Showing papers on "Creep published in 1985"

Strength of metals and alloys

Abstract: This book presents the papers given at a conference on the mechanical properties of metals and alloys. Topics considered at the conference included hardening, anisotropy and texture, phase transformations, creep resistance, plasticity, deformations, microstructure, fracture properties, fatigue, wear resistance, temperature effects, stress analysis, and recrystallization.

Creep of crystals : high-temperature deformation processes in metals, ceramics, and minerals

Abstract: Preface 1 Mechanical background 2 The agents of deformation: lattice defects 3 Phenomenological and thermodynamical analysis of quasi-steady-state creep 4 Dislocation creep models 5 The effect of hydrostatic pressure on deformation 6 Creep polygonization and dynamic recrystallization 7 Diffusion creep, grain-boundary sliding and superplasticity 8 Transformation plasticity 9 Scaling and classification References Indexes

Concrete creep at variable humidity: constitutive law and mechanism

Abstract: The previously formulated rate-type aging creep law based on Maxwell chain is generalized to variable humidity and is calibrated by extensive comparisons with test data from the literature. The main object of attention is the Pickett effect, i.e., the apparent increase in creep due to drying simultaneous with loading. This effect is shown to have four sources, in their decreasing order of importance: (1) stress-induced shrinkage, (2) tensile strain softening due to progressive cracking, (3) irreversibility of unloading contraction after tensile strainsoftening, and (4) increase of material stiffness due to aging (hydration). The model, which is a special case of a previously advanced thermodynamic theory, depends on only one hypothesis about the microscopic physical mechanism of creep: The creep rate depends on the magnitude of the flux of microdiffusion of water between the macropores (capillary pores) and the micropores in the cement gel. By assuming this microdiffusion to be infinitely fast, the effect is reduced to a dependence of creep viscosities on the time rate of pore humidity, and this is further shown to be equivalent to stress-induced shrinkage, in which the shrinkage coefficient defining the ratio of the increments of shrinkage strain and pore relative humidity depends on stress. In three dimensions, the shrinkage coefficient thus becomes a tensor. For thermodynamic reasons, there must also exist stress-induced thermal expansion. Although tensile cracking is found to make significant contribution to the Pickett effect, it is far from sufficient to explain in fully. The theory agrees with test data on basic creep, creep of specimens with reduced water content at hygral equilibrium (predried), shrinkage, swelling, and creep at drying under compression, tension, or bending. The strainsoftening model used for tensile cracking is the same as that used previously to fit test data from fracture tests, direct tensile tests, and deflection tests of reinforced beams.

Time-dependent deformation of metals

Abstract: The basic characteristics of timedependent deformation of metals are described in terms of dislocation properties. At high temperatures, diffusion controlled climb of edge dislocations is the rate limiting process, whereas at low temperatures, other forms of recovery involving cross-slip of screw dislocations operate. A composite model of plastic flow is used to describe the coupling between these recovery processes. The model is patterned after the persistent slip band structures observed in cyclically deformed fcc single crystals. Screw dislocations are allowed to move in the cell interiors and to deposit edge dislocations into the adjoining walls. Cross-slip and climb lead to dislocation rearrangement and annihilation in the two regions. These processes are coupled not only through the dislocation microstructure, but also through the mechanics of the composite structure. The model is used to describe various deformation properties of metals, including stage II, stage III, and stage IV strain hardening and saturation of the flow stress. The coupling of cross-slip and climb controlled recovery processes leads to gradual transitions in strain hardening and gives a natural account of the transition from low temperature deformation to high temperature creep. The model also leads to polarized dislocation structures, internal stresses, and anelastic creep properties.

Creep and densification during sintering of glass powder compacts: ii. effects of particle size

Abstract: The simultaneous creep and densification of glass powder compacts was studied as a function of low applied uniaxial stress, temperature, and particle size. The creep rate can be expressed as the sum of the contribution from the applied stress that varies linearly with stress, and a contribution due to anisotropic densification that varies linearly with the densification rate. For a constant applied stress, the ratio of the creep rate to the densification rate is almost independent of both termperature and density. While these observations are consistent with the model of Scherer for the viscous sintering of glass, other observations show significant deviations from the model. Both the densification rate and the creep viscosity, which has an exponential dependence on porosity, show much stronger dependence on density compared with theoretical predictions.

