Showing papers on "Creep published in 1969"
TL;DR: In this paper, a set of three-dimensional constitutive equations is proposed which is consistent with nonlinear behavior of some metals and plastics, and which enables all properties to be evaluated from uniaxial creep and recovery data.
Abstract: Starting with specific constitutive equations, methods of evaluating material properties from experimental data are outlined and then illustrated for some polymeric materials; these equations have been derived from thermodynamic principles, and are very similar to the Boltzmann superposition integral form of linear theory. The experimental basis for two equations under uniaxial loading and the influence of environmental factors on the properties are first examined. It is then shown that creep and recovery data can be conveiently used to evaluate properties in one equation, while two-step relaxation data serve the same purpose for the second equation. Methods of reducing data to accomplish this characterization and to determine the accuracy of the theory are illustrated using existing data on nitrocellulose film, fiber-reinforced phenolic resin, and polyisobutylene. Finally, a set of three-dimensional constitutive equations is proposed which is consistent with nonlinear behavior of some metals and plastics, and which enables all properties to be evaluated from uniaxial creep and recovery data.
1,004 citations
354 citations
TL;DR: In this article, the influence of stress, temperature, grain size and subgrain size on the steady state creep rate of polycrystalline unalloyed tungsten has been examined based on the extensive published data available.
283 citations
251 citations
238 citations
TL;DR: The Parkfield earthquake of June 27, 1966 and its aftershock and creep sequences are examined as a detailed example of fault slippage that includes both types, seismic and aseismic as discussed by the authors.
Abstract: Field and experimental evidence are combined to deduce the mechanism of slip on shallow continental transcurrent faults, such as the San Andreas in California. Several lines of evidence portray the central section of the San Andreas fault as a very smooth and flat surface, with a very low frictional strength in comparison to the breaking strength of intact rock. The Parkfield earthquake of June 27, 1966, and its aftershock and creep sequences are examined as a detailed example of fault slippage that includes both types, seismic and aseismic. It is shown from considerable number of field data that during the main shock a region from about 4 to 10 km in depth slipped approximately 30 cm. In response to this slippage, creep and aftershocks were generated. The creep and aftershocks are not directly interrelated, but they are microscopically identical processes of time-dependent brittle friction occurring in parallel in different regions. The creep occurred by time-dependent stable frictional sliding in the 4-km-thick surface layer; the aftershocks, by time-dependent stick-slip at the ends of the initial slipped zone. This model is in good agreement with laboratory results which show that slippage should occur by stable (aseismic) friction in the upper 4 km, by stick-slip accompanied by earthquakes from about 4 to 12 km, and by stable sliding or plastic friction below 12 km on the fault. One feature not observed in the laboratory is the episodic nature of creep. These episodes can be predicted with an accuracy of about 1 week.
175 citations
TL;DR: In this paper, a single crystal loaded in uniaxial compression parallel to the basal plane was qualitatively similar to the classical creep curve; creep rate at all stages was very much faster than for polycrystalline ice under the same conditions.
Abstract: Creep tests on homogeneous, isotropic polycrystalline ice gave an apparent activation energy for creep of 16.4 kcal/mol (68.8 kJ/mol) over the temperature range −10° to −60° C. Above −10° C the Arrhenius relation for temperature dependence is invalid, and creep rate becomes progressively more temperature dependent as the melting point is approached. Between −20° and −50° C the apparent activation energy for creep of a single crystal of ice was found to be 16.5 kcal/mol (69.1 kJ/mol). A complete creep curve for a single crystal loaded in uniaxial compression parallel to the basal plane was qualitatively similar to the classical creep curve; creep rate at all stages was very much faster than for polycrystalline ice under the same conditions. Creep tests on polycrystalline ice at 0° C gave a stress/strain-rate relation for that temperature, but its precise meaning is unclear, since recrystallization complicated the results.
154 citations
TL;DR: In this paper, the geometry of the deformation produced by the climb of edge dislocations is defined and the conditions under which dislocation climb, by itself or in conjunction with glide, can produce a general change of shape.
Abstract: The geometry of the deformation produced by the climb of edge dislocations is defined and the conditions are determined under which dislocation climb, by itself or in conjunction with glide, can produce a general change of shape. The results are important in understanding the high-temperature deformation (T > 0.5Tm) of single crystals and polycrystals of materials with fewer than five independent slip systems. Some of these are discussed in detail. In particular the high-temperature creep of sapphire single crystals pulled parallel to (0001) is shown to be quantitatively consistent with Nabarro (Phil. Mag., 16, 231, 1967) creep by climbing dislocations.
