Showing papers on "Strain rate published in 1968"

General stress-strain-time function for soils

Abstract: The stress-strain-time behavior of soils may assume a variety of form depending on such factors as soil type, soil structure, stress history, drainage conditions, and type of loading. Generalizations concerning creep in soils suggested by various investigators are sometimes contradictory. However, test results show that there exist linear relationships between logarithm of strain rate and logarithm of time for a given stress and between logarithm of strain rate and stress at any given time, provided the creep stress level is between the practical limits of about 30% to 90% of the initial soil strength. These findings lead to a simple three-parameter general stress-strain-rate-time relationship. Integration of this relationship yields creep curves which are observed to agree in form with the experimental results.

Mechanisms of plastic deformation of olivine

Abstract: Olivine deformed experimentally at 5-kb confining pressure undergoes plastic flow by slip on several systems. The active slip system is determined from microscopic study of slip bands, deformation lamellas and kink bands. At temperatures less than 1000°C the slip systems are {110} [001], (100) [001], and (100) [010]. At 1000°C, slip on {0kl} [100], pencil glide in [100], the system shown here to be predominant in naturally deformed terrestrial olivine, occurs in addition to the system {110} [001] and becomes the predominant mechanism with decreasing strain rate. Dependence of glide mechanism on temperature and strain rate is also observed in diopside. Data of the sort acquired in these experiments may be used to place limits on the possible temperatures and strain rates of naturally deformed rocks.

Fine Structure of Scalar Fields Mixed by Turbulence. I. Zero‐Gradient Points and Minimal Gradient Surfaces

Abstract: Turbulent mixing of passive scalar properties such as temperature or concentration is discussed From physical and geometrical considerations it is concluded that the smallest scale features of a steady‐state turbulent scalar distribution are primarily determined by the number and distribution of points in the fluid for which the scalar gradient vector is zero, and surfaces for which the gradient magnitude is minimal Mechanisms for the production, destruction, and motion of such “zero gradient points” and “minimal gradient surfaces” are examined Initially, zero gradient points must be produced from regions of uniform scalar gradient, but the vast majority result from secondary splitting due to strain induced eccentricities of the closed isoscalar surfaces surrounding maximum or minimum points An expression for the velocity of surfaces of constant scalar value is derived and used to interpret the Obukhov‐Corrsin length scale (D3/e)14 as the minimum size eddy capable of generating a zero gradient point from a region of uniform gradient, where D is the molecular diffusivity of the scalar and e is the mean viscous dissipation rate A steady‐state separation distance between zero gradient points of order (D/γ)12 is inferred for scalar fields of arbitrary diffusivity where γ is the root mean square rate of strain of the fluid Previous theories such as that of Batchelor, Howells, and Townsend have assumed the local strain rate has no effect on scalar structure for large Prandtl number fields

Mechanical Properties of a Sand-Ice System

Abstract: The mechanical properties of a saturated frozen sand were studied using constant axial stress creep tests and constant axial strain-rate tests. Observation of the effects on deformational behavior produced by the addition of sand particles to the ice matrix led to an analysis useful in predicting creep rates for a constant stress and stress levels for a constant strain rate. The relationship between strain rate and stress for polycrystalline ice considers a strain dependent hardening term and a strain energy dependent softening term. Parameters of the equation include stress and temperature. Stress factors, dependent on sand volume concentration and friction and dilatancy of the sand, are used in conjunction with the equation for polycrystalline ice to give predictions of the mechanical behavior of the sand-ice system. These are in reasonable agreement with experimental data.

The influence of grain size on the nature of portevin-lechatelier yielding

Abstract: When the grain size dependence of dislocation density is applied to Cottrell's theory of the Portevin–Lechatelier effect, the result indicates that the critical strain ∊0, necessary to initiate serrated flow, should depend on the grain size μ, at constant strain rate, according to the relationship ∊0 = (constant)μ p , where p=n/(m + β). The parameters n and (m + β) are evaluated by dislocation density measurements and strain rate experiments respectively to give ∊0 ∝ μ½. Direct experimental determination of ∊0 as a function of grain size is in agreement with that predicted theoretically, indicating that the effect arises from the influence of grain size on the average velocity of mobile dislocations. The period and amplitude of the serrations decrease with increasing grain size. This effect is consistent with the idea that each serration corresponds to a yield point followed by a local Luders strain. The period and amplitude of the serrations decrease with increasing grain size. This effect is co...

