Showing papers on "Stress relaxation published in 1970"
01 Jan 1970
TL;DR: In this article, the authors interpreted the stress-time relation during stress relaxation in terms of a power relation between dislocation velocity and the effective stress, which gave not only the velocity-stress exponent, but also the internal stress in the work-hardened state.
Abstract: The stress-time relation during stress relaxation is interpreted in terms of a power relation between dislocation velocity and the effective stress. Such interpretation gives not only the velocity-stress exponent, but also the internal stress in the work-hardened state. The velocitystress exponents thus obtained agree with those obtained from etch pit dislocation velocity measurements. The variation of internal stress with strain shows that work hardening is largely due to the development of internal stresses. The temperature dependence of flow stress arises from the increase of effective stress with decreasing temperature, consistent with a thermally activated process for dislocation motion.
155 citations
TL;DR: In this article, a mathematical model of annealing is proposed that takes account of both stress and structural relaxation, and the model fits the experimentally observed evolution of stresses during linear cooling, and suggests that the actual rate, at any given temperature, of structural relaxation is about 4 times lower than that of stress relaxation.
Abstract: Laboratory simulation of the industrial process of annealing sheet glass has yielded data on the genesis of stresses in initially stress-free glass. The experimental results differed from expectations based on classical annealing theory in that stresses began to develop in the annealing range even when the glass was being cooled at a constant rate, i.e. even in the absence of any changes of temperature gradients within the glass. Typically, these stresses account for 40% of the total residual stress in glass annealed according to a linear schedule. The remaining 60% are the well-known thermoelastic stresses that arise later in the annealing process from the decay of temperature gradients in the glass. The stresses observed to arise in glass as it is being cooled at a constant rate are attributed to volume relaxation effects which, in parts of the annealing range, generate stresses rapidly enough that they are not dissipated by stress relaxation. A mathematical model of annealing is proposed that takes account of both stress and structural relaxation. The model fits the experimentally observed evolution of stresses during linear cooling. It also suggests that (with the activation energies of stress and structural relaxation about the same) the actual rate, at any given temperature, of structural relaxation is about 4 times lower than that of stress relaxation.
143 citations
TL;DR: In this article, a theoretical and experimental treatment of the strength of unidirectional fiber-reinforced composite loaded in tension in the fiber direction is presented, where the fibers are treated as having variable strength which results in fiber fractures prior to composite failure.
103 citations
TL;DR: In this paper, the microdynamics of elevated temperature deformation in single and polycrystalline Fe-3 % Si is examined in terms of measured minimum effective stresses, which are obtained through the use of a new experimental technique based upon the response of dislocations to the internal and applied stresses.
98 citations
TL;DR: In this paper, a phenomenological theory of steady state ceep is formuated which explicitly includes the development of an average internal back stress, which is based on the premise that dislocation glide and recovery are separate kinetic processes driven by different components of the applied stress, the effective and internal stress respectively, and it is shown that the predicted applied stress dependence for high temperature creep can be predicted in a straightforward manner from the stress dependence of strain hardening, recovery and glide if the relative magnitudes of the internal and effective stresses are known.
89 citations
TL;DR: In this article, the deformation kinetics of zirconium and Zircaloy-2 strips, resulting from oxidation at 500°C in dry oxygen, were measured as a function of time.
Abstract: The deformation kinetics of zirconium and Zircaloy-2 strips, resulting from oxidation at 500°C in dry oxygen, were measured as a function of time. The stress distribution in the oxidized specimens was analyzed using a stress model based on the theory of elasticity and was correlated with the oxidation kinetics. The magnitude of the stress generated during oxidation differed significantly between zirconium and Zircaloy-2; while a simple linear relationship appeared to exist between the stress and the oxide thickness on zirconium, the stresses in oxides on Zircaloy-2 increased very rapidly and discontinuously to a maximum at a film thickness near the transition in the kinetic curve. In order to characterize the stress distribution in the oxide film, vacuum annealing experiments were performed on preoxidized specimens. These indicated that the stress relaxation which occurs during vacuum annealing resulted from dissolution in the metal of the highly stressed inner layers of the oxide film. Although these measurements have provided valuable information on the stresses generated during the oxidation of zirconium and Zircaloy-2, they do not require a causal relationship between the stresses and any oxidation process (e.g., transition).
