Shear Stress Release Research Articles

Dislocation Analysis of 3C-SiC Nanoindentation with Different Crystal Plane Groups Based on Molecular Dynamics Simulation

To explore the deformation law of nanoindentation dislocations of different crystal plane groups of 3C-SiC by cube indenter. The molecular dynamics simulation method is used to construct the different crystal plane family models of 3C-SiC, select the ensemble, set the potential function, optimize the crystal structure, and relax the indentation process. The radial distribution function, shear strain, and dislocation deformation of nanoindentation on (001), (110), and (111) planes were analyzed, respectively. In the radial distribution function, the change in g r in the (110) crystal plane is the most obvious. Shear strain and dislocation occur easily at the boundary of square indentation defects. During the indentation process, the shear strain is enhanced along the atomic bond arrangement structure, (001) crystal plane shear strain is mainly concentrated around and below the indentation defects and produce a large number of cross dislocations, (110) the crystal plane shear strain is mainly concentrated in the shear strain chain extending around and below the indentation defect, which mainly produces horizontal dislocations, and (111) the crystal plane shear strain is mainly concentrated in four weeks extending on the left and right sides in the direction below the indentation defect and produces horizontal and vertical dislocations. The direction of shear stress release is related to the crystal structure. The crystal structure affects the direction of atomic slip, resulting in the results of sliding in different directions. The final dislocation rings are different, resulting in different indentation results.

Calculation Model for Stick-Slip Deformation in Weak Horizontal Structural Surface Formation after Water Inflow.

In the process of reservoir exploration, the weak horizontal structural surface easily slips and cracks, and casing shearing often occurs during the formation process, which significantly affects the economic viability and effective development of an oilfield. Additionally, repeated damage at the same casing position indicates the possibility of numerous cracks in the weak structural surface. In this study, we propose the geomechanical stick–slip theory to verify the above phenomenon. The stress calculation method for the weak horizontal structural surface in the upper part of the reservoir is devised under the influence of inter-regional pore pressure differences. Based on the process of accumulation–release–reaccumulation–rerelease in the formation and deformation processes, we construct the calculation model of shear stress release and slips on the cracked surface. While considering the influence of water inflow on the cracked surface, the pressure exerted on the formation stick–slip is analyzed. Then, the model was verified by the data of block X in the Daqing oilfield. The results demonstrate that under wet conditions, the maximum static and dynamic frictional stresses on the cracked surface decrease significantly, and this makes the cracked surface more prone to a greater slip degree. After the weak horizontal structural surface cracks and slips occur for the first time, the pressure difference between regions required for formation of the next slip decreases significantly. With the continuous formation of slips, the slip range gradually expands with an increase in inter-regional pressure variance. The research work in this study provides a theoretical basis for the prevention and control of casing damage in oil development zones.

Geological and Geomorphological Evidence for Activity along the Motuo Fault, Eastern Side of the Namche Barwa Syntaxis, Tibetan Plateau

Abstract The Motuo fault (MTF) strikes along the Yarlung Zangbo suture zone on the eastern boundary of the Namche Barwa syntaxis. The movement pattern and Quaternary activity of the MTF remain unclear, which hampers efforts to undertake meaningful seismic hazard assessments near the southeastern part of the Tibetan plateau and to understand the tectonic evolution of the Namche Barwa syntaxis. In this study, the MTF is shown to feature left-lateral strike-slip movements with offset gullies and mountain ridges and appears to have ruptured during the late Pleistocene to Holocene, as evidenced from geological, paleoseismic, and radiocarbon dating investigations. Specifically, at least three surface-rupturing paleoseismic events are revealed; two events occurred after 2606 B.P. and after 18.2 ka. Combining this information with previous Global Positioning System results in southeastern Tibet, we suggest that, as a boundary fault, the MTF regulates the movements of the Namche Barwa and Chayu blocks. The velocity difference between the two blocks advancing to the north is the main mechanism of left-lateral strike-slip motion along the MTF. The accumulation and release of shear stress between the two blocks have led to strong activity along the MTF, since the late Quaternary.

