State Of Plastic Flow Research Articles

Study on tensile properties of copper/graphene/2D-SiC composite materials: A molecular dynamics simulation

Van der Waals heterostructures composed of graphene and 2D-SiC generally exhibit outstanding electrical and mechanical properties. However, these structures are prone to brittle failure during damage, which greatly limits their application in practical engineering. Conversely, copper-based composite materials have been discovered to significantly enhance material toughness. Therefore, a novel copper/graphene/2D-SiC composite material is designed and molecular dynamics simulation is performed to investigate its mechanical properties under uniaxial tension. The results show that these composite materials possess the advantages of both high strength and toughness due to the heterostructure layer and the copper layer. Further analysis of the deformation mechanism of the copper layer indicates that it is mainly attributed to short-range atomic slip and the generation of Shockley partial dislocations. The heterostructure layer can partially impede the propagation of dislocations, enhancing the overall tensile strength of the composite materials. After the copper layer enters the plastic flow state, the variation of bond lengths and bond angles suggests that the heterostructure layer bears the tensile load, and the 2D-SiC layer is more prone to fracture due to higher stiffness compared to the graphene layer. Additionally, the influence of temperature and strain rate on the tensile properties is investigated. It is observed that increasing temperature could gradually reduce the tensile strength of the composite materials while the strain rate, within a certain range, has little effect.

Rheological properties of Mg2SiO4 glass: A molecular dynamics study

We report an initial investigation of the rheology of Mg2SiO4 glass through classical molecular dynamics simulations. We performed simple shear tests at different temperatures (1–300 K), shear rates (108–1010 s–1) and pressures (0–10 GPa), from which we investigate the atomic rearrangements induced by loading. At low strain, atomic rearrangements, as detected by a non-affine displacement criteria, nucleate randomly in the glass. They then give rise to the formation of shear bands once the steady state of plastic flow is reached. We show that the flow stress follows a Herschel-Buckley law modified to account for the thermal activation of plastic events.

Superlubric-pinned transition of a two-dimensional solid dusty plasma under a periodic triangular substrate.

The superlubric-pinned transition in the depinning dynamics of a two-dimensional (2D) solid dusty plasma modulated by 2D triangular periodic substrates is investigated using Langevin dynamical simulations. When the lattice structure of the 2D solid dusty plasma perfectly matches the triangular substrate, two distinctive pinned and moving ordered states are observed as the external uniform driving force gradually increases from zero. When there is a mismatch between the lattice structure and the triangular substrate, however, on shallow substrates, it is discovered that all of the particles can slide freely on the substrate even when the applied driving force is tiny. This is a typical example of superlubricity, which is caused by the competition between the substrate-particle and particle-particle interactions. If the substrate depth increases further, as the driving force increases from zero, there are three dynamical states consisting of the pinned state, the disordered plastic flow state, and the moving ordered state. In an underdense system, where there are fewer particles than potential well minima, it is found that the occurrence of the three different dynamical states is controlled by the depth of the substrate, which is quantitatively characterized using the average mobility.

Mechanical Properties and Strengthening Mechanism of Dredged Silty Clay Stabilized by Cement and Steel Slag.

