Showing papers on "Creep published in 1994"

Tensile-compressive yield asymmetries in high strength wrought magnesium alloys

Abstract: Owing to magnesium's very low density (of 17 g/cm[sup 3]) there is potential for producing Mg alloys with higher specific strengths than those of existing aluminium alloys for applications where weight savings are at a premium Magnesium alloys can also have greater creep resistance and superior elevated temperature specific tensile properties than those of Al However, the hexagonal crystal structure of magnesium severely limits its available slip systems and preferred orientations can develop during working processes, slip occurring predominantly on the basal planes which leads to their alignment parallel to the direction of metal flow As a geometric consequence of the c/a ratio of magnesium being less than ideal, twinning can theoretically only occur in compression, parallel to the basal planes, which would be expected to result in there being large asymmetries between the tensile and compressive yield stresses in textured wrought products The compressive yield stresses in wrought magnesium alloys have been previously found in forgings to the 06--07 times those of the tensile Such an asymmetric yield behavior, resulting in a poor compressive yield stress, would prohibit many potential applications for wrought Mg alloys The aim of this paper is to investigate how significant the problem of yieldmore » asymmetry can be in two typical high strength wrought Mg alloys, the Mg-Y-Nd mischmetal based alloy, WE54, and the Mg-Zn-Cu based alloy, ZC71« less

A unified approach to grain boundary sliding in creep and superplasticity

Abstract: Rachinger grain boundary sliding is a characteristic of high temperature deformation in both creep when the grain size is large ( d > λ ) and superplasticity when the grain size is small ( d λ ), where d and λ are the grain size and the subgrain size, respectively. An analytical procedure is used to determine the rate equation for Rachinger sliding when d > λ . Data for superplastic metals are examined to give the rate equation for Rachinger sliding when d λ . A unified model is developed for Rachinger sliding at all grain sizes, where the rate of sliding is controlled by the rate of accomodation through intragranular slip. It is demonstrated that the predictions of this model are in good agreement with experimental data under both creep and superplastic conditions.

High Temperature Component Life Assessment

Abstract: Introduction. Processes of deformation and fracture at high temperatures. Stress analysis of uncracked bodies. Stress analysis of cracked bodies. Models for creep crack initiation and growth. Creep-fatigue crack growth. Experimental determinations of high temperature crack growth. Practical applications. Index.

Drying creep of concrete: constitutive model and new experiments separating its mechanisms

Abstract: A new experimental method which allows the direct separation of the components of drying creep due to microcracking and stress-induced shrinkage is developed, demonstrated and validated. The basic idea is to compare the curvature creep of beams subjected to the same bending moment but very different axial forces. The results confirm that drying creep has two different sources: microcracking and stress-induced shrinkage. The latter increases continuously, whereas the former first increases and then decreases. The test results are fitted using a finite element model. The results validate the present model for drying creep. The microcracking is described by an established model, and the free (unrestrained) shrinkage of a material element is shown to depend approximately linearly on the humidity drop.

Equilibrium pore surfaces, sintering stresses and constitutive equations for the intermediate and late stages of sintering—II. Diffusional densification and creep

Abstract: Constitutive equations for the intermediate and late stages of sintering are derived assuming grain boundary diffusion to be the controlling transport mechanism. The surface of the pore space is assumed to be in equilibrium. Numerical results for the shape of equilibrium surfaces are taken from a companion paper [J. Svoboda, H. Riedel and H. Zipse, Acta metall. mater.42, 435 (1994)]. For open porosity the contacts between grains turn out to be nearly circular in all cases, so that the diffusion problem in the contact areas can be solved readily. The macroscopic response of the powder compact can then be built up of the mechanical and the surface tension forces acting across the contacts. The analysis is carried out for a random, or homogeneous distribution of contacts around each particle, and for simple cubic, body-centered cubic, and face-centered cubic grain arrangements. The cubic viscosities are averaged using various methods, the best of which appears to be the self-consistent method.

Mechanisms-based creep constitutive equations for an aluminium alloy

Abstract: A number of mechanisms-based constitutive equations were assessed in an effort to describe the creep behaviour of an aluminium alloy at 150°C. It was found that a sinh function of stress, r...

