Small Strain Rates Research Articles

Molecular dynamics study of coupled layer thickness and strain rate effect on tensile behaviors of Ti/Ni multilayered nanowires* *Project supported by the National Natural Science Foundation of China (Grant No. 11572259), the Natural Science Foundation of Shaanxi Province, China (Grant No. 2019JQ-827), and the Scientific Research Program Funded by Shaanxi Provincial Education Department, China (Grant No. 19JK0672).

Novel properties and applications of multilayered nanowires (MNWs) urge researchers to understand their mechanical behaviors comprehensively. Using the molecular dynamic simulation, tensile behaviors of Ti/Ni MNWs are investigated under a series of layer thickness values (1.31, 2.34, and 7.17 nm) and strain rates (). The results demonstrate that deformation mechanisms of isopachous Ti/Ni MNWs are determined by the layer thickness and strain rate. Four distinct strain rate regions in the tensile process can be discovered, which are small, intermediate, critical, and large strain rate regions. As the strain rate increases, the initial plastic behaviors transform from interface shear (the shortest sample) and grain reorientation (the longest sample) in small strain rate region to amorphization of crystalline structures (all samples) in large strain rate region. Microstructure evolutions reveal that the disparate tensile behaviors are ascribed to the atomic fractions of different structures in small strain rate region, and only related to collapse of crystalline atoms in high strain rate region. A layer thickness-strain rate-dependent mechanism diagram is given to illustrate the couple effect on the plastic deformation mechanisms of the isopachous nanowires. The results also indicate that the modulation ratio significantly affects the tensile properties of unequal Ti/Ni MNWs, but barely affect the plastic deformation mechanisms of the materials. The observations from this work will promote theoretical researches and practical applications of Ti/Ni MNWs.

Hybrid Auxetic Structures: Structural Optimization and Mechanical Characterization

With their increased energy absorption capacity, auxetic materials are perfectly fit to develop new, enhanced lightweight crash absorbers for cars. Herein, the mass distribution along the struts is optimized via finite element analysis with a parameterized optimization. Four different auxetic unit cells are taken from the literature and their struts parameterize, the models simulate, and the mass specific energy absorption capacity optimizes. The two models with the highest energy absorption capacity are then selected for experimental investigation and produced by additive manufacturing from a polymer. To further enhance the mechanical properties, the specimens are electrochemically coated with nickel and the polymer molten out by pyrolysis. Those Ni/polymer hybrids are subjected to quasistatic and dynamic impact experiments. Only a small strain rate sensitivity can be detected under dynamic loading, namely, a higher plastic collapse and higher plateau stress. The hollow struts are folding instead of bending, which render them much weaker than predicted by the simulation. In conclusion, it is possible to improve existing crash absorber elements with tailored auxetic hybrid structures. They absorb higher amounts of energy without changing their stiffness under dynamic loading while saving mass and cost.

Molecular origin of sliding friction and flash heating in rock and heterogeneous materials

It is generally believed that earthquakes occur when faults weaken with increasing slip rates. An important factor contributing to this phenomenon is the faults’ dynamic friction, which may be reduced during earthquakes with high slip rates, a process known as slip-rate weakening. It has been hypothesized that the weakening phenomenon during fault slip may be activated by thermal pressurization of pores’ fluid and flash heating, a microscopic phenomenon in which heat is generated at asperity contacts due to high shear slip rates. Due to low thermal conductivity of rock, the heat generated at the contact points or surfaces cannot diffuse fast enough, thus concentrating at the contacts, increasing the local contact temperature, and reducing its frictional shear strength. We report the results of what we believe to be the first molecular scale study of the decay of the interfacial friction force in rock, observed in experiemntal studies and attributed to flash heating. The magnitude of the reduction in the shear stress and the local friction coefficients have been computed over a wide range of shear velocities V. The molecular simulations indicate that as the interfacial temperature increases, bonds between the atoms begin to break, giving rise to molecular-scale fracture that eventually produces the flash heating effect. The frequency of flash heating events increases with increasing sliding velocity, leaving increasingly shorter times for the material to relax, hence contributing to the increased interfacial temperature. If the material is thin, the heat quickly diffuses away from the interface, resulting in sharp decrease in the temperature immediately after flash heating. The rate of heat transfer is reduced significantly with increasing thickness, keeping most of the heat close to the interface and producing weakened material. The weakening behavior is demonstrated by computing the stress–strain diagram. For small strain rates there the frictional stress is essentially independent of the materials’ thickness. As the strain rate increases, however, the dependence becomes stronger. Specifically, the stress–strain diagrams at lower velocities V manifest a pronounced strength decrease over small distances, whereas they exhibit progressive increase in the shear stress at higher V, which is reminiscent of a transition from ductile behavior at high velocities to brittle response at low velocities.

