Constant Stress Conditions Research Articles

Coal permeability evolution characteristics: Analysis under different loading conditions

AbstractThe drainage and utilization of coalbed methane (CBM) resources can not only ensure the safe production of coal mines but also can reduce greenhouse gas emissions and protect the environment. Coal permeability is the key factor that affects the CBM drainage efficiency. To understand the coal permeability evolution characteristics, coal specimens reconstituted by coal powder with different particle sizes were prepared and their permeability under loading conditions was investigated. The results indicate that the coal permeability evolution laws measured by different gases are completely different under constant hydrostatic pressure conditions due to the influence of effective stress and the coal matrix sorption‐induced deformation. Under constant effective stress conditions, the helium permeability of coals is almost unchanged if the effects of the Biot's coefficient of coals are ignored, but the methane permeability of coals decreases with increasing gas pressure. In the complete stress–strain process, the variation of coal permeability as the axial strain increases at different stages is almost completely different and the coal permeability in the residual plastic flow stage increases by 2.4–71 times, 1.5–39 times, and 2.8–116 times that of the initial state under the different boundary conditions. Furthermore, it is also found that the coal permeability evolution laws are determined by the effective stress, the sorption‐induced deformation of coal matrices, and the malconformation of gas adsorption in coals. This study can help us better understand the seepage characteristics of gas in coals and guide the CBM development. © 2020 Society of Chemical Industry and John Wiley & Sons, Ltd.

Thermo-poromechanics-based viscoplastic damage constitutive model for saturated frozen soil

This study is primarily intended to present a viscoplastic damage theory for saturated frozen soil based on thermo-poromechanics. On the basis of the irreversible process of thermodynamics, an elasto-viscoplastic damage constitutive theoretical framework for saturated frozen soils is formulated by use of the Lagrangian saturation and solid-fluid interface interactions, for which the upscaling microscopic properties of saturated frozen soil in plasticity are provided to elucidate the macro stress-strain relation. Furthermore, a corresponding constitutive model for simulating the cryogenic triaxial stress state of saturated frozen soils is presented to take the influence of creep and loading strain rate into account. Compared with the cryogenic triaxial tests on saturated frozen soils, it is demonstrated that the time-dependent model proposed here can simulate the salient properties of frozen soils under constant shear stress and loading strain rate conditions. The constitutive theory proposed here provides a potential basis for analyzing the stress deformation considering the effects of creep and loading strain rate for saturated frozen soils in cold regions.

An Improved Coal Permeability Model with Variable Cleat Width and Klinkenberg Coefficient

The evolution of permeability in coal seams plays an important role in the exploitation of coalbed methane, and the Klinkenberg effect is an important factor affecting the permeability. In most studies, the Klinkenberg coefficient is considered a constant although it is closely related to the cleat width which changes dynamically due to the competing processes of change in effective stress and adsorption induced deformation. In this work, based on the double strain spring model and the effective stress principle, a new stress–strain relationship is proposed for cleat space. In addition, the influence of the adsorption induced swelling effect on coal deformation is considered. By considering both stress-induced strain and adsorption-induced strain, the cleat width model and Klinkenberg coefficient model are established. Finally, we develop an improved permeability model with variable cleat width and Klinkenberg coefficient based on cubic law. This improved permeability model is compared with experimental data and several classical models under conditions of constant effective stress and constant confining stress. Furthermore, the relationship between Klinkenberg coefficient and pore pressure under different conditions is analyzed. Our results indicate that the improved apparent permeability model matches the experimental data well and has a wider range of applications.

