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Showing papers on "Creep published in 1973"


Journal ArticleDOI
TL;DR: In this article, a new mechanism for superplastic deformation is described and modelled, which differs fundamentally from Nabarro-Herring and Coble creep in a topological sense: grains switch their neighbors and do not elongate significantly.

1,307 citations


Journal ArticleDOI
TL;DR: In this paper, the authors present the theory of SAXS and its application in the case of glassy phases with phases in internal equilibrium (SAXS-PE) and show that it can be applied to a variety of properties of amorphous polymers.
Abstract: (The Nature of Polymer Glasses, Their Packing Density and Mechanical Behaviour).- The Nature of Polymeric Glasses.- The common glassy polymers.- The softening of polymer glasses.- Polymer melts and rubbers.- The crystallisation of polymers.- Amorphous isotactic polymers.- The morphology of amorphous polymers.- Packing Volume in the Glassy State.- The expansion volume of amorphous polymers.- Free volume concepts derived from viscosity theories.- Viscosity and free volume in polymers.- Geometrical factors affecting the possible value of the free volume at Tg.- Bernal's random close packed volume.- The Rigidity of Polymer Glasses.- Large Deformations and Fracture.- References.- 1 The Thermodynamics of the Glassy State.- 1.1 Introductory Thermodynamic Considerations.- 1.2 Glassy Solidification and Transition Phenomena.- 1.2.1 General considerations and transitions of different order.- 1.2.2 Glassy solidification with one or several internal parameters.- 1.2.3 Experimental results.- 1.2.4 Position of the equilibrium curve below the glass temperature.- 1.2.5 Zero point volume of a polymer.- 1.3 Results of the Thermodynamic Theory of Linear Relaxation Phenomena.- 1.4 Glassy Mixed Phases.- 1.4.1 The glassy solidification of polymer solutions.- 1.4.2 The glassy solidification of cross-linked systems. The coexistence of glassy phases with phases in internal equilibrium.- 1.5 The Mobility and Structure of Glassy Phases.- References.- 2 X-Ray Diffraction Studies of the Structure of Amorphous Polymers.- 2.1 Introduction.- 2.2 The Interaction of X-rays With Matter.- 2.2.1 Scattering by a free electron.- 2.2.2 Interference among scattered waves.- 2.2.3 Atomic scattering factor.- 2.2.4 Compton scattering.- 2.3 Order and Orientation in Polymers.- 2.3.1 Order.- 2.3.2 Orientation.- 2.4 Diffraction of X-rays by Amorphous Materials.- 2.5 Small Angle X-ray Scattering.- 2.5.1 Introduction.- 2.5.2 Experimental requirements for SAXS.- 2.5.3 Outline of the theory of SAXS.- 2.5.4 Some applications of SAXS.- 2.6 The Radial Distribution Function for Amorphous Polymers.- References.- 3 Relaxation Processes in Amorphous Polymers.- 3.1 Introduction.- 3.2 Molecular Motion in Polymeric Melts and Glasses.- 3.2.1 General description of relaxational processes.- 3.2.2 Relaxational processes at the crystal melt temperature.- 3.2.3 Relaxations in the amorphous state above Tg and below Tm.- 3.2.4 Relaxational processes at the glass transition.- 3.2.5 Relaxations in the glassy state.- 3.3 Secondary Relaxation Regions in Typical Organic Glasses.- 3.3.1 Secondary relaxation regions in Polyvinylchloride.- 3.3.2 Secondary relaxation regions in polystyrene.- 3.3.3 Secondary relaxations in polymethylmethacrylate.- References.