Showing papers on "Creep published in 1974"

On the Penetration of Water into Hot Rock

Abstract: Summary This paper develops a theory for the mechanism of penetration of water into hot rock by considering the simplest possible one-dimensional model. The concept of a cracking front is used to separate the convective regime in cracked porous rock from the conductive boundary layer below it. Rock in the boundary layer cools, shrinks and builds up horizontal tensile stress as resistance to creep rises. Cracking occurs when the tensile stress slightly exceeds the overburden pressure and results in the stable downward propagation of a polygonal pattern of sub-vertical cracks. Further cooling shrinkage opens these cracks to the percolation of water so that the effective bulk permeability is a strong function of both crack spacing and temperature. The critical crack spacing is determined by the competing processes of pattern enlargement through stress relief by favoured cracks and the subdivision of columns due to the radial temperature gradients associated with cooling through the cracks themselves. A rudimentary treatment of the transient creep associated with the thermal contraction of virgin rock suggests cracking temperatures between 800 and 1000 °K at depths of several kilometres and a similar treatment of thermally activated crack propagation produces a relationship between front velocity and crack spacing. That permits an approximate solution of the whole problem, resulting in a formula for front velocity where φ is a fudge factor and the temperatures are: TW—hot hydrothermal, Tφ—overburden cancelling creep stress, T0—sink, T1—original. Maximizing through TW permits the calculation of u and all dependent parameters for any given Tφ, itself close to the cracking temperature. If Tφ= 800 °K, the front velocity is 32m/yr, the crack spacing is 4.8cm and the hot hydrothermal temperature is about 462 °K. The power output of the convecting system is about 5 kWm−2. Downward propagation of the cracked region seems to be limited by static fatigue failure of the columns under stress, some distance behind the front. The qualitative aspects of the model are in general agreement with ridge crest heat-flow and seismic data, though the sheeted dike complex near the surface of new crust is created by rapid cooling of each new dike by water circulation rather than by a process comparable to the one-dimensional model. Subsequent, deeper, water penetration may occur by processes similar to those described in the model.

Low-stress high-temperature creep in olivine single crystals

Abstract: Gem quality single crystals of dry olivine were plastically deformed in compression at differential stresses between 50 and 1500 bars, with zero confining pressure, and at temperatures between 1428° and 1650°C. When these new data are combined with existing creep data for dry olivine-bearing rock, a flow law of the form e˙ = ƒ(σ) exp (−Q/RT) is obtained, whereƒ(σ) is an empirical function that is not simply proportional to σn for constant n and Q is equal to 125 ± 5 kcal/mol. The flow law covers the stress range 50–10,000 bars and the temperature range 1100°–1700°C and is consistent with all published data to 1 decade in strain rate.

Creep Rupture of Structures

Abstract: A study is made of the failure times of structural components which operate at temperatures sufficiently high to cause material deterioration due to creep rupture. Expressions are derived which give lower bounds on failure times and which take into consideration the different stress criteria known to affect rupture mechanisms. The formulae are used to predict failure times of a variety of components, and it is found convenient, from a practical point of view, to express the times in terms of an equivalent representative rupture stress. By using this stress, failure times are obtained directly from uniaxial stress rupture data. It is found in the examples studied that the values for the representative rupture stress are almost independent of the constants used to define the deformation and rupture processes. Experimental evidence supports the prediction of the theory; for example, copper bars in torsion show better rupture characteristics than bars of aluminium alloy. The position is reversed in notched tensile specimens, with the aluminium specimens showing better characteristics than those of copper. It can be deduced that it is the form of rupture mechanism which affects behaviour rather than ductility as might be expected, since the creep ductility of the aluminium alloy is much less than that for copper.

Steady-state irradiation creep

Abstract: A dislocation climb model of the low-temperature, steady-state, irradiation creep of non-fissile materials is presented. The predicted creep rate is a linear function of the applied stress and the irradiating flux and is independent of temperature.

