Bulletin of the American Physical Society
21st Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 64, Number 8
Sunday–Friday, June 16–21, 2019; Portland, Oregon
Session K4: MS: Strength & Spall IV |
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Chair: Carl Trujillo, Los Alamos National Laboratory (LANL) Room: Pavilion West |
Tuesday, June 18, 2019 2:00PM - 2:15PM |
K4.00001: Fast Strength Model Parameter Optimization and Model Comparison Using Bayesian Statistics Ayan Biswas, David Walters, Devin Francom, Earl Lawrence, Darby Luscher, Sky Sjue, James Ahrens A variety of flow stress models exist with new models constantly being developed. These models aim to approximate the strength of materials in a variety of regimes from quasi-static loading through shock scenarios. All models contain an array of parameters which need to be tuned to the material under study. Some models perform well under limited conditions, requiring adjustment of the parameters when venturing outside of those predefined ranges. Other models perform well over a wide range of conditions with a set of parameters, but may be outperformed by other models optimized on a tighter range of conditions. Recent research by Los Alamos demonstrated the ability to optimize the Johnson Cook model using a set of 3 plate-impact experiments on Aluminum. They utilized Bayesian statistics and emulation to determine optimal parameters for the model with a quantification of parameter uncertainty. We have advanced this capability to incorporate multiple data types available to the modeler (e.g. velocimetry from plate-impact tests and stress-strain data from SHPB tests). Statistically robust comparisons of the performance and uncertainty of different flow stress models were carried out for different scenarios (e.g. small datasets vs. a suite of experiments of different types). [Preview Abstract] |
Tuesday, June 18, 2019 2:15PM - 2:30PM |
K4.00002: The Integration Schemes of the Preston-Tonks-Wallace (PTW) Viscoplasticity Model JeeYeon Plohr The Preston-Tonks-Wallace (PTW) viscoplasticity model is valid in the wide range of strain, strain rate, and temperature. In this paper, we examine how the Preston-Tonks-Wallace (PTW) flow stress was constructed: the closed form expression of the flow stress, which is known as the PTW model was obtained by integrating the differential form of the hardening law under the assumption that the strain rate is constant. We consider there are cases, like explosively driven deformation and high-velocity impacts, where this is not true. As a case study, we choose a gaussian function as a strain rate history and compare two different ways to use the PTW model. First, we integrate the differential form of the PTW model numerically, coupled with this non-constant strain rate function. Second, we use the closed form of the PTW (, which was already integrated for the fixed strain rate) and plug in the strain rate value at each discretized time. We draw the conclusion that based on the physical and mathematical arguments, one should solve the system of ODEs consisting of the differential form of the PTW model and the particular strain rate history of interest rather than using the closed form expression when the strain rate variation is large. [Preview Abstract] |
Tuesday, June 18, 2019 2:30PM - 2:45PM |
K4.00003: Shear Band Insertion for Capturing Strain Localization Jonathan Margraf The localization of shear strain into narrow bands is a common failure mechanism under dynamic loading, and we present an approach to capture this behavior in a numerical framework. The approach employs a mixture theory for embedding the band material. An iterative solver for traction balance is used, and this strategy for treating material interfaces has advantages across a wider class of problems. The traction balance methodology solves the appropriate compatibility and stress equilibrium conditions between the shear band and the bulk material within a given computational zone. The propensity for strain localization thus affects the macroscopic behavior of the zone without the need to fully discretize the shear band's geometric features that are often on the scale of micrometers. For general loading scenarios, the code must be able to detect elements in which shear bands might form and the orientation of such bands. A novel shear band insertion approach has been developed for this task. Example applications of the traction balance methodology coupled with shear band insertion will be shown. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 (LLNL-ABS-768682). [Preview Abstract] |
Tuesday, June 18, 2019 2:45PM - 3:00PM |
K4.00004: Modeling High Rate Stress Upturn for Brittle Materials Yehuda Partom High rate stress upturn in \underline {ductile materials} for strain rates between 10$^{\mathrm{3}}$ and 10$^{\mathrm{4}}$/s has been known since the 1980s, and previously we've shown how to model this behavior, based on our overstress approach to dynamic viscoplasticity. It turns out that \underline {brittle materials} also undergo high rate stress upturn, but at a lower strain rate of 1-10/s. Most available data on high rate stress upturn of brittle materials are for concrete (from obvious reasons), and they are usually represented as DIF (strain rate), where DIF$=$Dynamic Increase Factor. Here we model high rate stress upturn for brittle materials using our overstress brittle response (OBR) model. Our OBR model includes: 1) damage onset function for both compression and tension; 2) damage accumulation rate as function of overstress; 3) damage onset reduction as function of damage level down to the fully damaged state; 4) plastic flow of the fully damaged material as for a granular material; and 5) finite limit of the damage accumulation rate. This last item stems from the finite rate of fracturing, and from the finite speed of crack growth, and it turns out that this is the feature that leads to the high rate stress upturn response. To demonstrate how our model works, we compute several examples in cylindrical symmetry, with different ratios of axial to radial flow velocities. [Preview Abstract] |
Tuesday, June 18, 2019 3:00PM - 3:15PM |
K4.00005: Features of Fourth Power Behavior of Structured Shock Waves in Selected Solids Dennis Grady Three solid materials that exhibit steady structured shock-wave fourth-power dependence of shock pressure step versus strain rate are examined in further detail. They are, respectively the compound aluminum oxide, the metal uranium and the molecular crystal HMX. For Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ and HMX, both polycrystalline and single crystal structured wave data are examined. For both materials the plot of polycrystal and single crystal data overlay in the pressure versus strain rate plot. Further, for polycrystalline Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ and uranium the data span of steady-wave widths exceeds the crystal grain size by one to two orders of magnitude. Structured wave data for unalloyed alpha uranium and uranium six percent niobium are compared in the pressure versus strain rate presentation. Both metals exhibit fourth-power pressure versus strain rate behavior, however, U6Nb reveals markedly reduced shock viscosity relative to alpha uranium. Underlying implications and possible sources of the experimental observations are explored. [Preview Abstract] |
Tuesday, June 18, 2019 3:15PM - 3:30PM |
K4.00006: ABSTRACT WITHDRAWN |
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