Showing papers on "Stress relaxation published in 1989"

Mechanical properties of model polyethylenes: tensile elastic modulus and yield stress

Abstract: Etude sur un polyethylene lineaire et un copolymere ethylene-butene-1, entre 175 o K et 260 o K

Modeling of stress effects in silicon oxidation

Abstract: A new set of models for the stress effects in silicon oxidation is proposed. The most essential feature is the nonlinear shear-stress-dependent oxide viscosity which models a high-stress relaxation phenomenon similar to plastic deformation. The models are shown to agree well with the most comprehensive and quantitative experimental data available-the thickness of oxide grown on both convex and concave cylindrical silicon structures. The model parameters are extracted by fitting the simulation of two-dimensional growth rates to the experimental values. The extracted values for the linear viscosity of wet oxide are shown to be close to independent data for the equilibrium viscosity of silica. Furthermore, the proposed oxide viscosity model with the extracted model parameters can successfully explain both the occurrence and the magnitude of the intrinsic stress in planar silicon oxidation. >

Strength, elasticity and viscoelastic properties of cerebral aneurysms.

Abstract: Tissue strength and stiffness of cerebral aneurysm walls obtained intraoperatively or at autopsy were evaluated by uniaxial strain/stress measurements. For comparison, corresponding measurements were also made on autopsy specimens of intracranial arteries. The maximum stress that the aneurysm tissue could tolerate, the yield stress, was found to be slightly lower than in arteries, which is likely due to the content of immature forms of collagen. The material stiffness, as determined by division of the yield stress by the corresponding strain, was also smaller in aneurysms than in arteries. The stress resistance of aneurysms and arterial tissue decreased over a period of several hours. The relaxation curves were found to be identical in aneurysms and arteries. The stress tolerated by aneurysm walls was found to be in the range of the stress that is imposed in vivo by the blood pressure. Arteries resisted stresses corresponding to pressures 5–10 times higher than physiological values. It is suggested that the balance of tissue strength and the stress imposed by the blood pressure is causally related to aneurysm growth.

A Modified Maxwell and a Nonexponential Model for Characterization of the Stress Relaxation of Agar and Alginate Gels

Abstract: Compressive stress relaxation curves of agar and alginate gels of different gum concentration (1–3% and 0.5–2%, respectively) were fitted by a two parameter nonexponential empirical model and a three term modified Maxwell model with two fixed relaxation times (10 and 100s). The asymptotic portion of the residual (unrelaxed) stress calculated by the two models for each gel had a similar magnitude, and therefore could serve as an objective measure of the gels degree of solidity. The coefficients of the modified Maxwell model provided a simple but meaningful means to compare, in quantitative terms, the differences in the relaxation time spectra that were associated with the gels' stiffness and strength.

Mechanical relaxations in polypropylene: A new experimental and theoretical approach

Abstract: Accurate dynamic mechanical measurements have been performed on semicrystalline isotactic polypropylene over wide ranges of temperature and frequency. A mechanical model has been used to analyze experimental results in order to separate the behavior of amorphous and crystalline phases. The two main α and β relaxation processes have been analyzed. The β relaxation, related to the glass-rubber transition of the amorphous fraction, has been studied with the help of a physical model. The behavior is similar to that of a wholly amorphous polymer, with two characteristics: a high rubbery plateau, indicating a crosslinking effect by the crystalline phase, and a strong effect of interfaces in shear strain. Experimental data suggest the α relaxation originates within the crystalline phase and that it can be attributed to diffusion of defects. The amorphous phase plays an important role in this process, because it has to adapt itself by cooperative movements to respect the compatibility of deformations of the two phases. The formalism developed here rationalizes experimental results obtained with samples having different thermal histories.

Thermal relaxation of pseudomorphic Si-Ge superlattices by enhanced diffusion and dislocation multiplication

Abstract: Pseudomorphic Si/Si‐Ge strained layer superlattices are metastable and will relax to lower‐energy, less strained, states on thermal annealing. Such relaxation may occur by the generation of misfit dislocation or by compositional homogenization of the superlattice. The particular mechanism adopted is shown to depend on the initial dislocation density of the structures. In cases where a significant portion of the strain is accommodated by an array of misfit dislocations in the as‐grown state there is a propensity to relax by generating additional misfit dislocations. In the case of structures where only a very small fraction of the misfit is relieved by dislocations in the as‐grown state, additional relaxation does not involve the multiplication of dislocations but proceeds by the interdiffusion of Si and Ge towards the homogenizing of the superlattice structure. This strain‐enhanced diffusion has previously been observed in metals and we confirm its existence in semiconductor systems as well. The implication of the above observations on device structures and the growth of such layers is discussed.

