Dynamic Recovery Mechanism Research Articles

Inter-domain lightpath provisioning policy in support of hierarchical multi-target QoS

As the DWDM distributed multi-domain optical networks has been advocated to be the next generation networks, there is a pressing need to propose a set of effective light path provisioning policies. Meanwhile, with the growth of service types, different types of service should be set in different priority levels and pursuit individual QoS requirements. Thus, a new lightpath provisioning policy in support of hierarchical multi-target QoS is proposed, which is self-adaptive to different services, able to support priority, and designed with a dynamic blocking recovery mechanism.

Evaluating multiple reliability indices of regional networks in wide area measurement system

The paper presents a hierarchical structure of Wide Area Measurement System (WAMS) that can satisfy requirements of reliable real-time data transfer. The methods of calculating multiple reliability indices of WAMS regional networks in the protection and recovery survival mechanisms are developed and an equivalent two-state model of a WAMS regional network is presented. The numerical results of an example indicate that the meshed WAMS regional network can provide a very high reliability level. The reliability of the regional network can be enhanced by selecting an appropriate location of Phasor Data Center (PDC) or by improving availability of crucial optic fiber links in the protection mechanism. In the dynamic recovery mechanism, however, these measures may not be effective and thus the key consideration should be put on how to timely restore data transfers after a communication contingency.

Hot deformation behavior of an Al–5.7 wt.%Mg alloy with erbium

The hot deformation behavior of an Al–5.7 wt.%Mg alloy with erbium was studied by compressive deformation tests in the strain rate range of 0.001–10 s −1 and temperature range of 300–500 °C. The constitutive equations were presented considering the values of A and β as a function of strain in the exponential function form. The error of predicted and measured data was less than 9%. Dynamic recrystallization was restrained by precipitate with Er and dynamic recovery dominated in the hot deformation. The mechanism of dynamic recovery was dislocation movement and the development of microstructure during deformation can be characterized by the Z parameter, and then the relationship between subgrains and deformation parameters have been also established.

Mechanism of Dynamic Visual Acuity Recovery With Vestibular Rehabilitation

Mechanism of Dynamic Visual Acuity Recovery With Vestibular Rehabilitation

Mechanism of Dynamic Visual Acuity Recovery With Vestibular Rehabilitation

Schubert MC, Migliaccio AA, Clendaniel RA, Allak A, Carey JP. Mechanism of dynamic visual acuity recovery with vestibular rehabilitation. Objective To determine why dynamic visual acuity (DVA) improves after vestibular rehabilitation in people with vestibular hypofunction. Design Combined descriptive and intervention study. Setting Outpatient department in an academic medical institution. Participants Five patients (age, 42–66y) and 4 age-matched controls (age, 39–67y) were studied. Patients had vestibular hypofunction (mean duration, 177±188d) identified by clinical (positive head thrust test, abnormal DVA), physiologic (reduced angular vestibulo-ocular reflex [aVOR] gain during passive head thrust testing), and imaging examinations (absence of tumor in the internal auditory canals or cerebellopontine angle). Intervention Vestibular rehabilitation focused on gaze and gait stabilization (mean, 5.0±1.4 visits; mean, 66±24d). The control group did not receive any intervention. Main Outcome Measures aVOR gain (eye velocity/head velocity) during DVA testing (active head rotation) and horizontal head thrust testing (passive head rotation) to control for spontaneous recovery. Results For all patients, DVA improved (mean, 51%±25%; range, 21%–81%). aVOR gain during the active DVA test increased in each of the patients (mean range, 0.7±0.2 to 0.9±0.2 [35%]). aVOR gain during passive head thrust did not improve in 3 patients and improved only partially in the other 2. For control subjects, aVOR gain during DVA was near 1. Conclusions Our data suggest that vestibular rehabilitation increases aVOR gain during active head rotation independent of peripheral aVOR gain recovery.

