Stage Of Stress Relaxation Research Articles

Determination of In concentration in InGaAs/GaAs 0 0 1 epilayers in the early stage of anisotropic stress relaxation

A method has been developed for determination of In concentration in strained layers of InGaAs/GaAs heterostructures. Chemical composition and lattice strain were evaluated from the reciprocal space maps obtained for asymmetric reflections. It was observed that beginning of relaxation of the In 0.13Ga 0.87As/GaAs (0 0 1) system with lattice misfit Δ a/ a = 9.3 × 10 −3 and the critical thickness t cMB = 15 nm can be detected for layers of thickness exceeding t ≅ 70 nm ≅ 4.5 t cMB. The principal relaxation mechanism is due to the slipping of 60° misfit dislocations on the tilted (1 1 1) glide planes. The accuracy of indium concentration measurements was estimated to Δ x = ±0.01.

Stress relaxation at high temperatures and the role of delayed elasticity

Closed-loop controlled isothermal ‘constant-strain’ stress relaxation tests (SRT) at different strains led to the conclusion that SRT cannot provide the anticipated viscous strain rate over a wide stress range; the stress exponent for power-law creep was found to depend on strain. The recovery phase after SRT showed that delayed elasticity, not the viscous flow, controls relaxation processes during the primary stages of stress relaxation irrespective of the level of constraint. Viscous flow controls stress relaxation only at longer times exhibiting ‘apparently’ asymptotic approach to a quasi-stable stress rate. Limitations of SRT can be explained by formulating the relaxation processes on the basis of a three-component rheological equation, derived from independent isothermal constant-stress strain relaxation and recovery tests (SRRT). Theoretical predictions were made for the time-dependent recoverable (delayed elastic) and permanent components of strain as functions of time and imposed strain. Experimental results exhibiting the short-coming of conventional experimental and analytical approaches and the strength of present formulations are illustrated by using a titanium-based, α–β, Ti–6Al–2Sn–4Zr–6Mo alloy at 600 °C (0.45 T m) as a test material.

Molecular-dynamics study of dynamic vortex defects as the mechanism of relaxation in loaded solids

Processes in a loaded solid have been studied by the method of molecular dynamics. It is established that dynamic vortex structures can form both in the stage of active loading and in the stress relaxation stage. In the latter case, atomic displacements can result in the formation of a periodic system of correlated vortex threads. The lifetime of such dynamic defects may reach tens of nanoseconds, while their characteristic dimensions are on the nanometer scale. It is demonstrated that the system of vortex threads can change the sign of the angular velocity so that atomic displacements taking place within different time intervals virtually compensate each other. The formation of analogous dynamic vortexlike structures in the active stage of loading does not exhibit a periodic character.

Time-dependent stress?strain behaviour due to viscous properties of geogrid reinforcement

A series of conventional and unconventional tensile tests were performed on six types of geosynthetic reinforcement to evaluate their viscous properties. It is shown that the isochronous model, according to which the load (or the stress) is a unique function of instantaneous strain and elapsed time, is unable to explain the major viscous aspects of the test results. In particular the model is unable to predict the stress–strain behaviour and rupture strength observed after loading is restarted at a constant strain rate following a creep loading or stress relaxation stage. It is argued that the widely prevailing concept that creep is a degrading phenomenon requiring the design strength to be reduced with an increase in the design lifetime is not realistic under typical field conditions where the stress–strain property does not deteriorate noticeably with time. It is also argued that this concept stems from the isochronous model. A non-linear three-component model is proposed to simulate the viscous aspects of the strength and deformation characteristics of the tested geosynthetic reinforcements. It is shown that this rheology model can simulate very well the experimental results obtained by the present study.