81 citations
TL;DR: Stress relaxation is used as an indirect method for determining the velocity-stress exponent for dislocation motion in LiF and NaCl as mentioned in this paper, with the knowledge of this exponent, the applied stress can be separated into its effective stress and internal stress components.
68 citations
TL;DR: In this paper, it is suggested that transient creep at high temperatures arises principally as a result of the dispersal of entanglements by the climb mechanism, with a rate constant that depends on stress and temperature in the same way as does the secondary creep rate.
Abstract: It is suggested that transient creep at high temperatures arises principally as a result of the dispersal of entanglements by the climb mechanism. The dispersal of the entanglements is assumed to follow a unimolecular reaction kinetics with a rate constant that depends on stress and temperature in the same way as does the secondary creep rate. The analysis shows that the strain (e) versus time (t) relation can be represented by e=e 0 +e. 3 t+ β−1 K [1- exp (−K e 3 t)] , where e0 is the instantaneous strain on loading, e 3 , the secondary creep rate, K e 3 the rate constant, and β the ratio of initial to secondary creep rate. The experimental creep data on several b.c.c. and f.c.c. metals and alloys correlate quite well with the proposed mechanism. The constants β and K were found to be independent of temperature and stress. The proposed formulation becomes inapplicable for correlating creep data in polycrystals at low stresses because of the significant contribution of grain-boundary sliding to the total creep at these stress levels.
60 citations
TL;DR: In this paper, two sets of photoelastic experiments employing stress freezing were performed in order to evaluate closure and precatastrophic extension of cracks in plates under cylindrical bending.
Abstract: Two sets of photoelastic experiments employing stress freezing were performed in order to evaluate closure and precatastrophic extension of cracks in plates under cylindrical bending. Data were compared with the Hartranft-Sih theory in order to obtain quantitative results. It was found that crack closure on the compressive side of the plate produced somewhat larger local elastic stresses on the tensile side of the plate for straight through cracks than predicted by the mathematical model. When precatastrophic extension was allowed on the tension side of the plate, accompanying stress relaxation neutralized the stress intensification effect of closure. For short, nearly square cracks, three-dimensional effects appeared to reduce the stress intensification effects due to closure.
46 citations
TL;DR: In this paper, the authors used differential (step-stress) creep tests and constant axial strain-rate tests on two frozen soils, Sault Ste. Marie clay and saturated Ottawa sand.
Abstract: Soil strength, defined in terms of the stress to cause a stated strain-rate, was studied using differential (step-stress) creep tests and constant axial strain-rate tests on two frozen soils, Sault Ste. Marie clay and saturated Ottawa sand. Incremental increases in mean stress, with deviatoric stress constant produce measurable changes in secondary creep rates. Plots of a stress term versus the logarithm of creep rates give straight lines. The linear relationship indicates that creep rate increases exponentially with increases in stress difference and decreases exponentially with increase in the mean stress. The temperature effect on creep behavior is described by an exponential function of the reciprocal of temperature. For a specific creep rate and temperature, several stress circles give a Mohr-Coulomb envelope. The angle of internal friction evaluated for the sand-ice samples falls in the range of values expected for dry sand and appears to be independent of creep rate and temperature. The cohesion is dependent on creep rate.
46 citations
TL;DR: In this paper, the Hugoniot elastic limit (HEL) was determined by reducing quartz gauge measurements, and the results showed that the resolved shear stress on the active slip systems for uniaxial strain (shock loading) conditions to that for static loading (static loading) indicates a strain-rate effect.
