Dependence Of Shear Stress Research Articles

Features of Solid-Solution Hardening and Temperature Dependence of the Critical Shear Stress in Binary and Multicomponent Alloys

The paper analyses the hardening of binary and multicomponent solid solutions (including high-entropy alloys (HEAs)); addresses the notion of a compositional–cluster structure of binary solid solutions with unlimited solubility to propose an equation describing the concentration dependence of the critical shear stress; presents findings from a comparative analysis of the temperature dependences for critical shear stress (yield stress) for a series of binary and multicomponent solid solutions and pure metals with b.c.c. and f.c.c. lattices; considers potential mechanisms, which lead to a ‘plateau’ on the temperature dependence of critical shear stress for binary and multicomponent solid solutions and for pure metals; discusses the specifics of athermal hardening of HEAs and proposes a relatively simple equation for assessing their athermal hardening; and addresses the capabilities of using the x-ray diffraction to determine the root-mean-square displacements of atoms from ideal positions at crystal-lattice sites and crystal-lattice microdistortions in multicomponent solid solutions.

Local characteristics of bubbly flow in rods assembly 3 × 3

This study focuses on the experimental investigation of local characteristics of upward bubble flow in a 3 × 3 square rod bundle assembly. The experiments were conducted at Reynolds numbers ranging from 4000 to 11000 and gas void fractions up to 10%. The distribution of the gas phase around the central rod of the assembly was measured using a frontal conductivity probe, while the wall shear stress on the central rod was obtained through the electrodiffusion method. It is shown that the gas phase distribution does not exhibit the symmetry characteristic of the assembly cross-section. Furthermore, as the distance from the spacer grid increases, a reconfiguration of void fraction profiles is observed. This explains the non-monotonic dependence of wall shear stress and wall shear stress fluctuations on the distance to the spacer grid.

EXPERIMENTAL STUDY OF NON-EQUILIBRIUM RHEOLOGICAL EFFECTS DURING THE FLOW OF PARAFFINIC OILS OF TIMAN-PECHORA BASIN. THIXOTROPY, SUPER ANOMALOUS VISCOSITY, OSCILLATIONS OF SHEAR STRESS

Laboratory experiments were carried out to study the non-stationary and non-equilibrium flow regimes of some paraffinic and high paraffinic crude oils of the Timan-Pechora Basin in a rotational viscometer. The presence of thixotropic properties (shear thinnig and hy steresis loops on the flow curves) is shown. The threshold character of the dependence of the area of the hysteresis loop on the oil tempera ture is demonstrated. Experiments have shown that under certain conditions on the flow curves of the studied oils, there are areas of a decrease in shear stress with an increase in shear rate, similar to areas of a «super anomalous» viscosity (SAV) in plastic lubricants. The shape and size of these areas, i.e. the intensity of the «superanomality» manifestation strongly depends on the experiment conditions and are poorly reproducible from experiment to experiment. In experiments, the SAV phenomenon at temperatures below the gelation temperature was always observed when the sample of paraffinic oil was at rest for a sufficiently long time before deformation. If, however, it was in the gap between the viscometer cylinders after the end of the next measurement cycle for a short time (no more than a few minutes), then at the next cycle of recording the flow curve, the SAV phenomenon can be practically imperceptible, although the hysteresis remains. Experiments show that if a sample of high paraffinic oil is deformed at a constant low shear rate, i.e. constant angular velocity of rotation of the device cylinder, and a temperature below the temperature of gelation, then on the falling curve of the dependence of shear stress on time («shear thinning»), oscillations in shear stress with a period equal to the period of rotation of the viscometer cylinder can be observed.