Creep behavior due to interface diffusion in unidirectional fiber-reinforced metal matrix composites under general loading conditions: a micromechanics analysis

In this paper, the creep behavior due to interface diffusion in unidirectional fiber-reinforced metal matrix composites under general transverse loading conditions is studied. A micromechanics model is adopted to estimate the average stresses in fibers, which is demonstrated to be of reasonable accuracy by comparing with finite element analysis. The creep deformation due to interface diffusion is characterized by a tensor-form creep strain rate, which is related to the components of average stress in fibers and possesses incompressibility. The derived creep strain tensor is then introduced into fibers as eigenstrain to estimate the effect of creep strain on stress redistribution in composites. Eventually, the macroscopic creep strain of composites under constant external loads and the stress relaxation at fixed displacements due to interface diffusion are calculated by the incremental creep analysis procedures based on the average field theory. The effects of loading conditions on the overall creep behavior are examined in detail. Results of this study show that under the fixed strain condition, the initial biaxial stress ratio actually varies rather than remains unchanged during the stress relaxation process. In both constant stress and constant strain conditions, stresses in fibers have a propensity to approach hydrostatic stress state, thus suppressing the interface diffusion. These findings were not reported in previous researches and may bring new understanding on diffusion-induced creep deformation in metal matrix composite materials.

A Mathematical Model of Gas and Water Flow in a Swelling Geomaterial—Part 1. Verification with Analytical Solution

Gas generation and migration are important processes that must be considered in a safety case for a deep geological repository (DGR) for the long-term containment of radioactive waste. Expansive soils, such as bentonite-based materials, are widely considered as sealing materials. Understanding their long-term performance as barriers to mitigate gas migration is vital in the design and long-term safety assessment of a DGR. Development and the application of numerical models are key to understanding the processes involved in gas migration. This study builds upon the authors’ previous work for developing a hydro-mechanical mathematical model for migration of gas through a low-permeable geomaterial based on the theoretical framework of poromechanics through the contribution of model verification. The study first derives analytical solutions for a 1D steady-state gas flow and 1D transient gas flow problem. Using the finite element method, the model is used to simulate 1D flow through a confined cylindrical sample of near-saturated low-permeable soil under a constant volume boundary stress condition. Verification of the numerical model is performed by comparing the pore-gas pressure evolution and stress evolution to that of the results of the analytical solution. The results of the numerical model closely matched those of the analytical solutions. Future studies will attempt to improve upon the model complexity and investigate processes and material characteristics that can enhance gas migration in a nearly saturated swelling geomaterial.

A Mathematical Model of Gas and Water Flow in a Swelling Geomaterial—Part 2. Process Simulation

Gases can potentially generate in a deep geological repository (DGR) for the long-term containment of radioactive waste. Natural and engineered barriers provide containment of the waste by mitigating contaminant migration. However, if gas pressures exceed the mechanical strength of these barriers, preferential flow pathways for both the gases and the porewater could form, providing a source of potential exposure to people and the environment. Expansive soils, such as bentonite-based materials, are widely considered as sealing materials. Understanding the long-term performance of these seals as barriers against gas migration is an important component in the design and the long-term safety assessment of a DGR. This study proposes a hydro-mechanical mathematical model for migration of gas through a low-permeable swelling geomaterial based on the theoretical framework of poromechanics. Using the finite element method, the model is used to simulate 1D flow through a confined cylindrical sample of near-saturated low-permeable soil under a constant volume boundary stress condition. The study expands upon previous work by the authors by assessing the influence of heterogeneity, the Klinkenberg “slip flow” effect, and a swelling stress on flow behavior. Based on the results, this study provides fundamental insight into a number of factors that may influence two-phase flow.

Coal Permeability Evolution Under Different Water-Bearing Conditions

The seepage problem of coal-bed methane (CBM) has attracted increasing attention from the research and industry communities. Coal permeability, which is a key parameter for CBM production, has been extensively studied through both laboratory and field tests, mathematical modeling, and numerical simulations. However, relatively little work has been done to investigate the effect of matrix water content on coal permeability evolution. Because most coal seams contain water, it may have direct impact on the CBM seepage capacity. Therefore, in this paper, CBM seepage at different matrix water contents was investigated experimentally. Results demonstrate that coal permeability decreases with the increase in water content. It was also found that both pore pressure and water content can greatly affect CBM migration characteristics and that the sorption strain and permeability were mostly controlled by pore pressure. In addition, water can have an impact on gas adsorption and sorption-induced deformation, which can further affect seepage channel width, leading to change in coal permeability. In general, coal permeability was observed to decrease exponentially with the increase in water content and pore pressure. Moreover, a coal adsorption model and an adsorption-permeability model were established, considering water content and excess adsorption under constant external stress and triaxial strain conditions. The proposed permeability model showed good agreement with the experimental results. The present study provides for better understanding of coal permeability evolution in a water-bearing condition and for developing an improved model to simulate the CBM migration process under such condition more accurately.

