Showing papers on "Creep published in 1995"

Strength of the lithosphere: Constraints imposed by laboratory experiments

Abstract: The concept of strength envelopes, developed in the 1970s, allowed quantitative predictions of the strength of the lithosphere based on experimentally determined constitutive equations. Initial strength envelopes used an empirical relation for frictional sliding to describe deformation along brittle faults in the upper portion of the lithosphere and power law creep equations to estimate the plastic flow strength of rocks in the deeper part of the lithosphere. In the intervening decades, substantial progress has been made both in understanding the physical mechanisms involved in lithospheric deformation and in refining constitutive equations that describe these processes. The importance of a regime of semibrittle behavior is now recognized. Based on data from rocks without added pore fluids, the transition from brittle deformation to semibrittle flow can be estimated as the point at which the brittle fracture strength equals the peak stress to cause sliding. The transition from semibrittle deformation to plastic flow can be approximated as the stress at which the pressure exceeds the plastic flow strength. Current estimates of these stresses are on the order of a few hundred megapascals for relatively dry rocks. Knowledge of the stability of sliding along faults and of the onset of localization during brittle fracture has improved considerably. If the depth to the bottom of the seismogenic zone is determined by the transition to the stable frictional sliding regime, then that depth will be considerably more shallow than the depth of the transition to the plastic flow regime. Major questions concerning the strength of rocks remain. In particular, the effect of water on strength is critical to accurate predictions. Constitutive equations which include the effects of water fugacity and pore fluid pressure as well as temperature and strain rate are needed for both the brittle sliding and semibrittle flow regimes. Although the constitutive equations for dislocation creep and diffusional creep in single-phase aggregates are more robust, few data exist for plastic deformation in two-phase aggregates. Despite the fact that localization is ubiquitous in rocks deforming both in brittle and plastic regimes, only a limited amount of accurate experimental data are available to constrain predictions of this behavior. Accordingly, flow strengths now predicted from laboratory data probably overestimate the actual rock strength, perhaps by a significant amount. Still, the predictions are robust enough that uncertainties in geometry, mineralogy, loading conditions and thermodynamic state are probably the limiting factors in our understanding. Thus, experimentally determined rheologies can be applied to understand a broad range of topical problems in regional and global tectonics both on the Earth and on other planetary bodies.

Creep and shrinkage prediction model for analysis and design of concrete structures - model B3

Abstract: A model for the characterization of concrete creep and shrinkage in the design of concrete structures is recommended. It is simplier, agrees better with the experimental data and is justified better theoretically than the previous models. The model complies with the general guidelines recently formulated by RILEM TC 107. Justification of the model and various refinements are to be published shortly in two parts.

Experimental constraints on the dynamics of the partially molten upper mantle: Deformation in the diffusion creep regime

Abstract: Experiments have been conducted to investigate the effect of melt on the creep behavior of water-free olivine aggregates deformed in the dislocation creep regime. The influence of the melt phase is modest at melt fractions less than ∼0.04. However, at melt fractions > 0.04, the creep rate of melt-added samples is enhanced by more than an order of magnitude relative to melt-free aggregates. This unexpectedly large influence of melt on strain rate arises because deformation occurs by grain boundary sliding (GBS) accommodated by a dislocation creep process. Four observations support this hypothesis. (1) The strain rate enhancement observed in the dislocation creep regime can be related to the stress concentration caused by the reduction in the solid-solid grain boundary area. (2) Both melt-free and melt-added samples exhibit strain rates indicating that deformation is limited by slip on (010)[100], the easiest slip system in olivine. (3) The GBS mechanism occurs near the transition between diffusion and dislocation creep. (4) Grains in specimens deformed in the GBS regime are not significantly flattened, even after ∼50% shortening. In melt-free aggregates, a transition from the GBS mechanism to dislocation creep limited by slip on (010)[001], the hardest slip system, is observed with an increase in grain size. A transition to (010)[001] limited creep was not observed for partially molten aggregates because grain growth was inhibited by the presence of melt. The results of this study indicate that the viscosity of the upper mantle may decrease by at least an order of magnitude if the retained melt fraction exceeds 0.04 or if the onset of melting results in a reduction in grain size and a concomitant transition from (010)[001] to (010)[100] limited creep.

