Showing papers on "Creep published in 1986"

Introduction to continuum damage mechanics

Abstract: 1 - Introduction.- 2 - Creep and Fracture under Uniaxial Stress.- 3 - Creep and Fracture under Multiaxial Stress.- 4 - Crack Growth under Creep Conditions.- 5 - Damage Model for Ductile Fracture.- 6 - Fatigue Damage.- References.- Notations.

Relaxation in Glass and Composites

Abstract: VISCOELASTICITY IN OXIDE GLASSES. The Glass Transition. Elasticity and Viscoelasticity. Linear Viscoelastic Behavior in Oxides. Experimental Studies of Relaxation and Creep. Thermorheological Simplicity. Unstabilized Glass. The Visoelastic Analogy. The Sandwich Seal. STRUCTURAL RELAXATION Fictive Temperature. Phenomenological Model of Structural Relaxation Time. Universality of Glass Transition Phenomena. RELAXATION IN COMPOSITES. Relaxation in the Sandwich Seal. Composite Cylinder. Composite Sphere. Split Ring Composite. Interrelating Setting Temperatures. Elastic-Clad Composites. Glass-to-Glass Composites. Approximate Methods of Viscoelastic Analysis. Appendixes. Index.

Superplasticity of yttria-stabilized tetragonal ZrO2 polycrystals

Abstract: The uniaxial tensile deformation behavior of 3 molpercent Y/sub 2/O/sub 3/-tetragonal ZrO/sub 2/ polycrystals was studied at temperatures up to 1500 C in ambient atmosphere. The tensile specimens which were elongated at constant displacement rates from 0.00011/s to 0.00055/s at 1450 C exhibited unusually large deformation. The superplastically deformed specimens retained a bending strength of 976 MPa at room temperature. Steady creep rates measured at a constant load in the range of relatively small strains were described by a stress component, n, of 2 and by an activation energy of 586 + or - 40 kJ/mol. Large deformations during the accelerating creep were accompanied by extensive formation of cavities. 19 references.

Weakening of rock salt by water during long-term creep.

Abstract: The rheological properties of rock salt are of fundamental importance in predicting the long-term evolution of salt-based radioactive waste repositories and strategic storage caverns, and in modelling the formation of salt diapirs and associated oil traps1,2 The short-term, high-stress rheology of rock salt is well known from laboratory experiments; however, extrapolation to appropriately low stresses fails to predict the rapid flow seen in certain natural structures Furthermore, experiments have failed to reproduce the recrystallized microstructure of naturally deformed salt Here we report experiments indicating that the above discrepancies can be explained by taking into account the influence of trace amounts of brine Trace brine is always present in natural salt but sometimes escapes during experiments Our tests on dry dilated salt show more or less conventional dislocation creep behaviour, but brine-bearing samples show marked weakening at low strain rates This is associated with dynamic recrystallization and a change of deformation mechanism to solution transfer creep Because natural rock salt always contains some brine, these results cast substantial doubt on the validity of presently accepted dislocation creep laws for predicting the long-term rheological behaviour of salt in nature

Influence of grain size on the mechanical behaviour of some high strength materials

Abstract: Combining in an additive or synergetic manner the most potent strengthening mechanisms available in an alloy is the art of the metallurgist. The various models proposed in the literature in order to interpret the Hall-Petch relation are critically reviewed by comparison with experimental data. The pile-up models and the work hardening theories must include the inner structure of the grain in the case of alloys hardened by a second phase. Similarly, the properties and structure of the grain boundaries are influenced by impurities or the presence of particles. Ultra-fine grain sizes can provide ductility to high strength materials when surface preparation eliminates microcracks. In steady-state creep equations, introducing the influence of grain size in complex alloys by incorporating the Hall-Petch stress as one component of the internal stress helps in rationalizing the existence of an optimal grain size where creep resistance is maximized. Slower crack growth rates can be obtained by controlling the grain boundary structure as well as grain size. Fatigue tests at room temperature clearly point out the interest of small grain sizes for reducing crack initiation, usually associated, however, with lower propagation threshold and somewhat faster growth rates.