Interfacial Interaction and Fatigue Behavior of Asphalt Mastics

Rheological characteristics of five road bitumens with different group composition and penetration at 25оС from 60 to 115х0.1 mm and asphalt binders based on them with volume content of filler (mineral powder of MP1 grade) – 0.275 (mass ratio bitumen: filler – 1:1) have been determined in the range from 30 to -10оС on dynamic shear rheometer. The influence of temperature and frequency on the parameter of interfacial interaction K-B-G* and thickness of adsorbed layer of original and thermal oxidative aging samples of asphalt mastic has been investigated. It is shown that in all samples K-B-G* decreases with decreasing temperature and increasing test frequency. A decrease in K-B-G* and adsorbed layer thickness in mastics after aging was observed in the case of bitumen with Gestel colloidal index CI=0.46–0.53, defined as CI=(S+A)(/R+Ar), and was stability of K-B-G* and adsorbed layer thickness in the case of bitumen with CI=0.61. No relationship was found between group chemical composition of bitumen and adsorbed layer thickness in original mastics. In aged mastic the greater thickness of adsorbed layer has samples based on bitumen with higher content of asphaltenes. The peculiarities of fatigue behavior of bitumen and mastic in the linear amplitude sweep test were investigated. The correlation between the thickness of the adsorbed layer and the angle of slope of the curves of dependence of the maximum shear stress (τmax) on the complex modulus (G*) was noted for aged samples.

Characterization of Interlaminar Friction during the Forming Processes of High-Performance Thermoplastic Composites

Friction is a pivotal factor influencing wrinkle formation in composite material shaping processes, particularly in novel thermoplastic composites like polyetheretherketone (PEEK) and low-melting polyaryletherketone (LM-PAEK) matrices reinforced with unidirectional carbon fibers. The aerospace sector lacks comprehensive data on the behavior of these materials under forming conditions, motivating this study’s objective to characterize the interlaminar friction of such high-performance thermoplastic composites across diverse temperatures and forming parameters. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were employed to analyze the thermomechanical behaviors of PEEK and LM-PAEK. These data guided friction tests covering room-to-forming temperatures. Horizontal pull-out fixed-plies tests were conducted to determine the friction coefficient and shear stress dependency concerning temperature, pressure, and pulling rate. Below the melting point, both materials adhered to Coulomb’s law for friction behavior. However, above the melting temperature, PEEK’s friction decreased while LM-PAEK’s friction increased with rising temperatures. These findings highlight the distinct responses of these materials to temperature variations, pulling rates, and pressures, emphasizing the need for further research on friction characterization around glass transition and melting temperatures to enhance our understanding of this phenomenon.

On the rheology of thixotropic and rheopexic suspensions: accounting of the trimers formation

Semi-empirical equations are derived that describe the dependence of shear stress on shear rate during the flow of a one-component suspension. The suspension is considered as consisting of three fractions: single grains of the solid phase, their dimers and trimers, between which reversible dimerization and trimerization reactions occur. In this case, dimerization and trimerization are considered as reactions with invariable rate constants, and dissociation of dimers and trimers as reverse reactions with rate constants that increase linearly with shear rate. The derived equations are based on the Krieger-Doherty formula generalized to the case of a multicomponent suspension. The equations describe well pseudoplasticity, dilatancy, thixotropy and rheopexy, as well as suspensions with variable behaviour (when pseudoplastic behaviour is replaced by dilatant behaviour, and thixotropic behaviour is replaced by rheopexic one and vice versa).

ДОСЛІДЖЕННЯ В’ЯЗКІСНО-ТЕМПЕРАТУРНИХ ВЛАСТИВОСТЕЙ ВАЖКИХ НАФТ ЯБЛУНІВСЬКОГО РОДОВИЩА УКРАЇНИ

Viscosity-temperature properties of high-viscosity oils from 3 wells of the Yablunovsky field (Poltava region, Ukraine) were studied using a rotary viscometer. According to the nature of the curves of the dependence of dynamic viscosity and shear stress on the shear rate, the nature of the flow of these oils is established, which is of practical importance for effect evaluating of various methods of action on the rheological behavior of these oils during their extraction and transportation.

