Changes In Yield Stress Research Articles

Effect of neutron irradiation on the tensile properties of zircaloy-2 and zircaloy-4

The effect of fast-neutron damage on the room-temperature tensile properties of zircaloy-2 and zircaloy-4 irradiated at low temperature ( < 100 °C) up to a fluence of 1.43 × 10 20 n/cm 2 ( > 1 MeV) and at elevated temperature (320–360 °C) up to 1.53 × 10 21 n/cm 2 (> 1 MeV) was investigated. The irradiation conditions at elevated temperature simulated those of fuel elements. The behaviour of irradiation-hardening of zircaloy-2 was similar to that of zircaloy-4. Considerable irradiation-hardening and loss of ductil'ty were observed. The rate of hardening for low-temperature irradiation was higher than that for elevated-temperature irradiation. Irradiation-induced changes in yield stress, Δσ y , showed saturation at a fluence of 4 × 10 19 n/cm 2 (> 1 MeV) for zircaloy-2 and 5 × 10 19 n/cm 2 (> 1 MeV) for zircaloy-4 for low temperature, while no saturation was observed after elevated-temperature irradiation up to a fluence of 1.53 × 10 21 n/cm 2 (>1 MeV) . Low-temperature irradiation showed Δσ y ∞ . (φt) case1 3 below saturation while elevated-temperature irradiation showed Δσ y ∞ . (φt) 0.1 . The amount of irradiation-hardening decreased in zircaloy-4 as the pre-irradiation yield stress was increased either by increasing the amount of cold work or decreasing the grain size. The results are discussed in terms of recent results in electron microscopy of irradiated materials.

Free volume and the annealing and antiplasticizing of bisphenol A polycarbonate

AbstractAnnealing below the glass transition temperature and the addition of an antiplasticizer are each known to increase the yield stress and the density of glassy bisphenol A polycarbonate. We have studied the combined effects of annealing and antiplasticizing to see if the individual effects would interfere with one another, as might be expected if the same free volume were involved in each. We find that the effects, even when they individually approach saturation, remain additive. This suggests that the free volume in the glass has at least two parts, one of which is affected by annealing and the other by antiplasticization. The available data indicate that the rate at which the yield stress changes with measured volume is different for the two parts.

&amp;beta;黄銅の降伏応力の異常な温度依存性

Tensile tests were performed on β-brass at room temperature ∼500°C and at the strain rate of 4.4×10−5∼2.2×10−2 sec−1. Mechanical properties varied markedly and non-monotonously with testing temperature and strain rate as follows: With increasing temperature, the yield stress once increases and then decreases, resulting in the formation of a peak of yield stress in the range of intermediate temperature. With increasing strain rate, this peak shifts to a higher temperature and the peak value increases. The strain rate dependence of yield stress is small below the temperature of this peak, but it becomes larger above that temperature. In the temperature range where the yield stress increases anomalously with elevating temperature, that is, the range below the temperature of the peak, the yield stress changes with the strain rate in a usual manner; the greater the rate, the higher becomes the stress. On the other hand, above this temperature, the yield stress shows a normal dependence on both temperature and strain rate. Among the various strengthening mechanisms of the ordered alloys, Brown’s model seems to be most favorable to explain the pronounced anomalous temperature dependence of yield stress.

Precipitation Hardening of Aluminum-2.2wt% Lithium Alloy after Neutron Irradiation

An age-hardening was observed in aluminum-2.2wt% lithium alloy specimens irradiated to a neutron dose of 1.2×1019 cm−2 at 45°C. The specimens were annealed for 4 hr at 350°C prior to the irradiation. Upon heating at temperatures of 155°, 170° and 191°C, the yield stress of the irradiadted specimens increased in two stages following the recovery of radiation-induced hardening, while the yield stress of unirradiated specimens remained unchanged.The changes in yield stress of the irradiated specimens can be explained as follows. Precipitate particles in the overaged state were destroyed and resolved into the matrix by the irradiation. And the irradiation temperature was too low for the alloy to proceed the precipitation process during the irradiation. Hardening observed in the as-irradiated specimens was due to lattice defects produced by irradiation. Following the recovery of the hardening, precipitation hardening took place in two stages corresponding to the precipitation of spherical and rod-shaped particles.