Cavitation in creep

Abstract: Metals often fail in service under creep conditions because of the formation of cavities on the grain boundaries which are approximately normal to the applied stress. This phenomenon of creep cavitation is becoming of increasing technological importance. As a result a complete understanding of it is desirable so that alloys with improved cavitation resistance can be designed. This paper reviews the development of our present understanding of the phenomenon which is one of nucleation, growth and linkage leading to failure. Several mechanisms of nucleation, such as at grain-boundary ledges or precipitates, have been postulated and experimental evidence in support of each has been cited. Similarly, deformation- or vacancy-controlled growth mechanisms have been discussed. It is apparent from the literature thatno single mechanism is applicable, indeed, the work discussed here suggests that several mechanisms may operate and each may become dominant at different stages of the creep life. Finally, the status of research into nickel-base superalloys is reviewed with reference being made to such effects as regenerative heat-treatment.

Rate-type creep law of aging concrete based on maxwell chain

Abstract: It is shown that the linear creep law of concrete can be characterized, with any desired accuracy, by a rate-type creep law that can be interpreted by a Maxwell chain model of time-variable viscosities and spring moduli. Identification of these parameters from the test data is accomplished by expanding into Direchlet series the relaxation curves, which in turn are computed from the measured creep curves. The identification has a unique solution if a certain smoothing condition is imposed upon the relaxation spectra. The formulation is useful for the step-by-step time integration of large finite element systems because it makes the storage of stress history unnecessary. For this purpose a new, unconditionally stable numerical algorithm is presented, allowing an arbitrary increase of the time step as the creep rate decays. The rate-type formulation permits establishing a correlation with the rate processes in the microstructure and thus opens the way toward rational generations to variable tempeature and water content. The previously developed Kelvin-type chain also permits such a correlation, but its identification from test data is more complicated.

High‐Temperature Strength Behavior of Hot‐Pressed Si3N4: Evidence for Subcritical Crack Growth

Abstract: The high-temperature strength of commercial hot-pressed Si3N4 was obtained for (1) two materials with different impurity contents, (2) the weak and strong material directions, (3) air and Ar ambients, and (4) different stressing rates. Strength degradation occurred at a lower temperature for the less pure material; both material directions exhibit the same rate of strength degradation. The testing ambient did not affect strength. The strength at temperatures ∼1200°C depended strongly on stressing rate. The presence of rough, crack-shaped topographical features on the fracture surface and the observation of large cracks that formed during stressing are reported as evidence for subcritical crack growth at high temperatures. It is hypothesized that accelerated creep caused by grain-boundary sliding at preexisting crack fronts is the mechanism responsible for the observed subcritical crack growth.

Quasi-static extension of a tensile crack contained in a viscoelastic-plastic solid

Abstract: Final stretch criterion of failure is applied to the problem of quasi-static extension of a crack embedded in an elastic-plastic or viscoelastic-plastic matrix. The slow growth under subcritical conditions in a rate-sensitive Tresca solid is shown to be a superposition of creep rupture and McClintock's ductile growth. This type of growth occurs at subcritical magnitude of the imposed K-factor and can be accounted for only through a recognition of inelastic properties of solids. In the subcritical range there is no unique value for K sub c independent of geometrical configuration and flaw size. Not only the produced states of stress and strain are dependent on the loading path, but also the material resistance to fracture turns out to be a function of the history of loading that precedes catastrophic failure. A nonlinear integro-differential equation of motion is derived for a crack progressing through a viscoelastic medium with some limited ability to plastic flow. Examples of numerical integration are given incorporating both monotonic and cyclic loading programs.

Low temperature creep of Ti-6 Al-4 V

Abstract: Creep tests were conducted at 295 K on Ti-6 Al-4 V in the solution treated and aged (4 h at 815 K) condition, and in the as-welded condition Some aged specimens were tested after pre-straining Creep stresses ranged from 40 to 90 pct of the aged material yield strength Results showed that creep was of the primary or transient kind in all cases, and was much greater in welded than in aged material In general, pre-strains reduced creep, although a strain larger than 10-3 was needed to do this at the highest creep stress Activation areas A* were between 10 and 20 b2, and thus were similar to tensile results on titanium and its alloys The microstructural rationale applied to Ti-5 Al-25 Sn in earlier work, based on the character of dislocation sources, proved successful in understanding the effects of prestrain in this work

Grain‐boundary diffusion and boundary widths in metals and ceramics

Abstract: A technique is presented to calculate grain‐boundary widths and grain‐boundary diffusion coefficients in metals and nonmetals using grain growth, sintering, diffusion and/or creep data. High‐purity metals yield boundary widths of the order of a few atomic diameters, whereas ionic materials yield ``effective boundary widths'' orders‐of‐magnitude wider.