On the Rheology of Thixotropic and Rheopexic Suspensions

Semi-empirical equations are derived that describe the dependence of shear stress on shear rate during the flow of a one-component suspension. The suspension is considered as consisting of two fractions: single grains of the solid phase and their dimers, between which a reversible dimerization reaction occurs. In this case, the dimerization of single grains is considered as a reaction with an invariable rate constant, and the dissociation of dimers is considered as an inverse reaction with a rate constant that increases linearly with the shear rate. The equations are based on the Krieger−Doherty formula, generalized to the case of a multicomponent suspension.

Influence of Dislocation Substructure on Size-Dependent Strength of High-Purity Aluminum Single-Crystal Micropillars

In order to understand the influence of dislocation substructures on the size-dependent strength (smaller is stronger) of micron-sized metals, we have fabricated single-crystal cylindrical micropillars with various diameters approximately ranging from 1 to 10 μm, which were prepared on the surface of the fully annealed sample and the subsequently cold-rolled samples of high-purity aluminum (Al). The annealed micropillars exhibited a size dependence of the resolved shear stress required for slip. The shear stress (τi) normalized by shear modulus (G) and the specimen diameter (d) normalized by Burgers vector (b) followed the correlation of τi/G=0.33(d/b)−0.63. The size-dependent strength was reduced by cold-rolling, resulting in lower power-law exponents (0.26~0.31) for the correlation in the cold-rolled specimens. The fine dislocation substructures introduced by the cold-rolling could be associated with the reduced size-dependent strength, which can be rationalized using the stochastic model of the dislocation source length in an assumption of homogenously distributed dislocations existing in the experimental micropillars. The inhomogeneous dislocation substructure with various dislocation cell sizes would contribute to a variation in the measured strength depending on the location, likely due to the probability of exiting the dislocation cell walls (local variation in dislocation density) in the micropillars fabricated on the cold-rolled Al samples.

Seismic amplification factors for low plasticity medium-stiff soil deposits

The seismic response of low plasticity medium-stiff soil deposits has been investigated through non-linear one-dimensional analyses carried out with reference to a set of 30 soil profiles, with bedrock depths varying in the range 20–60 m. More than 300 horizontal acceleration time histories recorded on almost flat rock outcropping sites have been used as input motions with amplitude, energy and frequency content varying in wide intervals, which should lead to outcomes of general validity. In the modelling of soil hysteretic behaviour, the backbone curve was described by a modified hyperbolic model calibrated against well-established stress-dependent relationships describing the variation of shear modulus and damping with the strain level. The analysis results show that the stratigraphic amplification factor is significantly influenced by the peak outcrop acceleration and bedrock depth while it is almost unaffected by magnitude sorting of the data; accordingly, original empirical predictive models have been proposed to estimate the stratigraphic amplification factor as a function of the peak outcrop acceleration and bedrock depth. Also, it has been demonstrated that, regardless of the amplitude of the input motion and whatever is the sorting of data with reference to magnitude and bedrock depth, a direct proportionality exists between the stratigraphic and the spectral amplification factors, implying an almost constant value of the spectral shape ratio. Finally, the relevance of using stress-dependent shear modulus and damping curves has been pointed out to allow a reliable evaluation of the stratigraphic and spectral amplifications.

Stress-Dependent Shear Strength of Resedimented Nile Silty Clay

The variation of clay undrained shear strength with consolidation stresses is of paramount importance to geotechnical engineers. For several decades, the normalized shear strength of clays has been the cornerstone of geotechnical design in many applications, such as staged construction of embankments. Recently, different trends of shear strength dependency on consolidation stresses have emerged indicating nonlinear variation. Resedimentation has been a useful and reliable method to investigate stress dependency by generating lab test specimens of fine-grained soils with a minimum sample to sample variation for benchmark studies. Previous work on resedimented Nile silty clay (RNSC) has shown unique features, such as high sand fraction and contradicting index properties used for soil classification. The shear strength of Nile silty clay (NSC) and the effects of the consolidation stresses are explored through a laboratory program on resedimented samples from material excavated within the Nile River flood plains. A portion of the material was processed using mechanical energy, which exhibited a significant increase in the liquid limit and a larger clay-sized fraction of particles from grain-size distribution tests. The measured engineering parameters are compared with comparable studied soils and established empirical correlations to provide insights on mechanical behaviors of the NSC. NSC is found to have a rapid decrease in the undrained shear strength ratio and a rapid increase in the lateral stress ratio with increasing consolidation stress.

