Showing papers on "Stress relaxation published in 1999"

Crystallite coalescence: A mechanism for intrinsic tensile stresses in thin films

Abstract: We examined the stress associated with crystallite coalescence during the initial stages of growth in thin polycrystalline films with island growth morphology. As growing crystallites contacted each other at their bases, the side-walls zipped together until a balance was reached between the energy associated with eliminating surface area, creating a grain boundary and straining the film. Our estimate for the resulting strain depends only on interfacial free energies, elastic properties, and grain size and predicts large tensile stresses in agreement with experimental results. We also discuss possible stress relaxation mechanisms that can occur during film growth subsequent to the coalescence event.

Stress reduction and bond stability during thermal annealing of tetrahedral amorphous carbon

Abstract: A comprehensive study of the stress release and structural changes caused by postdeposition thermal annealing of tetrahedral amorphous carbon (ta-C) on Si has been carried out. Complete stress relief occurs at 600–700 °C and is accompanied by minimal structural modifications, as indicated by electron energy loss spectroscopy, Raman spectroscopy, and optical gap measurements. Further annealing in vacuum converts sp3 sites to sp2 with a drastic change occurring after 1100 °C. The field emitting behavior is substantially retained up to the complete stress relief, confirming that ta-C is a robust emitting material.

Thermal and viscoelastic property of epoxy-clay and hybrid inorganic-organic epoxy nanocomposites

Abstract: The properties of nanostructured plastics are determined by complex relationships between the type and size of the nanoreinforcement, the interface and chemical interaction between the nanoreinforcement and the polymeric chain, along with macroscopic processing and microstructural effects. In this article, we investigated the thermal and viscoelastic property enhancement on crosslinked epoxy using two types of nanoreinforcement, namely, organoion exchange clay and polymerizable polyhedral oligomeric silsesquioxane (POSS) macromers. Glass transitions of these nanocomposites were studied using differential scanning calorimetry (DSC). Small-strain stress relaxation under uniaxial deformation was examined to provide insights into the time-dependent viscoelastic behavior of these nanocomposites. Since the size of the POSS macromer is comparable to the distance between molecular junctions, as we increase the amount of POSS macromers, the glass transition temperature Tg as observed by DSC, increases. However, for an epoxy network reinforced with clay, we did not observe any effect on the Tg due to the presence of clay reinforcements. In small-strain stress relaxation experiments, both types of reinforcement provided some enhancement in creep resistance, namely, the characteristic relaxation time, as determined using a stretched exponential relaxation function increased with the addition of reinforcements. However, due to different reinforcement mechanisms, enhancement in the instantaneous modulus was observed for clay-reinforced epoxies, while the instantaneous modulus was not effected in POSS–epoxy nanocomposites. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1993–2001, 1999

Understanding thixotropic and antithixotropic behavior of viscoelastic micellar solutions and liquid crystalline dispersions. I. The model

Abstract: A simple model consisting of the Upper Convected Maxwell constitutive equation and a kinetic equation for destruction and construction of structure, first proposed by Fredrickson in 1970, is used here to reproduce the complex rheological behavior of viscoelastic systems that also exhibit thixotropy and rheopexy under shear flow. The model requires five parameters that have physical significance and that can be estimated from rheological measurements. Several steady and unsteady flow situations were analyzed with the model. The model predicts creep behavior, stress relaxation and the presence of thixotropic loops when the sample is subjected to transient stress cycles. Such behavior has been observed with surfactant-based solutions and dispersions. The role of the characteristic time for structure built up, λ, in the extent and shape of the thixotropic loops is demonstrated.

Thickness dependence of structural and electrical properties in epitaxial lead zirconate titanate films

Abstract: We have studied the effect of misfit strain on the microstructure and properties of ferroelectric lead zirconate titanate thin films We have changed the misfit strain by varying the film thickness and studied the thickness effect on the domain formation of epitaxial PbZr02Ti08O3 (PZT) films grown by pulsed laser deposition on (001) LaAlO3 substrates with La05Sr05CoO3 (LSCO) electrodes The nominal thickness of the PZT films was varied from 60 to 400 nm with the LSCO electrode thickness kept constant at 50 nm X-ray diffraction experiments show that the films relax via the formation of a domains, the fraction of which increase with the ferroelectric film thickness The c-axis lattice constant of PZT films calculated from the 002 reflection decreases with increasing film thickness and approaches the bulk value of ∼0413 nm in the films thicker than 300 nm Cross-sectional transmission electron microscopy images reveal that the a-domain fraction and period increase with increasing film thickness The relaxation of misfit strain in the film is accompanied by systematic changes in the polarization properties, as well as the switching fields, quantified by the coercive field and the activation field

