Bulletin of the American Physical Society
16th APS Topical Conference on Shock Compression of Condensed Matter
Volume 54, Number 8
Sunday–Friday, June 28–July 3 2009; Nashville, Tennessee
Session L4: ED-3b: Loading for Plasticity Measurements |
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Chair: Thibaud de Resseguier, ENSMA Room: Hermitage D |
Tuesday, June 30, 2009 3:30PM - 3:45PM |
L4.00001: A Novel Method of Resolving Ignition Threshold in Steven Test Using Hybrid Drop Weight-Hopkinson Bar Vasant Joshi Sensitivity of energetic material is traditionally evaluated by a go-no-go ignition condition. In a conventional drop weight test, sand paper is used to pin the sample in place. In Steven test the base metal directly acts on solid explosive. Although comparison of different explosives using blast overpressure in Susan or Steven test as a function of projectile impact velocity may be valid for a range of velocity, these tests, aimed at obtaining ignition have unintentional changes in friction conditions, which may contribute to initiation. There has been little effort to evaluate the effect of frictional contribution. Individual contribution of shear, strain, strain rate and friction in attaining ignition cannot be separated in any of these tests. We propose to quantify shear by lubricating the samples in an instrumented Hybrid Hopkinson bar, wherein the combination of shear and strain rate leading to ignition condition can be assessed. The current configuration of Hybrid Hopkinson Bar apparatus is being modified to conduct a subscale Steven test to evaluate the contribution of frictional changes leading to ignition. This will allow a number of tests to be conducted in lab scale, instead of expensive full scale tests. Use of real time diagnostic techniques in combination with data acquisition and reduction methods to simultaneously quantify mechanical properties and ignition conditions will be presented. [Preview Abstract] |
Tuesday, June 30, 2009 3:45PM - 4:00PM |
L4.00002: Stress Waves in Elastic and Viscoelastic Bars Majid Aleyaasin, Ahonsi Bright, John Harrigan, Jeremy Millett Strain-gauged circular rods are used for Split Hopkinson Pressure Bar (Kolsky Bar) testing and as load cells for Direct Impact testing. Strain measurements along the bar are used to determine the stress and displacement histories at the end of the bar. In recent studies an experimentally measured propagation coefficient has been used to account for wave dispersion and attenuation in polymer bars and to overcome the difficulties associated with modeling the stress waves in the bars. However, the accuracy of the technique relies on the accuracy of the propagation coefficient used to model the wave propagation in the bars. The propagation coefficient can be determined experimentally or, if the bar material properties are known accurately, it can be derived using a suitable wave model for the bars. Both elastic and viscoelastic bars are considered and experimental results, analytical wave models and finite element analysis are discussed and compared. Experimentally derived propagation coefficients are compared with those predicted by higher order rod theories. [Preview Abstract] |
Tuesday, June 30, 2009 4:00PM - 4:15PM |
L4.00003: Dynamic Punch Test Amos Gilat, Jeremy Seidt A dynamic punch test is introduced. The test is conducted by placing a punching device between the incident and transmitter bars of a compression split Hopkinson bar apparatus. The punch has a rounded end that penetrates into the specimen which is a thin round plate clamped around the circumference. The force of the punch and the relative motion between the punch and the specimen holder are determined from the waves recorded on split Hopkinson bars. Digital image correlation technique is used to verify the displacements determined from the waves. Results are shown from tests on specimens made of 2024-T351 aluminum. The results can be used for the development and validation of continuum failure models for high stain rates applications. Many existing failure models relate stress triaxiality (ratio of the pressure and the von Mises stress) to equivalent failure strain, and some models have been modified to include the Lode parameter. The coefficients in the failure models are determined from experiments in which specimens are subjected to a combined state of stress. Such experiments are relatively easy to conduct in low (quasi-static) strain rate applications, but are very difficult to conduct at high strain rate. [Preview Abstract] |
Tuesday, June 30, 2009 4:15PM - 4:30PM |
L4.00004: Gap Formations Along Specimen-Bar Interfaces in Numerical Simulations of SHPB Tests on Elastic Materials Soft in Shear Martin N. Raftenberg, Mike Scheidler Simulations of split Hopkinson pressure bar (SHPB) tests on elastic materials were performed using LS-DYNA. The specimens were much stiffer in dilatation than in shear. A compressible form of Mooney-Rivlin elasticity was applied with parameters evaluated from ballistic gelatin data. The bars were aluminum. The velocity prescribed on the incident bar increased over a rise time until attaining a steady-state value corresponding to a nominal strain rate of 2500/s. The rise time was varied to observe effects of pulse shaping. All calculations were 2D axisymmetric. A penalty-based contact algorithm was applied at the specimen-bar interfaces. This algorithm introduced a stiffness and a viscosity parameter. In sensitivity studies we varied the radius of the bars, the specimen's mesh, and the two contact parameters. In all calculations with the Mooney-Rivlin model, gaps formed at both specimen-bar interfaces over a wide range of strains. This gap phenomenon appears not to have been previously reported in the SHPB literature. We replaced the Mooney-Rivlin model with linear elasticity in order to explore whether the gaps were associated with material nonlinearity. We fixed Young's modulus at a value much smaller than that of aluminum. For sufficiently large Poisson ratios, we again observed gap formations at both specimen-bar interfaces. [Preview Abstract] |