Effect of grain size on mechanical properties of nanocrystalline materials

Abstract: The possibility of a dislocation mechanism in the deformation process of nanocrystalline materials is reviewed and analyzed. The present theoretical calculation, by taking the anisotropic characteristic of crystallographic symmetry and different choices of critical shear strength into account, results in a reasonable limit in grain size for applying dislocation pile-up theory to nanocrystalline materials. The deviation from the Hall—Petch relationship is rationalized in terms of a small number dislocation pile-up mechanism. A composite model is proposed to evaluate the strength of nanocrystalline materials. It is shown that this model can be used for interpreting the various cases observed in Hall—Petch studies. An analytical expression for assessing the creep rate of nanocrystalline materials by a diffusion mechanism, including triple line diffusion, is derived. It is predicted that the creep rate due to triple line diffusion will exhibit a stronger grain size dependence than that due to grain boundary diffusion.

Design for creep

Abstract: Keywords: structures ; etirement ; dilatation Reference Record created on 2005-11-18, modified on 2016-08-08

Current understanding of creep behaviour of near γ-titanium aluminides

Abstract: The literature dealing with creep of near γ-TiAl alloys is reviewed, including single and multiphase compositions. The characteristics of creep in the primary or transient, minimum strain rate or pseudo-steady state, and tertiary creep regimes are presented. Considerable experimental evidence indicates that a temperature-stress domain exists in which creep is controlled by dislocation climb. For multiphase compositions, the effect of microstructure on creep is discussed, with emphasis on the predominantly lamellar microstructure, which exhibits superior creep resistance. The influence of composition on creep resistance is presented in terms of intrinsic and extrinsic effects, and the associated role of β phase is discussed. Models for the creep of lamellar structures are reviewed. The current state of knowledge regarding creep of near γ-TiAl alloys precludes quantitative conclusions; however preliminary design of creep resistant microstructures is possible.

The relationship between indentation and uniaxial creep in amorphous selenium

Abstract: Ultralow load indentation techniques can be used to obtain time-dependent mechanical properties, termed indentation creep, of materials. However, the comparison of indentation creep data to that obtained during conventional creep testing is difficult, mainly due to the determination of the strain rate experienced by the material during indentation. Using the power-law creep equation and the equation for Newtonian viscosity as a function of stress and strain rate, a relationship between indentation strain rate,{center_dot}{epsilon}{sub {ital l}}={ital @};Dh/{ital h}, and the effective strain rate occurring during the indentation creep process is obtained. Indentation creep measurements on amorphous selenium in the Newtonian viscous flow regime above the glass transition temperature were obtained. The data was then used to determine that the coefficient relating indentation strain rate to the effective strain rate is equal to 0.09, or{center_dot}{epsilon}=0.0{center_dot}{epsilon}{sub {ital l}}.

Viscoelastic properties of polymers. 4. Thermorheological complexity of the softening dispersion in polyisobutylene

Abstract: The viscoelastic behavior of a high molecular weight polyisobutylene has been revisited to investigate the unique high-frequency peak (or shoulder) seen in the loss tangent behavior in the glass to rubber softening dispersion. Creep measurements made in the temperature range -74 to +27 °C were initially reduced to a curve, which, upon transformation from the time to the frequency domains, yielded a loss tangent peak with no hint of any shoulder. The original creep compliance curves were rescrutinized independently, and slight variations in the derivatives of the curves revealed the elusive high-frequency loss peak. Additional dynamic measurements were made to connect the transformed creep data to some of the original Fitzgerald, Grandine, and Ferry data [J.Appl.Phys. 1953, 24, 911]. The combined results cover over 9 decades of frequency. This extensive range revealed that the mechanisms contributing to the high-frequency peak have a different temperature dependence than that of those contributing to the main loss peak.

Fatigue rupture of wallaby tail tendons

Abstract: Wallaby tail tendons fail after repeated application of stresses much lower than would be needed to break them in a single pull. We show that this a fatigue phenomenon, distinct from the creep rupture that occurs after prolonged application of a constant stress. The two phenomena are disctinguished by experiments in which tensile stress is cycled at different frequencies, ranging from 1 to 50 Hz.