Concrete structures: Stresses and deformations

Abstract: Preface to the third edition Acknowledgements Note The SI system of units and British equivalents Notation 1 Creep and shrinkage of concrete and relaxation of steel 11 Introduction 12 Creep of Concrete 13 Shrinkage of Concrete 14 Relaxation of prestressed steel 15 Reduced relaxation 16 Creep superposition 11 The aging coefficient ?: definition 18 Equation for the aging coefficient ? 19 Relaxation of concrete 110 Step-by Step calculation of the relaxation function for concrete 111 Age-adjusted elasticity modulus 112 General 2 Stress and strain of uncracked sections 21 Introduction 22 Sign convention 23 Strain, stress and curvature in composite and homogeneous cross-sections 24 Strain and stress due to non-linear temperature variation 25 Time-dependent stress and strain in a composite section 26 Summary of analysis of time-dependent strain and stress 27 Examples worked out in British units 28 General 3 Special cases of uncracked sections and calculation of displacements 31 Introduction 32 Prestress loss in a section with one layer of reinforcement 33 Effects of presence of non-prestressed steel 34 Reinforced concrete section without prestress: effects of creep and shrinkage 35 Approximate equations for axial strain and curvature due to creep 36 Graphs for rectangular sections 37 Multi-stage prestressing 38 Calculation of displacements 39 Example worked out in British units 310 General 4 Time-dependent internal forces in uncracked structures: analysis by the force method 41 Introduction 42 The force method 43 Analysis of time-dependent changes of internal forces by the force method 44 Movement of supports of continuous structures 45 Accounting for the reinforcement 46 Step-by-step analysis by the force method 47 Example worked out in British units 48 General 5 Time-dependent internal forces in uncracked structures: analysis by the displacement method 51 Introduction 52 The displacement method 53 Time-dependent changes in fixed-end forces in a homogeneous member 54 Analysis of time-dependent changes in internal forces in continuous structures 55 Continuous composite structures 56 Time-dependent changes in the fixed-end forces in a composite member 57 Artificial restraining forces 58 Step-by-step analysis by the displacement method 59 General 6 Analysis of time-dependent internal forces with conventional computer programs 61 Introduction 62 Assumptions and limitations 63 Problem statement 64 Computer programs 65 Two computer runs 66 Equivalent temperature parameters 67 Multi-stage loading 68 Examples 69 General 7 Stress and strain of cracked sections 71 Introduction 72 Basic assumptions 73 Sign convention 74 Instantaneous stress and strain 75 Effects of creep and shrinkage on a reinforced concrete section without prestress 76 Partial prestressed sections 77 Flow chart 78 Example worked out in British units 79 General8 Displacements of cracked members 81 Introduction 82 Basic assumptions 83 Strain due to axial tension 84 Curvature due to bending 85 Curvature due to a bending moment combined with an axial force 86 Summary and idealized model for calculation of deformations of cracked members subjected to N and/or M 87 Time-dependent deformations of cracked members 88 Shear deformations 89 Angle of twist due to torsion 810 Examples worked out in British units 811 General 9 Simplified prediction of deflections 91 Introduction 92 Curvature coefficients, k 93 Deflection prediction by interpolation between uncracked and cracked states 94 Interpolation procedure: the 'bilinear method' 95 Effective moment of inertia 96 Simplified procedure for calculation of curvature at a section subjected to M and N 97 Deflections by bilinear method: members subjected to M and N 98 Estimation of probable defection: method of 'global coefficients' 99 Deflection of two-way s

Rheology and structure of olivine‐basalt partial melts

Abstract: Recent experiments demonstrate that solution-precipitation (pressure-solution) processes in the liquid basalt-olivine system are exceedingly rapid. The effect of liquid basalt on the diffusional (Newtonian) creep rate of polycrystalline olivine is significant; fine-grained, chemically and texturally equilibrated, partially molten assemblages of olivine plus basalt deform at rates which are a factor of 2–5 faster than polycrystalline olivine specimens without the liquid basalt second phase. Measured values of the activation energy for creep suggest that while the kinetics of deformation in this partially molten system are affected by short-circuit diffusion through melt-filled triple junctions, the deformation process is rate-limited by matter transport through melt-free grain boundaries. A simple geometrical model, based on a dynamic equilibrium partial-melt morphology which consists of melt-free olivine grain boundaries coupled with an interconnected melt network along triple junction channels, adequately describes our deformation results. The extent to which the liquid phase penetrates from the triple junctions into the grain boundaries of texturally equilibrated partial melts determines the contribution of solution-precipitation mechanisms to the creep rate; the critical physical parameters, therefore, are those which affect the solid-liquid and solid-solid interfacial energies, such as various composition-related chemical activities in the liquid phase and the composition and spatial distribution of mineral phases in the crystal residuum.