Effect of Copper Segregation at Low-Angle Grain Boundaries on the Mechanisms of Plastic Relaxation in Nanocrystalline Aluminum: An Atomistic Study.

The paper studies the mechanisms of plastic relaxation and mechanical response depending on the concentration of Cu atoms at grain boundaries (GBs) in nanocrystalline aluminum with molecular dynamics simulations. A nonmonotonic dependence of the critical resolved shear stress on the Cu content at GBs is shown. This nonmonotonic dependence is related to the change in plastic relaxation mechanisms at GBs. At a low Cu content, GBs slip as dislocation walls, whereas an increase in Cu content involves a dislocation emission from GBs and grain rotation with GB sliding.

Bridging microscale to macroscale mechanical property measurements of FeCrAl alloys by crystal plasticity modeling

FeCrAl alloys are candidates for accident tolerant fuel cladding of light water reactors. In this work, a microstructure- and temperature-dependent crystal plasticity model is employed to bridge microscale to macroscale mechanical property measurements of FeCrAl alloys. With the visco-plastic self-consistent (VPSC) polycrystal plasticity framework, a mechanism-based single crystal plasticity (MSCP) model adopts the Arrhenius type rate equation to describe the dependence of the critical resolved shear stress for dislocation slips on their temperature-dependent intrinsic frictional resistance and the microstructure-dependent irradiation hardening. The intrinsic frictional resistance associated with {110}<111> and {112}<111> slip systems were measured by in-situ micromechanical testing on unirradiated/irradiated samples at 25-500 °C. The irradiation hardening is estimated by the Bacon-Kocks-Scattergood (BKS) model with density and size of radiation-induced defects measured from microstructural characterization. Several features associated with thermo-mechanical behavior of unirradiated/irradiated polycrystalline FeCrAl alloys are captured. High density of deformation-induced dislocations and radiation-induced defects results in obvious hardening at room temperature, which is weakened at high temperature, and facilitates damage evolution during deformation. Moreover, both high temperature and radiation-induced defects, which facilitate dislocation multiplication, trigger large hardening rate. The proposed method together with application of accelerator-based ion irradiation technique is a surrogate approach to simulate neutron damage, improving the efficiency associated with evaluation of mechanical properties of FeCrAl alloys exposed to temperature, stress and radiation conditions.

Microscopic activated dynamics theory of the shear rheology and stress overshoot in ultradense glass-forming fluids and colloidal suspensions

We formulate a particle and force level, activated dynamics-based statistical mechanical theory for the continuous startup nonlinear shear rheology of ultradense glass-forming hard sphere fluids and colloidal suspensions in the context of the elastically collective nonlinear Langevin equation approach and a generalized Maxwell model constitutive equation. Activated structural relaxation is described as a coupled local-nonlocal event involving caging and longer range collective elasticity which controls the characteristic stress relaxation time. Theoretical predictions for the deformation-induced enhancement of mobility, the onset of relaxation acceleration at remarkably low values of stress, strain, or shear rate, apparent power law thinning of the steady-state structural relaxation time and viscosity, a nonvanishing activation barrier in the shear thinning regime, an apparent Herschel–Buckley form of the shear rate dependence of the steady-state shear stress, exponential growth of different measures of a yield or flow stress with packing fraction, and reduced fragility and dynamic heterogeneity under deformation were previously shown to be in good agreement with experiments. The central new question we address here is the defining feature of the transient response—the stress overshoot. In contrast to the steady-state flow regime, understanding the transient response requires an explicit treatment of the coupled nonequilibrium evolution of structure, elastic modulus, and stress relaxation time. We formulate a new quantitative model for this aspect in a physically motivated and computationally tractable manner. Theoretical predictions for the stress overshoot are shown to be in good agreement with experimental observations in the metastable ultradense regime of hard sphere colloidal suspensions as a function of shear rate and packing fraction, and accounting for deformation-assisted activated motion appears to be crucial for both the transient and steady-state responses.

