Showing papers on "Creep published in 1980"

Intergranular fracture during power-law creep under multiaxial stresses

Abstract: Approximate methods are used to calculate the growth of grain-boundary cavities by power-law creep, under multi axial stress states. The time to fracture at constant stress is given by t f =t n+ɑ/n+1)ess1n(1/(n+1)f i )where t n is the nucleation time (the time at which the voids first appear), ess is the steady-state creep rate, n is the creep exponent and f i is the original area fraction of cavities. The quantity α is defined by:ɑ=1/sinh-{2(n-½)/(n½)P/σe}where p is the hydrostatic pressure and σe the von Mises equivalent stress. Differential equations are given which allow the times and the strains to failure under variable loading histories to be calculated.

Tensile Cracks in Creeping Solids

Abstract: The aim of the paper is to answer the question: which loading parameter determines the stress and strain fields near a crack tip, and thereby the growth of the crack, under creep conditions? As candidates for relevant loading parameters, the stress intensity factor K I , the path-independent integral C*, and the net section stress σ n e t have been proposed in the literature. The answer, which is attempted in this paper, is based on the time-dependent stress analysis of a stationary crack in Mode I tension. The material behavior is modeled as elastic-nonlinear viscous, where the nonlinear term describes power law creep. At the time t = 0, load is applied to the cracked specimen, and in the first instant the stress distribution is elastic. Subsequently, creep deformation relaxes the initial stress concentration at the crack tip, and creep strains develop rapidly near the crack tip. These processes may be analytically described by self-similar solutions for short times t. An important result of the analysis is that small-scale yielding may be defined. In creep problems, this means that elastic strains dominate almost everywhere except in a small "creep zone" which grows around the crack tip. If crack growth ensues while the creep zone is still small compared with the crack length and the specimen size, the stress intensity factor governs crack growth behavior. If, however, the calculated creep zone becomes larger than the specimen size, the stresses become finally time-independent and the elastic strain rates can be neglected. In this limiting case, the stress field is the same as in the fully-plastic limit of power law hardening plasticity that has been treated in the literature. The loading parameter that determines the near tip fields uniquely is then the path-independent integral C*. It should be emphasized that K 1 and C* characterize opposite limiting cases. Which case applies in a given situation can be decided by comparing the creep zone size with the specimen size and the crack length. Criteria for small-scale yielding are worked out in several alternative forms. Besides several methods of estimating the creep zone size, a convenient expression for a characteristic time is derived also, which characterizes the transition from small-scale yielding to extensive creep of the whole specimen.

Plastic Deformation of Fine‐Grained Alumina (Al2O3): I, Interface‐Controlled Diffusional Creep

Abstract: Plastic deformation in fine-grained alumina polycrystals (grain size 1 to 15 μm) was studied. At least three distinct deformation mechanisms are important: diffusional creep, basal slip, and unaccommodated grain-boundary sliding. The first and most important of these processes is addressed in this paper. Analysis of the deformation dynamics suggests that both lattice and grain-boundary diffusion are important in the diffusional creep. Aluminum, rather than oxygen, lattice and grain-boundary diffusion are rate-controlling because oxygen diffusion is very slow in the lattice but very rapid in grain boundaries. Significantly, the diffusional creep can become interfacecontrolled at low stresses, causing the often-reported non-Newtonian creep behavior of fine-grained alumina.

Dynamic recrystallization of olivine single crystals during high-temperature creep

Abstract: High-temperature creep experiments were made on olivine single crystals under compressional stress to large strains. At strains larger than about 40 to 60%, dynamic recrystllization occurs and cellular wall dislocation structure is formed. Recrystallized grain size dg(µm) and cell wall spacing ds(µm) are dependent upon applied stress σ(MPa) as: and

Linearization of Relaxation and Creep Curves of Solid Biological Materials

Abstract: Stress relaxation curves of many solids can be normalized and presented in the linear form of F(0)t/[F(0)−F(t)]=k1+k2t, where F(t) is the decaying force and k1,k2 are constants The reciprocal of k2 denotes the asymptotic value of the relaxed portion of the initial stress Since in active biological materials equilibrium conditions in the conventional sense are difficult to determine, the asymptotic values can be used to calculate residual moduli that are representative of the material short‐term rheological characteristics Similarly, creep curves of solids in which the strain e(t) stabilizes or practically stabilizes with time (eg, under small loads or in compression) can be presented by t/e(t)=k1+k2t, where 1/k2 is the asymptotic strain It is shown that the relationships between asymptotic moduli, so calculated, and the strain in relaxation or the stress in creep can carry information that is relevant to structural changes that occur during deformation of the material such as development of hydrosta