106 citations
TL;DR: In this paper, the effect of a dispersion of particles on high-temperature creep has been examined by comparing the creep and recovery properties of Nimonic 80A with those of a single-phase nickel-20% chromium alloy at 750° C.
Abstract: The effect of a dispersion of particles on high-temperature creep has been examined by comparing the creep and recovery properties of Nimonic 80A with those of a single-phase nickel–20% chromium alloy at 750° C. Over most of the creep curve, the strain at any instant can be described accurately as ɛ = ɛ0 + ɛ T (l − e−mt) + έ s t, where ɛ0 is the instantaneous strain on loading, ɛ t the total transient strain, έ s the steady-creep rate, and m a constant relating to the rate of exhaustion of transient creep. Measurements of the rate of recovery during transient and steady-state creep suggest that this equation applies when the creep rate is proportional to the rate of recovery. Deviations from this equation occur during the initial 10–15% of the transient stage, and are attributable to a rapid increase in the coefficient of work-hardening. Although the activation energy for creep and the stress-dependence of the creep rate of the Nimonic alloy are higher than the values obtained for the single-phase...
93 citations
TL;DR: In this paper, the Burgers vector was shown to be parallel to the 〈110〉 axes by transmission electron microscopy and slip was observed on {100, {110}, or {111} planes, depending on the orientation of the stress axis; it always occurred on the most favorably oriented slip system.
Abstract: High-temperature deformation in ZrC single crystals was studied. Seeded crystals were grown by a direct rf-coupling floating-zone process. Yield stresses were measured from 1080° to 2000°C as a function of stress axis orientation. The Burgers vector was shown to be parallel to the 〈110〉 axes by transmission electron microscopy. Slip was observed on {100}, {110}, or {111} planes, depending on the orientation of the stress axis; it always occurred on the most favorably oriented slip system. The dependence of steady-state creep rate on the applied stress indicated that recovery occurred by a dislocation climb mechanism. Examination of the dislocation structure in deformed crystals by transmission electron microscopy supported this conclusion.
01 Jan 1969
TL;DR: In this paper, the secondary creep rates of all coarse-grained fcc metals are given by s 6 DGb (a. )n Es = 2.5 x 10 kT \ /G where D = the diffusivity, G = the shear modulus of elasticity, b = the Burgers vector, kT = the Boltzmann constant times the absolute temperature, and a = the applied tensile stress, the constant n increases from 4.4 for Al,to 5.3 for Ag in a consistent manner with increasing value of G
Abstract: Creep of metals and alloys at high temperatures is usually diffusion-controlled. Nevertheless, the creep behavior is often sensitive to various structural and substructural details. The secondary creep rates, E , of all coarse-grained fcc metals are given by s 6 DGb (a. )n Es = 2.5 x 10 kT \ /G where D = the diffusivity, G = the she'ar modulus of elasticity, b = the Burgers vector, kT = the Boltzmann constant times the absolute temperature, and a = the applied tensile stress, The constant n increases from 4.4 for Al,to 5.3 for Ag in a consistent manner with increasing value of Gb/f where r is the stacking fault energy. Whereas, many alloys give the same trends, others in which the velocity of glide of dislocations is limited by solute atom diffusion, give . E ~ 0,5 DGb s kT At the steady state the substructure consists of subgrains demarked by low-angle boundaries and more or less randomly meandering dislocations within the subgrains. The density, p, of dislocations during steady state creep depends only on the applied stress and is given by the same relationship that applies to metals during low-temperature strain hardening, namely
IBM1
TL;DR: The authors of as discussed by the authors showed that the stress required to operate dislocation sources within a grain, at a grain boundary, and at surfaces is larger than the intrinsics tresses observed in polycrystalline films.
Abstract: The stress required to operate dislocation sources within a grain, at a grain boundary, and at surfaces is found to be larger than the intrinsics tresses observed in polycrystalline films. It is therefore unlikely that a dislocation flow mechanism can relieve stresses in films. Grain boundary sliding and diffusional creep can, however, relieve stresses in films and equations describing the kinetics of stress relaxation are derived. It is suggested that stress relief occurs primarily by a diffusion-creep mechanism. Growth of hillocks during annealing of a film is briefly discussed in terms of the diffusion-creep mechanism.
01 Dec 1969
TL;DR: In this paper, the authors report that under different states of stress, CREEP is SOMEWHAT SMALLER THAN under UNIAXIAL STRESS, while under multiaxial STRESS CREEP'S RATIO is LOWER.