New, lead-free solders

Abstract: Two specific examples of the new lead-free alloys are described. In the Sn-3.5%Ag-l%Zn alloy (m.p.~217°C), the eutectic precipitate morphology is refined by the relatively small amount of zinc addition and as a result, a high-strength, high-ductility solder with significantly improved creep resistance is obtained. As a temperature equivalent drop-in replacement for the Pb-Sn eutectic alloy (m.p.~183°C), Sn-Zn-In based alloys (m.p.~185°C) have been developed. The addition of indium to the Sn-Zn binary system improves the wetting characteristics of the alloy and lowers the melting temperature.

Development and evaluation of the strategic highway research program measurement and analysis system for indirect tensile testing at low temperatures

Abstract: The indirect tensile testing mode is a practical method for obtaining tensile properties of bituminous mixtures from cylindrical specimens. An indirect tensile creep testing and analysis system is shown analytically to be superior to conventional methods in obtaining fundamental properties of asphalt mixtures at low temperatures in a previous study. Before extensive testing of a wide range of mixtures, however, the ability of the system to obtain accurate, reliable results was unknown. Since the introduction of the new measurement and analysis system, hundreds of field cores and laboratory compacted specimens have been tested and analyzed. Improvements to the system resulting from the testing experience are described, including how analyses of data collected from these tests were used to evaluate the accuracy of the system. Poisson's ratio values obtained with the new system were found not only to be reasonable but also necessary for the accurate determination of creep compliance of the asphalt mixtures tested--a result consistent with analytical findings. In addition, creep compliances obtained from mixtures of well-known components were found to agree with expected values and trends. Input of properties measured on 20 field sections with the new system into a mechanics-based thermal cracking model resulted in accurate predictions of thermal cracking in the field. The measurement and analysis system presented is part of the test selected by the Strategic Highway Research Program to support new mixture specifications for thermal cracking; it has been incorporated into the SUPERPAVE software.

Tensile Creep Behavior of Alumina/Silicon Carbide Nanocomposite

Abstract: Tensile creep and creep rupture behaviors of alumina/17 vol% silicon carbide nanocomposite and monolithic alumina were investigated at 1,200 to 1,300 C and at 50 to 150 MPa. Compared to the monolithic alumina, the nanocomposite exhibited excellent creep resistance. The minimum creep rate of the nanocomposite was about three orders of magnitude lower and the creep life was 10 times longer than those of the monolith. The nanocomposite demonstrated transient creep until failure, while accelerated creep was observed in the monolith. It was revealed that rotating and plunging of intergranular silicon carbide nanoparticles into the alumina matrix increased the creep resistance with grain boundary sliding.

On Brinell and Boussinesq indentation of creeping solids

Abstract: As an alternative to traditional tensile testing of materials subjected to creep, indentation testing is examined. Axisymmetric punches of shapes defined by smooth homogeneous functions are analysed in general at power law behaviour both from a theoretical and a computational point of view. It is first shown that by correspondence to nonlinear elasticity and self-similarity the problem to determine time-dependent properties admits reduction to a stationary one. Specifically it is proved that the creep rate problem posed depends only on the resulting contact area but not on specific punch profiles. As a consequence the relation between indentation depth and contact area is history independent. So interpreted, the solution for a flat circular cylinder (Boussinesq) is not only of intrinsic interest but serves as a reference solution to generate results for various punch profiles. This is conveniently carried out by cumulative superposition and in particular ball indentation (Brinell) is analysed in depth. A carefully designed finite element procedure based on a mixed variational principle is used to provide a variety of explicit results of high accuracy pertaining to stress and deformation fields. Universal relations for hardness at creep are proposed for Boussinesq and Brinell indentation in analogy with the celebrated formula by Tabor for indentation of strain-hardening plastic materials. Quantitative comparison is made with a diversity of experimental data attained by earlier writers and the relative merits of indentation strategies are discussed.

Ceramic composite material

Abstract: A novel ceramic composite material consisting of single crystal α-Al₂O₃ and polycrystal Y₃Al₅O₁₂ and a ceramic composite material consisting of polycrystal α-Al₂O₃ and polycrystal Y₃Al₅O₁₂, each having a high mechanical strength and creep behavior particularly at high temperature are provided.