A Model for stress generation and relief in oxide — Metal systems during a temperature change

Abstract: When an oxidized metal is cooled from a high temperature, stresses are produced at the metal-scale interface, owing to the difference in thermal expansion rates of the oxide and metal. Such stresses become time- and temperature-dependent if the scale or underlying metal creeps as cooling occurs. A model is presented which describes thermally induced stresses in the scale and accounts for partial stress relaxation by creep of the metal substrate and/ or the scale. The expected stresses are a function of the material parameters: thermal expansion coefficients, elastic modulii, and creep rates of both metal and scale. To illustrate a range of behaviors, we have presented example calculations for three Cr2O3 forming metals, Ni-30Cr, pure Cr, and MA-754. The effect of stress relaxation during thermal cycling was also examined briefly. In these examples, creep of the Cr2O3 scale was not expected to be important.

Magnesium-lithium alloys in metal matrix composites — A preliminary report

Abstract: Procedures are described for the fabrication of fibrous composites based on magnesiumlithium alloys as a matrix Such composites have been produced containing planar random and aligned (continuous and discontinuous) fibres of carbon, alumina and silicon carbide For all of these, except silicon carbide whiskers, significant fibre degradation occurred, during fabrication or subsequent heat treatments, either by chemical reaction or by grain boundary penetration of lithium Further consequences of the high atomic mobility exhibited by the matrix are manifest in the mechanical behaviour of the composites Although considerable property enhancement is possible by fibre reinforcement, a significant diffusional contribution to the stress relaxation mechanisms results in a dependence of work hardening rate and failure strain on temperature and strain rate, even around room temperature and at relatively high strain rates It is concluded that, although the system presents many practical difficulties, it is worthy of further study

Method of treating a sample of an alloy

Abstract: A method of treating a sample of an alloy which is capable of transforming between martensitic and austenitic phases, to render the alloy pseudoelastic, the method comprising: (a) annealing the alloy at a temperature which is greater than the stress relaxation temperature (T SR ) of the alloy and less than the temperature at which the alloy is fully recrystallized (T x ); and (b) deforming the sample at a temperature which is greater than about the maximum temperature at which the alloy can be made to transform from its austenitic phase to its martensitic phase by the application of stress (M d ), and less than the stress relaxation temperature.

Strain relaxation kinetics in Si1–xGex/Si heterostructures

Abstract: Strain relaxation in Si1–xGex/Si superlattices and alloy films is studied as a function of ex situ anneal treatment with the use of x-ray diffraction and Raman spectroscopy. Samples are grown by molecular-beam epitaxy at an unusually low temperature (≈365 °C). This results in metastably strained alloy and superlattice films significantly in excess of critical thicknesses previously reported for such structures. Significant strain relaxation is observed upon anneal at temperatures as low as 390 °C. After a 700 °C, 2 h anneal, superlattices are observed to relax less fully (~43% of coherent strain) than corresponding alloys (~84% of coherent strain). Also, the strain relaxation kinetics of a Si1–xGex alloy layer is studied quantitatively. Alloy strain relaxation is approximately described by a single, thermally activated, first order kinetic process having activation energy Ea=2.0 eV. The relevance of our results to the microscopic mechanisms responsible for strain relaxation in lattice-mismatched semiconductor heterostructures is discussed.

Effect of physical aging on tensile stress relaxation and tensile creep of cured EPON 828/epoxy adhesives in the linear viscoelastic region

Abstract: The effect of physical aging on viscoelastic properties was studied for several cross-linked epoxies in the glassy state. Tensile creep and tensile stress relaxation were measured during isothermal physical aging, following rapid quenching of samples annealed above the glass-transition temperature (Tg). The momentary creep curves measured at 21°C, 45°C, and 61°C below Tg for different epoxies could be fitted to an empirical equation for the creep compliance D(t): Values for β and to were obtained, and the dependence of to on the aging time was determined. Shift factors were calculated to investigate changes in molecular mobility during physical aging. The momentary stress relaxation was measured on the same epoxy materials as used for the creep studies. The stress relaxation curves were fitted to the following equation for the tensile modulus E(t): Values for α and to were obtained. The influence of physical aging on-to was again studied by calculating shift factors as a function of the aging time. The results are compared with the results of the creep tests and discussed in the context of current molecular theories of physical aging of glassy polymers.

