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Showing papers on "Stress relaxation published in 1993"


Book
14 Dec 1993
TL;DR: In this article, the authors discuss various mechanical properties of fiber-filled composites, such as elastic moduli, creep and stress relaxation, and other mechanical properties such as stress-strain behavior and strength.
Abstract: Mechanical Tests and Polymer Transitions * Elastic Moduli * Creep and Stress Relaxation * Dynamical Mechanical Properties * Stress-Strain Behaviour and Strength * Other mechanical Properties * Particulate-Filled Polymers * Fiber- Filled Composites and Other Composites.

3,166 citations


Book
01 Jan 1993
TL;DR: In this article, the authors discuss the properties of linear Viscoelastic Behaviour as a function of frequency and temperature, and investigate the relationship between the frequency and the temperature.
Abstract: Preface. 1 Structure of Polymers. 2 The Deformation of an Elastic Solid. 3 Rubber-like Elasticity. 4 Principles of Linear Viscoelasticity. 5 The Measurement of Viscoelastic Behaviour. 6 Experimental Studies of Linear Viscoelastic Behaviour as a Function of Frequency and Temperature: Time-Temperature Equivalence. 7 Anisotropic Mechanical Behaviour. 8 Polymer Composites: Macroscale and Microscale. 9 Relaxation Transitions: Experimental Behaviour and Molecular Interpretation. 10 Creep, Stress Relaxation and Non-linear Viscoelasticity. 11 Yielding and Instability in Polymers. 12 Breaking Phenomena. Appendix 1. Appendix 2. Answers to Problems. Index.

1,086 citations


Journal ArticleDOI
TL;DR: This work directly imaged the evolution of surface cusps during strained-layer epitaxy and found high stress concentrations at the cusp tip have important implications for strain relaxation in the film via dislocation nucleation.
Abstract: We have directly imaged the evolution of surface cusps during strained-layer epitaxy The cusps arise naturally as a result of gradients in the surface chemical potential High stress concentrations at the cusp tip have important implications for strain relaxation in the film via dislocation nucleation

232 citations


Journal ArticleDOI
M.D. Thouless1, J. Gupta1, J.M.E. Harper1
TL;DR: In this paper, an analysis based on existing models of creep is presented that predicts the stresses developed in thin blanket films of copper on Si wafers subjected to thermal cycling, and the results are portrayed on deformation-mechanism maps that identify the dominant mechanisms expected to operate during thermal cycling.
Abstract: The reliability of integrated-circuit wiring depends strongly on the development and relaxation of stresses that promote void and hillock formation. In this paper an analysis based on existing models of creep is presented that predicts the stresses developed in thin blanket films of copper on Si wafers subjected to thermal cycling. The results are portrayed on deformation-mechanism maps that identify the dominant mechanisms expected to operate during thermal cycling. These predictions are compared with temperature-ramped and isothermal stress measurements for a 1 μm-thick sputtered Cu film in the temperature range 25–450 °C. The models successfully predict both the rate of stress relaxation when the film is held at a constant temperature and the stress-temperature hysteresis generated during thermal cycling. For 1 μm-thick Cu films cycled in the temperature range 25–450 °C, the deformation maps indicate that grain-boundary diffusion controls the stress relief at higher temperatures (>300 °C) when only a low stress can be sustained in the films, power-law creep is important at intermediate temperatures and determines the maximum compressive stress, and that if yield by dislocation glide (low-temperature plasticity) occurs, it will do so only at the lowest temperatures (<100 °C). This last mechanism did not appear to be operating in the film studied for this project.