The high moisture content and low strength of dredged soft soils result in significant difficulties in directly reutilizing them in engineering. Improving their mechanical properties effectively and achieving re-utilization with the maximum benefit in engineering is the key to disposing of dredged soils with high moisture content. This study investigated the influences of cement and steel slag ratio, moisture content, the maximum particle size of steel slag, and curing age on the compressive strength of dredged silty clay in a plastic flow state. The performance improvement of dredged silty clay stabilized with cement and steel slag was discussed by comparing to related previous studies. The strengthening mechanism of dredged soils stabilized with cement and steel slag was explored by microstructural observation. The results show that when the ratio of cement to steel slag was 9:6; namely, using steel slag to replace 40% of cement, the strength properties of dredged silty clay stabilized by cement and steel slag could ensure the minimum requirements of the project greater then 100 kPa, and their economics could achieve the best results. The finer the particle size of steel slag was, the better the stabilization effect was. The compressive strength of dredged silty clay stabilized by cement and steel slag with particle sizes of less than 0.075 mm was 1.06 times, 1.10 times, and 1.16 times that of 0.25 mm, 1 mm, and 2 mm and increased linearly over curing ages earlier than 28 days. The compressive strength of dredged silty clay stabilized by cement and steel slag cured for 28 days was 2.44 times, 1.59 times, and 1.36 times that of 3, 7, and 14 days, respectively. The evolution of microstructural characteristics showed that the internal pore sizes of dredged soil decreased the structural compactness increased significantly due to the formation of more calcium silicate hydrate and other agglomerated flocculent gel materials from the further reaction between steel slag and cement hydration products. The results of this study can provide technological parameters for the re-utilization of dredged soil stabilized with cement and steel slag.

Study on creep characteristics and evolution model of slide body in D area of Qinyu landslide

The geological conditions are complex in the Longnan mountain area, and the landslide disasters are widely distributed and numerous, among which the large accumulation layer landslide is especially developed. This kind of landslide is mostly distributed along the river basin and has the characteristics of multi-phase and multistage. The slide body is mostly in the state of creep plastic flow, and it is easy to revive. In this paper, the creep characteristics and evolution model of the secondary landslide were studied based on the field geological survey and the numerical calculation method, in order to provide reference for the monitoring and treatment of the landslide in this area. The results show that the shear stress and shear strain concentration zone is mainly located near the previous sliding surface of the landslide, and the creep type is deep creep; The deformation of the sliding body in zone D show that the deformation of the trailing edge and the middle front is relatively small, while the deformation of the middle and the leading edge is large; At the present stage, the overall deformation mode of the slide body in the D area of Qinyu landslide is a slow creeping deformation mode of traction from the front, down-shifting from the middle to the back.

Continuous and discontinuous transitions in the depinning of two-dimensional dusty plasmas on a one-dimensional periodic substrate.

Langevin dynamical simulations are performed to study the depinning dynamics of two-dimensional dusty plasmas on a one-dimensional periodic substrate. From the diagnostics of the sixfold coordinated particles P_{6} and the collective drift velocity V_{x}, three different states appear, which are the pinning, disordered plastic flow, and moving ordered states. It is found that the depth of the substrate is able to modulate the properties of the depinning phase transition, based on the results of P_{6} and V_{x}, as well as the observation of hysteresis of V_{x} while increasing and decreasing the driving force monotonically. When the depth of the substrate is shallow, there are two continuous phase transitions. When the potential well depth slightly increases, the phase transition from the pinned to the disordered plastic flow states is continuous; however, the phase transition from the disordered plastic flow to the moving ordered states is discontinuous. When the substrate is even deeper, the phase transition from the pinned to the disordered plastic flow states changes to discontinuous. When the depth of the substrate further increases, as the driving force increases, the pinned state changes to the moving ordered state directly, so that the disordered plastic flow state disappears completely.

Mechanical study on the wellbore stability of horizontal wells in natural gas hydrate reservoirs

Currently, the exploration of natural gas hydrates (NGHs) is regarded as a worldwide research focus. Due to multiple characteristics of NGH reservoirs (such as weak bonding, low strength and high degradability), wellbore instability is extremely likely to occur during drilling operations. Based on the thermodynamic characteristics of NGHs, a heat–fluid–solid coupling numerical model of horizontal wells in NGH reservoirs that considers the decomposition of hydrates was established. Additionally, the evolution of stress fields and plastic zones induced by changes in the temperature–pressure field and the mechanical properties of NGHs before and after their decomposition were analyzed. The results showed that stress was reduced in the zone where NGHs were completely decomposed, due to the weakening of elastic parameters, and that the strata close to the wellbore were in a state of plastic flow because of stress concentration there. Moreover, the stress was dramatically increased in the transition zone for the decomposition of NGHs due to the change in the mechanical properties of the strata. Hence, the secondary stress concentration was formed in the strata where NGHs did not undergo decomposition, and part of this strata would be subjected to plastic deformation. With the further decomposition of NGHS, the extent of the strata with completely decomposed NGHs in the plastic stage was also increased, and the secondary stress concentration effect formed in the zone with NGHs that did not undergo decomposition, in the strata far from the wellbore. If NGHs were decomposed under the selected drilling conditions, the small wellbore was more affected by the decomposition of hydrates, with a greater risk of collapse. Moreover, the effect of the wellbore size became more significant with increased drilling time; thus, selecting a large wellbore for drilling was more favorable for the stability of the wellbore.