Abstract: CREEP DATA ARE PRESENTED FOR CONCRETE UNDER DIFFERENT STATES OF STRESS, WITH EACH STRESS COMPONENT INDEPENDENTLY CONTROLLED. CREEP IN ALL THREE PRINCIPAL DIRECTIONS IS REPORTED. UNDER MULTIAXIAL STRESS, CREEP IS SOMEWHAT SMALLER THAN UNDER UNIAXIAL STRESS. EFFECTIVE CREEP POISSON'S RATIO UNDER MULTIAXIAL STRESS IS LOWER (0.09 TO 0.17) THAN UNIAXIAL CREEP POISSON'S RATIO (0.17 TO 0.20) AND DEPENDS ON THE RELATIVE MAGNITUDE OF PRINCIPAL STRESSES. EMPIRICAL EXPRESSIONS FOR CREEP POISSON'S RATIO ARE DERIVED. DATA ON CREEP RECOVERY SHOW THE DEPENDENCE OF CREEP RECOVERY ON PRECEDING CREEP AS WELL AS ON STRESS REMOVED. UNDER MULTIAXIAL COMPRESSION THE RECOVERABLE PROPORTION OF CREEP IS LARGER THAN UNDER UNIAXIAL STRESS. /AUTHOR/
TL;DR: In this article, a comprehensive model is proposed to explain the creep of Zircaloy during neutron irradiation, which is based on the growth-directed Roberts-Cottrell yielding creep and the formation of point defect loops preferentially on certain planes in response to the applied stress.
TL;DR: A friction and wear analysis of ten thermoplastic polymers selected because their individual physical and mechanical properties suggested good wear and creep resistance is presented.
Abstract: Certain thermoplastic polymers possess numerous desirable physical and mechanical properties important for total joint replacement. Their modulus of clasticity and density are close to the cartilage bone composite. Their elasticity would provide valuable damping for the transmission of shock forces. As a class of materials, polymers do not have the compression and tensile strength of metals, but the high strength-to-weight ratio of some polymers makes them especially attractive as a material for the socket half of the joint. The socket shape could be easily molded or machined and at a low cost. As a prime requisite they must have sufficient wear and creep resistance for a lifetime of cyclic loading and sliding. In addition they must be biologically tolerated, and the joint environment must not degrade or otherwise adversely affect the physical and mechanical properties of the polymer.
The purpose of this paper is to present a friction and wear analysis of ten thermoplastic polymers selected because their individual physical and mechanical properties suggested good wear and creep resistance. Polyimides with and without graphite have particular potential as a bearing material. The toxicological aspects of these materials are under study and will be the subject of a subsequent report.
TL;DR: In this article, tensile and compressive tests on pure ice single crystals at various temperatures down to −90°C are reported, and a dislocation velocity of the order of 0.5×10−8 m s−1 is deduced for ice at −50°C.
Abstract: Results obtained from tensile and compressive tests on pure ice single crystals at various temperatures down to −90°C are reported. At −50°C tensile creep tests give a continually increasing creep rate until fracture, as observed at higher temperatures. The stress dependence of the strain-rate is discussed. Fracture stress increases with decreasing temperature. Results from constant strain-rate compressive tests are compared with theoretical curves computed from Johnston’s (1962) theory of dislocation multiplication. A dislocation velocity of the order of 0.5×10−8 m s−1 is deduced for ice at −50°C.
TL;DR: In this paper, a kinetic study of the densification of hafnium diboride in high-pressure hot-pressing was conducted and an activation energy of 22,900 cal/mole was obtained for the rate-controlling step in the creep process.
Abstract: The fabrication temperature was the principal variable in a kinetic study of the densification of hafnium diboride in high-pressure hot-pressing. Densification studies for conventional hot-pressing were reviewed and correlated with the high-pressure hot-pressing results. The consolidation of HfB2 in the open pore region during high-pressure hot-pressing is attributed to particle rearrangement caused by grain boundary sliding and fragmentation. The final stage of densification (relative density >90%) was analyzed in terms of the Nabarro-Herring vacancy creep model. An activation energy of 22,900 cal/mole was obtained for the rate-controlling step in the creep process.
TL;DR: In this paper, Steinkopff et al. presented numerical formulae for storage and loss compliance from the course of the creep compliance for linear viscoelastic materials.