Does partial melting reduce the creep strength of the upper mantle

Abstract: The seismic low-velocity zone has been thought to correspond to a zone of partial melting that significantly reduces the creep strength, making it possible for the plates to decouple from the underlying mantle1,2. This view is examined here in light of recent experimental results which show that water has more effect on the creep strength of olivine3–6 than does partial melting7,8 and that the former depends on the fugacity of water5,6. When there is a limited amount of water, partial melting will result in depletion of water from olivine, and therefore have a hardening, rather than a softening, effect.

Prediction of the long-term creep compliance of general composite laminates

Abstract: An accelerated viscoelastic characterization procedure for use with polymer-based composite materials is presented which employs short term test data obtained using unidirectional specimens to predict the long term viscoelastic behavior of general composite laminates. This procedure is here illustrated using the Schapery (1966, 1969) nonlinear theory as the required viscoelastic constitutive model, as well as classical lamination theory for the lamination scheme. The technique is applied to T300/5208 graphite/epoxy.

Finite Deformation of Soft Tissue: Analysis of a Mixture Model in Uni-Axial Compression

Abstract: The dynamic finite deformational behavior of a biphasic model for soft hydrated tissue is examined. In the case of uni-axial confined compression the displacement and stress fields are derived for steady-state permeation, creep, and stress-relaxation. It is shown how to use the results of this analysis to obtain the constitutive relations, as well as the associated material parameters, from the corresponding experiments. It is also shown that the solutions from the theory go much farther, giving a detailed account of the deformation and interaction of the fluid and solid phases in the tissue.

Effect of Shear Stress on Sintering

Abstract: The effect of small uniaxial stresses on the sintering of CdO powder compacts was studied using a loading dilatometer. Compacts of two different green densities were sintered at 1123 K and subjected to stresses between 0 and 0.25 MPa. Densification and creep occur simultaneously, and the effects of these two processes can be separated. Between relative densities of 0.5 and 0.9, the dependence of the uniaxial creep rate on density can be described in terms of a stress intensification factor which depends exponentially on the porosity but is independent of the grain size. Comparison of the densification and creep rates permits definition of the sintering stress, which is found to decrease with increasing density, and verification of the Zener relation. The stress and grain size dependence of the creep rate, and the grain size dependence of the densification rate, support grain-boundary diffusion as the rate-controlling step in both processes.

Composites for use in posterior teeth: mechanical properties tested under dry and wet conditions.

Abstract: The purpose of this investigation was to determine some mechanical properties of eight different posterior composites, and how they are affected by water sorption. Creep characteristics in compression were expressed as compliance/log time functions. Stress-strain relationship at a constant loading rate was determined both in compression and flexure, and presented as elastic modulus, ultimate strength, and ultimate strain. Water sorption increased creep values for all materials. Generally, the materials with the highest water sorption had the highest increase in creep. Water sorption decreased the elastic modulus and ultimate strength values. The creep values decreased and the elastic modulus increased with increasing quantity of inorganic fillers.

Internal Oxidation of Ag-In Alloys: Stress Relief and the Influence of Imposed Strain,

Abstract: The kinetics of internal oxidation of silver-indium alloys containing 3.5, 5.9, and 9.8 at.% In in air at temperatures 773 to 973 K were established by TGA with no load applied to the specimens. Silver nodules free of oxide particles were observed to form at the surface during internal oxidation. The volume of these silver nodules was comparable to the total volume increase caused by internal oxidation. The alloys were also creep tested during oxidation in air at creep rates varying from 10−7 to 5×10−5 s−1 at 773, 873, and 973 K. The parabolic rate constants kp for the internal oxidation of the solute were determined from the measured widths of the internal oxidation zones. A small increase in kp was observed with increased strain rate. The large volume change associated with internal oxide formation resulted in a stress gradient between the stress-free surface and the internal oxidation front which is under a high compressive stress. Stress relief occurred by transport of silver to the surface. A Nabarro-Herring creep type mechanism based on lattice diffusion of Ag cannot account for the high rate of silver transport to the surface. Pipe-diffusion controlled creep is proposed as the mechanism of stress accommodation by silver diffusion.

