Dynamical Recovery Research Articles

Hierarchical Shrinkage Gaussian Processes: Applications to Computer Code Emulation and Dynamical System Recovery

Hierarchical Shrinkage Gaussian Processes: Applications to Computer Code Emulation and Dynamical System Recovery

Extraordinary strain rate softening of metals in torsion

Generally, the strength of metals increases with increasing strain rate. Here, an extraordinary strain rate softening was observed in torsion, i.e., the following stress decreased and the twist angle to failure increased with increasing torsion strain rate, which was contrary to the strain rate hardening in tension. This condition was attributed to the unexpectedly suppressed dynamical recovery by decreased strain rate in torsion, that is, the introduced dislocation density elevated and the average misorientation dropped with decreasing torsion strain rate. This study is significant for revealing the mechanical behaviors and guiding the applications of metals in torsion under high strain rate.

Dynamical Recovery of Complex Networks under a Localised Attack

In real systems, some damaged nodes can spontaneously become active again when recovered from themselves or their active neighbours. However, the spontaneous dynamical recovery of complex networks that suffer a local failure has not yet been taken into consideration. To model this recovery process, we develop a framework to study the resilience behaviours of the network under a localised attack (LA). Since the nodes’ state within the network affects the subsequent dynamic evolution, we study the dynamic behaviours of local failure propagation and node recoveries based on this memory characteristic. It can be found that the fraction of active nodes switches back and forth between high network activity and low network activity, which leads to the spontaneous emergence of phase-flipping phenomena. These behaviours can be found in a random regular network, Erdős-Rényi network and Scale-free network, which shows that these three types of networks have the same or different resilience behaviours under an LA and random attack. These results will be helpful for studying the spontaneous recovery real systems under an LA. Our work provides insight into understanding the recovery process and a protection strategy of various complex systems from the perspective of damaged memory.

Nonequilibrium charge-density-wave order beyond the thermal limit

The interaction of many-body systems with intense light pulses may lead to novel emergent phenomena far from equilibrium. Recent discoveries, such as the optical enhancement of the critical temperature in certain superconductors and the photo-stabilization of hidden phases, have turned this field into an important research frontier. Here, we demonstrate nonthermal charge-density-wave (CDW) order at electronic temperatures far greater than the thermodynamic transition temperature. Using time- and angle-resolved photoemission spectroscopy and time-resolved X-ray diffraction, we investigate the electronic and structural order parameters of an ultrafast photoinduced CDW-to-metal transition. Tracking the dynamical CDW recovery as a function of electronic temperature reveals a behaviour markedly different from equilibrium, which we attribute to the suppression of lattice fluctuations in the transient nonthermal phonon distribution. A complete description of the system’s coherent and incoherent order-parameter dynamics is given by a time-dependent Ginzburg-Landau framework, providing access to the transient potential energy surfaces.

Fault diagnosis via a dynamical sparse recovery method and application to a gearbox system

With the ever-increasing complexity and importance of industrial systems, diagnosis techniques allowing to detect, locate, and identify any abnormalities in the system as early as possible have attracted a lot of attention over the past years. In this article, we present a diagnosis method for nonlinear dynamical systems, called sparse recovery diagnosis, based on a dynamical algorithm that estimates a sparse fault vector from few system measurements. The term sparse means that many faults can be considered, but only few of them can occur simultaneously. To illustrate the performances of this diagnosis method, we apply it to a gear power transmission. This dynamical system is among the most important mechanical components in industrial systems. The gear power transmission model considered in this article is composed by a two-stage gear for which we take into account the torsional effect of the gears. Different sensor and mechanical faults perturbing its operating mode will be modeled and detected by the sparse recovery diagnosis method.

The Influence of Temperature and Deformation Ratio on Structure Formation and Physico-Mechanical Properties of Powder Materials

The paper presents the materials on researching the use of current conductor copper waste for getting new functional materials. The changes of maximum deformation ratio, density, microstructure and hardness of powder copper preforms at different temperatures and deformation rates have been studied. The result is that the value of the maximum deformation ratio depends on kinetics of dynamic weakening processes. The change of the microstructure depending on the temperature and deformation ratio has been scrutinized. It has been established that the most fine-grained structure was got at the temperatures of dynamical recovery and recrystallization. The temperature intervals of stamping for manufacturing products by pressing are recommended. On the basis of experimental researches a resource-saving manufacturing method of producing axisymmetric details of high density from powder copper material with the use of maximum deformation ratios has been elaborated.