A constitutive model for the stress–strain–time behaviour of ‘wet’ clays

Abstract: A constitutive model is developed to describe the stress–strain–time behaviour of ‘wet’ clays subjected to three-dimensional states of stress and strain. The model is based on Bjerrum's concept of total strain decomposition into an immediate (time-independent) part and a delayed (time-dependent) part generalized to three-dimensional situations. The classical theory of plasticity is employed to characterize the time-independent stress–strain behaviour of cohesive soils using the ellipsoidal yield surface of the modified Cam Clay model presented by Roscoe and Burland. The time-independent strain is divided into an elastic part and a plastic part. The plastic part is evaluated using the normality condition and the consistency requirement on the yield surface. The time-dependent (creep) component of the total strain is evaluated by employing the normality rule on the same yield surface as in the time-independent model and the consistency requirement which requires that the creep strain rate reduces to phenome...

The development of γ-γ′ lamellar structures in a nickel-base superalloy during elevated temperature mechanical testing

Abstract: Changes in the morphology of the γ′ precipitate were examined during creep and tensile testing at temperatures between 927 and 1038 °C in [001]-oriented single crystals of a model Ni-Al-Mo-Ta superalloy. In this alloy, the γ′ particles link together to form lamellae, or rafts, which are aligned with their broad faces perpendicular to the applied tensile axis. The dimensions of the γ and γ′ phases were measured as the lamellar structure developed and were related to time and strain in an attempt to trace the changing γ-γ′ morphology. The results showed that directional coarsening of γ′ began during primary creep, and the attainment of a fully developed lamellar structure did not appear to be directly related to the onset of steady-state creep. The rate of directional coarsening during creep increased as the temperature was raised and also increased as the stress level was raised at a given testing temperature. The raft thickness remained equal to the initial γ′ size from the start of the creep test up through the onset of tertiary creep for all testing conditions. It was found that extensive rafts did not develop during the shorter testing times of the tensile tests, and that tensile testing of pre-rafted structures did not alter the morphology of the rafts. The overall behavior of the alloy was a clear indication of the stability of the finely-spaced γ-γ′ lamellar structure.

Elevated temperature creep-rupture behavior of the single crystal nickel-base superalloy NASAIR 100

Abstract: The creep and rupture behavior of [001] oriented single crystals of the nickel-base superalloy NASAIR 100 was investigated at temperatures of 925 and 1000 °C. In the stress and temperature ranges studied, the steady state creep rate, time to failure, time to the onset of secondary creep, and the time to the onset of tertiary creep all exhibited power law dependencies on the applied stress. The creep rate exponents for this alloy were between seven and eight, and the modulus-corrected activation energy for creep was approximately 350 kjoule/mole, which was comparable to the measured activa-tion energy for Ostwald ripening of the γ′ precipitates. Oriented γ′ coarsening to form lamellae perpendicular to the applied stress was very prominent during creep. At 1000 °C, the formation of a continuous γ-γ′ lamellar structure was completed during the primary creep stage. Shear through the γ-γ ' interface is considered to be the rate limiting step in the deformation process. Gradual thickening of the lamellae appeared to be the cause of the onset of tertiary creep. At 925 °C, the fully developed lamellar structure was not achieved until the secondary or tertiary creep stages. At this temperature, the γ-γ′ lamellar structure did not appear to be as beneficial for creep resistance as at the higher temperature.

Creep Deformation of an Alumina Matrix Composite Reinforced with Silicon Carbide Whiskers

Abstract: Whisker-reinforced ceramic composites with enhanced fracture toughness properties are being developed. The creep behavior of such a composite was studied. The introduction of silicon carbide whiskers significantly improves the creep resistance of polycrystaline alumina.