- 4 Creep in Glassy Polymers.- 4.1 Introduction.- 4.2 Phenomenological Theory of Creep.- 4.2.1 Linear theory.- 4.2.2 Nonlinear theory-creep equations.- 4.2.3 Nonlinear theory-superposition rules.- 4.3 Apparatus and Experimental Methods.- 4.3.1 General principles.- 4.3.2 Special experimental requirements.- 4.3.3 Special experiments.- 4.4 Creep Phenomena in Glassy Polymers.- 4.4.1 Typical creep behaviour.- 4.4.2 Creep at elevated temperatures.- 4.4.3 Creep in anisotropic samples.- 4.4.4 Recovery behaviour.- 4.4.5 Creep under intermittent stress.- 4.4.6 Creep under abrupt changes of stress.- 4.5 Final Comments.- References and Bibliography.- 5 The Yield Behaviour of Glassy Polymers.- 5.1 Introduction.- 5.2 Exact Definitions.- 5.2.1 Stress.- 5.2.2 Strain.- 5.2.3 The deformation-rate tensor.- 5.2.4 The yield point.- 5.2.5 Nomenclature for deformation processes.- 5.3 Mechanical Tests.- 5.3.1 The tensile test.- 5.3.2 The uniaxial compression test.- 5.3.3 The plane strain compression test.- 5.3.4 Tests in simple shear.- 5.3.5 Machine elasticity.- 5.3.6 Drawing at constant load.- 5.4 Characteristics of the Yield Process.- 5.4.1 The yield point and the yield stress.- 5.4.2 The yield strain.- 5.4.3 Strain softening and orientation hardening.- 5.4.4 The strain-rate dependence of the yield stress.- 5.4.5 The temperature dependence of the yield stress and the yield strain.- 5.4.6 The effect of hydrostatic pressure on the yield stress and yield strain.- 5.4.7 The effect of polymer structure on the yield stress.- 5.4.8 Volume changes at yield.- 5.4.9 The Bauschinger effect.- 5.5 Inhomogeneous Deformation.- 5.5.1 The reasons for inhomogeneous deformation.- 5.5.2 The principle of maximum plastic resistance.- 5.5.3 The geometry of inhomogeneous deformation.- 5.5.4 Strain inhomogeneities in polymers.- 5.6 Structural Observations.- 5.6.1 Birefringence.- 5.6.2 Electron microscopy.- 5.7 Yield Criteria for Polymers.- 5.7.1 The Tresca yield criterion.- 5.7.2 The von Mises yield criterion.- 5.7.3 The Mohr-Coulomb yield criterion.- 5.7.4 The modified Tresca criterion.- 5.7.5 The modified von Mises criterion.- 5.7.6 Choice of a yield criterion for polymers.- 5.8 Molecular Theories of Yielding.- 5.8.1 Reduction of the Tg by the applied stress.- 5.8.2 Stress-induced increase in free volume.- 5.8.3 Break-down of entanglements under stress.- 5.8.4 The Eyring model.- 5.8.5 The Robertson model.- 5.8.6 The theoretical shear strength-Frank's modification of the Frenkel model.- 5.8.7 Disclinations.- References.- 6 The Post-Yield Behaviour of Amorphous Plastics.- 6.1 General.- 6.2 The Phenomena of' strain Softening'.- 6.2.1 Stress hardening.- 6.3 Plastic Instability Phenomena.- 6.3.1 Plastic instability in tension.- 6.3.2 Plastic instability in different stress fields.- 6.4 The Adiabatic Heating of Polymers Subject to Large Deformations.- 6.4.1 Reversible thermoelastic effect.- 6.4.2 Thermal effects in large plastic deformation.- 6.4.3 The experimental measurement of temperature changes during deformation.- 6.5 Orientation Hardening.- 6.5.1 Orientation hardening as a physical process.- 6.5.2 Factors affecting orientation hardening.- 6.5.3 A model for large polymer deformations.- 6.6 Large Deformation and Fracture.- 6.6.1 Crack propagation as a deformation process.- 6.6.2 Crazing as a plastic instability phenomenon.- 6.6.3 The growth of voids in a polymer glass.