The effects of confining pressure and stress difference on static fatigue of granite

Abstract: Samples of Barre granite were creep tested at room temperature at confining pressures up to 2 kilobars. The time to fracture increased with decreasing stress difference at every pressure, but the rate of change of fracture time with respect to the stress difference increased with pressure. At 87% of the short-term fracture strength, the time to fracture increased from about 4 minutes at atmospheric pressure to longer than one day at 2 Kb of pressure. The inelastic volumetric strain at the onset of tertiary creep, delta, was constant within 25% at any particular pressure but increased with pressure in a manner analogous to the increase of strength with pressure. At the onset of tertiary creep, the number of cracks and their average length increased with pressure. The crack angle and crack length spectra were quite similar, however, at each pressure at the onset of tertiary creep.

Practical prediction of time-dependent deformations of concrete

Abstract: Proposed is a practical model for predicting creep and shrinkage of concrete from the composition of concrete mix, strength, age at loading, conditions of environment, size and shape, etc. The main features are: double power law for basic creep, square-root hyperbolic law for shrinkage, diffusion-type size dependence of humidity effects, additive drying creep term related to shrinkage, and activation energy treatment of thermal effects. Optimization techniques are used to fit numerous test data available in the literature. The work is a continuation of previous investigations and consists of several parts. This first part deals with shrinkage.

Compressive creep of Si 3 N 4 /MgO alloys

Abstract: Highly localized strain fields are observed at grain boundaries in crept specimens of Si3N4/MgO alloys which were frozen under stress. These fields disappear upon annealing. Unresolved asperities between the grain pairs appear to give rise to the strain field during deformation. Viscoelastic effects responsible for primary creep and strain recovery are explained in terms of grain-boundary sliding on the glassy interphase which is accommodated by the elastic strain arising at the asperities. Each boundary containing an asperity can be modelled as a simple Kelvin element. The spectrum of these boundaries within the bulk gives rise to a spectrum of relaxation times that is observed for the strain recovery effect. The highly stressed region at the asperity also gives rise to the higher chemical potential required to drive diffusional creep. Although the source of the asperities was not observed, the possibility of opposing ledges of either single or multiple interplanar height is discussed.

The effect of particle shape on the mechanical properties of filled polymers

Abstract: The existing models for predicting the elastic moduli of polymers dispersed with particles of shape other than spheres and continuous fibres are reviewed. The applicability and limitation of these equations are discussed. The emphasis of the review is to seek a unified understanding and approach to the effect of particle shape at finite concentration on the elastic moduli, thermal expansion coefficient, stress concentration factor, viscoelastic relaxation modulus and creep compliance of filled polymers. The effects of anisotropic particle shape on mechanical properties of polymeric composites are clearly illustrated. Attention is also drawn to the relationship between elastic moduli, thermal expansion, creep elongation and stress relaxation moduli.

A quantitative metallographic assessment of structural degradation of type 316 stainless steel during creep‐fatigue

Abstract: — Optical and scanning electron microscopy have revealed the existence of grain boundary cavities in a series of specimens which had been cyclically deformed in the strain range ±0.25% with hold times ranging from 0 to 1000 min. The way in which these defects increase in size and number has been measured and found to correlate with the creep strain accumulated during the hold periods. A further observation is that a critical amount of deformation is required before either fatigue or creep type damage is nucleated. These observations lead to an alternative method to the currently accepted linear damage summation rule for estimating a lower bound of the creep-fatigue endurance.

