Local Stress Perturbations Research Articles

Variations in the present-day stress field and its implications for hydrocarbon development in the Hassi Terfa field, Hassi Messaoud, Algeria

Understanding the contemporary stress field and its variations is important in hydrocarbon production. These fluctuations are significant for borehole stability and trajectory, hydraulic stimulation, fault reactivation, and optimal well completion. In this study, we provide the first geomechanical model to determine the orientation and magnitude of the present-day stress of the Hassi Terfa field using geophysical logs to understand the origin of the spatial variation of the stress state. The breakouts from acoustic image logs provide an average maximum horizontal stress (SHmax) orientation of N115°, consistent with the Africa–Eurasia plate motion, which imposes the largest first-order stress source. However, our analysis demonstrates local stress perturbations near faults. This may mean that the stress field in the Hassi Terfa is superimposed by a third-order source of stress (<100 km). Our model demonstrates variations in stress magnitude with depth interpreted to be lithology-dependent, which act as a natural barrier against fracture propagation. We propose a new approach to quantify these perturbations via the stress regime index R′, using the magnitude of the principal stress components. The results demonstrate different layer-to-layer faulting regimes. These perturbations will limit the vertical propagation of hydraulic fracturing to the overlying formation, which enables it to access a larger reservoir volume and improve production. In general, a strike–slip regime with a slight normal slip component is interpreted for the study area. We develop the first-ever quantitative stress map using the relative principal stress magnitude parameter (Aϕ) to present the lateral variation of the stress regime throughout the Saharan platform. Results demonstrate a consistent strike–slip to normal/strike–slip regime with a uniform SHmax orientation. This map will help operators achieve optimal well direction and optimize hydraulic stimulation. Our results have considerable implications for hydrocarbon production in the central and southeastern Saharan platform.

A benchmark study for quasi-static numerical upscaling of seismic wave attenuation and dispersion in fractured poroelastic rocks

Numerical simulation is an effective tool for comprehending fluid flow behavior and solid skeleton deformation within porous media, as well as obtaining valid seismic information. The elastic response of seismic waves, serving as an intermediary linking material properties and seismic attributes, is influenced by a combination of the wave-induced fluid flow (WIFF) effects and stress interactions. However, the impact of stress interactions is frequently overlooked in numerous studies, primarily due to the complex distribution of subsurface fractures makes it difficult to clarify the effects caused by local stress perturbations. Therefore, we present a numerical upscaling procedure as a benchmark process to quantify seismic wave attenuation and dispersion in fractured porous media simulations. The procedure is based on COMSOL Multiphysics and allows for the simulation of the energy dissipation resulting from the compression of a model with arbitrary forms of fluid saturation, geometry, and petrophysical parameters by seismic waves. We analyze the fracture models with different spatial distributions and infills to understand the mechanism. Our simulation results show the efficient and accurate capabilities of the numerical upscaling procedure in simulating the acoustic properties of fractured porous media and demonstrate the physical processes at various frequencies. In addition, our study yielded corresponding results: 1) the diverse spatial distribution of fractures not only gives rise to peak anomalies but also influences both the attenuation peaks and characteristic frequencies of seismic waves, thus reflecting the attenuation phenomena occurring at different scales; 2) various infills exhibit distinct sensitivities to fracture morphology and stress interactions.

Volatile-consuming reactions fracture rocks and self-accelerate fluid flow in the lithosphere

Hydration and carbonation reactions within the Earth cause an increase in solid volume by up to several tens of vol%, which can induce stress and rock fracture. Observations of naturally hydrated and carbonated peridotite suggest that permeability and fluid flow are enhanced by reaction-induced fracturing. However, permeability enhancement during solid-volume-increasing reactions has not been achieved in the laboratory, and the mechanisms of reaction-accelerated fluid flow remain largely unknown. Here, we present experimental evidence of significant permeability enhancement by volume-increasing reactions under confining pressure. The hydromechanical behavior of hydration of sintered periclase [MgO + H2O → Mg(OH)2] depends mainly on the initial pore-fluid connectivity. Permeability increased by three orders of magnitude for low-connectivity samples, whereas it decreased by two orders of magnitude for high-connectivity samples. Permeability enhancement was caused by hierarchical fracturing of the reacting materials, whereas a decrease was associated with homogeneous pore clogging by the reaction products. These behaviors suggest that the fluid flow rate, relative to reaction rate, is the main control on hydromechanical evolution during volume-increasing reactions. We suggest that an extremely high reaction rate and low pore-fluid connectivity lead to local stress perturbations and are essential for reaction-induced fracturing and accelerated fluid flow during hydration/carbonation.

