Showing papers on "Stress relaxation published in 2000"
TL;DR: In this paper, the melt-state linear viscoelastic properties for a series of intercalated nanocomposites are examined and the linear dynamic oscillatory moduli and the stress relaxation moduli are in quantitative agreement and suggest that at short times the relaxation of the nanocom composites is essentially unaffected by the presence of layered-silicate.
Abstract: The melt-state linear viscoelastic properties for a series of intercalated nanocomposites are examined. The nanocomposites are based on a short disordered polystyrene−polyisoprene diblock copolymer and varying amounts of dimethyldioctadecylammonium modified montmorillonite. The linear dynamic oscillatory moduli and the stress relaxation moduli are in quantitative agreement and suggest that at short times the relaxation of the nanocomposites is essentially unaffected by the presence of the layered-silicate. However, at long times (or equivalently low frequency), the hybrids exhibit dramatically altered viscoelastic behavior. Hybrids with silicate loadings in excess of 6.7 wt % exhibit pseudo-solidlike behavior, similar to that observed in previous studies of exfoliated end-tethered nanocomposites. On the basis of simple phenomenological arguments, the long time behavior is attributed to the presence of anisotropic stacks of silicate sheets randomly oriented and forming a percolated network structure that i...
566 citations
TL;DR: In this article, the role of aging in the rheology of soft materials was studied theoretically and several generalized rheological response functions suited to aging samples were defined, in which time translation invariance was lost.
Abstract: We study theoretically the role of aging in the rheology of soft materials. We define several generalized rheological response functions suited to aging samples (in which time translation invariance is lost). These are then used to study aging effects within a simple scalar model (the "soft glassy rheology" or SGR model) whose constitutive equations relate shear stress to shear strain among a set of elastic elements, with distributed yield thresholds, undergoing activated dynamics governed by a "noise temperature," x. (Between yields, each element follows affinely the applied shear.) For 1 < x < 2 there is a power-law fluid regime in which transients occur, but no aging. For x < 1, the model has a macroscopic yield stress. So long as this yield stress is not exceeded, aging occurs, with a sample's apparent relaxation time being of order its own age. The (age-dependent) linear viscoelastic loss modulus G[double-prime](omega,t) rises as frequency is lowered, but falls with age t, so as to always remain less than G[prime](omega,t) (which is nearly constant). Significant aging is also predicted for the stress overshoot in nonlinear shear startup and for the creep compliance. Though obviously oversimplified, the SGR model may provide a valuable paradigm for the experimental and theoretical study of rheological aging phenomena in soft solids. ©2000 Society of Rheology.
352 citations
TL;DR: In this paper, the authors show that the viscoelastic properties of materials are determined by transient or dynamic methods, such as stress relaxation and creep, and that the material response cannot be determined as a function of frequency.
Abstract: An ideal solid material will respond to an applied load by deforming finitely and recovering that deformation upon removal of the load. Such a response is called “elastic”. Ideal elastic materials obey Hooke's law, which describes a direct proportionality between the stress (σ) and strain (γ) via a proportionality constant called modulus (G), i.e., σ=Gγ. An ideal fluid will deform and continue to deform as long as the load is applied. The material will not recover from its deformation when the load is removed. This response is called “viscous”. The flow of simple viscous materials is described by Newton's law, which constitutes a direct proportionality between the shear stress and the shear rate ( γ ), i.e., σ=η γ . The proportionality constant η is called the shear viscosity. From energy considerations, elastic behavior represents complete recovery of energy expended during deformation, whereas viscous flow represents complete loss of energy as all the energy supplied during deformation is dissipated as heat. Ideal elastic and ideal viscous behaviors present two extreme responses of materials to external stresses. As the terms imply, these are only applicable for “ideal” materials. Real materials, however, exhibit a wide array of responses between viscous and elastic. Most materials exhibit some viscous and some elastic behavior simultaneously and are called “viscoelastic”. Almost all foods, both liquid and solid, belong to this group. The viscoelastic properties of materials are determined by transient or dynamic methods. The transient methods include stress relaxation (application of constant and instantaneous strain and measuring decaying stress with respect to time) and creep (application of constant and instantaneous stress and measuring increasing strain with time). Though such methods are fairly easy to perform, there are several limitations. Major among them is that the material response cannot be determined as a function of frequency.
256 citations
TL;DR: In this article, the authors derived the solutions of the equations of viscoelastic deformation under the boundary conditions of flat-punch indentation of a viscous half-space.