Effect of Fillers and Voids on Compressive Yield of Epoxy Composites

Abstract: Two series of epoxy composites, filled and porous, were loaded in compression up to yield under several constant strain rates. Results indicate a linear relationship between yield stress and log strain rate, characterized by a constant slope for the different com positions, and almost linear dependence of yield stress on filler content. A simplified empirical formula was derived for the yield stress of filled composites as function of filler content and strain rate. Similar dependence was found for porous composites.The strain-rate dependence indicates non-Newtonian visco plastic behaviour obeying the corresponding Eyring equation. The filler content was found to have almost no effect on the activation volume terms, and its main effect is apparently with regard to the activation energy characteristics of the yielding process.

Mechanics of Orthogonal Machining: Allowing for the Effects of Strain Rate and Temperature on Tool-Chip Friction

Abstract: An analysis of orthogonal machining is given in which the mean angle of friction usually used to describe the frictional condition at the tool-chip interface is replaced by the average shear stress along this interface. If this shear stress is expressed as a function of strain rate and temperature, using a velocity-modified temperature, there is good agreement between theory and experiment.

Creep of Frozen Sands

Abstract: UNCONFINED COMPRESSIVE CREEP STRENGTHS AND STRAINS WERE MEASURED FOR FROZEN SATURATED OTTAWA SAND (20-30) AND MANCHESTER FINE SAND. THE CREEP TESTS WERE CONDUCTED AT APPROXIMATE STRESS LEVELS OF 60, 35, 20 AND 5 PERCENT OF THE CONVENTIONAL UNCONFINED COMPRESSIVE STRENGTH. TESTING TEMPERATURES WERE 15, 25, 29 AND 31 F. IT WAS FOUND THAT THE UNCONFINED COMPRESSIVE CREEP STRENGTH OF THE FROZEN SAND CAN BE PREDICTED USING VIALOV'S STRENGTH FORMULA AND THAT CREEP STRAIN CAN BE PREDICTED USING TWO SHORT-TERM, HIGH-STRESS- LEVEL CREEP TESTS. EQUATIONS ARE GIVEN TO PREDICT TOTAL STRAIN. FOR STRESSES BELOW THE LONG-TERM STRENGTH, THE STRAIN RATE IS DIRECTLY PROPORTIONAL TO THE RECIPROCAL OF TIME DURING STRESS ACTION UNTIL COMPLETE STABILIZATION OCCURS. /AUTHOR/

On the interaction between internal gravity waves and a shear flow

Abstract: The theory of wave action conservation is summarized, and its interpretation in terms of the working, against the rate of strain of the basic flow, of an interaction stress associated with the waves is discussed. Usually this interaction stress is identical with the radiation stress of a uniform plane wave. The problem of internal gravity wave propagation in an incompressible, stratified Boussinesq liquid is considered in detail for a more general basic flow than has hitherto been treated, and the interaction stress is derived. One component of the interaction stress tensor is only equal to the corresponding component of the radiation stress tensor if we include in the latter, in addition to the Reynolds stress, a term associated with the redistribution of matter, on the average, by the wave. Two other components of the radiation stress tensor are modified in a similar manner, but the corresponding components of the interaction stress tensor are undefined, and so no comparison is possible.

The modulus and yield stress of glassy poly(methyl methacrylate) at strain rates up to 103 inch/inch/second

Abstract: Specimens of poly(methyl methacrylate) (PMMA) were compressed at nominally constant strain rates ranging from 10−5 to 103 in./in./sec. at 0, 22, 50, and 115°C. A plot of stress at a small fixed strain (2%) and constant temperature versus logarithm of strain rate is sigmoidal in shape and, furthermore, time-temperature superposition could be used to construct a master curve of stress versus temperature-compensated strain rate. It is suggested that the sigmoidal curve is a manifestation of the β transition in PMMA, and this is supported by the measured value of activation energy and the strain rate value at the point of inflection on the curve. By contrast, yield stress varies linearly with log . Time-temperature superposition could not be applied. Rationalizing on the basis of the high stress form of the Eyring equation, yielding is by a yet unspecified molecular mechanism in which activation volume has the order of magnitude of a monomer unit but increases with increasing temperature.

