Showing papers on "Strain rate published in 1985"

Higher-order derivative correlations and the alignment of small-scale structures in isotropic numerical turbulence

Abstract: In a three dimensional simulation higher order derivative correlations, including skewness and flatness factors, are calculated for velocity and passive scalar fields and are compared with structures in the flow. The equations are forced to maintain steady state turbulence and collect statistics. It is found that the scalar derivative flatness increases much faster with Reynolds number than the velocity derivative flatness, and the velocity and mixed derivative skewness do not increase with Reynolds number. Separate exponents are found for the various fourth order velocity derivative correlations, with the vorticity flatness exponent the largest. Three dimensional graphics show strong alignment between the vorticity, rate of strain, and scalar-gradient fields. The vorticity is concentrated in tubes with the scalar gradient and the largest principal rate of strain aligned perpendicular to the tubes. Velocity spectra, in Kolmogorov variables, collapse to a single curve and a short minus 5/3 spectral regime is observed.

Shock viscosity and the prediction of shock wave rise times

Abstract: The present study is focused on viscouslike behavior of solids during large‐amplitude compressive stress‐wave propagation. Maximum strain rate in the plastic wave has been determined for 30 steady‐ or near steady‐wave profiles obtained with velocity interferometry methods. The materials include six metals, aluminum, beryllium, bismuth, copper, iron, and uranium, and two insulating solids, magnesium oxide and fused silica. A plot of Hugoniot stress versus maximum strain rate for each material is adequately described by η=aσmh. The exponent m is approximately 4 for all materials while the coefficient a is material dependent. A model is developed which incorporates the observed trends of the shock viscosity data in a three‐dimensional framework. Finite‐difference calculations using the model reproduce the experimental wave profile data.

Viscous Sintering on a Rigid Substrate

Abstract: Two analyses are presented for the sintering kinetics of a porous glass layer on a rigid substrate. The first treatment uses a continuum model, with constitutive equations and the free strain rate derived from an appropriate microstructural model. Predictions are obtained for the sintering kinetics and the magnitude of the tensile stress in the layer. During sintering, shrinkage is not permitted in the plane of the substrate, but the resulting microstructural anisotropy is ignored by the model. A second treatment represents the sintering layer by tubes whose axes are normal to the substrate. The densification kinetics of this model are in reasonable agreement with the results of the continuum model. Therefore, the effects of microstructural anisotropy (pore orientation) are likely to be small, and either model can be used

Kinetics of solution hardening

Abstract: The flow stress of solution hardened single crystals and polycrystals is analyzed with respect to its dependence on temperature and strain rate. An evaluation of literature data, especially at low temperatures and low concentrations in fcc alloys, reveals that the interaction between dislocations and discrete, atomic-sized obstacles (or fixed clusters of them) cannot be responsible for solution hardening. A “trough” model is favored in which the effect of the solutes is postulated to be equivalent to a continuous locking of the dislocations along their entire length, during every waiting period. The macroscopic features of this model are similar to Suzuki’s chemical-hardening model. It can also explain the strong interaction of solution hardening and strain hardening at elevated temperatures, as well as basic features of dynamic strain-aging, in particular its strain dependence.

Analysis of local delaminations and their influence on composite laminate behavior

Abstract: An equation was derived for the strain energy release rate, G, associated with local delamination growth from a matrix ply crack. The critical GC for edge delamination onset in 25/902s graphite epoxy laminates was measured and used in this equation to predict local delamination onset strains in 25/90ns, n = 4, 6, 8 laminates. A simple technique for predicting strain concentrations in the primary load bearing plies near local delaminations was developed. These strain concentrations were responsible for reduced laminate nominal failure strains in laminates containing local delaminations. The influence of edge delamination and matrix crack tip delamination on laminate stiffness and strength was compared.

Effects of strain rate on delamination fracture toughness of graphite/epoxy

Abstract: The objective of this paper is to evaluate various experimental techniques and analysis methods for the characterization of interlaminar fracture toughness, and to determine the effects of strain rate on that property for a graphite/epoxy composite. Mode I interlaminar fracture was investigated by means of a double-cantilever beam (DCB) specimen for AS-4/3501-6 graphite/epoxy. Hinged tabs were used to insure unrestrained rotation at the free ends. Specimens were loaded at quasi-static deflection rates of up to 8.5 mm/s corresponding to crack extension rates of over 51 mm/s. Crack extension was monitored by means of strain gages mounted on the surface of the specimen, or a conductive-paint circuit attached to the edge of the DCB specimen. Continuous records were obtained of load, deflection, and crack extension for determination of the strain energy release rate. The latter was expressed as a power law of the crack extension velocity. Results indicate that the strain energy release rate increases with crack velocity by up to 28 percent for the range of rates considered.