Characterization of hot deformation behavior of 30Si2MnCrMoVE low-alloying ultra-high-strength steel by constitutive equations and processing maps

Isothermal compression tests of as-forged 30Si2MnCrMoVE low-alloying ultra-high-strength steel were carried out on a Gleeble 3500 thermal simulator at the deformation temperatures of 950–1150 °C and strain rates of 0.01–10 s−1. Based on the classical stress–dislocation density relationship and the kinematics of the dynamic recrystallization, the constitutive equations of the work hardening dynamical recovery period and dynamical recrystallization period were developed by using the work hardening curve and Avrami equation, which shows good agreement with the experimental value. Processing maps at the strain of 0.90 were constructed based on dynamic material model and were analyzed combined with microstructure observation under different conditions. The optimum parameter based on the processing maps was obtained and verified by a supplementary experiment. The power dissipation maps and instability maps at strains of 0.05–0.90 were also constructed, and the evolution law was analyzed in detail. The established constitutive equation and hot processing maps can provide some guidance for hot working process.

Irreversible contact process on complex networks with dynamical recovery probability

Contact process is widely used to describe the spreading of virus and information in real-world system. Previous studies on contact process always assumed that the infected node uniformly contacts one neighbor and becomes recovery with a constant recovery probability. In this paper, we propose a novel irreversible spreading model, in which an infected node preferencely contacts a neighbor according to the degrees of neighbors and becomes recovery with a dynamical probability. We propose a heterogeneous mean-field approach to describe the spreading dynamics. On both homogeneous and heterogeneous networks, we find that preferencely contacting the small degree nodes can promote the spreading dynamics, while the spreading dynamics will be greatly suppressed when each susceptible node provides the same volume of resources to the infected nodes. Our suggested theory can well predict the numerical simulations.

Dynamical sparse signal recovery with fixed-time convergence

Arising in a large number of application areas, sparse recovery (SR) has been exhaustively investigated and many algorithms have been proposed. Different from the numerical methods realized by iterative algorithm, the recent continuous approach is realized through analog circuit, which takes advantage of short time-delay and fast convergence. However, the existing continuous method for SR still has the space to further improve the convergence speed. Consequently, in this paper, we propose a new dynamical continuous system to solve the sparse signal recovery problem with fixed-time convergence property.

Constitutive modeling and critical strain of dynamic recrystallization in SA508Gr.4N steel for advanced pressure vessel materials

Using a Gleeble–1500D thermal–mechanical simulator, the hot-deformation behavior and critical strain in the dynamic recrystallization of SA508Gr.4N steel were investigated by compression tests from 1050 to 1250 °C with strain rates from 0.001 to 0.1 s−1. Stress–strain curves were fitted by a nonlinear fitting method. Based on these tests, the flow stress constitutive equations of the work-hardening dynamical recovery period and dynamical recrystallization period were established for SA508Gr.4N steel. The stress–strain curves of SA508Gr.4N steel predicted by the established models are in a good agreement with the experimental ones. Curves of lnθ − e and − ∂(lnθ)/∂e − e (where θ is the work-hardening rate and e is true strain) were plotted from experimental data. A critical strain (ec) and a peak strain (ep) of dynamic recrystallization were obtained and exhibited a linear relationship, i.e., ec = 0.386ep. The predicted model of ec could be described by the equation of ec = 1.604 × 10−3Z0.127.

Dynamical recovery of SU(2) symmetry in the mass-quenched Hubbard model

We use non-equilibrium dynamical mean-field theory with iterative perturbation theory as an impurity solver to study the recovery of $SU(2)$ symmetry in real-time following a hopping integral parameter quench from a mass-imbalanced to a mass-balanced single-band Hubbard model at half-filling. A dynamical order parameter $\gamma(t)$ is defined to characterize the evolution of the system towards $SU(2)$ symmetry. By comparing the momentum dependent occupation from an equilibrium calculation (with the $SU(2)$ symmetric Hamiltonian after the quench at an effective temperature) with the data from our non-equilibrium calculation, we conclude that the $SU(2)$ symmetry recovered state is a thermalized state. Further evidence from the evolution of the density of states supports this conclusion. At the same time, we find the order parameter in the weak Coulomb interaction regime undergoes an approximate exponential decay. We numerically investigate the interplay of the relevant parameters (initial temperature, Coulomb interaction strength, initial mass-imbalance ratio) and their combined effect on the thermalization behavior. Finally, we study evolution of the order parameter as the hopping parameter is changed with either a linear ramp or a pulse. Our results can be useful in strategies to engineer the relaxation behavior of interacting, quantum many-particle systems.