Analysis on pre-peak deformation and energy dissipation characteristics of sandstone under triaxial cyclic loading

Based on conventional triaxial cyclic loading–unloading compression tests, the axial and lateral elastic–plastic strains of sandstone under confining pressures of 5, 10, 20 and 30 MPa were analyzed. The results showed that the lateral stress–strain plastic hysteresis loop is more obvious than the axial direction under cyclic loading. The axial and lateral elastic strains of the specimens are nonlinearly related to the axial deviatoric stress, and the axial and lateral plastic strains of the specimens show obvious non-linear plastic hardening characteristics. The Duncan–Chang model is used to analyze the elastic stress–strain characteristics of sandstone during loading. Without considering the initial stage of fracture compaction, the tangent elastic modulus of the sandstone specimens decreases continuously with the increase of the axial elastic strain and the Poisson’s ratio increases continuously with the increase of lateral elastic strain under the same confining pressure. Based on the plastic strain characteristics of sandstone specimens, the corresponding plastic flow equation is established, which can preferably describe the axial and lateral strain characteristics of the specimen. The plastic flow rate of the specimens increases nonlinearly with the increase of the axial deviatoric stress, which approaches the ideal plastic flow state near the peak strength.

Discrete element analysis of structural characteristics of stepped reinforced soil retaining wall

Based on the laboratory large scale model test of the stepped reinforced soil retaining wall (RSRW), the discrete element numerical model of stepped RSRW is set up by particle flow code. The displacement of particles, stress distribution, force chain distribution, and potential failure surface of the stepped RSRW are analyzed by simulating the loading process. The results show that the geogrid can confine the horizontal displacement of soil and reinforce soil effectively. The geogrid endures the soil pressure of upside and transmits the pressure to surrounding soil, and improves the distribution of vertical soil pressure of the stepped RSRW. The number of strong force chains in the reinforced area is obviously higher than in the unreinforced area, which verifies that the geogrid can strengthen the soil. The soil particles have been in a plastic flow state and formed a continuous sliding surface at the front of retaining wall, but there was no obvious crack in the retaining wall. The potential failure surface of the stepped RSRW can be determined by observing the changes of force chain structure. The numerical simulation result of the potential failure surface is in good agreement with the theoretical calculation.

Fractional-order model of the compressive strength of hydraulic concrete in a real temperature and humidity environment

Concrete is a kind of composite material that is in a plastic flow state in its initial stage, gradually hardening with the cement hydration reaction and then finally reaching a relatively stable condition. In addition, the compressive strength of concrete is related to not only the curing age but also the curing temperature and humidity. The Kelvin model with a software element is used to describe the growth rule of the compressive strength of concrete. The equivalent age theory is used to consider the coupling effect of the temperature and humidity, and then a fractional-order model of the compressive strength of hydraulic concrete considering this coupling effect is proposed. Next, tests of the compressive strength of concrete under three different curing conditions for different concrete ages are performed, and a fractional-order model of the compressive strength is implemented using the test data. The analysis results show that the fitting effect of the fractional-order model can reach a high level of accuracy with only a few model parameters, and with a better fit than the combinatorial exponential model or combinatorial modified logarithmic model.

