Showing papers on "Creep published in 1997"

Microprestress-Solidification Theory for Concrete Creep. I: Aging and Drying Effects

Abstract: A new physical theory and constitutive model for the effects of long-term aging and drying on concrete creep are proposed. The previously proposed solidification theory, in which the aging is explained and modeled by the volume growth (into the pores of hardened portland cement paste) of a nonaging viscoelastic constituent (cement gel), cannot explain long-term aging because the volume growth of the hydration products is too short-lived. The paper presents an improvement of the solidification theory in which the viscosity of the flow term of the compliance function is a function of a tensile microprestress carried by the bonds and bridges crossing the micropores (gel pores) in the hardened cement gel. The microprestress is generated by the disjoining pressure of the hindered adsorbed water in the micropores and by very large and highly localized volume changes caused by hydration or drying. The long-term creep, deviatoric as well as volumetric, is assumed to originate from viscous shear slips between the opposite walls of the micropores in which the bonds or bridges that cross the micropores and transmit the microprestress break and reform. The long-term aging exhibited by the flow term in the creep model is caused by relaxation of the tensile microprestress transverse to the slip plane. The Pickett effect (drying creep) is caused by changes of the microprestress balancing the changes in the disjoining pressure, which in turn are engendered by changes of the relative humidity in the capillary pores. Numerical implementation, application and comparison with test data is relegated to a companion paper that follows in this issue.

Room temperature creep behavior of nanocrystalline nickel produced by an electrodeposition technique

Abstract: Deformation processes of nanocrystalline (6–40 nm) nickel produced by an electrodeposition technique were studied. First, the results of unidirectional tensile tests were discussed with respect to the deviation from the Hall-Petch relationship. It was suggested that such a mechanical behavior exhibited by nanocrystalline materials could be described by a composite model proposed previously. Further experimental work on static and dynamic creep tests under the load control condition showed that nanocrystalline nickel electrodeposits exhibited a significant room temperature creep behavior. It appeared that grain boundary sliding and diffusive matter transport within the intercrystalline region played an important role in terms of deformation mechanisms of nanocrystalline materials. The contributions of dynamic creep to stress-strain behavior and, in turn, to the assessment of the Hall-Petch relationship for nanocrystalline materials are discussed.

Creep, stress relaxation, and plastic deformation in Sn-Ag and Sn-Zn eutectic solders

Abstract: Because of the high homologous operation temperature of solders used in electronic devices, time and temperature dependent relaxation and creep processes affect their mechanical behavior In this paper, two eutectic lead-free solders (965Sn-35Ag and 91Sn-9Zn) are investigated for their creep and stress relaxation behavior The creep tests were done in load-control with initial stresses in the range of 10-22 MPa at two temperatures, 25 and 80°C The stress relaxation tests were performed under constant-strain conditions with strains in the range of 03-24% and at 25 and 80°C Since creep/relaxation processes are active even during monotonie tensile tests at ambient temperatures, stress-strain curves at different temperatures and strain rates provide insight into these processes Activation energies obtained from the monotonic tensile, stress relaxation, and creep tests are compared and discussed in light of the governing mechanisms These data along with creep exponents, strain rate sensitivities and damage mechanisms are useful for aiding the modeling of solder interconnects for reliability and lifetime prediction Constitutive modeling for creep and stress relaxation behavior was done using a formulation based on unified creep plasticity theory which has been previously employed in the modeling of high temperature superalloys with satisfactory results

Nanoindentation creep of single-crystal tungsten and gallium arsenide

Abstract: An extensive study of indentation creep on the nanometre scale has been made on single-crystal indium, tungsten and gallium arsenide. We use the force modulation technique which gives a direct measure of contact stiffness and, being insensitive to thermal drift, allows the accurate observation of creep in small indents to be carried out over long time periods: We show that strain rate indices similar to those for macroscopic creep can be obtained for indium. Stress relaxation negative creep is also observed in a manner similar to macroscopic tests. Indentation of tungsten and gallium arsenide shows a distinct pop-in at a critical load, before which the deformation is essentially elastic and after which it is elastoplastic with significant dislocation multiplication. The creep behaviour is quite different before and after pop-in, clearly demonstrating the role of mobile dislocations in creep, even in nanometre-sized volumes of materials.