Flow of Polycrystalline Ice at Low Stresses and Small Strains

Abstract: Tensile creep tests from −3° to −13°C at stresses between 2×105 and 2×106 dyn/cm2 with millimeter range grain‐size polycrystalline ice showed that the strain rate was proportional to the stress, and that the viscosity was proportional to the grain size squared. The flow process had an apparent activation energy of 12 kcal/mol. Calculated diffusion coefficients were several orders of magnitude larger than directly measured diffusion coefficients for single crystals. Enhancement of the diffusion rate in a wide band adjacent to grain boundaries is believed to account for the observed behavior. Previously reported data for ice sintering are interpreted on the same model.

Thermally activated deformation in dilute zirconium--oxygen alloys.

Abstract: The effect of temperature and strain-rate on the yield and flow stress of zirconium, containing oxygen in the range of 0.09 to about 2.5 at. % has been examined at temperatures between 77 and 700° K. The above measurements were coupled with strain rate change tests and stress-relaxation experiments, so that all the deformation parameters associated with thermal activation could be evaluated. The results of these measurements showed that the thermal component of flow stress increases with increasing oxygen content of the alloy. Two stages of activation were found to occur for all these alloys. The first stage was associated with the unpinning of dislocation loops from impurity pinning points. At higher temperatures, because of the unpinning of many impurity pinning points, a second stage of activation associated with a different mechanism was found to occur. The variation of certain parameters associated with this stage of deformation strongly suggests the Peierls-Nabarro force as the controlling criterion.

High‐Temperature‐Creep Mechanism of TiNi

Abstract: Polycrystalline specimens of the intermetallic compound TiNi were tested in tension in the temperature range 700°–1000°C. The specimens were observed to deform uniformly over the gage length until the very end of deformation, when it failed by necking. It was noted that the steady‐state flow stress was independent of the strain and was a function only of the temperature and strain rate. The shear strain rate γ was given by γ∝τn where n=3.0±0.2, r was the shear stress, and n was independent of temperature. The apparent activation energy for creep was found to be 60.0±3.0 kcal/mole and was independent of stress. The result strongly suggests that a viscous creep process, as first formulated by Weertman, is probably the rate‐controlling mechanism for this compound in the stress and temperature range investigated.

Strain rate history effects for polycrystalline aluminium and theory of intersections

Abstract: T his work presents an application of the dislocation theory of intersections to the experimental results obtained at changed strain rates under conditions of pure shear for polycrystalline aluminium. Simultaneously, strain-rate history effects have been shown, and discussed in terms of dislocation behaviour.

The Plate Impact Configuration for Determining Mechanical Properties of Materials at High Strain Rates

Abstract: A review of the latest free surface motion and stress measuring instrumentation is presented along with a description of the plate impact one-dimensional strain configuration for determining mechanical properties under stress wave propagation conditions. A discussion is included of results of some other investigators who have used this configuration to determine dynamic yield stress and the degree of strain rate sensitivity.

Stress Softening and Strain Softening of Poly(Methyl Methacrylate) in Yielding under Constant Load

Abstract: The strain rate of poly(methyl methacrylate) during yielding under constant load was determined experimentally for various tensile loads at temperatures from 70° to 100°C. In the constant‐load test, the polymer strains uniformly beyond the yield point up to several times the yield strain. The yield point is evident as a minimum of the strain rate. Intermittent superposition of load increments affects the strain rate reversibly. The observed strain‐rate increase under constant load in the uniform‐strain post‐yield region, results essentially from strain softening with some additional softening from the stress increase due to specimen thinning. The stress influence on the strain rate is expressible as a stress‐shift factor of the strain rate. A theoretical stress‐shift factor is derived from Doolittle's viscosity equation and the assumption that Poisson's volume dilatation contributes entirely to the free volume. Good agreement between the predicted and the observed stress‐shift of the strain rate is found. An apparent anomaly of the stress‐shift factor at 100°C is probably caused by recovery from strain softening in the vicinity of the glass transition temperature.