Modeling interaction between loading-induced and creep strains of rockfill materials using a hardening elastoplastic constitutive model

An elastoplastic constitutive model that takes into account the stress–strain relationship and creep-induced hardening behavior of rockfill materials is proposed in light of previous experimental observations. It is assumed that the mechanical response during loading and the final amounts of creep strains under a constant stress state are independent of the strain rate. The focus of the proposed model is the coupling effect between loading and creep, including the influence of loading history on subsequent creep strains and the influence of creep history on subsequent loading behavior. An extended yield function, which allows flexible control over the shape of yield surfaces, is used not only to distinguish among loading, unloading, and neutral loading, but also to manipulate the creep-induced hardening using a plastic strains–based hardening parameter. A stress-dependent dilatancy equation is used, instead of a plastic potential function, to define the directions of plastic strains during loading. The hardening law is established based on three different types of experimental results. Only routine experiments are required for calibration of model parameters, and the model can be used in a reduced form according to the available test results. The model is verified using typical experimental data and is found to be capable of capturing important behavior of rockfill materials, such as pressure-dependent strength, shear contraction and dilation, and creep-induced stiffening.

Impact of shale matrix mechanical interactions on gas transport during production

Shale gas has played an increasingly important role in world energy supply in recent years. The gas transport process in the shale matrix is particularly important as it affects the long-term production behavior of shale gas. The shale matrix consists of inorganic minerals and embedded organic matter with mass transfer influenced by mechanical interactions. We develop a micro-scale discrete coupled model to explicitly account for mass transfer and mechanical interactions, and how these interactions affect the gas transport characteristics of both components. Specifically, the model comprises organic matter embedded within inorganic minerals. The proposed model is implemented and solved within the framework of COMSOL Multiphysics (Version 5.4). The model is first verified against the results of a desorption-diffusion-seepage coupled experiment and then extended to field scale. It is found that the impacts of gas pressure and effective strain are different for the gas seepage in inorganic minerals and gas diffusion in organic matter. Gas pressure may enhance the permeability of the inorganic phase due to the gas slippage effect while compactive effective strain decreases permeability during the gas depletion process. For gas diffusion in the organic matter, surface diffusion decreases and effective diffusion coefficient increases with declining gas pressure. While the impacts of effective strain on the effective diffusion are dependent on the external boundary conditions, the effective diffusion coefficient is lower than the initial value under constant stress conditions and larger than the initial value under constant volume conditions. The proposed model provides a complementary method to conventional continuum dual-media approaches and provides a clear understanding of the interactions between the two components. Field-observed oscillatory gas production data is readily history-matched and key mechanisms explained with the proposed approach presented in this work.

Acceleration of pearlite transformation in a high-carbon steel by uniaxial compressive stress confirmed by volume measurements

Abstract Effect of external stress on pearlite transformation in a high-carbon steel was examined in terms of the measurement of volume change during isothermal holding. Two dimensional measurement of the steel holding at various temperatures under constant stress condition clarified anisotropic transformation plasticity, which means that the volume measurements are necessary to examine the transformation behaviors quantitatively. According to the measurement of volume change, it was found that the larger loading brings the shorter incubation time when compared at the same transformation temperature. These changes can be represented by the Johnson-Mehl-Avrami-Kolmogorov type equation with four constants when the transformation temperature is adequately deviated from the bainite start temperature.