211 citations


Journal ArticleDOI
TL;DR: In this paper, the concept of critical thickness, hc, for domain formation is developed for a single twin band in which the c axis of the tetragonal domains is either related by a 90° rotation about an axis in the plane of the film or by a normal about the surface normal, and the critical thickness depends only on the relative coherency strain between the substrate and film and the ratio of the domain wall energy to the stored elastic energy.
Abstract: Twin related domain formation is examined as a strain relaxation mechanism for a heteroepitaxial tetragonal film on a cubic substrate. Elastic relaxations are calculated for a single twin band in which the c axis of the tetragonal domains is either related by a 90° rotation about an axis in the plane of the film or by a 90° rotation about the surface normal. In all cases, the strain energy change is evaluated for both the film and the substrate. A domain pattern map is developed that predicts single domain and multiple domain fields depending on the relative misfit strains and domain wall energy. The concept of a critical thickness, hc, for domain formation is developed. For cases in which the c axis is rotated 90° about an axis in the plane of the film, the critical thickness depends only on the relative coherency strain between the substrate and film and the ratio of the domain wall energy to the stored elastic energy. For the case of a pattern consisting of energetically equivalent domains with the c a...

205 citations


Journal ArticleDOI
TL;DR: In this paper, the authors studied the thermal strains and stresses due to the thermal expansion coefficient difference in GaN(0001)/α-Al2O3(0001) layered structures.
Abstract: Thermal strains and stresses due to the thermal expansion coefficient difference in GaN(0001)/α-Al2O3(0001) layered structures are studied by varying the film thickness of GaN from 0.6 to 1200 µm. The strain in GaN is greater in films of less than a few microns thickness. It is decreased in films of thickness from several to about a hundred microns, and is almost completely relaxed in those thicker than 100 µm. The stresses and strains in the heterostructure are calculated using a model in which relaxation due to cracking in the sapphire is considered. Three relaxation mechanisms of the thermal strain are found for different film thicknesses as follows: (a) only lattice deformation ( 20 µm).

204 citations


Journal ArticleDOI
TL;DR: The nonlinear viscoelastic properties of the anteromedial bundle of porcine anterior cruciate ligament (ACL) were characterized by using a new analytical approach based on the quasi-linear vis coelastic theory, which provides an accurate description of this nonlinear stress relaxation behavior.

169 citations


Journal ArticleDOI
Kunihiro Osaki1
TL;DR: In this article, the damping function of the shear relaxation modulus, h(γ), has been investigated and the results show that the dampness of polystyrene is due to a slip or an instability of deformation in the material.
Abstract: Published data of the damping function of the shear relaxation modulus, h(γ), are reviewed. This is the ratio of the relaxation modulus measured at a finite magnitude of shear, γ, to that at the limit of γ = 0. Majority of the data are in accord with the universal function of the Doi-Edwards tube model theory, in which the damping or the decrease of h(γ) is attributed to the contraction along the tube of extended polymer chains. The weaker damping seems to be attributed to 1) comb-branching such as in LDPE; 2) lack of entanglement in too short chains; 3) bimodal molecular weight distribution. However, a star-branching does not cause a deviation from the tube model theory and a broadness of molecular weight distribution is not a major origin of a weaker damping. A star-branched polystyrene with 15 arms exhibits no strain dependence: h(γ) = 1. For highly entangled systems with more than 50 entanglement points per molecule, the strain dependence is stronger than that of the Doi-Edwards theory. This could be due to a slip or an instability of deformation in the material.

165 citations


Journal ArticleDOI
TL;DR: In this paper, a new mechanism of work hardening is proposed to explain the athermal hardening in Stage IV of f.c. and diamond cubic crystals, which is related to a cellular dislocation microstructure in which during Stage III, hardening by dislocation accumulation and recovery by various mechanisms occurs primarily in the cell walls.
Abstract: A new mechanism of work hardening is proposed to explain the athermal hardening in Stage IV of f.c.c. and diamond cubic crystals. The mechanism is related to a cellular dislocation microstructure in which during Stage III, hardening by dislocation accumulation and recovery by various mechanisms occurs primarily in the cell walls. Hardening of the cells is through the build-up of long range misfit stresses that result when the primary dislocation flux cuts trough the geometrically required dislocation density of the cell walls that is associated with the lattice misorientations between cells. Experiments show that these misorientations increase monotonically with increasing strain. There is no recovery in the cells. At the end of Stage III, hardening in the cell walls saturates, but the hardening due to misfit stresses in the cells continues unabated, giving rise to the rate independent hardening of Stage IV. Eventually this hardening is also terminated in Stage V when the misfit stresses inside cells reach a critical level that triggers rate dependent stress relaxation in the cells by secondary glide processes. The new mechanism makes successful predictions for Stage IV processes, including: hardening rate, plastic resistance levels, the gradual increase in hardening rate with plastic resistance, the residual lattice strains on unloading that can be measured with X-ray peak distortions and broadening, and for the Baushinger effect.