Effects of high hydrostatic pressure on the mechanical behavior of a crystalline polymer–polyoxymethylene

AbstractThe true stress‐true strain behavior of polyoxymethylene, n(‐CH2O), as an example of a bulk semi‐crystalline polymer, has been investigated for constant hydrostatic environmental pressures from 1 atmosphere to 8 kilobars with the principal objectives of elucidating the factors controlling flow and fracture. Experiments were conducted in uniaxial tension at room temperature and constant strain rate. The tensile observations were supplemented by measurements of bulk compressibility and stress relaxation behavior at pressure.In contrast with metals and inorganic compounds, the modulus, yield stress and fracture stress of POM increase strongly with pressure by a factor of approximately three at 8 kilobars. The modulus increase is shown from the stress relaxation measurements to be associated with a pressure‐induced increase in the β‐transition temperature which points to the potential usefulness of the concept of pressure‐temperature super‐position of mechanical behavior. The characteristics of the pressure dependence of the yield stress demonstrate that yield criteria based on continum mechanics considerations, including the Mohr or Coulomb‐Navier criterion, are not valid for general deformation (non‐plane strain) conditions in this polymer. The concept of a critical volume change determining the initiation of yielding is suggested to be applicable to semi‐crystalline polymers. Comparison with analogous changes in yield stress with temperature points to an increasing contribution to the control of yielding by the initially disordered regions with increasing pressure or decreasing temperature. The fracture behavior observed at pressure eliminates the concepts of a critical stress as a fracture criterion for POM and of a simple reduction in normal stress at points of stress concentration as the principal effect of the applied pressure on fracture.

Pressure Dependence of the Plastic Flow Stress of Alkali Halide Single Crystals

The change in the plastic yield stress of alkali halide single crystals due to applied hydrostatic pressure has been measured. The pressure dependence of the yield stress (δσ/σ) ranges from ∼9×10−2/kbar in soft RbI to ∼0 in LiF. The experimental results suggest that in soft crystals point-defect generation and/or diffusion is effective in limiting dislocation mobility in the early stages of deformation, whereas in crystals hardened by irradiation or cold work, elastic interactions are rate limiting.

Radiation Hardening in Platinum

AbstractSome measurements of the hardening introduced by fast neutrons in pure platinum are reported. It is found that the change in yield stress of neutron irradiated platinum varies as (Φt)1/2, in agreement with a lattice hardening mechanism. The rate of increase in damage, as observed with field ion microscopy in irradiated platinum, is compared with the rate of increase of yield stress for fixed total damage. This also indicates that the hardening is solely associated with depleted zones. Annealing studies of irradiated platinum, show that when vacancies are mobile, dislocation loops are formed from the vacancy clusters; the yield stress then varies as (Φt)1/3. Finally the total damage in the specimens is removed in the same temperature range where clusters of vacancies are removed.

Variation of Yield Stress With Strain and Strain Rate in Mild Steel

Knowledge of the yield stresses characterizing steel strip is essential to a correct discussion of the cold rolling process. Various authors have noted that yield stress rises, in general, with increasing strain rate. However, these results have been noted in compression and tension tests, and not in situations directly similar to cold rolling, though this of course does not necessarily obviate the conclusions reached. Previously, work has been done to deform steel strip in Steckel rolling, using strip of an initial thickness of 0.025 in. If we consider the work to be a function of the deformation only, we may find the yield stresses involved by differentiating with respect to strain, obtaining it as a function of strain and strain rate. The derived curves agree well with results obtained from Manjoine and others on the variation of yield stress. It can also be shown that the differential change in yield stress drops off almost exponentially with strain, showing that the effect is almost nonexistent for medium and high strains.

Slip character and the ductile to brittle transition of single-phase solids

Abstract A transition from brittle to ductile behaviour of single-phase polycrystals is shown experimentally to be a consequence of a change in slip character rather than of a change in yield stress. In the case of a b.c.c. Fe-Co alloy, deformation by planar glide induces brittleness, whereas the alloy is ductile if it deforms exclusively by wavy glide. For a series of copper-base solid solutions loaded in mercury, the degree of embrittlement increases with planarity of glide. The Cottrell-Petch equation can account for such behaviour provided the appropriate parameter, k y, is re-considered in terms of the number of operable slip systems and the propensity for cross slip.

Effect of vacancy clusters on yielding and strain hardening of copper

Abstract Clustering of excess vacancies in copper single crystal specimens was followed by resistivity, x-ray small angle scattering, and stress—strain curve observations. No change in yield stress or strain hardening rate was associated with excess vacancies when they existed primarily as single vacancies, divacancies or perhaps slightly larger aggregates. An increase in the critical resolved shear stress at the yield that was independent of the orientation of the tensile axis accompanied the growth of vacancy clusters in the size range 10 A to 30 A. Larger clusters also were associated with orientation dependent changes in the strain hardening characteristics.