Investigation of hydrogeomechanical parameters of loess massifs in conditions of technogenic underflooding and development of technical recommendations for strengthening of bases of foundations

Based on the analysis of actual data and the results of numerical modeling, dependencies of deformations of the investigated residential complex due to technogenic moistening of a loess massif of soils are investigated. It was established that a dynamics of subsidence of bench marks in time is closely correlated with a moistening mode. In order to form a picture of dynamics of development of moistening phases of the massif and a quantitative estimation of main factors of their formation, a numerical model of the loess massif was built, based on the finite element approximation of the section of built-up area of the residential complex. Stability of a soil massif was estimated by the character of development of plastic deformation zones. Analysis of a stress-strain state of a rock massif indicated that water-saturated soils are partially in a state of plastic flow in a base. The development of shear deformations is most characteristic within a zone of the main moistening, as well as a boundary of its front. Development of rupture disturbances at the edge of the contour of buildings corresponds with the formation of tear cracks. An intensification of subsidence of buildings with simultaneous frontal and subvertical technogenic moistening of loess soils can be noted. Model estimations of a stress-strain state of the pile foundation, considering the uneven subsidence that occurred along the perimeter of residential buildings, show that the elastic mode of their deformation has not been exhausted. Options of redistribution of loads from residential sections onto an additional pile field, regulated base moistening and grouting of soils are considered as engineering measures to prevent further deformation of the residential complex. Stabilization of a soil base by means of high-pressure cementation is the most acceptable in the present conditions. Technological scheme of cementation of the soil base is recommended, as well as measures after the base stabilization, such as monitoring of further deformations of the complex itself and parking structures, and possibilities of constructing auxiliary drainage.

Probing the limits of metal plasticity with molecular dynamics simulations.

Ordinarily, the strength and plasticity properties of a metal are defined by dislocations-line defects in the crystal lattice whose motion results in material slippage along lattice planes. Dislocation dynamics models are usually used as mesoscale proxies for true atomistic dynamics, which are computationally expensive to perform routinely. However, atomistic simulations accurately capture every possible mechanism of material response, resolving every "jiggle and wiggle" of atomic motion, whereas dislocation dynamics models do not. Here we present fully dynamic atomistic simulations of bulk single-crystal plasticity in the body-centred-cubic metal tantalum. Our goal is to quantify the conditions under which the limits of dislocation-mediated plasticity are reached and to understand what happens to the metal beyond any such limit. In our simulations, the metal is compressed at ultrahigh strain rates along its [001] crystal axis under conditions of constant pressure, temperature and strain rate. To address the complexity of crystal plasticity processes on the length scales (85-340 nm) and timescales (1 ns-1μs) that we examine, we use recently developed methods of in situ computational microscopy to recast the enormous amount of transient trajectory data generated in our simulations into a form that can be analysed by a human. Our simulations predict that, on reaching certain limiting conditions of strain, dislocations alone can no longer relieve mechanical loads; instead, another mechanism, known as deformation twinning (the sudden re-orientation of the crystal lattice), takes over as the dominant mode of dynamic response. Below this limit, the metal assumes a strain-path-independent steady state of plastic flow in which the flow stress and the dislocation density remain constant as long as the conditions of straining thereafter remain unchanged. In this distinct state, tantalum flows like a viscous fluid while retaining its crystal lattice and remaining a strong and stiff metal.

ТРЕЩИНООБРАЗОВАНИЕ В ЛИСТОВОМ ЭЛЕМЕНТЕ КОНСТРУКЦИИ ПРИ НЕОДНОРОДНОМ НАПРЯЖЕННОМ ПОЛЕ

We give a mathematical description of calculation model for cracking in sheet structural element under a non-uniform stress field. The model of pre-fracture zones in state of plastic flow under constant stresses was accepted. The boundary value problem for interaction of zones of weakened interparticle material bonds in the sheet structural element under influence of the inhomogeneous stress field is reduced to a system of singular integral equations. The integral equations further reduce to a system of nonlinear algebraic equations for solution of which we use the method of successive approximations. Sizes of the prefracture zones and limit value of external loads at which in the sheet structural element the cracking occurs are found.

