Long-term Relaxation Research Articles

Positron annihilation lifetime spectroscopy (PALS) studies of gamma irradiated As2Se3 films used in MIR integrated photonics

Chalcogenide glass (ChG) materials are increasingly being utilized in infrared planar photonic devices due to their tailorable refractive indices and compatibility with other on-chip materials and manufacturing methodologies. The performance and reliability of such integrated photonic components in land or space-based environments where radiation impact structure and properties is important to their ultimate deployment. Bulk glasses and thermally evaporated thin films of As2Se3 were subjected to a gamma exposure of 229Mrad to compare bulk/film material response and the impact of film thickness on such response. Positron annihilation lifetime (PAL) spectroscopy was employed to compare the effect of gamma irradiation on the different forms of the glass. The average positron lifetime was found to be longer for the films than the bulk glass in unirradiated specimens, and the films showed more significant increases in lifetimes with gamma exposure. This effect was more pronounced for thinner films, which showed, for example, an increase in positron lifetime. Positron lifetime was measured over the course of 20days after exposure, over which the effect of the gamma radiation was found to relax at room temperature. Long-term or incomplete relaxation of the gamma-induced changes were observed in the films, while the bulk glass positron lifetime returned to pre-exposed values. A proposed mechanism for these observations based on coordination topological defects (CTD) is presented to explain the initial and longer term response, and the possible consequence when such films are integrated in device geometries.

On Impedance Spectroscopy of Supercapacitors

Supercapacitors are often characterized by responses measured by methods of impedance spectroscopy. In the frequency domain these responses have the form of power-law functions or their linear combinations. The inverse Fourier transform leads to relaxation equations with integro-differential operators of fractional order under assumption that the frequency response is independent of the working voltage. To compare long-term relaxation kinetics predicted by these equations with the observed one, charging-discharging of supercapacitors (with nominal capacitances of 0.22, 0.47, and 1.0 F) have been studied by means of registration of the current response to a step voltage signal. It is established that the reaction of devices under study to variations of the charging regime disagrees with the model of a homogeneous linear response. It is demonstrated that relaxation is well described by a fractional stretched exponent.

Analysis on the Creep Mechanism of 12Cr-Mo-W-0.25V Steel Based on Long-Term Stress Relaxation Test Data

The current paper investigates on the creep behavior of 12Cr-Mo-W-0.25V heat resistant steel base on the long-term stress relaxation test data. It is shows that the stress relaxation curve can be divided into 2 stages: the high stress stage has higher apparent activation volume of 79~350 b3 and the low stress stage is 35~78 b3. Besides, the Helmholtz free energy at the high stress stage is 827~1034 kJ/mol which is higher than 210~252 kJ/mol of the low stress stage. Taking both apparent activation volume and activation energy into account, it is assumed that the high stress stage is mainly controlled by dislocation slip and the low stress stage is more related to diffusion.

F-centers mechanism of long-term relaxation in lead zirconate-titanate based piezoelectric ceramics. 2. After-field relaxation

The paper presents results of experimental study of the dielectric constant relaxation during aging process in Pb(Zr,Ti)O3based solid solutions (PZT) after action of external DC electric field. The said process is a long-term one and is described by the logarithmic function of time. Reversible and nonreversible relaxation process takes place depending on the field intensity. The relaxation rate depends on the field strength also, and the said dependence has nonlinear and nonmonotonic form, if external field leads to domain disordering. The oxygen vacancies-based model for description of the long-term relaxation processes is suggested. The model takes into account the oxygen vacancies on the sample's surface ends, their conversion into [Formula: see text]- and [Formula: see text]-centers under external effects and subsequent relaxation of these centers into the simple oxygen vacancies after the action termination. [Formula: see text]-centers formation leads to the violation of the original sample's electroneutrality, and generate intrinsic DC electric field into the sample. Relaxation of [Formula: see text]-centers is accompanied by the reduction of the electric field, induced by them, and relaxation of the dielectric constant, as consequent effect.