Abstract: The indentation of standard viscoelastic solids, that is, the three-element viscoelastic material, by an axisymmetric, flat-ended indenter has been investigated theoretically. Under the boundary conditions of flat-punch indentation of a viscoelastic half-space, the solutions of the equations of viscoelastic deformation are derived for the standard viscoelastic material. Their generality resides in their inclusion of compressible as well as incompressible solids. They cover the two transient situations: flat-punch creep test and load-relaxation test. In experimental tests of their applicability, nanoindentation and microindentation probes under creep and relaxation conditions yielded a modulus from 0.1 to 1.1 GPa and viscosity from 1 to 37 Gpa · s for a crosslinked glassy polyurethane coatings. For bulk polystyrene, the values vary from 1 to 2 GPa and from 20 to 40 Gpa · s, respectively. The analysis here provides a fundamental basis for probing elastic and viscous properties of coatings with nanoindentation or microindentation tests. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 10–22, 2000
241 citations
Journal Article•
TL;DR: The generalised Maxwell model was successfully employed to model the mechanical behaviour in the linear viscoelastic domain, and the fluid-like viscoELastic behaviour of liver tissue was confirmed by stress relaxation experiments.
194 citations
TL;DR: In this paper, a linear theory of wave propagation in saturated, unconsolidated granular materials, including marine sediments, is developed based on two types of shearing, translational and radial, which occur at grain contacts during the passage of a wave.
Abstract: A linear theory of wave propagation in saturated, unconsolidated granular materials, including marine sediments, is developed in this article. Since the grains are unbonded, it is assumed that the shear rigidity modulus of the medium is zero, implying the absence of a skeletal elastic frame. The analysis is based on two types of shearing, translational and radial, which occur at grain contacts during the passage of a wave. These shearing processes act as stress-relaxation mechanisms, which tend to return the material to equilibrium after the application of a dynamic strain. The stress arising from shearing is represented as a random stick-slip process, consisting of a random succession of deterministic stress pulses. Each pulse is produced when micro-asperities on opposite surfaces of a contact slide against each other. The quantity relevant to wave propagation is the average stress from all the micro-sliding events, which is shown to be a temporal convolution between the deterministic stress, h(t), from a single event and the probability, q(t), of an event occurring between times t and t+dt. This probability is proportional to the velocity gradient normal to the tangent plane of contact between grains. The pulse shape function, h(t), is derived by treating the micro-sliding as a strain-hardening process, which yields an inverse-fractional-power-law dependence on time. Based on two convolutions, one for the stress relaxation from translational and the other from radial shearing, the Navier–Stokes equation for the granular medium is derived. In a standard way, it is split into two equations representing compressional and shear wave propagation. From these wave equations, algebraic expressions are derived for the wave speeds and attenuations as functions of the porosity and frequency. Both wave speeds exhibit weak, near-logarithmic dispersion, and the attenuations scale essentially as the first power of frequency. A test of the theory shows that it is consistent with wave speed and attenuation data acquired recently from a sandy sediment in the Gulf of Mexico during the SAX99 experiment. If dispersion is neglected, the predicted expressions for the wave speeds reduce to forms which are exactly the same as those in the empirical elastic model of a sediment proposed by Hamilton. On this basis, the concept of a “skeletal elastic frame” is interpreted as an approximate, but not equivalent, representation of the rigidity introduced by grain-to-grain interactions.
186 citations
TL;DR: In this article, a simple model is presented that predicts the kinetics of tensile stress evolution during the deposition of thin films that grow by the Volmer-Weber mechanism, while concurrent stress relaxation was assumed to occur via a microstructure-dependent diffusive mechanism.
Abstract: A simple model is presented that predicts the kinetics of tensile stress evolution during the deposition of thin films that grow by the Volmer–Weber mechanism. The generation of a tensile stress was attributed to the impingement and coalescence of growing islands, while concurrent stress relaxation was assumed to occur via a microstructure-dependent diffusive mechanism. To model the process of island coalescence, finite element methods were employed and yielded average tensile stresses more consistent with experimental observations than those predicted using previously reported analytical models. A computer simulation was developed that models the process of film growth as the continuous nucleation of isolated islands, which grow at a constant rate to impinge and coalesce to form a continuous polycrystalline film. By incorporating the finite element results for stress generation and a microstructure-dependent stress relaxationmodel, the simulation qualitatively reproduced the complex temperature-dependent trends observed from in situ measurements of stress evolution during the deposition of Ag thin films. The agreement includes simulation of the decreasingstress relaxation rate observed during deposition at increasing temperatures.