Near MAP Dynamical Delay Estimator and Bayesian CRB for Coded QAM Signals

This paper presents an off-line algorithm for dynamical time delay recovery for which the whole observation block is used. The time offset varies over the observation interval following a random walk model. The proposed synchronizer applies to data-aided (DA), non-data-aided (NDA), and code-aided (CA) modes. Theoretical performance of the off-line technique is derived and compared with simulation results. The Bayesian Cramer-Rao Bound (BCRB) is also evaluated for DA, NDA, and CA modes and for both the off-line and on-line scenarios. Simulation results show the improvement brought by the off-line and the CA schemes. The presented algorithm outperforms the conventional on-line estimator, which only considers the current and previous observations, and its mean square error approaches the BCRB.

Effects of strain amount and strain rate on grain structure of a novel high Co nickel-based polycrystalline superalloy

The development of nickel-based superalloys with high Co addition has aroused widespread interest recently owing to their potential application in hot sections of gas turbine. However, how hot working parameters impact the grain evolution of these superalloys remains to be studied systematically. In this work, high-throughput double cone (DC) specimen, which yielded gradient strain at different regions in a single sample after hot compression, were designed to investigate the thermo-mechanical behaviors of a novel superalloy. Superslovus treatment was subsequently conducted to investigate the susceptibility to abnormal grain growth (AGG). By means of finite element simulation, transmission electron microscope (TEM), and electron backscatter diffraction (EBSD), it was found that slow strained DC was preferred to obtain a well-distributed grain structure after supersolvus annealing, owing to its adequate dynamical recrystallization (DRX) and dynamical recovery (DRV) during deformation.

Dark-ages Reionization and Galaxy formation simulation – I. The dynamical lives of high-redshift galaxies

We present the Dark-ages Reionization and Galaxy-formation Observables from Numerical Simulations (DRAGONS) program and Tiamat, the collisionless N-body simulation program upon which DRAGONS is built. The primary trait distinguishing Tiamat from other large simulation programs is its density of outputs at high redshift (100 from z=35 to z=5; roughly one every 10 Myr) enabling the construction of very accurate merger trees at an epoch when galaxy formation is rapid and mergers extremely frequent. We find that the friends-of-friends halo mass function agrees well with the prediction of Watson et al. at high masses, but deviates at low masses, perhaps due to our use of a different halo finder or perhaps indicating a break from "universal" behaviour. We then analyse the dynamical evolution of galaxies during the Epoch of Reionization finding that only a small fraction (~20%) of galactic halos are relaxed. We illustrate this using standard relaxation metrics to establish two dynamical recovery time-scales: i) halos need ~1.5 dynamical times following formation, and ii) ~2 dynamical times following a major (3:1) or minor (10:1) merger to be relaxed. This is remarkably consistent across a wide mass range. Lastly, we use a phase-space halo finder to illustrate that major mergers drive long-lived massive phase-space structures which take many dynamical times to dissipate. This can yield significant differences in the inferred mass build-up of galactic halos and we suggest that care must be taken to ensure a physically meaningful match between the galaxy-formation physics of semi-analytic models and the halo finders supplying their input.

Temperature and Electrical Field Dependence of the Ambipolar Mobility in N-Doped 4H-SiC

In this study, we present results on electrical characterization of bipolar pn-diodes to investigate the temperature and electrical field dependent behavior of ambipolar mobility in n-doped 4H-SiC. Therefore, static current-voltage measurements to calculate the specific differential resistance and dynamical reverse recovery measurements to determine the mean carrier concentration were carried out for different temperatures and forward current densities. The specific differential resistance of the drift layer decreased from 10 mΩcm2 at 80 Acm-2 to 6.6 mΩcm2 at 180 Acm-2, whereas the mean carrier concentration only increased from 4.1015 cm-3 to 8.1015 cm-3, indicating a decreasing ambipolar mobility. The calculated reduction of the ambipolar mobility from 800 cm2V-1s-1 to 650 cm2V-1s-1 in dependence on the current density has to be attributed to an increasing electric field from 150 Vcm-1 to 250 Vcm-1 and increasing carrier scattering due to higher carrier concentrations. For example, at a constant conduction current density of 160 Acm-2, the ambipolar mobility decreases from 710 cm2V-1s-1 at 300 K to 650 cm2V-1s-1 at 450 K.