Abstract: Single‐crystal specimens (7‐ and 12‐mm thick) of sodium chloride were impacted with flat‐nosed, gas‐driven projectiles, and the Hugoniot elastic limit (HEL) was determined by reducing quartz gauge measurements. The HEL for the [100], [110], and [111] crystal directions was 0.26, 0.77, and 7.4 kbar, respectively. Stress—time profiles for specimens shocked in the [100] and [111] direction show evidence of stress relaxation behind the elastic precursor. This phenomenon is more pronounced in 12‐mm‐thick specimens. The ratio of the resolved shear stress on the active slip systems for uniaxial strain (shock loading) conditions to that for uniaxial stress (static loading) indicates a strain‐rate effect. This ratio increases from 3.1 for loading in the [100] direction to 8 for the [110] direction and to 21 for the [111] direction. The anisotropy of the HEL with crystal direction is related to the resolved shear stress on the primary and secondary slip systems in single‐crystal sodium chloride. The large HEL for shock loading in the [111] direction is a consequence of the resolved shear stress on the primary slip systems being zero. Thus, for deformation by slip to occur, a secondary slip system (or systems) must be activated which will require a higher resolved shear stress. The experimental data for single crystals of copper and beryllium can also be explained in terms of the resolved shear stress.
TL;DR: In this paper, results from different techniques on amorphous polybisphenol-A carbonate (NMR, dielectric, mechanical) in the temperature range -150 C to room temperature were reviewed.
TL;DR: In this paper, the thermal expansion behavior of unidirectional, balanced angle-ply, and complex laminated graphite-epoxy composites is studied, and the expansion coefficient is determined as a function of fiber orientation and is found to follow the simple equation αθ=αL cos2θ+αT sin 2θ.
Abstract: The thermal expansion behavior of unidirectional, balanced angle‐ply, and complex laminated graphite‐epoxy composites, is studied. The expansion coefficient of the unidirectional composites is determined as a function of fiber orientation and is found to follow the simple equation αθ=αL cos2θ+αT sin2θ. Results obtained on angle‐ply composites are in agreement with the predictions based on Halpin and Pagano's analysis. The applicability of the same approach to a panel of more complex construction is discussed. Hysteresis effects, observed in this panel, may be caused by interlayer stress relaxation.
TL;DR: In this article, the creep behavior of poly(ethyl acrylate) and poly(methyl methacrylate)-interpenetrating polymer networks was investigated using a modified Tobolsky-Aklonis-Dupre glass-rubber theory, and it was found that the breadth of the transition could be attributed to a near continuum of phase compositions in the material, each phase composition making its specific contribution to the relaxation spectrum.
Abstract: The creep behavior of a series of poly(ethyl acrylate)–poly(methyl methacrylate) interpenetrating polymer networks was investigated. For comparison purposes, some stress relaxation data were included. Master curves containing a single broad transition covering approximately 20 decades of time were found for midrange compositions. Although the time–temperature superposition principle and the WLF equation should not strictly apply, reasonable agreement was found over a large portion of shift factor versus temperature plots. Application of a modified Tobolsky-Aklonis-Dupre glass–rubber theory suggested that the breadth of the transition could be attributed to a near continuum of phase compositions in the material, each phase composition making its specific contribution to the relaxation spectrum. Whether or not these phase regions are so small as to arise from random concentration fluctuations in an otherwise compatible polymer pair remains unknown.
01 Sep 1970
TL;DR: In this article, the internal stress developed during creep deformation was measured by a modified stress-relaxation technique, and the magnitude of internal stress is related to the scale and nature of the dislocation substructure.
Abstract: Polycrystalline copper of 99·99% purity has been deformed in tensile creep at a variety of strain rates and temperatures. The internal stress developed during creep deformation was measured by a modified stress-relaxation technique. The magnitude of the internal stress remains constant during steady-state deformation but the ratio of internal stress to applied stress depends critically on the creep rate and test temperature. At low creep rates the internal stress closely approaches the value of the applied stress but as the creep rate increases the magnitude of the effective stress becomes greater. The magnitude of the internal stress is related to the scale and nature of the dislocation substructure.