Predicting Fracture Network Development in Crystalline Rocks

The geometric properties of fractures influence whether they propagate, arrest, or coalesce with other fractures. Thus, quantifying the relationship between fracture network characteristics may help predict fracture network development, and perhaps precursors to catastrophic failure. To constrain the relationship and predictability of fracture characteristics, we deform eight one centimeter tall rock cores under triaxial compression while acquiring in situ X-ray tomograms. The tomograms reveal precise measurements of the fracture network characteristics above the spatial resolution of 6.5 µm. We develop machine learning models to predict the value of each characteristic using the other characteristics, and excluding the macroscopic stress or strain imposed on the rock. The models predict fracture development more accurately in the experiments performed on granite and monzonite, than the experiments on marble. Fracture network development may be more predictable in these igneous rocks because their microstructure is more mechanically homogeneous than the marble, producing more systematic fracture development that is not strongly impeded by grain contacts and cleavage planes. The varying performance of the models suggest that fracture volume, length, and aperture are the most predictable of the characteristics, while fracture orientation is the least predictable. Orientation does not correlate with length, as suggested by the idea that the orientation evolves with increasing differential stress and thus fracture length. This difference between the observed and expected relationship between orientation and length highlights the influence of mechanical heterogeneities and local stress perturbations on fracture growth as fractures propagate, link, and coalesce.

Imaging Elastodynamic and Hydraulic Properties of In Situ Fractured Rock: An Experimental Investigation Exploring Effects of Dynamic Stressing and Shearing

AbstractWe describe laboratory experiments to elucidate the relationship between nonlinear elasticity and permeability evolution in fractured media subjected to local stress perturbations. This study is part of an effort to measure fluid pathways and fracture properties using active‐source acoustic monitoring during fluid injection and shear of rough fractures. Experiments were conducted with L‐shaped samples of Westerly granite fractured in situ under triaxial conditions with deionized water subsequently circulated through the resulting fractures. After in situ fracturing, we separately imposed oscillations of the applied normal stress and pore pressure with amplitudes ranging from 0.2 to 1 MPa and frequencies from 0.1 to 40 Hz. In response to normal stress and pore pressure oscillations, fractured Westerly granite samples exhibit characteristic transient softening, acoustic velocity fluctuations, and slow recovery, together with permeability enhancement or decay, informing us about the coupled nonlinear elastodynamic and poromechanical rock properties. Fracture interface properties (contact asperity stiffness, aperture) are then altered in situ by shearing, which generally decreases the measured elastic nonlinearity and permeability change for both normal stress and pore pressure oscillations.

Stress/strain variability in fractured media: a fracture geometric study

The presence of natural fractures in fractured media plays a vital role in the in-situ stress state, which is predominantly influenced by tectonic stresses and local perturbations. Fracture orientation, wellbore stability/orientation, and permeability anisotropy are strongly dependent on local stress variations. In this study, a discrete fracture network (DFN) was generated using the stochastic approach to investigate the correlation between geometrical properties of the fracture network (fracture density and length) and local stress variability. Afterward, the FLAC2D software was employed to propagate the stress redistribution in the simulation process. Considering the tensorial nature of stress, a stress field was calculated under orthogonal static boundary conditions. A tensor-based mathematical formulation was applied to compute total stress variability, shear strain distribution, and total displacement in different fracture network realizations. The results demonstrated that the mean local stress perturbation, effective variance (as an indicator of total stress variability), shear strain distribution, and total displacement increased with the rise of stress ratio and fracture density. The aforementioned parameters decreased as the power-law length exponent of the DFN generator increased. Overall, it is concluded that the stress/strain variability and total displacement in a dense fracture network and large length fractures have their maximum values.

Aseismic deformations perturb the stress state and trigger induced seismicity during injection experiments

SUMMARY Fluid injections can trigger seismicity even on faults that are not optimally oriented for reactivation, suggesting either sufficiently large fluid pressure or local stress perturbations. Understanding how stress field may be perturbed during fluid injections is crucial in assessing the risk of induced seismicity and the efficiency of deep fluid stimulation projects. Here, we focus on a series of in situ decametric experiments of fluid-induced seismicity, performed at 280 m depth in an underground gallery, while synchronously monitoring the fluid pressure and the activated fractures movements. During the injections, seismicity occurred on existing natural fractures and bedding planes that are misoriented to slip relative to the background stress state, which was determined from the joint inversion of downhole fluid pressure and mechanical displacements measured at the injection. We then compare this background stress with the one estimated from the inversion of earthquake focal mechanisms. We find significant differences in the orientation of the stress tensor components, thus highlighting local perturbations. After discussing the influence of the gallery, the pore pressure variation and the geology, we show that the significant stress perturbations induced by the aseismic deformation (which represents more than 96 per cent of the total deformation) trigger the seismic reactivation of fractures with different orientations.