A method to experimentally investigate injection-induced activation of fractures

Natural fractures are generally well developed in most hydrocarbon and geothermal reservoirs, which can produce complex fracture networks due to the activation of fractures during hydraulic stimulation. The present paper is devoted to developing a method to investigate the activation characteristics of fracture under injection-shearing coupled condition at laboratory scale. The fluid is injected into the single-fractured granite until the fracture is activated based on the triaxial direct shear tests. The results show that injection process can significantly influence the shear stress distribution field, resulting in release of shear stress and relative slip between the opposite sides of the fractured surface. The injection-induced activation of fracture is strongly dependent on the stress states. When the normal stress increases, the injection-induced activation pressure increases, and the comparatively high normal stress can restrain the fracture activation. The fracture deformation mechanisms during fluid injection are also discussed preliminarily with the experimental data. The sensitivity of shear stress to fluid injection increases with increase of shear stress level, while it decreases under high normal stress. The results can facilitate our understanding of the natural fracture activation behavior during fluid pressure stimulation.

Stress-induced structural phase transition of 3C–SiC with TLK structure in a nano-abrading process

TLK (Terrace-Ledge-Kink) structure such as surface steps is often found on the surface of SiC materials. Removal behavior of SiC substrates with TLK structure has not been studied in a fixed abrasive polishing under nano-abrasion conditions. In this paper, the effect of TLK structure on the abrasion temperature, stress, and structural transition of SiC substrates during the nano-abrading process is pursued by molecular dynamics (MD) simulation. The results show that the average stress and temperature decrease with the increase of ledge numbers in the TLK structure. It is found that crystalline-to-amorphous (C–A) phase transition occurs. The high-density amorphorization of SiC is induced by a high shear stress, which transforms to low-density amorphous SiC with the release of shear stress. Furthermore, unlike previous reports, the pressure-induced 6-fold coordinated structure in 3C–SiC in this study, however, does not possess a high-pressured phase transition (HPPT) to rocksalt structure; instead, a disordered structure is found. The ductile domain removal goes through two stages: first through the dislocation nucleation and propagation at the initial abrading stage and is then followed by the plastic flow of the amorphous atoms after sliding over the ledge. The presence of TLK structures benefits the plastic deformation of the SiC substrate.

Tensile overpressure compartments on low-angle thrust faults

Hydrothermal extension veins form by hydraulic fracturing under triaxial stress (principal compressive stresses, σ1 > σ2 > σ3) when the pore-fluid pressure, Pf, exceeds the least compressive stress by the rock’s tensile strength. Such veins form perpendicular to σ3, their incremental precipitation from hydrothermal fluid often reflected in ‘crack-seal’ textures, demonstrating that the tensile overpressure state, σ3′ = (σ3 − Pf) < 0, was repeatedly met. Systematic arrays of extension veins develop locally in both sub-metamorphic and metamorphic assemblages defining tensile overpressure compartments where at some time Pf > σ3. In compressional regimes (σv = σ3), subhorizontal extension veins may develop over vertical intervals <1 km or so below low-permeability sealing horizons with tensile strengths 10 < To < 20 MPa. This is borne out by natural vein arrays. For a low-angle thrust, the vertical interval where the tensile overpressure state obtains may continue down-dip over distances of several kilometres in some instances. The overpressure condition for hydraulic fracturing is comparable to that needed for frictional reshear of a thrust fault lying close to the maximum compression, σ1. Under these circumstances, especially where the shear zone material has varying competence (tensile strength), affecting the failure mode, dilatant fault–fracture mesh structures may develop throughout a tabular rock volume. Evidence for the existence of fault–fracture meshes around low-angle thrusts comes from exhumed ancient structures and from active structures. In the case of megathrust ruptures along subduction interfaces, force balance analyses, lack of evidence for shear heating, and evidence of total shear stress release during earthquakes suggest the interfaces are extremely weak (τ < 40 MPa), consistent with weakening by near-lithostatically overpressured fluids. Portions of the subduction interface, especially towards the down-dip termination of the seismogenic megathrust, are prone to episodes of slow-slip, non-volcanic tremor, low-frequency earthquakes, very-low-frequency earthquakes, etc., attributable to the activation of tabular fault–fracture meshes at low σ3′ around the thrust interface. Containment of near-lithostatic overpressures in such settings is precarious, fluid loss curtailing mesh activity.Graphical abstract.

