Range Stress Field Research Articles

Accumulation Mechanism of Marine Shale Gas Reservoir in Anticlines: A Case Study of the Southern Sichuan Basin and Xiuwu Basin in the Yangtze Region

The study of tectonics is one of the important aspects of shale gas preservation. It is vital for understanding how to determine the enrichment regularity of marine shale gas in anticlines. This paper focuses on typical shale blocks in the southern Sichuan Basin and shale in the Upper Ordovician and the Lower Silurian. In this study, triaxial unloading tests, permeability tests perpendicular and parallel to the stratification plane, FIB-HIM tests, and inclusion analyses are carried out with real drilling data. The enrichment regularity of marine shale gas in anticlines is studied by considering 2 aspects: the angle of the limbs and the burial depth. For anticlines with adjacent synclines, the migration regularity of shale gas is considered by 3 aspects: the dynamics, channels, and processes of migration. This study reveals that a limb angle greater than 120° reflects relatively good conditions for shale gas preservation, while limb angles lesser than 70° indicate relatively poor conditions. This study also suggests that during the process of uplift, large-scale concentrated fractures will form at a certain depth range and horizontal stress field, resulting in the large loss of shale gas. The regression equation of the fractured depth (H) and the horizontal stress (S) is presented as H=15.404S−754.41 (with a correlation coefficient R2=0.6834). The stratification plane and the organic pores form the migration channel of natural gas that is horizontal to the stratification plane in shale. Under the condition of both anticlines and contiguous synclines, shale gas escapes through fractures resulting from extrusion along the anticline and the uplift effect. In addition, driven by differences in the formation pressure coefficients, shale gas is capable of migrating in a short-distance stair-type style from synclines to the adjacent anticlines. Thus, if the drilling costs allow, the well locations should be placed in the more deeply buried synclines.

A QM/MM approach for low-symmetry defects in metals

Concurrent multiscale coupling is a powerful tool for obtaining quantum mechanically (QM) accurate material behavior in a small domain while still capturing long range stress fields using a molecular mechanical (MM) description. We outline an improved scheme for QM/MM coupling in metals which permits the QM treatment of a small region chosen from a large, arbitrary MM domain to calculate total system energy and relaxed geometry. In order to test our improved method, we compute solute–vacancy binding in bulk Al as well as the binding of Mg and Pb to a symmetric Σ5 grain boundary. Results are calculated with and without our improvement to the QM/MM scheme and compared to periodic QM results for the same systems. We find that our scheme accurately and efficiently reproduces periodic QM target values in these test systems and therefore can be expected to perform well using more general geometries.

Long range stress fields and cavitation along a shear band in a metallic glass: The local origin of fracture

Nanomechanical properties along a single shear band in a Zr-based metallic glass were studied. Spatial mapping of both indentation hardness and modulus reveal complex long-range softening patterns that are indicative of internal stress fields along the shear band. These internal stresses reach values of the order of the yield strength of the tested metallic glass. Time dependent stress relaxation along the shear band is observed, and shear-band cavitation at the micron scale is found. Both the cavitation and the internal stresses are attributed to the non-planar shear plane that during shear-band propagation leads to the development of off-axis stress components relative to the shear direction. The cavities are a signature of a shear-band-to-crack transition, which is supported by stress fields known to develop ahead of mixed mode I and II crack tips.

Structure of Triple Junctions of Grains, Nanoparticles in them and Bending - Torsion in Metal Nanopolycrystals

The paper presents the results of the transmission electron microscopic (TEM) investigation of the structure and phase composition of nanocrystalline copper obtained by the severe plastic deformation using the high pressure torsion (HPT) method. Special attention is paid to the triple junctions of grain boundaries. It was established that the triple junctions contained the partial disclinations and particles of the secondary phases. The dependences of such junction fractions on the deformation were measured of the formation at nanocrystalline copper. The phase analyses of the secondary phase structure were carried out, the sizes of the phase particles and their volume fractions were determined. The bending – torsion of the crystal lattice arising near the triple junctions was measured. The problem of the long – range stress field screening was considered.