Mechanical failure of fresh nuclear grade iron–chromium–aluminum (FeCrAl) cladding under simulated hot zero power reactivity initiated accident conditions

The reactivity initiated accident (RIA) is a postulated accident in light water reactors initiated by a rapid increase in reactivity which causes an increase in fission rate and fuel temperature. One potential mode of fuel system failure during RIA is pellet cladding mechanical interaction (PCMI) due to the rapid thermal expansion of the fuel pellet. Simulated PCMI experiments were performed by rapidly pressurizing iron-chromium-aluminum (FeCrAl) cladding tube samples using a hydraulic modified burst test system, causing the specimens to burst under a biaxial loading path. Deformation and rupture of the specimens were tracked with a telecentric lens and high-speed camera system. Outer surface strains were calculated using digital image correlation (DIC) on speckle patterns painted on the specimens’ outer surfaces. Experiments were conducted at approximately 275 °C, representative of hot zero power RIA conditions. The failure hoop strain was DIC-calculated between approximately 1.8–3.4%, corresponding to quasi-static energy depositions of approximately 110–260 calories per gram UO2 assuming initial pellet-cladding contact. These conditions are very similar to the proposed energy deposition limit of 150 calories per gram UO2 for unirradiated zirconium-based cladding in US Nuclear Regulatory Commission (NRC) Draft Regulatory Guide 1327. A very small strain rate dependence was observed in the data, with the magnitude of the failure hoop strain decreasing slightly with increasing strain rate. This observed dependence may be relevant because the FeCrAl cladding strain rate will be approximately 20% higher than in zirconium-based cladding due to the harder neutron spectrum and resultant shorter neutron generation time.

Hot compression deformation behavior of Mg–Y–Zn alloys containing LPSO phase

The effect of the volume fraction of the long periodic stacking ordered (LPSO) phase on elevated temperature compressive deformation behavior is investigated using four types of Mg–Y–Zn alloys (Mg-0.4Y-0.2Zn, Mg–2Y–1Zn, Mg–5Y-2.5Zn, Mg–9Y–6Zn, where the numbers indicate at.%) produced by casting. The LPSO phase volume fraction is found to increase with increases in the concentrations of Y and Zn. The results obtained from compression tests at elevated temperatures reveal that the responses of these alloys to compression behavior, i.e., the stress vs. strain behavior, are changed by variations in the test conditions, such as the temperature and strain rate. The flow stress of the dilute solid-solution Mg-0.4Y-0.2Zn alloy containing a few volume proportions of the LPSO phase exhibits a small strain rate dependence on flow stress. In contrast, the flow stress clearly decreases with lower strain rate and higher temperature in the other three Mg–Y–Zn alloys, irrespective of the LPSO volume fraction. Observations of the deformed microstructures indicate that deformation twins are formed in the dilute alloy; however, the other three alloys show different microstructural features. Alternative for deformation twin formation, deformation kink bands are produced in these alloys; some of the existences of the LPSO phase (at least several dozens percent) are necessary to induce deformation kink band formation and to control the rate-controlling mechanism. Normalized plots obtained from previous hot compression testing together with the present results suggest that the volume fraction of the LPSO phase is unlikely to affect compressive deformation behavior.