Heat flux and wall shear stress in large-aspect-ratio turbulent vertical convection

Thermally driven flows are ubiquitous in nature. An important and longstanding question is how heat transfer depends on the control parameters of the flow. In this Letter, the author presents a theoretical analysis that answers such question for large aspect-ratio turbulent thermal convection in a fluid between two vertical walls maintained at different temperatures. This analysis yields the dependence of heat flux and wall shear stress on the Rayleigh and Prandtl numbers in the high Rayleigh-number limit.

Initiation and Mechanisms of Plasticity in Bimetallic Al-Cu Composite

We studied the shear deformation of a laminar Al-Cu composite with (100) and (110) interfaces with a shear perpendicular to the lamellae in comparison with pure single crystal Al and Cu at strain rates of 109 s−1 and 108 s−1 and different initial pressures in the range from −3 GPa to +50 GPa. The results of molecular dynamics (MD) for the plasticity initiation are generalized by means of an artificial neural network (ANN) trained by MD data for the (100) interface, and a rate sensitivity parameter identified using MD data for different strain rates. The ANN-based approach allows us to extrapolate MD data to much lower strain rates, which are more relevant for typical dynamic loadings. The considered problem is of interest as an example of the application of the developed ANN-based approach to bimetallic systems, whereas previously it was tested only for pure metals; in addition, Al-Cu composites are of practical interest for technology. The interface between metals reduces the shear strength of the composite in comparison with both pure metals. At an initial pressure below 10 GPa, the plasticity begins in the aluminum part of the composite, while at higher pressures, the plasticity of the copper part starts first. At a pressure above 40 GPa, a phase transition in the aluminum part governs the plasticity development. All this leads to a nonmonotonic pressure dependence of the critical shear stress of the Al-Cu composite in the case of (100) and (110) interfaces without misorientation. Misorientation decreases the critical stress of the nucleation of lattice dislocation and makes the pressure dependence of this stress monotonic. Deformation modes, with a defect-free copper part and a strain-accommodating aluminum part are observed in the MD and can be useful for technological applications related to deformable conducting materials.

Study of the laws of internal friction in magnetic fluids with a strongly developed thixotropic nanostructure

Due to their unique physical properties, magnetic fluids are promising for use in bearings, seals, sliding guides, and other devices of modern technology. Some restrictions on their use are imposed by the tendency of magnetic fluids to lose colloidal stability and structure formation in strong magnetic fields. Increasing the stability of a colloid by reducing the size of the dispersed particles of the magnetic fluid is limited by the Heisenberg uncertainty relation, on the condition of maintaining their ferromagnetic state. The search for ways to reduce internal friction in technical devices with magnetic fluids having a highly developed thixotropic nanostructure is important from a practical point of view. Using a device, simulating the operation of a magnetohydrostatic bearing, the rheological characteristics of a fluid nanostructured by a magnetic field, which is a colloidal system with a dispersed phase of magnetite particles (10 vol.%) and a dispersion medium of silicon organic fluid PESV-2, were studied. The dynamic viscosity of the magnetic fluid was about 0.05 Pa.s at 20°C. It has been established that the process of structuring a magnetic fluid in an external field can last hundreds of hours and depends mainly on the viscosity of the dispersion medium and the concentration of magnetite. It has been revealed that the motion of a cylinder with a terminal velocity begins only at shear stresses exceeding the limiting static stress and proceeds at a constant velocity. The breakdown of the structure begins after the shear stress exceeds the critical value. The critical stress is introduced to compare the strength of the structure of different fluids. The value of the critical stress was determined with an accuracy of up to 50 Pa by analyzing the curves of the change in the sliding speed with time. It has been established that the temperature dependence of the critical shear stress is very sharp and close to exponential.