Cracking in heat-releasing perforated material

A model of cracking in fuel isotropic material weakened by a doubly periodic system of cooling cylindrical channels of round cross section is mathematically described. It is assumed that with increasing intensity of the heat in the material its elastic properties become temperature dependent, and nucleation of cracks and failure of the material occur. It is assumed that the zone of cracks nucleation is a finite length layer containing a material with partially broken connections between separate structural elements in plastic flow state at constant stress. The determination of unknown parameters characterizing the initiating crack is reduced to the solution of a singular integral equation. An analysis of the ultimate state of the zone of the weakened interparticle bonds of the material is realized on the basis of the criterion of critical opening of prefracture zone faces.

TECTONOPHYSICAL SIGNS OF THE FORMATION OF STRONG EARTHQUAKE FOCI IN SEISMIC ZONES OF CENTRAL ASIA

Vibrations of the Earth crust and variations in the physical fields of the Earth atmosphere and ionosphere are continuously monitored by a variety of techniques and specialized facilities across the world. Nevertheless, most catastrophic earthquakes even in this century have occurred in “incidental” or “unexpected” places in “unpredicted” time. Earthquake predictions have errors as the current knowledge of focal mechanisms of strong (M≥8) earthquakes is still insufficient. It is believed today that the most common source of earthquakes is movement of rock blocks along a fault/megafracture. Such movements take place in a stepwise pattern with high or reduced friction, depending on the presence of fluids, hitches on the fault planes and other factors. Modern seismic forecasting is based on the concept of precursors. The author considers geological and geophysical settings in areas of dynamic influence of faults, wherein 8>М>7.5 earthquakes took place. Based on earthquake recurrence curves constructed for such areas, four tectonic criteria for formation of strong earthquake sources are identified: structural (large seismically active faults), kinematic (large amplitudes of the fault wing’s displacements), rheological (physical properties of the fault infill material, such as low viscosity of the intra-fault medium) and dynamic (high rates of the fault wing’s displacements) criteria. These criteria should be in the focus of quantitative studies in order to provide a solid scientific basis for long-term forecasting of strong earthquakes. Curves constructed for the criteria can show changes in the physics of earthquake foci in case of strong seismic events. With account of the tectonophysical features of faults associated with strong seismic events, the following conclusions are drawn. (1) In the continental lithosphere, catastrophic earthquakes (M≥8) occur in areas of dynamic influence of the major faults in the lithosphere in case of relatively high amplitudes of displacements of boundary blocks (i.e. fault wings). (2) In the relatively stable stress field, high amplitude displacements take place in case of reduced viscosity/quasi-viscosity of the medium comprising the internal structure of faults. (3) Reduced viscosity of the intra-fault medium is related to the physical conditions of transition of rocks in fault zones (mainly along the fault planes) in the state of quasi-plastic or plastic flow (unilateral pressure in excess of hydrostatic pressure, and relatively decreasing strength properties of the intra-fault medium with increasing length of the fault wings). (4) Reduced viscosity of the fault zone leads to an increase in the displacement rate of the fault wings in the constant stress field. The latter factor is the main one, transforming seismically active faults with M≤7.5 seismic events into faults of similar characteristics, but with earthquakes of higher energy, M≥8. Focal mechanisms of such earthquakes are associated with conditions for a potential increase of the displacement amplitude regardless of the presence of fluids, hitches on the fault planes and other poorly predictable factors. In-depth studies of the internal structure of faults with M≥8 earthquakes, their foci, conditions of the temporal regime of the seismic process before and after strong seismic events can discover a key to understanding the origin of earthquake sources, the criteria of energy release, and the occurrence of earthquakes with maximum energy. Further steps to develop the geological and geophysical (including tectonic) criteria for prediction of strong earthquakes should be focused on more detailed research of seismic zones wherein strong earthquakes were recorded.

