Shear-strain-rate effects in a high-strength aluminum alloy

Methods of investigating the plastic flow of materials at high rates are reviewed, and experiments are described in which thin-walled tubular specimens were subjected to dynamic torsional loading. These experiments were performed using a modified version of a torsional Hopkinsonbar apparatus used in earlier work. The results show that, at strain rates of order 10³ sec⁻¹, the stress-strain curve of the alloy tested does not differ significantly from that found at 10⁻³ sec⁻¹. In tests involving the propagation of a stress increment, however, it was found that the speed of propagation was that of elastic shear waves, indicating that the initial response of the material is essentially rate dependent.     

Techniques for measuring stress-strain relations at high strain rates

The qualitative dependence of the mechanical behavior of some materials on strain rate is now well known. But the quantitative relation between stress, strain and strain rate has been established for only a few materials and for only a limited range. This relation, the so-called constitutive equation, must be known before plasticity or plastic-wave-propagation theory can be used to predict the stress or strain distribution in parts subjected to impact stresses above the yield strength. In this paper, a brief review of some of the experimental techniques for measuring the stress, strain, strain-rate relationship is given, and some of the difficulties and shortcomings pointed out. Ordinary creep or tensile tests can be used at plastic-strain rates from 10^?8 to about 10^?1/sec. Special quasi-static tests, in which the stress- and strain-measuring devices as well as the specimen geometry and support have been optimized, are capable of giving accurate results to strain rates of about 10²/sec. At higher strain rates, it is shown that wave-propagation effects must be included in the design and analysis of the experiments. Special testing machines for measuring stress, strain and strain-rate relationships in compression, tension and shear at strain rates up to 10⁵/sec are described, and some of the results presented. With this type of testing machine, the analysis of the data requires certain assumptions whose validity depends upon proper design of the equipment. A critical evaluation of the accuracy of these types of tests is presented.     

Measuring dynamic-strain profile and history with a drum camera

A low-speed drum camera has been used to continuously record the strain profile in several different types of impact tests. The drum camera was selected because it is inexpensive and simple to use, and it overcomes many of the inherent disadvantages of other dynamic-recording techniques. The use of the drum camera in recording the strain profile in tensile and torsional impact is illustrated, as well as the determination of impact velocity in collinear plastic impact of two bars. A detailed discussion of determination of the strain profile from the tensile-test film record is presented, including an analysis of the accuracy achieved. It was found that the drum camera is capable of measuring strains to an accuracy of approximately 1 percent for a gage length of 0.05 in. Strains varying from 1 to 100 percent were measured in the tensile test, with a nominal strain rate of 20 sec^?1. Strain rates of up to 2000 sec^?1 were successfully recorded in the torsional impact tests.     

Impulsive Tension of Hexane and Glycerol under Shock‐Wave Loading

The strength of hexane and glycerol was measured under impulsive tension produced by interaction of a triangular compression pulse with a free surface. The tests were performed for strain rates of 10⁴–10⁵ sec^–1. It is established that the strength of hexane is equal to 14 MPa and does not depend on the strain rate, whereas the strength of glycerol increases from 57 to 142 MPa with an order of magnitude increase in the strain rate. The possibility of using the model of homogeneous nucleation to interpret the data obtained is discussed.     

Optical and mechanical properties of birefringent polymers

The stress-strain behavior and corresponding birefringence of several polymers have been investigated within a limited range of temperatures (from ?65 to 70°F) and strain rates (from 0.0027 to 0.1613 sec^?1). One of these materials, a polyethylene resin, has been studied in more detail to ascertain the existence of a simple relationship between stress history, temperature, strain rate and birefringence. When the results were compared with the photoviscoelastic relations developed by E. H. Dill for a simple rheological material, it was concluded that the polyethylene tested does not completely satisfy this model. Polyethylene as well as the other materials investigated—nylon, a polyester, cellulose acetate butyrate, cellulose nitrate—exhibits a linear relation between birefringence and strain, independent of rate within the limits of the present experimental range.     