Interacting inclined strike-slip faults in a layered medium

Two inclined, interacting, strike-slip faults, both buried, situated in a viscoelastic layer, resting on and in welded contact with a viscoelastic half space, representing the lithosphere-asthenosphere system, is considered. Solutions are obtained for the displacements, stresses and strains, using a technique involving the use of Green’s functions and integral transforms, for three possible cases - the case when no fault is slipping, the case when one fault is slipping and the other is locked and the case when both the faults are slipping. The effect of sudden movement across one fault on the shear stress near the fault itself and near the other faults has been investigated. Some situations are identified where a sudden movement across one fault results in the release of shear stress near the other fault, reducing the possibility of seismic movements across it. Other situations are also identified where a sudden movement across one fault increases the possibility of seismic fault movements. A detail study may lead to an estimation of the time span between two consecutive seismic events near the mid points of the faults. It is expected that such studies may be useful in understanding the mechanism of earthquake processes and may be identified as an earthquake precursor.

Morphological changes in polycrystalline Fe after compression and release

Despite a number of large-scale molecular dynamics simulations of shock compressed iron, the morphological properties of simulated recovered samples are still unexplored. Key questions remain open in this area, including the role of dislocation motion and deformation twinning in shear stress release. In this study, we present simulations of homogeneous uniaxial compression and recovery of large polycrystalline iron samples. Our results reveal significant recovery of the body-centered cubic grains with some deformation twinning driven by shear stress, in agreement with experimental results by Wang et al. [Sci. Rep. 3, 1086 (2013)]. The twin fraction agrees reasonably well with a semi-analytical model which assumes a critical shear stress for twinning. On reloading, twins disappear and the material reaches a very low strength value.

Mechanical differences of sickle cell trait (SCT) and normal red blood cells.

Sickle cell trait (SCT) is a condition in which an individual inherits one sickle hemoglobin gene (HbS) and one normal beta hemoglobin gene (HbA). It has been hypothesized that under extreme physical stress, the compromised mechanical properties of the red blood cells (RBCs) may be the underlying mechanism of clinical complications of sickle cell trait individuals. However, whether sickle cell trait (SCT) should be treated as physiologically normal remains controversial. In this work, the mechanical properties (i.e., shear modulus and viscosity) of individual RBCs were quantified using a microsystem capable of precisely controlling the oxygen level of RBCs' microenvironment. Individual RBCs were deformed under shear stress. After the release of shear stress, the dynamic cell recovery process was captured and analyzed to extract the mechanical properties of single RBCs. The results demonstrate that RBCs from sickle cell trait individuals are inherently stiffer and more viscous than normal RBCs from healthy donors, but oxygen level variations do not alter their mechanical properties or morphology.

Stress rotations and the long-term weakness of the Median Tectonic Line and the Rokko-Awaji Segment

We used a field analysis of rock deformation microstructures and mesostructures to reconstruct the long-term orientation of stresses around two major active fault systems in Japan, the Median Tectonic Line and the Rokko-Awaji Segment. Our study reveals that the dextral slip of the two fault systems, active since the Plio-Quaternary, was preceded by fault normal extension in the Miocene and sinistral wrenching in the Paleogene. The two fault systems deviated the regional stress field at the kilometer scale in their vicinity during each of the three tectonic regimes. The largest deviation, found in the Plio-Quaternary, is a more fault normal rotation of the maximum horizontal stress to an angle of 79° with the fault strands, suggesting an extremely low shear stress on the Median Tectonic Line and the Rokko-Awaji Segment. Possible causes of this long-term stress perturbation include a nearly total release of shear stress during earthquakes, a low static friction coefficient, or lowelastic properties of the fault zones comparedwith the country rock. Independently of the preferred interpretation, the nearly fault normal orientation of the direction of maximum compression suggests that the mechanical properties of the fault zones are inadequate for the buildup of a pore fluid pressure sufficiently elevated to activate slip. The long-term weakness of the Median Tectonic Line and the Rokko-Awaji Segment may reside in low-friction/low-elasticity materials or dynamic weakening rather than in preearthquake fluid overpressures.