- 6.6.4 The nucleation of voids.- References.- 7 Cracking and Crazing in Polymeric Glasses.- 7.1 Introduction.- 7.2 Fracture Mechanics.- 7.2.1 Linear fracture mechanics.- 7.2.2 Measurements of KIC for glassy polymers.- 7.2.3 Crack-opening displacement.- 7.2.4 Energy balance approach.- 7.2.5 Measurements of surface work.- 7.2.6 Fracture stress.- 7.3 Fatigue Fracture.- 7.3.1 Fatigue failure by heat build-up.- 7.3.2 Fatigue crack propagation.- 7.4 Crazing.- 7.4.1 Crazing of glassy plastics in air.- 7.4.2 Environmental crazing.- 7.4.3 Theoretical aspects.- 7.5 Molecular Fracture.- 7.5.1 Kinetic theories of fracture.- 7.5.2 Experimental evidence for bond fracture.- 7.6 Conclusion.- References.- 8 Rubber ReinForced Thermoplastics.- 8.1 Introduction.- 8.2 Rubber Reinforced Glassy Polymers of Commercial Importance.- 8.2.1 Based on polystyrene.- 8.2.2 Based on styrene acrylonitrile copolymer (SAN).- 8.2.3 Based on Polyvinylchloride.- 8.3 Methods of Manufacture.- 8.3.1 Physical blending.- 8.3.2 Interpolymerisation process.- 8.3.3 Latex interpolymerisation.- 8.4 Incompatibility in Polymer Mixtures.- 8.5 Identification of Two Phase Rubber Reinforced Systems.- 8.6 Dispersed Phase Morphology.- 8.6.1 Toughened polystyrene.- 8.6.2 ABS copolymers.- 8.6.3 Polyvinylchloride.- 8.7 Optical Properties.- 8.7.1 Matching of the refractive index.- 8.7.2 Reduction in particle size.- 8.8 Mechanical Properties.- 8.8.1 Tensile properties.- 8.8.2 Dynamic mechanical properties.- 8.8.3 Impact properties.- 8.8.4 Structure-property relationships.- References.- 9 The Diffusion and Sorption of Gases and Vapours in Glassy Polymers.- 9.1 Introduction.- 9.2 Ideal and Non-ideal Sorption and Diffusion of Fixed Gases.- 9.2.1 Ideal diffusion and sorption of fixed gases.- 9.2.2 Non-ideal sorption and diffusion of fixed gases.- 9.3 The Effect of the Glass Transition on Gas and Vapour Diffusion in Polymers.- 9.4 Relaxation Controlled Transport and Related Crazing of Polymeric Glasses by Vapours.- 9.4.1 Introduction ..- 9.4.2 Relaxation controlled transport and solvent crazing.- 9.5 Some Effects of Crystallinity and Orientation on the Transport of Gases and Vapours in Glassy Polymers.- 9.5.1 Effect of crystallinity.- 9.5.2 The effect of orientation.- References.- 10 The Morphology of Regular Block Copolymers.- 10.1 Introduction.- 10.1.1 General.- 10.1.2 Microphase separation.- 10.2 Techniques Used for the Study of the Morphology of Block Copolymers.- 10.2.1 Low angle X-ray scattering.- 10.2.2 Electron microscopy.- 10.2.3 Other techniques.- 10.3 Variables Controlling the Morphology.- 10.3.1 Chemical variables.- 10.3.2 Physical variables.- 10.4 Studies with Specific Systems.- 10.4.1 Systems with liquid.- 10.4.2 The pure copolymers.- 10.5 Theories of the Morphology of Block Copolymers.- 10.5.1 Objectives.- 10.5.2 Principles of calculation.- 10.6 Implications of Theories and Comparison With Experiment.- 10.6.1 Influence of block molecular weight ratio.- 10.6.2 Effect of block molecular weights.- 10.6.3 Molecular orientation in the phases.- 10.6.4 Interfacial region.- 10.6.5 Effect of temperature on domain size.- 10.7 Mechanical Properties and Deformations.- 10.8 Crystallinity.- References.- Appendix I Glass Transition Temperatures and Expansion Coefficients for the Glass and Rubber States of some Typical Polymeric Glasses.- Appendix II Conversion Factors for SI Units.