175 citations
TL;DR: In this article, the influence of strain history on rock specimen deformation during multi-level loading and unloading cyclic uniaxial compression creep tests is studied with a creep testing machine.
173 citations
TL;DR: In this paper, the influence of crystallinity on the ductile-brittle transition in a bulk lanthanum-based metallic glass was investigated, and the impact test results showed a significant decrease, by ∼90%, in impact toughness with the introduction of a small percentage of the crystallinity.
152 citations
TL;DR: In this article, a mechanism of strain relief of H+ ion implanted and annealed pseudomorphic Si1−xGex/Si(100) heterostructures grown by molecular beam epitaxy is proposed and analyzed.
Abstract: A mechanism of strain relief of H+ ion implanted and annealed pseudomorphic Si1−xGex/Si(100) heterostructures grown by molecular beam epitaxy is proposed and analyzed Complete strain relaxation was obtained at temperatures as low as 800 °C and the samples appeared free of threading dislocations within the SiGe layer to the limit of transmission electron microscopy analysis In our model, H filled nanocracks are assumed to generate dislocation loops, which glide to the interface where they form strain relieving misfit segments On the basis of this assumption, the conditions for efficient strain relaxation are discussed
135 citations
TL;DR: In this paper, the authors investigated the tensile strain relaxation in AlxGa1−xN layers of different compositions epitaxially grown on GaN/sapphire.
Abstract: Relaxation of tensile strain in AlxGa1−xN layers of different compositions epitaxially grown on GaN/sapphire is investigated. Extended crack channels along 〈211¯0〉 directions are formed if the aluminum content exceeds a critical value, which decreases with increasing layer thickness. This process is found to limit the average strain energy density to a maximum value of 4 J/m2. By calculating the stress distribution between cracks and the strain energy release rate for crack propagation, the relaxed strain as measured by x-ray diffraction is correlated to the crack density, and the onsets of crack channeling and layer decohesion are fitted to a fracture toughness of 9 J/m2. Moreover, the crack opening at the surface is found to linearly increase with the stress. Annealing of samples above the growth temperature introduces additional tensile stress due to the mismatch in thermal expansion coefficients between the layer and substrate. This stress is shown to relieve not only by the formation of additional cr...
TL;DR: In this article, a phenomenological constitutive model with no adjustable parameters appropriate for the transient behavior of droplets and blends is presented, where the time evolution of the droplet anisotropy tensor during droplet relaxation under quiescent conditions is described using a frame-invariant formulation.
Abstract: We report a phenomenological constitutive model with no adjustable parameters appropriate for the transient behavior of droplets and blends. The time evolution of the droplet anisotropy tensor during droplet relaxation under quiescent conditions is described using a frame-invariant formulation that approximately imposes constancy of droplet volume. The Doi–Ohta theory [J. Chem. Phys. 95, 1242 (1991)] is then adapted to transient flows in which breakup and coalescence do not occur by replacing the Doi–Ohta relaxation terms with this relaxation description. Model predictions are compared to results of visualization of single droplets in step shear and startup of steady shear and to measurement of concentrated blend rheology in step shear and startup of steady shear. The model quantitatively described the relaxation after step strain of single droplets to axisymmetric and then to isotropic shapes. With the inclusion of the rational ellipsoidal closure for affine deformation [Wetzel and Tucker, Int. J. Multip...
TL;DR: In this paper, the authors measured the stress evolution during metal-organic chemical vapor deposition of AlGaN/GaN heterostructures on sapphire and correlated in situ stress measurements with ex situ microstructural analysis to determine directly a critical thickness for cracking and the subsequent relaxation kinetics of tensile-strained AlxGa1−xN grown on GaN.
Abstract: We have directly measured the stress evolution during metal-organic chemical vapor deposition of AlGaN/GaN heterostructures on sapphire. In situ stress measurements were correlated with ex situ microstructural analysis to determine directly a critical thickness for cracking and the subsequent relaxation kinetics of tensile-strained AlxGa1−xN grown on GaN. Cracks appear to initiate the formation of misfit dislocations at the AlGaN/GaN interface, which account for the majority of the strain relaxation.