Very low creep, ultra high moduls, low shrink, high tenacity polyolefin fiber having good strength retention at high temperatures and method to produce such fiber

Abstract: By poststretching, at a temperature between about 135° and 160° C., a polyethylene fiber, which has already been oriented by drawing at a temperature within 5° C. of its melting point, an ultra high modulus, very low creep, low shrink, high tenacity polyolefin fiber having good strength retention at high temperatures is obtained. The poststretching can be in multiple stages and/or with previous annealing. The poststretching should be done at a draw rate of less than 1 second -1 . Tensile modulus values over 2,000 g/d for multifilament yarn are consistently obtained for ultrahigh molecular weight-polyethylene, with tensile strength values above 30 g/d while at the same time dramatically improving creep (at 160° F. (71.1° C.) and 39,150 psi load) by values at least 25% lower than fiber which has not been poststretched. Shrinkage is improved to values less than 2.5% of the original length when heated from room temperature to 135° C. Performance at higher temperature is improved by about 15° to 25° C.

Creep of chemically vapour deposited SiC fibres

Abstract: The creep, thermal expansion, and elastic modulus properties for chemically vapour deposited SiC fibres were measured between 1000 and 1500°C. Creep strain was observed to increase logarithmically with time, monotonically with temperature, and linearly with tensile stress up to 800 MPa. The controlling activation energy was 480 ± 20 kJ mol−1. Thermal pretreatments near 1200 and 145O° C were found to significantly reduce fibre creep. These results coupled with creep recovery observations indicate that below 1400°C fibre creep is anelastic with negligible plastic component. This allowed a simple predictive method to be developed for describing fibre total deformation as a function of time, temperature, and stress. Mechanistic analysis of the property data suggests that fibre creep is the result of β-SiC grain boundary sliding, controlled by a small percentage of free silicon in the grain boundaries.

Creep crack growth under non-steady-state conditions

Abstract: The problem of characterizing crack growth under small scale creep and transition conditions is of great practical significance. This paper defines a crack tip parameter C, for characterizing creep crack growth behavior under wide range creep conditions from small scale creep to steady-state creep. Under steady-state creep conditions, C, is shown to reduce to the familiar C*-integral. The physical basis for the C, parameter is also provided. Wide range creep crack growth data on A470 Class 8 steel using two specimen geometries were obtained and correlated with the C, parameter in the temperature regime of 482 to 538°C. The levels of creep deformation in the various specimens tested to obtain the crack growth rate data ranged from small scale creep to essentially steady-state creep conditions.

On transient rheology and glacial isostasy

Abstract: The effect of transient creep on the inference of long-term mantle viscosity is investigated using theoretical predictions from self-gravitating, layered earth models with Maxwell, Burgers' body, and standard linear solid rheologies. The interaction between transient and steady-state rheologies is studied. The responses of the standard linear solid and Burgers' body models to transient creep in the entire mantle, and of the Burgers' body and Maxwell models to creep in the lower mantle are described. The models' responses are examined in terms of the surface displacement, free air gravity anomaly, wander of the rotation pole, and the secular variation of the degree 2 zonal coefficient of the earth's gravitational potential field. The data reveal that transient creep cannot operate throughout the entire mantle.

"Creep currents" in single frog atrial cells may be generated by electrogenic Na/Ca exchange.

Abstract: The objective of these experiments was to test the hypothesis that the "creep currents" induced by Na loading of single frog atrial cells (Hume, J. R., and A. Uehara. 1986. Journal of General Physiology. 87:833) may be generated by an electrogenic Na/Ca exchanger. Creep currents induced by Na loading were examined over a wide range of membrane potentials. During depolarizing voltage-clamp pulses, outward creep currents were observed, followed by inward creep currents upon the return to the holding potential. During hyperpolarizing voltage-clamp pulses, creep currents of the opposite polarity were observed: inward creep currents were observed during the pulses, followed by outward creep currents upon the return to the holding potential. The current-voltage relations for inward and outward creep currents in response to depolarizing or hyperpolarizing voltage displacements away from the holding potential all intersect the voltage axis at a common potential, which indicates that inward and outward creep currents may have a common reversal potential under equilibrium conditions and may therefore be generated by a common mechanism. Measurements of inward creep currents confirm that voltage displacements away from the holding potential rapidly alter equilibrium conditions. Current-voltage relationships of inward creep currents after depolarizing voltage-clamp pulses are extremely labile and depend critically upon the amplitude and duration of outward creep currents elicited during preceding voltage-clamp pulses. An optical monitor of mechanical activity in single cells revealed (a) a similar voltage dependence for the outward creep currents induced by Na loading and tonic contraction, and (b) a close correlation between the time course of the decay of the inward creep current and the time course of mechanical relaxation. A mathematical model of electrogenic Na/Ca exchange (Mullins, L.J. 1979. Federation Proceedings. 35:2583; Noble, D. 1986. Cardiac Muscle. 171-200) can adequately account for many of the properties of creep currents. It is concluded that creep currents in single frog atrial cells may be attributed to the operation of an electrogenic Na/Ca exchange mechanism.