A triangular finite element using Laplace transform for viscoelastic laminated composite plates based on efficient higher-order zigzag theory

To predict the time-dependent behaviors of viscoelastic laminated composites, a three-node multilayered plate element is developed based on the efficient higher-order plate theory (EHOPT), which was originally proposed by Cho and Parmerter. With the help of the Laplace transform, the integral form of the constitutive equation in the time domain is reduced to an algebraic equation in the Laplace domain. Thus, the structures and advantages of the EHOPT can be preserved in viscoelastic laminated composites in the Laplace domain. Since the time dimension is transformed to Laplace domain, the finite element discretization is only used in the spatial domain. A nonconforming three-node triangular element is employed to implement the viscoelastic EHOPT for finite element analysis. To pass the proper bending and shear patch tests in arbitrary mesh configurations, the modified shape function developed by Specht is applied and converted into Laplace domain. Therefore, the final numerical results, which is obtained by using inverse Laplace techniques, always converge to the corresponding analytical solutions. In order to verify the efficiency and accuracy of the present study, some numerical examples for long-term creep and relaxation processed are performed. The present viscoelastic finite element of composite laminates provides a powerful tool to accurately investigate the responses of the viscoelastic and time-dependent mechanical behaviors of composite laminates.

Application of TTSP to wood-development of a vertical shift factor

Abstract The applicability of the time-temperature superposition principle (TTSP) to wood has been investigated aiming at the prediction of long-term mechanical properties of wood by both horizontally and vertically shifting of short-term stress relaxation data obtained by experiments. The expression of TTSP considering the vertical shift factor (b T ) for wood is proposed the first time. The results showed that: (1) TTSP applied to poplar and the master curve that was obtained from 1 h of tests at 283.2, 303.2, 320.2, 343.2, and 363.2 K in a relative humidity (RH) of 60% could predict the stress relaxation behavior for approximately 42 years at 283.2 K and 60% RH. (2) There was a linear correlation between lga T and T -1, lg a T =6590.40 T -1 -23.64 (R2=0.994), which followed the Arrhenius equation well, while the apparent activation energy was 34.6 kcal mole-1. (3) The b T had a linear relationship with temperature, and the relation was lgb T =0.0013T-0.37 (R2=0.999). (4) The long-term relaxation curve of the long-term verification test had high goodness of fit with the master curve. The results can be interpreted that the TTSP expression considering the b T proposed in this paper is rational.

Physical ageing and lifetime prediction of polymer membranes for gas separation processes

The free volume in glassy polymers for gas separation processes is affected decidedly by physical ageing. In this work, we analyse the long-term volume relaxation of glassy polymer membranes using statistical data analysis in order to predict the lifetime of these membranes with minimal experimental effort. Several lifetime definitions are proposed and discussed in their use for the prediction quality. A principal objective of this work is the statistical analysis of the phenomenological ageing model developed by Struik using the bootstrapping method. We analyse the stepwise reduction of the number of data points used in model fitting. Based on these results we propose an optimal modelling and experimental strategy directed at the prediction of thickness dependent volume relaxation. In particular, the influence of the duration of pointwise measurements is discussed in detail. Our investigation is concluded by the dimensional analysis of the Struik equation, resulting in a master curve concept. The analysis of the master curve reveals a weak dependence of the lifetime of a membrane on its initial condition. The lifetime is proportional to the relaxation time for a criterion based on the absolute permeability and inverse proportional to the relaxation time for a criterion based on the derivative of the permeability for the chosen threshold parameters.

Osoblyvosti protikannya strumu pry ul'trazvukovomu navantazhenni v syl'no kompensovanykh nyz'koomnykh krystalakh CdTe:Cl

To elucidate the mechanism of influence of ultrasound on the temperature, T, dependence of conductivity (q(T)) in low-resistance CdTe:Cl (NCl ≈ 1024 m−3) single crystals of the n-type, the Hall effect and the relaxation kinetics of q(t) at the ultrasound (fUS ∼ 10 MHz, WUS ∼ 104 W/m2) switching-on and -off have been studied in a temperature interval from 77 to 300 K. A completely reversible dynamical influence of ultrasound is revealed for the first time. It has different characters for the low (LT, T < 180 K) and high (HT, T > 200 K) temperature intervals. Acoustically stimulated changes in the HT region are found to be in-significant: the mobility of charge carriers decreases a little, and long-term processes of q(t) relaxation are not observed. In the LT region, the relative acoustically stimulated changes grow; in particular, the duration of q(t) relaxation processes increases, and they reveal a two-stage character. To explain this phenomenon, the model of a heterogeneous semiconductor containing clusters of impurity defects in vicinities of dislocations is applied. A mechanism is proposed that relates the “instant” increase of q(t) with the acoustically stimulated reduction of the amplitude of fluctuations of the large-scale potential owing to the enlargement of the effective electronic radius of dislocation impurity clusters. Long-term (50–500 s) temperature-dependent relaxation processes are governed by the diffusive reconstruction of the point-defect structure in the cluster bulk, including the transformation of acceptor (V2−CdCl+Te)− complexes into neutral (V2−Cd2Cl+Te)0 ones.