Effect of strain rate on the tensile failure of woven reinforced polyester resin composites

Abstract: Tensile stress-strain curves a r e obtained at s t rain rates of %lo-'+, ~ 1 0 and c1000/s for carbon, Kevlar and glass fibre woven-reinforced polyester resin compos i tes and the variation of modulus, tensile strength and fracture strain with strain rate is determined. A change in the failure mode at the highest strain rates is observed for the glass but not the carbon or Kevlar reinforced composites.

Mechanical properties of polypropylene filled with calcium carbonate

Abstract: The tensile behavior of polypropylene (PP) filled with calcium carbonate particles has been studied using a tensile test. In particular, the effect of strain rate, filler content, and filler size upon the elastic modulus, yield stress, and strain of surface-modified and unmodified particles-filled PP were investigated. The results indicated that the elastic modulus and yield stress of an unmodified system were increased with an increase of strain rate and filler content, and with a decrease of filler size. The yield strain was decreased with an increase of filler content, and with a decrease of filler size, but did not depend on the strain rate. Although the dependence of elastic modulus on the filler size was maintained even by the surface-modified fillers, that dependence on the strain rate and filler content was decreased by such fillers. This may be because the modifier is present at the interface of filler and polymer matrix.

Temperature and strain rate dependence of stress-strain behavior in a nickel-base superalloy

Abstract: The temperature and strain rate dependence of yield and work hardening behavior in elevated temperature stress-strain testing was investigated in the nickel-base superalloy UDIMET 115. This alloy was heat treated to produce a bimodal distribution of hyperfine and coarse γ1 precipitates. Yield behavior is shown to be controlled primarily by dislocation pair cutting of the hyperfine precipitates. Stage II work hardening appears to be governed mainly by the dislocation networks generated on the coarse precipitates as these are bypassed under the Orowan mechanism. Analysis of Stage III work hardening using ϕvs σ plots shows that a steady state exists for the stress-strain deformation of this system. Results are also reported on extremely strong serrated flow at moderately elevated temperatures and a high temperature ductility minimum.

Constitutive Model for Dynamic Loading of Concrete

Abstract: Constitutive properties of concrete under dynamic loading are necessary for the rational analysis of concrete structures subject to impact and impulsive loads. The constitutive model presented herein models microcracking through the use of a continuous damage parameter for which a vectorial representation is adopted. The rate of increase of the damage is dependent on the state of strain as well as on the time rate of strain. The constitutive equations are derived from the strain energy function which is influenced by the accumulated damage. The constitutive model is calibrated using uniaxial tension (or flexural) and uniaxial compression test data. The calibrated model is then used to predict certain other load responses of concrete.

Superplastic Deformation in Fine-Grained MgO 2Al2O3 Spinel

Abstract: Fine-grained polycrystals of MgO 2A12O3 spinel were deformed to large strains at strain rates ranging from 1O-5 to 1O-3 s-1 and at temperatures from 1723 to 1885 K. These polycrystals were ductile at low strain rates and high temperatures; the ductility was especially remarkable at temperatures near the solvus. The mechanism of deformation, which was determined by measuring the change of flow stress with grain size, was dislocation creep, with the stress exponent in the power-law creep equation having a value of 2.1 ± 0.4. Despite deformation of several hundred percent, the grain size remained equiaxed, and deformation led to the evolution of a grain size which depended only on the strain rate and temperature. The initial microstructure had an influence on whether the poly-crystal would fracture or flow; a small initial grain size and a supersaturated solid solution were conducive to ductile flow. The ductility is attributed to dynamic recrystallization. It is proposed that the onset of fracture and the onset of dynamic recrystallization are competitive processes. Conditions which promote dynamic recrystallization also promote ductile flow.