824 citations


Journal ArticleDOI
TL;DR: In this article, a numerical model is developed for the welding and subsequent loading of a fabricated structure, which treats the weld process as a thermo-mechanical problem, and the model includes finite strain effects during isothermal loading, so that it may be used in the modeling of distortion sensitive structure.

255 citations


Journal ArticleDOI
TL;DR: In this article, the shape of the cavity surfaces is determined based on the assumption that this is governed by surface diffusion, and that steady state conditions prevail, so that the cavity advances as a profile retaining a constant shape near its tip.

230 citations


Journal ArticleDOI
TL;DR: In this article, the effect of superplastic deformation on the grain growth kinetics was studied using both constant crosahead speed and creep tests, the grain size was measured as a function of deformation time and strain over a wide range of strain rates.

156 citations


Journal ArticleDOI
01 Apr 1973
TL;DR: In this paper, a composite plot of dimensionless parameter•/γkT/DGb vsτ/G revealed a transition from a high stress region (with slope ∼4.9 for lead and ∼6.6 for tin) to a low stress region with slope ∼1 for both metals, at stresses of ∼2 × 10-5G for lead, and ∼10− 5G for tin.
Abstract: High-temperature creep has been studied in lead and tin in the stress range of 10−6 to 10−4G over a range of temperatures near the melting point. A composite plot of dimensionless parameter•/γkT/DGb vsτ/G revealed a transition from a high stress region (with slope ∼4.9 for lead and ∼6.6 for tin) to a low stress region (with slope ∼1 for both metals), at stresses of ∼2 × 10-5G for lead and ∼10−5G for tin. The creep behavior in the low stress region is identical to that attributed to creep of the Harper-Dorn type. In addition, earlier work on aluminum in the low stress region is confirmed using polycrystalline as well as single crystal samples.

154 citations


Journal ArticleDOI
01 Jan 1973
TL;DR: In this paper, single crystals of a precipitation-hardened nickel-base super alloy, Mar-M200, were tested in constant load creep at 1575°F and it was found that pairs of α/2 (110) dislocations controlled deformation in both primary and steady-state creep.
Abstract: Single crystals of a γ′ precipitation-hardened nickel-base super alloy, Mar-M200, were tested in constant load creep at 1575°F. It was found that shear of the γ′ precipitate by pairs of α/2 (110) dislocations controlled deformation in both primary and steady-state creep. This contrasts with 1400°F creep behavior where shear of γ′ is dominated by α/3 (112) dislocations in primary creep, but by pairs of α/2 (110) dislocations in steadystate creep. The orientation dependence of the steady-state creep rate at 1575°F is explained by the nature of dislocation junction reactions for the different orientations. Crystals along the [001]-[1•11] boundary have the greatest creep resistance because of the formation of stable dislocation networks at the matrix-particle (γ⊃ interfaces, whereas the lower creep resistance of crystals oriented along the [001]-[0•11] boundary is a consequence of the low probability for the formation of stable junction reactions. Finally, evidence, in the form of resolvable α/2 (110) dislocation pairs within the γ′ precipitate, is presented for a reduction in the local antiphase boundary energy of γ′ at high temperatures.

136 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used available measurements of creep rates and dimensions of ice shelves, in conjunction with equations derived in a companion paper (Thomas, 1973), to evaluate the flow law parameters B and n for stresses down to 104 N m−2.
Abstract: Available measurements of creep rates and dimensions of ice shelves are used, in conjunction with equations derived in a companion paper (Thomas, 1973), to evaluate the flow law parameters B and n for stresses down to 104 N m−2. The results show good agreement with laboratory work at higher stresses. Adoption of these values of B and n enables us to examine the restraining effects on an ice shelf of obstructions such as areas of grounding.

129 citations


Journal ArticleDOI
TL;DR: In this paper, a critical review of high-temperature flow mechanisms in ceramics and their relation to empirical flow laws is presented, and the authors conclude that these fine-grained materials creep primarily by a quasi-viscous grain-boundary sliding mechanism which is unlikely to predominate in the deep interior.

128 citations


Journal ArticleDOI
TL;DR: In this paper, a simple method is presented by which the linear creep function of concrete can be approximated, with any desired accuracy, by Dirichlet series with variable coefficients, and smooth fits of the best known data on creep at constant temperature and water content are demonstrated.
Abstract: A simple method is presented by which the linear creep function of concrete can be approximated, with any desired accuracy, by Dirichlet series with variable coefficients. Smooth fits of the best known data on creep at constant temperature and water content are demonstrated. It is shown that the approximation is equivalent to the Kelvin chain model with age-dependent properties. Other approximations leading to the Kelvin chain are also presented. It is found, however, that no Kelvin chain approximation can avoid negativeness of some spring moduli for some periods of time, which precludes physical interpretation of hidden strains. But representations with Maxwell chain are free from this deficiency. The Dirichlet series approximation allows formulation of an efficient algorithm of step-by-step time integration of creep problems, for which arbitrary increase of the time step is possible and storage of the stress history can be dispensed with.