Reverse ultrasonic changes of electrical conductivity in CdTe:Cl crystals

Acousto-stimulated changes of electrical conductivity in low ohmic resistance n-type CdTe:Cl monocrystals are reversible and can be seen only during the ultrasonic influence. For the first time the long-term relaxation processes of the conductivity σ(t) at ultrasound-on as well as at ultrasound-off have been found out. The relaxation σ(t) is not monotonous generally, but, at least, has two stages. Acousto-active metastable defects are resposible for “instant” jump-like changes of the temperature dependence of conductivity σ(t). The acousto-stimulated diffusion processes where the defining role belongs to dislocations, are responsible for long-term relaxation the temperature dependence of conductivity σ(t). The main rebuilding process at ultrasound-on consists of the transformation of the charged acceptor complex into a neutral one.

Model of the distraction callus tissue behavior during bone transport based in experiments in vivo

Bone transport studies have measured the forces related to bone segment distraction (Brunner et al., 1994; Hyodo et al., 1996). However, no distraction force distribution between callus and docking-site was reported. Besides, most of these works have not provided continuous and long-term force relaxation measurements. The fit of the relaxation curves allows for modeling the mechanical behavior of the callus tissue during distraction osteogenesis, particularly in bone transport, where the resistance of the soft tissue and muscle is reduced compared with the bone lengthening. Bone transport experiments were carried out in sheep in which the distraction force was monitored continuously in vivo. The daily force relaxation curves were fitted, and two experimental models of the mechanical behavior of the callus tissue were obtained, assuming the total daily force relaxation or the accumulation of the residual forces. According to these models, the residual force 24h after each distraction step was a maximum of 71.6N, and the peak distraction force increased with the number of steps from 7–34N to 41–246N. The maximum residual force values that were predicted are much lower than those measured during bone lengthening in the literature. These results indirectly differentiate the influence of the surrounding soft tissues during bone transport compared with bone lengthening. Moreover, experimental measurements showed that distraction force through the docking-site was negligible with respect to distraction force through the callus. Experimental models of the callus tissue allow for an understanding of the mechanobiology of distraction osteogenesis and for predicting outcomes in its application processes.

Effects of Stress and Relaxation on Central Pain Modulation in Chronic Whiplash and Fibromyalgia Patients Compared to Healthy Controls

Background: Compelling evidence has demonstrated that impaired central pain modulation contributes to persistent pain in patients with chronic whiplash associated disorders (WAD) and fibromyalgia (FM). However, there is limited research concerning the influence of stress and relaxation on central pain modulation in patients with chronic WAD and FM. Objectives: The present study aims to investigate the effects of acute cognitive stress and relaxation on central pain modulation in chronic WAD and FM patients compared to healthy individuals. Study Design: A randomized crossover design was employed. Setting: The present study took place at the University of Brussels, the University Hospital Brussels, and the University of Antwerp. Methods: Fifty-nine participants (16 chronic WAD patients, 21 FM, 22 pain-free controls) were enrolled and subjected to various pain measurements. Temporal summation (TS) of pain and conditioned pain modulation (CPM) were evaluated. Subsequently, participants were randomly allocated to either a group that received progressive relaxation therapy or a group that performed a battery of cognitive tests (= cognitive stressor). Afterwards, all pain measurements were repeated. One week later participant groups were switched. Results: A significant difference was found between the groups in the change in TS in response to relaxation (P = 0.008) and cognitive stress (P = 0.003). TS decreased in response to relaxation and cognitive stress in chronic WAD patients and controls. In contrast, TS increased after both interventions in FM patients. CPM efficacy decreased in all 3 groups in response to relaxation (P = 0.002) and cognitive stress (P = 0.001). Limitations: The obtained results only apply for a single session of muscle relaxation therapy and cognitive stress, whereby no conclusions can be made for effects on pain perception and modulation of chronic cognitive stress and long-term relaxation therapies. Conclusions: A single relaxation session as well as cognitive stress may have negative acute effects on pain modulation in patients with FM, while cognitive stress and relaxation did not worsen bottom-up sensitization in chronic WAD patients and healthy persons. However, endogenous pain inhibition, assessed using a CPM paradigm, worsened in chronic WAD and FM patients, as well as in healthy people following both interventions. Key words: Chronic pain, central sensitization, endogenous pain inhibition, temporal summation of pain, cognitive stressor, relaxation, fibromyalgia, whiplash-associated disorders

