Showing papers on "Strain rate published in 1971"

Stress-strain behavior of styrene-acrylonitrile/glass bead composites in the glassy region

Abstract: The effects of temperature, strain rate and filler content on tensile properties of SAN/glass bead composites are studied. A point of discontinuity on the stress-strain curves for unannealed composites is investigated, annealing results in smooth curves with no discontinuities. A simple model for the filler effect on yield stress is suggested and shown to be in a good agreement with experimental data. A double shifting procedure to account for the temperature and filler effects on yield stress as a function of strain rate is proposed. A single master curve that can be represented by the equation: relates composite yield stress to strain rate, temperature and filler volume fraction.

Stress relaxation and mechanical behavior of metals

Abstract: Stress relaxation tests have been made in the temperature range 200° to 400°C on two materials, the Mg-Al eutectic alloy and commercial purity zirconium. The materials represent respectively high and low homologous temperature regimes. The novel features of the tests are the use of high speed, high sensitivity digital measurement techniques and the direct reduction of the data to stress-strain rate curves. It was possible in this way to obtain phenomenological information on the material behavior over a very large range of strain rate with very few specimens. Comparison was made with results obtained by more conventional differential strain rate tests. The test lends itself well to establishing the phenomenology of mechanical behavior of metals.

The measurement of strain-rate sensitivity in superplastic alloys

Abstract: The currently used methods of determining strain-rate sensitivities,m, of superplastic alloys from strain-rate change tests are examined and found to be unsatisfactory. The stressrelaxation method is also investigated and is shown to be applicable to the superplastic Al-Cu eutectic. However, them-values derived by this method, and by a variant of the strain-rate change technique, are significantly lower than those obtained using the conventional strain-rate change tests and this is explained in terms of microstructural factors.

Yielding Behavior of Glassy Amorphous Polymers

Abstract: The effect of thermal history on yielding behavior of atactic polystyrene, isotactic polystyrene, polycarbonate, and polymethyl methacrylate was examined by correlating the deformed microstructure with measured density changes and compressive stress‐strain studies. Electron micrographs of bulk polymers and thin films demonstrate the tendency for well‐annealed materials to undergo localized shear deformation at 100–1000‐A interspacings, and density measurements show an over‐all density increase of about 0.15% upon plastic deformation. Rapid cooling from the melt decreases both the material density (about−0.04%) and the tendency for plastic strain to localize into narrow bands. Compressive stress‐strain studies in which the strain rate, test temperature, and thermal history were systematically varied show a semilogarithmic relationship between nominal strain rate and yield stress. These data were analyzed according to an Eyring‐type exponential model where the ``shear activation volume'' and the ``activatio...

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.

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.

Dynamic Compressive Strength and Failure of Steel Reinforced Epoxy Composites

Abstract: Results of an experimental program systematically evaluating the deformation and fracture of steel wire reinforced epoxy composite systems are presented. The program involved mechanical testing in the strain rate range 10-5 to 103/sec and impact testing of composite specimens using massive elastic targets at strain rates approximately 104/sec. Specific results presented include static and dynamic prop erties, strain rate sensitivity, information on the nature and character of dynamic fracture, influence of specimen geometry and reinforce ment spacing, and high speed photographs of dynamic failure modes. Further, a simplified energy criterion is proposed for predicting failure modes and critical velocities.

Dependence of Strain-Rate Effects on Deformation Mechanism and Rock Type

Abstract: Results are reported for sixty-nine triaxial compression tests run on a marble, a lithographic limestone, a very fine-grained sandstone, and a siltstone at room temperature, dry, confining pressures of 1,000 and 2,000 bars, and at strain rates of $10^{-3}$, $10^{-4}$, $10^{-5}$, $10^{-6}$, and $10^{-7}$/second A decrease in strain rate from $10^{-3}$ to $10^{-7}$/second at 2,000 bars confining pressure caused a decrease in strength at 2 percent strain of about 33 percent for the marble and 84 percent for the limestone, but had no significant effect on either the sandstone or siltstone Similar results were obtained for the tests run at 1,000 bars Over the same range of strain rates, the strength at 10 percent strain decreased by about 10 percent for the marble at 2,000 bars, but increased slightly more than 11 percent at 1,000 bars The limestone showed a small increase at 2,000 bars Again, there was no significant change for the sandstone and siltstone at either confining pressure The observed incre

The influence of strain rate dependent work hardening on the necking strain in α-titanium at elevated temperatures

Abstract: Titanium has a “blue brittle” effect like that observed in steels. At the temperature of minimum ductility, the neck in the tensile specimen is very sharply defined. Above the blue brittle temperature, the tensile elongation of this metal undergoes a very significant increase that is due to the development of a diffuse neck. Diffuse necks are also observed in superplasticity. However, the conditions that cause the diffuse neck above the blue brittle temperature in titanium are different from those encountered in superplasticity. In superplasticity, the necking phenomenon is associated with a direct dependence of the flow stress on the strain rate, whereas in the present case it is primarily due to rate dependent work hardening associated with dynamic strain aging.

