Showing papers on "Creep published in 1979"

Dynamic recrystallization during creep of single-crystalline halite: An experimental study

Abstract: Single crystals of pure and impure halite have been dynamically recrystallized during compression creep at temperatures between 250° and 790°C and stresses between 1.5 and 120 bars. Recrystallization was found to occur by two different mechanisms: at lower temperatures and stresses the new grains result from the rotation of subgrains without grain boundary migration (rotation recrystallization), and at higher temperatures and stresses the final texture results from the migration of the high-angle grain boundaries of the rotated subgrains. Migration recrystallization was shown to occur for critical stress and temperature conditions, allowing rapid grain boundary migration. A curve separates the two domains in the σ, T plane and moves to higher temperatures and stresses for crystals of higher impurity content; for natural crystals, only rotation recrystallization can occur. In each recrystallization regime the recrystallized grain size is uniquely related to the applied stress, thus yielding two different geopiezometers, which should not be applied indiscriminately to natural tectonites to determine lithospheric or mantle deviatoric stresses. The experimental results are interpreted by the Lucke et Stuwe theory for impurity-controlled grain boundary migration.

Microscopic mechanisms and mechanics of craze growth and fracture

Abstract: Isolated air crazes have been produced in thin films of polystyrene (PS) bonded to copper grids by straining these in tension The craze thickness profile, r(x), was directly determined from a series of transmission electron micrographs taken along the craze Local values of the craze fibril volume fraction v 1 and fibril extension ratio λ were established at frequent intervals along the craze by optical densitometry of the micrographs The craze surface displacement profile w(x), craze surface stress profile S(x) and the true stress a t in the craze fibrils are computed from these parameters The λ(x) profile provides conclusive evidence that the craze increases in thickness as it grows in length by drawing new polymer from the craze surface into the fibrils rather than by creep of the existing fibrils The S profile exhibits a modest maximum at the craze tip but falls slowly over a distance of about 15 μm behind the craze tip to a value about 10% below the applied tensile stress The λ of the d

The frequency dependence of Q in the Earth and implications for mantle rheology and Chandler wobble

Abstract: For most solids the ‘high temperature background’ attenuation dominates at low frequencies and temperatures greater than about one-half the melting temperature. It is likely to be important in the mantle at seismic frequencies. The same mechanism also contributes to transient creep at low stresses and low total strains. A relaxation spectrum is found which satisfies the frequency dependence of laboratory Q and the time dependence of transient creep data. This makes it possible to provide a physical interpretation of the parameters in Jeffrey's modified Lomnitz creep function. Q is predicted to increase as ω^α in the lower Q regions of the mantle. At high and low frequencies Q should increase as ω and ω^(−1), respectively. The location of the ω^α band depends on temperature and therefore shifts with depth. At high temperatures, seismic waves are on the low-frequency side of the absorption band and Q decreases with frequency. Far from the melting point and at sufficiently high frequencies Q should increase linearly with frequency. We use Chandler wobble, tidal and free oscillation data to estimate that α is ∼ 1/5 to 1/3, consistent with laboratory measurements of transient creep and internal friction at high temperature. A preliminary attempt is made to estimate the transient creep response of the mantle from Q measurements. The inferred viscosity agrees well with direct measurements. The effect of anelasticity is to lengthen the calculated period of the Chandler wobble by 5-20 days, depending on the Chandler wobble Q. A Q of 300 for the wobble, which is within the experimental uncertainty of recent determinations, gives the observed period after correcting for the effect of the oceans.

