Long-term Relaxation Research Articles

Experimental and numerical research of stress relaxation behavior of magnetorheological elastomer

The paper presents experimental research and viscoelastic modeling of stress relaxation response of isotropic magnetorheological elastomer (MRE). The isotropic MRE has been prepared based on silicone matrix filled by magnetically micro-sized carbonyl iron particles. Effects of constant strain level and external magnetic field on the stress relaxation behavior of the MRE were carefully investigated by single- and multi-step relaxation tests in shear mode using double-lap shear specimens. Results revealed that the stress relaxation response of the MRE was dependent on the applied constant strain and external magnetic field. The relaxed stress and modulus of the MRE increased with increasing the constant strain level. In addition, the values of absolute stress and modulus in the relaxation periods enhanced with the rise of magnetic flux density. A four-parameter fractional derivative viscoelastic model was used to describe the stress relaxation behavior of the MRE. The studied model was fitted well to experimental data of the MRE in both single- and multi-step relaxation tests. The fitting of shear-stress relaxation modulus for the MRE is in a very good agreement with the experimental one. Effects of applied constant strain and magnetic field intensity on the fitted parameters were discussed. Moreover, the model can be applied to predict accurately the long-term relaxation behavior of the MRE.

Erroneous or Arrhenius: A Degradation Rate-Based Model for EPDM during Homogeneous Ageing.

To improve the predictive capability of long-term stress relaxation of elastomers during thermo-oxidative ageing, a method to separate reversible and irreversible processes was adopted. The separation is performed through the analysis of compression set after tempering. On the basis of this separation, a numerical model for long-term stress relaxation during homogeneous ageing is proposed. The model consists of an additive contribution of physical and chemical relaxation. Computer simulations of compression stress relaxation were performed for long ageing times and the results were validated with the Arrhenius treatment, the kinetic study and the time-temperature superposition technique based on experimental data. For chemical relaxation, two decay functions are introduced each with an activation energy and a degradative process. The first process with the lower activation energy dominates at lower ageing times, while the second one with the higher activation energy at longer ageing times. A degradation-rate based model for the evolution of each process and its contribution to the total system during homogeneous ageing is proposed. The main advantage of the model is the possibility to quickly validate the interpolation at lower temperatures within the range of slower chemical processes without forcing a straight-line extrapolation.

Analysis and modeling of uniaxial compressive creep of MMA-modified unsaturated polyester polymer concrete

Creep and shrinkage are the major time-dependent phenomena occurring in concrete. Because they significantly affect the long-term dimensional changes and relaxation of restrained stresses in concrete, it is of great importance to accurately consider these characteristics in the structural analysis and design process. In this study, laboratory experiments were conducted to examine the creep under uniaxial compression and linear setting shrinkage of unsaturated polyester (UP) polymer concrete modified with three different levels of methyl methacrylic (MMA) monomer (i.e., 10, 20, and 30 wt.%). The creep of UP-MMA polymer concrete was measured as per ASTM C512 under a sustained compressive stress of 20% of the compressive strength for 90 days, while the linear setting shrinkage was monitored based on the JSCE method for 7 days. Results indicated that as the MMA content increased, the 90-day creep increased although the differences were non-significant. On the contrary, the linear setting shrinkage tended to decrease with an increase in MMA content. The compressive strength was found to decrease by 7.2% as the MMA content increased from 10 wt.% to 30 wt.%. Additionally, the present study compared the measured creep coefficients with those estimated by ACI 209 and CEB-FIP Model Code 1990. It was revealed that the current creep models for Portland cement concrete are inappropriate in predicting the creep of UP-MMA concrete. A new predictive model for creep applicable to the UP-MMA concrete is proposed.

Photoconductivity along the Polymer/Polymer Interface

The effect of photoexcitation on the electrical conductivity of the interface between two polydiphenylenephthalide (PDP) polymer films is discovered. It is shown that ultraviolet radiation makes the main contribution to the generation of photoconductivity. Long-term relaxation of photoconductivity is observed after extinction of illumination. Based on electrophysical measurements and comparing the results of optical studies, it is concluded that ultraviolet radiation has an effect on the dipole moment magnitude.