Vortex pinning and creep in high-temperature superconductors with columnar defects

Abstract: Columnar defects introduced by irradiation with very energetic heavy ions are the most effective pinning centres for flux lines in high-temperature superconductors. This correlated disorder generates large increases in the critical current densities and expansion of the irreversible regime in and the various Bi- and Tl-based compounds. In single crystals and thin films, the pinning enhancement is strongly angular-dependent, and maximizes when the applied magnetic field is parallel to the amorphous latent tracks. In contrast, in the much more anisotropic Bi- and Tl- based materials this unidirectional anisotropy is very small due to the quasi two-dimensional character of the vortices. Some of the extensive experimental studies on this topic are reviewed. Measurements of the temperature, field and defect density dependence of the persistent currents and their time relaxation in YBCO are presented. The analysis of these results based on recent theoretical models permits the identification of various pinning and creep regimes. Studies in other compounds, including technologically relevant Bi-based tapes, are summarized. The influence of the angular dispersion (splay) of the tracks on vortex pinning and creep is discussed.

On the creep behaviour of grain boundary engineered nickel 1

Abstract: Grain boundaries described by low-Σ CSL relationships (ie Σ ≤ 29) have previously been shown to be resistant to grain boundary sliding, cavitation and fracture The present work reports on efforts to reduce creep rates in conventional polycrystalline nickel by increasing the frequency with which these ‘special’ interfaces occur in the microstructure Suitable thermomechanical processing was employed to enhance the frequency of ‘special’ grain boundaries (Σ ≤ 29) in 9999% Ni from 13 to 66%, resulting mostly from the formation of twins (23) and crystallographically-related 29 and 227 boundaries This 53% increase in the fraction of low-2 boundaries produced reductions of 16-fold in the steady-state creep rate and six-fold in the primary creep strain Microstructures having ‘special’ boundary frequencies of less than 50% exhibited significant cavitation almost exclusively along ‘random’ boundaries (ie Σ > 29) at or near triple points No gross cavitation was evident in microstructures containing ‘special’ boundary fractions of 66% Such improvements in creep properties provide considerable promise for the application of a ‘grain boundary engineering’ approach to developing interfacial materials for structural applications

Nonlinear mechanical response of high density polyethylene. Part I: Experimental investigation and model evaluation

Abstract: The nonlinear behavior of high density polyethylene (HDPE) is investigated for samples cut from thick-walled HDPE pipe. Extensive experimental work has been performed to characterize the non-linear time-dependent response of the material tested under uniaxial compression. Tests were conducted under conditions of constant strain rate, creep, stress relaxation, constant loading rate, abrupt change of strain rate, creep-recovery, cyclic strain rate, and various combinations of these loading conditions. Creep and stress relaxation response after strain reversal and the effect of the transient response on the following stress-strain behavior is examined. Permanent strains for the test specimens and their dependence on loading histories are investigated. Specimens cut at various orientations from the pipe are used to quantify the small amounts of local anisotropy in the pipe specimen. The experimental work has been used to develop both nonlinear viscoelastic (NVE) and viscoplastic (VP) constitutive models in a companion paper. Both the test results and the corresponding model predictions are reported in this paper. It is found that the VP model reproduces the nonlinear viscoelastic-viscoplastic behavior of HDPE very well provided that the current strain is not below the maximum strain imposed (there is no strain reversal). The NVE model predicts the material behavior reasonably well for some loading conditions, but inadequately for others.

Prediction of Flexural Fatigue Strength of CRFP Composites under Arbitrary Frequency, Stress Ratio and Temperature:

Abstract: This paper deals with fatigue strength of a class of CFRP laminates satisfying certain hypotheses and proposes a prediction method of the fatigue strength at an arbitrary combination of frequency, stress ratio and temperature. Three-point bending tests were conducted for satin-woven CFRP laminates T400/3601 under static, creep and fatigue loadings. The experimental data support the prediction method and the hypotheses.