TL;DR: In this paper, a simple model for the interfacial free energy of a semicoherent interface is used to develop expressions for interface stresses, which are surface thermodynamic quantities associated with solid-solid interfaces.
Abstract: A simple model for the interfacial free energy of a semicoherent interface is used to develop expressions for interface stresses, which are surface thermodynamic quantities associated with solid–solid interfaces. An analysis of the thermodynamics of thin film epitaxy is presented that incorporates the effects of free surface and interface stresses, and an expression for the critical thickness for thin film epitaxy is obtained. Based on this analysis, the concept of effective pressures exerted by the thin film free surface and film–substrate interface is introduced. If it is assumed that misfit dislocations are generated at the film–substrate interface as a result of glide of threading dislocations, the thermodynamics and kinetics of stress relaxation can be discussed in terms of a balance of Peach–Koehler forces acting on the threading dislocations owing to the surface and interface pressures as well as to the coherency stress. An example is given that shows that, if the film has a relatively large surface pressure that opposes lattice matching, the dependence of the coherency strain on film thickness can be very different from that obtained from conventional analyses which ignore the effect of the free surface; specifically, the largest equilibrium coherency strain of the same sign as the misfit can be much smaller than the total misfit, and an “anomalous” coherency strain of sign opposite that of the misfit can be thermodynamically favorable at small film thicknesses. The analysis used to obtain the critical thickness for thin film epitaxy is extended to give an expression for the critical thickness for misfit dislocation generation at the interface between a substrate and a superlattice thin film. It is shown that this critical thickness depends on a superlattice pressure associated with the interlayer interface stress in addition to the free surface and film–substrate interface pressures.
TL;DR: The experimentally measured Brownian motion conforms to the model predictions at long times (low frequencies) and is an additional confirmation of the essentially Maxwellian stress relaxation behavior of wormlike micelle solutions at low frequencies.
Abstract: A simple model of Brownian motion in a single relaxation time Maxwell fluid is described and compared to diffusing wave spectroscopy measurements of colloidal motion in representative viscoelastic fluids, namely, CTAB/KBr wormlike micelle solutions. The experimentally measured Brownian motion conforms to the model predictions at long times (low frequencies) and is an additional confirmation of the essentially Maxwellian stress relaxation behavior of wormlike micelle solutions at low frequencies. Surprisingly, the Maxwell model predicts a plateau onset time which, while capable of reducing the measured mean-square displacements to a master curve, also grossly underestimates the actual plateau onset time. The predicted rescaling is shown to be essentially that also predicted by the Doi-Edwards tube model for polymer solutions under good solvent (excluded volume) conditions where a more proper accounting of the short-time dynamics is made. This indicates that the success of the predicted Maxwell model plateau onset time rescaling is purely fortuitous.
TL;DR: In this article, the authors measured the flow stress of tungsten single crystals, oriented for single slip, in dependence on temperature and strain rate in the temperature regime 60 to 800 K by successive tensile deformations at constant strain rate and in correlated stress-relaxation tests.
Abstract: The flow stress of high-purity tungsten single crystals, oriented for single slip, was measured in dependence on temperature and strain rate in the temperature regime 60 to 800 K by successive tensile deformations at constant strain rate and in correlated stress-relaxation tests. Three regimes in the dependence of the flow stress and its strain-rate sensitivity on the temperature were detected. They constitute a challenging feature of the mechanical properties of high-purity b.c.c. metals. The interpretation of the results is based on the motion of a 0 /2(111) screw dislocations which is rate-controlled by thermally activated formation of kink pairs on screw dislocations. The kink-pair theory of the flow stress developed by A. Seeger provides excellent tools to interprete the temperature and strain-rate dependence of the flow stress of b.c.c. metals. Comparison between theory and experiment enables quantitative tests of the theory and allows the determination of kink properties. It is found that for low stresses (regime I, T > 600 K) the fundamental process of kink-pair formation takes place on {211} plane with a formation enthalpy of two isolated kinks of a kink pair 2H k ≅ 2.1 eV. For intermediate stresses (regime II, 220 K ≤ T ≤ 600 K) the fundamental kink-pair formation process takes place on {211} with 2H k ≅ 1.75 eV. For high stresses (regime III, T ≤ 220K) the fundamental process of kink-pair formation takes place on {110} glide planes with 2H k ≅ 1.3eV.