Stochastic approach to plasticity and yield in amorphous solids

We focus on the probability distribution function (PDF) P(Δγ;γ) where Δγ are the measured strain intervals between plastic events in a athermal strained amorphous solids, and γ measures the accumulated strain. The tail of this distribution as Δγ→0 (in the thermodynamic limit) scales like Δγ(η). The exponent η is related via scaling relations to the tail of the PDF of the eigenvalues of the plastic modes of the Hessian matrix P(λ) which scales like λ(θ), η=(θ-1)/2. The numerical values of η or θ can be determined easily in the unstrained material and in the yielded state of plastic flow. Special care is called for in the determination of these exponents between these states as γ increases. Determining the γ dependence of the PDF P(Δγ;γ) can shed important light on plasticity and yield. We conclude that the PDF's of both Δγ and λ are not continuous functions of γ. In slowly quenched amorphous solids they undergo two discontinuous transitions, first at γ=0(+) and then at the yield point γ=γ(Y) to plastic flow. In quickly quenched amorphous solids the second transition is smeared out due to the nonexisting stress peak before yield. The nature of these transitions and scaling relations with the system size dependence of 〈Δγ〉 are discussed.

420 粒子法を用いたAl-Cu接合のFSWシミュレーション(OS4-(5),OS4 オーガナイズドセッション≪材料・組織と加工≫)

In FSW, unconjugated material undergoing stirring with heat generated by the rotation of the tool it is difficult to observe the state of plastic flow directly. In this study, it was visualized by particle method simulating the flow properties and the temperature distribution of the material at the time of Al-Cu bonding by the metal during processing is regarded as a highly viscous fluid by using a particle method. Also, when comparing the results with and without cooling due to backing strip, defects and burrs at the bonded compared to none in the case of cooling there will be less revealed. Therefore, potential cooling effect by backing gold is large and involved parameters to joining by FSW have been suggested.

Cracking in a circular disk under mixed boundary conditions

A model of cracking in a circular disk with mixed boundary conditions on the boundary is suggested. It is assumed that the cracking process zone is a finite length layer containing a material with partially disturbed bonds between separate structural elements in the plastic flow state at constant stress. Limit equilibrium analysis of the zone of weakened interparticle bonds of the material (prefracture) is performed on the basis of the criterion of critical opening of prefracture zone faces.

The new clay mud and its improvement effects of tunnels

During tunneling process by earth pressure balance (EPB) shield, the strata containing large amounts of sand and gravel are often encountered. The excavated soil in the shield chamber has poor plastic flow and larger permeability, so it leads to the difficult construction of the EPB tunnel. In order to ensure tunneling advance successfully, the excavated soil must be improved by adding modified materials to change its physical and mechanical properties, and make it to have a good plastic flow state, low friction angle and low permeability. Soil improvement techniques can guarantee the stability of excavation face, achieve the balance advance of shield tunnel, reduce machinery load and the ground settlement, and improve the excavation speed. The existing mud used in engineering practice is the dispersing mud and water system whose compositions are bentonite and water. Both the composition and function are simple. The existing mud has various problems, such as large amounts of waste mud emission, difficulties in controlling mud indicators, large areas for mud treatment and large uses of new mud materials and environmental pollution. In this paper, a kind of environmentally friendly clay mud was developed, and the laboratory and field tests were conducted to verify its adaptability and superiority.

Parameter Design of Anti-Slide Pile Control Method in Floor Heave

Floor heave is a kind of deformation form that rock and soil mass of roadway floor enter a plastic flow state under surrounding stress. Installing anti-slide piles is an effective method to control floor heave. Considering the stress conditions and the characteristic of slip line fields in geometry size when floor heave emerges, some parameters used to design anti-slide piles are proposed to control floor heave, which will provide theoretical basis for further designs of roadway support.