Showing papers on "Creep published in 1982"
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01 Oct 1982
TL;DR: Deformation-mechanism maps: the plasticity and creep of metals and ceramics as discussed by the authors, Deformation-Mechanism Maps of metal deformation: the deformation and the creep of metal and ceramic.
Abstract: Deformation-mechanism maps: the plasticity and creep of metals and ceramics , Deformation-mechanism maps: the plasticity and creep of metals and ceramics , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی
2,342 citations
TL;DR: In this article, a composite system consisting of SiC yarn reinforced lithium aluminosilicate (LAS) glass-ceramic, containing ZrO2 as the nucleation catalyst, has been found to be reproducibly fabricated into composites that exhibit exceptional mechanical and thermal properties to temperatures of approximately 1000° C.
Abstract: Silicon carbide fibre reinforced glass-ceramic matrix composites have been investigated as a structural material for use in oxidizing environments to temperatures of 1000° C or greater. In particular, the composite system consisting of SiC yarn reinforced lithium aluminosilicate (LAS) glass-ceramic, containing ZrO2 as the nucleation catalyst, has been found to be reproducibly fabricated into composites that exhibit exceptional mechanical and thermal properties to temperatures of approximately 1000° C. Bend strengths of over 700 MPa and fracture toughness values of greater than 17 MN m−3/2 from room temperature to 1000° C have been achieved for unidirectionally reinforced composites of ∼ 50 vol% SiC fibre loading. High temperature creep rates of 10−5 h−1 at a temperature of 1000° C and stress of 350 MPa have been measured. The exceptional toughness of this ceramic composite material is evident in its impact strength, which, as measured by the notched Charpy method, has been found to be over 50 times greater than hot-pressed Si3N4.
446 citations
TL;DR: In this paper, the authors compared the frequency-depth distribution of earthquakes (DEFREQ) in several tectonic areas and concluded that it is basically the temperature and the water content of the entire crust, not the properties of a particular layer, which determine the shape of maxima of the DEFREQ curves.
Abstract: Based on published results of experiments on low-temperature, low-pressure frictional sliding and creep at higher temperature and pressure, theoretical curves of yield strength versus depth corresponding to maximum crustal stresses (STRESSMAX) are calculated. These curves are compared to the frequency-depth distribution of earthquakes (DEFREQ) in several tectonic areas. Both sets of curves have a very similar form and show a prominent peak. From the similarity it is concluded that it is basically the temperature and the water content of the entire crust, not the properties of a particular layer, which determine the shape of maxima of the DEFREQ curves. The peaks of DEFREQ curves are generally at 5- to l0-km depth and agree with STRESSMAX peaks of wet upper crust only. At this depth range the high stresses provide an increased ‘cracking potential’, resulting in an increased number and an increased stress drop of earthquakes. More and stronger barriers/asperities seem to exist at these depths, causing large earthquakes to nucleate in this high-strength region. The stresses required to overcome the strength maximum are built up from below by ductile creep. The lower crust is considered to be a stress and viscosity minimum; in orogenic zones, strong interaction with plumes from the mantle may take place, and lateral movements are probable.
432 citations
TL;DR: In this paper, the authors derived constitutive equations for creep rate and densification rate of polycrystalline aggregates and showed that the rate may be controlled either by the interface kinetics of dissolution/precipitation of the crystal from the liquid or by the diffusion rate of matter through the liquid.
Abstract: Polycrystalline aggregates which contain some liquid in grain interfaces can deform by matter transport, through the liquid phase. If the applied stress is multiaxial, then the deviatoric component of the stress will produce creep, that is, a change in shape of the aggregate without any change in volume, while the mean stress will cause the aggregate to densify. Constitutive equations for creep rate and densification rate are derived. It is shown that the rate may be controlled either by the interface kinetics of dissolution/precipitation of the crystal from the liquid or by the diffusion rate of matter through the liquid. The first process can be related to the kinetics of crystal growth from solution, and the second, to the viscosity of the liquid. The results of the analysis are used to interpret experiments on densification rate of a rock salt polycrystalline aggregate immersed in saturated brine.
338 citations
TL;DR: In this article, recent developments in the understanding of creep fracture in both uniaxial tension and multiaxonial stress states are outlined. Simple analytical equations for void growth are given.
293 citations
TL;DR: In this article, the transition from high-temperature creep to brittle fracture in Maryland diabase was investigated as a function of confining pressure and strain rate, and experiments were conducted at 1000°C.