TL;DR: In this article, the authors investigated the dynamics of dislocation motion in a series of copper-tin solid solution alloys and concluded that the parameters used to describe the dynamics were dependent both on the strength and on the concentration of local obstacles.
Abstract: Stress relaxation and strain-rate change experiments have been used to investigate the dynamics of dislocation motion in a series of copper-tin solid solution alloys. The magnitude of the thermal component of the flow stress and exponent m∗ of the Gilman-Johnston velocity-stress relationship were observed to vary in a complex manner with solute content and the degree of plastic strain. For dilute solid solutions the thermal and athermal components of the flow stress remain proportional and the value of m∗ increases continuously with plastic strain. In more concentrated solid solutions the magnitude of both m∗ and the thermal component are essentially constant. The results are rationalized in terms of the waiting time necessary for a dislocation to penetrate an array of random obstacles of different strengths. It is concluded that the parameters used to describe the dynamics of dislocation motion are dependent both on the strength and on the concentration of local obstacles in a manner analogous t...
TL;DR: In this paper, the creep properties of polycrystalline copper and the alloys Cu-14 wt.% Zn and Cu-26 wt.%.% have been studied at stress levels between 70 kN/m2 and 27 MN/m 2 at temperatures of 873 and 1093°K, in each case, there was an abrupt change in the stress vs. strain rate relation.
TL;DR: In this paper, the creep properties of niobium-stabilised stainless steels of carbon contents in the range 0.01 to 0.05% carbon can be explained by the general recovery theory of creep.
Abstract: The creep properties of niobium-stabilised stainless steels of carbon contents in the range 0.01 to 0.05% carbon can be accounted for by the general recovery theory of creep. The high stress dependencies of recovery and creep rate can be adequately explained through an internal friction stress or impedance term, retarding recovery. Measurement of this friction term by dislocation density and stress relaxation techniques provides the correct stress dependencies when applied to the modified recovery theory.
TL;DR: In this paper, Biaxial stress tests were performed on thin-wall tubes of polycrystalline 2024-T81 aluminum at temperatures of 150°C and 250°C.
Abstract: Biaxial stress tests were performed on thin-wall tubes of polycrystalline 2024-T81 aluminum at temperatures of 150°C and 250°C. The nominal metallurgical stabilization temperature for this alloy is 190°C. Transient and steady-state creep strain rates exhibited a considerable dependence on load path history. For a prescribed history it is possible to determine unique surfaces of constant creep strain rate. For the zero history, involving a single loading from the origin to a prescribed point in stress space, surfaces of constant steady-state strain rate, at elevated temperature, have the same shape as room temperature yield surfaces of moderate offset. In the temperature and small strain regions considered here, room-temperature yield surfaces were found to be unaffected by elevated temperature deformation. The changes in shape of room-temperature yield surfaces, due to room-temperature plastic deformation caused corresponding changes in the elevated temperature surfaces of constant steady-state creep rate. At a given stress point, an outward local motion of the yield surface resulted in a corresponding outward local motion of the steady-state creep rate surfaces. The experimental determination of surfaces of constant flow potential was also attempted.
TL;DR: In this article, it was shown that the rate of dislocations in polycrystalline polycrystals is grain size dependent and can be expressed in terms of a modified Hall-Petch equation with a slope dependent on the degree of plastic strain.
TL;DR: In this article, the authors compared relaxation modulus curves for monodisperse and polydisperse polystyrenes and showed that the significant effect of molecular weight distribution on these curves is shown.
Abstract: : Relaxation modulus curves for monodisperse and polydisperse polystyrenes are compared and the significant effect of molecular weight distribution on these curves is shown. An empirical three-parameter equation is shown to describe well the stress relaxation behavior of monodisperse polystyrenes. New data for high molecular weight monodisperse polystyrenes are presented and used to extend the applicability of previous quantitative relations for higher molecular weights. The stress relaxation technique is shown to be sensitive to the type of plasticizer used at low concentrations. (Author)
01 Jan 1970
TL;DR: In this article, it was found that CREEP is due to ORIENTED INTERNAL MOISTURE DIFFUSION caused by a free ENERGY GRADIENT and to slow deformation of the ELASTIC SKELETON of the GEL.