Effect of Creep Damage on the Tensile Creep Behavior of a Siliconized Silicon Carbide

Abstract: The tensile creep behavior of a siliconized silicon carbide was investigated in air, under applied stresses of 103 to 172 MPa for the temperature range of 1100° to 1200°C. At 1100°C, the steady-state stress exponent for creep was approximately 4 under applied stresses less than the threshold for creep damage (132 MPa). At applied stresses greater than the threshold stress for creep damage, the stress exponent increased to approximately 10. The activation energy for steady-state creep at 103 MPa was approximately 175 kJ/mol for the temperature range of 1100° to 1200°C. Under applied stresses of 137 and 172 MPa, the activation energy for creep increased to 210 and 350 kJ/mol, respectively, for the same temperature range. Creep deformation in the siliconized silicon carbide below the threshold stress for creep damage was determined to be controlled by dislocation processes in the silicon phase. At applied stresses above the threshold stress for creep damage, creep damage enhanced the rate of deformation, resulting in an increased stress exponent and activation energy for creep. The contribution of creep damage to the deformation process was shown to increase the stress exponent from 4 to 10.

Structure and mechanics of starfish body wall.

Abstract: The structure of the dorsal body wall of the starfish Echinaster spinulosus was studied using polarized light microscopy of frozen tissues, scanning electron microscopy and histology. The collagen fibres of the body wall form a three-dimensional orthogonal web. Voids in the web contain ossicles and papulae. The orthogonal web delivers dimensional stability but allows shear necessary for ray torsion. The ossicles and fibres interact to load the fibres in tension and the ossicles in compression. Strain rates of the dorsal body wall were measured on live animals during typical movements. Uniaxial tension tests of the body wall yielded Young's moduli of 267 MPa (longitudinal), 249 MPa (transverse) and 353 MPa (bias); curves were essentially linear. The body wall was approximately linearly viscoelastic and showed hysteresis at 0.01 Hz. Stress relaxation over five decades of time (in seconds) yielded relaxation spectra with peaks in relaxation time at 2.96-3.35, depending on test direction. Stress relaxation caused the connective tissue to soften. The surface of fractured stress-relaxed tissue revealed wispy, dissociated fibril tufts, whereas unrelaxed fractures produced blunt-ended fibre bundles. Neural control was necessary for body wall integrity.

Mechanical behaviors of 60/40 tin-lead solder lap joints

Abstract: Creep-fatigue interactions have been shown to play a critical role in the failure of surface mount solder joints. Monotonic, fatigue, and creep-fatigue data for 60/40 tin-lead solder lap joints at room temperature are presented. Monotonic tests performed include rapid-strain shear, creep, and stress relaxation tests. Fundamental data for the strain range partitioning (SRP) approach to creep-fatigue life-prediction are presented. SRP is found to model adequately the solder creep-fatigue response for the range of frequencies (0.007 to 0.5 Hz) and waveforms tested. Instantaneous plastic strains and viscoplastic strains are found to be equally damaging, in any combination, for isothermal room-temperature creep-fatigue. Fatigue lives obtained for 0.5-Hz tests performed under load control correlate with the lives to 50% load drop obtained for tests performed under strain control. >

High temperature inelastic deformation under uniaxial loading - Theory and experiment

Abstract: The elevated-temperature uniaxial inelastic deformation behavior of an Ni-base alloy, B1900 + Hf, is investigated by performing isothermal tensile, creep, cyclic, stress relaxation, and thermomechanical fatigue tests. The range of strain rates examined is from 10 to the -7th to 100 per sec, while the test temperatures range from 25 to 1093 C. This extensive constitutive data base has been used for evaluating the unified constitutive models of Bodner and Partom (1972) and of Walker (1972) which apply for the small-strain regime. Comparison of test results with independent model predictions indicates good agreement over a broad range of loading conditions, demonstrating the applicability of the unified-constitutive-equation approach for describing the strongly nonlinear and temperature-dependent response of meals under a wide range of deformation and thermal histories. Thus the results give confidence that the unified approach is an effective and efficient approach in which complex history-dependent thermoviscoplastic flow can be represented within a single inelastic strain-rate term.