An experimental study on stress relaxation behaviour of high strength steel wire: Microstructural evolution and degradation of mechanical properties

In this study, a series of experiments including stress relaxation, nanoindentation, tension loading experiments, and transmission electron microscope (TEM) examination were performed to investigate microstructural evolution, the stress relaxation behavior, and the influences of stress relaxation on mechanical properties of high strength steel wire. The stress relaxation experiments were performed on a single steel wire for 1000 h at three initial stress levels to characterize the stress relaxation behavior of the tested material. Microstructural evolution of the specimens deformed by stress relaxation was observed and analyzed using TEM examination. Long-term relaxation behavior was then calculated and investigated. Finally, the influences of stress relaxation on material properties were studied by performing nanoindentation and tension loading experiments on the deformed relaxation specimens. The degradation of mechanical properties under stress relaxation was interpreted through a micro-mechanism regarding the microstructural evolution and the grain boundary strengthening. The results from the present study can be used for practical designs as well as for assessing the stress relaxation behavior of high strength steel wire.

Laser-induced modification of optical properties of glassy selenium films synthesized by vacuum thermal evaporation

The influence of laser radiation at the wavelength of 632.8 nm on the optical properties of ~ 1 µm thick Se (a-Se) glassy films synthesized by vacuum thermal evaporation on quartz substrates has been studied. It is established that an insignificant decrease in their transmittance coefficients (T) and optical gap energy (Eg) is observed after the laser irradiation of the obtained films with a power density of 80 W⋅cm−2. For the first time, an increase in the reflection coefficient (R) and refractive index (n) of the films was detected in the course of long-term relaxation of their amorphous matrix to a more stable state caused by phase changes in a-Se films. It was established that a significant decrease in transmittance and reflection coefficients, refractive index, and Eg occurs after the laser irradiation of a-Se films with a higher power density (5300 W⋅cm−2).

Performance of industrial wedge-anchors for pre-stressing BFRP bars: Experimental and numerical studies

The application of fibre reinforced polymer (FRP) bars as a pre-stressing element in concrete has shown excellent performance in the past few decades. Although, aramid, carbon and glass FRPs have been included in the ACI committee report, basalt fibre reinforced polymer (BFRP) bars have been excluded. This is due to insufficient research available on the application of BFRP as a pre-stressing element and availability of a reliable ready-to-use industrial anchor compatible with BFRP bars. This paper investigates the performance of industrial wedge-anchors to pre-stress BFRP bars. Anchor-efficiency and long-term relaxation of the bars have been estimated using load-cells and strain-gauges. The anchor-slips during the stressing of the bar have been monitored using digital image correlation technique and compared with linear-potentiometers. The test results showed an anchor-efficiency between 59% and 79% and indicated that the tested anchors can pre-stress the BFRP bars beyond the maximum initial pre-stress limit defined for FRP tendons as per ACI guidelines. The measured loss due to stress-relaxation after 1000 h was 12% and the data was extrapolated to predict million-hour stress-loss. The experimental data was utilised to develop a non-linear finite element model to perform parametric studies on the anchor system.

Design and Optimization of Bidirectional Tunable MEMS All-Silicon Evanescent-Mode Cavity Filter

This article presents a bidirectional electrostatic MEMS tuner that enables active voltage compensation of integrated evanescent-mode cavity-based tunable filters. Unlike conventional tunable resonators tuned by unidirectional electrostatic diaphragms, the bidirectional design can compensate for residual displacement errors. This corrective tuning can successfully counterbalance long-term viscoelastic relaxation commonly found in large-displacement MEMS diaphragms. Consequently, it can significantly improve stability and reliability without introducing fabrication complexity. A detailed RF-MEMS codesign and a specific optimization design flow are also discussed. A proof-of-concept, two-pole filter is fabricated, which demonstrates a measured bidirectional tuning range of 18.9–39.6 GHz. The forward (main) actuation tunes the filter from 21.3 to 39.6 GHz with 120 V, whereas the reverse (corrective) actuation tunes it from 21.3 down to 18.9 GHz with 2 V. The measured filter insertion loss varies from 3.14 to 0.78 dB. The extracted unloaded quality factor is 265–510, which is comparable with the state-of-the-art unidirectional filters of this type.