Structural models for human spinal motion segments based on a poroelastic view of the intervertebral disk.

Abstract: Analytical and finite element models (FEMs) were used to quantify poroelastic material properties for a human intervertebral disk. An axisymmetric FEM based on a poroelastic view of disk constituents was developed for a representative human spinal motion segment (SMS). Creep and steady-state response predicted by FEMs agreed with experimental observations, i.e., long-time creep occurs with flow in the SMS, whereas for rapid steady-state loading an "undrained," nearly incompressible response is evident. A relatively low value was determined for discal permeability. Transient and long-term creep FE analyses included the study of deformation, pore fluid flow, stress, and pore fluid pressure. Relative fluid motion associated with transient creep is related to nuclear nutrition and the overall mechanical response in the normal disk. Degeneration of the disk may be associated with an increase in permeability.

Constitutive equation of wood at variable humidity and temperature

Abstract: Theoretical analysis of the effects of variations in moisture content and temperature on the creep of wood is presented. Thermodynamics of the processes of diffusion of water in wood microstructure is discussed and distinction is drawn between macrodiffusion and microdiffusion. The constitutive relation for steady states of moisture content and temperature is formulated on the basis of Maxwell chain model whose viscosity coefficients depend on moisture content and temperature. It is shown that the apparent acceleration of creep due to simultaneous drying (or wetting) as well as heating (or cooling) may be modeled as additional, stress-induced shrinkage (or swelling) and stress-induced thermal expansion (or contraction), described by shrinkage and thermal expansion coefficients that depend on the absolute values of the rates of pore humidity and temperature. Certain other sources of irreversibility of creep are also discussed.

Localization of dilatancy in Ohshima granite under constant uniaxial stress

Abstract: The localization of dilatancy during creep of Ohshima granite under uniaxial compression was observed. Hypocenters of 3933 acoustic emission (AE) events were accurately located. It was found that the mechanical behavior of Ohshima granite was controlled by the localization of microcracks. During the stage of loading up to the creep stress, which is 83% of the average short-term fracture strength, the hypocenters of AE events were randomly distributed throughout the specimen. As soon as the primary creep began, abrupt migration and clustering of AE hypocenters into several near-surface zones were observed. AE events formed volumetric concentrations. This migration and clustering strongly suggested the rapid localized development of dilatancy at the very beginning of the primary creep stage. The distribution of AE hypocenters observed in this stage was unchanged until final faulting. By the end of the primary creep stage, AE events began to concentrate into one of these clusters, while the activities of other clusters gradually decayed. This change spread broadly and continuously in time during the creep. In the most active cluster, clustering of AE events by itself gave rise to more AE events. The shape of this cluster was spheroidal with the long axis parallel to the loading axis. No evidence directly related to planar focusing of dilatancy was found. Surface strains were mapped. The axial strain distributions in the loading interval showed that the state of stress within the sample was homogeneous. A large change in both axial and circumferential strain fields occurred during the early stage of the primary creep. After this drastic change, the pattern of strain distribution remained unchanged in the subsequent stage of the creep. The accelerated increase in one of the circumferential strain gauges during the tertiary creep stage showed strongly localized deformation preceding faulting. The development of localized dilatancy identified by hypocenter locations was confirmed by the surface strain mapping. The position where the anomalous acceleration in circumferential strain was observed was close to the active, remaining cluster. The migration and clustering of AE hypocenters to the circumferential surface at the beginning of primary creep in the uniaxial unjacketed specimen were explained in terms of stress corrosion. Since the surface is the most favorable for accessing atmospheric moisture, stress corrosion in the vicinity of the surface was facilitated.