Time-dependent stress–strain behaviour due to viscous properties of geogrid reinforcement

A series of conventional and unconventional tensile tests were performed on six types of geosynthetic reinforcement to evaluate their viscous properties. It is shown that the isochronous model, according to which the load (or the stress) is a unique function of instantaneous strain and elapsed time, is unable to explain the major viscous aspects of the test results. In particular the model is unable to predict the stress–strain behaviour and rupture strength observed after loading is restarted at a constant strain rate following a creep loading or stress relaxation stage. It is argued that the widely prevailing concept that creep is a degrading phenomenon requiring the design strength to be reduced with an increase in the design lifetime is not realistic under typical field conditions where the stress–strain property does not deteriorate noticeably with time. It is also argued that this concept stems from the isochronous model. A non-linear three-component model is proposed to simulate the viscous aspects of the strength and deformation characteristics of the tested geosynthetic reinforcements. It is shown that this rheology model can simulate very well the experimental results obtained by the present study.

The first stage of stress relaxation in tensile strained In1-xGaxAs1-yPy films

The first stage of stress relaxation in tensile strained In1-xGaxAs1-yPy films

The first stage of stress relaxation in tensile strained In1-xGaxAs1-yPyfilms

Abstract The first stages of stress relaxation have been studied in a series of 2% tensile strained (In, Ga)(As, P) films of various compositions by transmission electron microscopy. Ternary In1-x Ga x As and In1-x Ga x P films relaxed by a combination of twinning and cracking, while the quaternary In1-x Ga x As1-y P y film relaxed only by twinning. The differences between the behaviours of the ternary and quaternary systems were correlated with the surface topography of the films. The surface nucleation of 90° partial dislocations was shown to be feasible at the growth temperature of the films (480°C) in the presence of surface roughness but, in the absence of a critical roughness, cracking was the preferred mechanism for coherency loss. The critical film thicknesses observed for stress relaxation by either twinning or cracking were shown to be in good agreement with the calculated values. The twins themselves often terminated inside the substrate (InP) rather than at the substrate-film interface. This b...

Stress dependence on stress relaxation creep rate during tensile holding under creep-fatigue interaction in 1Cr-Mo-V steel

A quantitative analysis of the stress dependence on stress relaxation creep rate during hold time under creep-fatigue interaction conditions has been conducted for 1Cr-Mo-V steel. It was shown that the transient behavior of the Norton power law relation is observed in the early stage of stress relaxation in which the instantaneous stress is relaxed drastically, which occurs due to the initial loading condition. But after the initial transient response in a 5 hour tensile hold time, the relations between strain rate and instantaneous stress represented the same creep behavior, which is independent of the initial strain level. The value of stress exponent after transition was 17 which is the same as that of the typical monotonic creep suggested from several studies for 1Cr-Mo-V steel. Considering the value of the activation energy for the saturated relaxation stage, it is suggested that the creep rate is related to instantaneous stress and temperature by the Arrhenius type power law.

Early stage-stress relaxation in compact bone

The early stage of stress relaxation, up to 10 s after strain application, in compact bone was investigated in order to find the limit of the applicability of the empirical equation (Sasaki et al., 1993. Journal of Biomechanics 26, 1369–1376), E(t)=E 0{A 1 exp[−(t/τ 1) β]+A 2 exp(−t/τ 2)}, A 1+A 2=1, 0<β⩽1, where E 0 is the relaxation modulus value at t=0, A 1 and A 2 are portions of the first term (the KWW relaxation) and the second term(the Debye relaxation), respectively, and τ 1 and τ 2 are relaxation times of respective relaxation processes. The relaxation modulus was generally well fitted to the empirical equation, although magnification revealed that in the short-time response region there was an inconsistency between the empirical equation and the measured data. The residual deviation increases with the decrease in time of the initial datum point which was used for the regression analysis. This tendency was concluded to be due to material properties. These results indicate the existence of a new relaxation in the short time response region ( t<2–3 s) of the stress relaxation process that cannot be described by the empirical equation.