Overview no. 117 The structure of constitutive laws for the sintering of fine grained materials

Abstract: This paper examines the general structure of constitutive laws for the sintering of fine grained materials for situations where power-law creep and grain-boundary diffusion are the dominant mechanisms of deformation and densification. The description of grain-boundary diffusion accepts an interface reaction. Constitutive laws for general multiaxial stress states are expressed in terms of scalar potential functions. Bounds to the potential are obtained which take into account the coupling between power-law creep and diffusion.

In situ measurement of the glass transition temperature of polymers with compressed fluid diluents

Abstract: In situ measurement of the creep compliance of poly (methyl methacrylate) (PMMA) and poly (ethyl methacrylate) (PEMA), equilibrated with a pressurized CO 2 phase, is used to determine the T g . Corrections due to dilation of the polymer by CO 2 as well as the buoyancy are assessed. Both polymer systems exhibit a recently discovered phenomenon, retrograde vitrification, in which a liquid polymer becomes a glass with an increase in temperature. The experimental results are predicted semi-quantitatively in terms of the temperature and pressure effects on the solubility of the compressed fluid in the polymer

Rheological and microstructural transitions in colloidal crystals

Abstract: The flow of suspensions of 229-nm, charge-stabilized, polystyrene latex particles is directly linked with suspension microstructure. Transitions from long-range orientational order at rest to a steady-state, polycrystalline phase are associated with exceeding the dynamic yield stress. Application of stresses larger than a critical value nucleates a sliding layer microstructure which is accompanied by a discontinuous drop in suspension viscosity. Over a range of shear rates, polycrystalline and sliding layer microstructures coexist. At elevated shear rates, stresses and microstructures fluctuate in time. Shear thickening accompanies the complete loss of long-range orientational order. The transitions in rheology and microstructure are reversible except at very high shear rates where shear thickening accompanies irreversible flocculation. 48 refs., 20 figs., 1 tab.

Mathematical Modelling of Inelastic Deformation

Abstract: Preliminaries Mechanisms of plasticity and creep Thermodynamics of Elastic-inelastic Deformation Elementary models of small Deformations Computational aspects Fraction models for increasing complexity Large strain plasticity

High-temperature deformation of 6061 Al

Abstract: The creep behavior of powder metallurgy (PM) 6061 Al, which has been used as a metal matrix alloy in the development of discontinuous silicon carbide reinforced aluminum (SiCAl) composites, has been studied over six orders of magnitude of strain rate. The experimental data show that the steady-state stage of the creep curve is of short duration; that the stress dependence of creep rate is high and variable; and that the temperature dependence of creep rate is much higher than that for self-diffusion in aluminum. The above creep characteristics are different from those documented for aluminum based solid-solution alloys but are similar to those reported for discontinuous SiCAl composites and dispersion-strengthened (DS) alloys. Analysis of the experimental data shows that while the high stress dependence of creep rate in 6061 Al, like that in DS alloys, can be explained in terms of a threshold stress for creep, the strong temperature dependence of creep rate in the alloy is incompatible with the predictions of available threshold stress models and theoretical treatments proposed for DS alloys.

Magnetic field and force analysis of high T/sub c/ superconductor with flux flow and creep

Abstract: This paper describes a new method for the magnetic force analysis of a high T/sub c/ superconductor based on the flux flow and creep model. The introduction of the artificial conductivity, which is used in the conventional method, is not needed. The CPU time requirement of the calculations is considerably lower than that in the case of the conventional method. Thereby the vibration of a levitated permanent magnet was numerically analyzed by taking into account the flux flow and creep. >

The steady-state crystal size of deforming ice

Abstract: Previous studies have established that, together with the development of a preferred crystal-orientation fabric in ice undergoing creep deformation to high strains, there also develops a tertiary equilibrium crystal size, ie the crystal size, rather than affecting the creep rate, is a result of the deformation to large strains Equilibrium crystal size is considered here as a “balance” between crystal growth with time as a function of temperature and crystal change as a result of temperature dependent deformation The temperature effects in these two processes (Arrhenius relation) are similar and consideration of the activation energies for the two processes indicates that it may be appropriate to cancel them, yielding a dependence of equilibrium crystal size on stress alone The results from laboratory experiments of steady-state crystal size plotted as a function of stress support the above proposition The possibility of using the relationships between steady-state crystal size and deviatoric stress as a polar ice-mass piezometer is discussed

Step model of solution-precipitation creep

Abstract: Polycrystals which have intergranular thin liquid film deform by solution-precipitation creep. The strain rate is expressed by the velocity of surface step and its density. The velocity of step is proportional to the solubility and stress. It is controlled by resistance at kink, diffusion in the adsorption layer, and diffusion in the liquid film. The density of step is, (a) constant; (b) proportional to stress; or (c) increased by two dimensional nucleation. The constitutive equations were derived for diffusion-controlled and interface-reaction controlled creep. The step model was applied to creep and superplasticity of fine-grained ceramics with intergranular liquid film, and found to successfully explain the experimental results.

