Showing papers on "Creep published in 1971"

On grain boundary sliding and diffusional creep

Abstract: The problem of sliding at a nonplanar grain boundary is considered in detail. The stress field, and sliding displacement and velocity can be calculated at a boundary with a shape which is periodic in the sliding direction (a wavy or stepped grain boundary): a) when deformation within the crystals which meet at the boundary is purely elastic, b) when diffusional flow of matter from point to point on the boundary is permitted. The results give solutions to the following problems. 1) How much sliding occurs in a polycrystal when neither diffusive flow nor dislocation motion is possible? 2) What is the sliding rate at a wavy or stepped grain boundary when diffusional flow of matter occurs? 3) What is the rate of diffusional creep in a polycrystal in which grain boundaries slide? 4) How is this creep rate affected by grain shape, and grain boundary migration? 5) How does an array of discrete particles influence the sliding rate at a grain boundary and the diffusional creep rate of a polycrystal? The results are compared with published solutions to some of these problems.

The friction and creep of polycrystalline ice

Abstract: The work described in this paper falls into two parts. The first is concerned with a study of the deformation of polycrystalline ice (crystal size ca. 1 mm) in uniaxial compression and when subjected to indentation. The uniaxial compression experiments covered strain ratesfrom 10 -9 to 10 -2 s -1 and temperatures from 0 to —48 °C. It is shown that over the whole range of experimental conditions the secondary creep behaviour can be described by a single relation of the type where o is the applied stress, Q an activation energy an d A, a and n are suitable constants. This reduces to the more familiar power law over more restricted portions of the experimental curve. Over the whole range n has a value close to 3 but Q has two distinct values: 120 J mol -1 above — 8 °C: 78 J mol -1 below —8 °C. The in dentation hardness experiments cover loading-times from 10 -4 to 104 s and a temperature range of 0 to — 25 °C. The hardness behaviour may be linked with the creep properties using the analysis of Atkins, Silverio & Tabor (1966) which assumes that the rate-determining process is the diffusion of the hemispherical elastic-plastic zone surrounding the indenter into the undeformed material ahead. There is very good agreement between the hardness data and the creep parameters determined in the uniaxial compression experiments. In addition the hardness experiments enable experiments to be carried out at much higher compressive stresses.

Viscoelastic Characterization of a Nonlinear Fiber-Reinforced Plastic

Abstract: The nonlinear, viscoelastic behavior of a unidirectional, glass fiber-epoxy composite material is characterized by using isothermal, uniaxial creep and recovery tests together with a constitutive equation based on thermodynamic theory. The nonlinear constitutive equation for uniaxial loading is described first, and then fourth-order tensor transformations relating principal linear viscoelastic creep compli ances, uniaxial creep compliance, and fiber angle are summarized. Following a discussion of experimental aspects, creep and recovery data obtained from several different specimens (each having a differ ent fiber orientation relative to the loading axis) are reduced using a graphical shifting procedure and tensor transformations to evaluate all material properties, including the principal creep compliances. As a check on the constitutive theory, the data are shown to be in ternally consistent. Some simplicity in the analytical representation of the data is found; viz. the nonlinear, uniaxial creep compli...

Calculation of Stresses and Strains in Four-Point Bending Creep Tests

Abstract: An analysis of strains and stresses in four-point bending creep tests in the limit of small beam deflections resulted in a general equation which relates the load-point deflection, the applied load, the creep exponent (N), and the geometrical parameters of the loading system. Measurements of load-point deflection rates, which are experimentally easy to accomplish in ceramic systems, vs the applied load lead to the direct determination of the creep exponent and the creep compliance in a steady-state creep test. The creep compliance is a function of the temperature, grain size, and all other factors except stress. The elastic equation relating the load-point deflection and the outer fiber strain is strictly valid for viscous creep and approximately valid for nonviscous creep (i.e. N>1) if the ratio of the distance between the support points to the distance between the load points is not very large.