Rate laws for water‐assisted compaction and stress‐induced water‐rock interaction in sandstones

Abstract: Mineral-water interactions under conditions of nonhydrostatic stress play a role in subjects as diverse as ductile creep in fault zones, phase relations in metamorphic rocks, mass redistribution and replacement reactions during diagenesis, and loss of porosity in deep sedimentary basins. As a step toward understanding the fundamental geochemical processes involved, using naturally rounded St. Peter sand, we have investigated the kinetics of pore volume loss and quartz-water reactions under nonhydrostatic, hydrothermal conditions in flow-through reactors. Rate laws for creep and mineral-water reaction are derived from the time rate of change of pore volume, sand-water dissolution kinetics, and (flow rate independent) steady state silica concentrations, and reveal functional dependencies of rates on grain size, volume strain, temperature, effective pressure (confining minus pore pressure), and specific surface areas. Together the mechanical and chemical rate laws form a self-consistent model for coupled deformation and water-rock interaction of porous sands under nonhydrostatic conditions. Microstructural evidence shows a progressive widening of nominally circular and nominally flat grain-grain contacts with increasing strain or, equivalently, porosity loss, and small quartz overgrowths occurring at grain contact peripheries. The mechanical and chemical data suggest that the dominant creep mechanism is due to removal of mass from grain contacts (termed pressure solution or solution transfer), with a lesser component of time-dependent crack growth and healing. The magnitude of a stress-dependent concentration increase is too large to be accounted for by elastic or dislocation strain energy-induced supersaturations, favoring instead the normal stress dependence of molar Gibbs free energy associated with grain-grain interfaces.

The effect of soldering process variables on the microstructure and mechanical properties of eutectic Sn-Ag/Cu solder joints

Abstract: Fundamental understanding of the relationship among process, microstructure, and mechanical properties is essential to solder alloy design, soldering process development, and joint reliability prediction and optimization. This research focused on the process-structure-property relationship in eutectic Sn-Ag/Cu solder joints. As a Pb-free alternative, eutectic Sn-Ag solder offers enhanced mechanical properties, good wettability on Cu and Cu alloys, and the potential for a broader range of application compared to eutectic Sn-Pb solder. The relationship between soldering process parameters (soldering temperature, reflow time, and cooling rate) and joint microstructure was studied systemati-cally. Microhardness, tensile shear strength, and shear creep strength were measured and the relationship between the joint microstructures and mechani-cal properties was determined. Based on these results, low soldering tempera-tures, fast cooling rates, and short reflow times are suggested for producing joints with the best shear strength, ductility, and creep resistance.

Creep Behavior of Fiber-Reinforced Polymeric Composites: A Review of the Technical Literature:

Abstract: This report provides a review of the technical literature related to the creep behavior of fiber reinforced polymer (FRP) composites. The review presented here was directed toward those papers that...

Creep strengthening in a discontinuous SiC-Al composite

Abstract: High-temperature strengthening mechanisms in discontinuous metal matrix composites were examined by performing a close comparison between the creep behavior of 30 vol pct SiC-6061 Al and that of its matrix alloy, 6061 Al. Both materials were prepared by powder metallurgy techniques. The experimental data show that the creep behavior of the composite is similar to that of the alloy in regard to the high apparent stress exponent and its variation with the applied stress and the strong temperature dependence of creep rate. By contrast, the data reveal that there are two main differences in creep behavior between the composite and the alloy: the creep rates of the composite are more than one order of magnitude slower than those of the alloy, and the activation energy for creep in the composite is higher than that in the alloy. Analysis of the experimental data indicates that these similarities and differences in creep behavior can be explained in terms of two independent strengthening processes that are related to (a) the existence of a temperature-dependent threshold stress for creep, τ0, in both materials and (b) the occurrence of temperature dependent load transfer from the creeping matrix (6061 Al) to the reinforcement (SiC). This finding is illustrated by two results. First, the high apparent activation energies for creep in the composite are corrected to a value near the true activation energy for creep in the unreinforced alloy (160 kJ/mole) by considering the temperature dependence of the shear modulus, the threshold stress, and the load transfer. Second, the normalized creep data of the composite fall very close to those of the alloy when the contribution of load transfer to composite strengthening is incorporated in a creep power law in which the applied stress is replaced by the effective stress, the stress exponent,n, equals 5, and the true activation energy for creep in the composite,Q c , is equal to that in the alloy.

Creep rupture of wallaby tail tendons.

Abstract: The tail tendons from wallabies (Macropus rufogriseus) suffer creep rupture at stresses of 10 MPa or above, whereas their yield stress in a dynamic test is about 144 MPa. At stresses between 20 and 80 MPa, the time-to-rupture decreases exponentially with stress, but at 10 MPa, the lifetime is well above this exponential. For comparison, the stress on a wallaby tail tendon, when its muscle contracts isometrically, is about 13.5 MPa. Creep lifetime depends sharply on temperature and on specimen length, in contrast to strength and stiffness as observed in dynamic tests. The creep curve (strain versus time) can be considered as a combination of primary creep (decelerating strain) and tertiary creep (accelerating strain). Primary creep is non-damaging, but tertiary creep is accompanied by accumulating damage, with loss of stiffness and strength. 'Damage' is quantitatively defined as the fractional loss of stiffness. A creep theory is developed in which the whole of tertiary creep and, in particular, the creep lifetime are predicted from measurements made at the onset of creep, when the tendon is undamaged. This theory is based on a 'damage hypothesis', which can be stated as: damaged material no longer contributes to stiffness and strength, whereas intact material makes its full contribution to both.