Compression-impact testing of aluminum at elevated temperatures

This paper presents and experimental technique for determining compressive stress-strain curves well into the plastic range of relatively soft metals at strain rates from 300 to 2000 sec^?1 at six temperatures from 30 to 550° C. More than 100 curves were obtained on annealed 1100° F aluminum. The strain-rate dependence in these tests could be fitted quite well either by a power function (log-log plot) or by a semilogarithmic plot, but the power function gave a better correlation of the present data with that obtained at lower strain rates by Alder and Phillips.¹     

Experimental analysis of dynamic mechanical properties for artificially frozen clay by the split Hopkinson pressure bar

A device for impact compression experiments is the split Hopkinson pressure bar with a refrigerating attemperator. Data for incident and reflected waves are obtained by the measuring technique with strain gauges, and data for transmitted waves are obtained by the measuring technique with semiconductor gauges. Static compression tests of frozen clay are conducted at an identical temperature and different strain rates of 0.001 and 0.01 sec ⁻¹ . Dynamic stress-strain curves are obtained at strain rates of 360–1470 sec ⁻¹ . The low and high temperatures correspond to high and low strain rates, respectively. It is shown that both the temperature and strain rate affect the frozen soil deformation process. Different dynamic stress-strain curves obtained at the same temperature but different strain rates are found to converge. The test results indicate that frozen soil has both temperature-brittleness and impact-brittleness.     

Critical Stresses in Spalling and Dynamic Rupture of Metals

Results of dynamic rupture tests of a series of metals obtained using a composite Hopkinson bar and shock-wave loading of plane specimens are described. It is shown that the actual rupture strength at a strain rate of 5 · 10³ sec⁻¹ is very close to the spall strength at higher strain rates. Results of testing the same metals using a composite Hopkinson bar within a temperature range of 20–350°C are given. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 46, No. 6, pp. 103–107, November–December, 2005.     

High-strain-rate tests on titanium 6-6-2 utilizing a unique rate-testing machine

This paper presents the design of a unique materials-testing system capable of medium strain rates of from 10^?4 to 10²/s. The design incorporates both closed-loop hydraulic operation with that of open-loop pneumatic operation. A novel design permits accurate specimen alignment and a stiff frame which exceeds 17×10⁶ lb/in. (11.7×10⁴ MPa). The mechanine is able to perform conventional tension/compression tests, fatigue tests and, with slight modification, biaxial-stress-tube tests and triaxial-stress tests. The accurate alignment capability coupled with high frame stiffness and the pneumatic operation enables the testing of brittle materials with rigid grips. Titanium 6-6-2 was tested in both tension and compression at strain rates from 10^?4 to about 10/s at four selected temperatures. The material showed a slight strain-rate sensitivity. Yield stress was shown to increase with strain rate while ductility decreased at each test temperature.     

Synchronous Full-Field Strain and Temperature Measurement in Tensile Tests at Low,Intermediate and High Strain Rates

Tensile tests with simultaneous full-field strain and temperature measurements at the nominal strain rates of 0.01, 0.1, 1, 200 and 3000 s^?1 are presented. Three different testing methods with specimens of the same thin and flat gage-section geometry are utilized. The full-field deformation is measured on one side of the specimen, using the DIC technique with low and high speed visible cameras, and the full-field temperature is measured on the opposite side using an IR camera. Austenitic stainless steel is used as the test material. The results show that a similar deformation pattern evolves at all strain rates with an initial uniform deformation up to the strain of 0.25–0.35, followed by necking with localized deformation with a maximum strain of 0.7–0.95. The strain rate in the necking regions can exceed three times the nominal strain rate. The duration of the tests vary from 57 s at the lowest strain rate to 197 μs at the highest strain rate. The results show temperature rise at all strain rates. The temperature rise increases with strain rate as the test duration shortens and there is less time for the heat to dissipate. At a strain rate of 0.01 s^?1 the temperature rise is small (up to 48 °C) but noticeable. At a strain rate of 0.1 the temperature rises up to 140 °C and at a strain rate of 1 s^?1 up to 260 °C. The temperature increase in the tests at strain rates of 200 s^?1 and 3000 s^?1 is nearly the same with the maximum temperature reaching 375 °C.     