Effects of radiation modeling on non-premixed sooting flames simulations under oxyfuel conditions

Radiative heat modeling has a central role for the simulation of soot formation in flames. Radiation heat transfer, gas-phase chemistry and soot formation are strongly coupled and highly dependent on temperature. Thus, it is of primary importance that simplified radiative models are able to reproduce detailed simulations at conditions found in flames of interest. In this work, the capabilities of the superposition WSGG radiation model is compared to the LBL spectral integration for a set of laminar non-premixed counterflow flames of ethylene burning with oxygen-rich oxidizer. The numerical solutions for both thermal radiation approaches are fully coupled with detailed chemistry and a discrete sectional model for soot formation. The role of thermal radiation and the importance of radiation absorption on soot formation were explored through systematic replacement of N2 by CO2 on either the fuel or on the oxidizer. In addition, the validity of the simplified optically thin approximation for modeling such flames was also discussed. The spectral superposition WSGG model is able to accurately describe general flame structure and soot predictions respective to the LBL integration for both the reference flame and for the cases of CO2 addition in the fuel and in the oxidizer mixtures. For the CO2 addition flames, the WSGG model tends to overestimate the radiation emission on high temperature regions and to overestimate the radiation absorption on low temperatures towards the reactant with CO2 addition. However, these discrepancies were not sufficient to significantly influence the flame structure. Some non-negligible discrepancies are observed only at very small strain rates. Finally, the evaluation of the soot spectral radiation coefficient (Cκ,s) reveled an important influence on the soot volume volume fraction for low levels of CO2 addition.

Experimental characterization of the compressive mechanical behaviour of Ti6Al4V alloy at constant strain rates over the full elastoplastic range

Full range constant strain rate tests are required for accurately characterizing initial yield point, strength differential effect and direct identification of constitutive laws describing the plastic behavior of materials. These tests require the use of a closed-loop control in order to achieve the constant strain rate, however this feature is not available in many laboratories. A method for full range constant strain rate with testing machines that can be configured for user-defined displacements of the cross head prior to testing is presented here. Tests performed at a constant die speed include a variable strain rate response for the specimen involved. Significant deformation rate variation occurs between the elastic and plastic range with consequences for initial yield point identification. To overcome this drawback, appropriate user-defined displacements can be computed and applied, allowing for both tensile and compression tests to be performed at a constant strain rate. The method is validated using a compression test of Ti6Al4V alloy at room temperature, as well as a 3D digital image correlation (DIC) system exhibiting a constant strain rate value equal to 10−3 s−1, for both elastic and plastic ranges. A non-negligible inhomogeneous strain field was measured on the surface of the compression specimen using DIC and was corroborated by numerical modeling. Results identified the source of the non-homogeneous strain field, thereby proposing a quantitative indicator of plastic anisotropy. The initial yield stress and strain hardening rates of the alloy at several temperatures were obtained with both testing method, conventional constant cross-head speed, and the constant strain rate; these were then used to determine the influence of the small strain rate variations on the mechanical response of Ti6Al4V alloy.

Hot Deformation Characteristics of 18Cr-5Ni-4Cu-N Stainless Steel Using Constitutive Equation and Processing Map

The hot deformation of 18Cr-5Ni-4Cu nitrogen-alloyed austenitic stainless steel was tested with a Gleeble-1500D simulator in the temperature range of 1273–1473 K and in the strain rate range of 0.01–10 s−1. The Zener-Hollomon parameter method was used to construct a constitutive equation for high-temperature plastic deformation. The energy dissipation diagram of the material was calculated based on dynamic material modelling (DMM). The microstructural variations were characterized via X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) with energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM). The thermodynamic calculation results showed that the addition of nitrogen to 18Cr-5Ni-4Cu steel promoted the formation of Cr2N and gas phases and expanded the austenite phase region; these results were consistent with the XRD test results of the solid solution sample. The hot deformation activation energy after nitrogen addition was 556.46 kJ∙mol−1. The processing map predicted that the optimum hot working regimes were in the temperature range of 1416–1461 K, where ln ε ˙ was 0.75–1 on the power dissipation map. At high temperatures and a small strain rate, dynamic recrystallization easily occurred. The TEM analyses showed that nano-scale M23C6 and Cr2N precipitated at the grain boundary, and NbC with a diameter of approximately 150 nm appeared along the grain boundary, resulting in grain boundary strengthening. The phase precipitation results were consistent with the Thermo-Calc calculation results. The nitrogen solid solution in the steel promoted the precipitation of nitrides, which caused grain boundary strengthening. Thus, the grain boundary stress increased and wedge-shaped grain boundary cracks formed.