Abstract: The transition from high-temperature creep to brittle fracture in Maryland diabase was investigated as a function of confining pressure and strain rate. Experiments were conducted at 1000°C. Confining pressure was varied to 450 MPa and strain rates from 2×10−3 s−1 to 4×10−6 s−1 At fixed strain rate, the rock strength first increased with pressure, reached a maximum, and then decreased with increasing pressure. Finally, with high pressures, the strength reached an asymptotic value which was the steady state creep strength at that temperature and strain rate. The positive pressure sensitive domain corresponded to brittle behavior, the negative pressure sensitivity domain to a transitional behavior, and the pressure insensitive domain to dislocation creep. The boundary between the last two domains occurred where the strength of the rock was about equal to the confining pressure. Similar variations in strength have been reported in the literature for carbonates and silicates, although not associated with a transitional behavior. Unfaulted specimens deformed in the transitional field showed microcracks and plastically deformed minerals. The boundaries of the transitional domain were extrapolated to geological conditions corresponding to the oceanic lithosphere. The transition depth near ridges is in close agreement with the base of the seismogenic layer (Tapponnier and Francheteau, 1978). However, the old oceanic lithosphere is probably totally brittle over its whole thickness.
289 citations
TL;DR: In this paper, it is shown that viscous gliding occurs in solid solution alloys over a limited range of stresses, and there is a transition to dislocation climb (class M) at both lower and higher stress levels.
262 citations
01 Jan 1982
TL;DR: In this article, a model for the description of the damage concept in the three-dimensional case and its consequences, as regards the induced elastic anisotropy, will be investigated in order to allow the evaluation of this model by comparison with experimental results or numerical simulation.
Abstract: The concept of damage has been introduced by Rabotnov and Kachanov in 1958 for the description of creep rupture. Since then this concept has been used to account for various types of rupture occuring in metals and other kinds of solids: creep rupture, fatigue, plastic rupture. For the one-dimensional case a survey of the basic ideas as well as some recent results can be found in [1]. The three-dimensional case has been far less investigated and many conceptual problems remain to be solved. Most of these problems are related to the anisotropy of the damage and of the resulting properties, for instance the elastic response. This paper is devoted to an approach to these problems: a model will be presented for the description of the damage concept in the three-dimensional case and its consequences, as regards the induced elastic anisotropy, will be investigated in order to allow the evaluation of this model by comparison with experimental results or numerical simulation.
250 citations
TL;DR: In this article, the authors investigated the creep behavior of a Mg-0.8% Al solid solution alloy over the temperature range from 473 to 810 K. The results showed the occurrence of three different mechanisms, with the creep process dependent on the testing temperature and stress level.
TL;DR: The influence of orientation on the stress rapture properties of MAR-M247 single crystals was studied in this article, where the Schmid factors for the operative slip system and the lattice rotations which the crystals underwent during deformation were used to rationalize the behavior of single crystals.
Abstract: The influence of orientation on the stress rapture properties of MAR-M247 single crystals was studied. Stress rupture tests were performed at 724 MPa and 774 °C where the effect of anisotropy is prominent. The mechanical behavior of the single crystals was rationalized on the basis of the Schmid factors for the operative slip systems and the lattice rotations which the crystals underwent during deformation. The stress rupture lives at 774 °C were found to be greatly influenced by the lattice rotations required to produce intersecting slip, because second-stage creep does not begin until after the onset of intersecting slip. Crystals which required large rotations to become oriented for intersecting slip exhibited a large primary creep strain, a large effective stress level at the onset of steady-state creep, and consequently, a short stress rupture life. Those crystals having orientations within about 25° of the [001] exhibited significantly longer lives when their orientations were closer to the [001]-[011] boundary of the stereographic triangle than to the [001]-[1l 1] boundary, because they required smaller rotations to produce intersecting slip and the onset of second-stage creep. Thus, the direction off the [001], as well as the number of degrees off the [001], has a major influence on the stress rapture lives of single crystals in this temperature regime.
TL;DR: In this paper, the crack growth properties of various microstructures developed in one heat of Inconel 718 alloy were investigated at 650 °C under air and vacuum environments.
TL;DR: In this paper, the authors synthesize the results from published literature on separate topics and apply them to fracture in polycrystalline ceramics, with an emphasis on quantifying properties and phenomena of engineering interest such as modulus of rupture, creep crack-growth and superplastic flow.
TL;DR: In this article, the authors measured the stress due to thermal expansion anisotropy in polycrystalline Al2O3 and found that the maximum stress varied from 80 to 100 MPa, depending on the thermal history of the sample.