Evolution of coherent islands in Si 1 − x Ge x / Si ( 001 )

Abstract: The evolution of strain driven coherent islands is examined using sensitive real time stress measurements during heteroepitaxial growth of Si{sub 1{minus}x}Ge{sub x}/Si(001), combined with {ital ex situ} microscopy. We show that the sequence of morphological transitions at low mismatch strain is qualitatively identical to that for pure Ge heteroepitaxy on Si(001). In particular, films with strains less than 1{percent} undergo Stranski-Krastanov-like island-on-layer growth, followed by an extended regime of [501]-faceted hut clusters that eventually transform into higher aspect ratio dome clusters. The hut and dome islands are fully coherently strained and do not exhibit lateral composition modulation. Quantitatively, the relevant island length scales are significantly increased at low strain. Scaling of the morphological transitions with strain is directly demonstrated using the real time stress data. We further show that the apparent formation of a ripplelike surface morphology at low strain is actually a consequence of kinetic limitations on adatom diffusion, and does not necessarily signify the presence of a surface instability. {copyright} {ital 1999} {ital The American Physical Society}

Influence of AlN nucleation layers on growth mode and strain relief of GaN grown on 6H–SiC(0001)

Abstract: We study the growth mode and strain state of GaN layers grown either directly on 6H–SiC(0001) or on thin (5 nm), coherently strained AlN nucleation layers. Using a combination of structural, optical, and vibrational characterization methods, we show that the 3.4% compressive lattice mismatch strain is fully relieved in the former case, whereas in the latter case a significant amount (0.3%) remains even after 1 μm of growth. This finding is clarified by in situ reflection high-energy electron diffraction and transmission electron microscopy. We demonstrate that the strain state of the GaN layer is determined by its growth mode, which in turn is governed by the degree of wetting of the underlayer rather than by lattice mismatch.

Application of Time-Based Fractional Calculus Methods to Viscoelastic Creep and Stress Relaxation of Materials

Abstract: The quasi-static viscoelastic response of polymeric materialsis investigated utilizing constitutive models based on fractionalcalculus Time-based fractional calculus analysis techniques areemphasized Analytic solutions to quasi-static boundary value problemsin which the viscoelastic behavior is characterized by thefour-parameter fractional calculus-based solid model are given Varioussets of data from the literature are fit with existing and newfractional calculus-based constitutive equations

The effect of constraint on creep fracture assessments

Abstract: This paper describes a preliminary examination of the effect of in-plane constraint on creep crack growth under widespread creep conditions using the Q stress. Plane strain is assumed. Damage models for fracture of the process zone based on both ductility exhaustion and stress rupture are shown to predict a variation of the crack growth rate with Q. Lower levels of constraint lead to lower crack growth rates for a given C*. The results are used to outline a high temperature failure assessment diagram approach to constraint-dependent creep crack growth.

Internal stresses during film formation of polymer latices

Abstract: The lateral stresses that latex dispersions undergo upon forming films have been investigated by an optical lever technique. Dilational lateral stress induced by capillary pressure, tensile stress caused by the uniaxial shrinkage imposed by the substrate, and stress relaxation on longer time scales were observed. Failure of film formation because of cracking events was reflected in irregular substrate deflection. The stress evolution in pigment-filled paints below the critical pigment volume concentration displayed two maxima in tensile stress which we associate with a separation of time scales between the drying of polymeric binder and the particle rearrangement to eliminate voids.

Strain relaxation in graded composition InxGa1−xAs/GaAs buffer layers

Abstract: A model to compute the strain relaxation rate in InxGa1−xAs/GaAs single layers has been tested on several compositionally graded buffer layers The existence of a critical elastic energy has been assumed as a criterion for the generation of new misfit dislocations The surface strain accuracy results are within 25×10−4 The influence of different grading laws and growth conditions on residual strain, threading dislocation density, misfit dislocation confinement, and surface morphology has been studied The probability of dislocation interaction and work hardening has been shown to strongly influence the mobility and the generation rate of the dislocations Optimization of the growth conditions removes residual strain asymmetries and smoothes the surface roughness