The effect of strain rate on the stress–strain curve of oriented polymers. I. Presentation of experimental results

Abstract: The stress-strain curves of viscose, nylon 66, poly(ethylene terephthalate), polyacrylonitrile, and polypropylene have been determined at a large number of different strain rates between 10−4 and 330 sec−1 The shape of these stress-strain curves and its change with strain rate is shown to depend upon whether the material is tested above or below its glass temperature The stress-strain curves of materials tested below their glass temperature consists of an initial straight portion followed by a yield point at a few per cent strain The breaking strain is only slightly affected by strain rate, and the energy to rupture increases with increasing rate For materials tested above their glass temperature the initial portion of the stress-strain curves in nonlinear, and the yield strain is much higher than for the other materials There is a small range of strain rate, in which the breaking strain falls sharply to the yield strain with increasing rate, and the energy to rupture also decreases Outside this range the energy to rupture increases with increasing rate

Spreading and contraction at the boundaries of free turbulent flows

Abstract: The unstrained free turbulent flow generated by a composite grid has been found to possess a distinct, deeply indented boundary separating non-turbulent fluid from turbulent fluid of virtually homogeneous intensity. At this boundary, contraction, in the sense of a reduction in the volume of turbulent fluid, takes place at a rate which is independent not only of the intensity and degree of anisotropy of the turbulence, but also of the relative depth of the surface indentations. The apparent outward spreading of the turbulence is solely due to the increasing amplitude of the boundary bulges.The subsequent plane straining of the flow in a constant area distortion shows that a reasonably high degree of anisotropy, combined with straining by the mean flow, is required before entrainment of non-turbulent fluid can occur at the boundary. Under these simple straining conditions, the rate of growth of the volume of turbulent fluid appears to be independent of the relative depth of indentations in the turbulence front.It is considered that in free turbulent shear flows, the potential-turbulent flow boundary will advance into the non-turbulent fluid at a rate which depends on the local mean rate of strain and the local degree of anisotropy adjacent to the front. Contraction is expected to occur on those occasions when the boundary lies in a region of zero mean rate of strain, or when the anisotropy of the turbulence is low. The increased rate of spread due to large eddy activity is thought to be mainly caused by an increase in the probability of the potential-turbulent flow boundary being in a position where the mean rate of strain and local anisotropy are both high, rather than being entirely due to an increase in the surface area of the front.

A note on the limit loads of non-standard materials

Abstract: The paper discusses the collapse theorems for materials which do not exhibit normality of the strain rate vector to the yield surface in the superposed stress and strain rate spaces. The consequences of the lack of normality on the designs for minimum weight are considered.

On the distribution of cavities during creep

Abstract: The true and apparent angular distributions of grain boundary cavities have been measured for OFHC copper deformed over a range of creep rates at 415°C. No effect of strain rate was observed, the f...

The effect of strain rate on the stress–strain curve of oriented polymers. II. The influence of heat developed during extension

Abstract: The temperature changes which take place in a yarn during extension are considered. From thermodynamical considerations and the heat-transfer coefficient it is shown that extension of the yarns studied will take place isothermally at strain rates below 0.04 sec.−1 and adiabatically at rates above 4 sec.−1 It is not possible to make an accurate estimate of the magnitude of the temperature rise during adiabatic extension, because of the lack of information on internal energy changes during irreversible extension, but by assuming these to be zero it is estimated that the temperature is likely to rise by 20–30°C. at strains above 10%. Results from a study of the effect of strain rate on the stress-strain curves of five different yarns show in all these materials a range of strain rate in which the stress that produces a given strain increases less rapidly with strain rate than elsewhere. For viscose and poly(ethylene terephthalate) this effect occurs in the expected range of strain rate, and its magnitude is of the correct order for it to be attributed to the temperature rise resulting from the transition from isothermal to adiabatic extension. For the other materials the transition does not seem likely to provide a complete explanation of this effect. There is no evidence that the transition significantly affects the breaking properties.

Experimental Approach to a Theory of Plasticity at Elevated Temperatures

Abstract: The problem of providing an experimental basis for an isothermal theory of plasticity of metals at elevated temperatures has been examined. The key objectives are considered to be the establishment of a steady‐state function and the discovery of a nonsteady deformation law. Constant stress and constant strain rate are judged the best loading conditions for both of these tasks since they provide paths, under the simplest thermomechanical histories, to the steady‐state deformation. The use of plots of strain rate and its time derivative for constant stress tests and plots of stress and its time derivative for constant strain rate tests is advocated as a means of best displaying the transient nonsteady behavior. It is even suggested that the functions represented by such plots may be independent of the loading history prior to the period of constant stress or constant strain rate applications. Constant stress and constant strain rate tests are limited in the early (nonsteady) period by lack of control of structure on initial loading, and generally in the later period by the nonuniformity of deformation. It is suggested that the former difficulty may be overcome by employing loading rates sufficiently rapid to permit sensible constancy of structure. Nonuniformity of deformation imposes a severe restriction on the examination of steady‐state deformation.