PLASTIC BEHAVIOR AND CONSTITUTIVE MODELING OF ARMOR STEEL OVER WIDE TEMPERATURE AND STRAIN RATE RANGES

Plastic behavior of 603 armor steel is studied at strain rates ranging from 0.001 s-1 to 4500 s-1 , and temperature from 288 K to 873 K. Emphasis is placed on the effects of temperature, strain rate, and plastic strain on flow stress. Based on experimental results, the JC and the KHL models are used to simulate flow stress of this material. By comparing the model prediction and the experimental results of strain rate jump tests, the KHL model is shown to have a better prediction of plastic behavior under complex loading conditions for this material, especially in the dynamic region.     

Characterization of Anisotropic Polymeric Foam Under Static and Dynamic Loading

An orthotropic polymeric foam with transverse isotropy (Divinycell H250) used in composite sandwich structures was characterized at various strain rates. Uniaxial experiments were conducted along principal material axes as well as along off-axis directions under tension, compression, and shear to determine engineering constants, such as Young??s and shear moduli. Uniaxial strain experiments were conducted to determine mathematical stiffness constants, i. e., C _ij . An optimum specimen aspect ratio for these tests was selected by means of finite element analysis. Quasi-static and intermediate strain rate tests were conducted in a servo-hydraulic testing machine. High strain rate tests were conducted using a split Hopkinson Pressure Bar system built for the purpose using polymeric (polycarbonate) bars. The polycarbonate material has an impedance that is closer to that of foam than metals and results in lower noise to signal ratios and longer loading pulses. It was determined by analysis and verified experimentally that the loading pulses applied, propagated along the polycarbonate rods at nearly constant phase velocity with very low attenuation and dispersion. Material properties of the foam were obtained at three strain rates, quasi-static (10^?4 s^?1), intermediate (1 s^?1), and high (10³ s^?1) strain rates. A simple model proposed for the Young??s modulus of the foam was in very good agreement with the present and published experimental results.     

Extensional flow of polystyrene melt

Summary The extensional flow of molten polystyrene was studied in the strain rate range of 7.8×10^–4 sec^–1 to 2.2×10^–2 sec^–1 at a test temperature of 300F (149 C) Extensional viscosity was compared to shear viscosity measured at the same degree of stress and temperature and found to be from 3 to 350 times greater in magnitude but much less stress dependent.     

Relationship between Local Strain Jumps and Temperature Bursts Due to the Portevin-Le Chatelier Effect in an Al-Mg Alloy

Local strain and temperature of an AA5754-O aluminum alloy sheet have been full-field measured during monotonous tensile tests carried out at room temperature. Sharp strain increases and temperature bursts which are locally generated by the Portevin-Le Chatelier phenomenon have been measured at the same point for two strain rates: V2?=?1.9?×?10^?3?s^?1 and V10?=?9.7?×?10^?3?s^?1. A relationship, which is based on the underlying physical mechanisms, has been established between the strain and the temperature and experimentally verified for the highest strain rate V10. The discrepancy between the theoretical and experimental results for the lowest strain rate V2 suggests that the localized plastic deformations do not follow an adiabatic transformation. Such a set-up seems to offer a direct and experimental method to check the adiabatic character of localized plastic deformations.     

The mechanisms governing the activation of dislocation sources in aluminum at different strain rates

This article examines the time to activate Frank–Read sources in response to macroscopic strain rates ranging from 10¹ s⁻¹ to 10¹⁰ s⁻¹ in aluminium under athermal conditions. We develop analytical models of the bowing of a pinned dislocation segment as well as numerical simulations of three dimensional dislocation dynamics. We find that the strain rate has a direct influence on both the activation time and the source strength of Frank–Read sources at strain rates up to 10⁶ s⁻¹, and the source strength increases in almost direct proportion to the strain rate. This contributes to the increase in the yield stress of materials at these strain rates. Above 10⁶ s⁻¹, the speed of the bowing segments reaches values that exceed the domain of validity of the linear viscous drag law, and the drag law is modified to account for inertial effects on the motion of the dislocation. As a result the activation times of Frank–Read sources reach a finite limit at strain rates greater than 10⁸ s⁻¹, suggesting that Frank–Read sources are unable to operate before homogeneous nucleation relaxes elastic stresses at the higher strain rates of shock loading. Elastodynamic calculations are carried out to compare the contributions of Frank–Read sources and homogeneous nucleation of dislocations to plastic relaxation. We find that at strain rates of 5×10⁷ s⁻¹ homogeneous nucleation becomes the dominant generation mechanism.     