Interrelation of Creep and Relaxation: A Modeling Approach for Ligaments

Abstract: Experimental data (Thornton et al., 1997) show that relaxation proceeds more rapidly (a greater slope on a log-log scale) than creep in ligament, a fact not explained by linear viscoelasticity. An interrelation between creep and relaxation is therefore developed for ligaments based on a single-integral nonlinear superposition model. This interrelation differs from the convolution relation obtained by Laplace transforms for linear materials. We demonstrate via continuum concepts of nonlinear viscoelasticity that such a difference in rate between creep and relaxation phenomenologically occurs when the nonlinearity is of a strain-stiffening type, i.e., the stress-strain curve is concave up as observed in ligament. We also show that it is inconsistent to assume a Fung-type constitutive law (Fung, 1972) for both creep and relaxation. Using the published data of Thornton et al. (1997), the nonlinear interrelation developed herein predicts creep behavior from relaxation data well (R > or = 0.998). Although data are limited and the causal mechanisms associated with viscoelastic tissue behavior are complex, continuum concepts demonstrated here appear capable of interrelating creep and relaxation with fidelity.

Interpretation of creep behaviour of a 9Cr–Mo–Nb–V–N (T91) steel using threshold stress concept

Abstract: The creep behaviour and the microstructural evolution of a 9Cr–Mo–Nb–V (T91) steel were extensively evaluated by means of short term constant load creep tests and TEM analysis. Statistical analysis of the microstructural data revealed that the precipitated phases M23 C6 (where M is a metal, mainly Cr or Fe) and MX (where M is Nb or V, and X is C and/or N) were subject to coarsening during creep exposure. The coarsening law and its dependence on applied stress were identified, and the model was used to predict the magnitude of the Orowan stress at the time corresponding to the minimum creep rate. The minimum creep rate dependence on applied stress at 873 K was described by incorporating the threshold stress concept in a power law with stress exponent n = 5. In the resulting phenomenological model, the strengthening effect of the dispersed phases was thus expressed by a threshold stress proportional to the Orowan stress.

Yield Stress and Time‐dependent Rheological Properties of Mango Pulp

Abstract: Mango pulp was tested for time-independent and time-dependent flow properties using a coaxial cylinder rheometer. Mango pulp is a pseudoplastic liquid with yield stress, and exhibits thixotropic properties. The yield stress calculated using the Casson or Bingham plastic models, had markedly higher values than those determined by stress relaxation, controlled stress experiments, or from stress-strain plots. The yield stress of mango pulp tested in this experiment was sensitive to temperature and decreased rapidly as temperature increased. The time-dependent model of Weltman was found to be most applicable (|r| ≥ 0.991, p ≤ 0.01) for mango pulp.

Ag/cu(111) structure revisited through an extended mechanism for stress relaxation

Abstract: The Ag/Cu(111) system can be considered as a model one concerning the atomic structure of one monolayer deposited on a substrate in the case of strong size mismatch. Thus, it has been the subject of many experimental [Auger electron spectroscopy, low-energy electron diffraction and scanning tunneling microscopy (STM)] and theoretical studies, in particular within N-body potentials. Although most results agreed both with the existence of an $n\ifmmode\times\else\texttimes\fi{}n$ superstructure accommodating the size mismatch and with a strong corrugation of the Ag adlayer, the morphologies---derived from STM on the one hand and numerical simulations on the other hand---were not found to be consistent. Here we revisit the previous theoretical study, taking into account different additional mechanisms (Ag and Cu vacancy formation, partial dislocation loop) to relax the interfacial stress. As a result, we obtain that the most efficient relaxation mechanism is the formation of partial dislocation loops in the first Cu substrate layer, requiring the formation of four or five Cu vacancies per unit cell in this plane. This leads to a strong damping of the corrugation in the Cu underlayers, and a perfect agreement is reached between observed and calculated surface morphology.

Rheological modelling of complex fluids: IV: Thixotropic and “thixoelastic” behaviour. Start-up and stress relaxation, creep tests and hysteresis cycles

Abstract: Structural rheological modelling of complex fluids developed in Part I of this series and applied to shear thickening systems (Parts II & III), is now used to improve such a modelling in the case of unsteady behaviour, that is, in the presence of thixotropy. The model is based on an explicit viscosity-structure relationship, η ( S ), between the viscosity and a structural variable S. Under unsteady conditions, characterized by a reduced shear, Γ ( t ), shear-induced structural change obeys a kinetic equation (through shear-dependent relaxation times). The general solution of this equation is a time-dependent function, S ( t ) ≡ S [ t, Γ ( t )]. Thixotropy is automatically modelled by introducing S [ t, Γ ( t )] into η ( S ) which leads directly to η ( t ) ≡ η [ t, Γ ( t )], without the need for any additional assumptions in the model. Moreover, whilst observation of linear elasticity requires small enough deformation i.e. no change in the structure, larger deformations cause structural buildup/breakdown, i.e. the presence of thixotropy, and hence leads to a special case of non-linear viscoelasticity that can be called “thixoelasticity”. Predictions of a modified Maxwell equation, obtained by using the above-defined η ( S ) and assuming G = G 0 S (where G 0 is the shear modulus in the resting state defined by S = 1) are discussed in the case of start-up and relaxation tests. Similarly modified Maxwell-Jeffreys and Burger equations are used to predict creep tests and hysteresis loops. Discussion of model predictions Maynly concerns (i) effects of varying model variables or/and applied shear rate conditions and (ii) comparison with some experimental data.

