Showing papers on "Creep published in 1977"

Flow Stress, Subgrain Size, and Subgrain Stability at Elevated Temperature

Abstract: Well defined subgrain boundaries dominate the microstructural changes occurring during plastic flow of polycrystalline metals at elevated temperature. The quantitative influence of subgrain size on elevated-temperature plastic flow is considered. Based on the results of tests under constant-stress and constant-structure conditions, an equation is developed which predicts the creep rate as a function of subgrain size, stress, diffusion coefficient, and elastic modulus. In general, the subgrain size is a unique function of the current modulus-compensated flow stress, but if fine subgrains can be introduced and stabilized, large increases in creep strength may result. The applicability of the phenomenological relation developed to the behavior of dispersion-strengthened materials (where the second-phase particles may predetermine the effective subgrain size) is discussed. When subgrain effects are included, it is shown that the creep rate is less dependent on stacking fault energy than has been previously thought.

Impression creep; a new creep test

Abstract: A new indentation creep test is introduced in which the indenter is a circular cylinder with a flat end Unlike conventional indentation tests, a steady-state velocity is observed in this new test shortly after a transient period during which the indenter makes a shallow impression on the surface of the specimen; hence the name “impression creep” This steady-state velocity is found to have the same stress and temperature dependences as the conventional undirectional creep tests using bulk specimens Three possible mechanisms are analysed in detail, bulk diffusion, surface diffusion, and dislocation creep They have different stress and indenter-size (radius) dependences Experimental results on succinonitrile crystals are reported

Stochastic process for extrapolating concrete creep

Abstract: Creep of concrete is modeled as a process with independent increments of locally gamma distribution. The process is transformed to a stationary gamma process. The mean prediction agrees with the deterministic double power law established previously. Infinite divisibility of the increment distribution is assumed. This is justified by additivity of deformations and of stresses, and also by considerations of the microscopic mechanism of creep, assuming creep to be due to migrations of widely spaced solid particles along micropore passages whose length is statistically distributed. The treatment of creep as a stochastic process allows extracting considerable information from measurements even on one specimen, although a greater number of specimens is preferable. The main use of the model is in extrapolation of short time creep data into long times, and calculation of confidence limits. Methods of determining process parameters from creep test data are given. Monte Carlo simulations demonstrate reasonable agreement with test data.

Mechanics of materials: An introduction to the mechanics of elastic and plastic deformation of solids and structural components

Abstract: Introduction Notation Unsymmetrical bending Struts Strains beyond the elastic limit Rings, discs and cylinders subjected to rotation and thermal gradients Torsion of non-circular and thin-walled sections Experimental stress analysis Circular plates and diaphragms Introduction to advanced elasticity theory Introduction to the Finite Element Method Contact stress, residual stress and stress concentrations Fatigue, creep and fracture Miscellaneous topics Appendices Index.

Creep and plasticity of hexagonal polycrystals as related to single crystal slip

Abstract: The role of slip on basal, prismatic and pyramidal systems of hexagonal single crystals in determining inelastic polycrystalline behavior is studied using a uniform strain-rate upper bound and a self-consistent method. Steady power-law creep is considered. Included as a limiting case is rigid-perfectly plastic behavior, for which the upper bound to the yield stress of the polycrystal coincides with the Bishop-Hill bound for these materials. When the resolved shear stress needed to produce a given level of slip on the pyramidal systems is large compared to that on the other systems the upper bound lies well above the self-consistent estimate. Scif-consistent theory indicates that overall inelastic deformation of a polycrystal is possible without pyramidal slip. Implications for hexagonal materials, including ice, are discussed.