Correlation of Properties of Materials to Debye and Melting Temperatures

Abstract: Many solid state properties are correlated to simple parameters like the atomic mass, the interatomic distance and some measure of the strength of the interatomic interaction, e.g. the melting temperature Tm or some Debye temperature θD. Such empirical relations are investigated for elastic properties, melting temperature, thermal expansion, vacancy formation energy, grain boundary and surface energy, cohesive energy, heat of fusion, activation energies for bulk, grain boundary, surface and dislocation pipe diffusion, viscosity, activation energy for creep, recrystallization temperature and other properties. When experimental data are available, both elements and diatomic compounds (alkali halides, oxides, carbides, III-V semiconductors) are considered. The correlations to Tm, θD(T = 0) and a high temperature effective θD are compared. It is found that for all practical purposes Tm gives the best correlation. For diatomic compounds, the average mass is not uniquely defined and various averages are discussed. The purpose of the paper is not only to present empirical relations but also to give a short account of the possibilities to calculate the considered parameters using solid state theory in its most advanced present state.

Prestrain, Cavitation, and Creep Ductility

Abstract: Specimens of Nimonic 80A have been prestrained in tension at room temperature by various amounts up to 15% elongation. Each prestrained specimen was crept at 750° C at one of two stresses, 154 N/mm2 or 460 N/mm2, which gave unprestrained creep lives of 1800 and 5 h, respectively. Some of the tests were interrupted after 160 h but most specimens were fractured. At both stresses, the material suffered progressive loss of creep strength, life, and fracture ductility as prestrain increased, while quantitative optical metallography revealed a concomitant dramatic increase in the number of grain-boundary cavities. Cavity density and size measurements have established that creep ductility is reduced because of increased cavity density and not because of any increase in cavity growth rate. It is suggested that the loss in creep strength is probably due to an increase in the mobile dislocation content with cold work.

The Strength of Rigid Polymer Materials

Abstract: : The book contains information on methods dealing with the strength of polymer materials; fundamental information from the field of the mechanics of elasto-visco- plastic bodies; and demonstration of methods for analyzing the simplest designs of rods, plates, and shells. Creep of anisotropic and non-linear viscoelastic material, the theory of reinforcement, and the destruction and fatigue of polymer materials. (Author, modified-PL)

The yield-point loads of singly-notched pin-loaded tensile strips

Abstract: P lane stress and plane strain yield-point loads have been calculated for single-edge notched strips when tensile loading is applied through pins. These yield-point loads are lower than those for fixed-grip loading, and the percentage drop is larger in plane stress than in plane strain. The calculations were required for assessing various creep and fatigue crack propagation tests. Excellent agreement was found between experimental yield-point loads and the plane stress calculations based on the von Mises yield criterion. However, no such agreement was found when further tests were carried out on doubleedged notched specimens. Possible reasons for this discrepancy are discussed. The plane strain calculations may also be used for the ‘double cantilever bend’ specimen by a simple substitution.

Time dependent strain following faulting of a porous medium

Abstract: A fracture that redistributes shear stress in a porous medium produces local fluid pore pressure changes. Decay of this pore pressure due to flow causes strain. Using the theory of Biot, I calculate the time dependent stress, strain, and pore pressure fields after a plane fracture whose offset varies sinusoidally in the direction of slip. The results permit construction of arbitrary one-dimensional slip functions by Fourier synthesis. In particular, I investigate the simple crack made from two edge dislocations. The fracture surface is slowly reloaded after an initial shear stress drop, and the shear strength of the material adjacent to the fracture varies with time. This behavior is useful in explaining earthquake foreshocks, aftershocks, and delayed creep. For the simple crack the frequency of aftershocks associated with the reloading decays like t−1.

Time-dependent reinforced concrete slab deflections

Abstract: A finite element analysis to determine time-dependent deflections of reinforced concrete slabs, including the effects of cracking, creep, and shrinkage, is presented. The analysis employs a 16-degree-of-freedom layered rectangular plate bending element, and uses a numerical time integration scheme to evaluate creep and shrinkage strains determined from CEB parameters. The initial strain technique is used in the solution procedure. Predicted results are compared with the limited experimental results in the literature.