Dynamic Error Recovery in theATLAS TDAQ System

This paper describes the new dynamic recovery mechanisms in the ATLAS Trigger and DataAcQuisition (TDAQ) system. The purpose of the new recovery mechanism is to minimize the impact certain errors and failures have on the system. The new recovery mechanisms are capable of analyzing and recovering from a variety of errors, both software and hardware, without stopping the data-gathering operations. An expert system is incorporated to perform the analysis of the errors and to decide what measures are needed. Due to the wide array of sub-systems there is also a need to optimize the way similar errors are handled for the different sub-systems. The main focus of the paper is to consider the design and implementation of the new recovery mechanisms and how expert knowledge is gathered from the different sub-systems and implemented in the recovery procedures.

UNIFIED TERMINOLOGY FOR STRAIN INDUCED BOUNDARIES

The terminology of strain induced or altered boundaries should reflect the mechanism that created them and the function they perform; both are related to the regions they surround or separate. Dislocation glide under applied stress is the primary mechanism in creep (higher rate) and hot, warm and cold working, being less influenced by dynamic recovery mechanism (DRV) as temperature T falls and strain rate rises. Dependent on the stress, boundaries contain dislocations that are geometrically necessary for themisorientations as well as dipoles that aremore dense in cold working; cell walls are rather incidental whereas higher-angle block walls enclose cell clusters developing a different slip. After DRV in hot forming, polygonized walls consist almost entirely of regularly arrayed, low-energy, geometrically-needed dislocations; such subgrain boundaries (SGB) during steady state straining continually rearrange in a stable stress-defined substructure. Following the Taylor-defined multiple-slip in polycrystals, grains divide into deformation bands slipping on different systems and rotating differently to create texture components. They are separated by transition boundaries that are permanent and increase in misorientation with strain to become largely indistinguishable from grain boundaries (GB). Original GB are disturbed by lattice dislocations and extended under strain; at high T, they migrate locally to become serrated by SGB.La terminologie des joints induits ou altérés par déformation devrait refléter le mécanisme qui les a créés et la fonction qu’ils performent; ils sont tous deux reliés aux régions qu’ils entourent ou séparent. Le glissement de dislocation sous contrainte externe est le mécanisme primaire du fluage (taux plus élevé) alors que la déformation à chaud, après préchauffage ou à froid, est moins influencée par le mécanisme de restauration dynamique (DRV) à mesure que la température T baisse et que la vitesse de déformation s’élève. Dépendant de la contrainte, les joints contiennent des dislocations qui sont nécessaires géométriquement pour les désorientations ainsi que des dipôles qui sont plus denses lors de la déformation à froid; les parois de cellule sont plutôt accessoires alors que les parois de bloc à grand angle enferment des grappes de cellules développant un différent glissement. Après la DRV lors du formage à chaud, les parois polygonisées consistent presque entièrement de dislocations en arrangements réguliers, à basse énergie, géométriquement nécessaires; de tels sous-joints de grains (SGB) se réarrangent continuellement, lors de la déformation en régime permanent, en une substructure stable à contrainte définie. Suivant les glissements multiples dans les polycristaux, tels que définis par Taylor, les grains se divisent en bandes de déformation glissant dans différents systèmes et pivotant différemment pour créer des composantes texturées. Elles sont séparées par des joints de transition qui sont permanents et qui augmentent en désorientation avec la déformation pour devenir largement indiscernables des joints de grain (GB). Les GB originaux sont dérangés par les dislocations du réseau et sont étirés sous déformation; à haute T, ils migrent localement pour devenir dentelés par des SGB.