The Temperature Dependence of the Viscoelastic Behavior of Poly(vinyl acetate)

Abstract: The torsional creep and creep recovery behavior of a high molecular weight (Mw=6.5×105) poly(vinyl acetate) is reported for the temperature range, 37.5°C to 154°C. The results for the recoverable compliance, Jr(t), reveal the usual two dispersions seen between glassy and steady state behavior. The intermediate rubbery plateau compliance, JN, has a value of 2.8×10−7 cm2 dyn−1, reflecting a molecular weight per entangled unit of 8,500. Though the specimen studied was a fraction, the terminal dispersion was extensive. The steady-state recoverable compliance, Je°, was approximately 80 times larger than JN. Because there was considerable information on the terminal dispersion, it was possible to independently determine its temperature dependence, which was found to be indistinguishable from that of the viscous deformation. However, this temperature dependence is significantly different from that of the softening dispersion. The differences observed are in the same sense as that found previously for the softening dispersion and viscous flow dependences of polystyrene.

Morphological changes of carbides during creep and their effects on the creep properties of inconel 617 at 1000 °C

Abstract: Creep tests have been correlated with microstructural changes which occurred during creep of Inconel 617 at 1000 °C, 24.5 MPa. The following results were obtained: 1) Fine intragranular carbides which are precipitated during creep are effective in lowering the creep rate during the early stages of the creep regime (within 300 h). 2) Grain boundary carbides migrate from grain boundaries that are under compressive stress to grain boundaries that are under tensile stress. This is explained in terms of 1 the dissolution of relatively unstable carbides on the compressive boundaries, 2 the diffusion of the solute atoms to the tensile boundaries and 3 the reprecipitation of the carbides at the tensile boundaries. The rate of grain boundary carbide migration depends on grain size. 3) M23C6 type carbides, having high chromium content, and M6C type carbides, having high molybdenum content, co-exist on the grain boundaries. M23C6 type carbides, however, are quantitatively predominant. Furthermore, M6C occurs less frequently on the tensile boundaries than on the stress free grain boundaries. This is attributed to the difference of the diffusion coefficients of chromium and molybdenum. 4) The grain boundaries on which the carbides have dissolved start to migrate in the steady state creep region. The creep rate gradually increases with the occurrence of grain boundary migration. 5) The steady state creep rate depends not so much on the morphological changes of carbides as on the grain size of the matrix.

Nucleation and growth of creep cavities in a Type 347 steel

Abstract: The nucleation and growth of intergranular creep cavities has been studied in a Type 347 austenitic steel tested at 550 and 650°C over a limited range of stress. It was found that the number of cavities per unit volume is linearly related to the time-dependent creep strain, being independent of stress or temperature over the conditions tested. The results indicate that cavity nucleation is controlled by a deformation process. Studies on cavity growth show that the cavity diameter increases approximately linearly with time, with a stress dependence for growth similar to that for secondary creep. These results are consistent with a diffusion growth model when a simple deformation-controlled nucleation law is incorporated. Using this growth model the observed stress and time dependence for the total void volume can be predicted, and the rupture life has been estimated reasonably well using a simple fracture criterion.

An experimental study of uniaxial creep, cyclic creep and relaxation of aisi type 304 stainless steel at room temperature

Abstract: Following previous work ( Krempl , 1979), a servocontrolled testing machine and strain measurement at the gage length were used to study the uniaxial rate(time)-dependent behavior of AISI Type 304 stainless steel at room temperature. The test results show that the creep strain accumulated in a given period of time depends strongly on the stress-rate preceding the creep test. In constant stress-rate zero-to-tension loading the creep strain accumulated in a fixed time-period at a given stress level is always higher during loading than during unloading. Continued cycling causes an exhaustion of creep ratchetting which depends on the stress-rate. Periods of creep and relaxation introduced during completely reversed plastic cycling show that the curved portions of the hysteresis loop exhibit most of the inelasticity. In the straight portions, creep and relaxation are small and there exists a region commencing after unloading where the behavior is similar to that at the origin for virgin materials. This region does not extend to zero stress. The results are at variance with creep theory and with viscoplasticity theories which assume that the yield surface expands with the stress. They support the theory of viscoplasticity based on total strain and overstress.