TL;DR: In this article, a telechelic associating polymers in aqueous solutions self-assemble into starlike flowers in the dilute regime and develop a fully connected network of flowers above some threshold concentration φ* (∼1 wt %).
Abstract: We report on telechelic associating polymers in aqueous solutions that self-assemble into starlike flowers in the dilute regime and develop a fully connected network of flowers above some threshold concentration φ* (∼1 wt %). The peculiarity of these telechelics is that the end caps are partially fluorinated. Small-angle neutron scattering has been used to investigate the form and structure factors of the starlike aggregates, and linear rheology was performed in order to identify the viscoelastic features of the physically cross-linked network. Taking advantage of the long network relaxation times, we use step-strain experiments of amplitudes γ comprised between 0.01 and 3 to explore the nonlinear regime of shear deformations. In the linear regime, the stress relaxation function is well described by a stretched exponential of the form G(t) = G0 exp(−(t/τ0)α), where G0 is the elastic modulus, τ0 the relaxation time, and α an exponent close to 1 (α ∼ 0.8). With increasing deformations (γ > 0.4), the elastic...
TL;DR: In this article, the authors studied the growth of self-assembled quantum dot on GaAs and showed that the growth process is characterized by the equilibrium between InAs and floating In dominated by the stress energy.
Abstract: In segregation effects during InAs growth on GaAs(001) and critical thickness for InAs self-assembled quantum dots are studied using a real time, in situ technique capable of measuring accumulated stress during growth. Due to a large (∼50%) surface In segregation of floating In, self-assembled dot formation takes place when less than one monolayer of InAs is pseudomorphically grown on GaAs. A picture of the growth process is discussed on the basis of the equilibrium between InAs and floating In dominated by the stress energy.
TL;DR: The large strain dynamic behaviour of brain tissue and silicone gel, a brain substitute material used in mechanical head models, was compared and it was concluded that silicone gel material parameters are in the same range as those ofbrain tissue.
Abstract: The large strain dynamic behaviour of brain tissue and silicone gel, a brain substitute material used in mechanical head models, was compared. The non-linear shear strain behaviour was characterised using stress relaxation experiments. Brain tissue showed significant shear softening for strains above 1% (approximately 30% softening for shear strains up to 20%) while the time relaxation behaviour was nearly strain independent. Silicone gel behaved as a linear viscoelastic solid for all strains tested (up to 50%) and frequencies up to 461 Hz. As a result, the large strain time dependent behaviour of both materials could be derived for frequencies up to 1000 Hz from small strain oscillatory experiments and application of Time Temperature Superpositioning. It was concluded that silicone gel material parameters are in the same range as those of brain tissue. Nevertheless the brain tissue response will not be captured exactly due to increased viscous damping at high frequencies and the absence of shear softening in the silicone gel. For trend studies and benchmarking of numerical models the gel can be a good model material.
TL;DR: In this article, it was shown that the bombardment of high quality cubic (c-) BN films with 300 keV Ar+ ions leads to a strong relaxation of their compressive stresses without destroying the cubic phase if the total ion fluence is kept below an upper limit.
Abstract: It is shown that the bombardment of high quality cubic (c-) BN films with 300 keV Ar+ ions leads to a strong relaxation of their compressive stresses without destroying the cubic phase if the total ion fluence is kept below an upper limit. In addition, it was found that on top of such a stress-relieved film a further pure c-BN layer can be grown, but it builds up compressive stress again. Based on both results, a procedure is developed to grow thick (>1 μm) c-BN films (>80% c-BN) exhibiting low residual stress and long term stability under ambient conditions.
TL;DR: In this article, step strain experiments and dynamic light scattering measurements are perfomed to characterize the dynamic behavior of an oil-in-water droplet microemulsion into which is incorporated a telechelic polymer.
Abstract: Step strain experiments and dynamic light scattering measurements are perfomed to characterize the dynamic behavior of an oil-in-water droplet microemulsion into which is incorporated a telechelic polymer. At sufficient droplet and polymer concentrations, above the percolation threshold, the system is viscoelastic and its dynamic structure factor consists of two steps for the relaxation of concentration fluctuations: the fast one is dominated by the diffusion but the slower one is almost independent of the wave vector. The terminal time of the stress relaxation τR and the slow time of the dynamic structure factor τS are both presumably controlled by the residence time of a sticker in a droplet: consistently, τR and τS are of the same order, they both vanish at the percolation threshold according to power laws but with different exponents. We discuss these features in terms of deviations at the transition, from the usual mean field description of the dynamics of transient networks.