Effects of film thickness and lattice mismatch on strain states and magnetic properties of La0.8Ca0.2MnO3 thin films

Abstract: The effects of strain relaxation on the crystallographic domain structure and on the magnetic and transport properties of epitaxial colossal magnetoresistive La0.8Ca0.2MnO3 (LCMO) thin films have been studied. LCMO films in the thickness range of 100–4000 A were grown on (001) SrTiO3 and (001) LaAlO3 substrates, which impose an in-plane tensile and an in-plane compressive biaxial stress in the films, respectively. On (001) SrTiO3 substrates, the films can be grown coherently up to a thickness ∼250 A, then strain relaxation occurs at a thickness of ∼500 A. In contrast, even the 100 A film grown on (001) LaAlO3 is partially relaxed, and the critical thickness for complete strain relaxation is ∼750 A. The very thin films (<250 A) show a pure (001)T normal orientation for growth on SrTiO3 and a pure (110)T texture for growth on LaAlO3. As thickness increases, the lattice strain relaxes, resulting in mixed (001)T and (110)T textures for growth on both substrates. Both the Curie and peak resistivity temperature...

Dynamics of sorbitol and maltitol over a wide time-temperature range

Abstract: The relaxation behaviour of two molecular glass-forming systems, namely sorbitol and maltitol, are investigated in the large temperature range relevant to the glass-transition. These data are obtained by combining three techniques, i.e. low-frequency mechanical spectroscopy, medium and high frequency dielectric spectroscopy, and viscosity measurements. This procedure allows to determine the relaxation map of these polyols on a wide time range [10-9-107 s]. Two different relaxation processes can be observed. The principal α-relaxation process exhibits a complex behaviour, comprising a non-Arrhenius temperature dependence above T g (supercooled liquid state), and an Arrhenius behaviour below T g (glassy state). A secondary β-relaxation is observed at higher frequencies with an Arrhenius temperature dependence. The secondary process appears in the same time-temperature range in both polyols. Consequently the molecular root of this relaxation is most likely the same in these complementary chemical systems. On the other hand, the time scale on which the α and β processes cross is very different for these two polyols. We relate this feature to the differences in the relative contributions of intra and inter-molecular interactions due to the different chemical architecture of these polyols.

Materials with Internal Variables and Relaxation to Conservation Laws

Abstract: .The theory of materials with internal state variables of Coleman & Gurtin [CG] provides a natural framework to investigate the structure of relaxation approximations of conservation laws from the viewpoint of continuum thermomechanics. After reviewing the requirements imposed on constitutive theories by the principle of consistency with the Clausius‐Duhem inequality, we pursue two specific theories pertaining to stress relaxation and relaxation of internal energy. They each lead to a relaxation framework for the theory of thermoelastic non‐conductors of heat, equipped with globally defined “entropy” functions for the associated relaxation process. Next, we consider a semilinear model problem of stress relaxation. We discuss uniform stability and compactness for solutions of the relaxation system in the zero‐relaxation limit, and establish convergence to the system of isothermal elastodynamics by using compensated compactness. Finally, we prove a strong dissipation estimate for the relaxation approximations proposed in Jin & Xin [JX] when the limit system is equipped with a strictly convex entropy.

Relaxation of strained Si layers grown on SiGe buffers

Abstract: Thin strained Si layers grown on SiGe layers graded to 20% Ge were studied for resistance to relaxation. It was observed that in the presence of ∼105/cm2 threading dislocations from the underlying graded layers, the barrier to misfit dislocation formation is sufficiently reduced to induce relaxation in Si layers even when the layer thickness is less than the predicted critical thickness. Raman spectroscopy revealed that elastic strain accumulation in the uniform SiGe layers is a significant contributor to strain relaxation in the Si cap layers. Upon annealing, thermal mismatch causes the Si layers to relax further, but most of the strain relaxation is accommodated by elastic strain increase in the SiGe layers. This prevents the rampant increase in defect density that would otherwise accompany the strain relaxation. Annealing in an oxidizing ambient appears to pin pre-existing threading dislocations causing nucleation of new threading dislocations and short misfit segments to relieve the thermal mismatch s...