Thermal History-Dependent Current Relaxation in hBN/MoS2 van der Waals Dimers.

Combining atomically thin layers of van der Waals (vdW) materials in a chosen vertical sequence is an emerging route to create devices with desired functionalities. While this method aims to exploit the individual properties of partnering layers, strong interlayer coupling can significantly alter their electronic and optical properties. Here we explored the impact of the vdW epitaxy on electrical transport in atomically thin molybdenum disulfide (MoS2) when it forms a vdW dimer with crystalline films of hexagonal boron nitride (hBN). We observe a thermal history-dependent long-term (over ∼40 h) current relaxation in the overlap region of MoS2/hBN heterostructures, which is absent in bare MoS2 layers (or homoepitaxial MoS2/MoS2 dimers) on the same substrate. Concurrent relaxation in the low-frequency Raman modes in MoS2 in the heterostructure region suggests a slow structural relaxation between trigonal and octahedral polymorphs of MoS2 as a likely driving mechanism that also results in inhomogeneous charge distribution in the MoS2 layer. Our experiment yields an aspect of vdW heteroepitaxy that can be generic to electrical devices with atomically thin transition-metal dichalcogenides.

Magnetic field effect on the slow relaxation of photoconductance in tunnel coupled quantum dot arrays

The effect of magnetic field on the long-term photoconductance relaxation in two-dimensional arrays of Ge tunnel-coupled quantum dots grown on Si by molecular-beam epitaxy is studied. It was shown that the relaxation process can be slowed down as well as accelerated by magnetic field. The sign of changing the relaxation rate depends on the localization radius and the quantum dot occupancy. To explain an unusual acceleration effect we proposed a model based on the difference in probabilities of carrier transitions to single- or double-occupied quantum dots due to Zeeman effect . • The magnetic field effect on slow conductance relaxation in quantum dots was studied. • The relaxation rate increases or decreases depending on quantum dot occupancy. • The acceleration of relaxation is explained by hop facilitation due to Zeeman effect.

Indentation creep vs. indentation relaxation: A matter of strain rate definition?

An extensive study of the different methods able to measure material creep properties in indentation is proposed. A systematic comparison of the fast creep, long-term creep, constant strain rate and long-term relaxation tests is performed on amorphous selenium. Even though measured apparent activation energy and strain rate sensitivities are very similar for all tests, a discrepancy is highlighted between the creep/constant strain rate tests and the relaxation tests. This effect seems to be related to the strain rate definition which should be a function of the strain rate sensitivity m. In this paper, it is proposed to use a representative strain rate ϵ̇r as proposed by Kermouche et al. (2008). An excellent agreement is found between all the methods, including uniaxial compression when expressing the creep behavior as a function of representative stress and strain rate.

The investigation of viscoelastic mechanical behaviors of bolted GLARE joints: Modeling and experiments

The viscoelastic properties of bolted GLARE joints, which are rarely reported, also play a significant role in the mechanical performance, especially for long-term services. Glass fiber reinforced resin matrix composite layers of GLARE show temperature dependence and viscoelastic response in thermal environments. Thus, a novel viscoelastic model based on the spring-based method is proposed to predict the relaxation behavior of bolted GLARE joint in this work. The Maxwell–Wiechert model with n dashpots in parallel is introduced into this model, and the specific meaning of each parameter in the model is discussed in detail. This model is established to link the viscoelastic theory to the macroscale relaxation behavior of bolted GLARE joints, which matches well with the experimental results. It is observed that the long-term relaxation results of bolted GLARE joints under uniaxial tension deformation present obvious differences at different experimental temperatures illustrating that the relaxations of bolted GLARE joints have strong temperature dependence. Finally, the time-temperature superposition principle (TTSP) is incorporated into this model to predict the long-term relaxation effect on the bolted GLARE joints according to the relaxation experiments at different temperatures. This work provides a guideline for the design bolted GLARE joint used in the extreme environment for a long-term service.