Creep of Metallic Materials in Bending

The creep behavior of metallic materials in bending has received limited attention because of the complexity of the stress state and the nontrivial correlation with an equivalent uniaxially deformed state. Furthermore, conventional creep testing methods, i.e., under uniaxial tension and compression, which have a constant stress state during creep and well-established data interpretation protocols, are adequate for studying the creep properties of most materials. Hence, the creep properties of metallic systems have rarely been evaluated in bending. Currently, there is an increase in the demand for testing in-service components and materials having microstructures on small length scales (e.g., a few tens to hundreds of micrometers). In this situation, the material for testing is often in short supply and the dimensions of the relevant samples are very small, thus limiting the feasibility of uniaxial tests. This warrants development of alternate testing techniques, such as indentation and bending, that are ideally suited for testing small-volume samples. In particular, cantilever bending is quite attractive given the possibility of obtaining multiple data points covering a range of stresses from a single sample and the ease of sample fabrication, alignment, and gripping while testing at a small length scale. In addition, the mechanics of power-law creep in bending is well understood and developed. We present herein a review of seminal literature on power-law creep in bending, a topic which has been investigated for almost 90 years now, and an outlook on adopting bending of cantilevers as a mainstream methodology for characterizing the creep behavior of metallic systems.

Damage mechanism analysis of a high-strength dual-phase steel sheet with optimized fracture samples for various stress states and loading rates

In this study, it is aimed to investigate the damage mechanisms of a DP1000 steel with very fine-grain microstructure and further study the effect of the stress states and loading rates on the damage mechanism. To ensure clear and systematic conclusions are drawn about the effect of stress state on the damage mechanism, careful study is performed on achieving a constant stress state of the fracture initiation spot during the entire deformation history by designing a fracture specimen family for various stress states. The specimens feature an identical dog-bone specimen layout with only differences at the detailed notches and cuts to ease the experimental and analysis effort. After testing the optimized fracture specimens from quasi-static to intermediate loading rates, the scanning electron microscopy is employed to analyze the damage mechanism of the investigated material. Both dimple and shear fracture modes are observed in DP1000 and the underlying damage mechanisms are ferrite-martensite interphase debonding and martensite cracking. By systemic analysis, it is found that the stress state and loading rate have distinct influences on triggering the dominant fracture mode and the underlying damage mechanism. Furthermore, the damage mechanism of DP1000 is compared with DP600 steel and significantly different behavior is observed for two types of dual-phase steels.

The Characteristics of Creep in Metallic Materials Processed by Severe Plastic Deformation

Processing through the application of severe plastic deformation (SPD), as in equal-channel angular pressing (ECAP), provides an opportunity for achieving exceptional grain refinement to the submicrometer or even the nanometer level. After SPD processing, these materials may be conveniently used for evaluating the effect of a reduced grain size on the creep behaviour at elevated temperatures under testing conditions of constant load or constant stress. This report provides an overview of the creep properties of ECAP-processed metals with an emphasis on the microstructural characteristics developed by SPD, on their thermal stability and especially on the creep mechanisms that control their flow behaviour. For convenience, these properties are generally compared with the creep behaviour of coarse-grained (CG) samples of the same materials tested under identical conditions.

Induced water stress affects seed germination response and root anatomy in Robinia pseudoacacia (Fabaceae)

The different germination behaviours of the seeds under induced water limitations may be related to the different adaptive capacities acquired at the diverse collection sites, as a response to the different environmental parameters. The island of Pianosa resulted the most performant in term of germination responses and the co-occurrence of xeromorphic anatomical evidences at root level confirmed this trend. Regeneration from seeds is an important co-determinant in the invasion ecology of black locust. In the attempt of providing new information on its invasion potential in Mediterranean Europe under the future scenario of global warming, we investigated the effects of induced water-deficit regimes on: (1) seed germination performance and (2) root growth and anatomy. Ripe seeds were collected from four populations established in Tuscany (Central Italy): mechanically scarified seeds were incubated in a range (− 0.2/− 0.8 MPa) of constant water stress conditions at 21 °C. The final germination rate drastically declined with increasing induced water-deficit conditions, with the highest value at control and at − 0.2 MPa (ca 50–97%), and the lowest at − 0.6 MPa (ca 10–33%). The mean germination time decreased with increasing water stress. At root level, xeromorphism relies on the combination of different anatomical traits which co-optimize water uptake/loss: thinner roots, higher number of xylem vessels, vessels with small-sized lumen and thinner cell walls. Seeds collected in sites characterized by different environmental parameters display a noteworthy difference in the germination dynamics: as far as the beginning and ongoing of germination, as well as the germination response in time is concerned, the seeds from the island of Pianosa showed the highest performance; the major arid conditions in Pianosa could have caused a “stress imprint” able to facilitate a fast and protective response to future drought events. As a whole, our results confirmed the great phenotypic plasticity of black locust as a response to variable water availability and provided evidence for the potential high germination capacity in drier environments, as seems to be the future Mediterranean Europe.