ANISOTROPY AND ELEVATED TEMPERATURE DEFORMATION MECHANISMS IN AN Al-Li-Cu-Mn ALLOY

The 2090 experimental alloy was deformed in torsion in the temperature (T) range of 300 through 500 °C at strain rates (ε of 0.01 to 5 s-1. The results were analyzed using the equation A (sinh α σ)n = Z = ε exp (QHW/RT) where Z is the Zener-Hollomon parameter. For the stress (σ) multiplier α = 0.052 kJ/mol, the activation energy (QHW) of 219 kJ/mol was determined. During hot working, the mechanism of dynamic recovery was operating. The average subgrain size (d) changed with the conditions of deformation according to the relationships d-1 = 0.057 lnZ –1.62 and σm = 155 d-1 – 5.33. The need to reduce the anisotropic behaviour with a better understanding of deformation mechanisms was the overall goal of this research. Heterogeneous precipitation on dislocations and grain boundaries was studied using transmission electron microscopy (TEM). The nature and density of dynamically formed precipitates depend on the temperature and strain rate and contribute to an irregular temperature dependence of ductility. A large volume density of T1 precipitates in different orientations formed at all temperatures, but especially during slow strain rates at 400 and 500 °C. Nonuniform distribution of the T1 phase has a significant influence on the ductility and anisotropy of the 2090 experimental alloy.On a déformé par torsion l’alliage expérimental 2090 dans la gamme de température (T) de 300 à 500 °C, à des vitesses de déformation (ε) de 0.01 5 s-1. On a analysé les résultats en utilisant l’équation: A (sinh α σ)n = Z = ε exp (QHW/RT), où Z est le paramètre de Zener-Hollomon. On a déterminé l’énergie d’activation (QHW) de 219 kJ/mol pour le multiplicateur (α = 0.052 kJ/mol) de contrainte (σ). Lors du travail à chaud, le mécanisme de restauration dynamique opérait. La taille moyenne (d) du sous-grain changeait avec les conditions de déformation d’après les relations: d-1 = 0.057 lnZ –1.62 et σm = 155 d-1 – 5.33. Le but global de cette recherche était le besoin de réduire le comportement anisotrope avec une meilleure compréhension des mécanismes de déformation. On a étudié la précipitation hétérogène sur les dislocations et aux joints de grain en utilisant la microscopie électronique à transmission (TEM). La nature et la densité des précipités formés dynamiquement dépendent de la température et de la vitesse de déformation et contribuent à une dépendance irrégulière de la ductilité sur la température. Une grande densité volumique de précipités T1 avec différentes orientations s’est formée à toutes les températures, mais plus spécialement aux basses vitesses de déformation à 400 et 500 °C. Une distribution non uniforme de la phase T1 a une influence importante sur la ductilité et sur l’anisotropie de l’alliage expérimental 2090.

A Dynamic Packet Recovery Mechanism for Realtime Service in Mobile Computing Environments

This paper analyzes the characteristics of packet losses in mobile computing environments based on the Gilbert model and then describes a mechanism that can recover the lost audio packets using redundant data. Using information periodically reported by a receiver, the sender dynamically adjusts the amount and offset values of redundant data with the constraint of minimizing the bandwidth consumption of wireless links. Since mobile computing environments can be often characterized by frequent and consecutive packet losses, loss recovery mechanisms need to deal efficiently with both random and consecutive packet losses. To achieve this, the suggested mechanism uses relatively large, discontinuous exponential offset values. That gives the same effect as using both the sequential and interleaving redundant information. To verify the effectiveness of the mechanism, we extended and implemented RTP/RTCP and applications. The experimental results show that our mechanism, with an exponential offset, achieves a remarkably low complete packet loss rate and adapts dynamically to the fluctuation of the packet loss pattern in mobile computing environments.

Strength and plastic flow in “in situ” TiC reinforced aluminum composites

The deformation behavior of TiC particulate-reinforced aluminum composites (Al-TiCp) was investigated in this work using pure aluminum as the reference matrix material. Uniaxial compression tests were carried out at 293 and 623 K and at two strain rates (3.7×10−4 and 3.7×10−3 s−1). Yield strengths of up to 127 MPa were found in composites containing 10 vol pct TiC particulates, which were almost 4 times the yield strength of pure Al. In addition, at 623 K, relatively small reductions in yield strength were found, suggesting that this property was rather insensitive to temperature for the temperatures investigated in this work. Nevertheless, at 623 K, increasing the rate of straining from 3.7×10−4 s−1 to 3.7×10−3 s−1 lowered the yield strength, particularly in 10 vol pct TiCp-Al composites. Two stages of work hardening were identified in pure Al and a 10 vol pct TiCp composite during plastic flow through the modified version of the Hollomon equation (σ = Kɛn ± Δ). In particular, the work-hardening exponents found in pure Al shifted from high to low values as the extent of plastic strain was increased while the opposite was true for the 10 vol pct TiCp composite. Finally, at 623 K, dynamic recovery mechanisms became dominant at plastic strain levels >0.2 in 10 vol pct TiCp-Al composites, with the effect being minor at room temperature.