Creep at low temperatures: unzipping of dislocations, inertia, and criticality processes

We consider creep processes in plastically deformed materials as a critical process such as proposed by Zaitsev (Physica A 189 (1992) 411). We give arguments that the low-temperature creep rate necessarily has two different modes. During the first (fast) stage of stress relaxation, dislocations surmount many barriers simultaneously due to their inertia and a low viscosity of the electron liquid. In the second (slow) stage the depinning of dislocations is mostly due to thermal activation. Our experiments on the mechanical relaxation process in lead alloys and zinc are consistent with the proposed mechanism of depinning of dislocations.

Characterization study of strained InxGa1−xAs/GaAs superlattices

InxGa1−xAs/GaAs strained layer superlattices were studied by means of double-crystal x-ray diffractometry (DCXD), transmission electron microscopy (TEM), and selected area x-ray photoelectron spectroscopy (SAXPS) depth profiling techniques. The quality of superlattices, strain in the sublayers, and their thickness and chemical composition were evaluated. The effect of the real superlattice structure on the shape of x-ray rocking curves has been revealed. It was concluded that full identification of nonperiodic defects and the initial stage of stress relaxation can be determined by combining DCXD and TEM methods. The results of SAXPS depth profiling were found to be in good quantitative agreement with DCXD and TEM data. The linear dependence of depth resolution Δz on the sputtering depth, caused by sputtering induced development of heterointerface roughness was determined in SAXPS profiles.

A rheological, optical, and x‐ray study of the relaxation of orientation of nematic PBZT

AbstractThe ultimate properties of liquid crystalline polymers depend upon the achievement of high orientation, usually by means of flow fields. The properties are limited by disorientation which can occur before the product is solidified. Such cooperative orientation and disorientation phenomena also underlie the complex fluid rheology and product microstructure of these materials. The orientation and subsequent disorientation can be followed dynamically by optical and x‐ray techniques. Normally, monitoring of orientation is possible only by “fast” techniqes, such as birefringence, these are not applicable to opaque and strongly scattering liquid‐crystalline systems. To enable examination of the full dynamic response of concentrated nematic solutions of poly(1,4‐phenylene‐2,6 benzobisthiazole) (PBZT) the Daresbury Synchrotron Radiation Source was used. PBZT is among the most rigid macromolecules and serves as a good model for other materials of its class. The orientation process determined optically and from x‐rays is correlated with fluid rheology and availabel theoretical approaches. During relaxation from near perfect mesogen alignment three principal stages of the disorientation process were identified corresponding to solvent disorientation (first stage of stress relaxation), banding (slow stress relaxation and mesogen disorientation), and finally a very slow banding to polydomain transition. © 1993 John Wiley &amp; Sons, Inc.

Tensile Stress Relaxation in High-Strength Spring Steel Wire

Abstract Stress relaxation data were obtained in tension tests using the vibrating string technique (measuring the resonance frequency) in spring steel wire of ASTM Grade A 228 (also known as music wire). The steel wire of 0.56 mm diameter had a 0.29% offset tensile yield strength of 1689 MPa. Tests were conducted at low temperatures in the range of 23 to 140°C, and at initial stress levels of up to 75% of the 0.20% offset yield strength of the wire. The test duration was to 4000 h at 23°C and to 100 h at temperatures to 140°C. The test results indicate a faster rate of stress relaxation during the early stage of stress relaxation, and subsequently a slower rate of stress relaxation rate with increasing time, as is usually observed. An empirical equation has been determined that fits the experimental data very well under the given test conditions.

Vulcanization of triple ethylene-propylene rubber and properties of rubbers vulcanized with the sulphur-perozide system and bis-(tetraethyldiaminophosphoryl) disulphide

The application of bis-(tetraethyldiamidophosphoryl)disulphide together with the sulphur-peroxide vulcanizing system increases the strength, resistance to heat ageing under stress and frost resistance of triple ethylene-propylene rubbers (TEPR). The slow stage of physical stress relaxation in rubbers vulcanized with this system is characterized by shorter relaxation times and lower activation energies as compared to peroxidic vulcanizates.