Investigation on dynamic strain aging behaviour of ferritic-pearlitic ductile cast irons

The influence of ferritic and pearlitic microstructures to dynamic strain aging (DSA) of ductile iron was investigated by means of strain-controlled cyclic tests performed at temperatures ranging from room temperature to 450°C, strain rates of 10−5 to 10−2 s−1 and at three different strain amplitudes along with tensile tests. Results indicated that all tested microstructures exhibit strain aging effects at elevated temperatures in some form, similar to ferritic-pearlitic steels. Additionally, static tensile tests show hardening at intermediate temperatures, minima in elongation and serrations in stress–strain curves. Cyclic data indicates DSA related changes in stresses at 250°C and 350°C at smaller strain rates, while the effects become more suppressed at higher strain rates.

Distribution of ductile deformation around the Main Central Thrust zone at the frontal part of nappe in southeastern Nepal Himalaya

AbstractA geological survey and morphological analysis of quartz grains were performed to investigate the distribution of ductile deformation caused by the Himalayan Main Central Thrust (MCT) around Dhankuta, southeastern Nepal. The MCT was mapped as the lithological boundary between the gneiss of the Higher Himalayan Crystalline (HHC) as the hanging wall and the inverted metamorphic sequence of the Lesser Himalayan Sediments (LHS) as the footwall. The MCT was found to truncate various stratigraphic levels of LHS and cuts a map‐scale gentle fold developed in the LHS. Ductile deformation was quantified by fractal dimension between size and perimeter of dynamically recrystallized quartz grains in bedded metaquartzite intercalated in both HHC and LHS. Serrate and polygonal shapes of quartz indicate large and small strain rates, respectively, when the temperature during ductile deformation was assumed to be uniform. A peak of strain rate was found at the lithological boundary with the peak width of ca. 500 m. Such a thin shear zone is favorable for producing frictional heat to promote the inverted metamorphism in LHS.

Parameter study of variable order fractional model for the strain hardening behavior of glassy polymers

In this work, a novel approach of variable order fractional derivative model of viscoelasticity is provided to describe the strain hardening behavior of amorphous glassy polymers, which has been considered as a combined viscoelastic phenomenon in recent studies. The proposed model contains only four parameters as the order function is assumed to linearly vary with time. To validate the model, experimental tests of constant true strain rate uniaxial compression are conducted on PC to get the stress–strain data at different temperatures and strain rates. Comparison between the model prediction and experimental data show that the model can well describe the strain hardening behavior. It is also indicated by the linearly decreasing order function that strain hardening is a continuous stiffening process of material property because the smaller fractional order means that the property of material is closer to solid. Furthermore, a study on parameter is performed to investigate the physical significance of model parameters. It is shown that the slope of order change determines the rate of strain hardening and the intercept of order function mainly affects the initial stress of strain hardening. Finally, the rule of order function under various temperatures and strain rates reveals that during strain hardening the mechanical property of amorphous glassy polymers is softer but stiffens faster at higher temperatures and smaller strain rates.

Indentation size and loading rate sensitivities on mechanical properties and creep behavior of solid bitumen

Creep behavior of rocks could impair fracture conductivity and wellbore stability during gas production from highly matured organic-rich shales in South China, of which the organic matter is mainly in the form as solid bitumen and is thought to be a major contributor for the creep deformation. To get a better insight into this phenomenon, this paper for the first time characterizes the mechanical properties and creep behavior of a millimeter-sized solid bitumen sample by using quasi-static state creep tests and Dynamic Mechanical Analysis in nanoindentation, and reports their dependences on indentation size and loading rate, respectively. Mechanical properties (including hardness and Young's modulus) are found to be negatively related with both indentation size and loading rate. The extremely small creep strain rate sensitivity (m) of solid bitumen indicates a localized shear flow inside. And m exhibits slightly positive dependences on indentation size and loading rate. The potential mechanisms controlling the deformation of solid bitumen under indentation are also discussed.