Dynamic deformation of aluminium and copper at elevated temperatures

Abstract: T he mechanical behaviour of two annealed face-centred cubic metals, aluminium and copper, at elevated temperatures has been investigated experimentally to determine their rate sensitivity. The results indicate that hot compression is a thermally-activated process. The experimental value of the activation energy for aluminium obtained from the present work is almost similar to that for creep and self-diffusion. This indicates that hot dynamic compression may be a diffusion-controlled thermally-activated process. However, the experimental activation energy of 74 kcal/mole obtained from the present work for copper is higher than that observed both for creep and self-diffusion. Experimental evidence (D. Hardwick and W.J. McG. Tegart in 1961) suggests that recrystallization is the rate-controlling process in dynamic deformation, while in creep it is usually recovery in the form of sub-grain formation. For aluminium specimens the dislocation density decreases with increasing temperature; for increasing strain rate the dislocation density remains almost constant, while the size of dislocation cells diminishes. If the dislocation density did not change substantially, one should conclude (as did L. Taborský in 1969) that the rise in flow stress caused by high strain rate is due to the reduction of the size of dislocation cell structure and by the increased rate of movement of the moving individual dislocations.

High temperature rhelogy of westerly granite

Abstract: The creep of Westerly granite has been measured up to the melting point (720°C) under 4–5 kb external pressure and 1 kb water pressure for differential stresses from 128 to 643 bars. Under these conditions the cracks that affect creep behavior in rocks at room pressure are probably unimportant. The anelastic strain (e′) normalized against the instantaneous elastic strain (e0) can be represented by a single curve e′/e0 = (σ/643 bars)0.66 (t/τ)1/2, where τ = (2.2 × 10−14 sec) exp (78 kcal/mole/RT) for 10−2 < e/e0 < 2 and 128 bars < σ < 643 bars. Wave attenuation was computed for the experimental conditions observed. The data suggest that the effective viscosities computed from glacial uplift data may be smaller than those corresponding to large strains by as much as two orders of magnitude. Poor agreement is found with previous high-temperature creep results on rock specimens at room pressure.

Crack Propagation in a Linearly Viscoelastic Strip

Abstract: The tip velocity of a crack propagating through a viscoelastic material depends on geometry, applied load and its history, and material properties A consideration of the work done by the unloading tractions at the crack tip shows that, for a large crack propagating through an infinitely long strip under constant lateral strain, the rate of propagation can be calculated from a knowledge of the intrinsic fracture energy (a material constant), the material creep compliance, and an additional size parameter This parameter vanishes from the analysis if the material is elastic, and the familiar instability criterion is obtained in this case Comparison with experimental data is provided and the consequences of step loadings are examined

Deformation of UO2 at High Temperatures

Abstract: The effects of temperature, strain rate, and grain size on the mechanical properties of UO2 were investigated using the four-point bending technique. Strain rates were varied by two orders of magnitude, and test temperatures up to 1800°C were used. Data are presented on the ultimate tensile stress, yield stress, and plastic strain-to-fracture. Below the brittle-to-ductile transition temperature, Tc, the material fractured in a brittle manner, with no macroscopic plastic deformation. Between Tc and a second transition at a higher temperature, Tt, a small amount of plastic deformation was measured before fracture. Beyond Tt, the strength of UO2 decreased continuously, and extensive plasticity was observed. This high-temperature plasticity was characterized by a thermally activated rate-controlling process; this behavior is consistent with observations of creep behavior under high stresses. The following phenomenological equations for the strain rate fit the data for the material with 8-μm grain size above Tt: and where σp and σ88f are the proportional limit and steady-state flow stress, respectively, and temperature T is in °K.

Dislocations associated with optical features in naturally-deformed olivine

Abstract: Transmission electron microscopy has been used for the direct observation of dislocations in naturally-deformed olivine. The dislocations are arranged in arrays forming low-angle sub-boundaries which have been identified with features observed in the optical microscope. Comparison of this dislocation substructure with that observed in olivine, and in metals, experimentally deformed under various conditions, suggests that the deformation in nature has occurred by creep. Possible mechanisms of creep, involving the cooperative glide and climb of dislocations, are discussed.

The Fracture Energy and Some Mechanical Properties of a Polyurethane Elastomer

Abstract: The energy required to form a unit of new surface in the fracture of a polyurethane elastomer is determined. The rate sensitivity of the material has been reduced by swelling it in toluene. This paper primarily describes the experimental work of measuring the lower limit of the fracture energy. With this value and the creep compliance as a basis, the rate dependence of fracture energy for the unswollen material has been determined. It is thus shown that the dependence of the fracture energy on the rate of crack propagation can be explained by energy dissipation around the tip of the crack. Good agreement between the theoretically and experimentally determined relationships for the rate-sensitive fracture energy is demonstrated.