An experimental study of room-temperature rate-sensitivity, creep and relaxation of AISI type 304 stainless steel

Abstract: T he spuniaxial viscoplastic behavior of AISI Type 304 stainless steel was investigated by tensile tests at various strain-rates (10−8−10−2s−1), and by short-term creep and relaxation tests up to 5 h. Instantaneous large changes in strain-rate were also performed during monotoniC and cyclic loading. A servocontrolled testing machine and displacement measurement on the specimen gage length were used for all tests. The results show significant rate-sensitivity, creep and relaxation. Test histories involving loading and unloading with positive loads up to 15% strain show that the relaxation behavior in the plastic range depends only on the strain-rate preceding the relaxation test and is independent of the strain magnitude. Also, the relaxation behavior is uniquely related to the stress changes corresponding to instantaneous large changes in the strain-rate during tensile tests. Completely reversed strain-controlled loading gradually changes the stress change/strain-rate change behavior. Annealed specimens and specimens loaded to a cyclic steady-state differ not only in their work-hardening characteristics but also in their rate-dependent behavior. In the cyclic steady-state, different hysteresis loops are traced for different strain-rates with fully reversible transitions from one hysteresis loop to another under strain-rate changes. These results support the notion that the viscoplastic behavior can be represented by piecewise nonlinear viscoelasticity.

Contact pressure dependence of wear rates of ultra high molecular weight polyethylene

Abstract: Wear rates of orthopaedic polyethylene in 37 degrees C water are not very contact stress dependent below 1000 psi (6.9 MPa) but above that level they accelerate substantially. The pressure dependence overall follows an exponential function. Creep in the contact pressure range of 3--17 MPa and above is a much larger factor than wear in indentation effects. For accurate measurement of wear rates by depth measurements it is necessary to permit creep to proceed under static load until it ceases before beginning the wear test.

Dependence of the thickness of the elastic oceanic lithosphere on age

Abstract: Most bathymetric profiles of oceanic trenches show a well-defined outer rise seaward of the trench. The widths of the observed outer rises are used with an elastic flexure model to determine the effective elastic thickness (EET) of the oceanic lithosphere as a function of the age of the lithosphere being subducted. A model for the EET is proposed in which its upper boundary is taken at a prescribed depth and its lower boundary at a prescribed temperature. It is assumed that the base temperature is the temperature at which elastic stresses are relaxed by dislocation creep on geological time scales. Using a laboratory-derived creep law this relaxation temperature is found to be 700±100°C this result is relatively insensitive to time and stress levels. Using accepted thermal models for the oceanic lithosphere, this relaxation temperature gives values of the EET which are in good agreement with the observations.

Irradiation creep by climb-enabled glide of dislocations resulting from preferred absorption of point defects

Abstract: A mechanism of irradiation creep arising from the climb-enabled glide of dislocations due to stress-induced preferred absorption of radiation-produced point defects is proposed. This creep component is here termed preferred absorption glide, PAG. PAG-creep operates in addition to the previously studied components of creep from climb by stress-induced preferred absorption, (SI)PA-creep, and the climb-enabled glide due to excess absorption of interstitials on dislocations during swelling, I-creep. A formulation of the various climb and climb-enabled glide processes which includes earlier results is presented. PAG-creep is comparable in magnitude to PA-creep in the parameter range of applications. While the PA-creep rate and the I-creep rate are linear in stress, the PAG-creep rate is quadratic in stress and thus dominates at high stresses.

The use of strain energy as a yield and creep criterion for lightly overconsolidated clays

Abstract: Triaxial, stress controlled, drained tests on four undisturbed, overconsolidated clays from eastern Canada show that the strain energy can be used with advantage to define the limit state surface o...

Mechanical behavior of superplastic ultrahigh carbon steels at elevated temperature

Abstract: Ultrahigh carbon (UHC) steels have been investigated for their strength and ductility characteristics from 600 to 850°C. It has been shown that such UHC steels, in the carbon range 1.3 to 1.9 pct C, are superplastic when the microstructure consisted of fine equiaxed ferrite or austenite grains (∼1 μm) stabilized by fine spheroidized cementite particles. The flow stress-strain-rate relations obtained at various temperatures can be quantitatively described by the additive contributions of grain boundary (superplastic) creep and slip (lattice diffusion controlled) creep. It is predicted that superplastic characteristics should be observed at normal forming rates for the UHC steels if the grain size can be stabilized at 0.4 μm. The UHC steels were found to be readily rolled or forged at high strain-rates in the warm and hot range of temperatures even in the as-cast, coarse grained, condition.