Confinement effects on the orientation of graphene in multilayer polymer nanocomposites during lamination: A molecular dynamics simulation

As a stacking form of ultrathin polymer films, multilayer nanostructures are of great interest in various applications. Coarse-grained molecular dynamics simulations were carried out to understand the confinement effects on the orientation of graphene sheets in multilayer polymer films during lamination. The influences of position, strain rate, expansion type, and arrangement pattern on the orientation of graphene sheets in the near-boundary and internal layers were considered. It was found that the confinement effects induce the graphene sheets to gradually orient parallel to the plane of the layer during lamination. Such effects are more obvious in the near-boundary layer than in the internal layer. With the same strain, a smaller strain rate gives rise to more obvious effects on the in-plane orientation of graphene sheets in both the near-boundary and internal layers. Biaxial expansion also makes the graphene sheets orient more parallel to the plane of the layer. This work is helpful for better understanding the lamination process and the designing of multilayer nanocomposites.

Dependence of strain rate on hydrogen-induced hardening of austenitic stainless steel investigated by nanoindentation

Abstract The behavior of hydrogen-induced hardening of 310S and 304 austenitic stainless steels was investigated by using nanoindentation constant strain rate (CSR) and strain rate jump tests. Hydrogen increases the hardness at all strain rates, and the effect of hydrogen-induced hardening becomes stronger with increasing the strain rate in 310S steel. Additionally, hydrogen increases the strain rate sensitivity in 310S steel, and enhances pile-up around indentation with decreasing the strain rate. The interaction between hydrogen and mobile dislocation is responsible for the dependence of hydrogen-induced hardening on the strain rate. Hydrogen can keep pace with mobile dislocation and enhance the movement of mobile dislocation at a small strain rate (0.005 s−1), while hydrogen lags behind mobile dislocation and inhibits the movement of mobile dislocation at a large strain rate (0.5 s−1). Since hydrogen suppresses the α′-martensite transformation, hydrogen-induced hardening of 304 becomes weaker than that of 310S.

Recent geodynamics: from crustal movements to monitoring the critical objects

The evolution of the views on the subject and methods of recent geodynamics over the past fifty years is outlined. Metrological provision of the results obtained by repeated observations by means of ground-based and satellite geodesy is discussed. The substantial dependence of the main characteristics of recent geodynamical processes on the degree of spatiotemporal detail of the observational systems is demonstrated.
 A possible solution of the paradoxes of large and small strain rates which were detected in the studies at geodynamical sites in seismically active and aseismic regions is proposed. To explain the anomalous deformational activity on the platform faults, the mechanism of parametric excitation is suggested. According to this mechanism, the time fluctuations in the internal parameters of a fault zone (stiffness, pore pressure, friction coefficient) create local strain anomalies under quasi static external loading.
 The results of strain monitoring are demonstrated by the example of a shelf oil field. It is substantiated that the geodynamical testing sites are a universal instrument for exploring recent deformational processes which offers a unified framework for establishing the spatiotemporal structure of different-scale geodynamical phenomena addressed in the fundamental and applied studies.

Dynamic recrystallization kinetics and microstructure evolution of 7055 aluminum alloy during hot compression

7055 aluminum alloy is widely used in manufacture of key components in fields of aerospace. Hot forming is the vital process for manufacture of components. Dynamic recrystallization (DRX) plays a significant role in grain refinement. Hot compressions and metallographic tests are conducted. Equations of DRX kinetics are fitted by least square method and finite element models (FEM) coupled with DRX kinetics equations are established to study the effects of forming parameters on microstructure evolution. The results show that true stress increases with ascending strain rate and decreases with ascending temperature. Large strain, small strain rate and high temperature are beneficial to sufficient DRX fraction and grain refinement. Deformation energy and thermally activated motion of atoms and molecules resulting from large strain and high temperature contribute to dynamic nucleation; meanwhile, small strain rate provides sufficient time for growth of recrystallized grains. The FEM results agree with experiments.

