Showing papers on "Creep published in 1975"

Tensile and creep strengths of hot-pressed Si3N4

Abstract: Tensile, creep, and stress-rupture data for Norton HS-110 and HS-130 hot-pressed Si3N4 are presented. It is shown that the strength of the material is controlled by the grain-boundary glass phase. At elevated temperatures, >1000‡ C, deformation is controlled by grain-boundary sliding. A model based on the concept of geometrically necessary wedge cracks is then developed which accounts for the observed effects of strain rate, stress, temperature, environment, and impurity content. It is concluded that the currently available hot-pressed Si3N4 is creep strain limited.

The stress-induced point defect-dislocation interaction and its relevance to irradiation creep

Abstract: The general interaction energy between an applied stress and a misfitting inhomogeneity is presented and used to deduce the accurate elastic interaction between edge dislocations and point defects in a body subjected to uniaxial tension. It is shown that the external stress does, as conjectured by Heald and Speight (1974), cause a preferential drift of interstitials and vacancies to dislocations with particular orientations to the external stress axis. Moreover, for a physically plausible range of the inhomogeneity representation of the point defects the particular preferences found are consistent with those necessary to validate the mechanism of irradiation creep proposed by Heald and Speight.

Creep behaviour in the superplastic Pb–62% Sn eutectic

Abstract: The creep behaviour of the superplastic Pb-62% Sn eutectic was investigated for grain sizes from 5·8 to 14·5 μm and at temperatures in the range from 336 to 422 K. The results showed a sigmoidal relationship between strain rate and stress. At intermediate strain rates (∼ 10−5–10−2 sec−1), the stress exponent was ∼1·65, the exponent of the inverse grain size was ∼2·3, and the activation energy was similar to the value anticipated for grain boundary diffusion. At very low strain rates (≲10−5 sec−1), the stress exponent was ∼3·0, the exponent of the inverse grain size was ∼2·3, and the activation energy was similar to the value anticipated for lattice self-diffusion. The results are not entirely consistent with either of the two major theories of superplasticity, but suggest instead the sequential operation of two different deformation processes.

The Nucleation and Growth of Cavities in Iron during Deformation at Elevated Temperatures

Abstract: A fractographic technique has been used in a quantitative study of intergranular cavities formed in alpha-iron during creep at 700°C. By studying the role of grain-boundary sliding and the effect o...

Transient behavior of diffusion-induced creep and creep rupture

Abstract: Steady state solutions to three types of diffusion problems: creep, grain boundary sliding and intergranular crack growth, have been published in the literature. This paper considers, in detail, the events which occur between the time when the external stress is applied and the time when the steady state is eventually reached. The time constant of the transient has been calculated. It is shown how the grain boundary tractions change with time from the initial “elastic” configuration (when sliding has been elastically accommodated) to the steady state “diffusional” configuration (when the sliding rate is diffusionally accommodated). This requires infusion of excess grain boundary dislocations; the distribution of these dislocations is calculated. The results are applied to problems of diffusional creep, grain boundary sliding and intergranular crack growth.

Frequency sensitivity of fatigue processes in polymeric solids

Abstract: One is faced with an interesting challenge when trying to explain the effect of test frequency on polymer fatigue performance. While hysteretic heating arguments appear sufficient to explain a diminution of fatigue resistance with increasing cyclic frequency in unnotched test samples, the enhancement of fatigue resistance in many polymers with increasing cyclic frequency in notched samples is still not clearly understood. In large measure, this is due to contradictory trends in fre-quency-sensitive material properties which affect the fatigue process. In this paper, a number of proposed fatigue models dealing with the time and strain rate dependence of elastic modulus, yield strength, creep and localized crack tip heating are examined and confronted with available data from the literature. Additional fatigue crack propagation data for poly(methyl methacrylate), poly (vinyl chloride), polystyrene, poly-carbonate, nylon 66, poly(vinylidene fluoride) and poly(2,6-dimethylphenylene oxide) were obtained and are reported herein. These data were obtained over a maximum frequency range of 0.1 to 100 Hz and, for selected polymers, with various waveforms. Frequency sensitivity is shown to be greatest in those polymers that show a high tendency for crazing. Relative fatigue behavior is found to reflect a competition between strain rate and creep effects. Where creep effects dominate, the total crack growth rate may be viewed as consisting of the summation of pure fatigue and creep components, respectively. Finally, the β transition appears to have a role, with frequency sensitivity being at a maximum for polymers where the β transition at room temperature occurs in the range of the experimental test frequency.

The creep fracture of wrought nickel-base alloys by a fracture mechanics approach

Abstract: Creep fracture in the 500 to 750°C temperature range was by an intergranular crack growth process involving the formation of microcracks in grain boundaries slightly ahead of the main crack. The crack growth was proportional to an exponential power of the stress intensity. Wide differences in cracking behavior were seen between different alloys, but their differences were due primarily to material processing history and not to compositionper se. Transverse sample orientation and coarser grain sizes significantly improved the resistance to cracking. Both slow crack growth and the final fast fracture toughness changed appreciably with test history. A good correlation was found between the notch properties and the creep behavior of an unnotched sample loaded to the yield strength.