The production and utility of recovered dislocation substructures

Abstract: The production of dislocation substructures by cold working and recovery, fatigue, creep and hot working are reviewed. The relationships of subgrain size and dislocation density to the causal parameters of strain, strain rate, strain amplitude, temperature, stress and time (as applicable) are presented for each process. The importance of dislocation mechanisms such as climb, cross-glide, annihilation and subboundary formation are explained. The relative capabilities and limitations of each mode of creation with respect to both external processing and internal mechanisms are explored. The effects of the metal's stacking fault energy, of solid solution and of particle dispersion on structure and behavior are presented. The properties of the different kinds of substructures for room temperature and creep service are examined. The need for modification of the Petch relationship between yield strength and subgrain size is explored. The thermal stability is shown to be an important factor for creep service. It is concluded that the most suitable modes of substructure preparation are either cold working and recovery or hot working both from the view point of fitting into current industrial practice and from that of dependable, useful service properties.

Diffusional Creep of Polycrystalline Materials

Abstract: Introduction Comparison between Experiment and Theory The Role of Grain Boundary Sliding The Limitation of Diffusional Creep by Interfacial Processes Comparison with Experimental Results Assiciated Phenomena References

The use of the C∗ parameter in predicting creep crack propagation rates:

Abstract: The applicability of the C∗ parameter for the prediction of creep crack propagation rates is considered. A new method for estimating C∗ is presented, the results from which show good agreement with those from an existing technique. Experimental results from creep crack growth tests, conducted on a 1 Cr Mo V steel using both compact tension and single edge notch bend specimens, indicate that good correlation with C∗ is obtained once the effects of stress redistribution become negligible. Finally, comparisons are drawn between C∗ and other possible correlating parameters, and the limitations of each approach are discussed.

Creep of Nimonic 80A in torsion and tension

Abstract: With the object of observing the influence of stress system, the creep of Nimonic 80A has been studied in torsion and tension over the range of effective stress σ from 100 to 500 MPa and at the temperature 750°C For a given σ, tertiary creep and fracture occur sooner in tension than in torsion, while nucleation of cavities is faster The cavities evidently accelerate creep strain and, since creep strain produces cavities, the behaviour as regards both strain and cavitation is autocatalytic The results are expressed and explained in terms of (σ1/σ), where σ1 is the maximum principal stress, and a predictive law for fracture life is derived from constitutive relations connecting strain, cavity volume, and stress

Critical dilatant volume of rocks at the onset of Tertiary creep

Abstract: During uniaxial compression creep tests at 24°C, quartzite and granite specimens were subjected to stresses ranging from 73% to 95% of their uniaxial fracture strengths. Fracture times ranged from 10 to 1.6×106 s. Strains both parallel (ez) and perpendicular (er) to the maximum stress direction were recorded continuously in order to compute volumetric strain. Tertiary creep is signified by a continuous increase of strain rate in either direction. The amount of time spent in tertiary creep prior to failure was different for the two rock types. Inelastic volumetric strain at the onset of tertiary creep was computed and found to be nearly constant, independent of stress level. The magnitude of inelastic strain at the onset of tertiary creep appears to be related to fracture strength and rock type. For quartzite the value is about 3×10−3, and for the granite it is about 1×10 −3. The hypothesis that the tertiary creep onset is dependent upon the induced dilatant volume formed by microcracking is examined in relation to critical crack density models of creep rupture.

Acoustic emission and creep in rock at high confining pressure and differential stress

Abstract: Two samples each of Weber Sandstone and Westerly Granite were tested under triaxial compression at 1 kb confining pressure. Axial load was increased in steps, and the acoustic emission generated in the samples during primary creep was monitored. The rate v of acoustic emission events was found to decrease exponentially at each stress level, obeying the law log v = β − α N , where N is the total number of microseismic events that have occurred and α and β are constants. By assuming that acoustic emission is proportional to inelastic deformation, this relation can be compared to empirical creep laws. It is similar to the relation given by Lomnitz and fits the data more closely than other creep laws that were functions of time rather than number of acoustic emission events. The value of α was found to decrease systematically with increasing differential stress, and in one experiment became negative before sample failure.

