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 L2: CP-1: Polymers |
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Chair: Dana Dattelbaum, Los Alamos National Laboratory Room: Hermitage AB |
Tuesday, June 30, 2009 3:30PM - 4:00PM |
L2.00001: Non-Equilibrium Volumetric Response of Shocked Polymers Invited Speaker: Polymers are well known for their non-equilibrium deviatoric behavior. However, recent investigations involving both high rate shock experiments and equilibrium measured thermodynamic quantities have reminded us that the volumetric behavior also exhibits a non-equilibrium response. An area where this work should be important is the impact of glassy polymers. At the time of impact and near the impact surface, the polymer's volumetric response will be described as being Hugoniot-like, $i.e$., standard shock Hugoniot jump conditions apply. However, at later times, release waves from neighboring free surfaces will cause the polymer's volumetric response to be far from Hugoniot. In this talk, experiments showing the non-equilibrium behavior will be described. Following that discussion, a continuum-level theory is proposed that will allow us to bridge the equilibrium and non-equilibrium behaviors with a single model that can go seamlessly from one regime to the other.\\[4pt] In collaboration with Philip Rae and Dana Dattelbaum, Los Alamos National Laboratory. [Preview Abstract] |
Tuesday, June 30, 2009 4:00PM - 4:15PM |
L2.00002: Pressure-Sensitivity and Constitutive Modeling of an Elastomer at High Strain Rates Tong Jiao, Rodney Clifton, Stephen Grunschel Pressure-shear plate impact experiments have been conducted to study the mechanical response of an elastomer (polyurea) at very high strain rates:$10^5\,-10^6\,s^{-1}$. To measure the pressure-sensitivity of polyurea's shearing resistance, an impact configuration was designed to reduce the pressure during the shear wave loading of the sample by having an unloading longitudinal wave reflected from the rear surface of the target assembly arrive at the sample at the midpoint of the shear wave pulse. A similar impact configuration was designed to reduce the pressure on the sample before the shear wave arrives by having the unloading longitudinal wave arrive even earlier. In the first case the sample is sheared at high strain rates at both high and low pressure during a single experiment. In the second case the sample is sheared at high strain rates and low pressures. Based on experimental data, a constitutive model has been developed. This model features a hyperelastic spring working in parallel with an elastic spring and a viscoplastic dashpot in series. The viscoplastic dashpot is modeled by means of a thermal activation model in which the activation energy is taken to be pressure dependent. Good agreement between the measured and computed wave profiles is obtained over the entire range of pressures investigated in the experiments. [Preview Abstract] |
Tuesday, June 30, 2009 4:15PM - 4:30PM |
L2.00003: Dynamic-tensile-extrusion response of fluoropolymers Eric N. Brown, George T. Gray III, Carl P. Trujillo The quasistatic and dynamic response of two fluoropolymers--polytetrafluoroethylene (PTFE) and polychlorotrifluoroethylene (PCTFE)--have been extensively characterized. The two polymers exhibit significantly different failure behavior under tensile loading at moderate strain rates. Polytetrafluoroethylene resists formation of a neck and exhibits significant strain hardening. Independent of temperature or strain rate, PTFE sustains true strains to failure of approximately 1.5. Polychlorotrifluoroethylene, on the other hand, consistently necks at true strains of approximately 0.05. Here we investigate the influence of this propensity to neck or not between PCTFE and PTFE on their response under Dynamic-Tensile-Extrusion. Similar to the Taylor Impact Rod, Dynamic-Tensile-Extrusion is a strongly integrated test, probing a wide range of strain rates and stress states. The results of the Dynamic-Tensile-Extrusion technique are compared with two classic techniques. Both polymers have been investigated using Tensile Split Hopkinson Pressure Bar. [Preview Abstract] |
Tuesday, June 30, 2009 4:30PM - 4:45PM |