113 citations


Journal ArticleDOI
TL;DR: In this paper, the authors measured the growth rate of crack growth in single edge-notched specimens of a single Mo steel and found that the time required to initiate a crack from a notch is not significant compared with the total times for specimens to fail.
Abstract: Crack growth rates have been measured in single edge-notched specimens of % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D% aebbfv3ySLgzGueE0jxyaibaiGc9yrFr0xXdbba91rFfpec8Eeeu0x% Xdbba9frFj0-OqFfea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs% 0dXdbPYxe9vr0-vr0-vqpWqaaeaabiGaciaacaqabeaadaqaaqGaaO% qaaiaaikdadaWcbaWcbaGaaGymaaqaaiaaisdaaaaaaa!3E80!\[2\tfrac{1}{4}\]Cr 1 Mo steel loaded in tension at 565 dgC. It is shown that the creep crack growth rate in tests of up to about 2,000 hours duration can be described in terms of the sharp crack stress intensity factor. A correlation is obtained over five orders of magnitude in crack growth rate and the relationship of these results to conventional creep tests is discussed. For the particular test conditions the time required to initiate a crack from a notch is not significant compared with the total times for specimens to fail. Under some conditions high rates of stable crack growth rate can be obtained. The work has obvious potential in the assessment of the integrity of components which contain defects and operate under creep conditions. A possible application of the data to the analysis of a particular thermal fatigue situation is suggested.

Journal ArticleDOI
TL;DR: In this article, a rate theory of the growth of irradiation-produced interstitial loops in uniaxially stressed materials which also contain voids, coherent and/or incoherent precipitates and network dislocations is derived and the resulting creep behaviour is determined.
Abstract: A rate theory of the growth of irradiation-produced interstitial loops in uniaxially stressed materials which also contain voids, coherent and/or incoherent precipitates and network dislocations is derived and the resulting creep behaviour is determined. Allowance is made for the preferential nucleation of loops oriented perpendicular to the stress axis and for the subsequent enhanced growth of these same loops. It is shown that the first of these mechanisms dominates and gives a creep rate which can be correlated with the volume swelling rate. The magnitude of the effect is shown to be consistent with the limited amount of available experimental data appropriate to the void-swelling temperature regime.

Journal ArticleDOI
TL;DR: The steady creep rate for polycrystalline copper at 686, zinc at 385, and iron at 923 K varies as the nth power of the applied stress (σ) as discussed by the authors.
Abstract: The steady creep rate, e s , for polycrystalline copper at 686, zinc at 385, and iron at 923 K varies as the nth power of the applied stress (σ). The value of n was 4.8 for copper and zinc but was 8.3 for iron. Stress-relaxation tests and constant-stress tests in which the stress was progressively reduced gave values of a ‘friction’ stress, σ0, such that for all materials examined ɛ ˙ s ∝ ( − σ 0 ) 4 In all cases, decreasing the stress by a small amount, (∆σ, ≃ 0.1 σ) during steady-state creep resulted in an incubation period of zero creep rate, after which the creep rate increased to a new steady value. When the stre...

Journal ArticleDOI
TL;DR: The dependence of the steady creep rate, e s, on stress, σ, and temperature, T, for Nimonic 80A at 1023K can be described as A σ n exp (Q c / R T ).
Abstract: The dependence of the steady creep rate, e s , on stress, σ, and temperature, T, for Nimonic 80A at 1023K can be described as ɛ ˙ s = A σ n exp ( Q c / R T ) The values of the stress exponent, n (∼ 8.3) and the activation energy for creep, Qc , (≃460 kJ/mol) are considerably higher than those for the nickel-20% chromium matrix material. These anomalously high values of n and Qc are accounted for in terms of the stress- and temperature-dependence of the friction stress, σ0, which is determined by a technique involving consecutive small stress reductio...