Combined logarithmic and exponential function model for fitting postseismic GNSS time series after 2011 Tohoku-Oki earthquake

The time series of a postseismic deformation is commonly fitted by a logarithmic or exponential decay function. However, the high-quality postseismic Global Navigation Satellite System (GNSS) time series of the 2011 Mw 9 Tohoku-Oki earthquake indicates that a single decay function cannot be used to represent the postseismic behaviour. We therefore combined the logarithmic (log) and exponential (exp) decay functions and developed methods for obtaining global solutions using nonlinear least squares calculations for such complex functions. Our models significantly improved the fitting performance of the postseismic time series and the prediction performance of the evolution of postseismic deformation. The solutions obtained by the proposed models and methods enabled distinction between the contributions of the log and exp functions, and explanation of characteristic phenomena such as the subsidence that occurs immediately after an earthquake is reversed to an uplift. The analysis of the solutions may suggest that there has been a continuous increase in the contribution of viscoelastic relaxation to postseismic deformation in eastern Japan, whereas the contribution of afterslip has rapidly decreased. The short-term prediction performance and the universal applicability of the proposed models to the Tohoku-Oki earthquake have contributed to the detection of a slow-slip event in the Tokai region. Rather than the existence of a unique single relaxation time for each surface site, our results suggest a unique single relaxation time for each postseismic deformation mechanism at a given subsurface location. Although the predictions were highly dependent on the assigned steady velocities and the long-term relaxation time constants, they indicate that the coseismic subsidence of the Yamoto station in Miyagi prefecture will recover around the year 2020. The estimated relaxation time constants of the present models appeared to be uniform throughout eastern Japan.

Performance peculiarities of carbon-nanotube-based thin-film saturable absorbers for erbium fiber laser mode-locking

We have studied the saturation behavior of single-walled carbon-nanotube-based saturable absorbers (SWCNT-SAs) at different temperatures and SWCNT concentrations in the polymer matrix and tried to relate it to the mode-locked erbium-doped fiber laser performance. It has been observed that a modulation depth of SWCNT-SA transmission which we relate to the ground state recovery time monotonically decreases upon temperature growth. Comparing laser performance with different SWCNT-SAs, we suppose that the defects in the CNT lattice promote recovery acceleration resulting in shorter pulse generation as obtained for boron-nitride-doped SWCNTs. Moreover, for the first time, to the best of our knowledge, we have observed and studied the long-term relaxation of SWCNT-SA saturated transmission exhibiting temperature-dependent behavior.

To a Method of Polarization-Depolarization Currents for Diagnosis of Dielectric Isolation

Fractional derivative formalism is proposed as the mathematical foundation of the polarization-depolarization current method for the diagnosis of dielectric isolation. Physical basis of the new approach is the observed deviation of the long-term relaxation from the Debye exponential law. We found that this behavior is consistent with the solution of the fractional differential equation: exponential behavior turns into the power dependence in the long-time asymptotics, and this part of the relaxation curve is more sensitive to the material state. The results of calculations for the polarization-depolarization currents in an oil-paper capacitor are in agreement with the specially performed experiments.

F-center mechanism of long-term relaxation in lead zirconate-titanate-based piezoelectric ceramics. 1. After-heating relaxation

Results of experimental investigation of relaxation aging processes in Pb(Zr,Ti)O3-based solid solutions after termination of external actions are presented. Heating, DC electric field, uniaxial pressure and some of their combinations were taken as external actions. In the main part of the present paper, we use heating as external action. The said processes are long-time one and are described by the logarithmic function of time.Reversible and nonreversible relaxation processes take place depending on the action intensity. The relaxation rate depends on the action intensity also, and the said dependence has nonlinear and nonmonotonic form if external action leads to domain disordering.The oxygen vacancies-based model for description of the long-time relaxation processes is suggested. The model takes into account oxygen vacancies on the sample’s surface ends, their conversion into [Formula: see text]- and [Formula: see text]-centers under external effects (due to the liberation of the pyroelectric charge) and subsequent relaxation of these centers into the simple oxygen vacancies after the actions termination. The initial sample’s state is electroneutrality one. F-center formation leads to the violation of the original sample’s electroneutrality, and generates DC electric field into the sample. Relaxation of F-centers is accompanied by decreasing of electric field, induced by them, and dielectric constant relaxation as consequent effect.