Dynamics and stability of a power-law film flowing down a slippery slope

A power-law fluid flowing down a slippery inclined plane under the action of gravity is deliberated in this research work. A Newtonian layer at a small strain rate is introduced to take care of the divergence of the viscosity at a zero strain rate. A low-dimensional two-equation model is formulated using a weighted-residual approach in terms of two coupled evolution equations for the film thickness h and a local velocity amplitude or the flow rate q within the framework of lubrication theory. Moreover, a long-wave instability is shown in detail. Linear stability analysis of the proposed two-equation model reveals good agreement with the spatial Orr-Sommerfeld analysis. The influence of a wall-slip on the primary instability has been found to be non-trivial. It has the stabilizing effect at larger values of the Reynolds number, whereas at the onset of the instability, the role is destabilizing which may be because of the increase in dynamic wave speed by the wall slip. Competing impressions of shear-thinning/shear-thickening and wall slip velocity on the primary instability are captured. The impact of slip velocity on the traveling-wave solutions is discussed using the bifurcation diagram. An increasing value of the slip shows a significant effect on the traveling wave and free surface amplitude. Slip velocity controls both the kinematic and dynamic waves of the system, and thus, it has the profound passive impact on the instability.

Recent Geodynamics: from Crustal Movements to Monitoring Critical Objects

This paper addresses the evolution of the views on the subject and methods of geodynamics over the past fifty years. The problems of the metrological provision of the results of repeated observations by the ground-based and satellite geodesy are discussed. It is shown that the basic characteristics of the recent geodynamical processes substantially depend on the degree of spatiotemporal detail of the observational systems. A variant of the solution is suggested for the problem of paradoxes of large and small strain rates which was detected during the studies at the geodynamical sites in seismically active and weakly seismic regions. For explaining the anomalous deformational activity of the platform’s faults, the mechanism of parametric excitation of the processes is suggested according to which the time fluctuations in the internal parameters of a fault zone (stiffness, pore pressure, coefficient of friction) under quasi-static external loading form local strain anomalies. The results of strain monitoring are demonstrated by the example of a shelf oil field. The assertion is substantiated that the concept of geodynamical testing sites is a universal form for investigating the recent deformational processes which offers a unified framework for determining the spatiotemporal structure of the geodynamical phenomena of different scales that are used in the fundamental and applied studies.

Effect of hydrogen and strain rate on nanoindentation creep of austenitic stainless steel

The effect of hydrogen and strain rate on nanoindentation creep of austenitic stainless steel was investigated by nanoindentation loading and creep tests. The loading segment mainly reflects dislocation nucleation, but creep mainly indicates dislocation movement. The strain rate significantly influences dislocation nucleation, resulting in increased creep displacement with strain rate. The interaction between hydrogen and dislocation is responsible for the increased creep displacement in 310S steel. Hydrogen greatly enhances the displacement burst after small strain rate due to increased dislocation nucleation rate. Hydrogen slightly enhances creep of 304 steel as hydrogen diffuses through strain-induced α′ martensite to accumulate under the indenter before creep and enhances dislocation movement.

Deformation behaviour of re-entrant carbon honeycomb structures

Recent successes of researchers in the fabrication of new carbon structures of different morphologies and dimensions open up broad prospects for the development of carbon materials with qualitatively new properties. Such promising materials are cellular structures (or carbon aerogels) that have high deformability, good adsorption and conductivity characteristics. In this work, we consider a carbon structure consisting of re-entrant honeycomb cells, which allow the structure to be auxetic. The stability and deformation behaviour of re-entrant carbon honeycomb structures is studied by molecular dynamics simulation. The effect of structural parameters on the stability of carbon aerogel is discussed. It is shown that structural changes significantly depend on the strain rate and the loading direction. At a high strain rate in tension along the z-axis, all cells open slightly at the same time. On the contrary, at small strain rates, certain cells open, while others almost completely collapse, which is expected to result in changes in physical properties, for example, in conductivity. During tension along the x-axis, a transformation from a re-entrant honeycomb to a traditional honeycomb took place.