Anelasticity and viscoplasticity in metallic glasses

Low level constant-load tensile creep measurements and bend stress relaxation measurements were made on the metallic glasses Fe 40Ni 40B 20 and Fe 32Ni 36Cr 14P 12B 6 at various temperatures. The anelastic (recoverable) and viscoplastic (non-recoverable) components of flow were separated using pre-annealing treatments of various lengths of time, and multiple stress reversals for bend stress relaxation. The influence of the structural relaxation on the anelasticity was studied by continuous measurements of apparent viscosity. It was found that, while viscoplastic deformation drastically decreases with annealing, the anelastic behavior is only weakly affected. The viscosity versus time curves for pre-annealed samples are shifted on the time axis with respect to those for as-quenched samples. The relaxation rates calculated from the final stages of stress relaxation are higher than those estimated for pre-annealing periods. All these findings strongly imply that the spectrum of activation energies for anelastic relaxation is very broad and little affected by thermal treatments.

Studies of fibrin film. I. Stress relaxation and birefringence.

AbstractMeasurements of stress relaxation in uniaxial extension and associated time‐dependent birefringence have been made on bovine fibrin film, prepared by gentle compaction of coarse fibrin clots, containing 13–22% fibrin plasticized with either aqueous buffer or glycerol. Both unligated and ligated (i.e., with α‐α and γ‐γ ligation by fibrinoligase, factor XIIIa) films were studied. Both types showed two stages of stress relaxation, with time scales of approximately 10 and 103–104 s, respectively, with a plateau region between. In the plateau, the nominal (engineering) stress for ligated glycerol‐plasticized film is proportional to In λ, where λ is the stretch ratio, up to λ ≅ 2, and it decreases with increasing temperature. For unligated glycerol‐plasticized film, the stresses are smaller by a factor of one‐half to one‐third. For ligated film, the second stage of relaxation is relatively slight, and recovery after release of stress is often nearly complete. For unligated film, the second stage involves a substantial drop in stress, and after recovery there is a significant permanent set. A second relaxation for ligated film reproduces the first, but for unligated film it reproduces the first only if the initial relaxation is terminated before the second stage; otherwise, the second relaxation shows a weaker structure. The behavior of water‐plasticized film is similar to that of glycerol‐plasticized except that the second stage of relaxation occurs at shorter times. During the first stage of stress relaxation, up to about 100 s, the birefringence and the stress‐optical coefficient increase; during the plateau zone of stress relaxation, the birefringence of ligated films is approximately constant and is proportional to 2λ2/(λ2 + 1) − 1, where λ is the stretch ratio. This dependence is predicted by a two‐dimensional model in which rodlike elements in the plane of the film are oriented with independent alignment. During the final stage of stress relaxation, the birefringence of ligated films decreases slightly; that of unligated films decreases substantially, but less rapidly than the stress, corresponding to a further increase in the stress‐optical coefficient. With additional information from small‐angle x‐ray scattering reported in an accompanying paper, the first stage of relaxation is attributed to partial release of bending forces in the fibers by orientation, accompanied by increased birefringence. The second stage is attributed, for ligated films, to an internal transition in the fibrin units accompanied by elongation of some of the fibers; and in the unligated films, to a combination of the latter transition with slippage of protofibrils lengthwise within the fiber bundles that causes some loss of orientation, which diminishes the birefringence.