Abstract: IT IS SUGGESTED THAT CREEP IS DUE TO ORIENTED INTERNAL MOISTURE DIFFUSION CAUSED BY A FREE ENERGY GRADIENT AND TO SLOW DEFORMATION OF THE ELASTIC SKELETON OF THE GEL INDUCED BY VISCOUS DEFORMATION OF ADSORBED WATER. SHRINKAGE ACTUALLY REALIZED IS AFFECTED BY THE RELIEF OF THE STRESS IN THE ELASTIC SKELETON BY CREEP. THE NATURE OF MOISTURE DIFFUSION SUGGESTS THAT CREEP STRAINS MAY NOT BE SUPERPOSABLE UNDER MULTIAXIAL STRESS. CREEP RECOVERY IS SHOWN TO DEPEND BOTH ON CREEP AT THE TIME OF UNLOADING AND ON MAGNITUDE OF ELASTIC STRAIN RECOVERY. A RHEOLOGICAL MODEL IS PROPOSED. SOME SUPPORTING EXPERIMENTAL DATA ARE PRESENTED. /AUTHOR/
TL;DR: In this paper, it was shown that the work of structure breakdown increases with increasing shear rate, and its rise is related linearly to development of the viscosity anomaly.
Abstract: In the prestationary deformation regime relaxation times decrease both before and after passage through the ultimate shear strength, i. e., independent of whether the stresses at which relaxation began were growing or decreasing. The relaxation properties of the system change the most intensively before the ultimate shear strength is passed. This occurs at the deformation where the viscosity and the elastic modulus reach their maximum values. The viscosity grows during relaxation. Increasing the temperature intensifies recovery of the broken down structure. Retarded recovery of the strength properties of the polymer correlate with retardation of reestablishment of the initial structure, characterized by attainment of the initial Newtonian viscosity and retardation of high-elastic recovery. By resolving deformations into their reversible and irreversible components it was found possible to estimate the amount of work done in destroying the initial structure network of the polymer when passing from a state of rest to steady flow. The work of structure breakdown increases markedly with increasing shear rate, and its rise is related linearly to development of the viscosity anomaly.
TL;DR: In this paper, stress-relaxation and tensile tests have been performed at various creep strains to study the dynamic-recovery and work-hardening behavior of alpha-iron during creep.
Abstract: Stress-relaxation and tensile tests have been performed at various creep strains to study the dynamic-recovery and work-hardening behaviour of alpha-iron during creep. The work-hardening rate controls the creep deformation to a minor extent only; the observed creep rates at various stresses and temperatures can be explained mainly in terms of the dynamic-recovery rates in both the paramagnetic and the ferromagnetic temperature regions. The activation energies for dynamic recovery are in good agreement with those for creep, which are equal to the activation energies for self-diffusion. Thus, it is concluded that the creep deformation of alpha-iron is determined by a diffusion-controlled dynamic-recovery process. It is also proposed that the stress-relaxation method should be used in quantitative studies of dynamic recovery during creep.
TL;DR: In this article, the physical creep of vulcanized natural rubber vulcanizates has been studied, and it was shown that the nature of the crosslink structure is not a determining factor in the type of a vulcanizer.
Abstract: The physical creep of unfilled natural rubber vulcanizates, prepared with different vulcanizing systems, has been studied. For each of the three vulcanizing systems chosen there is a strong dependence of creep rate on crosslink density, but the rates for accelerated sulfur vulcanizates are two or three times higher than those of peroxide vulcanizates of similar crosslink density. Supplementary experiments, in which the crosslink structure of sulfur vulcanizates is modified either by chemical treatment or by variations in the vulcanizing conditions, show that the nature of the crosslink itself is not a determining factor in the type of vulcanizate. Other features, such as the type and quantity of extranetwork material arising from the vulcanizing process, contribute significantly to the viscoelastic behavior of accelerated sulfur vulcanizates.