Incorporating Density and Temperature in the Stretched Exponential Model for Predicting Stress Relaxation Behavior of Polymer Foams

This paper discusses an approach to incorporate density and temperature terms in the well-known stretched exponential (SE) model for predicting the stress relaxation behavior of polymer foams. We have developed this approach for closed-cell polyurethane foams (PUFs) and verified using experimental data for accuracy. The SE model was first examined using short-term experimental data to predict long-term stress relaxation behavior of PU solid (PUS). The corresponding model parameters were then extracted for PUS and two PUFs with different densities (PU404 and PU415) at three different test temperatures. Finally, an expression was developed in conjunction with the modified Gibson–Ashby relationship and the Arrhenius equation and validated for other foam density (PU420) and test temperatures. The predictions were found to be reasonably good with more than 90% accuracy.

Thermally Activated Deformation Behavior of ufg-Au: Environmental Issues During Long-Term and High-Temperature Nanoindentation Testing.

For testing time-dependent material properties by nanoindentation, in particular for long-term creep or relaxation experiments, thermal drift influences on the displacement signal are of prime concern. To address this at room and elevated temperatures, we tested fused quartz at various contact depths at room temperature and ultra-fine grained (ufg) Au at various temperatures. We found that the raw data for fused quartz are strongly affected by thermal drift, but corrected by use of dynamic stiffness measurements all the datasets collapse. The situation for the ufg Au shows again that the data are only useful with drift correction, but with this applied it turns out that there is a significant change of elastic and plastic properties when exceeding 200°C, which is also reflected by an increasing strain rate sensitivity.

Research on the time–temperature–damage superposition principle of NEPE propellant

To describe the relaxation behavior of NEPE (Nitrate Ester Plasticized Polyether) propellant, we analyzed the equivalent relationships between time, temperature, and damage. We conducted a series of uniaxial tensile tests and employed a cumulative damage model to calculate the damage values for relaxation tests at different strain levels. The damage evolution curve of the tensile test at 100 mm/min was obtained through numerical analysis. Relaxation tests were conducted over a range of temperature and strain levels, and the equivalent relationship between time, temperature, and damage was deduced based on free volume theory. The equivalent relationship was then used to generate predictions of the long-term relaxation behavior of the NEPE propellant. Subsequently, the equivalent relationship between time and damage was introduced into the linear viscoelastic model to establish a nonlinear model which is capable of describing the mechanical behavior of composite propellants under a uniaxial tensile load. The comparison between model prediction and experimental data shows that the presented model provides a reliable forecast of the mechanical behavior of propellants.

Localized contacts, stress concentrations and transient states in bent-lamination with viscoelastic adhesion. An analytical study

An analytical study is presented for the bent-lamination of curved layered beams, a process consisting in gluing the constituent plies together after they have been elastically bent against a constraining negative mould. Possible applications range from glued laminated timber manufacturing, to cold-lamination-bending of structural glass. After removal of the constraint, the shear coupling through the glue maintains the curvature only partially, because the laminate suffers an initial spring-back followed by a long-term relaxation. The model problem considered here is that of two Euler–Bernoulli beams coupled by a thin viscoelastic adhesive layer. Within a variational approach, we analytically describe the relationship between the mould shape and the shape of the curved beam, which is time-dependent due to the viscosity of the adhesive layer. Localized contacts with the mould and stress concentrations may occur, depending upon the type of profile that is initially imposed. Comparison of the cases of instantaneous or gradual release of the contact with the mould, evidences a remarkable reduction of the transient state of stress in the second case.

Intensification of the ion transport in an aluminum chloride electrolyte

It is found that the electrical conductivities of molten aluminum chloride electrolytes increase with the electric field and reach limiting values, which are higher than the low-voltage values by several hundreds of percent. Under the action of high-voltage microsecond pulses, the melts transform into a nonequilibrium state with a increased electrical conductivity and long-term relaxation.