TL;DR: In this paper, the authors measured the activation volume for inelastic deformations by measuring the rates of stress relaxation during isothermal annealing of polycrystalline Ag thin films on oxidized Si substrates.
15 Feb 2000-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the residual compressive stress in α-Al 2 O 3 oxide films formed on the surface of Fe ǫCrAl Kanthal A-1 heat-resisting alloy at 1200°C was measured as a function of the cooling rate from the oxidation temperature and the oxide thickness ratio.
Abstract: The residual compressive stress in α-Al 2 O 3 oxide films formed on the surface of FeCrAl Kanthal A-1 heat-resisting alloy at 1200°C is measured as a function of the cooling rate from the oxidation temperature and the oxidemetal thickness ratio Except for the high cooling rates of about 500–1000°C min −1 , relaxation in both the oxide and the metal significantly reduces the residual stress The oxide exhibits failure after cooling at intermediate rates (5–200°C min −1 ) but remains intact after cooling at either higher or lower rates The failure occurs by spontaneous buckling and spalling at room temperature under a constant residual stress in the film The extent of buckling and spalling does not decrease by reducing the residual stress in the film Surprisingly, at a given cooling rate, spalling is more pronounced on thinner metal plates when the oxide stress is smaller Moreover, it appears that the best spalling resistance usually corresponds to the highest compressive stress in the oxide
TL;DR: In this article, in situ measurements of the in-plane mechanical stress were made during and after ion irradiation at various temperatures in the range from 95 to 575 K, in order to study the magnitude of these effects, the mechanism behind them, as well as their interplay.
Abstract: The 4 MeV Xe ion irradiation of a thin thermally grown SiO2 film on a Si substrate leads to four different effects in which each manifests itself by a characteristic change in the mechanical stress state of the film: densification, ascribed to a beam-induced structural change in the silica network; stress relaxation by radiation-enhanced plastic flow; anisotropic expansion and stress generation; and transient stress relaxation ascribed to the annealing of point defects. Using sensitive wafer-curvature measurements, in situ measurements of the in-plane mechanical stress were made during and after ion irradiation at various temperatures in the range from 95 to 575 K, in order to study the magnitude of these effects, the mechanism behind them, as well as their interplay. It is found that the structural transformation leads to a state with an equilibrium density that is 1.7%–3.2% higher than the initial state, depending on the irradiation temperature. Due to the constraint imposed by the substrate, this trans...
TL;DR: In this paper, a physical stress-enhanced bond-breaking model is proposed to explain the oxide breakdown mechanism, which is very closely related to the Si-Si bond formation from the breakage of Si-O -Si bond.
Abstract: In this work, we demonstrate that for ultrathin MOS gate oxides, the reliability is closely related to the SiO/sub 2//Si interfacial physical stress for constant current gate injection (V/sub g//sup -/) in the Fowler-Nordheim tunneling regime. A physical stress-enhanced bond-breaking model is proposed to explain this. The oxide breakdown mechanism is very closely related to the Si-Si bond formation from the breakage of Si-O-Si bond, and that is influenced by the physical stress in the film. The interfacial stress is generated due to the volume expansion from Si to SiO/sub 2/ during the thermal oxidation, and it is a strong function of growth conditions, such as temperature, growth rate, and growth ambient. Higher temperatures, lower oxidation rates, and higher steam concentrations allow faster stress relaxation through viscous flow. Reduced disorder at the interface results in better reliability. Fourier transform infrared spectroscopy (FTIR) technique has been used to characterize stress in thin oxide films grown by both furnace and rapid thermal process (RTP). In conjunction with the Gibbs free energy theory, this model successfully predicts the trends of time-to-breakdown (t/sub bd/) as a function of oxide thickness and growth conditions. The trends of predicted t/sub bd/ values agree well with the experimental data from the electrical measurement.
TL;DR: In this paper, a simple shear flow was performed on (50/50) PB/PDMS poly(1-butene)/polydimethylsiloxane immiscible model blends and the results were compared to the predictions of the Doi-Ohta and Lee-Park models.
