhong depression we havefound SH : Sv: SK= 1. 38: 1. 00: 0. 57, where SH, SV and Sh are the maximum horizontal, vertical and minimum horizontal stress, resistively. This indicates that the present stress regime in this area is of strike-slipfaulting type. In the southern part of the depreSSion we have obtained SH: Sv: Sh=0. 80: 1. 00 1 0. 62, indicating a normal faulting stress regime in the shallow Part Of the crust.

Effect of hydrogen content and stress state on room-temperature mechanical properties of Ti-6Al-4V alloy

This work aims to investigate the effects of hydrogen content(in the range of 0%-0.5%,mass fraction)and stress state (tension and compression)on the room-temperature mechanical properties of Ti-6Al-4V alloy through mechanical properties tests. The effects of hydrogen content on microstructure evolution of Ti-6Al-4V alloy is also examined by optical microscopy,X-ray diffractometry,transmission electron microscopy and scanning electron microscopy.The results show that hydrogen content and stress state have important effects on the room-temperature mechanical properties of Ti-6Al-4V alloy.Tensile strength and ultimate elongation decrease with increasing the hydrogen content,while compressive strength and ultimate reduction are improved after hydrogenation.The reason is that the intergranular deformation dominates at the state of tension.Hydrogen atoms in solid solution and hydrides at grain boundaries increase with increasing the hydrogen content and they can promote the initiation and propagation of cracks along grain boundaries.While the intragranular deformation dominates at the state of compression.The plastic beta phase and hydrides increase with increasing the hydrogen content and they improve the ultimate reduction and compressive strength.

Experimental study of seepage characteristics of single rock fracture based on stress states and stress history

Through seepage tests under different loading and unloading confining pressures and different hydraulic gradients,the authors studied the effects of stress states and stress history on fracture permeability evolution for single granite fracture and sandstone fracture. The results show that there exists a linear relationship between the seepage discharge and osmotic pressure in sandstone fissure under each level of confining pressure. With the increasing in the confining pressure,the permeability of the fracture decreases,but the decreasing rate is changeing. During the unloading process,the fracture seepage velocity cannot be fully recovered to the size of the loading process. Therefore,in the unloading process of the confining pressure,the recovery of fracture permeability shows obvious hysteresis effects. The flow rate of the fracture remains unchanged during five cycles of loading and unloading processes of the confining pressure. In each cycle,the evolution character of the flow rate with the confining pressure remains unchanged. These experiments show that the seepage characteristics of sandstone and granite fractures are not the same under the same stress state.

Hydromechanical behaviors of andesite under different stress states during fluid injection

Water reinjection into the formation is an indispensable operation in many energy engineering practices.This operation involves a complex hydromechanical(HM)coupling process and sometimes even causes unpredictable disasters,such as induced seismicity.It is acknowledged that the relative magnitude and direction of the principal stresses significantly influence the HM behaviors of rocks during injection.However,due to the limitations of current testing techniques,it is still difficult to comprehensively conduct laboratory injection tests under various stress conditions,such as in triaxial extension stress states.To this end,a numerical study of HM changes in rocks during injection under different stress states is conducted.In this model,the saturated rock is first loaded to the target stress state under drainage conditions,and then the stress state is maintained and water is injected from the top to simulate the formation injection operation.Particular attention is given to the difference in HM changes under triaxial compression and extension stresses.This includes the differences in the pore pressure propagation,mean effective stress,volumetric strain,and stress-induced permeability.The numerical results demonstrate that the differential stress will significantly affect the HM behaviors of rocks,but the degree of influence is different under the two triaxial stress states.The HM changes caused by the triaxial compression stress states are generally greater than those of extension,but the differences decrease with increasing differential stress,indicating that the increase in the differential stress will weaken the impact of the stress state on the HM response.In addition,the shear failure potential of fracture planes with various inclination angles is analyzed and summarized under different stress states.It is recommended that engineers could design suitable injection schemes according to different tectonic stress fields versus fault occurrence to reduce the risk of injection-induced seismicity.

The stress state of geological structure and mining dynamic disaster in Fuxin basin

Further evidences show that most mining dynamic disasters are mainly oc- curred nearby NNE and near SN geological structures.In-situ stress measurement in Fuxin basin shows that the orientation of major compressed stress is near EW.At this stress field,geological structures with deferent strike have deferent stress state and dis- place mode.NNE and near SN geological structures are compressed to thrust and come into being high stress zone.NWW and NEE geological structures are tensile to separate and not prone to being low stress zone.NW structure is intervenient of them.So NEE and near SN structures are easy to occurre mining dynamic disasters and NWW and NEE structures is 'safety' comparatively.The mining dynamic disaster is controlled by stress state of geologic structure,which is determined by its strike.