162 citations


Journal ArticleDOI
TL;DR: The tensile creep behavior of Al-SiC metal matrix composites has been investigated and analyzed over the temperature range from 230 to 525°C as discussed by the authors, and it is shown that the plastic flow in these materials is lattice-diffusion controlled dislocation creep in the aluminum matrix.
Abstract: The tensile creep behavior of AlSiC metal matrix composites has been investigated and analyzed over the temperature range from 230 to 525°C. It is shown that plastic flow in these materials is lattice-diffusion controlled dislocation creep in the aluminum matrix. All data on AlSiC have been assessed by a creep relation developed for creep of metals at constant structure with the added contribution of a threshold stress. The threshold stress for creep in AlSiC composites is not a thermally-activated process and is shown to have a linear dependence with temperature becoming zero at 470°C. The threshold stress is higher for the whisker composites than for the particulate composites. The origin of the threshold stress is not well understood and cannot be explained by contemporary dislocation models involving dislocation bowing or unpinning around particles sites. The observed interparticle-interwhisker spacing is shown to influence the creep rate in the same way as observed for mechanical alloyed (MA) Al base materials.

152 citations


Journal ArticleDOI
TL;DR: In this article, the authors proposed a mechanism for interparticle sliding that is both viscous and frictional, based upon rate process theory, which leads to an expression for the sliding velocity of two contacting particles in terms of the ratio between the tangential and normal contact force components.
Abstract: Soil exhibit viscous creep behavior, and the creep strain rate is known to change greatly with time. Various possible reasons for the changing creep rate are considered. The writers propose a mechanism for interparticle sliding that is both viscous and frictional. The mechanism is based upon rate process theory and leads to an expression for the sliding velocity of two contacting particles in terms of the ratio between the tangential and normal contact force components. This interparticle sliding mechanism was incorporated into a numerical discrete element model of a large assembly of circular particles. Numerical simulations with the assembly displayed creep behavior that is very similar to soils. The creep rate increased with greater applied stress, decreased rapidly with time, and exhibited creep rupture at large stress levels. Changes in the creep rate are shown to result from the changes in contact forces that accompanied deformation of the entire assembly. Such deformation‐dependent changes are thou...

Journal ArticleDOI
TL;DR: The model for the KWW relaxation function requires the disordered glassy structure of collagen fiber, which is consistent with the results of the structural investigations, and the relaxation mechanism in bone and bone collagen was identified.

Journal ArticleDOI
TL;DR: In this article, the authors measured axial and volumetric strain from changes in the output of resistance foil strain gauges bonded to the rock surface and measured dc electrical resistivity parallel to the sample axis.
Abstract: Cylindrical samples of granite were deformed at 26oC, constant confining pressure (60 MPa), and constant pore pressure (20 MPa). Axial and volumetric strain were determined from changes in the output of resistance foil strain gauges bonded to the rock surface. In addition, dc electrical resistivity was measured parallel to the sample axis. During each experiment (typically lasting from 1–2 weeks), the deviatoric stress σd applied to the sample was cycled between 70% and 90% of the short–term failure strength. The bulk of the experiments were conducted in the secondary or “steady state” creep regime. Inelastic volumetric strain rate was found to obey the law , where and (compressive stresses are negative). The C coefficient represents a strain-hardening-like term. The stress dependence is of the same form as the stress dependence measured for mode I crack growth in double cantilever beam experiments. The observed creep behavior is analyzed in terms of stress corrosion and crack growth models including a formulation based on energy release rate for characteristic microcracks.