Plastic deformation of glassy amorphous polymers: influence of strain rate

Abstract: The compressive deformation of glassy atactic polymethyl methacrylate has been studied in the temperature range 150

Constitutive relationships for polymeric materials with power-law distributions of relaxation times

Abstract: The linear relaxation modulus of polydisperse polymer melts and solutions can often be approximated by a power law,ct −m over some range of time,t. If, in addition, the nonlinear rheology is given by a separable integral equation, with a strain-dependent factor typical of those observed experimentally, then some commonly observed empirical rules and equations can be readily derived as approximations, namely the Cox-Merz relationship between complex viscosity and steady-state shear viscosity, Bersted's predictions of steady shear stress and first normal-stress difference from a truncated spectrum of linear relaxation times, and the observation of Koyama and coworkers that the ratio of the nonlinear to the linear time-dependent elongational viscosity is independent of strain rate, over a range of strain rates outside the linear regime.

High temperature deformation of ultra-fine-grained oxide dispersion strengthened alloys

Abstract: The high temperature deformation properties of two oxide dispersion strengthened (ODS) alloys, MA 754 and MA 6000, with initial grain sizes of 0.67 µm and 0.26 µm, respectively, have been studied. Tensile tests have been conducted at 1173, 1273, and 1373 K at strain rates ranging from 2 X 10−4 to 5 s−1. Tension creep tests were conducted on MA 6000 at 1273 K to extend the strain rate regime to 3 X 10−8 s−1. Microstructures of both undeformed and deformed samples have been characterized by transmission electron microscopy. MA 754 exhibits a maximum elongation of 200 pct and a maximum strain rate sensitivity of 0.30 at 1273 K. MA 6000 is superplastic, exhibiting a maximum elongation of over 300 pct and a maximum strain rate sensitivity of 0.47 at 1273 K. The microstructure of MA 754 is unstable during deformation, showing recrystallized grains and grains which have grown to 1 µm in diameter. No evidence for ordinary recrystallization is found for MA 6000, and grain growth is slight. For both alloys, strain rates less than about 1 s−1 alter the initial microstructure and prevent grain coarsening on subsequent annealing at higher temperature. Deformation of the fine-grained MA 6000 can be described as a combination of power law creep and diffusional (Coble) creep, with a threshold stress caused by the presence of γt’ particles existing only for the diffusional creep process. Structural instabilities do not permit a simple description of deformation of MA 754.

The effect of a lathyritic diet on the sensitivity of tendon to strain rate.

Abstract: While the tensile failure properties of rat-tail tendon depend on strain rate, the sensitivity to strain rate decreases with age, especially during sexual maturation. The object of this study was to determine the effect of an experimental model of chronic lathyrism on age-dependent changes in the sensitivity of developing tendon strength to strain rate. Tensile failure experiments were conducted at high and low strain rate on tendons excised from test and control animals aged 1 to 6 mo. The tensile "yield" response of tendon was significantly affected by the diet resulting in a reduced tensile strength and failure strain. While the sensitivity of tendon failure to strain rate was slightly elevated by the experimental diet, age-dependent changes compared with controls. Since the diet supplement is thought to inhibit covalent crosslinking of collagen in the developing tendon, other factors are likely responsible for decrease in the sensitivity of tendon strength to strain rate during maturation.

Rheology of Glacier Ice

Abstract: A new method for calculating the stress field in bounded ice shelves is used to compare strain rate and deviatoric stress on the Ross Ice Shelf, Antarctica. The analysis shows that strain rate (per second) increases as the third power of deviatoric stress (in newtons/sq meter), with a constant of proportionality equal to 2.3 x 10 to the -25th.

The Effect of Straining on the Transport of Hydrogen in Iron, Nickel, and Stainless Steel

Abstract: Hydrogen permeation transients for low-carbon iron were measured at room temperature; pure nickel and 17Cr-12Ni austenitic stainless steel (SS) specimens were electrochemically charged with hydrogen. The results for unstrained specimens were compared with those obtained under conditions of continuous stretching. A slight increase in the permeation rate for iron was observed in the range of elastic deformation, with no change in effective diffusivity. Plastic deformation caused a substantial reduction of both the diffusivity and permeability of hydrogen. The magnitude of these effects depended on the amount of strain but was independent of the strain rate; this suggested they were caused by an enhanced trapping of hydrogen. Only a slight influence of plastic deformation on the effective diffusivity and permeability of hydrogen was observed in nickel and austenitic SS.