Domain partitioning material point method for simulating shock in polycrystalline energetic materials

Heterogeneous energetic materials (EMs) subjected to mechanical shock loading exhibit complex thermo-mechanical processes which are driven by the high temperature, pressure, and strain rate behind the shock. These lead to spatial energy localization in the microstructure, colloquially known as “hotspots” where chemistry may commence and possibly culminating in detonation. Shock-induced pore collapse is one of the dominant mechanisms by which localization occurs. In order to physically predict the shock sensitivity of energetic materials under these extreme conditions, we formulate a multiplicative crystal plasticity model with key features inferred from molecular dynamics (MD) simulations. Within the framework of thermodynamics, we incorporate the pressure dependence of both monoclinic elasticity and critical resolved shear stress into the crystal plasticity formulation. Other fundamental mechanisms, such as strain hardening and pressure-dependent melting curves, are all inferred from atomic-scale computations performed across relevant intervals of pressure and temperature. To handle the extremely large deformation and the evolving geometry of the self-contact due to pore collapse, we leverage the capabilities of the Material Point Method (MPM) to track the interface via the Lagrangian motion of material points and the Eulerian residual update to avoid the mesh distortion issue. This combination of features enables us to simulate the shock-induced pore collapse and associated hotspot evolution with a more comprehensive physical underpinning, which we apply to the monoclinic crystal β-HMX. Treating MD predictions of the pore collapse as ground truth,head-to-head validation comparisons between MD and MPM predictions are made for samples with identical sample geometry and similar boundary conditions for reverse-ballistic impact speeds ranging from 0.5kms−1 to 2.0kms−1. Comparative studies are performed to reveal the importance of incorporating a frictional contact algorithm, pressure-dependent elastic stiffness, and non-Schmid critical resolved shear stress in the mesoscale model.

Extended Barton–Bandis model for rock joints under cyclic loading: Formulation and implicit algorithm

AbstractIn this paper, the Barton–Bandis model for rock joints is extended to cyclic loading conditions, without any new material parameter. Also developed herein is an implicit solution algorithm for the extended Barton–Bandis model, which can also be used for the original Barton–Bandis model for which an implicit algorithm has been unavailable. To this end, we first cast the Barton–Bandis model into an incremental elasto‐plastic framework, deriving an expression for the elastic shear stiffness being consistent with the original model formulation. We then extend the model formulation to cyclic loading conditions, incorporating the dependence of shear stress and dilation on the joint position and the shearing direction. The extension is achieved by introducing a few state‐dependent variables which can be calculated with the existing material parameters. For robust and accurate utilization of the model, we also develop an implicit algorithm based on return mapping, which is unconditionally stable and guarantees the satisfaction of the strength criterion. We verify that the proposed model formulation and algorithm produce virtually the same results as the original Barton–Bandis model under monotonic shearing conditions. We then validate the extended Barton–Bandis model against experimental data on natural rock joints under cycling loading conditions. The present work thus enables the Barton–Bandis model, which has been exceptionally popular in research and practice, to be applicable to a wider range of problems in rock mechanics and rock engineering.

Atomistic simulation of shear deformation at bcc-Fe grain boundary and precipitation strengthening by Cr23C6

We investigate the temperature and the tilt angle dependence of the shear deformation behavior at a body-centered cubic (bcc)-Fe grain boundary (GB) by utilizing classical molecular dynamics simulations. We found that the shear deformation at a [001] axial bcc-Fe GB accommodates GB migration. The critical shear stress needed to induce GB migration was found to decrease with an increase in the temperature or molecular dynamics simulation time, because GB migration is a thermally activated process. The tilt angle dependence of the critical shear stress is successfully explained by the presence of a specific structure unit composed of under-coordinated atoms. It was suggested that Cr 23 C 6 precipitation within the bcc-Fe crystal decreases the critical stress required to generate dislocations, because of the interface between the precipitation and parent phases. On the other hand, Cr 23 C 6 precipitation on a [001] axial bcc-Fe GB was found to increase the critical shear stress, while the precipitation didn’t change the shear deformation mechanism. The degree of strengthening was verified by theoretical equation to evaluate the shear strengthening by the ordered precipitates.