Journal ArticleDOI
TL;DR: In this article, the biaxial modulus and coefficient of thermal expansion of ion-beam-sputtered amorphous Si and Ge thin films were determined from curvature changes induced by differential thermal expansion.
Abstract: The biaxial modulus and coefficient of thermal expansion of ion‐beam‐sputtered amorphous Si and Ge thin films were determined from curvature changes induced by differential thermal expansion. Viscous flow was measured by stress relaxation and was found to be Newtonian. The viscosity increased linearly with time as a result of structural relaxation, and its isoconfigurational activation enthalpy was 1.8±0.3 and 2.6±1.3 eV for amorphous Si and Ge, respectively. An atomistic model, based on a chain reaction of broken bond rearrangements, is proposed to describe the observation.

Journal ArticleDOI
TL;DR: It is shown that during the growth of InxGa1-xAs on GaAs, the strain-induced lattice distortion oscillates as a function of monolayer completion in both purely 2D and quasi-2D layer-by-layer growth regimes.
Abstract: It is shown that during the growth of InxGa1-xAs on GaAs, the strain-induced lattice distortion oscillates as a function of monolayer completion in both purely 2D and quasi-2D layer-by-layer growth regimes. This is explained by considering that nontetragonal elastic distortion occurs at the free edges of 2D monolayer islands. Numerical relaxation using a simplified model of interatomic forces gives the correct order of magnitude of the strain relaxation by this process.

Journal ArticleDOI
TL;DR: In this article, the authors measured the biaxial extensional flow behavior of polystyrene and polypropylene melts using the lubricated squeezing flow method under the condition of constant strain rate.
Abstract: Biaxial extensional flow behavior of polystyrene and polypropylene melts was measured utilizing the lubricated squeezing flow method under the condition of constant strain rate. The uniaxial extensional flow behavior of these samples was also measured using a Meissner‐type rheometer for comparison with the biaxial behavior. The biaxial stress growth coefficient η B + grows more slowly than the uniaxial stress growth coefficient η E + at long times. Deviation of η B + from the low strain rate asymptote is not so prominent as that of η E +. The deviation of η B + and the upturn of η E + from the low strain rate asymptotes are more apparent in broad distribution samples than in a narrow distribution sample. It is found that η B + decreases with strain rate at long times near steady state. The upturn behavior of η E + at high strain rates in a narrow distribution sample is discussed based on the Doi–Edwards theory and stress relaxation data. The upturn is clearly observed when the strain rate exceeds the contraction (retraction) rate τeq −1, where the contraction time τeq is evaluated from the relaxation data. Consideration on chain stretching in various multi‐axial extensional flows gives a prediction for deviation of stress growth coefficients from the low strain rate asymptotes in uniaxial, biaxial, planar, and ellipsoidal extensional flows.

Journal ArticleDOI
TL;DR: In this paper, the authors studied load-carrying capacity as an outcome of the competition between stress concentration due to the notch, and stress relaxation due to inelastic deformation.
Abstract: Holes are often drilled in a panel for cooling or fastening. For a panel made of a monolithic ceramic, such a hole concentrates stress, reducing load-carrying capacity of the panel by a factor of 3. By contrast, for a ductile alloy panel, plastic flow relieves stress concentration so that the small hole does not reduce load-carrying capacity. A panel made of ceramic-matrix composite behaves in the middle: matrix cracks permit unbroken fibers to slide against friction, leading to inelastic deformation which partially relieves stress concentration. Load-carrying capacity is studied in this paper as an outcome of the competition between stress concentration due to the notch, and stress relaxation due to inelastic deformation. The inelastic deformation is assumed to be localized as a planar band normal to the applied load, extending like a bridged crack. The basic model is large-scale bridging. A material length, δ0 E/σ0 , scales the size of the inelastic band, where σ0 is the unnotched strength, δ0 the inelastic stretch at the onset of rupture, and E Young’s modulus. Load-carrying capacity is shown to depend on notch size a, measured in units of δ0 E/σ0 . Calculations presented here define the regime of notch ductile-to-brittle transition, where ceramic-matrix composites with typical notch sizes would lie. Both sharp notches and circular holes are considered. The shape of the bridging law, as well as matrix toughness, is shown to be unimportant to load-carrying capacity.