Formation of listric normal faults by extensional duplexing: A case study from the active Langshan piedmont fault, NW China

The development of normal faults is controlled by many factors, among which mechanical layers or pre-existing fabrics play an important role in the growth of normal faults. It is widely accepted that normal faults are rarely continuous surfaces but are zones composed of fault segments, heterogeneously distributed fault rocks of various compositions, slip bands, blocks and other deformation elements. Listric normal faults, particularly those developed in metamorphic crystalline rocks or preexisting fabric-developed basements, may be composed of subparallel faults in cross-sections, linked by extensional duplex structures. Here, we choose a listric rift-bounding fault rooted in foliation developed basements, the Langshan normal fault in the northwest corner of the Ordos Block, to conduct an outcrop analysis of fault geometry and linkage behavior from cross-sectional views. The Langshan normal fault zone is composed of mixed regions of high- and low-angle segments linked by duplex structures at various scales. Mylonitic foliations play a significant role in the formation of fault segments and the evolution of the listric fault zones. Lines of evidence show that small en echelon faults initiated from preexisting foliations. Local stress perturbation between overstepping faults results in the formation of duplex structures and displays a high fracture density, which serve to the coalesce of fault segments. Consequently, the progressive localization of strain onto a large listric fault is achieved by progressive development of extensional duplexing among fault segments in cross-sections. These extensional duplexes imply a scale-invariant property, based on which we can extrapolate this pattern occurs on larger scales and is verified by seismic data. The case of Langshan piedmont fault demonstrates the possibility of a listric fault being formed by subparallel planner faults and highlights the effects of preexisting fabrics on the evolution of listric faults, especially in basement fabrics developed regions along the fault.

Dynamic Fault Interaction during a Fluid-Injection-Induced Earthquake: The 2017 Mw 5.5 Pohang Event

ABSTRACT The 15 November 2017 Mw 5.5 Pohang, South Korea, earthquake has been linked to hydraulic stimulation and fluid injections, making it the largest induced seismic event associated with an enhanced geothermal system. To understand its source dynamics and fault interactions, we conduct the first 3D high-resolution spontaneous dynamic rupture simulations of an induced earthquake. We account for topography, off-fault plastic deformation under depth-dependent bulk cohesion, rapid velocity weakening friction, and 1D subsurface structure. A guided fault reconstruction approach that clusters spatiotemporal aftershock locations (including their uncertainties) is used to identify a main and a secondary fault plane that intersect under a shallow angle of 15°. Based on simple Mohr–Coulomb failure analysis and 180 dynamic rupture experiments in which we vary local stress loading conditions, fluid pressure, and relative fault strength, we identify a preferred two-fault-plane scenario that well reproduces observations. We find that the regional far-field tectonic stress regime promotes pure strike-slip faulting, whereas local stress conditions constrained by borehole logging generate the observed thrust-faulting component. Our preferred model is characterized by overpressurized pore fluids, nonoptimally oriented but dynamically weak faults and a close-to-critical local stress state. In our model, earthquake rupture “jumps” to the secondary fault by dynamic triggering, generating a measurable non-double-couple component. Our simulations suggest that complex dynamic fault interaction may occur during fluid-injection-induced earthquakes and that local stress perturbations dominate over regional stress conditions. Therefore, our findings have important implications for seismic hazard in active georeservoir.

Local stress perturbations associated with the 2008 Wenchuan M 8.0 earthquake near the Longmenshan fault zone in the eastern margin of the Tibetan Plateau