On the elastic–viscoplastic behavior of nanocrystalline materials

A new constitutive law is introduced to quantify the macroscopic effect of grain boundary dislocation emission on the behavior of pure face center cubic nanocrystalline materials. It is postulated that an emitted dislocation ends its trajectory in the grain boundary opposite to the source causing mass transfer. Dislocation emission by grain boundary ledges, considered here as the primary grain-boundary sources, is modeled as a thermally activated mechanism and the penetration of an emitted dislocation is assimilated as a soft collision. The macroscopic behavior of the material is retrieved via the use of a secant self-consistent scheme. The material is seen as a two-phase composite where the inclusion phase represents grain cores, their behavior is driven by dislocation glide, and where the matrix phase, governed by the newly introduced dislocation emission and penetration mechanism, represents both grain boundaries and triple junctions. The long range stress field arising from the presence of grain boundaries is taken into account in the critical glide resistance stress at 0 K in the inclusion phase. The model is applied to polycrystal copper and results in pure tension and creep are compared to experiments. Good agreements between the experimental measurements and the model predictions are observed.

Structure of metal materials under bulk doping

The method of bulk doping of metallic materials is effected by super deep penetration of dispersed particles as a consequence of dynamic interaction with a barrier. Such a treatment has considerable potential, since it'increases the number of physic-mechanical and service properties of metals and alloys. The mechanism underlying the interaction of solid dispersed particle, with a metal matrix has been investigated by several authors. However, the problem of structural changes taking place in the matrix near particles and the trajectories of their motion has yet to be adequately studied. It would provide the possibility of understanding the mechanism of mass transfer of particles into metals with different compositions and structures. The process of hardening of the materials with different types of crystalline lattices by mean of explosive doping is considered as multifaceted. The similarities of, and the differences between, the mechanisms underlying the hardening of materials having body-centered (bcc) and face-centered cubic (fee) lattices are discussed. The role of long - range stress-fields in the formation of dislocation structure of metals subjected to explosive doping has been investigated. The intricate nature of the spatial configuration of stress fields near retarded particles and the role of these fields in hardening the treated metallic materials has been demonstrated.

Dislocations at ductile/plastic crack tips: in-situ TEM observations

In-situ observations of dislocation structures ahead of crack tips in TEM metal foils are reviewed. Two cases are compared in particular: Structure development during in-situ straining to failure of (i) electron-transparent foils ahead of the tip of a growing crack that spreads from the thinnest regions or perforations and (ii) initially non-transparent thick foils. In the latter case cracks formed only after substantial in-situ straining, and they propagated along dislocation cell walls via repeated stimulated crack nucleation ahead of the tip. This behavior was shown to adequately simulate bulk behavior and such cracks do not exhibit dislocation-free zones at their tips. By contrast, dislocation-free regions along ligaments formed by crack propagation and observed in thin (e.g. about 100 nm or less) TEM foils are found to be artifacts due to strong dislocation image forces. These image forces at the same time limit mutual dislocation interactions to the thickness of the foil, and rotate the dislocations to be normal to the foil plane, meanwhile straightening them. This behavior has no correspondence to conditions at real cracks in bulk materials. Theoretical expressions are derived for the dislocation densities ahead of crack tips that give rise to long-range and shorter range stress fields in mode I crack tip configurations, respectively.

Statistical models for cyclic strain hardening of copper single crystals

Models based on testing of copper single crystals and on the statistical theories of flow and cyclic work hardening are developed to explain the measurements of plastic strain amplitude as a function of acting shear stress amplitude, flow strength and number of cycles. The penetration probability of mobile dislocations through the microstructural obstacle distribution is analogous to the Weibull distribution depending on the ratio of stress amplitude to flow strength and other physical parameters. For the easy glide orientation crystals the results support the strain hardening model where new obstacles are stored as Orowan type loops and for multiple slip orientation crystals the forest cutting and the long range stress field theories seem probable.

Calculation of stress—strain curves by using 2 dimensional dislocation dynamics

A method is presented to derive stress-strain curves by using 2 dimensional dislocation dynamics. The simulation treats plastic deformation of single grains, in which one or more slip systems are active. The involved dislocations are regarded as infinite straight line defects which are embedded in an otherwise isotropic linear elastic medium. As the model is two dimensional only edge dislocations are incorporated. The calculation of the local stressfield considers the long range elastic stress contribution of all dislocations within the grain, of the externally imposed stressfields and of the short range stressfield imposed by large angle grain boundaries. Due to the net local value of the stressfield the dislocations may either glide or climb. Dislocation multiplication, annihilation and reactions are taken into account. Thermal activation is considered. The simulations allow to compute the evolution of the dislocation distribution, the stressfield and the plastic strain during a simulated deformation experiment. The latter quantities are used for the calculation of stress-strain curves.