Flow-induced alterations of ultrasonic signatures and fracture aperture under constant state of stress in a single-fractured rock

Fluid-fracture surface interaction, caused by different mechanisms, is one of the underlying reasons for permeability reduction over long period of time in different georesources, such as deep geothermal systems and shale gas/oil reservoirs. The sensitivity of the ultrasonic signatures (e.g., frequency content, velocity, amplitude, and attenuation) to the changes in fracture aperture caused by fluid-fracture surface interactions can be considered as a probe for flow-induced fracture aperture evolution. Flow-through tests on an artificially fractured phyllite specimen from a geothermal reservoir along with the concurrent measurements of ultrasonic signatures of P- and cross-polarized S-waves demonstrated the sensitivity of ultrasonic signatures to the evolution of fracture aperture/permeability under a constant state of stress (i.e., constant pore and confining pressures). In particular, the closure of the fracture and the decrease of permeability led to an increase of the P-wave velocity, a decrease of the P-wave attenuation, and an increase of the S-wave amplitude. In addition, time evolution of the time-frequency maps of the transmitted ultrasonic waves revealed that partitioning of the frequency content slightly changes because the fracture aperture/permeability is altered. Specifically, alterations in hydraulic aperture are reflected in the changes of time-frequency partitioning, whereas under constant hydraulic aperture, the time-frequency partitioning is unaltered.

WOMEN OF UKRAINIAN ANTI-NAZI UNDERGROUND REFLECTED IN HISTORICAL ANTROPOLOGY

Purpose. The purpose of this paper is to study the behaviour of women in extreme conditions, to establish various social roles of women, ways of their adaptation to extreme conditions on the example of Ukrainian anti-Nazi underground during Second World War. Theoretical basis. The authors derive from the fact that historical anhropology is a leading and promising area of historical research. Originality. For the first time, the authors have shown that in conditions of constant stress state and direct threat to life and health, the behaviour of underground women was determined by adequate survival strategies, i.e. by the life circumstances and personal qualities of the underground members. Conclusions. Permanent existence on the verge of life and death often led women to emotional actions, deviant forms of behaviour. Their actions could be controversial, and the motives – confusing. At the same time, the majority of women left in the Soviet underground chose as a strategy of survival the path of confrontation with occupiers, held true to their choices, often showed courage, ingenuity and self-sacrifice.

Correction of torque transfer lag in magnetic coupling rheological test system.

In this study, a mathematical model for the magnetic coupling transmission process was set up to solve the problem of torque transfer lag in magnetic-coupled rheological testing systems. This model was developed on the basis of torque balance in a magnetic coupling rotatory rheometer test system, which considered friction loss for the jewel bearing, as well as the inertia of both the motor and fixture. Taking the HAAKE-MARS60 high-pressure rheometer as an example, the rheological properties of Newtonian fluid at the initial measuring stage under controlling constant stress conditions were tested to verify the accuracy of the model and using the model to modify the rheological test results. The results show that the load torque mainly influences the alternating frequency of the rotational speed and the hysteresis degree of the inner and outer magnetic rings, the larger the load torque, the greater the deviation of the magnetic coupling test results of the rotational rheometer. The results of the apparent viscosity test are modified under different loading conditions, and the rheological curves of the initial shear phase of the Newtonian fluid are coincident with the steady-state test results, showing the true viscosity of the Newtonian fluid, which conforms to the cognition of Newtonian fluid rheology being independent of time. The results of the initial start test of gelling crude oil were modified, the yield stress and yield time were reduced, and the lower the test temperature and the higher the shear rate, the more obvious the correction effect was. Under different shear rate conditions, the yield strain corresponding to the modified yield stress was close and the yield strain was approximately not changed with the shear rate. The results can provide a basis for understanding the rheological properties of materials in the start-up transient process under high pressure conditions.