Strain-rate and strain-rate-history effects in several metals in torsion

Abstract: A machine for testing thin-walled tubes in torsion at shear-strain rates up to 25/sec is described Results of constant and variable-strain-rate tests are presented for 1100-0 aluminum, AISI 1020 steel, and 50-A titanium Results indicate that 1100-0 aluminum is very slightly strain-rate sensitive, but steel and titanium are noticeably sensitive to both strain rate and strain-rate history Variable-rate tests show that subsequent dynamic loading on a statically prestrained specimen causes an increase in the flow stress in steel and a decrease in the flow stress in titanium

The Portevin-Le chatelier effect in vanadium

Abstract: In order to examine the temperature and strain-rate dependences of the Portevin-Le Chatelier (P-L) effect, tensile tests were carried out, using vanadium containing 170 p.p.m. of carbon and 42 p.p.m. of oxygen. The results obtained were as follows. 1. The P-L effect is observed in the temperature range from about 200°C to 500°C for a strain rate of 3.3 × 10−5/sec. 2. The serrations observed after the initial Luders strain are classified into two types, relatively fine serrations (Type A) and coarser serrations (Type B) which appear intermittently and are observed clearly in the higher temperature region. 3. Type A serration is associated with a very high work-hardening rate, while the rate is smaller with Type B serration. 4. In the lower temperature region, the P-L effect is observed only when the deformation proceeds beyond some critical strain, and the serration increases in stress amplitude with strain, while, in the higher temperature region, Type A serration disappears abruptly at some crit...

Relationship between compressive yield and tensile behavior in glassy thermoplastics

Abstract: The effect of temperature and strain rate on the compressive yield behavior of polystyrene is compared with the effect of the same variables on crazing in tension. The results support the conclusion of other, more extensive work, which shows that crazing involves the same types of molecular processes as those which occur during deformation under compression and shear. An improved method of measuring compressive stress–strain curves is then described, and the compressive yield stress is also compared with an extrapolated tensile yield stress. The difference between the two is in line with concepts which assume a dependence of yield stress on the state of hydrostatic tension (or compression). It can be adequately described by the Mohr-Coulomb yield criterion. Application of this criterion also enables a theoretical stress strain curve in tension to be derived from other results in compression. Comparison of the tensile stress–strain curve so obtained with those which can be directly measured with other plastics, supports the hypothesis that crazing is favored by a marked decline in engineering stress during tensile elongation (plastic instability).

Deformation of Polyethylene at High Pressure

Abstract: The influence of pressure, deformation rate, and pressure fluid on the mechanical properties of high‐molecular‐weight polyethylene is examined. A miniature motor‐driven testing machine which operates completely enclosed in a high‐pressure vessel is employed. This device allows for the first time in any material measurement of the sensitivity of the flow stress σ to an instantaneous change of strain rate under superposed high hydrostatic pressure. From these data and the pressure dependence of σ it is possible to calculate the activiation volume V* for plastic flow as a function of pressure; it is suggested that V* is the appropriate parameter to describe the pressure dependence of a kinetic process such as plastic deformation. It is found that samples deformed in n‐pentane generally exhibit a reduced flow stress relative to those tested in water. In addition, it is observed that V* for specimens tested in pentane is significantly less than for those tested in water (∼230 A at 1 atm as opposed to ∼266 A3)....

Time and temperature dependence of the mechanical properties of polystyrene bead foam

Abstract: Tensile and compressive properties of polystyrene bead (PSB) foams at room temperature for strain rates from 10−3 to 105 min−1 can be represented as nearly linearly increasing functions of modulus or stress versus the logarithm of the strain rate. The shear modulus and tensile data, including failure properties, on 0.054 g/cc PSB foam at various temperatures and strain rates can be represented by master curves of log (stress or modulus) versus log (reduced strain rate). These master curves are formed by a time and temperature superposition method, wherein data at one temperature are superposed on data at another temperature by a shift along the log (strain rate) axis. These time–temperature shift factors are calculated using a form of the Arrhenius equation.

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.

A strain-rate dependent crack model

Abstract: A uniformly moving crack, preceded by a thin plastic zone, in which the yield stress is linearly dependent on the strain rate, is considered, in both plane and anti-plane strain. This model is used to calculate the variation of the stress intensity factor with crack velocity in the limit of small-scale yielding and the unsteady motion of a crack with a finite plastic zone in anti-plane strain. It is shown that this latter motion differs only slightly from what would be obtained if the yielding were small scale and that if the parameters are chosen appropriately the model can explain the observed relativelyslow velocities of crack propagation.

Tensile properties of 0.05 to 0.20 Pct C TRIP steels

Abstract: The uniaxial tensile properties of a series of TRIP steels of varying carbon contents and processing histories were determined over a wide range of test temperatures. The yield strengths at room temperature varied both with the deformation temperature (over the range 250° to 550°C) and with the carbon content (0.05 to 0.20 pct). Possible reasons for these variations are advanced. For all steels, the −100°C yield strengths were substantially lower than the 100°C yield strengths. The minima and maxima in the yield strengths vs temperatures curves were especially pronounced for the steels processed at the lowest deformation temperatures. Both the rate of work hardening and the elongation were influenced by the strain-induced austenite-to-martensite transformation. The rate of strain hardening and the rate of production of strain-induced martensite (per unit strain) increased with decreasing temperature.