The 2008 Wenchuan M 8.0 earthquake induced local stress perturbations near the Longmenshan (LMS) fault zone. Using in situ measurements and stress tensor inversions from focal mechanisms, we investigated the post-earthquake perturbed stress field at shallow depths (≤1.2 km), in the upper crust above a basal attachment (~5–18 km) as well as in the middle crust beneath a basal attachment (~19–32 km). At shallow depths in the Beichuan-Jiangyou section, the maximum principal stress (σ1) orientations changed from a pre-earthquake orientation of NWW to a post-earthquake orientation of NEE. However, near the Baoxing-An'xian and Qingchuan-Mianxian sections, the pre-earthquake σ1 orientation of NW-NWW was largely unaltered. Before the mainshock, in the upper crust above the basal detachment, the entire LMS fault zone exhibited a consistent σ1 orientation of NW-NWW. However, after the earthquake, some sections, such as the Beichuan-Nanba section, achieved σ1 orientations of NEE that were distinctly different from the pre-seismic stress field. In the middle crust beneath the basal detachment, the stress field was mostly unchanged. The crust beneath the basal detachment, along the eastern margin of the Tibetan Plateau, is dominated by NWW-oriented maximum principle stress and strain directions and a thrust faulting stress regime along the entire LMS fault zone; these observations are representative of the southeastward motion of the Bayan Har block and its ongoing collision with the South China block along the LMS thrust belt. The spatially heterogeneous stress field that occurred in the upper crust (≤18 km) after the mainshock was most likely caused by the co-seismic static stress changes associated with the Wenchuan earthquake. Prior to the onset of seismic activity, the deviatoric stress magnitudes were estimated at less than 22 MPa and 4 MPa at depths of 10 km and 20 km, respectively, suggesting that the mainshock fault zone has a relatively low fault strength to overcome, in order to initiate significant co-seismic rupture and aftershocks, especially in the crust beneath the basal detachment. These smaller deviatoric magnitudes are indicative of the weaker fault materials that resides in the LMS region.

Influence of fault roughness on surface displacement: from numerical simulations to coseismic slip distributions

SUMMARYField studies have characterized natural faults as rough, non-planar surfaces at all scales. Fault roughness induces local stress perturbations during slip, which dramatically affect rupture behaviour, resulting in slip heterogeneity. However, the relation between fault roughness and slip heterogeneity remains a key knowledge gap between current numerical and field studies. In this study, we analyse numerical simulations of earthquake rupture to determine how roughness influences final slip. Using a rupture catalogue containing thousands of dynamic rupture simulations on band-limited self-similar fractal fault profiles with varying roughness and background shear stress levels, we quantify how fault roughness affects the spectral characteristics of the resulting slip distribution. We find that slip distributions become increasingly more self-affine, that is, containing more short wavelength fluctuations as compared to the self-similar fault profiles, as roughness increases. We also find that, at very short wavelengths (&amp;lt;1 km), the fractal dimension of the slip distributions dramatically changes with increasing roughness, background shear stress, and rupture speed (sub-Rayleigh versus supershear). The existence of a critical wavelength around 1 km, under which more short wavelengths are either preserved or created, suggests the role of rupture process and dynamic effects, together with fault geometry, in controlling the final slip distributions. The same spectral analysis is performed on high-resolution coseismic surface slip distributions from a catalogue of real strike-slip earthquakes. Compared to numerical simulations, all earthquakes feature slip distributions that are much more self-affine than the slip distributions from numerical simulations. A different critical wavelength, here around 5–6 km, appears, potentially informing about a critical asperity length. While we show here that the relation between fault roughness and slip is much more complex than expected, this study is a first attempt at using statistical analyses of numerical simulations on rough faults to investigate observed coseismic slip distributions.

In situ stress states at KURT, an underground research laboratory in South Korea for the study of high-level radioactive waste disposal

The Korea Atomic Energy Research Institute Underground Research Tunnel (KURT) is an underground research laboratory in South Korea, built for investigations into the geological disposal of high-level radioactive waste. We characterize in situ stress states at KURT using data from a series of hydraulic fracturing (HF) tests and borehole image logs to depths of ~700 m in two boreholes. The tensile fractures induced by HF tests, plus several borehole stress indicators (e.g., drilling-induced tensile fractures and borehole breakouts) measured from image logs, consistently indicate an ESE–WNW oriented maximum horizontal principal compressive stress (SHmax). This site-scale SHmax orientation varies slightly from the regional-scale SHmax orientation, likely reflecting a local stress perturbation resulting from a fault network that traverses the site. Estimated magnitudes of the minimum horizontal principal compressive stress (Shmin), determined either from the shut-in pressures recorded during HF tests or from stress indicators on image logs, are comparable to the vertical stress, indicating that the stress regime at the KURT site straddles the boundary between strike-slip and reverse faulting. The depth-dependent trend in estimated SHmax magnitudes deviates at ~500 m depth, which we attribute to variations in the distribution of natural fractures in the granitic rock mass. This depth-dependent variation in SHmax magnitudes has implications for the slip stability of pre-existing fractures at the site. That is, at shallow depths, SHmax lies within the Coulomb stress limit for optimally oriented fractures and faults with a frictional coefficient of 0.6, whereas at greater depths, SHmax exceeds this limit, meaning that pre-existing fractures at shallow depth are more susceptible to slip reactivation. Our stress estimation results suggest that the site-scale stress state is strongly coupled with characteristics of natural fractures, emphasizing the importance of detailed geologic and stress data for subsurface utilization and its stability evaluation.