Achieving high strength and high ductility in nanostructured metals: Experiment and modelling

Engineering nanostructures in metallic materials such as nanograins and nanotwins can promote plastic performance significantly. Nano/ultrafine-grained metals embedded in coarse grains called bimodal metals and nanotwinned polycrystalline metals have been proved to possess extensively improved yield strength whilst keeping good ductility. This paper will present an experimental study on nanostructured stainless steel prepared by surface mechanical attrition treatment (SMAT) with surface impacts of lower strain rate (10 s^?1–10³ s^?1) and higher strain rate (10⁴ s^?1–10⁵ s^?1). Microstructure transition has been observed from the original γ-austenite coarse grains to α′-martensite nanograins with bimodal grain size distribution for lower strain rates to nanotwins in the ultrafine/coarse grained austenite phase for higher strain rates. Meanwhile, we will further address the mechanism-based plastic models to describe the yield strength, strain hardening and ductility in nanostructured metals with bimodal grain size distribution and nanotwinned polycrystalline metals. The proposed theoretical models can comprehensively describe the plastic deformation in these two kinds of nanostructured metals and excellent agreement is achieved between the numerical and experimental results. These models can be utilized to optimize the strength and ductility in nanostructured metals by controlling the size and distribution of nanostructures.     

Impulsive Tension of Ethanol under Shock Loading

This paper reports results from experimental studies of the strength of ethanol under impulsive tension due to interaction of a triangular compression pulse with the free surface. The experiments were performed in the range of strain rates 4 ⋅ 10⁴-4 ⋅ 10⁵ sec⁻¹. It is established that the failure of ethanol is a two-stage process. In the first stage at a negative pressure of about 14 MPa, pore formation begins, which proceeds at a rather low rate and is manifested as an inflection on the free-surface velocity profile. In the second stage, the porosity growth rate increases, resulting in formation of a spalling pulse. The possibility of using the model of homogeneous nucleation to interpret experimental data is discussed.__________Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 46, No. 4, pp. 29–38, July–August, 2005.     

Dynamic viscoelastic response of bone

The dynamic stress-strain characteristics of beef-femur bone as a function of elapsed post-mortem time have been determined directly using the split-Hopkinson-bar technique. Specimens were fabricated from dense cortical material of the posterior part of the femoral midshaft and and subjected to dynamic compressive loading (1～6) × 10⁸ psi/sec covering a wide range of strain rates (10～450 sec^?1). Eighty-two test runs were conducted using 43 bone specimens for a range of post-mortem ages (1～240 days). A linear-viscoelastic model describing the mechanical behavior of bone was obtained, including an estimate of the parameters immediately after death.     

Thermo-mechanical response of Al 6061 with and without equal channel angular pressing (ECAP)

The thermo-mechanical responses of Al 6061 before and after equal channel angular pressing (ECAP) at different strain rates and temperatures were measured. Al 6061 was solution heat treated before ECAP pressing at room temperature and subjected to up to three passes. After pressing, the billets were aged at 100 °C for 2 days. An as-received Al 6061-T651 was studied similarly to investigate the differences between processed and non-processed specimens. The responses of ECAP material were determined at −30, 22, 125 and 250 °C, and at strain rates from 10⁻⁵ to 2530 s⁻¹; the 6061-T651 specimens were subjected to uniaxial compressive loading at −31, 22, 85, 150, 230 and 315 °C, and strain rates ranging from 10⁻⁵ to 2200 s⁻¹. It was found that, the ECAP process increases the strength versus the T651 condition. Additionally, the Al 6061 ECAP is not sensitive to strain rate at room and lower temperatures, but the sensitivity increases as the number of passes and/or temperature are increased and this is the same for the non-processed material. Increasing the number of passes increases the flow stress at room and lower temperatures, has almost no effect at 125 °C and decreases at 250 °C. For both materials, the dynamic flow stress is higher than the stress at quasi-static strain rates even when the quasi-static strain rate regime is insensitive to strain rate. The Al 6061 has strong texture after one pass but steadily increases as the number of passes are increased. This is the first study that reports on the thermo-mechanical responses of ECAP and non-ECAP Al 6061 at such a wide range of strain rates, including dynamic, and temperatures.