Triaxial and Plane Strain Creep Rupture of an Undisturbed Clay

Abstract: A comparative study of the undrained creep rupture characteristics of a saturated, normally consolidated, undisturbed marine clay has been carried out under triaxial and plane strain conditions. Cr...

Recrystallization in Hot Working and Creep

Abstract: Experimental evidence indicates that metals may recrystallize during hot working. A model is presented that permits discussion of the influence of this process on stress/strain curves at high strain rates and on the dependence of strain on time for low strain rates (as in creep).

Theory of high temperature intercrystalline fracture under static or fatigue loads, with or without irradiation damage

Abstract: Hull and Rimmer’s theory of high temperature creep fracture by grain boundary void growth and Skelton’s theory of high temperature fatigue fracture have been extended The present theory includes the effect on void growth of volume diffusion and of vacancy generation by irradiation damage or plastic deformation within the grain interiors The growth rate of cylindrical as well as spherical voids is calculated We find that the presence of volume diffusion or of vacancy generation within the grain does not increase very much the growth rate of spherical voids but under certain conditions the growth rate of cylindrical voids is increased markedly Unlike Skelton, we find that vacancy production within the grains does not decrease the critical radius for the growth of a grain boundary void in push-pull fatigue (Likewise intra-granular vacancy production does not decrease the critical radius of grain boundary voids in creep) Since in push-pull fatigue void growth is observed in the case of radii smaller than the critical value it is concluded that the stress concentrations that are produced by grain boundary sliding (according to the theory of Raj, Ashby, and Gifkins) are responsible for this growth Whether voids aided by stress concentrations can continue to grow beyond a certain size depends critically upon the “roughness” of the grain boundary It is strongly recommended that grain boundary internal friction peak studies be carried out in connection with void growth and high temperature fracture experiments In particular, grain boundary voids are more likely to grow in push-pull fatigue (or in creep) if the value of the grain boundary sliding activation energy is close to that of volume self diffusion rather than if the sliding activation energy is close to the grain boundary self diffusion activation energy

Creep in Continua and Structures

Abstract: Creep mechanics is a young branch of solid mechanics Even though its foundations were established by experimental and theoretical studies early in this century, creep phenomena came to gain engineering importance only in the last few decades

Creep and Shrinkage Law for Concrete at Variable Humidity

Abstract: The drying creep effect is modeled by a nonlinear coupling between two types of hidden stresses, those in solids and those in (hindered adsorbed) water. Material parameters, as functions of pore humidity, of equivalent hydration period, and of relaxation time, are identified from the existing test data, using an optimization algorithm based on a least-square criterion and coupled with a finite element program for strain and stress history in a drying concrete cylinder. The ratios in which pore humidity modifies the creep parameters are found to be nearly the same for all concretes. Realistic estimates of stresses induced by shrinkage and by drying creep are also reached. The formulation is also extended to variable temperature. The success in fitting the data corroborates the underlying irreversible therodynamic theory of the creep mechanism, and especially confirms that creep at working stress levels is caused primarily by the diffusion of solids which, in turn, is facilitated by the presence of water and is nonlinearly accelerated by its migration.

Creep of Frozen Silt and Clay.

Abstract: Unconfined compressive creep strengths and strains were measured for remolded saturated frozen Hanover silt and Suffield clay. The creep tests were conducted at the approximate stress levels of 60, 35, 20 and 5 percent of the conventional unconfined compressive strength. Testing temperatures were 15, 25, 29, and 31 degrees F (minus 9.45, minus 3.89, minus 1.67, and minus 0.56 degrees C). An equation was found for the variation of unconfined compression peak strength with temperature. Unconfined compression creep strength could be estimated by Vialov's strength equation. Long-term creep strength was less than 45 percent of unconfined compression strength and could be as low as 10 percent of this strength. An equation for the increase in long-term creep strength with the decrease in soil temperature was also found. An expression was derived for obtaining a close estimate of the long-term creep strain. The results of this investigation were in general agreement with findings of other investigators for frozen silt and clay except that Hanover silt and Suffield clay did not display the classical creep curve shape.