Atmospheric Origins of Variability in the South Atlantic Meridional Overturning Circulation

Abstract Insights from the RAPID–MOCHA observation network in the North Atlantic have motivated a recent focus on the South Atlantic, where water masses are exchanged with neighboring ocean basins. In this study, variability in the South Atlantic meridional overturning circulation (SAMOC) at 34°S is attributed to global atmospheric forcing using an inverse modeling approach. The sensitivity of the SAMOC to atmospheric state variables is computed with the adjoint of the Massachusetts Institute of Technology general circulation model, which is fit to 20 years of observational data in a dynamically consistent framework. The dynamical pathways highlighted by these sensitivity patterns show that the domain of influence for the SAMOC is broad, covering neighboring ocean basins even on short time scales. This result differs from what has previously been shown in the North Atlantic, where Atlantic meridional overturning circulation (AMOC) variability is largely governed by dynamics confined to that basin. The computed sensitivities are convolved with surface atmospheric state variability from ERA-Interim to attribute the influence of each external forcing variable (e.g., wind stress, precipitation) on the SAMOC from 1992 to 2011. Here, local wind stress perturbations are shown to dominate variability on seasonal time scales while buoyancy forcing plays a minor role, confirming results from past forward perturbation experiments. Interannual variability, however, is shown to have originated from remote locations across the globe, including a nontrivial component originating from the tropical Pacific. The influence of atmospheric forcing emphasizes the importance of continuous widespread observations of the global atmospheric state for attributing observed AMOC variability.

A numerical study of stress variability in heterogeneous fractured rocks

We conduct numerical simulations to investigate the variability of local stresses in heterogeneous fractured rocks subjected to different far-field stress conditions. A realistic fracture network is constructed based on a real outcrop mapped at the Hornelen Basin in Norway. The heterogeneity of the rock material is modelled using a Weibull distribution of Young's modulus characterised by a homogeneity index m. As m decreases, the rock material becomes less homogeneous. The distribution of local stresses in the fractured rock under far-field stress loading is derived from a hybrid finite-discrete element model, and the stress variability is further analysed using a tensor-based formalism that faithfully honours the tensorial nature of stress data. The local stress perturbation is quantified using the Euclidean distance of each local stress tensor to the mean stress tensor, and the overall stress dispersion is measured using the effective variance of the entire stress tensor field. We show that the local stress field is significantly perturbed when the far-field stresses are associated with a high stress ratio and imposed at a critical direction in favour of intense sliding along preferentially-oriented fractures. The strong correlation between fracture sliding and local stress variability are further revealed from a scanline sampling analysis through the domain. Furthermore, larger perturbation of local stresses can be induced as the inhomogeneity of the rock materials increases (i.e. m decreases). Whether the stress field is dominated by fractures or matrix depends on the far-field stress state, material inhomogeneity, and fracture properties. If the rock material is highly heterogeneous, stress variability is controlled by the matrix when the far-field stress ratio is low; however, the stress distribution becomes more affected by fractures as the stress ratio increases. If the rock material is more homogeneous, the system tends to be more dominated by fractures even under a relatively low stress ratio.

Mechanical Factors Controlling the Development of Orthogonal and Nested Fracture Network Geometries

Orthogonal fracture networks form an arrangement of open well-connected fractures which have perpendicular abutment angles and sometimes show topological relations by which fracture sets abut against each other, thus forming a nested network. Previous modelling studies have shown that orthogonal fractures may be caused by a local stress perturbation rather than a rotation in remote stresses. In this study, we expand on the implications of these local stress perturbations using a static finite element approach. The derived stress field is examined to assess the development of implemented microfractures. The results show that the continuous infill of fractures leads to a gradual decrease in the local tensile stresses and strain energies, and, therefore, results in the development of a saturated network, at which further fracture placement is inhibit. The geometry of this fully developed network is dependent on the remote effective stresses and partly on the material properties. Saturated networks range from: (1) a set of closely spaced parallel fractures; (2) a ladder-like geometry; and (3) an interconnected nested arrangement. Finally, we show that our modelling results at which we apply effective tension, are equivalent to having a uniformly distributed internal pore fluid pressure, when assuming static steady state conditions and no dynamic fluid behaviour.