Journal ArticleDOI
TL;DR: In this paper, a mass transport equation which takes into account parallel diffusion paths for anions and cations was derived and applied to the diffusional creep of polycrystalline ionic solids.
Abstract: A mass transport equation which takes into account parallel diffusion paths for both anions and cations was derived and applied to the diffusional creep of polycrystalline ionic solids. From the results of the analysis, several limiting conditions were found for grain-boundary- and lattice-diffusion-controlled kinetics. These conditions depend on temperature, grain size, and type and concentration of cation impurities. Examples of these limiting situations are given for the creep of polycrystalline Fe-doped MgO and transition-metal-doped Al2O3. Summary A mass transport equation which takes into account parallel diffusion paths for anions and cations was derived and applied to the diffusional creep of polycrystalline ionic solids. The effect of grain size and cation impurities of variable valence in solid solution on the relative contributions of lattice and grain-boundary diffusion of different ionic species in polycrystalline MgO and Al2O3 was examined. Depending on the temperature, grain size, impurity level, and O2 partial pressure, several limiting conditions were found: Limit I: At very small grain sizes and reasonably small cation lattice diffusivities the creep rate will be controlled by the slower-moving ion in the grain-boundary regions (i.e. Coble creep). Limit II: For intermediate grain sizes and cation lattice diffusivities the creep rate will be controlled by cation lattice diffusion when anion transport is significantly faster near grain boundaries than in the lattice (i.e. Nabarro-Herring creep). Limit III: For an appropriate combination of large grain size and high cation lattice diffusivity the creep rate will be controlled by anion boundary diffusion (i.e. Coble creep). Well-defined examples of limits I and II have been observed in the creep of Fe-doped polycrystalline MgO, and tentative evidence exists for limit III. Most results of studies of creep in polycrystalline Al2O3 (doped and undoped) fall within limit II, with some overlap with limit III. The model developed in the present work explains much of the data in the literature in which creep rates correspond to cation lattice mobilities. It is concluded that in the creep of polycrystalline ionic solids anion transport near grain boundaries is rapid and can, in some circumstances, be rate-controlling. It should also be possible to apply this model to sintering and thermal-grooving data for such systems, particularly for Al2O3, in which cation lattice diffusion is frequently observed to be rate-controlling.32

Journal ArticleDOI
TL;DR: In this paper, it is shown that creep strain transients can be adequately described by invoking anelastic strain contributions to both primary creep and unloading transients at all stress levels.

Journal ArticleDOI
TL;DR: In this paper, the steady-state creep of transition-metal-doped polycrystalline Al2O3 was investigated at O2 partial pressures of 0.88 to 10−10 atm.
Abstract: Steady-state creep was studied in hot-forged polycrystalline Al2O3 (3 to 42 μm) of nearly theoretical density doped with≤1 cation % of Fe, Ti, or Cr. Tests were conducted at stresses between 10 and 550 kg/cm2 at 1375° to 1525°C under O2 partial pressures of 0.88 to 10−10 atm. Except in the 10-μm Fe-doped material tested at very small stresses, slightly nonviscous creep behavior was generally observed. The effects of Po2 on the creep rate indicated that increased concentration of a divalent (Fe2+) or quadrivalent (Ti4+) impurity in solid solution enhances the creep rate of polycrystalline Al2O3. The activation energies for the creep of Fe- and Ti-doped Al2O3 samples (148 and 145 kcal/mol, respectively) were significantly higher than that for Cr-doped material (114 kcal/mol). Taking into account the effects of Po2, temperature, and grain size, it was concluded that the steady-state creep of transition-metal-doped Al2O3 is controlled by cation lattice diffusion.

Journal ArticleDOI
TL;DR: In this paper, the effects of both anodic and cathodic polarization on accelerated creep rates of oxide-free copper wires immersed in an acetate buffer solution at 25°C were studied.

Journal ArticleDOI
TL;DR: In this paper, the Boltzmann superposition principle was used to establish the transient creep and recovery behavior in compression of two lithium zinc silicate glassceramics over the temperature range 590 to 750° C at stresses up to 91.4 MN m−2.
Abstract: The creep and recovery behaviour in compression of two lithium zinc silicate glassceramics is established over the temperature range 590 to 750° C at stresses up to 91.4 MN m−2. It is shown that the transient creep obtained is linearly viscoelastic and obeys the Boltzmann superposition principle. The activation energy of the rate-controlling process is the same as that found for secondary creep and is due to viscous flow of the residual glass phase. A simple method of analysis of the strain-time curves is presented, which can be modified to apply to stress relaxation tests.


Journal ArticleDOI
TL;DR: In this article, high-temperature hardness data for thirty-seven metals have been collected from the published literature, and seven of these metals were further tested to confirm and improve the hardness data.
Abstract: The high-temperature hardness data for thirty-seven metals have been collected from the published literature. Seven of these metals were further tested to confirm and improve the hardness-temperature data. All data were analysed to calculate the softening parameter B and the apparent activation energy for indentation B′. The values of B′ are compared with the activation energies for lattice and dislocation pipe diffusion to obtain an estimate of the stress coefficient, α, for creep. The B′ and α values have been improved by considering the effect of elastic modulus.