Continuous Retardation Spectrum for Solidification Theory of Concrete Creep

Abstract: The basic creep of concrete is the time-dependent strain caused by a sustained stress in absence of moisture movements. It is the strain observed on sealed specimens. Similar to other properties of concrete, it is dependent on the age of concrete, as a consequence of long-time chemical reactions associated with the hydration of cement. This paper formulates the solidification theory with a continuous retardation spectrum, and shows how this spectrum can be readily and unambiguously identified from arbitrary measured creep curves and how it then can be easily converted to a discrete spectrum for numerical purposes. The identification of the continuous spectrum is based on Tschoegl's work on viscoelasticity of polymers. Attention is limited to basic creep.

A dislocation based criterion for the raft formation in nickel-based superalloys single crystals

Abstract: A dislocation based criterion is proposed for describing the raft formation in single crystals of nickel-based superalloys subject to creep at high temperature. Those of the precipitate faces exhibiting dislocations after the onset of creep are thought to expand by directional coarsening of γ′ precipitates. The proposed criterion predicts correctly the raft morphology for creep along cristallographic directions of high and low symmetry for alloys exhibiting a positive or a negative misfit. A diffusional mechanism based on the effect of creep dislocations on the local γ phase chemical composition is suggested to account for the observed relationship between dislocation activity and directional coarsening of precipitates.

Cavitation Contributes Substantially to Tensile Creep in Silicon Nitride

Abstract: During tensile creep of a hot isostatically pressed (HIPed) silicon nitride, the volume fraction of cavities increases linearly with strain; these cavities produce nearly all of the measured strain. In contrast, compressive creep in the same stress and temperature range produces very little cavitation. A stress exponent that increases with stress ({dot {var_epsilon}} {proportional_to} {sigma}{sup n}, 2 < n < 7) characterizes the tensile creep response, while the compressive creep response exhibits a stress dependence of unity. Furthermore, under the same stress and temperature, the material creeps nearly 100 times faster in tension than in compression. Transmission electron microscopy (TEM) indicates that the cavities formed during tensile creep occur in pockets of residual crystalline silicate phase located at silicon nitride multigrain junctions. Small-angle X-ray scattering (SAXS) from crept material quantifies the size distribution of cavities observed in TEM and demonstrates that cavity addition, rather than cavity growth, dominates the cavitation process. These observations are in accord with a model for creep based on the deformation of granular materials in which the microstructure must dilate for individual grains t slide past one another. During tensile creep the silicon nitride grains remain rigid; cavitation in the multigrain junctions allows the silicate tomore » flow from cavities to surrounding silicate pockets, allowing the dilation of the microstructure and deformation of the material. Silicon nitride grain boundary sliding accommodates this expansion and leads to extension of the specimen. In compression, where cavitation is suppressed, deformation occurs by solution-reprecipitation of silicon nitride.« less

Threshold creep behaviour of discontinuous aluminium and aluminium alloy matrix composites: An overview

Abstract: Several sets of creep data for aluminium and aluminium alloy matrix composites reinforced by silicon carbide particulates, silicon carbide whiskers or alumina short fibres are analysed. It is shown that for this class of discontinuous composites the threshold creep behaviour is inherent. Applying the concept of threshold stress, the true stress exponent of minimum creep strain rate of approximately 5 follows from the analysis even when the matrix solid solution alloy exhibits Alloy Class creep behaviour, for which the value of 3 for the true stress exponent is typical. The creep strain rate in the discontinuous aluminium and aluminium alloy matrix composites is shown to be matrix lattice diffusion controlled. The usually observed high values of the apparent stress exponent of creep strain rate and the high values of the apparent activation energy of creep are then rationalized in terms of the threshold creep behaviour. However, the origin of the threshold stress decreasing with increasing temperature but not proportional to the shear modulus in creep of discontinuous aluminium and aluminium alloy matrix composites is still awaiting identification. The creep-strengthening effect of silicon carbide particulates, silicon carbide whiskers and alumina short fibres is shown to be significant, although the particulates, whiskers and short fibres do not represent effective obstacles to dislocation motion.

