Dana Mcgraw Dattelbaum

Control of structural topology, via a bottom-up approach, is now possible through the advent and continuing maturation of additive manufacturing techniques. For example, new classes of porous materials with increased strength-to- density ratios, novel thermal and acoustic properties, and even "metamaterial" properties such as negative Poisson ratios have been recently realized by tailoring deformation mechanisms and structural instabilities. It is the control of organizing features at the mesoscale (100s nm- mu m's) that has led to a revolution in tailoring materials mechanical properties and function. While many examples exist in which the low strain rate mechanical responses of materials have been tailored, extensions to dynamic, high strain rate, large strain conditions have been scarcely explored. Here, we present the results of traditional plate impact methods applied to organized polymer architectures, using velocimetry and X-ray phase contrast imaging at the Advanced Photon Source. These methods allowed for the characterization of the mechanisms of shock wave propagation, localization, and compaction in the structures. In particular, we have investigated the role of interfaces on stress localization within the structures. The experimental results will be discussed in the context of finite element simulations of the same structures, including progress on topological optimization for desired dynamic response.

The chemical reaction zone (CRZ) of detonating explosives is defined by the leading, inert shock front, which compresses the explosive to the von Neumann (vN) spike condition on the unreacted Hugoniot, and the Chapman-Jouget (CJ) sonic locus condition, according to the Zel'dovich/von Neumann/Doering (ZND) one-dimensional theory of detonation. The CRZ is often measured using optical velocimetry techniques at a windowed interface; the window affects the reaction zone dynamics due to wave interactions from the interface. Fluoropolymer windows are attractive as they provide a near impedance match to most common explosives with initial densities 0 rho = 1.8-2.0 g/cm(3). Poly(chlorotrifluoroethylene-co-vinylidene fluoride) (Kel-F 800, Lot 30013) was purchased from 3M Inc., St. Paul, Minnesota. Small (150 mm 150 mm 50 mm) billets were prepared by compression molding the polymer at 90 degrees C and similar to 50,000 psi by Afton Plastics. This method resulted in a semi-transparent, golden-colored billet from which window samples were machined and polished to an optical clarity for experiments. To extend the window correction for Kel-F 800 (see D. M. Dattelbaum et al., Proceedings of the 15th International Detonation Symposium (2014)), a series of gas gun-driven plate impact experiments were performed using both VISAR (532 nm) and PDV (1550 nm) velocimetry methods to extend the window correction to a larger range of initial shock pressures and densities.

Abstract Understanding the kinetics of phase transitions, including decomposition from reactants to products under extreme condition events is challenging. Capturing these processes require: 1) diagnostics that probe on the timescales and at energies capable of interacting with the dynamically evolving products, penetrating the opaqueness of the changing system; and 2) detectors sensitive enough to observe these events. Synchrotrons and free electron lasers provide ke-V-energy x-ray beams capable of penetrating the optical-opaqueness of the temporally evolving products. At the Dynamic Compression Sector at the Advanced Photon Source, the x-ray beam is coupled to single and two-stage gas guns capable of producing planar shocks at a range of projectile velocities while capturing in situ x-ray diffraction/scattering of the evolving material under dynamic compression. In this work, we demonstrate the utility of this approach in measuring the evolution of crystalline domains in shocked high-density polyethylene to P = 7.45 GPa, and have observed the compression and orientation of the polymer chains in real time.

Shock-driven reactions in simple molecules often occur with densification along the reaction coordinate. Acrylonitrile (CH2CHCN) is a simple molecule that undergoes shock-driven reaction on the principal Hugoniot. Using in-situ embedded electromagnetic gauging techniques and the LANL large bore two-stage gas gun, multi-wave structures were observed in acrylonitrile indicating the formation of higher density shock-induced reaction intermediates and products. Using the data from the embedded gauge experiments, time-resolved Raman spectroscopy experiments were conducted to elucidate reaction species in shocked acrylonitrile, but it was found that the conditions became optically opaque even during the first wave at modest input conditions. A description of the time-resolved Raman system coupled to the LANL large-bore twostage light gas gun is also provided.

The high pressure dynamic response of polymers is important to a wide variety of applications. The details of compressibility and reactivity can have a large effect on overall behaviors of dynamic systems even when polymers are used in small amounts. Polyethylene is of broad interest for a variety of applications, as an ingredient and as a pure material. It is also of significant interest as a model system to understand the correlating effects of polymer dynamics in a material with a relatively simple chemical composition that can have highly varied properties through the alteration of molecular weight and associated crystallinity of the material. Although a variety of Hugoniot and dynamic information is available for polyethylene, it is a challenge to obtain information on the product equation of state at pressures high enough to achieve decomposition. Following recent successes in producing deep release states in compressed epoxy material, a series of plate impact experiments was performed in the same configuration on high density and ultra-high molecular weight polyethylene at pressures where there is only limited Hugoniot data. The experimental wave profiles are presented and the Hugoniot states are compared to previous results. In ongoing work, the release profiles are intended to calibrate a product equation of state.