Book
10 Feb 1993
TL;DR: In this article, the authors propose a model for the elastic-plastic bending of prismatic beams and plane frames with respect to the time hardening properties of the material and its elastic-Bingham material.
Abstract: BASIC DEFINITIONS Stress and Strain State Stress tensor Strain tensor Finite Deformations Finite strain tensors in material or spatial coordinates Strain rates tensors Stress tensors in material or spatial descriptions FOUNDATIONS OF PLASTICITY Basic Equations of Perfect Plasticity Uniaxial stress-strain behavior Criteria for yielding in perfect plasticity Stress-strain relations for perfect plasticity Methods of reduction of equations of perfect plasticity Problems Basic Equations of Plastic Hardening Drucker's postulate and the associated flow rule Subsequent yield surfaces for hardening material Theories of plastic hardening Problems Methods of the Theory of Plasticity Analysis of the level of a cross-section Interaction curves on levels of a cross-section or a body Extremum theorems of limit analysis: statically or kinematically admissible solutions Shakedown analysis Integration along characteristics in plane strain problems Problems SOL UTIONS OF ELASTIC-PLASTIC PROBLEMS Elastic-Plastic Torsion and Bending Elastic-plastic torsion of prismatic bars Problems Elastic-plastic bending of prismatic beams and plane frames Problems Elastic-Plastic Analysis of Cylinders, Disks, and Plates Thick-walled tubes, spherical shells and disks Problems Limit analysis of Plates Problems FOUNDATIONS OF CREEP Basic Equations of Uniaxial Creep Models Creep phenomenon Schematizations of creep at constant uniaxial stress Modelling of creep at varying uniaxial stress Linear uniaxial viscoelastic models Modelling of viscoplastic materials Problems Creep Constitutive Equations Under Multiaxial Loading Classical multiaxial creep theories Developed multiaxial creep theories Linear multiaxial viscoelastic equations SOLUTION OF CREEP PROBLEMS Bending, Buckling, and Torsion of Bars Under Creep Conditions Bending and buckling of a prismatic bar made of the linear viscoelastic material Bending of a prismatic be am made of the piece-wise linear elastic/viscoplastic material Bending of a prismatic beam made of the time hardening material Torsion of a circular bar made of the elastic-Bingham material Problems Rotationally Symmetric Creep Problems Creep of a thick-walled tube General formulae for the rotationally-symmetric transient creep problems CREEP RUPTURE Constitutive Equations of Creep Rupture Creep rupture phenomenon Classical creep rupture theories Problems Rotationally Symmetric Creep Rupture Problems Mechanisms of brittle rupture of tubes and disks Design of disks with respect to creep rupture References Author Index Subject Index

Journal ArticleDOI
TL;DR: In this paper, a mesophase of hexagonally ordered cylindrical domains is considered, and shear deformation is applied perpendicular to the cylinder axes, and a cell dynamic approach is used to simulate the system under oscillatory and step-shear strains.
Abstract: Computer simulation is carried out for microphase-separated diblock copolymers under applied shear deformation. A mesophase of hexagonally ordered cylindrical domains is considered, and shear strains are applied perpendicular to the cylinder axes. Rheological responses and structural changes of the system under oscillatory and step-shear strains are studied by using a cell dynamic approach. For small strains, the usual behavior of ordered viscoelastic solids is seen, while for large strains, anomalous behavior is observed, including (i) in the case of oscillatory shears, a nonlinear stress response which is out of phase with the applied strain in the low frequency limit and (ii) in the case of step-shear, a double stress relaxation process in which the stress first approaches a pseudoequilibrium value and then much later relaxes further toward the final equilibrium value