Deformation Processes in the Intermediate Phase NiAl

Abstract: The activation energies and volumes of deformation have been evaluated from the effects of changes in strain rate and temperature on the flow stress of single-crystal and polycrystalline NiAl. The stress-dependence of these parameters, the appearance of slip-traces, and the agreement with a theoretical analysis are consistent with the rate-controlling mechanism at low temperatures being the nucleation of dislocation/kink pairs over a Peierls-Nabarro barrier. Deformation at high temperatures is complex and several mechanisms, including dislocation climb and cross-slip, appear to be important.

Solid-solution strengthening in Ni-C alloys

Abstract: “Pure” nickel and Ni-C solid-solution alloys of various grain sizes (ASTM no. 2 to 10), with eight different carbon concentrations in the range 0.008 to 0.304 wt pct, were strained in tension between 4° and 474°K at a strain rate of 8.3 × 10−5per sec. The critical resolved shear stress (CRSS) was independent of temperature in the range 200° to 474° K (athermal region). Below 200°K, the CRSS increased sharply with decreasing temperature, the increase being larger for alloys of high carbon concentration. Both the temperature-dependent and the athermal alloy hardening were found to be linear functions of carbon concentration. The strain-rate sensitivities of flow stress of alloys did not change with strain and were larger for alloys of higher carbon concentrations. The Hall-Petch relation was used to calculate the CRSS of Ni-C single crystals, Τ0f, so that the data can be compared with existing alloy hardening theories. The data are compatible with the solid-solution theory of Friedel in which the hardening is attributed to both elastic and electrical interactions between dislocation cores and solute atoms.

Magneto-Plastic Effect in Nickel Single Crystals

Abstract: It has been observed that the flow stress of a nickel single crystal can be lowered about 15% by applying an alternating magnetic field parallel to the specimen axis during plastic deformation at -194°C. This phenomenon, named by us as the magneto-plastic effect, can be interpreted qualitatively in terms of a concept that oscillating magnetic domain walls give a force on dislocations. In order to clarify this effect, its dependence on the temperature, plastic strain, strain rate, and on the strength and frequency of the alternating magnetic field applied have been measured. The activation volumes in relation to a thermally activated process of dislocation motion were also obtained from the stress relaxation curves measured after and before the application an alternating magnetic field, and they were compared mutually at various values of the shear stress.

Magneto-Plastic Effect in Nickel Single Crystals

Abstract: It has been observed that the flow stress of a nickel single crystal can be lowered about 15% by applying an alternating magnetic field parallel to the specimen axis during plastic deformation at -194°C. This phenomenon, named by us as the magneto-plastic effect, can be interpreted qualitatively in terms of a concept that oscillating magnetic domain walls give a force on dislocations. In order to clarify this effect, its dependence on the temperature, plastic strain, strain rate, and on the strength and frequency of the alternating magnetic field applied have been measured. The activation volumes in relation to a thermally activated process of dislocation motion were also obtained from the stress relaxation curves measured after and before the application an alternating magnetic field, and they were compared mutually at various values of the shear stress.

The effect of time and temperature on the mechanical behavior of epoxy composites

Abstract: A crosslinked epoxy resin consisting of a 60/40 weight ratio of Epon 815 and Versamid 140 and composites of this material with glass beads, unidirectional glass fibers and air (foams) were tested in tension, compression and flexure to determine the effect of time and temperature on the elastic properties, yield properties and modes of failure. Unidirectional continuous fiber-filled samples were tested at different fiber orientation angles with respect to the stress axis. Strain rates ranged from 10−4 to 10 in./in.-min and the temperature from −1 to 107°C. Isotherms of tangent modulus versus strain rate were shifted to form master modulus curves. The moduli of the filled composites and the foams were predictable over the entire strain rate range. It was concluded that the time-temperature shift factors for tangent moduli and the time-temperature shift factors for stress relaxation were identical and were independent of the type and concentration of filler as well as the mode of loading. The material was found to change from a brittle-to-ductile-to-rubbery failure mode with the transition temperatures being a function of strain rate, filler content, filler type and fiber orientation angle, indicating that the transition is perhaps dependent on the state of stress. In the ductile region, an approximately linear relationship between yield stress and log strain is evident in all cases. The isotherms of yield stress versus log strain rate were shifted to form a practically linear master plot that can be used to predict the yield stress of the composites at any temperature and strain rate in the ductile region. The time-temperature shift factors for yielding were found to be independent of the type, concentration and orientation of filler and the mode of loading. Thus, the composite shift factors seem to be a property of the matrix and not dependent on the state of stress. The compressive-to-tensile yield stress ratio was practically invariant with strain rate for the unfilled matrix, while fillers and voids raised this ratio and caused it to increase with a decrease in strain rate. The yield strain of the composites is less than the unfilled matrix and is a function of fiber orientation and strain rate.