Abstract: Numerical formulae are given for calculation of storage and loss compliance from the course of the creep compliance for linear viscoelastic materials. These formulae involve values of the creep compliance at times which are equally spaced on a logarithmic time scale. The ratio between succeeding times corresponds to a factor of two. A method is introduced by which bounds for the relative error of those formulae can be derived. These bounds depend on the value of the damping, tan δ. The calculation of the storage compliance is easier with the lower damping values. This calculation involves the value of the creep compliance at time t0=1/ω, and that of its derivative with respect to the logarithm of time in a rather narrow region around t0. In contrast the calculation of the loss compliance is more difficult with the lower damping values. This calculation involves the value of the derivative of the creep compliance with respect to the logarithm of time in a broad interval around t0. © 1969 Dr. Dietrich Steinkopff Verlag.
TL;DR: In this article, the authors derived the stress and temperature dependence of the stationary creep rate using a technique of repeated load and temperature changes with an appropriate evaluation of pure iron alloys with up to 4.3 at.
TL;DR: In this paper, the authors show that the strain/time relation for transient and steady-state creep can be derived from first-order reaction-rate theory under the assumption that transient creep obeys first order reaction rate theory.
Abstract: An analysis is presented which shows that the strain (ɛ)/time (t) relation ɛ = ɛ 0 + ɛ T { 1 − exp ( − t / τ ) } + ɛ ˙ s t where ɛ0 is the instantaneous strain on loading, ɛ T the magnitude of transient creep, τ the relaxation time of transient creep, and έ s the steady-state strain rate, can be derived by assuming that transient and steady-state creep obey first-order reaction-rate theory. The assumption is considere...
TL;DR: It was found that different alloys vary widely in their dynamic-creep characteristics; that higher mercury content demonstrates increased dynamic creep, and that static creep correlates well to dynamic creep.
Abstract: The dynamic creep of amalgam was investigated in view of its possible relationship to the clinically observed extrusion of amalgam restorations. It was found that different alloys vary widely in their dynamic-creep characteristics; that higher mercury content demonstrates increased dynamic creep, and that static creep correlates well to dynamic creep.
TL;DR: In this article, it was shown that cavity formation in copper is proportional to strain in the primary as well as in the secondary creep range, which is supported by experimental data in the early stages of creep.
Abstract: It is shown that cavity formation in copper is proportional to strain in the primary as well as in the secondary creep range. This proportionality is used in conjunction with the prediction of a constant rate of cavity growth by vacancy condensation, when cavities are widely spaced compared with their size, to indicate a volume increase due to cavitation which is proportional to the product of creep strain and time independently of the precise form of creep curve. This relationship is supported by experimental data in the early stages of creep.
TL;DR: In this article, an analysis of a long slope undergoing creep deformation is presented, based on assumptions that the soil flows viscously with no volume change, and that the deformation occurs when the shear stress due to gravity exceeds the soil residual shear strength.
Abstract: An analysis of a long slope undergoing creep deformation is presented. The analysis is based on assumptions that the soil flows viscously with no volume change, and that the creep deformation ensues when the shear stress due to gravity exceeds the soil residual shear strength. Expressions to describe the velocity field in the creep zone of the slope are derived. The results are discussed for the cases of with or without surcharge loads and results are presented in dimensionless charts. Because of the assumptions used, the theoretical results appear to be applicable to slopes of over-consolidated clay or clay shales. In order to determine the average coefficient of viscosity of the soil, field displacement versus time records of the slope undergoing creep is necessary. Two published soil creep case histories are utilized to verify the theoretical analysis and the results are reasonable.
TL;DR: In this paper, a series of filled and unfilled polythylenes were investigated and the creep and dynamic mechanical properties of the materials were reported, and the fillers were kaolin and wollastonite.
Abstract: The creep and dynamic mechanical properties are reported for a series of filled and unfilled polythylenes. The fillers (untreated and treated with silanes) were kaolin and wollastonite. The variables studied include filler concentration, silane treatment, water soaking, temperature, and load. Most of the creep data can be explained, and semiquantitatively predicted, in terms of the increase in elastic modulus of the materials as a result of the incorporation of the filler. However, there are some secondary effects due to filler‐polymer interactions which apparently change the properties of the polymer phase and result in a further reduction of creep and an increase in damping. The presence of the filler greatly reduces creep, increases the stiffness, and increases the loss modulus of polyethylenes. Silane treatment generally reduces creep and increases modulus somewhat. An unexpected long‐time aging effect at room temperature was found which makes the polyethylenes more creep resistant. If allowance is made for the aging effect, soaking in water increases the creep of filled polyethylenes.
TL;DR: In this article, the shape of the stress-temperature (τ vs. T) curve was analyzed according to several expressions discussed in the literature, and a reasonably good fit to the Fleischer expression, ( τ/τ 0 ) 1 2 = 1 − (T/T 0 ), 1 2, was obtained, but an equally good fit was found to the simpler relation (τ/τ0) = 1 + (T0/T).