Tuesday, June 30, 2009 4:30PM - 4:45PM |
L4.00005: Plasticity analysis from shockwave velocimetry Bryan Reed, James Stolken, Reed Patterson, Jeff Nguyen, Mukul Kumar Using shocks in tantalum as a test case, we are developing methods to extract plasticity information from shock velocimetry in the absence of impedance-matched windows. We have revisited literature data analysis approaches, combining them into a single formalism while generalizing the physics. Finding the deviatoric stress and plastic strain in a one-dimensional wave is reduced to the problem of determining (1) the material's equation of state and pressure-dependent shear modulus and (2) the particle velocity as a function of position and time. The results provide plastic stress-strain data while clarifying the links between rate-dependent plasticity and shock wave propagation. Plastic relaxation acts a distributed source of elastic rarefaction waves that superpose very nearly in step with the shocks. This mechanism is known to produce elastic precursor decay, but we also find that it may account for an unexpected slowing of the plastic shock speed at intermediate pressures. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Tuesday, June 30, 2009 4:45PM - 5:00PM |
L4.00006: Dynamic behaviors of various volume rate steel-fiber reinforced reactive powder concrete after high temperature burnt Baojun Pang, Liwen Wang, Zhenqi Yang, Runqiang Chi Dynamic strain-stress curves of reactive powder concrete under high strain rate (10/s-100/s) were determined by improved split Hopkinson pressure bar (SHPB) system. A plumbum pulse shaper was used to ensure the symmetrical stress in the specimens before fracture and avoid the fluctuation of test data due to input shaky stress pulse. A time modified method was induced for data processing in order to get accurate SHPB results. The results of experiment showed after high temperature burnt, different volume rate (0.0\%, 0.5\%, 1.0\%, 1.5\%) steel-fiber reinforced reactive power concrete had the same changing tendency of residual mechanics behaviors, e.g. after 400 centigrade burnt, the residual compression strength was about 70\% of material strength without burnt under 100/s. After 800 centigrade burnt, the compression strength is about 30\% under 100/s while the deformation ability increased. At meanwhile, steel fiber had improved the mechanism of reinforcing effect and toughening effect of concrete material after burnt. With increasing of steel fiber volume rate, dynamic residual behavior of samples was improved. Microcosmic characteristics and energy absorption were induced for explaining the experiment results. [Preview Abstract] |
Tuesday, June 30, 2009 5:00PM - 5:15PM |
L4.00007: The Dynamic Mechanical Properties of AL-6MG Alloy with Different States Mu-Sen Lin, Wei Zhang, Bao-Jun Pang The mechanical behavior of Al-6Mg alloy with three treatment states (H112 state, O state and cold extrusion state) have been investigated by using INSTRON machine and SHTB. Stress-strain curves of Al-6Mg alloy with three treatment states have been obtained at quasi-static strain rates 6$\times $10-4, 6$\times $10-3 and 6$\times $10-2 s-1 and dynamic strain rates 670$\sim $3050 s-1,respectively. The results show that three materials is low sensitive to the strain rate. A mild negative strain rate sensitivity was observed at strain rates 6$\times $10-4$\sim $ 6$\times $10-2 s-1 while a positive strain rate sensitivity was observed at strain rates 670$\sim $3050 s-1.At quasi-static condition, three materials exhibit serrated stress--strain curves. The Al-6Mg alloy of cold extrusion state displays highest strength and Al-6Mg alloy of O state displays most ductibility, but their advantages decline with the strain rate increasing at dynamic condition. The fracture surfaces of the tested specimens have been examined .The results show that Al-6Mg alloy of O state exhibit ductile fracture at all strain rates, however, H112 and cold extrusion states display ductile fracture at quasi-static condition but fragile fracture at high strain rates. [Preview Abstract] |
Tuesday, June 30, 2009 5:15PM - 5:30PM |
L4.00008: High Rate Plasticity under Pressure using a Windowed Pressure-Shear Impact Experiment Jeffrey Florando, Tong Jiao, Stephen Grunschel, Rodney Clifton, Louis Ferranti, Richard Becker, Roger Minich An experimental technique has been developed to study the strength of materials under conditions of moderate pressures and high shear strain rates. The technique is similar to the traditional pressure-shear plate-impact experiments except that window interferometry is used to measure both the normal and transverse particle velocities at a sample-window interface. Experimental and simulation results on copper and vanadium samples backed with a sapphire window will be presented to show the utility of the technique to measure flow strength under dynamic loading conditions. The samples were impacted with a Ta10W flyer at approximately 200 m/s. [Preview Abstract] |
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