Session C3: AETD: Optical Imaging of Dynamic Events

Nanosecond imaging techniques to characterize detonator breakout performance

Simultaneous ultra-high-speed framing and electronic steak photography have been applied to the working surface of cylindrical detonators in order to quantitatively assess shock-wave breakout kinematics. The framing camera successfully captured the evolution of two-dimensional cross-sections of the axial shock wave exiting an aluminum output cup of the detonator, and the streak camera collected one-dimensional, space-time breakout curvature with sub-nanosecond resolution. Both data sets have been analyzed to assess the geometrical characteristics of the leading shock wave exiting the cup. An existing and simple model equation for hyperbolic breakout of cylindrical high-explosive (HE) charges has been parameterized for nominal detonator-explosive performance and used to initially inform the geometry of the detonation wave within the functioning detonator. A more detailed investigation will be discussed that assesses what impact the inert aluminum cup has on the model assumptions and parameterization, as well as on one’s ability to extract HE-performance parameters when applying the model to detonator-breakout data.   

Explosive particle image velocimetry in cast polydimethylsiloxane

Extrudable high explosive XTX 8004 was hand-loaded into custom-cast polydimethylsiloxane (PDMS) witness blocks in a cylindrical charge geometry. In the PDMS casting process a thin volume of polymer containing sparsely distributed tracer particles was added to allow direct observation of radial-drive velocities using ultra-high-speed explosive particle image velocimetry (ExPIV). The shock wave image framing technique (SWIFT), a laser-backlit derivative of focused shadowgraphy, simultaneously captured both detonation-front position along the HE charge length, as well as the temporal evolution of leading shock fronts propagating radially outward from the charge interface. The visualized axisymmetric and self-similar shock-front geometries were used to initialize a three-dimensional ray-tracing scheme that estimates the density-gradient-based optical distortion realized when observing the object plane through the curved shock fronts in PDMS. After applying a corrective mapping the particle positions were compared across images, which provided a field measurement of particle velocity throughout the shocked PDMS. Novel aspects of data analysis will be discussed, along with preliminary results quantifying XTX 8004 drive into PDMS witness media.   

Small scale tests to determine divergence and spreading of explosive booster materials

Initiation and propagation of various PBXs was investigated using two modified versions of the historic Floret test. Modification one involves replacement of the traditional dent block with a PMMA window capable of 1) transmitting axial shock parameters (pressure, particle velocity) via PDV and 2) orthogonal high speed backlit videography to evaluate wave shape and corner turning criteria. This test is focused on booster materials (PBX 9701, PBXN-7 and PBXN-5) that are challenged by the inherent features of the main charge which may be insensitive, slow to run to detonation, have asymmetric wave propagation, and be hampered by cold temperature. Modification two probes the notion that HE are inherently more sensitive when heated. By changing the geometry to allow a stand-off configuration between flyer and acceptor, minimum spot size criteria can be evaluated. Since spot size is correlated to critical diameter, increased temperature shows reduced spot size/critical diameter. This test focused on a variety of materials, but primarily is focused on IHEs such as PBX 9502 which have large ambient temperature critical diameters (\textasciitilde 8 mm).   

Voitenko experiments with novel diagnostics detect velocities of 89 km/s.

Using a Voitenko accelerator, a short series of experiments were performed with the goal of attaining shock velocities in gases approaching 90 km/s. The basic apparatus comprised a hemispherical bowl filled with a gas at atmospheric pressure; a metal piston across its diameter; and a small bore evacuated shock tube at its apex. The evacuated shock tube was separated from the gas bowl by a thin diaphragm. A combination of a plane wave explosive lens and a high explosive pad accelerated the piston to a velocity of the order of 4 km/s and subsequently compressed the gas in the bowl. The thin diaphragm at the other end of the bowl then ruptured and the high pressure (shock compressed) gas escaped into the shock tube. A high speed digital framing camera recorded the light emission from the shock front, photon Doppler velocimetry monitored the piston, and for the first time, a 94-GHz microwave interferometer was used to monitor the position of the front. These diagnostics showed that the shock velocity in helium was 89 km/s in helium and there were interesting features, previously unreported, in the shock propagation detected by the microwave interferometer..   

Exploiting New Imaging Techniques to Provide Quantitative Data for Model Validation

Historically, data collected from explosives and shock physics experiments is qualitative and used as a snapshot to gain insight into the physics taking place. In order to advance the science of shock physics, experiments need to be designed that can directly validate the output from simulations, including their underlying material models. Recent advancements in electronics and subsequent imaging diagnostics can and should be leveraged to transition from qualitative, trend-based studies to repeatable, quantitative (digitized/digitizable) data collection and analysis. Within a single experiment, a rigorous process for data collection and analysis of concurrent, high-fidelity diagnostics is utilized producing large amounts of data. Three specific examples of these advanced diagnostics will be discussed: 1) digital holography for detonator fragmentation to obtain 3D positions and velocities, 2) micro-streak Schlieren imaging of bursting wires to visualize phase changes with time correlated energy deposition, and 3) x-ray phase contrast imaging with high-flux picosecond exposures to observe materials under extreme conditions. Utilizing these newly developed diagnostics from other fields, we have made large strides in data collection to gain insight for explosives research and shock physics.