Effects of lamellar spacing, volume fraction and grain size on creep strength of fully lamellar TiAl alloys

Abstract: Lamellar spacing, volume fraction of constituent phases and grain size are the major structural factors to be controlled in alloy design of fully lamellar TiAl alloys. Creep tests were carried out on binary TiAl alloys with fully lamellar structure, to elucidate effects of the three structural factors on their creep strength. Lamellar alloys give better creep strength than the constituent single phases at higher stresses, due to their higher yield stress. Creep rate of lamellar alloys decreases with decreasing lamellar spacing, suggesting that a fine lamellar structure is effective in improving creep strength. The decrease is remarkable at high stresses. At low stresses however, the difference in creep rate diminishes, due to the increasing contribution of dynamic recrystallization and interface sliding. Creep rate is insensitive to volume fraction of the constituent phases and aluminum concentration is free in alloy design of TiAl alloys as far as creep strength is concerned. Creep rate is also independent of grain size d when d>100 μm. Because of easy dynamic recrystallization in the lamellar alloys, creep rate may increase when d

Acoustic emission in single crystals of ice

Abstract: The dislocation dynamics during the creep deformation of single crystals of ice Ih was studied using acoustic emission (AE) measurements. The AE activity was recorded during uniaxial compression and torsion creep tests. The results were interpreted in terms of dislocation dynamics with the help of an AE source model relating the amplitude of an acoustic event to the number of dislocations involved in the event and to their velocity. This model was first validated by a comparison between the global AE activity and the global strain rate. Then, it was possible to evaluate the density of moving dislocations during creep deformation. Two regimes were revealed. Without significant polygonization, the density of mobile dislocations, deduced from AE, was proportional to the stress, but increased much faster after polygonization, in agreement with theoretical arguments. Finally, the power law distributions observed for AE amplitudes, the slow driving process, the very large number of interacting dislocations invo...

Development of Oxide Dispersion Strengthened Ferritic Steels for FBR Core Application, (I): Improvement of Mechanical Properties by Recrystallization Processing

Abstract: As to an oxide dispersion strengthened (ODs) ferritic steel cladding as the promising candidate for long-life core materials of the fast reactors, previously fabricated claddings had inferior internal creep rupture strength in hoop direction and inferior formability due to less ductility Those unexpected features of ODs claddings are substantially ascribed to the needle-like grain structure excessively elongated along the forming direction Controlling the grain morphology by applying the recrystallization method to ODs ferritic steel made possible to improve those inferior features The ranges of Y2O3 and excessive oxygen contents for possibly cold-rolling and recrystallization were revealed, and the effects of extruded temperature and deformation texture on recrystallization characteristics were evaluated The recrystallized ODs ferritic steel showed superior internal creep rupture strength and ductility It was demonstrated from those results that cold-rolling manufacturing of ODs cladding at room tem

Viscoelastic material functions of noncolloidal suspensions with spherical particles

Abstract: We have characterized various steady and time-dependent material functions of suspensions of a non-Newtonian binder, poly(dimethyl siloxane), incorporated with 10%–60% by volume of hollow and spherical glass beads The material functions included storage and the loss moduli, shear stress and first normal stress difference growth and relaxation, relaxation modulus upon step strain and creep and recovery behavior Both constant shear stress and shear rate experiments were carried out using multiple rheometers over a broad temperature range (−35 to 40 °C) while following sample fracture and wall slip effects With increasing volume fraction, φ, of the noncolloidal particles, the strain range, over which linear viscoelastic behavior is observed, became narrower and the relaxation time of the suspension increased Increasing solid content gave rise to the development of the yield stress and the dependence of large amplitude oscillatory shear properties on time and deformation history The yield stress values i

Microstructural stability of creep resistant alloys for high temperature plant applications

Abstract: Materials used for high temperature components in power generation, aerospace, chemical and process plant applications undergo changes in their microstructure during operation which will affect their mechanical properties. The 28 papers presented discuss the microstructural changes occurring in high temperature materials during service exposure and the processes and mechanisms leading to the observed degradation of their mechanical properties.

Toward an experimental study of deep mantle rheology: A new multianvil sample assembly for deformation studies under high pressures and temperatures