Grain boundary sliding in polycrystalline materials

Abstract: Delayed elastic phenomenon during high–temperature creep of polycrystalline materials is correlated with strain due to grain boundary sliding. This correlation has been used to develop a phenomenological viscoelastic model that includes the grain–size effect. With ice as a reference material, it is shown that the contribution of the grain-boundary sliding strain to the total strain, and its dependence on stress, time, temperature and grain diameter can be systematically analysed by the proposed model. The results appear to agree with the observed trends in other materials.

Effect of low-temperature annealing and deformation on the structure of metallic glasses by X-ray diffraction

Abstract: The changes in the atomic structure of amorphous Pd-Si and Ni-P alloys due to low-temperature annealing, cold-rolling and isothermal creep have been studied by the conventional X-ray diffraction. The present results on the effect of low-temperature annealing were consistent with those of amorphous Fe40Ni40P14B6 alloy studied by the energy dispersive X-ray diffraction method. In addition, the present results have indicated that the effect of cold-rolling causes small changes in the structure of amorphous Pd80Si20 alloy which are qualitatively different from the structural relaxation, and the effect of annealing plays a significant contribution in the structural change during an isothermal creep test.

Titanium alloys of the TiAl type

Abstract: Cast and forged titanium alloys suited for use at temperatures over 600° C. are based on TiAl gamma phase structure. Useful alloys have about 1.5% or greater tensile ductility at temperatures of 260° C. and below, thereby making them fabricable and suited for engineering applications. Disclosed are alloys having weight percent compositions of 31-36 aluminum, 0-4 vanadium, balance titanium (in atomic percent, about: 45-50Al, 0-3V, bal Ti). The inclusion of about 0.1 weight percent carbon improves creep rupture strength. To obtain high tensile strength, the alloys are forged at about 1025° C. and aged at about 900° C.; to obtain higher creep rupture strength and tensile ductility, a solution anneal at about 1150° C. is interposed before aging.

The effect of hold time on the fatigue properties of a β-processed titanium alloy

Abstract: Several recent papers have demonstrated that dwell periods at peak stress can significantly reduce the number of cycles to failure in LCF tests on titanium alloys and can cause enhanced growth rates in fatigue crack propagation tests. In all cases cleavage or quasi-cleavage facet formation has been intimately linked with the dwell sensitive fatigue response. The present paper demonstrates that facets can also form during creep deformation at ambient temperatures and it proposes that the LCF dwell effect and facet formation under cyclic conditions is dependent on time dependent plastic strain accumulation. If hydrogen contributes to the failure process it is suggested this must be through an interaction with dislocations. The significance of the model for dwell sensitive fatigue crack propagation is discussed.

Mechanisms controlling creep of single phase metals and alloys.

Abstract: The physical mechanisms that control creep of single phase metals and alloys are reviewed Attention is given to both steady state and non-steady state properties The evolution of dislocation substructure in the course of primary creep is followed for both pure metals and solid solution alloys in an effort to demonstrate its dominant role in creep It is shown that the subgrain size and dislocation density scale in a natural way with the applied stress The long range internal back stresses that are responsible for creep anelasticity are shown to be determined not only by the scale of the substructure but also by the bias of the applied stress Creep transients following load changes are also described and interpreted in terms of the nature and properties of the creep substructure In addition, the characteristics of steady state creep are described by referring to the extensive phenomenological evidence which supports the concept that steady state creep at high temperatures is controlled by lattice self diffusion It is suggested that the diffusion controlled recovery and thermally activated glide approaches to creep should be complimentary rather than competitive In general, dislocation motion can occur either as a result of thermal activation past obstacles or as a consequence of obstacle recovery We argue that the inherently inhomogeneous dislocation substructures that form during creep produce inhomogeneous stresses that enhance the rate of obstacle recovery in the “hard” regions of the creep substructure and inhibit thermally activated glide in the “soft” regions This “composite” model of creep allows us to explain both steady state and non-steady state creep properties in a semiquantitative way