Formation mechanism of a microscale domain and effect on transport properties in strainedVO2thin films onTiO2(001)

We investigated film thickness dependence of domain size and transport property in ${\mathrm{VO}}_{2}$ thin films on rutile ${\mathrm{TiO}}_{2}$ (001) substrates and identified formation mechanism of the microscaled domain. It was found that domain size decreased with increasing film thickness and the domain boundary consisted of cracks and dislocations, clarified by high-resolution transmission electron microscopy. The detailed images showed, the tensile-strained ${\mathrm{VO}}_{2}$ lattices received by ${\mathrm{TiO}}_{2}$ (001) were partially relaxed around the cracks and dislocations. The relaxed lattice is likely to return the original metal-insulator transition temperature of 340 K, whereas the tensile-strained lattice has the transition at 300 K in a ${\mathrm{VO}}_{2}/{\mathrm{TiO}}_{2}$ (001) system. Thus, the mixed states of strained and relaxed crystal lattice and the increase in dislocation density in thicker films cause the overly broad resistance behavior against temperature. Furthermore, the origin of the dislocations and the thickness dependence of the domain size could be explained by the energy release of shear stress generated by competition between the pinning layers at near-interface ${\mathrm{VO}}_{2}$ layers holding the tetragonal structure and the near-surface layers separated from the substrate attempting the lattice transformation to a monoclinic structure. This understanding enables us to more precisely design the size and configuration of these domains and their transport properties.

Molecular dynamics simulations of shock-induced deformation twinning of a body-centered-cubic metal

Despite a number of previous nonequilibrium molecular dynamics (MD) studies into plasticity in face-centered-cubic metals, and phase transitions in body-centered-cubic (bcc) metals, the plastic response to rapid compression of bcc metals remains largely unexplored. Key questions remain as to the relative importance of dislocation motion and twinning in shear stress release and consequent strength. We present here large scale MD simulations of shock compressed bcc metal, using an extended Finnis-Sinclair potential for tantalum, and demonstrate the presence of significant deformation twinning for pressures above the Hugoniot elastic limit for shock waves propagating along the $[001]$ direction. The twinned variants are separately identified by a per atom order parameter, allowing the strain and stress states of the rotated material to be studied. The atomic motion during twinning, and thus its mechanism, for this potential, is identified by use of a three-dimensional pair-correlation function.

Abstract 220: Shear Stress Induces the Expression of Connexin-43 in Human Coronary Smooth Muscle

Coronary collaterals normally do not accommodate blood flow unless an upstream occlusion diverts blood flow through them. This blood flow provides an initiating stimulus, shear stress on the vessel wall, to increase collateral vessel diameter and wall thickness. This process, termed arteriogenesis, requires chemoattraction of monocytes into the vessel wall and vascular smooth muscle proliferation. Therefore, communication of the shear signal from the endothelium to the underlying smooth muscle is necessary. Previously, we demonstrated that endothelial cells exposed to shear stress release hydrogen peroxide which signals the underlying smooth muscle to express the arteriogenic factor, placenta growth factor (PLGF). Additional signaling pathways are involved in arteriogenesis although the mechanisms of communication across the vessel wall and regulation are still largely unknown. Connexins (Cx) are transmembrane proteins that form hemichannels between cells which enable direct exchange of signaling molecules. Vascular connexins have been shown to modulate blood flow, angiogenesis, and are correlated with atherosclerosis; however, very little is known of their role in arteriogenesis, particularly in human coronary vasculature. In this study, human coronary artery endothelial cells (CAEC) were co-cultured on membranes with coronary artery smooth muscle cells (CASMC) to mimic the vessel wall structure. We hypothesized that co-cultures exposed to high shear stress would exhibit an altered expression pattern of vascular connexins relative to static and normal shear stress. We show that 2 h of high shear stress at the endothelial surface significantly increases (3 fold) the expression of Cx43 in coronary smooth muscle while Cx37 and Cx40 are undetectable under static and shear conditions. Shear stress did not drive a significant change in the expression of Cx37, Cx40, or Cx43 in coronary endothelial cells compared to static conditions. Taken together with our previously published work, the data suggest that expression of Cx43 by human coronary smooth muscle may play a role in shear signaling pathways for arteriogenesis.