Random creep and shrinkage in structures: Sampling

Abstract: This paper deals with uncertainty in the prediction of the effects of creep and shrinkage in structures, such as deflections or stresses, caused by uncertainties in the material parameters for creep and shrinkage, including the effect of random environmental humidity. This problem was previously analyzed using two‐point estimates of probability moments. Here the same problem is analyzed using latin hypercube sampling. This has the merit that the number of required deterministic structural creep analyses is reduced from 2n to approximately 2n, where n=numberofrandomparameters. A method of taking into account the uncertainty due to the error of the principle of superposition is also presented. The mean and variance of creep effects in structures are calculated and scatter bands are plotted for seven typical practical examples, and the results are found to be close to those obtained with two‐point estimates. Further, it is demonstrated that the distribution of creep effects is approximately normal if the cre...

The influence of cobalt, tantalum, and tungsten on the elevated temperature mechanical properties of single crystal nickel-base superalloys

Abstract: For alloys with the baseline refractory metal level of 3 percent Ta and 10 percent W, decreases in Co level from 10 to 0 percent resulted in increased tensile strength and creep resistance. Substitution of W for Ta resulted in decreased creep life at high stresses but improved life at low stresses. Substitution of Ni for Ta caused large reductions in tensile strength and creep resistance, and corresponding increases in ductility. For these alloys with low Ta plus W totals, strength was independent of Co level. The increases in tensile strength with increases in refractory metal content were related to the increases in gamma volume fraction and solid solution hardening. Increases in strength as Co level decreased were considered to be the result of coherency strain hardening from the increased lattice mismatch. Dislocation shear through the gamma-gamma interface is considered to be the rate limiting step in the deformation process.

Strain Softening with Creep and Exponential Algorithm

Abstract: A constitutive relation that can describe tensile strain softening with or without simultaneous creep and shrinkage is presented, and an efficient time-step numerical integration algorithm, called the exponential algorithm, is developed. Microcracking that causes strain softening is permitted to take place only within three orthogonal planes. This allows the description of strain softening by independent algebraic relations for each of three orthogonal directions, including independent unloading and reloading behavior. The strain due to strain softening is considered as additive to the strain due to creep, shrinkage and elastic deformation. The time-step formulas for numerical integration of strain softening are obtained by an exact solution of a first-order linear differential equation for stress, whose coefficients are assumed to be constant during the time step but may vary discontinuously between the steps. This algorithm is unconditionally stable and accurate even for very large time steps, and guarantees that the stress is always reduced exactly to zero as the normal tensile strain becomes very large. This algorithm, called exponential because its formulas involve exponential functions, may be combined with the well-known exponential algorithm for linear aging rate-type creep. The strain-softening model can satisfactorily represent the test data available in the literature.

High-temperature deformation of a polycrystalline alumina containing an intergranular glass phase

Abstract: The high-temperature creep and flexural deformation of a model alumina-glass ceramic is reported over the temperature range in which the glass phase softens (from a viscosity of ~107 to ~103 P). Under both loading conditions the deformation is accompanied by extensive cavitational damage throughout the material. The cavitation is inhomogeneously distributed and at high stresses can be localized into bands. In addition, crack propagation at high temperature results in the formation of a cavitational zone on either side of the fracture and measurements of its size are presented as a function of deformation temperature. On the basis of the microstructural observations of the deformed material, the sequence by which isolated cavities grow and link up prior to failure, is described.

Time-dependent Deformation of Composite Resins — Compositional Considerations:

Abstract: The compressive yield strength and creep of 21 dental composite resins were evaluated and correlated with filler volume percent and extent of cure in the resin. In general, yield and creep of composites were more dependent upon filler volume fraction than on extent of cure. However, the types of monomers and fillers used in the composite formulation appeared to play a major role in determining the compressive characteristics of the materials.

Duality in the Creep Rupture of a Polycrystalline Alumina

Abstract: Creep rupture experiments performed on a polycrystalline alumina have revealed a duality in fracture behavior associated with a crack blunting threshold. At high stresses, or in the presence of large preexisting flaws, fracture is dictated by the slow creep growth of flaws and exhibits substantial statistical variability. At lower stresses, preexisting flaws blunt, and failure is delayed. In this regime, failure occurs by the coalescence of creep damage, manifested as shear bands, and the failure strain becomes inversely dependent on the applied stress.