In this work a methodology for estimating the onset of reaction for porous polymer systems is presented. Shock Hugoniot data for a well-characterized polymer, polyurethane, is used to calibrate a functional relationship that captures the onset of reaction at several initial densities. In doing so, this function is then used to estimate the volume at which reaction is initiated in polyurethane at any initial density. The function is also extended to estimate the onset of reaction in less well-characterized polymer systems, polystyrene and SX358.

Reactive flow models have been used to design explosive trains and predict explosive response to various shock inputs. Parametrization of these models can be determined using short-duration shock data from thin flyers for ignition behavior and sustained pulse Pop-plot data for growth to detonation behavior. The latter was measured in an explosive using 4 experimental configurations with different data collection techniques. Two were gas-gun driven 1-D shock waves with either embedded particle velocity gauges or photon Doppler velocimetry at the end of different sample thicknesses. The other two were explosive donors to produce either a 1-D or quasi-1-D shock wave in wedge or cylindrical acceptors, respectively. Break out of the detonation wave in wedge samples was observed by streak camera, while embedded time of arrival gauges were used for cylindrical samples. Run-distances were compared between all 4 cases using a consistent method involving the intersection of two linear fits through data prior to and after transiting to detonation. All methods provided consistent data, indicating that one or more combinations of these methods are suitable for parameterizing a reactive flow model.

Polyethylene is a widely-encountered polymer that exhibits mechanical responses tailorable to a given application based on its network and chain structures (crystallinity) and molecular weight. Several earlier reports have provided shock Hugoniot data for polyethylene over a broad range of conditions to very high shock stresses, while other reports have focused on the unusual and discontinuous low pressure Hugoniot of crystalline forms of polyethylene. Very little has been published on dynamic failure, i.e. spall, in polymers in general, let alone PE specifically. In this paper, high density polyethylene with crystallinity > 40% was investigated both for the shock response using in situ electromagnetic gauges and the dynamic tensile failure using photon Doppler velocimetry (PDV). The evolution of particle velocity wave profile with wave propagation distance is presented to study the previously reported discontinuous Hugoniot at low pressures, providing evidence of a three-wave structure above a shock stress of 0.5 GPa. Above this region, the transition is overdriven, and a single shock wave is observed to shock stress greater than 10 GPa. Dynamic tensile experiments were conducted in the pressure region above where spall had been previously observed, and a shallow pullback signal with some ringing was observed.

Shock-driven reactions are commonplace. Examples include the detonation of high explosives, shock-driven dissociation of polymers, and transformation of carbon from graphite-to-diamond phases. The study of shock-driven chemical reactions is important for understanding reaction thresholds, their mechanisms and rates, and associated state sensitivities under the extreme conditions generated by shock compression, with the ultimate goal of understanding the full transformation of reactant to product. Reactions are distinguished by their thermicity - e.g. the volume and enthalpy changes along the reaction coordinate. A survey of the hallmarks of shock-driven reactivity for a variety of simple molecules are presented, including benzene and acrylonitrile. These "simple" molecules illustrate the nature of reactive flow through particle velocity wave profiles measured by in situ electromagnetic gauging applied to gas gun-driven plate impact experiments. Progress in applying bondspecific diagnostics is also described, including time-resolved Raman spectroscopy in gas gun experiments, and recent results of in situ x-ray diffraction of carbon at the Linac Coherent Light Source (LCLS) free electron laser.

Using gas-gun-driven plate impact techniques, we have measured the Hugoniot of the filled silicone elastomer DC745U cooled to -60 degrees C. DC745U consists of approximately 62 weight% poly-dimethyl-siloxane rubber and 38 wt% silicon dioxide filler. At similar to -50 degrees C, the poly-dimethyl-siloxane rubber in DC745U crystallizes with similar to 40% crystallinity. This is accompanied by a density change from 1.31 g/cm(3) at 23 degrees C to 1.45 g/cm(3) at -60 degrees C. Below the crystallization transition temperature, a measurable increase in the shock velocity was observed. This is coincident with a decrease in compressibility due to crystallization of the polydimethylsiloxane repeat units. The linear Us up Hugoniot also significantly changes from U-S = 1.62 + 1.74u(p) mm/mu s at 23 degrees C to U-S = 2.00 +/- 0.05 + (2.06 +/- 0.06)u(p) mm/mu s at -60 degrees C. Cooling to -60 degrees C and the associated crystalline phase transition therefore results in considerable stiffening. This is the first time, to our knowledge, that a polymer crystallization transition has been shown to affect shockwave properties in this way.