Correlation Between Fracture Network Properties and Stress Variability in Geological Media

AbstractWe quantitatively investigate the stress variability in fractured geological media under tectonic stresses. The fracture systems studied include synthetic fracture networks following power law length scaling and natural fracture patterns based on outcrop mapping. The stress field is derived from a finite‐discrete element model, and its variability is analyzed using a set of mathematical formulations that honor the tensorial nature of stress data. We show that local stress perturbation, quantified by the Euclidean distance of a local stress tensor to the mean stress tensor, has a positive, linear correlation with local fracture intensity, defined as the total fracture length per unit area within a local sampling window. We also evaluate the stress dispersion of the entire stress field using the effective variance, that is, a scalar‐valued measure of the overall stress variability. The results show that a well‐connected fracture system under a critically stressed state exhibits strong local and global stress variabilities.

Estimation of the influence of a defect on the operational suitability of a pipeline

A finite-element model of a pipe header with a defect in the metal wall was created in the ANSYS software complex. The boundary conditions consider the hinging of the ends of the section from linear displacements. The calculation is based on the procedure of the allowable conditional elastic stresses. Values of equivalent stresses arising in the defective pipeline from the operational loads are obtained. Analysis of the obtained stresses in comparison with the allowable values is given based on the calculated resistance of steel. It is established that a dent in the pipeline causes a local stress perturbation. Thus, an increase in equivalent stresses in the region of the maximum depth of the dent amounted to 1.3% in comparison to the vicinity of the defect. Maximum values of membrane, tangential, and equivalent stresses do not exceed the design resistance. A conclusion is drawn on the influence of the dent-type defect on the operational suitability of the pipeline under study.

Crustal stress field perturbations in the continental margin around the Korean Peninsula and Japanese islands

Seismic activity and focal mechanisms are governed by the effective stress field that is a combined result of regional tectonic processes and local stress perturbation. This study investigates the regional variation in the stress field in the eastern continental margin of the Eurasian plate around the Korean Peninsula and Japanese islands using a damped stress inversion technique based on the focal mechanism solutions of regional earthquakes. The dominant compressional stress is directed ENE–WSW around the Korean Peninsula and eastern China, E–W at the central East Sea and northern and southern Japan, NW–SE at central Japan, and N–S around the northern Nankai trough. The dominant compression direction changes rapidly in the East Sea and Japanese islands, which may be due to the combined effects of tectonic loading in the subduction zones off the Japanese islands and the India-Eurasia plate boundary. The crustal stress fields around the subduction zones off the Japanese islands present characteristic depth-dependent orientations. The orientations of the largest horizontal stress components, σH, in the subduction zones are subparallel with the plate convergence directions at shallow depths. The σH orientations are observed to rotate clockwise with the depth owing to slab subduction and lithospheric deformation. The regional stress field around the Japanese islands was perturbed temporally by the 2011 M9.0 Tohoku-Oki megathrust earthquake. The regional stress field was recovered in a couple of years. The stress field and tectonic structures are mutually affected, causing stress field distortion and a localized mixture of earthquakes in different faulting types.

The relation between fault movement potential and seismic activity of major faults in Northwestern Vietnam

The relation between fault movement potential and seismic activity of major faults in Northwestern Vietnam

Experimentally quantifying critical stresses associated with basal slip and twinning in magnesium using micropillars

Basal slip and {011¯2} twinning are two major plastic deformation mechanisms in hexagonal closed-packed magnesium. Here we quantify the critical stresses associated with basal slip and twinning in single-crystal and bi-crystal magnesium samples by performing in situ compression of micropillars with different diameters in a scanning electron microscope. The micropillars are designed to favor either slip or twinning under uniaxial compression. Compression tests imply a negligible size effect related to basal slip and twinning as pillar diameter is greater than 10 μm. The critical resolved shear stresses are deduced to be 29 MPa for twinning and 6 MPa for basal slip from a series of micropillar compression tests. Employing full-field elasto-visco-plastic simulations, we further interpret the experimental observations in terms of the local stress distribution associated with multiple twinning, twin nucleation, and twin growth. Our simulation results suggest that the twinning features being studied should not be close to the top surface of the micropillar because of local stress perturbations induced by the hard indenter.