Indentation analysis of biphasic articular cartilage : nonlinear phenomena under finite deformation

Abstract: The nonlinear indentation response of hydrated articular cartilage at physiologically relevant rates of mechanical loading is studied using a two-phase continuum model of the tissue based on the theory of mixtures under finite deformation. The matrix equations corresponding to the governing mixture equations for this nonlinear problem are derived using a total Lagrangian penalty finite element method, and solved using a predictor-corrector iteration within a modified Newton-Raphson scheme. The stress relaxation indentation problem is examined using either a porous (free draining) indenter or solid (impermeable) indenter under fast and slow compression rates. The creep indentation problem is studied using a porous indenter. We examine the finite deformation response and compare with the response obtained using the linear infinitesimal response. Differences between the finite deformation response and the linear response are shown to be significant when the compression rate is fast or when the indenter is impermeable. The finite deformation model has a larger ratio of peak-to-equilibrium reaction force, and higher relaxation rate than the linear model during the early relaxation period, but a similar relaxation time. The finite deformation model predicts a slower creep rate than the linear model, as well as a smaller equilibrium creep displacement. The pressure distribution below the indenter, particularly near the loaded surface is also larger with the finite deformation model.

Finite-Element Modeling of Deformation and Cracking in Early-Age Concrete

Abstract: The main nonlinear phenomena that govern the deformational behavior of early-age concrete are the evolution of the stiffness properties, the development of thermal strains, creep and cracking. A general approach for numerically simulating this type of behavior is presented. The thermomechanical problem is decoupled such that first a thermal analysis is carried out and then a stress calculation is performed. An interface program is used to map the results from the thermal analysis onto the input data required for the stress analysis. A brief review of the relations for the thermal-stress analysis is given, followed by a more elaborate treatment of the algorithm used for the combination of thermal strains, creep, and smeared cracking. To properly accommodate these effects in a finite-element analysis, a smeared-crack model is used that is rooted in a decomposition of the strain increments. The emphasis is on the general approach for properly and efficiently handling these phenomena. A special case, namely a power-type creep law, is elaborated. It is shown that this relationship reasonably fits experimental data. A detailed description of an example calculation that demonstrates the potential of the numerical simulation strategy follows.

Flow of colloidal aqueous silica dispersions

Abstract: Concentrated dispersions of colloidal silica particles in water behave as soft solids. Under low stresses they show viscoelastic behavior, i.e., elastic response at short times and viscous creep at long times. Visualization of the deformation field shows that in this case the shear is homogeneous throughout the sample. Under large stresses they yield and accommodate any rate of shear; the dynamic yield stress follows the osmotic pressure of the dispersion. Visualization of the flow field shows that in this case the shear localizes in internal slip layers. The mechanism for localization results from release of water into fractures which let the solid regions flow past each other. At rest the solid regions reabsorb this water and heal the fractures. In this sense the flow is self‐lubricating.

Creep and Stress Rupture Behavior of an Advanced Silicon Nitride: Part I, Experimental Observations

Abstract: Cylindrical buttuohead specimens of an advanced silicon nitride were tested in uniaxial tension at temperatures between 1422 and 1673 K. In the range 1477 to 1673 K, creep deformation was reliably measured using high-temperature contact probe extensometry. Extensive scanning and transmission electron microscopy has revealed the formation of lenticular cavities at two-grain junctions at all temperatures (1422–1673 K) and extensive triple-junction cavitation occurring at the higher temperatures (1644–1673 K). Cavitation is believed to be part of the net creep process. The stress rupture data show stratification of the Monkman–Grant lines with respect to temperature. Failure strain increased with increase in rupture time or temperature, or decrease in stress. Fractography showed that final failure occurred by subcritical crack growth in all specimens.