Journal ArticleDOI
TL;DR: In this article, an analysis of fault creep events based on models of propagating edge dislocations and propagating screw dislocation is described, and it is shown that the stresses involved in a creep event are of the order of 1 bar or less.
Abstract: An analysis of fault creep events based on models of propagating edge dislocations and propagating screw dislocations is described. From the displacement history of a creep event it is possible to calculate its stress history. The stresses involved in a creep event apparently are of the order of 1 bar or less. It is not possible to obtain a unique constitutive equation for fault gouge from the data of a creep event. Because of the sharp beginning of many creep events, however, the fault gouge that is involved in a creep event must exhibit an upper yield point effect. The ‘eventfulness’ of fault creep may be due to this yield point effect.

Journal ArticleDOI
01 Mar 1973
TL;DR: In this article, the substructure of AISI 316 stainless steel resulting from creep deformation has been quantitatively characterized using transmission electron microscopy, and a double triple node dislocation configuration was frequently observed in all specimens.
Abstract: The substructure of AISI 316 stainless steel resulting from creep deformation has been quantitatively characterized using transmission electron microscopy. The specimens were tested at temperatures and stresses ranging from 593° to 816°C and 8000 to 35,000 psi, respectively. Subgrains whose boundaries are predominantly (111) twist boundaries were formed in all tests at and above 704°C but were observed very infrequently at 650°C and were completely absent after creep at 593°C. The subgrain diameter,d, and the mobile dislocation density, ρ, were found to vary with the applied stress, σa, according to:d =kσa-1 and ρα σa2. Subgrain misorientation varys from less than 0.1 to 1 deg in each specimen seemingly independent of all parameters evaluated. A double triple node dislocation configuration was frequently observed in all specimens. Its relation to the deformation process is discussed in a mechanism involving the breaking of attractive dislocation nodes.

Journal ArticleDOI
TL;DR: In this paper, the mean effective stress σ ∗ was measured in steady state creep in an Al-5.5 at.% Mg solid solution and was found to reach values as low as about 0.4 at low applied stresses and temperatures and as high as 0.94 at both high temperatures and applied stresses.

Journal ArticleDOI
TL;DR: In this article, the dislocation substructure generated during compressive creep of MgO single crystals was investigated using both etchpit and transmission electron microscopy techniques, and the type of dislocations substructure is dependent on strain.
Abstract: The dislocation substructure generated during compressive creep of MgO single crystals has been investigated using both etch-pit and transmission electron microscopy techniques. The type of dislocation substructure is dependent on strain. During transient creep nearly parallel cell walls are found whereas in steady state creep a nearly equiaxed network of cells is built up. Measurements of the density ρ of dislocations not associated with cell walls and of the cell wall area per volume S v reveal the following trends: During primary creep both ρ and S v are functions of strain; ρ decreases while S v increases with increasing strain. During steady state creep ρ and S v are constant. The isothermal stress dependence of the steady state values of ρ and S v can be described by relations ρ ∼ σ1·4, and SV∼σ0·7.

Journal ArticleDOI
TL;DR: In this paper, a tensile and creep testing was performed on double shear type specimens in a normalized stress ( τ G ) range of ~5 × 10 −7 to ~5× 10 −3, and the experimental results in the employed stress range obeyed the follo′ing phenomenological effect.

Journal ArticleDOI
01 Apr 1973
TL;DR: In this article, three processing histories were studied: bar stock and forgings made in the α and β ranges, and it was concluded that the observed transient creep had arisen from exhaustion of the easiest sources at each stress level.
Abstract: Creep tests were performed on normal grade Ti-5 Al-2.5 Sn at 194, 299, 339, and 422 K. Three processing histories were studied: bar stock and forgings made in theα andβ ranges. Creep stresses ranged from 40 to 90 pct of the 299 K tensile yield stress, σy. Not all combinations of stress and temperature gave reliable creep data. At 60 and 80 pct σy, the forged materials were more creep resistant than the bar stock, while at 90 pct σy all three materials were alike. The apparent activation energy for creep, about 37 kJ/mol, was about one-fourth the energy for selfdiffusion. Activation areas were about 10⇃2; thus the rate-controlling process at the stresses used was probably the overcoming of interstitial obstacles. Observations on thin foils showed that the bar stock and α-forged material had equiaxed grain structures, while theβ-forged material consisted of massive martensite(α’) plates. Films of retainedβ appeared to be present in manyα’ boundaries; this restricted slip to individualα’ plates. A microstructural rationale was constructed, suggesting that each material contained dislocation sources which differed in ease of operation. Theβ-forged material was fitted into this rationale by the observation that dislocations inα’ boundaries could apparently act as sources. It was concluded that the observed transient creep had arisen from exhaustion of the easiest sources at each stress level.