Numerical simulation of stress fields of dislocation networks with special regard to interface dislocations

AbstractNumerical simulations of the stress fields of dislocation networks were carried out. Various types of dislocation configuration including single dislocations, dipoles, and interface dislocations were taken into consideration. The numerical description of dislocation statics with high spatial resolution provided a useful diagnostic means of scrutinising assumptions which are usually incorporated into analytical models of strain hardening and flow stress. It was shown that interface dislocations generated long range stress fields as predicted using analytical models, but contrary to the predictions of many models, their contribution to the total dislocation density was not negligible. The simulations confirmed that an adequate description of local stress fields, and hence of strain hardening and flow stress, can only be obtained if both the local densities of dislocations, and their character and arrangement are taken into account. Since the statics of complicated dislocation arrangements can no lon...

Origin of crystallographic tilt in InGaAs/GaAs(001) heterostructure

X-ray rocking curve measurements showed a significant crystallographic tilt in relaxed InGaAs layer grown on (001) GaAs. Transmission electron microscopy revealed that the origin of tilt is 60° dislocations generated having Burgers vectors of a same vertical edge component. Calculations using anisotropic elasticity show that this configuration of 60° dislocation array is energetically favorable when the tilt of epilayer is present as to remove the long range stress field induced by the vertical edge components at the interface.

3D simulation of the stress fields associated with disordered finite dislocation walls in face centred cubic crystals

Simulations of the 3-dimensional elastic stress fields of finite dislocation walls with spatial disorder were carried out. Each wall consisted of 40 dislocations with an average spacing of L = 40 nm. The dislocation lines involved were decomposed into piecewise straight segments representing translatoric vectors of a face centred cubic lattice. Spatial disorder of the walls was achieved by incorporating curved dislocations. The results substantiate that the disorder of the dislocation lines involved alters the stress fields associated with the walls. In the direct vicinity of the walls (distance ≤ 2πL) the shear stress is only weakly dependent on the degree of order and decays as predicted analytically for an infinite low angle tilt boundary. Within the intermediate range (2πL < distance ≤ 60πL) a maximum of the shear stress occurs which decreases with increasing degree of disorder. The long range stress fields (distance > 60πL) are only weakly affected by the type of disorder imposed in the present approach. The profile of the shear stress of the dislocation walls showing a maximum in the intermediate range is attributed to the fact that finite rather than infinite walls are considered. The decrease of the maximum occurring shear stress in the intermediate range with decreasing order is interpreted in terms of self screening effects.

X-ray pattern simulation in textured nanostructured copper

Computer simulations of X-ray patterns (XRP) of nanocrystalline (NC) copper are performed. Possible reasons are analyzed such as the crystallite size, the grain boundary (GB) width and the disposition of atoms in it, the character caused by long range stress fields produced by non-equilibrium GB elastic strains of the crystal lattice leading to changes of XRP and its features in crystallite size refinement. It is shown that the crystallite size and interatomic spacing variations in the crystal lattice caused by stress fields from non-equilibrium GBs are the main factors determining XRP, but the peak shift and the change of line form experimentally observed in NC materials could be explained by the presence of long range stress fields only.

Dislocation walls

The elastic properties of infinite perfect periodic planar dislocation walls are well known. It is however important to appreciate the effect of the departure from periodicity and the end effects for a finite wall. A weak uncorrelated disorder is shown to result in a long range stress field decreasing as (distance)−32 from the wall, consequences are discussed. Convenient approximate closed formulae are given and discussed for the stress field of bounded dislocation walls.

On the structure, stress fields and energy of nonequilibrium grain boundaries

The changes of grain boundary (GB) structure caused by the absorption of lattice dislocations are analysed. The typical result of the absorption is the disordering of networks of disclination dipoles/grain boundary dislocations (GBDs). It is shown that the variance of GBD spacings is a good quantitative measure of the degree of GB structure nonequilibrium. By the use of Monte Carlo technique the abating law of x − 1 2 for long range stress fields of disordered GBD networks is obtained. Excess energies of nonequilibrium boundaries are also calculated. In strongly excited state of GBs characterized by the maximum value of the variance of GBD spacings the excess energy can exceed the equilibrium GB energy. Such high-nonequilibrium boundaries can exist in ultrafine grained materials.