Surface‐Modified Nanostructured Piezoelectric Device as a Cost‐Effective Transducer for Energy and Biomedicine

Flexible polymer–metal oxide nanocomposites with multiwalled carbon nanotube (MWCNT) films are fabricated using poly(vinylidene fluoride) (PVDF) as the bulk matrix material with three‐dimensional (3D) lithium‐doped zinc oxide (Li‐ZnO) as a filler. Li‐ZnO is synthesized hydrothermally followed by surface modification with polyethylene glycol (PEG). PEG coating serves as an effective solution for avoiding costly electrical poling and enhances the proportion of the PVDF β phase, while MWCNTs act to increase conductivity and to reinforce the composite during mechanical stressing. The piezoelectric composite is fabricated with 12 wt% surface‐modified Li‐ZnO with 0.2 wt% MWCNT relative to the bulk PVDF. The fabricated composite is tested with different body motions and in different environments. The highest obtained value of open circuit voltage is 10.1 V and 2740 µA amperometric alternating current with bending motions. It is also observed that the electrical signal fluctuates by ≈200 µA due to microrelaxation and microstressing under constant stress conditions. The piezoelectric nanocomposite shows a linear response to a gradual increase in normal stress.

Study on the long-term behaviour of glass fibre in the tensile stress field

Abstract Glass fibre is a random network structure composed of [SiO4] tetrahedra. The structure contains a large number of defects, which act as crack initiation points. Under tensile stress, cracks undergo crack initiation, stable propagation, failure propagation, and fracture, and the stress that begins after unstable propagation is called the critical fracture stress. When the stress is less than the critical value, the crack is subject to the force of chemical bonds during the crack propagation process, and crack arrest occurs. When the stress is greater than the critical value, the glass fibre will undergo destructive fracture. In this paper, long-term tensile tests were carried out on glass fibre and a glass fibre composite under different constant tensile stress conditions. The fracture times of the glass fibre and glass fibre composite under different tensile stresses were obtained, the critical fracture stress of glass fibre was inferred, and the fracture mechanism was explained.

Mechanisms of wetting-induced loess slope failures

Frequent occurrence of landslides induced by rainfall or irrigation has seriously threatened the urban and rural development in the Loess Plateau, China. The increase in pore water pressure has been identified as a key factor for understanding wetting-induced loess slopes failures. However, experimental studies are limited regarding the increase in pore water pressure of the collapse loess from an initial negative value until failure occurs under constant total stress condition. An old landslide with progressive retreat development at an early stage of the construction of the Lvliang Airport was selected as a case study. Field surveys including exploration wells and boreholes revealed very fresh sliding shear planes and clearly visible cracks, suggesting the creeping movement of the old landslide. In case of heavy rain or long-term rainfall, this old landslide may be resurrected, threatening the stability of the airport site. To examine the mechanism of the failure induced by wetting for this unsaturated loess landslide, loess specimens were taken from the field, followed by performing a series of laboratory tests, including triaxial shear tests at constant matric suctions and wetting tests at constant deviator stresses. The test results revealed that the wetting-induced deformations of the loess included volume and shear deformations, reflecting compression and shearing behaviour induced by wetting. The failure behaviour of the loess along a wetting path was dependent on the stress level and the loss degree of matric suction as well as the hydro-mechanical path, and could be well described by the linear form of the Mohr-Coulomb strength theory. On this basis, the threshold value of the stress level was identified, which could be used to judge whether the wetting-induced failure of the loess occurs. The threshold value of matric suction at failure was also identified to analyse the loss degree of matric suction from stable conditions to failure. The mechanism of the failure of the soil due to wetting revealed from the present study could interpret the rainfall-induced landslide in unsaturated loess.