Abstract: The stress due to thermal expansion anisotropy in polycrystalline Al2O3 was measured. The broadening of spectroscopic R lines (692 and 693 nm, due to Cr3+ impurities) was used to measure the stresses (at 77 K) in samples with grain sizes of 50 to 150 μm that had been cooled, from 2150 K, at constant rates from 0.1 to 100 K/min. The maximum stress was found to vary from 80 to 100 MPa, depending on the thermal history of the sample. The results are compared to the predictions of a model based on stress relaxation by diffusional creep and are in good agreement for the dependence on cooling rate. No effect due to grain-size changes was observed due to the limited range of grain sizes accessible in this study.
TL;DR: In this article, the authors investigated the role of grain boundaries at grain boundaries in primary creep and strain recovery in Si3N4/MgO alloys which were frozen under stress.
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.
TL;DR: In this paper, the steady compression flow properties of oxide dispersion strengthened Al and an Al-magnesium solid solution have been studied in the temperature range 475-825 K. Transmission electron diffraction techniques were used to identify α-Al2O3 and MgO as the oxide dispersoids in D.S. Al and D.Mg, respectively.
01 Jan 1982
TL;DR: In this article, the authors investigated the stress-strain properties of frozen Oslo clay and found that there exists a fairly well defined critical shear stress level for a given freezing temperature, and the stresses do not exceed this stress level, the creep deformations are small and the creep rates are constant or decreasing with time.
Abstract: In connection with the building of a railway tunnel through the City of Oslo, temporary improvement of the soft clay by freezing was considered in order to reinforce the tunnel ceiling in an area where the rock cover was insufficient. The same solution was also considered for another part of the tunnel where the problem was to prevent bottom heave failure in a deep excavation. As this marine clay deposit has a very high salt content (about 25 g/l pore liquid), it was reasonable to believe that the strength of the frozen clay would be relatively low compared to previously published data concerning fresh clay soils. To investigate the stress-strain properties of frozen Oslo clay, a fairly comprehensive research programme was carried out, including two different temperature levels and using different testing techniques. The clay was tested in compression, tension, bending and shear, respectively. The tests were run very slowly, time to failure was normally 70-100 days. This paper presents the main test results, which indicate that there exists a fairly well defined critical shear stress level for a given freezing temperature. As long as the stresses do not exceed this stress level, the creep deformations are small and the creep rates are constant or decreasing with time. On the other hand, higher shear stresses lead to very extensive deformations or failure. (Author)
TL;DR: In this paper, failure and post-failure behavior of Westerly granite were investigated at pressures up to 400 MPa and temperatures up to 700°C in an internally heated gas apparatus.
TL;DR: In this article, the authors examined the behavior of several aluminum grades and provided information on the nature and origin of the Harper-Dorn creep in aluminum, and suggested that the large strains of large strain may be explained by multiplication of dislocations by cross-slip, which is consistent with the observation that the condition of the initial dislocation densities reluired for the occurrence of Harper-dorn creep is the same condition under lhich cross slip becomes an effective and dominant multiplication process.
TL;DR: The Al-5% Mg solid solution alloy exhibits three regions of flow at a testing temperature of 823 K as mentioned in this paper, and it is shown that the creep curves exhibit a distinct, but short, primary stage.
TL;DR: The creep and stress-relaxation predicted using the theoretical model were found to be very close to the experimental results observed.
TL;DR: In this article, the authors analyzed the time to fracture by coupled diffusion and power-law creep, using approximate methods, both for uniaxial tension and for axial tension plus a superimposed hydrostatic tension.
Abstract: Creep fracture by coupled diffusion and power-law creep is analysed, using approximate methods, both for uniaxial tension and for axial tension plus a superimposed hydrostatic tension. Over a wide range of stress and temperature voids grow predominantly by diffusion when they are small and by power-law creep when they are large, so that the time to fracture is determined by a contribution from each mechanism. A change of temperature, stress, or stress state changes the relative contribution of each mechanism. The equations for void growth are integrated to give both the times and strains to fracture.
TL;DR: In this article, a constitutive equation of creep for polycrystalline materials is developed by introducing a concept of a creep-hardening surface corresponding to the loading surfaces of plastic deformation.
TL;DR: A methodology for separating the creep strain from the cavitation strain is described in this paper, where a significant fraction of the strain in high-temperature deformation may arise from cavitation.
Abstract: A significant fraction of the strain in high-temperature deformation may arise from cavitation. A methodology is described for separating the creep strain from the cavitation strain. Such a distinction is necessary because cavitation and creep are mechanistically different processes.
Abstract: A numerical model for calculation of thermal stresses and strains during solidification of steel is presented. Creep deformations are included. Stresses and strains during the early stage of solidification are studied for different steels under different cooling conditions. The results obtained may explain some observed cracking in continuously cast steel.