Development of the MPC Omega Method for Life Assessment in the Creep Range

Abstract: A methodology for characterizing and assessing the behavior of materials after service in the creep range has been developed and used on a broad range of materials and components. It incorporates the results of relatively short-term tests and improved databases on materials properties. The essence of the method is the definition of a material performance characteristic which the authors refers to by the symbol {Omega}{sub p}. This coefficient effectively describes the rate at which a material`s ability to resist stress is degraded by strain. While {Omega}{sub p} is a function of stress, temperature, and mode of loading, it is amenable to parametric representation and is, therefore, useful in predicting life and strain accumulation. Time to failure and total accumulated strain are shown to be consequences of a characterizing strain rate, as defined herein, and an appropriate {Omega}{sub p} for the operating conditions and geometry of interest. Accumulated strain, future strain, current creep rate, remaining life, total damage, and damage rate are among the quantities which are easily calculated. The development of the method employs and extends the concepts of Larson-Miller, Monkman-Grant, Robinson, Theta Projection, Kachanov, and Norton.

Wrinkling of the oxide scale on an aluminum-containing alloy at high temperatures

Abstract: To survive in an oxygen-bearing gas, an alloy grows an oxide scale to cover itself. The scale thickens slowly at a high temperature, until the compressive stress generated by oxidation and cooling causes it to spall off. A widely known model assumes that the scale-alloy interface has unbonded flaws, which allow the scale to buckle under the compression, followed by delamination and cracking. For the scale to buckle under a typical stress, this model requires flaws with radii exceeding ten times the scale thickness. In general, such flaws are too large to be produced during oxidation. This paper proposes a different model. It is commonly observed that wrinkles appear at the high temperature, caused by the oxidation-induced compressive stress. Initially the interface remains bonded, and the alloy conforms to the shape change by creep or diffusion. If the high temperature is maintained long enough, voids will grow on the interface, and the scale will slide, fold and crack. Cooling can intervene at any point; for example, even when the scale only wrinkles without extensive voiding, large thermal stress on the wavy interface may cause the scale to spall off. The initial wrinkling is analyzed as a time-dependent bifurcation by using a variational principle. The base alloy is assumed to conform by metal diffusion along the interface, motivated by the strain energy release due to wrinkling. The results are discussed in connection with experimental observations.

Tensile creep behaviour of a silicon carbide-based fibre with a low oxygen content

Abstract: The high-temperature mechanical behaviour and microstructural evolution of experimental SiC fibres (Hi-Nicalon) with a low oxygen content (<0.5 wt%) have been examined up to 1600 °C. Comparisons have been made with a commercial Si-C-O fibre (Nicalon Ceramic Grade). Their initial microstructure consists of β-SiC crystallites averaging 5–10 nm in diameter, with important amounts of graphitic carbon into wrinkled sheet structures of very small sizes between the SiC grains. The fall in strength above 800 °C in air is related to fibre surface degradation involving free carbon. Crystallization of SiC and carbon further develops in both fibres subject to either creep or heat treatment at ∼1300 °C and above for long periods. The fibres are characterized by steady state creep and greater creep resistance (one order of magnitude) compared to the commercial Nicalon fibre. The experimental fibre has been found to creep above 1280 °C under low applied stresses (0.15 GPa) in air. Significant deformations (up to 14%) have been observed, both in air and argon above 1400 °C. The stress exponents and the apparent activation energies for creep have been found to fall in the range 2–3, both in air and argon, and in the range 200–300 kJ mol−1 in argon and 340–420 kJ mol−1 in air. The dewrinkling of carbon layer packets into a position more nearly aligned with the tensile axis, their sliding, and the collapse of pores have been proposed as the mechanisms which control the fibre creep behaviour.

Mesh-Dependence in Local Approach to Creep Fracture

Abstract: Mesh-dependence and its regularization in the local approach to creep fracture based on continuum damage mechanics and finite element method are discussed. The essential causes of the mesh-dependence are elucidated first in some detail. Then, the process of damage localization and its effects on the mesh-dependence are discussed by performing finite element analysis of a plate in uniform state of stress. The stress sensitivity in damage evolution equations is shown to be one of the major causes of the mesh-dependence. Finally, besides the non-local damage theory often employed in local approach, three new possibilities, including the stress limitation method and that of the modified stress sensitivity in the damage evolution equation, to secure the convergent results are proposed. The validity and the limitations of these methods are elucidated and compared with each other by analyzing creep fracture process of an axisymmetric thick-walled tube.