Marked discrepancy in pressure gradient between Doppler and catheter examinations on Medtronic Mosaic valve in aortic position

A 71-year-old woman underwent aortic valve replacement with 23 mm Medtronic Mosaic Ultra valve 4 years ago because of aortic stenosis. Although she had been asymptomatic since the operation, echocardiography showed 4 m/s of transprosthetic valve flow that implied early prosthetic valve failure. Catheter examination revealed that the mean transvalvular pressure gradient during systole was 15.1 mmHg on simultaneous pressure recording, and calculated valve area 1.82 cm(2). Her body surface area was 1.56 m(2). Prosthetic valve failure and prosthesis-patient mismatch were both denied. The discrepancy between Doppler study data and catheter data seemed to be due to fluid dynamical pressure recovery phenomenon. Net pressure difference between the left ventricle and the aorta may be significantly smaller than that estimated using Bernoulli's equation from transvalvular flow speed in some patients after aortic valve replacement.

POSS-enhanced shape-memory copolymer of polynorbornene derivate and polycyclooctene through ring-opening metathesis polymerization

A copolymer consisting of polycyclooctene (PCOE) and poly(5-norbornene- exo, exo-2,3-dicarboxylic anhydride) (PNBEDCA) was firstly synthesized through ring-opening metathesis polymerization, and then the obtained copolymer P(COE- co-NBEDCA) was readily modified by grafting reaction of NBEDCA units with polyhedral oligomeric silsesquioxanes (POSS) to afford a novel functionalized copolymer P(COE- co-NBEDCA- g-POSS), in which carboxyl and POSS as side chains attached to polynorbornene backbones. The copolymers were characterized by means of thermogravimetry, wide-angle X-ray diffraction, and stress–strain measurements. The shape memory effect of the copolymer was evaluated by dynamical mechanical analysis and shape recovery speed. Having the properties of the good shape fixity and the large shape recovery, the POSS-enhanced copolymer is expected to use as a potential shape memory material.

On the guaranteed accuracy of a dynamical recovery procedure for controls with bounded variation in systems depending linearly on the control

We obtain an upper bound for the accuracy of a modification of the finite-step dynamical reconstruction algorithm proposed by Yu. S. Osipov and A. V. Kryazhimskii for the unknown control in the dynamical system from inaccurate data on its trajectory We establish that, for the case in which the control is a function of bounded variation and the subspace of the image of the matrix of its coefficients has constant dimension, the asymptotic order in the metric of the space L1[a, b] of its accuracy with respect to the input error is 1/2.

Effects of heat treatment on microstructure and tensile deformation of Mg AZ80 alloy at room temperature

The plasticity of coarse and grain-refined Mg AZ80 alloys in the as-cast, γ-dissolved and homogenized states was investigated by specialized tensile testing at room temperature. Results indicate that microstructural parameters such as the activation volume and mean free path are important descriptors for these materials and capture the nature of the solute and second phase effect on strength and ductility. The as-cast alloys contain a microstructure consisting of α-Mg matrix, and divorced eutectic α-Mg/γ-Mg 17Al 12 phase with non-uniform Al solute content in the α-Mg. Dissolution of the majority of γ-phase occurs after annealing 5 h at 420 °C, and an almost uniform solid solution is obtained after 20 h at 420 °C. The yield strength is dependent upon the volume fraction of γ-phase and grain size. All alloys yield initially by basal slip and they exhibit different work hardening behaviour. The as-cast alloys show the fastest initial hardening and earliest saturation, and ultimately the lowest ductility. In contrast the solutionized alloys show a lower initial work hardening rate that is sustained, and enhanced ductility. The flow stress dependence of the strain rate sensitivity indicates that dynamical recovery processes associated with the dislocation–dislocation interactions, which develop in the as-cast alloys after small amount of deformation, lead to strain localizations and early failure. Results reveal that reducing the grain size and dissolving the γ-phase will enhance the ductility of AZ80 at room temperature.

Physical Simulation of Dynamic Recrystallization Behavior of 7050 Aluminum Alloy

This work aims to investigate the influence of hot deformation on dynamic recrystallization(DRX) behavior of 7050 aluminum alloy by means of physical simulation method. The hot compression tests were carried out on Gleeble 1500 machine in temperature range of 250-450°C and strain rate range of 0.01–10 s-1. Transmission electronic microscopy (TEM) was employed to observe and analyze the microstructure and DRX behavior in different deformation conditions. The results show that the effects of deformation temperature and strain rate on microstructural evolution of the alloy are remarkable. When temperature is lower than 350°C, only the dynamical recovery( DRV) occurs and typical sub-grains appear. In the range of 350-400°C, the incomplete DRX occurs and the recrystallized grain size increases with increasing temperature. The complete DRX occurs at 450°C and the fine equiaxied grains with high-angle boundaries develop resultantly. The DRX grain size increases as strain rate decreases. The desirable microstructure and properties can be obtained by optimizing the forging process parameters.