Bulletin of the American Physical Society
15th APS Topical Conference on Shock Compression of Condensed Matter
Volume 52, Number 8
Sunday–Friday, June 24–29, 2007; Kohala Coast, Hawaii
Session M3: Stress-Strength Measurements II |
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Chair: Rip Collins, Lawrence Livermore National Laboratory Room: Fairmont Orchid Hotel Plaza I |
Wednesday, June 27, 2007 10:30AM - 10:45AM |
M3.00001: Shear Strength Response of the Aluminium Alloy 6082-T6 During One-Dimensional Shock Loading John Harrigan, Jeremy Millett, Neil Bourne The measurement of shear strength via the use of lateral stress gauges has been shown to be a viable technique in a number of materials. An experimental investigation on the intermediate-rate behaviour and shock response of the aluminium alloy, 6082-T6, is reported here. Results obtained using the lateral stress gauge technique show that the shear strength increases with impact stress. The lateral stress behind the shock front is seen to be relatively flat, unlike many other face-centred cubic metals and alloys, where a decrease in lateral stress indicates an increase in shear strength. This unusual response may be a reflection of the high stacking fault energy of aluminium and its alloys resulting in a reduction of the work hardening ($i.e$. increases in dislocation and/or twin density). Further plate impact results show that the Hugoniot of 6082-T6 is in effect identical to that of the more widely known 6061-T6. Split Hopkinson pressure bar results are used to provide a fuller picture of the rate-dependant behaviour of 6082-T6 over a range loading rates and conditions. Key words: shear strength, aluminium alloy, one-dimensional shock [Preview Abstract] |
Wednesday, June 27, 2007 10:45AM - 11:00AM |
M3.00002: Constitutive Model Constants for Low Carbon Steels from Tension and Torsion Data Nachhatter Brar, Vasant Joshi, Bryan Harris Low carbon C1010 steel is characterized under tension and torsion to determine Johnson-Cook (J-C) strength model constants. Constitutive model constants are required as input to computer codes to simulate projectile (fragment) impact on structural components made of this material. J-C model constants (A, B, n, C, and m) for the alloy are determined from tension and torsion stress-strain data. Tension tests are performed at a strain rate of $\sim $1/s at room temperature. Tests at high strain rates are performed at high temperatures to 750$^{\circ}$C. J-C strength model constants determined from these data are: A=367 MPa, B=700 MPa, n=0.935, C=0.045, and m=0.643. Similar values for other low carbon steels (1006, 1008, and 1020) are compared. Torsion tests at quasi-static and high strain rates are performed at room and high temperatures. J-C model constants are evaluated from equivalent tensile stress-strain data obtained from torsion data using von Mises flow rule. These constants are compared to those determined from directly measured tensile data. [Preview Abstract] |
Wednesday, June 27, 2007 11:00AM - 11:30AM |
M3.00003: Strength of materials in the diamond anvil cell to 1 Mbar Invited Speaker: The yield strength of materials at high pressure has diverse applications to interpretation of static and dynamic experiments, understanding mechanical performance, and constraining the rheology of planetary interiors. In recent years, diamond anvil cell techniques have been developed to measure lattice strain under non-hydrostatic loading using synchrotron x-rays. These studies constrain yield strength and provide insights into elastic moduli, equation of state, and texture development. Here we report results using this method to characterize the strength of a suite of metals (e.g., W, Re) and ceramics (e.g. B$_{6}$O, MgSiO$_{3})$ to pressures up to 1 Mbar. In general, strength increases with compression at a rate greater than the shear modulus, implying significant strain hardening under diamond cell loading. For W, the strength under static loading is comparable to that observed under dynamic quasi-isentropic loading. At pressures of 20-80 GPa, metals typically exhibit strengths of 1-3{\%} of the shear modulus, $G$. Strong covalent oxides possess yield strengths that range from 4-8{\%} of $G$. The consistency of strength trends across material classes suggests that reasonable empirical predictions of strength in the multimegabar pressure regime can now be made. Future advances can be expected to improve the capabilities of the diamond cell as a deformation device with the goal toward eventual direct determination of rheological properties at very high P-T conditions. [Preview Abstract] |
Wednesday, June 27, 2007 11:30AM - 11:45AM |