Abstract: Relaxation experiments after simple shear flow were performed on (50/50) PB/PDMS poly(1-butene)/polydimethylsiloxane immiscible model blends and the results were compared to the predictions of the Doi–Ohta and Lee–Park models. Three situations of flow were examined: (i) first the variation of stress relaxation was followed in time at various step strain amplitudes, (ii) variation of stress relaxation as a function of the amplitude of preshear rate at a fixed strain, and (iii) at a fixed strain and preshear rate, the relaxation of the stress was studied as a function of the time elapsed between the end of the preshear and the step strain. After application of step strains of various magnitudes, the stress relaxation modulus G(t,γ) at short times was found to obey the Wagner time-strain separability [Wagner (1976)]. It was possible to separate linear effects from the nonlinear ones via a damping function h(γ) of sigmoidal form. After cessation of steady shear flow of different magnitudes, the linear stress ...
TL;DR: In this article, the authors compare results from two-step aging experiments in which the material is first annealed at a temperature near the nominal glass transition temperature of 120 °C, and the second step is tested at a lower temperature, following classical sequential aging techniques.
Abstract: The sub-glass-transition viscoelastic and physical aging responses of an amorphous poly(ethylene naphthalate) (PEN) have been studied using uniaxial tension stress relaxation experiments. It is known that PEN exhibits a strong β relaxation that overlaps the α relaxation in the experimental time and temperature ranges studied. In prior work, we had shown that both amorphous and semicrystalline PEN exhibit thermorheologically complex behaviors in that neither time−temperature nor time−aging time superposition apply to the materials. Here we compare results from two-step aging experiments in which the material is first annealed at a temperature near the nominal glass transition temperature of 120 °C. In the second step, the viscoelastic response of the material is tested at a lower temperature, following classical sequential aging techniques. We find, for samples first annealed at 100 °C, that the amorphous PEN shows time−temperature superposition behavior for constant annealing times. The results are interp...
TL;DR: In this article, the uniaxial tension and stress relaxation properties of micron-scale Al beams for microelectromechanical systems applications were investigated in a piezoactuator-driven test apparatus.
Abstract: We investigated the uniaxial tension and stress relaxation properties of micron-scale Al beams for microelectromechanical systems applications in a piezoactuator-driven test apparatus. Pure aluminum and Al-1.5 at. % titanium free-standing beams were fabricated using micromachining procedures. In the tensile tests, we found the yield strength of the Al beams to be approximately 95 MPa. We also observed a significant strengthening effect in the alloyed samples, which had a yield strength approximately 85% higher than the pure Al samples. In stress relaxation tests, we observed a substantial load-drop (about 56% after 10 min) in the Al films, and we propose here that grain boundary sliding is responsible for this relaxation. By comparison, the relaxation of the Ti-alloyed samples was only 15%. We believe that this difference results from the Al3Ti precipitates that form at the Al grain boundaries in the alloy samples.
TL;DR: The results obtained from the present study suggest that ground substance matrix, and interfiber and fiber-matrix interactions have important roles in the remodeling response of tendons to stress.
Abstract: With the aim of studying mechanisms of the remodeling of tendons and ligaments, the effects of stress shielding on the rabbit patellar tendon were studied by performing tensile and stress relaxation tests in the transverse direction The tangent modulus, tensile strength, and strain at failure of non-treated, control patellar tendons in the transverse direction were 1272 kPa, 370 kPa, and 405 percent, respectively, whereas those of the tendons stress-shielded for I week were 299 kPa, 108 kPa, and 404 percent, respectively Stress shielding markedly decreased tangent modulus and tensile strength in the transverse direction, and the decreases were larger than those in the longitudinal direction, which were determined in our previous study For example, tensile strength in the transverse and longitudinal direction decreased to 29 and 50 percent of each control value, respectively, after 1 week stress shielding In addition, the stress relaxation in the transverse direction of stress-shielded patellar tendons was much larger than that of non-treated, control ones In contrast to longitudinal tensile tests for the behavior of collagen, transverse tests reflect the contributions of ground substances such as proteoglycans and mechanical interactions between collagen fibers Ground substances provide lubrication and spacing between fibers, and also confer viscoelastic properties Therefore, the results obtained from the present study suggest that ground substance matrix, and interfiber and fiber-matrix interactions have important roles in the remodeling response of tendons to stress
TL;DR: In this paper, a viscoplastic constitutive model was extended to apply to the cyclic deformation analysis of the modified 9Cr-1Mo steel, which was applied to simulations of inelastic deformation behavior under monotonic tension, stress relaxation, creep, isothermal cyclic deformations including stress relaxation and anisothermal cyclics deformations.