Activation Volume for Creep in the Upper Mantle

Abstract: The activation volume for creep, V*, of olivine-rich rocks has been determined in pressure-differential creep experiments on dunite at temperatures from 1100° to 1350°C and confining pressures from 5 to 15 kilobars. Values of V* range from 10.6 to 15.4 cubic centimeters per mole with a mean value of 13.4 cubic centimeters per mole, near that expected for oxygen ion self-diffusion. The quantity V* is incorporated into existing flow equations; in combination with observations on naturally deformed mantle xenoliths, estimates are given of the variation with depth of stress, strain rate, and viscosity.

Dilatancy anisotropy and the response of rock to large cyclic loads

Abstract: An experimental study of cyclic fatigue of granite and diabase in triaxial compression was conducted. It was found that dilatancy progressively developed during cyclic loading of these rocks with a form in time very similar to a creep curve. Dilatancy was in all cases highly anisotropic. This anisotropy was controlled by the crack microstructure in the granite and by the residual stress in the diabase. The strike of the fracture plane was found typically to form normal to the axis of maximum dilatancy. Increasing dilatancy during cyclic loading was found to be caused by progressive dilatant creep plus additional damage produced by the cycling itself. The former leads to a pronounced loading rate effect on fatigue, the latter to a complex effect of cycle amplitude on fatigue. No stabilization of fatigue by pressure was found for confining pressures up to 300 MPa. The total dilatant strain at fracture was found to increase strongly with peak stress and decrease with increasing loading rate. Three types of cracking are shown to result in dilatancy: type 1, stress-induced cracking; type 2, stress corrosion cracking; and type 3, fatigue cracking. Rock fracture is sensitive to which type is prevalent.

AN INVESTIGATION OF THE CREEP‐FATIGUE‐ENVIRONMENT INTERACTION IN A Ni‐BASE SUPERALLOY

Abstract: — This work was conducted to study the significant increases in fatigue crack growth rates at low frequencies commonly observed at high temperature. Creep tests and fatigue tests at 0·01, 0·1 and 1·0 Hz were conducted at 650°C in air and in helium on samples of INCONEL alloy 718 in two different heat treatments. Creep crack growth rates were 50–100 times greater in air than in helium. Fatigue tests in helium showed little sensitivity to frequency, but tests in air showed considerable increases in crack growth rates at lower frequencies. The results indicate the air environment played a predominant role in both the creep and the fatigue tests. Oxygen diffusion into grain boundaries appeared responsible for the accelerated crack growth in air. An overaging heat treatment reduced the crack growth rates.

A non-linear uniaxial integral constitutive equation incorporating rate effects, creep and relaxation

Abstract: A previously proposed first order non-linear differential equation for uniaxial viscoplasticity, which is non-linear in stress and strain but linear in stress and strain rates, is transformed into an equivalent integral equation. The proposed equation employs total strain only and is symmetric with respect to the origin and applies for tension and compression. The limiting behavior for large strains and large times for monotonic, creep and relaxation loading is investigated and appropriate limits are obtained. When the equation is specialized to an overstress model it is qualitatively shown to reproduce key features of viscoplastic behavior. These include: initial linear elastic or linear viscoelastic response: immediate elastic slope for a large instantaneous change in strain rate normal strain rate sensitivity and non-linear spacing of the stress-strain curves obtained at various strain rates; and primary and secondary creep and relaxation such that the creep (relaxation) curves do not cross. Isochronous creep curves are also considered. Other specializations yield wavy stress-strain curves and inverse strain rate sensitivity. For cyclic loading the model must be modified to account for history dependence in the sense of plasticity.