Journal ArticleDOI
TL;DR: In this article, the stress-strain relations and creep behaviour of a dolerite, a microgranodiorite, and a dunite have been determined in uniaxial compression at temperatures up to 1120°C.
Abstract: Summary The stress-strain relations and creep behaviour of a dolerite, a microgranodiorite, and a dunite have been determined in uniaxial compression at temperatures up to 1120°C. All three rocks remained brittle at temperatures up to 1050"C, with approximately linear stress-strain relations and small fracture strains. Creep tests were carried out at stresses (< 10' Nm-2)* which were low compared to the uniaxial compressive strength at the same temperature. At these stresses and in experiments with a duration of -1 d only transient creep was observed, with the creep strain proportional to a fractional power of the time (Andrade creep), as was observed in earlier experiments at lower temperatures, and the creep rate decreased with the elapse of time. The apparent viscosity, therefore, increases with time, having a value in the range 10I2 - 10'' Nsm-2 after times of 10% and 10"- lo2' Nsm-' after times of -3y (by extrapolation)?. The activation energy for creep at a low stress of 130 x lo5 Nm-2 determined for microgranodiorite at temperatures greater than 800°C and for dolerite at temperatures greater than 940°C indicated that the creep was probably diffusion controlled. Using a theory due to Mott, the steady-state creep rate and the corresponding equivalent viscosity were calculated for the conditions of the experiments. The viscosities for pressures on the geotherms of Ringwood were then calculated, by making use of the proportionality between activation energy for diffusion and the absolute melting temperature. For stresses in the range lo6- 10' Nm-' dolerite has a viscosity of 10"- lo2' Nsm-2 at at depths of 60-80 kin in oceanic areas, and at depths greater than 120 km in shield areas; and microgranodiorite at the same stresses has a similar viscosity at depths in the range 80-130 km in shield areas. These estimates are in satisfactory agreement with present concepts of the lithosphere. At temperatures above 1050°C the dolerite and microgranodiorite exhibited partial melting. The dolerite remained brittle, with the partial melt being extruded from microfractures, but the microgranodiorite showed some ductility. We can postulate that even rocks deep in the mantle may be brittle if their temperatures are not more than N 1OOO"C. A discussion is given of the apparent conflict between the experimental observation of hot brittleness and the geological observation that even some comparatively cool silicate rocks have apparently deformed in a ductile manner.

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TL;DR: In this paper, it was shown that cyclic loading accelerates the non-elastic deformation of concrete and the resulting creep can be expressed as the sum of a mean-stress component and of a component which is dependent both upon the range of stress and upon the value of the mean stress.
Abstract: Synopsis It is shown that, in comparison with a static stress, cyclic loading accelerates the non-elastic deformation of concrete. The resulting creep can be expressed as the sum of a mean-stress component and of a component which is dependent both upon the range of stress and upon the value of the mean stress.

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TL;DR: In this article, it was found that similar densities of dislocations unbound in sub-boundaries exist in various metals in steady-state creep at the same value of the ratio of applied stress to elastic modulus.
Abstract: Some recent data obtained by investigating dislocation substructure formed in high-temperature creep of several metals are summarized, and some substructural aspects of both steady-state and primary creep are discussed on the basis of the data analysis. It is found that similar densities of dislocations unbound in sub-boundaries (free dislocations), ρ, exist in various metals in steady-state creep at the same value of the ratio of applied stress to elastic modulus. In steady state the mean subgrain diameter is proportional to the reciprocal of the square root of the total dislocation density ρc=ρ+ρSB ρSB being the density of dislocations forming sub-boundaries. Recovery in steady-state creep most probably takes place almost exclusively by annihilation of free dislocations and not by the entry of these dislocations into the sub-boundaries. The total dislocation density was found to increase in the course of primary creep. Most of the dislocations generated go to build the sub-boundaries as the den...