Paper 16: Effect of Thermal Cycling on Pre-Stressed Steam Pipelines

Investigations into the effect of temperature and pressure cycling on a pre-loaded model main steam line of 0·5% Cr-0·5% Mo-0·25% V steel have shown that a relatively large increase in length occurred over a few cycles at 100 per cent cold pull, and a decrease in length occurred at 50 per cent cold pull. Using high temperature platinum-tungsten strain gauges, it has been found that large changes in strain occur round an expansion loop in the model pipeline. It is suggested that large changes in the design stress distribution in a main steam line occur during the commissioning stages of a generating unit, making creep behaviour of the line during base loading difficult to predict. It is further suggested that failure in pipelines when a unit is working under a twoshift system is due to low cycle fatigue, as the strain changes occurring do not appear to approach an equilibrium minimum under cycling conditions. It would appear that strain changes, and hence the effect of low cycle fatigue, can be minimized by choosing a level of cold pull between 50 and 100 per cent, the actual level depending upon the physical characteristics of the pipe material, the more important ones being thermal expansion and the change of yield stress with temperature. Using the plastic strain range measured in the test a prediction can be made regarding the time to failure.

Experimental and Theoretical Investigation of the Plastic Deformation of Cantilever Beams Subjected to Impulsive Loading

The experimental techniques and the results obtained in a program to evaluate the assumptions of dynamic, rigid-plastic theory of beams are presented. The experiments used steel and aluminum-alloy cantilever beams subjected to either a rapid velocity change at the base or to an impulsive load at the tip. A rigid-plastic theory that includes the strain-rate dependence of the yield stress and geometry changes is outlined for the case of the tip impulsive loading. The predictions of this theory are in satisfactory agreement with the experimental results.

The embrittlement of molybdenum by neutron irradiation

To determine if the same type of irradiation embrittlement which Hull and Mogford observed in EN/sub 2/ steel occurred in other body-centered cubic metals, their method was applied to molybdenum. Twenty molybdenum tensile, specimens, each having a different grain size, were prepared. The specimens were placed in a sealed argon-filled aluminum canister and irradiated in BEPO for six months. The irradiated specimens were tested at a strain rate of 0.88 x 10/sup - 4/ sec-/sup -1/. Nine were completely brittle. All the remaining specimens showed pronounced yield points and increased yield stresses. The changes in yield stress were greatest for large grain sizes. These results showed clearly that the mechanism of irradiation embrittlement was more complex than in EN/sub 2/ steel, and no straightforward interpretation of the results could be given. (M.C.G.)

Mobility of Edge Dislocations in Silicon-Iron Crystals

A method has been developed whereby the velocity of motion of fresh dislocations introduced into a crystal by scratching the surface may be measured as a function of stress. The velocities of unpinned edge dislocations in 3¼% silicon-iron have been measured as a function of stress over a range of five orders of magnitude in velocity from 10−7 cm/sec to 10−2 cm/sec. The velocity has been found to be very sensitive to the applied stress in this velocity range. Measurements of velocity as a function of stress have been made for four temperatures in the range 78°K to 373°K. It has been found that over this range of temperature the yield stress changes with temperature in the same way as the stress to produce a constant velocity of dislocation motion. From the result it is concluded that the rise in yield stress at low temperatures exhibited by this silicon iron is due primarily to an increase in the lattice resistance to dislocation motion and not to an increase in the Cottrell locking force. Preliminary experiments on pinned dislocations suggest that there may also be an effect of temperature on the strength of pinning by impurities which would be superimposed on the increase in lattice resistance with decreasing temperature.