Creep and strain recovery in hot-pressed silicon nitride

Abstract: It is observed that creep response in hot-pressed silicon is characterized by two parallel phenomena; one accounts for a persistent non-recoverable plastic deformation and the other for a transient viscoelastic recoverable deformation. The persistent creep component is time-dependent, and apparently follows parabolic time kinetics. It is further observed that creep is characterized by a power law stress exponent of about 4 and an activation energy of 848 kJ mol−1. The viscoelastic recoverable component of strain is found to be independent of the total plastic strain in the material. The recovery rate at any given time is directly proportional to the preceding creep stress and therefore can be considered linear viscoelastic. The creep compliance of the viscoelastic transient is temperature-dependent with an activation energy of about 722 kJ mol−1. It is further observed that the viscoelastic recovery is characterized by a spectrum of retardation times and can be modelled by a series of Kelvin analogue models. Finally, the viscoelastic recovery and the viscoelastic component of subsequent creep appear to be inversely related and apparently obey Boltzman superposition. A model is developed for the creep and recovery behaviour of hot-pressed silicon nitride consistent with all experimental observations and based in relative grain motion accommodated by the fluid grain-boundary glass liquid flow, cavitation and wedge opening.

High-temperature fracture and diffusional deformation mechanisms in Si-Al-O-N ceramics

Abstract: A comparison has been made of hot-pressed Si-Al-O-N ceramics, with different impurity sintering aids (MgO and Mn3O4), in relation to microstructure, high-temperature creep and fracture. The Mn-containing ceramic exhibits a mechanism for creep of grainboundary sliding accompanied by cavitation at triple junctions, nucleated within an impurity silicate residue. The measured non-integral stress exponent (n∼1.5) and activation energyQ in the creep equation $$\dot \varepsilon $$ = const. σ n exp (-Q/kT) are typical of commercial silicon nitrides. A similar cavity-interlinkage is the principal mechanism for sub-critical crack growth, characterized by a low value for the stress-intensity exponent (n) in the relationV (crack velocity)=const.K 1 determined on double-torsion test specimens. Triple-junction silicate, and hence cavitation, is absent in the Mg-containing ceramic, which exhibits a Coble diffusional creep mechanism (stress exponentn=1). Sub-critical crack growth occurs only over a narrow range of stress intensity, near toK lC withn∼13 in theV-K 1 relation. A grain-boundary de-segregation caused mainly by extraction of impurities into an oxide film results in further improvement in creep and resistance to sub-critical crack growth.

A Mechanistic Model for Time-Dependent Fatigue

Abstract: Elevated-temperature failure of structural materials (e.g., austenitic stainless steels, low-alloy steels) used in energy-conversion systems can occur by fatigue, creep, or by interactive processes involving creep, fatigue, and environment. The fracture surfaces of these materials exhibit a variety of microstructural features depending upon the type of material, strain rate, temperature, environment, hold times, and sequence of waveshapes. These microstructural observations have been used as a guide in the formulation of generalized damage-rate equations that include interaction between a crack and cavities in a given environment. Crack-propagation rate as well as total life of a fatigue specimen have been calculated by integrating the damage-rate equations over the inelastic strain history of the specimen, and compared with experimental results.

On the hygrothermomechanical characterization of polyvinyl acetate

Abstract: As a part of a program to understand the mechanisms of failure in time-dependent adhesion and film bonding, the creep compliance of polyvinyl acetate (PVAc) in shear has been determined both as a function of temperature and absorbed moisture. Volumetric expansion as a function of temperature or moisture takeup was also measured. We find that practically realizable changes in moisture content affect both the creep compliance and the swelling of PVAc to a degree comparable to that resulting from realistic changes in temperature. For example, the creep rates (histories) at corresponding times for PVAc subjected to 92% relative humidity storage are accelerated by approximately four orders of magnitude over those found for the dry material. Moreover, we find within reasonable experimental error that water concentration affects the time scale of creep like temperature through a concentration-dependent shift factor. An attempt is made at discussing the interrelation of temperature- and moisture-induced volume changes.

The effect of MgO additions on the kinetics of hot pressing in Al2O3

Abstract: The kinetics of hot pressing of Al2O3 with and without MgO additives have been measured at 1475 and 1630° C and at 5 to 20 MPa using Al2O3 powders of different grain size and using different additive levels. Data obtained within the solid solution regime are interpreted in terms of diffusional creep processes. MgO additions accelerate densification within this regime; the consequent reduction in pore size, and hence in pore drag, can explain the function of MgO as sintering additive for Al2O3.