Enhancing the quality of Internet voice communication for Internet telephony systems

Internet real time voice communication involves transmitting digitized voice signals which can be lost in the packet-switched network environment of TCP/IP, thereby causing intermittent voice losses and quality degradations. Various methods such as silence substitution, waveform substitution, sample interpolation, Xor mechanism, embedded speech coding, and the combined rate and control mechanism have been proposed to enhance voice delivery and to minimize the quality impact caused by these losses. This paper proposes a quality-based dynamic voice recovery mechanism that combines network transmission control and voice recovery to deliver voice signals with optimal intelligibility and quality. This is accomplished by considering the subjective rating of different codecs that are used in the coding and transmission of digital audio and network packet loss conditions. The dynamic mechanism results in voice delivery that, at minimum, satisfies voice intelligibility while tolerating moderate packet loss caused by network congestion. This mechanism has been successfully incorporated into the Internet Telephone Software System developed at the School of Applied Science, Nanyang Technological University.

An Implicit integration scheme for generalized viscoplasticity with dynamic recovery

This paper is concerned with the development of a general implicit time-stepping integrator for the flow and evolution equations in a recent representative class of generalized viscoplastic models, involving both hardening and dynamic recovery mechanisms. To this end, the computational framework is developed on the basis of the unconditionally stable, backward Euler difference scheme. Its mathematical structure is of sufficient generality to allow a systematic treatment of several internal variables of the tensorial and scalar types. The matrix forms developed are directly applicable in general (three-dimensional) situations as well as subspace applications (i.e., plane stress/strain, axisymmetric, generalized plane stress in shells). The closed-form expressions for residual vectors and the algorithmic, (consistent) material tangent stiffness array are given explicitly, with the maximum matrix sizes “optimized” to depend only on the number of independent stress components, but not the number of internal state variables involved. Several numerical simulations are given to assess the performance of the developed schemes.

Modelling of work hardening and stress saturation in FCC metals

A work hardening theory has been developed based on a microstructural concept comprising three elements; the cell/subgrain size, δ, the dislocation density inside the cells, ρ i , and the cell boundary dislocation density or the sub boundary misorientation, ρ b or ϕ. The theory is based on a statistical approach to the storage of dislocations. This approach predicts that the slip length, L, scales with the inverse square root of the stored dislocation density, ρ −1/2, and also, predicts a substructure evolution which is consistent with the concept of microstructural scaling (similitude) at zero degree Kelvin, at stress τ< τ III . The model provides a solution to the basic `dislocation–book-keeping-problem' by defining a differential equation which regulates the storage of dislocations into (i) a cell interior dislocation network, (ii) increases in boundary misorientation and (iii) the creation of new cell boundaries. By combining such a solution for the dislocation storage problem with models for the dynamic recovery of network dislocations and sub-boundary structures, the result becomes a general internal state variable solution which has the potential of giving the flow stress as a function of strain for any combination of strain-rate and temperature. The theory predicts that the dislocation density ρ i inside the cells saturates at the end of the Stage II–III transition, a saturation effect which regulates the subsequent stress–strain behaviour which is then controlled by the continuous refinement of the cell/subgrain structure. The characteristic features of Stages III and IV are well accounted for by the model. The present work hardening model also provides the necessary basis for the construction of constitutive laws for the saturation stresses; τ III , τ IIIs and τ s and steady-state creep. Beyond Stage II breakdown only two rate controlling dynamic recovery mechanisms are involved, relating to dislocation network growth and subgrain growth. The activation energy of both growth reactions is that of self diffusion. Steady-state creep laws are presented covering the entire range from low temperature creep to Harper–Dorn creep.