M3.00004: Hydrocode Analysis of Lateral Stress Gauges in Shocked Tantalum Ernest Harris, Ron Winter Experiements published by other workers on the resistance change of manganin stress gauges embedded in a lateral orientation in Tantalum targets have been analysed using an Adaptive Mesh Refinement Hydrocode. It was found that for four experiments the shape of the time profile of the computed lateral stress in the mounting layer closely matched the shape of the experimental lateral stress profiles. However, the calculated lateral stresses at the gauge location in the mounting layer are significantly less than the stresses that would have been produced in the target if no gauge had been present. The perturbation caused by the gauge increased as the strength of the applied shock increased. When the perturbations are taken into account values of flow stress that are significantly smaller than those reported in the original research paper are derived. The work demonstrates that the lateral gauge technique can give valuable information on strength provided high resolution simulation is used to compensate for the perturbations caused by the gauges. [Preview Abstract] |
Wednesday, June 27, 2007 11:45AM - 12:00PM |
M3.00005: Perturbations Caused by Lateral Stress Gauges Ron Winter, Ernie Harris In principle, stress gauges mounted to measure lateral stresses in a shocked matrix allow the shear strength of the material to be determined. Interpreting the records from lateral stress gauges is hindered by the fact that the stress field in the insulating layer in which the gauges are mounted can differ significantly from the stress field that would be generated in the sample if no gauge were present. A series of high resolution Eulerian code calculations have been run which suggest that the stresses in the insulating layer vary with distance and time in a way that depends on the thickness of the layer, the shock strength, and the elastic and plastic properties of both the layer and the matrix. In particular, if the shock velocity in the matrix material is high the stress at a typical gauge position initially rises to a sharp peak then falls with time, but when the shock velocity in the matrix is low the stress rises relatively gradually throughout the time of interest. The shapes of the stress-time profiles predicted by the hydrocode compare well with the results of lateral gauge experiments on several different materials. [Preview Abstract] |
Wednesday, June 27, 2007 12:00PM - 12:15PM |
M3.00006: Split Hopkinson Pressure Bar and Direct Impact Testing of Rohacell Foam Elisavet Palamidi, John Harrigan, Qingming Li Rohacell foam is a low density close-cell polymethacrylimide rigid foam used as a core material in sandwich panels which are utilised in aircraft and marine constructions and in radiation applications. As such its dynamic properties are important. Two foam densities, Rohacell 51WF and 110 WF, have been tested at quasi-static and intermediate strain rates along their three principal directions. Typically, the foams have plateau strengths of 1 and 3 MPa in their strongest direction and corresponding densification strains of 0.66 and 0.6. Due to the low strength of the foams, the split Hopkinson pressure bar (SHPB) tests were carried out on low impedance PMMA bars. The propagation coefficient was determined experimentally to account for wave dispersion and attenuation in the bars. Wave separation was used to measure large strains. The foam properties appear broadly independent of strain rate. Direct impact tests were carried out to measure proximal and distal end forces at impact velocities of between 40 and 120 m.s$^{-1}$. With increasing impact velocity, the deformations are localized at a compaction front. The proximal stresses increase as predicted by the Rankine-Hugoniot conditions. [Preview Abstract] |
Wednesday, June 27, 2007 12:15PM - 12:30PM |
M3.00007: Hopkinson Bar Studies of a PBX Simulant Daniel Drodge, John Addiss, David Williamson, William Proud A Split Hopkinson Pressure Bar system was equipped with an environmental chamber for high and low temperature studies of a HTPB/sugar propellant simulant. Experiments were carried out, at a strain rate of $\sim $1600s$^{-1}$, to characterise the material response above and below the glass transition temperature. Other techniques were deployed, including high-speed photography with Digital Image Cross-Correlation analysis for flow visualisation, and a line-laser occlusion method, to determine the dynamic Poisson's ratio. This paper outlines the current state of research and details the important observations to date. [Preview Abstract] |
Wednesday, June 27, 2007 12:30PM - 12:45PM |
M3.00008: High-pressure Carbon Strength Model: first guess Daniel Orlikowski In support of National Ignition Facility experiments, there have been several sets of laser compression experiments performed at JANUS and OMEGA measuring the hugoniot of diamond phase carbon. In conjunction with those experiments, a theoretical effort to calculate, using robust density function theory (DFT), the equation of state (EOS) (A. Correa and L. Benedict) has also been performed. However, historically an adequate strength model is difficult to develop, due to a lack of data in general. Here, we give a simple interpretation of the experiments to develop a Steinberg-Guinian-like model based upon experimental observations and DFT calculations of the elastic moduli. We discuss this model and it comparision to particle velocity histories. [Preview Abstract] |
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