The Effect of a Dispersion of Cobalt Particles on High-Temperature Creep of Copper

Abstract: The dependence of the steady-state creep rate, es, on stress, σ, and temperature, T, for pure copper and a two-phase copper–4.04 wt.-% cobalt alloy at 712 K can be described asFor copper, the stress exponent, n, is ∼4.8 and the activation energy for creep, Q c, is ∼ 115 kJ/mol. For the copper–cobalt alloy, n≃5 and Qc≃140 kJ/mol at low stresses, whereas n≃12 and Qc≃210 kJ/mol at high stresses. This variation in n and Q c can be rationalized by measuring the ‘friction stress’, σo, using a technique involving consecutive small stress reductions during creep. Then, for both materialsThe activation energy, Q c *, is derived from the temperature-dependence of es at the same value of (σ – σo) rather than at constant σ as in the determination of Q c. For both materials, Q c*≃110 kJ/mol, indicating that processes occurring in the matrix are rate-controlling during creep of the two-phase alloy. The greater creep-resistance of the copper-cobalt alloy is attributable to the particle dispersion decreasing A* ...

Estimates of the creep rupture lifetime of structures using the finite element method

Abstract: Multi-axial creep constitutive laws are presented and discussed for materials which obey different rupture criteria. The finite element method is formulated and used to investigate the creep rupture behaviour of uni-axially loaded tension-plates containing central circular holes. Experimental values of rupture lifetimes and plate displacements for copper and aluminium plates are shown to be closely predicted by the calculated ones. The calculated shapes of zones of material deterioration or rupture are shown to be in close agreement with the results of metallographic examinations of copper and aluminium plates.

The influence of temperature and cyclic frequency on the fatigue fracture of cube oriented nickel-base superalloy single crystals

Abstract: Carbon-free single crystals of Mar-M200 were tested in pulsating tension, stress-controlled fatigue at temperatures and frequencies ranging from 1033 to 1255°K and 0.033 to 1058 Hz, respectively. The axis of loading was parallel to [001], the natural growth direction for directionally-solidified nickel-base alloys. Except for the lowest frequency at the higher temperatures where creep damage was extensive, crack initiation occurred at subsurface microporosity. Cracks initiated and propagated in the Stage I mode (crystallographic cracking on the {111} slip planes) at the lower temperatures and higher frequencies, whereas Stage (perpendicular to the principal stress axis) crack initiation and propagation was found at the higher temperatures and lower frequencies. Often a transition from Stage II to Stage I crack propagation was observed. It was established that Stage I cracking occurred under conditions of heterogeneous, planar slip and Stage II cracking under conditions of homogeneous, wavy slip. A thermally activated recovery process with an activation energy of 368 KJ/mole (88 Kcal/mole) determined the instantaneous slip character,i.e., wavy or planar, at the crack tip. In addition, it was found that an optimum frequency existed for maximizing fatigue life. At frequencies below the optimum, creep damage was detrimental, while at frequencies greater than the optimum, intense, planar slip was detrimental. The optimum frequency increased with increasing temperature.

The tensile and fatigue behaviour of Kevlar-49 (PRD-49) fibre

Abstract: The tensile, creep and tension-tension fatigue properties of Kevlar-49 fibre (formerly known as PRD-49) have been determined. The fracture morphology of the fibre has been examined and is shown to be complex due to considerable splitting. The fibre quickly stabilizes under a steady load but failure due to creep can occur when it is loaded very near to its simple tensile breaking load. Kevlar-49 has been found to fail by fatigue, and its fatigue lifetime is dependent on the amplitude of the applied oscillatory load as well as the maximum load to which the fibre is cycled.

The validity of various fracture mechanics methods at creep temperatures

Abstract: The application of stress intensity factors derived from linear elastic fracture mechanics (LEFM) to fracture at creep temperatures has been considered. From tensile creep rupture tests on single edge notched and notched centre hole specimens of solution treated A.I.S.I. type 316 stainless steel, it is shown that a LEFM approach is inapplicable to predicting creep crack growth rates, whilst the net section stress is found to correlate well with the crack growth rates. These observations have been explained by considering the creep relaxation that takes place at the notch root, smoothing out the local stresses and thus making the LEFM stress distribution inapplicable. The resulting stress distribution supports the observation that the net section stress is a successful criterion on which to predict creep rupture in stainless steel.

Viscoelasticity of polystyrene melts in tensile creep experiments

Abstract: An apparatus for measuring properties of polymer melts in tensile creep is described. Elongational viscosityµ, recoverable straine 0 , and complianceD 0 in the steady-state of deformation are determined from the creep curves of ten polystyrene samples with various molecular weights and molecular weight distributions. The elongational viscosityµ is three times the zero shear viscosity in the stress independent region. It decreases slightly for larger stresses. The temperature dependence of the viscosity is found to be the same in shear and elongation. The recoverable deformatione r 0 is independent of temperature but increases with stress. It follows thatµ 0 is an exponential function of molecular weight whereasD 0 reflects the molecular weight distribution.

On high creep activation energies for dispersion strengthened metals

Abstract: Creep studies on dispersion strengthened metals have often resulted in inexplicably high creep activation energies and high stress exponents. An analysis is presented which shows that the contribution of the temperature dependent elastic modulus to the apparent activation energy can be very large. Two cases are examined in which the difference between the apparent activation energy and the activation energy of self diffusion is almost entirely accounted for by the contribution of the temperature dependent elastic modulus. The modulus correction is particularly important at high temperatures and for materials with high stress exponents.