Post-transcriptional regulation of placenta growth factor mRNA by hydrogen peroxide

In tissues containing pre-existing collateral vessels, occlusion of an upstream supply artery results in diversion of blood flow through these vessels, protecting the distal tissue from ischemia. The sudden rise in blood flow through collateral vessels exerts shear stress upon the vessel wall, thereby providing the initial stimulus for arteriogenesis. Arteriogenesis, the structural expansion of collateral circulation, involves smooth muscle cell (SMC) proliferation which leads to increased vessel diameter and wall thickness. Since shear is sensed at the level of endothelial cells (EC), communication from EC to the underlying SMC must occur as part of this process. We previously reported that endothelial cells (EC) exposed to shear stress release hydrogen peroxide (H2O2), and that H2O2 can signal vascular SMC to increase gene and protein expression of placenta growth factor (PLGF), a known mediator of arteriogenesis. The purpose of the current study was to further elucidate the mechanism whereby PLGF is regulated by H2O2. We found that a single, physiological dose of H2O2 increases PLGF mRNA half-life, but has no effect on PLGF promoter activity, in human coronary artery SMC (CASMC). We further demonstrated that the H2O2‐induced increase in PLGF mRNA levels partially relies on p38 MAPK, JNK and ERK1/2 pathways. Finally, we showed that chronic exposure to pathological levels of H2O2 further increases PLGF mRNA levels, but does not result in a corresponding increase in PLGF secreted protein. These data suggest that PLGF regulation has an important translational component. To our knowledge, this is the first study to characterize post-transcriptional regulation of PLGF mRNA by H2O2 in vascular SMC. These findings provide new insights into the regulation of this important growth factor and increase our understanding of PLGF-driven arteriogenesis.

Low shear stress release procoagulant endothelial microparticles by a RhoK and ERK1/2-MAPK-dependent mechanism

Low shear stress release procoagulant endothelial microparticles by a RhoK and ERK1/2-MAPK-dependent mechanism

Non-coherent interfaces in diffuse interface models

We propose a description of incoherent and semi-coherent interfaces within the framework of diffuse interface modelling: accounting for the discontinuity of tangential displacements is an issue addressed by introducing a new field, called the incoherency field, located in the diffuse interface and able to release the shear stresses tangential to the interface. After having demonstrated the ability of the model to recover trends obtained by sharp interface models of sliding interfaces, the process of loss of coherency is investigated by assuming a local process which would result from the competition between the shear stress release at the interface and the interfacial energy increase.

Shear Stress Induces Synthetic‐to‐contractile Phenotypic Change of Smooth Muscle Cells via Paracrine Effect of Prostacyclin from Endothelial Cells and the PPAR‐α/δ Pathways

Endothelial cells (ECs) exposed to shear stress may exert paracrine effects to modulate the phenotype of smooth muscle cells (SMCs). We investigated the role of shear stress on ECs in SMC phenotypic modulation, using our EC/SMC co‐culture system in which these 2 types of cells were separated by a porous membrane. Application of shear stress (12 dynes/cm2) to EC/SMC induced SMC phenotypic change to a contractile state. This phenotypic change of SMCs was also observed by treating SMCs with media collected from ECs. For SMCs transfected with ligand‐binding domains of peroxisome proliferator activated receptor (PPAR)‐α, −δ, or −γ, treatment with media from ECs induced the ligand binding activities of these PPARs, suggesting that ECs exposed to shear stress release ligands of these PPARs. The shear‐induced SM22 expression in SMCs was inhibited by treating SMCs with antagonists of PPAR‐α (MK886) and −δ (sulindac sulfide), suggesting that PPAR‐α/δ pathways play a role in SMC phenotypic change. Shear stress to ECs induced their release of prostacyclin, which contributes to phenotypic modulation of SMCs. Our findings provide new insights into the role of shear stress on ECs in phenotypic modulation of SMCs.