Effect of intermediate principal stress on strength of soft rock under complex stress states

A series of numerical simulations of conventional and true triaxial tests for soft rock materials using the three-dimensional finite difference code FLAC3D were presented. A hexahedral element and a strain hardening/softening constitutive model based on the unified strength theory(UST) were used to simulate both the consolidated-undrained(CU) triaxial and the consolidated-drained(CD) true triaxial tests. Based on the results of the true triaxial tests simulation, the effect of the intermediate principal stress on the strength of soft rock was investigated. Finally, an example of an axial compression test for a hard rock pillar with a soft rock interlayer was analyzed using the two-dimensional finite difference code FLAC. The CD true triaxial test simulations for diatomaceous soft rock suggest the peak and residual strengths increase by 30% when the effect of the intermediate principal stress is taken into account. The axial compression for a rock pillar indicated the peak and residual strengths increase six-fold when the soft rock interlayer approached the vertical and the effect of the intermediate principal stress is taken into account.

THE DEPENDENCE OF WIND STRESS ON SEA STATE

Based on up to date literature, this paper details the evolution of wave dependence of wind stress.Some typical models of the dependence of wind stress on waves are described in detail. Although there isno universally accepted theory and model, recent studies indicate that the wind strees strongly dependson the development state of sea waves, i. e., young seas are rougher than mature seas, in other words, thewind stress decreases with increasing wave age.

Mutual impact of true triaxial stress, borehole orientation and bedding inclination on laboratory hydraulic fracturing of Lushan shale

Unconventional resources like shale gas has been the focus of intense research and development for two decades. Apart from intrinsic geologic factors that control the gas shale productivity (e.g. organic matter content, bedding planes, natural fractures, porosity and stress regime among others), external factors like wellbore orientation and stimulation design play a role. In this study, we present a series of true triaxial hydraulic fracturing experiments conducted on Lushan shale to investigate the interplay of internal factors (bedding, natural fractures and in situ stress) and external factors (wellbore orientation) on the growth process of fracture networks in cubic specimens of 200 mm in length. We observe relatively low breakdown pressure and fracture propagation pressure as the wellbore orientation and/or the maximum in situ stress is subparallel to the shale bedding plane. The wellbore orientation has a more prominent effect on the breakdown pressure, but its effect is tapered with increasing angle of bedding inclination. The shale breakdown is followed by an abrupt response in sample displacement, which reflects the stimulated fracture volume. Based on fluid tracer analysis, the morphology of hydraulic fractures (HF) is divided into four categories. Among the categories, activation of bedding planes (bedding failure, BF) and natural fractures (NF) significantly increase bifurcation and fractured areas. Under the same stress regime, a horizontal wellbore is more favorable to enhance the complexity of hydraulic fracture networks. This is attributed to the relatively large surface area in contact with the bedding plane for the horizontal borehole compared to the case with a vertical wellbore. These findings provide important references for hydraulic fracturing design in shale reservoirs.

Effect of Poisson’s ratio on stress state in the Wenchuan M_S8.0 earthquake fault

The Wenchuan Ms8.0 earthquake occurred on the Longmenshan fault which inclines at a dip angle exceeding 60 degrees. Since most thrust earthquakes occur on faults with dip angles of about 30 degrees, it is enigmatic why the Wenchuan earthquake occurred on such a steep fault. In this study we use a simple finite element model to investigate how the stress state in the fault changes with the variation of Poisson＇s ratio. The results show that, with the Poisson＇s ratio in the fault increasing, the magnitudes of the principal stresses increase and the maximum Shear stress decrease, and, especially, the angle between the maximum principal stress and the fault plane decreases, which will enhance the driving force to overcome the frictional resistance on the fault. The increase of Poisson＇s ratio in the fault may be an important factor to affect the occurrence of the fault earthquakes with large angles between maximum principal stress and fault plane.

DETERMINATION OF STATES OF STRESS AND STRAIN AT POLE OF HYDRAULIC BULGE ORTHOTROPIC SHEET SAMPLE

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2-D viscoelastic FEM simulation on stress state in the deep part of a subducted slab

The characteristics of the stress fields in deep subducting slabs are studied using viscoelastic plain strain finite element method. When introducing the new rheology structure given by Karato, et al into our computation, there emerge two regions with great shear stress just below the olivine-spinel phase transition zone, which encompass the low viscosity zone below the lower tip of the metastable wedge. Further, the directions of the main compressional stress of these two regions are all along the dip direction of the slab. These are in accordance with the seismic observations that there are two deep seismic zones in a slab and the directions of the main compressional stress in these two seismic zones are along the dip direction of the slab. Smaller effective viscosity probably caused by smaller grain size in the phase transformation zone does not have great influence on the stress state in the deep part of the slab. There is the maximum of shear stress at the transition region from olivine to spinel and the direction of the main compressional stress in this region is roughly perpendicular to the trend of the phase boundary no matter whether there exists metastable wedge, which nevertheless do not correspond to some well-known seismic observations.