Plastic Deformation in Epitaxial Ge Layers Grown on Single Crystal Semi‐Insulating GaAs

Abstract: The compatibility of Ge and , in terms of their temperature dependent mechanical properties, is described in this paper. Examination at room temperature of epitaxial Ge layers grown on wafers of single crystal, semi‐insulating , at substrate temperatures of 700°C by pyrolytic dissociation of , reveals evidence of plastic deformation immediately following the epitaxial deposition process. Plastic deformation is observed in epitaxial Ge layers grown at 350°C, by disproportionation of , only after a subsequent annealing cycle to at least 500°C. The low substrate temperature Ge films, however, show elastic bending of thin substrates after deposition and cooling to room temperature. Based on these observations, it is possible to estimate the differential thermal expansion coefficient between Ge and , and to use this result to estimate the critical shear stress for plastic deformation under low strain rate conditions . Interface dislocations are not observed.

Deformation mechanisms in hot working

Abstract: 1. During deformation at a constant temperature above 0.5Tm, there is a steady-state regime where both stress and strain rate remain constant independent of strain. 2. The interdependence of stress, strain rate and temperature during steady-state hot working and secondary creep are similar. 3. During the hardening stage, simultaneous dynamic recovery causes the dislocations to arrange into sub-boundaries. 4. During the steady state, the balance between hardening and recovery maintains the dislocation density constant as subgrains of constant size and misorientation. 5. The subgrains remain equiaxed, indicating that the sub-boundaries continuously break up and reform. It is proposed that this process inhibits recrystallization during high strain-rate deformation. 6. The size of the subgrains and the neatness of their boundaries increase as the temperature increases and the strain rate decreases.

Material Properties Measurements for Selected Materials

Abstract: Hugoniot equation of state measurements were made on Coconino sandstone, Vacaville basalt, Kaibab limestone, Mono Crater, pumice and Zelux (a polycarbonate resin) for pressures to 2 Mb. A single data point was obtained for fused quartz at 1.6 Mb. In addition to the hugoniot studies, the uniaxial compressive stress behavior of Vacaville basalt and Zelux was investigated at strain rates from about 10(exp -5)/sec to 10(exp 3)/second. The data presented include the stress - strain relations as a function of strain rate for these two materials.

Effects of molecular weight and strain rate on the flexural properties of polycarbonate

Abstract: The effects of changes in molecular weight (7000 to 22,000) and strain rate (0.0001 to 4 min.−1) on the flexural properties of polycarbonate have been examined in detail with the use of speciments of different molecular weight prepared by high-energy electron irradiation. The results have been plotted as surfaces which show the dependence of both stress and strain on molecular weight and strain rate, and these surfaces have been described in terms of brittle, transitional, and ductile regions. The relationships between stress or strain and molecular weight in the brittle region have been shown to be hyperbolic. A single failure locus has been found to include all the corresponding stress and strain data obtained at the various molecular weights and strain rates. In the low strength region this locus exhibits a proportionality between stress and strain, while at high strength values, strain becomes a logarithmic function of stress. Stress–molecular weight data obtained at the various rates have been superimposed to form a single composite curve, and the corresponding crossplots of stress–log rate have been treated similarly. It is concluded from these superpositions that an equivalence exists between changes in both molecular weight and strain rate such that a tenfold change in strain rate corresponds approximately to a change of 1000 in molecular weight. Strain-strain rate data obtained at the various molecular weights have also been superimposed in a similar manner. Modulus is shown to increase slowly with decrease in molecular weight and appears to be relatively insensitive to changes in strain rate.

Particle hardening in manganese-doped sodium chloride single crystals

Abstract: The effects of temperature and strain rate changes on the yield stress have been used to analyse the strengthening produced in NaCl single crystals containing 675 mole p.p.m. Mn++. The temperature dependence of the yield stress observed at low temperatures can be interpreted satisfactorily in terms of the thermally activated cutting of particles; the particles are considered to be trimers. The athermal yield stress can be explained in terms of contributions from precipitation hardening by the Orowan mechanism and from coherency stresses due to trimers.