Investigation of temporal variations in stress orientations before and after four major earthquakes in California

Orientations of the principal stresses before and after four major earthquakes in the greater San Francisco Bay Area were determined by inversions of 34 suites of focal mechanisms of about 1500 small earthquakes recorded by the Northern California Seismic Network over three decades. Stress orientations are expected to rotate due to the release of shear stress in a major earthquake. The degree of rotation can place some constraints on the ambient level of stress in the crust surrounding the mainshock. For the four earthquakes studied here, the 1986 Mt. Lewis, 1984 Morgan Hill, 1979 Coyote Lake, and 1989 Loma Prieta events, modest rotations of the maximum compressive stress S H to a higher angle (i.e., an orientation closer to fault-normal) appear to occur at the time of the mainshock. In some cases, S H eventually rotates back towards its original orientation. However, due to relatively large uncertainties obtained on the stress orientations, the constraints that can be inferred on the absolute levels of stress surrounding the mainshock regions are rather weak. By considering the largest stress change permitted by the confidence limits, we obtain lower bounds on the background deviatoric stress of 3–12 MPa, levels only slightly greater than the mainshock static stress drops.

Microseismic monitoring of geomechanical reservoir processes and fracture-dominated fluid flow

Abstract Microearthquakes (microseismic events) are induced during hydrocarbon and geothermal fluid production operations in naturally fractured reservoirs. They typically result from shear-stress release on pre-existing faults and fractures due to production/injection induced perturbations to the effective stress conditions. These stress changes may be due to reservoir depletion, flooding or stimulation operations. Over a number of years it has been shown that microseismic monitoring has the potential to provide valuable time-lapse 3D information on the geomechanical processes taking place within a reservoir. These processes include the distribution of fluid flow and pressure fronts within naturally fractured systems, production-related compaction and the reactivation of faults. With the advent of permanent reservoir monitoring systems (e.g. intelligent wells) microseismic monitoring has the potential to become a practicable means of time-lapse imaging of hydrocarbon reservoir processes remote from production/injection boreholes. This paper illustrates some of the ways in which microseismic monitoring can contribute to the development and management of hydrocarbon reservoirs through the presentation of examples from both hydrocarbon and geothermal reservoirs.

A model for fluid-injection-induced seismicity at the KTB, Germany

SUMMARY The 9.1 km deep KTB (Kontinentale Tiefbohrung, Germany) drilling hole is one of the best investigated deep-drilling sites in the world. Among other parameters, in situ measurements revealed continuous profiles of principal stresses, pore fluid pressure and fracture geometry in the vicinity of the borehole. The present study combines these parameters with hydraulic and seismicity data obtained during fluid-injection experiments conducted at the KTB to derive a conceptual model for fluid-injection-induced seismicity at the KTB. This model rests on the well constrained assumptions that (1) the crust is highly fractured with a permeable fracture network between 9 km depth and the Earth's surface and (2) the crust is in near-failure equilibrium, whereby a large number of fracture planes are under near-critical condition. During the injection experiment, the elevated pore fluid pressure remained well below the least principal stress and thus was too small to cause hydraulic opening of existing fractures. Consequently, the geometry of the fracture network was assumed to have not changed during fluid injection with induced seismicity occurring solely as a result of lowering of the effective normal stress, consistent with observed source mechanisms. The key parameter in the present model is the fracture permeability, which exhibits large spatial and directional variations. These variations are proposed to primarily control fluid migration paths and associated propagation of elevated fluid pressure during fluid injection. In contrast with common models based on isotropic fluid diffusion or spatially averaged permeability, highly permeable branches of the fracture network strongly affect the propagation of fluid pressure and prohibit the concept of a smooth ‘pressure front’. We find evidence that major fluid flow exists at comparatively low fluid pressure (below the critical pressure required to cause seismic failure) without being detected seismically. This might also explain the difference between 1011 J of hydraulic energy inserted into the system during fluid injection and ∼108 J of seismic energy: a major part of the hydraulic energy might be converted to potential energy of the ground water level caused by upward migrating fluid. From the fluid level response to changes of injection rate observed in a second borehole we estimate fluid signal velocities to be as large as 300 m d−1. Importantly, the suggested model also accounts for the occurrence of repeating earthquakes (multiplets), a large number of which were observed during the injection experiment. The present model also suggests that coseismic changes of the stress field caused by tectonic shear stress release are very local and of small magnitude. This is consistent with the observation that none of the larger induced events is followed by aftershock series that would be expected if coseismic processes had noticeably perturbed the local stress field.