ポリサルファイドゴム印象材の粘弾性に関する基礎的研究

The deformation of a polysulfide rubber base impression material can be considered to appear as an effect of relief of the internal stress occurring during the gelation of the material, or of imperfect recovery from the strain applied to it during and/or after the removal of the impression. However, the mechanism of these phenomena have ever been scarecely elucidated. The author tried to make clear the deformation mechanism of the polysulfide rubber base impession materials used generally for precision impression in recent years. For this purpose, the author rehologically studied into the cause of the stress relaxation behavior from gelation process through relatively stable stage of reaction after gelation. Two stress relaxation tests were conducted : shearing test and tensile test. Principal results of the study are as follows : (1) Stress relaxation of a polysulfide rubber base imprssion material during and/or after gelation can be classified into the physical and the chemical relaxation within a low strain range permitting the fomation of linearity. The former is mainly due to the physical flow of the filler or the unreacted part. The latter is provoked by the network chain exchange reaction relieving the internal stress. (2) Physical relaxation may be explained by a simple Maxwell's relaxation. The chemical relaxation due to the network chain exchange reaction may be considered to follow a primary reaction. (3) In the initial stage of stress relaxation, the combination of physical and chemical relaxation is observed. However, it may be considered that the physical relaxation is completed in a relatively short time and thereafter only the chemical relaxation is observed substantially. (4) Permanent set (P.S%) of materials can be theoretically calculated on the basis of the continuous stress relaxation curve along with the discontinuous stress relaxation curve. The permanent set (P.S%) seems to depend upon both physical and chemical relaxation in the initial tension stage, while mainly upon the chemical relaxation in the later tension stage. This means that a respective increase in physical and chemical relaxation leads to the increase in permanent set. (5) As the gelation progresses, the network chain density increases and the chemical relaxation time is therefore lengthened. As a result, the decrease in fluidity of the contained filler results in the longer physical relaxation time. However, upon substantial completion of the gelation, the network chain density has a practically constant value, and so both physical and chemical relaxation present in a constant time. (6) The increase in temperature of a specimen results in the increase in exchange reaction rate and therefore promotes the relaxation. And then the fluidity of the filler also increases. As a result, the permanent set of the impression material increases.

Passive stretch relationships in human uterine muscle

The passive length-tension relationship was studied using human myometrium. The strips of muscle were extended by step pulls of 3 mm. per extension and the tension recorded after the first or rapid stage of stress relaxation had occurred. The cross-sectional area was measured at the narrowest point of the muscle and tension was computed as grams per square millimeter.The material from pregnant patients gave curves which were flatter and rose less steeply than those from the nonpregnant. The alteration in the passive length-tension properties due to the pregnant state was also found to be present when 20 mm. strips were compared to 30 mm. strips from nonpregnant patients. This comparison was necessary due to what appeared to be elastic recoil in the excised muscle from pregnant patients.The passive length-tension relationship in the nonpregnant patient material was not altered by age, previous parity, or phase of the menstrual cycle. There were no differences noted in strips taken from various sites in the pregnant uterus.Histologic counts of the relative amounts of muscular and nonmuscular tissue between the pregnant and nonpregnant material could not account for the differences in the passive length-tension curves. The relative amounts of muscle and connective tissue were the same in all categories.

Kinetics and Temperature Dependence of the Chemical Stress Relaxation of Wool Fibers

The stress relaxation process of keratin fibers may be divided into two stages, the first rapid stage involving hydrogen bond breakdown and the second slower stage being connected with disulfide bond scission. The temperature dependence of the second disulfide-controlled stage of stress relaxation of wool fibers from extensions of 20 and 40% was investigated at pH 7.0, both in the presence and absence of the sulfhydryl- blocking agent N-ethylmalimide (NEMI). By plotting the relaxation modulus Er as a function of temperature, a transition at 70°C was observed for untreated wool fibers. An increase in the SH content by partial reduction of the wool cystine decreases this transition temperature, as was expected from previous evidence of the SH dependence of the transition temperature under stress obtained from stress-strain curves [20]. By normalizing the stress at any time with a stress value in the presence of the SH-group blocking agent, it was possible to eliminate the first stage of stress relaxation without losing any component of the second stage. The viscoelastic behavior of the second stage was found to approximate that of a single Maxwellian element. The controlling mechanism in 'this stage appears to be a disulfide interchange catalyzed by free. SH groups, with an activation energy of approximately 23 kcal mole, which is independent of the strain level.