Creep crack growth in alloy 718

Abstract: Subcritical crack growth behavior in Alloy 718 was studied under creep conditions at 538, 649, and 760°C (1000, 1200, 1400°F) and crack growth rates were correlated using both linear and nonlinear elastic fracture mechanics. The results show that for a given stress intensity value crack growth rate increases significantly with increase in temperature from 538 to 649°C but either decreases or increases slightly with further increase in temperature to 760°C. On the basis of these results it is concluded that creep crack growth results from a balance of two competing processes, diffusion of point defects which contributes to crack growth, and creep deformation process that causes retardation of crack growth and even its arrest. Significance of these concepts in relation to enhancing the resistance of a given material to creep crack growth is discussed in detail.

Creep of polycrystalline alumina, pure and doped with transition metal impurities

Abstract: Grain size effects were used to evaluate the relative contributions of aluminium lattice and oxygen grain boundary diffusion to the high temperature (1350 to 1550° C) steady state creep of polycrystalline alumina, pure and doped with transition metal impurities (Cr, Fe). Divalent iron in solid solution was found to enhance both aluminium lattice and oxygen grain-boundary diffusion. Large concentrations of divalent iron led to viscous Coble creep which was rate-limited entirely by oxygen grain-boundary diffusion. Nabarro-Herring creep which was rate-limited by aluminium lattice diffusion was observed in pure and chromium-doped material. Chromium additions had no effect on diffusional creep rates but significantly depressed non-viscous creep modes of deformation. Creep deformation maps were constructed at various iron dopant concentrations to illustrate the relative contributions of aluminium grain boundary, aluminium lattice, and oxygen grain-boundary diffusion to the diffusional creep of polycrystalline alumina.

Metallurgical factors affecting the crack growth resistance of a superalloy

Abstract: During creep loading of IN-792, grain boundary morphology in conjunction with grain size strongly affected crack propagation. Compositional variations and fabrication techniques showed no significant effect. A primary requirement for materials to be used in gas turbine engine discs is satisfactory resistance to crack growth resistance in the 650 to 760°C range. Both conventional smooth and machine notched stress-rupture samples and dead weight loaded fatigue precracked fracture toughness specimens were evaluated in this study. Creep fractures took place by grain boundary cracking followed by rapid transgranular fractures. Composition variations had only very slight effects on crack propagation. Materials hot worked from castings had the same properties as those made by powder metallurgy techniques. The primary factors influencing the crack growth behavior were the grain size and grain shape. Increasing grain size markedly improved the toughness. By slow cooling through the gamma prime solvus a serrated grain boundary structure was developed that also improved the cracking resistance. Earlier creep fracture toughness studies have shown that the slow crack growth behavior can be described by a critical strain model in which the crack propagation is controlled by the yield strength, grain size, and a critical strain parameter. The present results are consistent with this model, with serrated grain boundaries introducing a four-fold increase in the critical strain parameter over that of smooth grained material.

A comparison of creep, elasticity and strength of concrete in tension and in compression

Abstract: Synopsis A method of manufacturing a bobbin-shaped specimen for testing concrete in direct tension is described. Direct tensile strength is related to tensile and compressive strengths as determined by standard tests and their changes with age are described. Data on the modulus of elasticity in compression and in tension at two exposure conditions are given, and the influence of age upon creep and creep recovery is established. The differences in behaviour of strength, modulus of elasticity, creep and creep recovery are discussed by considering the effects of load and storage environment upon the adsorbed water of the cement paste, and the suggested creep mechanism is a combination of the seepage and viscous shear theories.

Computer simulation of impression creep by the finite element method

Abstract: The steady state impressing velocity of the punch during an impression creep test is calculated by the finite element method based on a single power law constitutive equation for the deformation of each and every element. The calculated impressing velocities and their stress dependence agree very well with the experimental values on succinonitrile crystals using empirical power laws obtained from unidirectional creep tests.