Hot deformation mechanisms of a solution-treated Al-Li-Cu-Mg-Zr alloy

Solution-treated 8090 and 8091 Al-based alloys were subjected to hot torsion testing in the temperature range of 300 °C through 500 °C at strain rates of 0.1 to 5 s−1, up to an equivalent strain of 4. The flow stresses for alloys 8090 (8091) were found to depend on strain rate through a sinh function with exponent 3.98 (2.37) and on temperature through an Arrhenius behavior with activation energy of about 287 (282) kJ/mol. Studies using transmission electron microscopy (TEM) have been performed with the aim of understanding the difference in deformation mechanisms at 500 °C, 400 °C, and 300 °C. During hot processing, the mechanism of dynamic recovery is operative. The change in average subgrain size (d) with the conditions of deformation,i.e., Zener-Hollomon parameter and steady-state flow stress (σs), was quantitatively characterized. Heat treatment at 550 °C induced the precipitation of Al3Zr particles which are resistant to dislocation shear. Furthermore, electron microscopic analyses have revealed a large number of helical dislocations, prismatic loops, and some Orowan loop formation after deformation at 500 °C and 400 °C. The density of these defects depends on the temperature of deformation and strain rate. At 300 °C, dynamic precipitation of T2 (Al6CuLi3) and T1 (Al2CuLI) phases strongly affected hot deformation behavior. In all cases, the microstructural analyses were consistent with a dual-slope description of the mechanical behavior during hot deformation.

Effect of strain rate on the mechanical response of a Ti3Al-Nb-Mo alloy

In this study, the effect of strain rate and temperature on the substructure evolution and mechanical response of a Ti3Al-Nb-Mo alloy (Ti-24.5Al-10.5Nb-l.5Mo) was investigated. The yield and flow stresses were found to decrease gradually with increasing temperature at the strain rate of 3000 s1 and no flow stress/temperature anomaly was observed. The variation of hardening behavior of Ti-24.5Al-10.5Nb-1.5Mo alloy with temperature and strain rate was found to be negligible, implying that the dynamic recovery mechanism for this alloy is rather athermal. Dynamic recovery of Ti-24.5Al-10.5Nb-l.5Mo alloy is thought to be quite difficult and slow because of its complicated dislocation core structure. Ti-24.5Al-10.5Nb-l.5Mo alloy exhibited predominantly planar dislocation debris after deformation. Following the room temperature deformation at low strain rate the majority of the dislocations were a-dislocations lying on the basal and 1storder pyramidal slip planes and the 2nd-order pyramidala/2+c slip on {1211}. Increasing the rate of deformation at room temperature to 6000 s1 resulted in the increaseda-slip on prism planes and the lessor amount of basal slip. At elevated temperatures, the substructure after high rate deformation was found to be similar to that following high rate deformation at room temperature except for an increased amount of basal slip and somewhat higher incidence of the 2nd-order pyramidal slip. The evidence of an increased amount of shearing of the βo phase was observed at elevated temperature, suggesting the decreased strength of the βo phase.

An analysis of splitting failures during the drawing of tungsten wire

A model is proposed to describe crack formation (referred to as wire splitting) that can occur during wire drawing of commercial tungsten lamp wire. In the proposed model, wire splitting is said to occur in response to residual tensile hoop stresses that are generated during wire drawing. The resistance of tungsten wire to crack formation is said to be controlled by the degree of polygonization that has taken place during wire drawing. It was found that wire splitting did not occur when high wire drawing temperatures were maintained throughout the seven wire drawing passes that were studied. Previous work in which dynamic recovery mechanisms were quantified for various wire sizes and drawing temperatures allowed us to show that the drawing temperature, for a given wire size, above which wire splits were not detected coincided well with the onset temperature of dynamic polygonization.