Numerical Analysis of a Rock Bolt Load Indicator Washer Neglecting the Stress Relaxation Effect

Rock bolts are components of tunnel support structures and their efficacy in reinforcement requires the appropriate estimation of the bolt pre-tension. However, cases of ineffective rock bolt application due to loosening are common which leads to the potential failure of the support structures, thus negatively affecting the tunnel stability. The effective employment of pre-tensioned bolt within support structures requires an appropriate installation tension as a result of sufficient bolt head/nut pre-load. Henceforth, this paper presents a load indicator washer (LIW) capable of withstanding assigned static loading as well as provides an appropriate pre-tension. The different LIW breadths are numerically analysed in the finite element code (ABAQUS) to estimate deformation when a designated 60-kN pre-load is applied. The numerical simulation employs a bi-linear isotropic hardening constitutive law governed by the von Mises yield criterion. The simulation also considers the nominal temperature of the nut-LIW-rock bolt connection. Therefore, a long-term relaxation effect on bolt pre-tension as a result of stress relaxation is neglected. The compressive force required to achieve the LIW-predetermined deformation is the bolt pre-tension. It is drawn from the simulation results that the 1.5-, 2.0-, 3.0-, 3.5-, and 4.0-mm-thick LIWs are completely flattened when subjected to 28-, 42.5-, 65-, 76-, and 88-kN pre-load respectively.

Фотопроводимость вдоль границы раздела полимер/полимер

Abstract The effect of photoexcitation on the electrical conductivity of the interface between two polydiphenylenephthalide (PDP) polymer films is discovered. It is shown that ultraviolet radiation makes the main contribution to the generation of photoconductivity. Long-term relaxation of photoconductivity is observed after extinction of illumination. Based on electrophysical measurements and comparing the results of optical studies, it is concluded that ultraviolet radiation has an effect on the dipole moment magnitude.

The Influence of Structural Defects in Silicon on the Formation of Photosensitive Mn4Si7–Si❬Mn❭–Mn4Si7 and Mn4Si7–Si❬Mn❭–M Heterostructures

The effect of the amorphous transition layer at the interface between Mn4Si7 and silicon doped with manganese on the photoelectric properties of heterostructures is considered. It is found that the precipitated Mn atoms on the silicon surface are grouped at high temperatures, forming drops of liquid manganese, which dissolve the near-surface layer of silicon, forming a liquid solution-melt of Mn with Si. As the solution solidifies, Mn4Si7 forms, and the Si–Si bonds under the silicide break due to intense diffusion of Si atoms; an elastically deformed Si region forms, which predetermines the evolution of the formation of photoelectric phenomena in the Mn4Si7–Si❬Mn❭–Mn4Si7 and Mn4Si7–Si❬Mn❭–M heterostructures. The microstructure and chemical composition of doped Si❬Mn❭ samples were studied by means of scanning electron microscopy and X-ray energy dispersive spectrometry using a Quanta 200-3D microscope, and the interface structure of the higher manganese silicide (HMS) and Si❬Mn❭ layer at the nanoscale was refined using the Fourier transform of local zones of high-resolution electron-microscopic images. It is concluded that in the process of diffusion doping of silicon with manganese, broken layers on the surface of the crystal deepen the embedding of manganese atoms, facilitate adsorption, dissolution, and diffusion of Mn in the volume of Si, and also enable the formation of an amorphous layer at the interface of higher manganese silicide and the Si❬Mn❭ layer. The presence of an amorphous transition layer facilitates the process of impact ionization of current carriers upon application of external voltage, as well as the formation of photoelectric phenomena: infrared quenching, temperature quenching, high photosensitivity, and long-term relaxation of residual conductivity.