Undrained Creep Behaviour of a Coastal Organic Silty Clay

Abstract: Synopsis Stress controlled laboratory test data on the creep behaviour of a coastal organic silty clay under undrained conditions at different isotropic consolidation pressures and stress levels are reported. During undrained creep the sustained deviator stress was maintained at 90, 70, 50 and 30% of the ultimate deviator stress determined in normal strength tests. The pore pressures developed during creep have been shown to be due both to the distortional stress and also to the secondary compression effect. It is considered that any theory developed for the prediction of stress paths for undrained strength and creep behaviour of sensitive soils should take into account the pore pressures developed during secondary compression under zero deviator stress. The magnitude of pore-water pressure build-up during secondary compression is shown to be time and structure dependent. On donne les resultats d'essais au laboratoire, sous contrainte controlee, sur la tendance au fluage d'ne argile limonique organique co...

Reaction‐Rate Model for Fracture in Polymeric Fibers

Abstract: Electron paramagnetic resonance (EPR) techniques were used to determine the number of free radicals produced during deformation leading to fracture of nylon 6 fibers. A reaction‐rate molecular model is proposed to explain some of the deformation and bond‐rupture behavior leading to fracture. High‐strength polymer fibers are assumed to consist of a sandwich structure of crystalline‐block and amorphous‐flaw regions along the fiber axis. In the flaw regions, tie chains connecting the crystalline blocks are assumed to have a statistical distribution in length. These chains are, therefore, subjected to different stresses. The length distribution was determined by EPR. The probability of bond rupture was assumed to be controlled by reaction‐rate theory with a stress‐aided activation energy and behavior of various loadings determined by numerical techniques. The model is successfully correlated with experimental stress, strain, and bond‐rupture results for creep, constant‐rate‐of‐loading, and cyclic‐stress tests.

Prediction of activation energies for creep and self-diffusion from hot hardness data.

Abstract: It is shown that the activation energy for creep or self-diffusion for pure metals can be determined from hot hardness data above 0.75T m by means of the expressionH/E=G expQ L/nRT· HereH is the hot hardness,E is the elastic modulus,G is a material constant,Q L is the lattice self-diffusion activation energy,R is the gas constant,T is the absolute temperature, andn is the stress exponent for creep assumed equal to five. Hot hardness data above 0.5T m plotted as logarithmH/E against reciprocal absolute temperature reveal two straight lines with a break observed at about 0.75T m. It is shown that the break occurs at a value of strain rate, ∈, over lattice self-diffusivity,D L, of about 109, a value associated with power-law breakdown for creep. These observations suggest two conclusions regarding the rate-controlling process during hot indentation testing of pure metals. Between 0.75 and 1.0T m, the deformation process is associated with lattice self-diffusion and creep flow in the power. law region. Between 0.5 and 0.75T m the rate-determining process is associated with dislocation pipe diffusion and creep flow in the power-law breakdown region.

The stiffness of polymers in relation to their structure

Abstract: In the majority of applications of polymers, we are interested in one or more of three basic mechanical properties-stiffness, strength and toughness. To these can be added creep, which becomes important in many engineering applications. Stiffness represents resistance to deformation, and is a much simpler property than strength and toughness, which relate to failure. Strength is the ultimate stress which a material can withstand, before it fails, whether by fracture or by excessive deformation, whilst toughness represents the work required to fracture a material

The mechanical behavior of isolated Avena coleoptile walls subjected to constant stress: properties and relation to cell elongation.

Abstract: In order to assess the role of the mechanical properties of the wall in auxin-induced cell elongation, a study has been made of the ability of isolated Avena coleoptile walls to extend (creep) when subjected to a constant applied stress. Creep occurs as a viscoelastic extension which has the following characteristics: the extension is proportional to log time and is partly reversible, and the extension rate has a Q(10) of about 1.05 and is markedly greater in auxin-pretreated walls. In nonconditioned walls the extension rate is proportional to applied stress, but pre-extension causes the appearance of an apparent yield strain. The similarity of creep and instantaneous plastic deformation in response to temperature or to pretreatment with auxin or KCN suggests that the instantaneous deformation is simply the viscoelastic extension which occurs at very short times. A comparison of these viscoelastic properties with the properties of auxin-induced cell elongation indicates that cell elongation requires more than just a physical extension of the wall. It is suggested that elongation occurs as a series of extension steps, each of which involves a viscoelastic extension preceded or accompanied by an auxin-dependent biochemical change in the wall properties.