Secondary and tertiary creep of glacier ice as measured by borehole closure rates

Abstract: Published and previously unpublished measurements of closure rates of five boreholes in polar ice caps are reviewed. The data cover effective shear stresses between 0.15 and 1.0 MN m−2, temperatures between −16° and −28°C, and strains up to 2.2. Curves of strain at the borehole wall (logarithm of the ratio of hole diameter to its initial diameter) against time show a stage of constant closure rate corresponding to secondary (steady state) creep of the ice followed by accelerating closure rate attributed to recrystallization of the ice (tertiary creep). Curves for low stresses also show an initial transient stage of decreasing closure rate. The onset of tertiary creep is largely determined by the strain; critical values range from 0.03 to 0.10, and the lower the temperature, the higher the critical value. Secondary creep rates in the different boreholes are consistent with each other; the data yield a creep activation energy of 54 kJ/mol and a flow law index close to 3. The borehole data reduced to a common temperature of −22°C are compared with the results of two laboratory experiments at this temperature. For a given stress the strain rates measured by Steinemann (1958a, b) are 2–3 times those in the boreholes, and for the experiments of Barnes et al. (1971) the factor is about 8. Differences between laboratory and glacier ice, probably in grain size, may explain the differences between the borehole data and the results of Steinemann. Some evidence is presented that the creep rates measured by Barnes et al. at this temperature may contain a significant component of transient creep; this might account for the large difference between their results and those of Steinemann. The ratio of tertiary to secondary creep rate increases approximately linearly with the strain. No steady state tertiary creep rate is observed even at a strain of 1.5, at which point the ratio of tertiary to secondary creep rate is about 10. However, the ice is not strained uniformly during borehole closure. Even if recrystallization has been completed in the ice near the borehole wall, the ice further away, having been strained less, may still be recrystallizing. This may account for the failure to observe steady state tertiary creep. Near the bottom of one borehole, creep rates (tertiary) are about 4 times those in the ice immediately above. The boundary between the two deformation regions corresponds closely to the boundary between ice deposited during the Wisconsin glaciation and ice deposited since that time. The crystals in the Wisconsin ice are smaller, much less variable in size, and more nearly equidimensional than those elsewhere. Moreover, the Wisconsin ice has a much higher microparticle content and a much lower content of salts of marine origin. It is suggested that one or more of these differences make the Wisconsin ice ‘softer’ than the remainder of the ice. The decrease in grain size is considered to be the most likely factor.

Impression creep of LiF single crystals

Abstract: Impression creep tests of LiF single crystals at 600–750[ddot]C and under punching stresses of 4-30 MN/m2 are reported. Both temperature and stress dependences are comparable with those of conventional compression creep tests under similar conditions. These dependences give a power law stress exponent of 4–5 and an activation enthalpy of 52 kcal/mole. A finite element numerical analysis shows consistency between the measured impressing velocity and the conventional power law constitutive equation assumed for the plastic deformation of each element. The dislocation structure developed under the punch as revealed by etch pits indicates that the depth of the plastic zone is comparable to the punch diameter.

Creep-fatigue interaction in austenitic stainless steels

Abstract: Low cycle fatigue failures occur by the initiation and controlled growth of a surface crack. The development of crack propagation models, based on continuum mechanics, have enabled successful predictions of fatigue life at both room and elevated temperatures. This paper attempts to extend such models to cover the situations in which creep damage, introduced during periods of stress relaxation, influences the rate of growth of the surface fatigue crack. Equations predicting fatigue life as a function of hold period are in good agreement with experimental data, for Type 304 stainless steel, Type 316 stainless steel and Incoloy-800.

Viscoelasticity of Solidifying Porous Material-Concrete

Abstract: The form of the age effect on linear creep and elasticity is established by treating the hydrating cement paste as a variable composite in which the volume fraction of solid grows while the material creeps under load. The formulation rests on three hypotheses: (1)The average strains in microelements solidified at various times are equal; (2)the properties of solidified matter (cement gel) are time-invariant; (3)the rate of migrations of solid particles causing creep in cement gel depends on the mean length of migration passages at the time of load application. The first hypothesis allows eliminating the microstresses from a system of two integral equations set up to relate stress and strain histories. A new form of the creep function is deduced, expressing the aging effect in terms of the growth of volume of cement gel and the increase of the migration passage length. It is shown that the double power law is obtained as a special case when the growth of gel volume is negligible, and a generalization consistent with growing gel volume is suggested and compared with creep data.