A Modeling Investigation of Thermal and Strain Induced Recovery and Nonlinear Hardening in Potential Based Viscoplasticity

Specific forms for both the Gibb’s and the complementary dissipation potentials were chosen such that a complete potential based multiaxial, isothermal, viscoplastic model was obtained. This model, in general, possesses three internal state variables (two scalars associated with dislocation density and one tensor associated with dislocation motion) both thermal and dynamic recovery mechanisms, and nonlinear kinematic hardening. This general model, although possessing associated flow and evolutionary laws, is shown to emulate three distinct classes of theories found in the literature, by modification of the driving threshold function F. A parametric study was performed on a specialized nondimensional multiaxial form containing only a single tensorial internal state variable (i.e., internal stress). The study was conducted with the idea of examining the impact of including a strain-induced recovery mechanism and the compliance operator, derived from the Gibb’s potential, on the uniaxial and multiaxial response. One important finding was that inclusion of strain-induced recovery provided the needed flexibility in modeling stress-strain and creep response of metals at low homologous temperatures, without adversely affecting the high temperature response. Furthermore, for nonproportional loading paths, the inclusion of the compliance operator had a significant influence on the multiaxial response, but had no influence on either uniaxial or proportional load histories.

High temperature deformation kinetics of Al—4Mg alloy

The flow behaviour and kinetics of deformation in the temperature and strain-rate range encountered are important and relevant in material processing since this information will be useful in the calculation of working loads and in identifying the mechanisms of deformation. An evaluation of the mechanism of high-temperature deformation suggests directions of control for enhanced workability and for developing superior properties. In this study, solid cylindrical specimens of Al—4Mg alloy were subjected to insiute compression tests in the temperature range of 550–800 K and at strain rates of 0.02, 0.2, 8 and 200 s −1. The flow stress, true strain, strain rate and temperature rise were evaluated from the transient measurements made during the tests. The flow stress values have been represented in terms of actual or adiabatic temperature instead of initial test temperature. The kinetics of high temperature deformation are expressed by means of some empirical rate equations. Based on the activation parameters, a mechanism of dynamic recovery involving cross-slip of dislocations has been identified to be the high temperature rate controlling mechanism in the Al—4Mg alloy.

Effet de la structure des joints de grains sur la déformation plastique des polycristaux

The interaction of lattice dislocations with high angle grain boundaries is considered from the point of view of recently discovered process of spreading of extrinsic grain boundary dislocations (EGBDs). The Lojkowski and Grabski model for the spreading kinetics is explained and the new experimental evidence is provided, showing that the spreading rate is a most sensitive available indicator of the state of grain boundary structure, allowing for the accurate determination of g.b. self diffusion coefficients and g.b. energy. With use of spreading kinetics measurements it is shown that as a result of spreading the grain boundary structure deviates from the equilibrium state, and that high temperature annealing transforms the structure back towards the equilibrium one. This process of transformation is called the recovery of grain boundary structure.The proof is delivered that the changes of grain boundaries structure produced by the spreading process exerts a strong effect on the value of the yield stress, and that the annihilation of dislocations through the spreading or EGBDs is an important mechanism of dynamic recovery, affecting the plastic flow at high temperatures. The recent results on Hall-Petch relation in stainless steel and aluminium and on Piobert-Luders bands formation in pure aluminium are presented and discussed in terms of changes in grain boundaries structure. It is concluded, that the changes in grain boundary state can influence the mechanical properties of polycrystal much more strongly than changes in grain size.

ChemInform Abstract: HOT WORKABILITY AND KINETICS OF HOT DEFORMATION IN ELECTROSLAG‐REFINED HEAT‐RESISTANT ALUMINUM ALLOY

AbstractDie Elektroschlackenaffination mittels synthetischer Schlacke (KCl‐MgCl2‐MgF2) bewirkt eine Verminderung der Makro‐ und Mikroporosität sowie eine Verfeinerung des Gefüges der Legierung (RR58 oder 2618), die eine feinere Dispersion der zweiten Phase zeigt.