Effects of intrinsic stress on submicrometer Nb/AlO/sub x//Nb Josephson junctions

Abstract: The intrinsic stress and its relaxation process are discussed for sputtered Nb films used for the electrodes in Nb/AlO/sub x//Nb Josephson junctions. The stress, optically measured for Nb films, depends on the Ar pressure during sputtering, and it changes from compressive to tensile when the Ar pressure is increased. From X-ray diffraction, the shift in the lattice constant was observed to be proportional to the film stress. Changes in the lattice constant were also clearly observed when Nb films were etched to fine patterns. This suggests that the intrinsic stress in Nb films is relaxed at peripheral areas after the patterning process, and that such relaxation is one cause of deterioration in current-voltage characteristics frequently observed in small Josephson junctions. This indicates that stress-free Nb should be used for submicrometer junctions. >

Growth of CdTe on GaAs by hot‐wall epitaxy and its stress relaxation

Abstract: CdTe(100) layers of the thickness range from 0.7 to 15 μm were grown on GaAs(100) substrates by hot‐wall epitaxy. The crystallinity of the layer was examined by reflection high‐energy electron diffraction, x‐ray rocking curve, and photoluminescence. The lattice relaxation were investigated by x‐ray analysis and optical reflectance spectra. CdTe layers with thickness up to 15 μm were under compressive biaxial stress. In addition to the split exciton lines (n=1), the emission from excited states (n=2) was observed.

Ply-Slip During the Forming of Thermoplastic Composite Parts

Abstract: A model for the ply-slip deformation behavior of a laminate composite is presented which allows prediction of the stresses, displacements, and strains developed during the forming process. The model takes into account the viscous nature of the matrix and the stress relaxation achievable via ply slippage. The stress response is inherently time dependent and is strongly affected by the forming rate. Results calculated for the isother mal three point bending of a unidirectional lay-up agree well with previous experimental data. An important parameter, the maximum system time constant, is identified and related to the material properties and part geometry. Comparisons with experiments and simulation results for nonisothermal forming are also discussed.

Creep-fatigue interactions in solders

Abstract: Solders in surface-mount-technology (SMT) joints are subjected to high homologous temperatures that cause the solder to deform and accumulate damage by both time-dependent and instantaneous mechanisms. The effects of frequency, stress range, mean stress, waveform (hold times on and off load), and mechanical history are investigated in bulk uniaxial solder in the as-cast condition. Strain is measured on a per-cycle basis as well as cumulatively, to separate the mechanisms of strain storage and damage storage. At hold times less than about 10 s, a significant part of the strain is recoverable, time dependent, and nondamaging. By this strain storage mechanism, life, measured either by cycles to failure or by time on load, can be increased by at least a factor of 5. At lower frequencies the damage is stored as a function of time on load and stress (creep damage). The effective stress for a cyclic creep test is defined in terms of the stress sensitivity of the solder in creep. A technique for measuring damage for life predictions is developed and examined. This technique involves the grain boundary embrittlement of partially fatigued specimens and the statistical analysis of the voids on the surfaces. >

Viscoelastic response of epoxy glasses subjected to different thermal treatments

Abstract: Isothermal aging after a quench from above the glass-transition temperature (Tg) to below it was studied as a function of cross-link density for model epoxy networks using small deformation stress relaxation experiments We found that time-aging time and time-cross-link density superposition principles described the changes observed in the Viscoelastic behavior of the epoxy networks The aging response in the nonlinear Viscoelastic regime was also studied using creep experiments for one of the networks It was found that upon aging near to the conventional glass-transition temperature, the time required for the glass to age into structural equilibrium was independent of the magnitude of the applied stress This result suggests that large stresses do not erase physical aging (or cause rejuvenation of the glasses)

Nonlinear Viscoelasticity of ABS Polymers in the Molten State

Abstract: Nonlinear viscoelastic properties were measured in the molten state for two series of ABS polymers with rubber particle contents of 5, 10, 15, and 20 wt%. Experimental results for the shear relaxation modulus G(t,γ) obtained at large step strains revealed a striking fact that the strain dependence of the modulus disappears in the region where a long‐time relaxation due to the particles is dominant. In single‐step stress relaxation, the G(t,γ) curves are composed of two contributions, entanglement relaxation of the matrix polymer at shorter time scales and long‐time relaxation due to the particles. In order to separate the above two contributions, double‐step stress relaxation experiments were done in which the first step strain −γ is applied at t=−t1 and the second step strain γ is added at t=0. It is shown that t1 should be comparable with the maximum relaxation time of the matrix polymer for proper evaluation of the particle contribution.