Deformation behavior of plasma-sprayed thick thermal barrier coatings

Abstract: A fundamental deformation study of several candidate diesel engine coating materials, independent of a substrate, has been conducted. Both plasma-sprayed 8%Y2O3–ZrO2 and CaTiO3 material specimens were subjected to various isothermal mechanical tests from room temperature to 800 °C to understand their basic constitutive behavior. In this work, it was found that all of the coating materials showed significant irreversible deformation behavior during cyclic loading. In addition, both time- and temperature-dependent behavior were observed through stress relaxation and strain-rate sensitivity tests. This inelastic behavior was seen to occur under all conditions, including tests at room temperature. In a diesel engine, this irreversible deformation behavior will give rise to detrimental tensile stresses when subjected to in-service compressive strain cycles. SEM observations suggest that the combined closing of pre-existing microcracks and sliding along microcrack surfaces results in the deformation behavior observed. In addition, a glassy phase found in the microstructure is thought to aid these microstructural deformation processes at high temperature.

Annealing behavior of p-type Ga0.892In0.108NxAs1−x (0⩽X⩽0.024) grown by gas-source molecular beam epitaxy

Abstract: P-type, Be-doped GaInNAs layers (1100 A thick) are grown on GaAs substrates by gas-source molecular beam epitaxy with a nitrogen radical beam source. High-resolution x-ray rocking curves show that the Ga0.892In0.108NxAs1−x peak shifts closer to the GaAs substrate peak with increasing N concentration, indicating reduced strain. After rapid thermal annealing (RTA) at 700 °C for 10 s, the Ga0.892In0.108As sample suffers strain relaxation, but the N-containing samples remain pseudomorphically strained, suggesting better thermal stability of GaInNAs. The wavelength of room-temperature photoluminescence redshifts from 0.988 to 1.276 μm, due to large band gap bowing, with N concentration increased from 0 to 0.024. Secondary ion mass spectrometry results show no Be diffusion, but hydrogen incorporation alongside N. The free carrier concentration is decreased by one order of magnitude mainly due to H passivation, but after RTA at 700 °C, it is increased to half that of GaInAs due to the reduced H concentration. Th...

Stress Relaxation of Sugi (Cryptomeria japonica D.Don) Wood in Radial Compression under High Temperature Steam

Abstract: To clarify the mechanism of the permanent fixation of compressive deformation of wood by high temperature steaming, stress relaxation and stress-strain relationships in the radial compression for Sugi (Cryptomeria japonica D.Don) wood were measured under steam at temperatures up to 200°C. The stress relaxation curves above 100°C were quite different in shape from those below 100°C, showing a rapid decrease in stress with increasing temperature. In the stress-strain relationships measured above 140°C, the stress reduced as pre-steaming time increased when compared at the same strain. The recovery of compressive deformation (strain recovery) was decreased with steaming time and reached almost 0 in 10 min at 200°C. The relationship between the residual stress and the strain recovery at the end of relaxation measurements could be expressed by a single curve regardless of time and temperature. The permanent fixation of deformation by steaming below 200 °C was considered to be due to chain scission of hemicelluloses accompanying a slight cleavage of lignin. In some cases, the increase in regularity of the crystalline lattice space of microfibrils or the formation of crosslinks between the cell wall polymers seemed to play an important role in the permanent fixation of compressive deformation.

Modeling high-temperature stress-strain behavior of cast aluminum alloys

Abstract: A modified two-state-variable unified constitutive model is presented to model the high-temperature stress-strain behavior of a 319 cast aluminum alloy with a T7 heat treatment. A systematic method is outlined, with which one can determine the material parameters used in the experimentally based model. The microstructural processes affecting the material behavior were identified using transmission electron microscopy and were consequently correlated to the model parameters. The stress-strain behavior was found to be dominated by the decomposition of the metastable θ′ precipitates within the dendrites and the subsequent coarsening of the θ phase, which was manifested through remarkable softening with cycling and time. The model was found to accurately simulate experimental stress-strain behavior such as strain-rate sensitivity, cyclic softening, aging effects, transient material behavior, and stress relaxation, in addition to capturing the main deformation mechanisms and microstructural changes as a function of temperature and inelastic strain rate.