Journal ArticleDOI
TL;DR: In this paper, the authors show that dislocations are probably not responsible for the plastic deformation of icosahedral quasicrystals at high temperatures with no final hardening stage.
Abstract: Plastic deformation of icosahedral AlCuFe has been studied by compression tests for temperatures ranging from room temperature to 750°C. Whereas at room temperature no ducibility is observed, the samples can be homogeneously deformed up to 130% at high temperatures with no final hardening stage. SEM observations reveal, at the ultimate stage of deformation, a vein pattern similar to the observed in amorphous metals but without slip bands or shear bands. Stress relaxation measurements shows to different regimes at low and high temperatures with a transition at 660°C. It is concluded from these experiments that dislocations are probably not responsible for the plastic deformation of icosahedral quasicrystals.

Patent
07 Apr 1993
TL;DR: An elastic nonwoven web is formed from elastic fibers composed of a blend of a styrene-poly(ethylene-propylene)-styrene thermoplastic elastomeric block copolymer or a mixture of a polyolefin and an extending oil as mentioned in this paper.
Abstract: An elastic nonwoven web is formed from elastic fibers composed of a blend of (1) a styrene-poly(ethylene-propylene)-styrene thermoplastic elastomeric block copolymer or a mixture of a styrene-poly(ethylene-propylene)-styrene elastomeric block copolymer and a styrene-poly(ethylene-butylene)-styrene elastomeric block copolymer, and (2) a tackifying resin in which the elastic nonwoven web has a stress relaxation of less than about 30 percent. The blend used to form the elastic nonwoven web and/or elastic fibers may also include a polyolefin and an extending oil.

Journal ArticleDOI
TL;DR: In this article, the authors consider stress relaxation in a strongly segregated lamellar mesophase, where block copolymers are in the "brush" state with junction points confined to the interface between the adjacent lamellae and blocks stretched out away from it.
Abstract: We consider stress relaxation in a strongly segregated lamellar mesophase, where block copolymers are in the "brush" state with junction points confined to the interface between the adjacent lamellae and blocks stretched out away from it. If the molecular weight of the blocks is large enough, they entangle with their neighbors as well as with blocks from the opposite brush. The number of entanglements of one particular chain with the opposite brush varies from chain to chain even in the monodisperse system. We demonstrate that this dispersion of the number of entanglements leads to a very broad spectrum of relaxation times and to an effectively power-law-like stress relaxation function log G - (log t)" with CY = 1/2 (a = 2) in the strong (weak) chain-stretching limit. We analyze various disentanglement mechanisms for diblocks and triblocks and the onset of the diffusion of copolymers along the interface. Another relaxation mechanism is due to the displacement of a block across the interface into the "enemy" phase. We conclude that for highly entangled copolymers it will be an important mode of stress relaxation (especially for triblock copolymers). For asymptotically long times the relaxation of stress along the perfectly ordered lamella will be liquidlike. In a system with defects in domain structure the relaxation of stress is controlled by the processes of equilibration of excess density along the layers. For the lamellar mesophase we found G(t) - t 4. 2, while for the cylindrical mesophase, G(t) - t

Journal ArticleDOI
TL;DR: Using a broadband mechanical spectrometer, the tensile modulus and stress relaxation of amorphous selenium were studied in the glass transformation range and are consistent with the existence of a cooperative transition, near 300 K, between ringlike and chain elements in the disordered chain model of Misawa and Suzuki.
Abstract: Using a broadband mechanical spectrometer, the tensile modulus and stress relaxation of amorphous selenium were studied in the glass transformation range. Bimodal spectra were recorded under isothermal conditions in the time range ${10}^{\mathrm{\ensuremath{-}}1}$ st${10}^{5}$ s. It was found that the structural equilibrations of the two contributions to the stress decay proceed independent of one another. The mechanical correlation functions of stabilized supercooled liquid Se can be described using a weighted sum of two stretched exponentials. The temperature dependences of the relative relaxation strengths are compared with results from a rotational isomeric state model. Our results are consistent with the existence of a cooperative transition, near 300 K, between ringlike and chain elements in the disordered chain model of Misawa and Suzuki. This transition has been kinetically inaccessible to previous measurements.