Material behavior of rubber sealing for proton exchange membrane fuel cells

Reliable sealing is necessary for the stable operation of proton exchange membrane fuel cell (PEMFC). In practical application, various materials have been tried in PEMFC sealing. However, the mechanical properties of these sealing materials, which play a key role in the sealing stability, have not been fully understood in PEMFC environment, especially after long-term operation. In this paper, according to the operating environment of PEMFC, sealing material experiments are carried out to explore the differences in mechanical behaviors of sealing materials, including silicone rubber (SR), fluororubber (FR), nitrile rubber (NBR) and ethylene-propylene-diene-terpolymer rubber (EPDM) and the variation of mechanical properties of these sealing materials is predicted as time goes on. The results indicate that compression rate has a great influence on sealing contact stress. SR and EPDM, with the variation of 0.15 MPa and 0.45 MPa in stress, show the best and worst mechanical stability at different compression rates, respectively. In terms of temperature, it is found that SR can adapt to different operating temperature of PEMFC and only 18% variation is found from 20 °C to 100 °C. Finally, based on Time-Temperature Superposition (TTS), high temperature experiments are conducted to predict long-term relaxation stress under PEMFC working condition. The analysis results are beneficial for choosing suitable sealing material, and it can also be applied to predict sealing ability in PEMFC.

Long-term behavior of GFRP reinforcing bars

Glass fiber reinforced polymer (GFRP) bars represent a valid solution as internal reinforcement of concrete members for some particular applications. GFRP reinforcing bars (rebars) have a high-strength-to-weight ratio and good resistance to corrosion. However, they may suffer of degradation when exposed to specific aggressive environments and when subjected to long-term sustained stress. To increase their durability, design guidelines available in the literature limit the stress level in the rebar. However, such limitations are based on few experimental results and represent conservative estimations of the bar long-term behavior. In this paper, the results of 9 short-term tensile tests and 17 long-term tensile tests on GFRP bars are presented. The long-term tests included relaxation and creep tests for 1000 and 2000 h considering five different initial applied stress levels. The results obtained are described by two new relaxation and creep functions able to reproduce the bar behavior from the application of the initial applied stress. The functions proposed allow for obtaining the long-term relaxation losses of the reinforcing bars for different stress levels.

Abstraction and operator characterization of fractal ladder viscoelastic hyper-cell for ligaments and tendons

This study investigates the viscoelastic behaviour of soft bio-fibres in association with their fractal structures. A spring-dashpot fractal network with the self-similar topology, named the Π-type fractal ladder hyper-cell (FLHC), is abstracted from the micro/nano-structure of ligaments and tendons (LTs). Its constitutive operator is derived by the Heaviside operational calculus, which is of intrinsic fractional order. In terms of this operator, the long-term viscoelastic relaxation of bio-fibres arising from the fractal ladder topology is expounded. In addition, the fractional-order viscoelastic constitutive equation is obtained based on the FLHC of LTs, and its results are consistent with those of available human knee and spinal LT relaxation experiments. Results on the constitutive equation of FLHCs are formulated into two propositions. The multidisciplinary invariance and implications from the fractal ladder pattern of bio-fibres are also discussed.

A new long-term indentation relaxation method to measure creep properties at the micro-scale with application to fused silica and PMMA

A new procedure is developed to measure strain rate sensitivity m and apparent activation volume V* using nanoindentation relaxation experiments. This procedure is based on the control of the dynamic contact stiffness as measured by the continuous stiffness measurement module (CSM). Load decrease is monitored while maintaining the contact stiffness constant. It allows for very stable measurements of load relaxation (up to 10 h) since the contact area between the tip and the sample surface is kept constant. This improvement is significant as it increases by two orders of magnitude the measurement time compared to classical constant displacement experiments. The load relaxation data are interpreted using an analogy to uniaxial tests in order to extract representative material’s parameters. An excellent agreement with literature data is found for fused silica and PMMA. Nanoindentation relaxation experiment is proved to be an accurate procedure to extract viscoplastic parameters of materials under very low strain rates.

Memristive Properties of PANI-Polysterene/PVDF-TrFE Interface

The article presents the results of an experimental study of the transport of charge carriers through semiconductor PANI-polystyrene/ ferroelectric PVDF-TrFE interface. Current-voltage characteristics of the structure under study have a typical form for memristors and may be explained by the movement of charge carriers in the internal switchable field of the crystal ferroelectric microregions located within a bulk volume of amorphous PVDF-TrFE matrix. This assumption is subject to XRD phase analysis, FTIR spectroscopy, and X-ray EDS microanalysis. A long-term (about 100 h) relaxation is detected for the resistance of the PANI-polysturene/PVDF-TrFE interface after the current-voltage characteristics measurement cycle that is associated with the processes of capture and release traps of charge carriers.