L2.00004: Modeling and characterization of PMMA for high strain-rate and finite deformations Eric Herbold, Jennifer Jordan, Michael Nixon, Naresh Thadhani The complex response of glassy polymers to high strain-rate dynamic loading necessitates accurate modeling of these events for comparison with experiments. The strain-rate and temperature sensitivity as well as the strain softening behavior are significant and must be considered for large deformations. Several constitutive relationships that account for these attributes will be discussed in terms of their applicability to modeling PMMA in gap tests. Validation experiments involving Comp B detonated in contact with 4 in. of PMMA will be compared with dynamic models of for glassy polymers. [Preview Abstract] |
Tuesday, June 30, 2009 4:45PM - 5:00PM |
L2.00005: Dynamic Properties of Polyurea 1000 W. Mock, Jr., S. Bartyczak, G. Lee, J. Fedderly, K. Jordan A gas gun has been used to investigate the shock response of the viscoelastic material Versathane P1000. This polyurea material is synthesized from a polyamine (Versalink P1000, Air Products) with a multi-functional isocyanate (Isonate 143L, Dow Chemical). Versalink P1000 has a nominal molecular weight of 1000. The morphology of the resulting polymer consists of aromatic hard segments in an aliphatic soft-segment elastic matrix. Sabots carrying 9.5-mm-thick metal disks were launched into target assemblies containing the polyurea material. A target assembly consisted of a three-layer sandwich configuration: a 0.9-mm-thick metal disk on the impact side, a 6.5-mm-thick polyurea disk, and a 9.5-mm-thick metal backup disk. The metal disks were either OFHC copper or 6061-T6 aluminum. Impact velocities ranged from 280 to 920 m/s. Impact planarity was 1 milliradian or less. Thin film 50-ohm manganin gauges (Dynasen) were epoxied between the metal/polymer and polymer/metal interfaces in each target assembly to measure the interface stresses as a function of time. The polyurea shock velocity was also determined for each experiment. Measured longitudinal stresses ranged from 5 to 45 kbar. A comparison of the measured initial stress values and calculated pressure values suggests that the shear strength increases with increasing stress. [Preview Abstract] |
Tuesday, June 30, 2009 5:00PM - 5:30PM |
L2.00006: Predicting the Highly Nonlinear Mechanical Properties of Polymeric Materials Invited Speaker: Over the past few years, we have developed models that calculate the highly nonlinear mechanical properties of polymers as a function of temperature, strain and strain rate from their molecular and morphological structure. A review of these models is presented here, with emphasis on combining the fundamental aspects of molecular physics that dictate these properties and the pragmatic need to make realistic predictions for our customers; the designer of new materials and the engineers who use these materials. The models calculate the highly nonlinear mechanical properties of polymers as a function of temperature, strain and strain rate from their molecular structure. The model is based upon the premise that mechanical properties are a direct consequence of energy stored and energy dissipated during deformation of a material. This premise is transformed into a consistent set of structure-property relations for the equation of state, EoS, and the engineering constitutive relations in a polymer by quantifying energy storage and loss at the molecular level of interactions between characteristic groups of atoms in a polymer. These relations are derived from a simple volumetric mean field Lennard-Jones potential function for the potential energy of intermolecular interactions in a polymer. First, properties such as temperature-volume relations and glass transition temperature are calculated directly from the potential function. Then, the `shock' EoS is derived simply by differentiating the potential function with respect to volume, assuming that the molecules cannot relax in the time scales of the deformation. The energy components are then used to predict the dynamic mechanical spectrum of a polymer in terms of temperature and rate. This can be transformed directly into the highly nonlinear stress-strain relations through yield. The constitutive relations are formulated as a set of analytical equations that predict properties directly in terms of a small set of structural parameters that can be calculated directly and independently from the chemical composition and morphology of a polymer. A number of examples are given to illustrate the model and also to show that the method can be applied, with appropriate modifications, to other materials. [Preview Abstract] |
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