軟鋼薄板のひずみ時効

The strain-aging processes in mild steel sheets which were stretched in tension and temperrolled, were compared with each other by means of measuring changes of yield stress, yield ratio, total elongation, yield-point elongation, and hardness, and further, the optimum temperrolling reduction was studied from a view-point of the strain-aging and the formability.The results obtained may be summarized as follows:(1) The values of yield stress which were measured immediately after temper-rolling, became lower than those of annealed sheets in a certain range of temper-rolling reduction. This may be caused by the comprehensive effect of hardening action due to inhomogeneous deformation, and saftening action by a kind of the Bauschinger effect, because the tensile test may result in effect, in a different kind of working for the temper-rolling.(2) The yield ratio of the temper-rolled sheets increased with the aging process, therefore, the formabitity of the sheets may decrease with the aging process.(3) The aging curves for total elongation and the yield-point elongation showed no appreciable change in the earlier stage of the strain-aging process and these curves showed different form from those for yield stress, yield ratio, and hardness. Total elongation and the yield-point elongation may have no unitary relation-ship to the strain-aging.(4) When the aging curves in the temper-rolled sheets were compared with those in the stretched sheets, the apparent retardation of the strain-aging was recognized in the temper-rolled sheet concerning the aging curves for yield stress, in spite of good agreement with the curves for hardness measurement. Such retardation of the strain-aging in the temper-rolled sheets has a practical meaning.(5) The activation energies which were calculated from changes in hardness and yield stress of the stretched and the temper-rolled sheets indicated a comparable good agreement with those of other workers. Therefore, the difference of the rate of the strain-aging by the method of pre-straining as stated above may be nothing but apparent, and the basic mechanism of the strain-aging may not be affected by the method of pre-straining.(6) From the point of view of formability and strain-aging, the optimum temper-rolling reduction is in a range of from 1% to 1.5%. In the case of laying special emphasis on the formability, rolling reduction will have to selected from a range of from 0.5% to 0.8%, and when the straining of formation of stretcher strain marking is to be emphasized, rolling reduction may have to be selected from larger values than 1.5%.

温度変動下のクリープおよびクリープ破断

Structure members which are subjected to load at high temperature are, in general, not in a steady condition of applied stress and temperature. Therefore, the studies on creep of structures under cyclic temperatures or cyclic stressing are important problem. In this paper, the influence of temperature history on creep is discussed. Experimental and analytical relationships of the creep and the rupture life under temperature cycling or under combined variation of stress and temperature to the static creep and the rupture life are presented.(1) The effect of temperature history on creep is composed of transitional change in internal yield stress, that is, recovery or aging, where the internal yield stress is defined as the local stress at which deformation begins without the aid of thermal fluctuation. The influence of the temperature history results in the existence of “induction period” or transitional increase in creep rate immediately after the change of temperature. However, these effects are negligible small in most metallic materials and the concept of “mechanical equation of state in solid”or the Robinson's hypothesis is applicable for preduction of creep or rupture life under cyclic temperatures.(2) By adopting the assumption the creep strain in the stage of transient or steady state creep under temperature cycling can be predicted analytically from the data of static creep test in transient stage, by introducing the equivalent steady temperature. The equivalent steady temperature means the temperature at which the transient creep strain under cyclic temperatures is equivalent to the strain in the static creep test. In like manner, creep rupture life under temperature cycling can also be estimated analytically from the data of static creep rupture test by introducing the equivalent steady temperature for creep rupture life. These analysises are usefull if severe metallurgical change such as microstructural phase change or corrosion does not occur. The assumption is also applicable for prediction of creep or the rupture life under combined variation of stress and temperature. However, under the condition that the stress amplitude which is synchronized with temperature cycle is large, the influence of recovery must be considered.

Ductile-brittle transition in steels irradiated with neutrons

Abstract The yield and fracture stresses of unirradiated and irradiated End steel have been measured at 20°, −78°, and −196°c. From the change in yield stress, [sgrave], (shear), with grain size, 2d, the values of [sgrave] i and kv in a Petch type equation, [sgrave]v=[sgrave] i +kv d −1/2, have been determined. The term kv is unaffected by irradiation to within 5% and [sgrave] i increases at a rate approximately proportional to the cube root of the dose. The effect of irradiation on the grain size transition from ductile to brittle fracture has been determined experimentally at -196°c and estimated from the change in [sgrave] i and the value of kv for tests at -78° and 20°c. The results are in accord with the predictions of Cottrell's relation ([sgrave] i d 1/2+kv ) kv =βγμ for the transition. The transition surface relating grain size, yield stress and temperature has been obtained.

Concerning the Effect of Strain and Rate of Strain on Tensile Tests at Normal and Elevated Temperatures

A few idealized cases of plastic flow have been analyzed in an attempt to show the influence of constant coefficients of work-hardening and pure viscosity on tests made at normal and elevated temperatures. Although it is true that the assumed linear expression might approximately fit cases of very low stresses and small strains, of the order of a percent or less, it cannot be used to explain test results in which annealing and chemical changes occur, as they often do in practice. It is believed, however, that the cases discussed do bring certain fundamental considerations into sharper definition than in the past. They indicate that more emphasis should be placed by experimenters on the changes in yield stress (in the hardness) that occur in materials even without stress under exposure to time and high temperature.