Journal ArticleDOI
TL;DR: In this paper, the authors examined the dilatational and shear rheological properties of spread films of PEO/PMMA graft (comb) copolymers at the air/water, toluene-n-heptane/water and toluenes n -heptanes/water interfaces.

Journal ArticleDOI
TL;DR: In this paper, the tensile creep and creep-recovery behavior of a hot-pressed unidirectional SiC-fiber/Si3N4-matrix composite was investigated at 1200 C in air, in order to determine how various sustained and cyclic creep loading histories would influence the creep rate, accumulated creep strain, and the amount of strain recovered upon specimen unloading.
Abstract: The tensile creep and creep-recovery behavior of a hot-pressed unidirectional SiC-fiber/Si3N4-matrix composite was investigated at 1200 C in air, in order to determine how various sustained and cyclic creep loading histories would influence the creep rate, accumulated creep strain, and the amount of strain recovered upon specimen unloading. The data accumulated indicate that the fundamental damage mode for sustained tensile creep at stresses of 200 and 250 MPa was periodic fiber fracture and that the creep life and the failure mode at 250 MPa were strongly influenced by the rate at which the initial creep stress was applied. Cyclic loading significantly lowered the duration of primary creep and the overall creep-strain accumulation. The implications of the results for microstructural and component design are discussed.

Journal ArticleDOI
TL;DR: In this article, two high temperature chambers for X-ray diffraction were designed, allowing us to determine the stresses in both the oxide and the substrate with the sin2 ϕ technique, at high temperatures or room temperature and during heating-cooling sequences.
Abstract: In order to characterize the respective importance of the growth stresses, thermal stresses and stress relaxation developed in oxide scales, two high temperature chambers for X-ray diffraction were designed, allowing us to determine the stresses in both the oxide and the substrate with the sin2 ϕ technique, at high temperatures or room temperature and during heating-cooling sequences. It was applied to NiNiO. At room temperature after oxidation, NiO is subjected to compressive stresses whose level depends on the substrate thickness and on the oxidation time and temperature. In the substrate, compressive stresses are mainly due to internal oxidation. During oxidation at 900 °C, the oxide scale is subjected to slight tensile stresses which can be due partially to anionic diffusion, internal oxidation or the heating process. During heating-cooling sequences, the stresses in the scale decrease with increasing temperature and become negligible when the oxidation temperature is reached. The reversibility of the stress-temperature curve indicates that no stress relaxation occurs. The stresses found at room temperature are due only to thermal stresses and fit well the theoretical calculation of thermal stresses in NiO scale based on the newly determined thermal expansion coefficients of Ni and NiO. All these results show that the stresses found at room temperature are mainly generated during cooling and that the effect of the Pilling-Bedworth ratio or of factors playing a role during isothermal growth is negligible.

Journal ArticleDOI
TL;DR: In this article, a test method is presented for simulating warp yams during weaving, where the residual force at infinite time in a relaxation test from a model for the tensile curve of spun yams is calculated.
Abstract: During weaving, warp yams are subjected to dynamic loading before stress relaxation during a loom stop. A test method is presented for simulating this behavior. There are three types of relaxation curves—simple relaxation, inverse relaxation, and mixed relaxation—depending on the previous load history of the yam. One can calculate the residual force at infinite time in a relaxation test from a model for the tensile curve of spun yams. There seems to be good correlation with the results of tests made using this method.