The Influence of Alumina Dispersions on the Diffusion-Creep Behaviour of Polycrystalline Copper

Abstract: The creep behaviour of powder-compacted copper and copper containing 0.5, 1.0 and 1.5 vol.-%alumina has been investigated in the range where the stress-directed lattice diffusion of vacancies is expected to control. Addition of alumina particles is shown to change the initial flow characteristics from Newtonian, where the creep rate έ is proportional to the applied stress σ, to Bingham type, where έ is proportional to an effective stress σέ. This effective stress is defined by σέ = σ − σ0, where σ0 is a threshold value that increases linearly with volume fraction of alumina and below which no deformation occurs. The proportionality constant between έ and σE is shown to be identical to that between έ and σ and is given by the Nabarro-Herring equation as BΩD/d2 kT, where B is a numerical constant, Ω the atomic volume, D the lattice self-diffusion coefficient, D the grain size, and kT has its usual meaning. Above the threshold stress, creep rate decreases with strain and this curvature is more pronou...

Discontinuous precipitation of M23C6 in austenitic steels

Abstract: Grain boundary precipitation of M23C6 has been studied in a 20% Cr-35% Ni stainless steel with two grain sizes during creep deformation at 700°C as well as during an ordinary ageing treatment at 700°C. A special etching technique was applied which showed how the grain boundary precipitation gave rise to depletion of alloying elements in a zone of uniform thickness, independent of the carbide distribution, and with a gradual decrease of the depletion towards the grain interior. At some places the carbide precipitation and grain boundary migration co-operated and in these cases there was a sharp change in alloying content across the grain boundary. This process was more frequent in creep tested samples and the degree of co-operation was larger in the coarse-grained material where even a few cases of lamellar, eutectic-like precipitation was observed. Such a grain size dependence is expected theoretically and is caused by the large difference in diffusivity between carbon and the other alloying elements. It is proposed that the various degrees of co-operation between carbide precipitation and grain boundary migration are all examples of discontinuous precipitation. The various proposed mechanisms for grain boundary migration during discontinuous precipitation are discussed on the basis of the present results.

High temperature creep in copper

Abstract: Creep in copper of 99·99% purity in the temperature interval 550-1025°K was investigated by the isothermal tests technique. Two dislocation mechanisms operating in parallel were detected. In the region of lower creep rates-Region 1-the apparent activation energy of creep depends linearly on stress: [image omitted] where Q01 = 47·4 ± 2·8 kcal mol-1 and B1 = 3·2 ± 0·5 kcal mol-1 Kg-1 mm2. The value of Q01 is close to the activation enthalpy of lattice self-diffusion in copper. Consequently, creep is controlled by lattice self-diffusion. The stress exponent n ≡ n1 = δ In ∊/δ In σ is a function of both stress and temperature. Generally, n1 decreases, reaches a minimum and increases again with the increasing stress. The lowest value of n1 ≃ 5·4. Non-conservative motion of jogs on screw dislocations dependent on lattice self-diffusion was suggested to be a rate-controlling process in Region 1.In the region of higher creep rates-Region 2-the apparent activation energy Qc2, is considerably higher than the activat...

Stress‐time superposition of creep data for polypropylene and coupled glass‐reinforced polypropylene

Abstract: Glass-reinforced thermoplastics are being considered in many structural applications and fabricators require design information on these materials. Basic creep data are, in many cases, the most useful for design purposes. The work reported here concerns the development of methods of increasing our efficiency in generating creep data. The methods developed are applicable to polypropylene and coupled, glass-reinforced polypropylene. A stress-time superposition procedure has been found valid for extending creep data generated on coupled glass-reinforced polypropylene at several glass levels and at temperatures of 23 and 80°C.

The effect of combined low-cycle fatigue and creep on the life of austenitic stainless steels

Abstract: Experiments regarding the effect of simultaneous fatigue and creep on life have been performed for a 20 pct Cr-35 pct Ni stainless steel at 700°C. It was found that a life fraction rule, comprising an interaction term besides the terms of pure fatigue and creep damage, adequately accounts for the observed lives. The interaction term contains the product of the fatigue and creep damage, and may physically be interpreted as the damage generation due to the interaction of the fatigue and creep processes. There is evidence that the surface crack density represents a measure of the total damage. The cracks created during the combined fatigue and creep test were both transgranular and intergranular. They were probably nucleated intergranularly and later, during their growth, were transformed to transgranular cracks. As the creep damage of the test was increased gradually, more cracks remained intergranular throughout the test.