Weak-beam TEM study of the h.c.p. to f.c.c. martensitic phase transformation lamellae in CoNi

Abstract The thermically induced h.c.p. to f.c.c. martensitic phase transformation was studied in Co-32 at.% Ni single crystals by the weak-beam method of transmission electron microscopy (TEM). In this alloy the mean transformation temperature is at RT and therefore both phases are stable at RT. At the beginning of the transformation new f.c.c. lamellae grow from the thick into the thin region of the specimen. At the front of the lamella a Shockley partial dislocation must lie on every other close-packed plane. The dislocation contrast of the front observed by weak-beam images is similar to that of a single partial dislocation, even when the lamella is up to 50 atom layers thick. The minimum thickness of the lamellae is 6 to 10 close-packed planes; the lamellae grow in thickness by ledges. The lack of long range stress fields and the observation of lamellae of discrete height seem to indicate that at the front of the lamellae all three coplanar Burgers vectors of the partial dislocations are present and ...

The effect of extrinsic grain boundary dislocations on the grain size strengthening in polycrystals

The grain size dependence of flow stress at room temperature in the regime of small strains (0–2%) has been examined in as-annealed and 2% pre-strained and subsequently annealed specimens of 316L stainless steel. Grain sizes in the range of 3.4–22.4 μm were considered. The stress-strain curves exhibit linear hardening characteristic beyond 0.2% plastic strain. The analysis of the Hall-Petch parameters show a linear increase in σ 0( ϵ) and a linear decrease in K( ϵ) with strain in the as-annealed specimens. The increase in σ 0( ϵ) has been associated with both, the work-hardening processes in the grain interior and the long range stress field of extrinsic grain boundary dislocations (EGBDs). The EGBDs also act as sites of stress concentration thereby making it easier to generate dislocations in the vicinity of grain boundaries. Therefore, K( ϵ) which is function of the stress required to generate dislocations decreases with increasing strain. The observed drop in flow stress as a result of annealing of pre-stained specimens at 800°C has been related with the annihilation of dislocations at and in the vicinity of grain boundaries. Annealing at 550°C (this temperature is sufficient for the delocalization of the cores of EGBDs) does not have any significant effect on the density of dislocation at and in the vicinity of grain boundaries. Therefore, no significant change in the flow stress was obsereved.

Local structural incompatibilities in amorphous, quasicrystalline and crystalline solids

The local structural incompatibilities, defined in terms of the atomic level stresses, are found not only in liquids and glasses, but also in many of the crystalline solids and qusicrystals. The effect of these local structural incompatibilities on the medium range structural order through long range stress fields is discussed.

Effect of nitrogen on low cycle fatigue behavior of austenitic stainless steel.

The effect of nitrogen on low cycle fatigue behavior of 18.5Cr-15Ni austenitic stainless steel has been investigated over a temperature range from room temperature to -162°C. Results obtained are as follows:(1) N induces cyclic hardening, followed by cyclic softening to saturation over the whole temperature range.(2) The cyclic softening at room temperature may be attributed to dislocation rearrangement from planar arrays to cellular ones, while at -162°C, it may be attributed to the increase in mobile dislocation density.(3) Based on these results, it is proposed that a zone of short range order (SRO) may be formed by the addition of N, and the interaction between dislocations and the SRO-zone may be responsible for the observed cyclic behaviors. Especially at -162°C the cyclic behavior is controlled by a thermally activated process of dislocation motion in a short range stress field.(4) N improves the fatigue life over the whole temperature range by reducing cyclic hardening and promoting cyclic softening.

Theory and experiment of martensitic nucleation in ZrO2 containing ceramics and ferrous alloys

An in situ TEM experiment on martensitic nucleation was performed using sub-micron ZrO2 particles which were initially defect free. Extrinsic, spontaneous and heterogeneous nucleation was induced by introducing dislocation loops with a strong shear component and Hertzian contact stresses into selected particles. The critical characteristics of these defects which were required to trigger barrierless nucleation, in terms of the loop radius, Burgers vector, contact stress, and contact area were in good agreement with an analytic elasticity model. Along with dislocation walls, these defects which have a short range stress field can also account for intrinsic nucleation in ZrO2 containing ceramics and ferrous alloys. The essential features of nucleating defects, which may be classified into intrinsic or extrinsic ones depending on their origins, and short range or long range ones depending on their stress fields, are analyzed and contrasted in view of their experimental evidence.