Journal ArticleDOI
TL;DR: In this paper, the authors generalized the linear elastic fracture model based on an R-curve (a curve characterizing the variation of the critical energy release rate with the crack propagation length) to describe the rate effect and size effect observed in concrete, rock or other quasibrittle materials.
Abstract: The equivalent linear elastic fracture model based on an R-curve (a curve characterizing the variation of the critical energy release rate with the crack propagation length) is generalized to describe both the rate effect and size effect observed in concrete, rock or other quasibrittle materials. It is assumed that the crack propagation velocity depends on the ratio of the stress intensity factor to its critical value based on the R-curve and that this dependence has the form of a power function with an exponent much larger than 1. The shape of the R-curve is determined as the envelope of the fracture equilibrium curves corresponding to the maximum load values for geometrically similar specimens of different sizes. The creep in the bulk of a concrete specimen must be taken into account, which is done by replacing the elastic constants in the linear elastic fracture mechanics (LEFM) formulas with a linear viscoelastic operator in time (for rocks, which do not creep, this is omitted). The experimental observation that the brittleness of concrete increases as the loading rate decreases (i.e. the response shifts in the size effect plot closer to LEFM) can be approximately described by assuming that stress relaxation causes the effective process zone lenght in the R-curve expression to decrease with a decreasing loading rate. Another power function is used to describe this. Good fits of test data for which the times to peak range from 1 sec to 250000 sec are demonstrated. Furthermore, the theory also describes the recently conducted relaxation tests, as well as the recently observed response to a sudden change of loading rate (both increase and decrease), and particularly the fact that a sufficient rate increase in the post-peak range can produce a load-displacement response of positive slope leading to a second peak.

Journal ArticleDOI
TL;DR: In this paper, a phenomenological model was developed to derive the Raman shift of the Si−Si mode as a function of the germanium content for the two limiting cases, the pseudomorphically strained layer, and the alloy-like stress-free layer.
Abstract: Epitaxial Si1−xGex layers, grown by molecular beam epitaxy (MBE) and by chemical vapor deposition (CVD) on Si(001) substrates, with thicknesses between 20 and 50 nm and Ge contents from 4% to 23% were investigated by micro Raman backscattering, x‐ray double crystal diffractometry, and transmission electron microscopy. A quite simple phenomenological model was developed to derive the Raman shift of the Si–Si mode as a function of the germanium content for the two limiting cases, the pseudomorphically strained layer, and the alloy‐like stress‐free layer. A measure for the degree of relaxation can be obtained from the measured Raman shift and from the independently determined germanium content, using the results of the model. The degree of relaxation was determined for a number of CVD‐ and MBE‐grown Si1−xGex layers. The as‐grown pseudomorphic layers relax partially after annealing at 900 °C. The Raman scattering allows the monitoring of the development of relaxation during the semiconductor device processing.

Journal ArticleDOI
TL;DR: In this paper, a biaxial extensional viscosity, ηbe, was computed for non-brittle, soft samples and the average ratios of recovered to initial instantaneous compliance were between 0.31 and 0.59, and increased with sample temperature.
Abstract: Eleven traditionally produced butter samples were evaluated for rheological properties at large deformations by lubricated and nonlubricated uniaxial compression with varying sample geometries and compression rates, by stress relaxation in shear and creep experiments in compression at 5–20C each. Compression forces required to deform cylindrical samples in nonlubricated conditions were higher than in lubricated cases. Correction of shape changes in nonlubricated stress-strain curves resulted in good agreement with lubricated experiments at low compression rates up to stress maxima or yielding. As frictional forces were relatively smaller at high deformation rates, lower stresses were obtained in nonlubricated conditions. Samples with higher height/diameter ratios showed more pronounced stress maxima. A biaxial extensional viscosity, ηbe, was computed for nonbrittle, soft samples. Typical slopes in logarithmic plots of viscosity against strain rate were approximately -1. At strains of about 0.01 creep compliance depended on compression stress. The average ratios of recovered to initial instantaneous compliance were between 0.31 and 0.59, and increased with sample temperature. A calculated ηBE from creep compression showed fair agreement with ηBE from static compression experiments. At comparable strains, a conversion of creep compliance to the stress relaxation modulus by an approximate method showed a good fit of experimental results.