Abstract: A new sample assembly for the multianvil high-pressure apparatus has been developed that results in high-strain plastic deformation at high pressures and temperatures with minimal deformation during the initial pressurization stage. In this assembly, the sample is a thin disk which is sandwiched between two pistons and oriented at 45° to their long axis. The sample and pistons are surrounded by a Pt tube and a polycrystalline MgO cylinder. Upon pressurization, a uniaxial stress develops because of the anisotropy of mechanical properties. Deformation during initial pressurization, which occurred in previous studies, is minimized by locating soft materials at the ends of the pistons and by the simple shear deformation geometry (as opposed to uniaxial compression) that allows sliding at the sample-piston interfaces at low pressures. Large plastic strains, up to ∼100% shear strain, have been achieved in (Mg,Fe) 2 SiO 4 phases at high pressures (up to 15 GPa) and high temperatures (up to 1900 K). A theoretical analysis has been made to evaluate the relative contributions to sample deformation from the relaxation of elastic strain in the sample column and from continuing advancement of the multianvil guide blocks. The observed dependence of strain on time, pressure and temperature suggests that deformation in the present experiments occurred mostly as a relaxation process rather than at a constant strain rate caused by continuous piston movement. A comparison of the creep strength of olivine inferred from the strain relaxation data at ∼15 GPa and ∼1900 K with low-pressure data provides an estimate of the activation volume for creep of V* = 14 ( ±1) x 10 -6 m 3 mol -1 . The theoretical analysis shows that constant strain rate deformation could result from the advancement of the guide blocks after complete stress relaxation, although the total strain will be much less than that attained in the relaxation process. Possible applications of this technique to studies of high-pressure rheology and deformation microstructures in high-pressure minerals are discussed, and strategies for future deformation experiments under high pressures and temperatures are proposed.

The role of coincidence-site-lattice boundaries in creep of Ni-16Cr-9Fe at 360 °C

Abstract: The objective of this study is to understand and quantify the role of the coincidence-site-lattice boundary (CSLB) population on creep deformation of Ni-16Cr-9Fe at 360 °C. It is hypothesized that an increase in the CSLB population decreases the annihilation rate of dislocations in the grain boundary, leading to an increase in the internal stress and a decrease in the effective stress. The result is a reduction in the creep strain rate. The role of CSLBs in deformation is, thus, to increase the internal stress by trapping run-in lattice dislocations at the grain boundaries as extrinsic grain boundary dislocations (EGBDs), creating backstresses on following dislocations rather than annihilating them, as in the case of high-angle boundaries (HABs). The hypothesis was substantiated by showing (1) that dislocation absorption kinetics differ substantially between a CSLB and an HAB, and (2) that the CSLB fraction strongly affects the internal stress in the solid. Dislocation absorption kinetics were measured by comparing EGBD density in transmission electron microscopy (TEM). Results showed that CSLBs contain an EGBD density which is 3 times higher than HABs at 1.25 pct strain. Internal stress was measured by the stress dip test and was found to be ≈ 30 MPa higher in the CSLB-enhanced sample. Steady-state creep rates of Ni-16Cr-9Fe in 360 °C argon were also found to be strongly affected by the grain boundary character distribution. Increasing the CSLB fraction by approximately a factor of 2 resulted in a decrease in steady-state creep rates by a factor of 8 to 26 in coarse-grain (330 µm) samples and a factor of 40 to 66 in small-grain (35 µm) samples. It is postulated that annihilation of EGBDs only occurs at triple lines where at least two HABs intersect. By using a geometric relationship to evaluate the probability of EGBDs annihilating at a triple line, the model predicts a non-linear dependence of the creep rate with CSLB fraction, yielding excellent correlation with measurement. The model provides a physical basis for measurements which show that increasing the CSLB fraction by only moderate amounts can greatly reduce the steady-state creep rate in Ni-16Cr-9Fe.

A Damage Mechanics Treatment of Creep Failure in Rock Salt

Abstract: Recent progress in the formulation of a constitutive model for describing coupled creep and damage development in rock salt is summarized. The constitutive model is based on the assumption that both dislocation slip and creep damage in the form of microcracks with possible wing-tips contribute to the macroscopic inelastic strain rate. The relevant kinetic equations, flow law, and damage evolution equation are presented. Evaluations are made of the coupled creep and fracture model against the measured creep response of clean salt from the Waste Isolation Pilot Plant (WIPP) site. In addition, the development of creep damage and the rupture of WIPP salt subjected to either triaxial compression or indirect tension have been analyzed to evaluate several creep rupture criteria.

Modelling precipitation sequences in power plant steels Part 1 – Kinetic theory

Abstract: The ability of steels to resist creep deformation depends on the presence in the microstructure of carbides and intermetallic compounds which precipitate during tempering or during elevated temperature service. The precipitation occurs in a sequence which leads towards thermodynamic equilibrium. The present paper deals with an extension of the Johnson-Mehl-Avrami theory for overall transformation kinetics. The modification permits the treatment of more than one precipitation reaction occurring simultaneously, afeature which isfound to be essential for representing the reactions observed experimentally in a wide range of secondary hardening steels.