Creep and fracture of a vitreous-bonded aluminium oxide

Abstract: Creep and creep-rupture behaviour of a commercial grade of glass-bonded, 96% aluminum oxide was characterized as a function of temperature and applied stress. The creep data were fitted to the classical empirical relation usually used to describe this phenomenon. The apparent activation enthalpy, ΔH = 926 kJ mol−1, and the stress exponent,n = 4.8, lie at the high end of the range reported for two-phase materials, primarily as a result of structural modifications that occur during creep. A stress-modified Monkman-Grant relationship was fitted to the creep-rupture data to give a stress exponent of −4.2. None of the available theories of creep rupture provided a satisfactory description of the present set of data. Analytical electron microscopy was used to characterize the composition and structure of this material. In the as-received material the intergranular phase was a glass of nearly uniform composition. During high-temperature exposure, devitrification of the glass resulted in the formation of various crystalline phases within the intergranular region of the material. Devitrification depended on both the proximity to the surface, where it was most pronounced, and on the state of stress. In this regard, flexural creep samples exhibited extensive crystallization within the tensile region of the flexural specimens, but little crystallization within the compressive cross-section. From the composition of the retained glass, estimates of the viscosity of the glass at the grain boundaries were made and used, in combination with microstructural information, to compare the creep behaviour with available theories of creep. The results of this paper are consistent with percolation and solution precipitation mechanisms of creep deformation. By contrast, cavitation did not seem to play a major role in the creep deformation process.

A finite element model of skin deformation. II. An experimental model of skin deformation.

Abstract: Skin flap design has traditionally been based on geometric models which ignore the elastic properties of skin and its subcutaneous attachments This study reviews the theoretical and experimental mechanics of skin and soft tissues (I) and proposes a mathematical model of skin deformation based on the finite element method (III) Finite element technique facilitates the modeling of complex structures by analyzing them as an aggregate of smaller elements This paper gives the results of an animal model developed to study the deformation and mechanical properties of skin, including its viscoelastic properties (hysteresis, creep, and stress relaxation) A new skin extensometer, constructed with digital stepper motors and controlled with a microcomputer, is described to measure these properties for both skin and its subcutaneous attachments Deformation grids quantitated from photographs with a digitalizing tablet are presented, and computer software is introduced to standardize and analyze them (II) The mathematical model is used to simulate wound closures such as the ellipse and rectangular advancement flap In addition, a series of mathematical experiments performed to simulate deformation of a strip of skin are described; the relationships between the various elastic constants are investigated; and a comparison of these simulations with actual deformation is presented Limitations of the model and areas for future investigation are discussed (III)

Estimation of power-law creep parameters from bend test data

Abstract: Power-law creep parameters of brittle ceramic materials are commonly deduced from load-point displacement data generated by four-point bend experiments, under the assumption that tensile and compressive behaviours obey the same constitutive law. However, because of microcracking and cavitation, it is now well recognized that this premise may not always be valid. The present paper presents an analysis which takes the differences into account. Governing equations are first derived for the location of the neutral axis of a beam under bending which does not in general pass through the centroid of the cross-section, and for the creep response in terms of both curvature rate and load-point displacement rate as functions of the applied moment and power-law creep parameters. Numerical solutions are obtained for any given set of material constants over a wide range of applied moments. It is shown from the plots of creep response against applied moment on a logarithmic scale that even linear curves over a narrow range of applied moment do not necessarily imply identical stress exponents, and that non-linear curves concave upward signify a profound difference in stress exponent between tension and compression. An example is given of applying the present analysis to a set of load-point displacement data on glass-alumina beam specimens crept at 1100° C. The results show that the conventional method over/underestimates the creep rates in compression/tension by two orders of magnitude, indicating a need for using the more accurate analysis presented here. Several recommendations are offered to improve the estimation of power-law creep parameters from bend test data.