Bulletin of the American Physical Society
15th APS Topical Conference on Shock Compression of Condensed Matter
Volume 52, Number 8
Sunday–Friday, June 24–29, 2007; Kohala Coast, Hawaii
Session G6: Explosives/Mechanical Response I |
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Chair: Darla Thompson, Los Alamos National Laboratory Room: Fairmont Orchid Hotel Promenade I/II |
Tuesday, June 26, 2007 10:30AM - 10:45AM |
G6.00001: An initial investigation of the sub-microsecond features of dynamic crack propagation in PMMA and the RDX-based explosive PBX 9205 Peter Washabaugh, Larry Hill A dynamic crack propagating in a brittle material releases enough thermal energy to produce visible light. The dynamic fracture of even macroscopically amorphous materials becomes unsteady as the crack propagation velocity approaches the material wave-speeds. The heat generated at a crack-tip, especially as it jumps, may be a mechanism to initiate a self-sustaining reaction in an energetic material. Experiments were conducted in specimens to simulate an infinite plate for 20 $\mu $s. The initial specimens were 152 mm square by 6 mm thick acrylic sheets, and were fabricated to study non-steady near-wave-speed crack propagation. A variant of this specimen embedded a 25 mm x 3 mm PBX 9205 pellet to explore the influence of dynamic Mode-I cracks in these materials. The crack was initiated by up to 0.2 g of Detasheet placed along a precursor 50 mm long notch, with a shield to contain the reaction products and prevent propagation along the fractured surfaces. The crack was studied by means of a streak camera and a Fourier-filter of the light reflecting off the newly minted surfaces. The sub-microsecond behavior of holes initiating, preceding and coalescing with the main crack were observed in the PMMA samples. The embedding and mechanical loading of explosives by this technique did not initiate a self-sustaining reaction in preliminary testing. [Preview Abstract] |
Tuesday, June 26, 2007 10:45AM - 11:00AM |
G6.00002: Brazilian disc testing of a UK PBX below the glass transition temperature. David Williamson, Stewart Palmer, William Proud Previous research at room temperatures and quasi-static loading rates have shown that PBX Brazilian disc specimens fail by the localisation of strain in binder rich areas, followed by debonding of the binder from the larger filler crystals. Crystal failure is rare and the process is predominantly intergranular. In contrast, we show that for a UK PBX when the temperature is reduced to below the glass transition of its binder system, failure becomes predominantly transgranular. Such data are required for the development and validation of PBX micromechanical models. This paper outlines the current state of research and details the important observations to date. [Preview Abstract] |
Tuesday, June 26, 2007 11:00AM - 11:30AM |
G6.00003: Recent Developments In Shear Ignition Of Energetic Materials Using Hybrid Drop Weight-Hopkinson Bar Invited Speaker: The sensitivity and mechanical behavior and of energetic material is highly dependent on its constituents. Cast as well as cured (like PBX) types of explosives have mechanical properties significantly different from metals and the assumption of isotropic behavior may not be valid beyond a finite strain. While Split Hopkinson Pressure Bar (SHPB) can be successfully used to obtain mechanical properties of these soft explosives at strain rates up to 5,000/sec, ignition conditions are seldom achieved in SHPB tests. In very sensitive explosives, if ignition occurs in very small sample at extremely high strain rates, it would be very difficult to calculate the energy and energy rate that led to successful ignition. In contrast to the SHPB test, the drop-weight test, uses gravitational acceleration to impart a nearly constant velocity at the instant of impact. The Drop weight test is intended to obtain ignition, only as a go-no go condition, due to the variation of velocity, either due to change in the initial height or small changes in friction or drag. Due to lack of quantifiable parameters, the result from this test is not suitable for modeling, which is important in development of new explosive formulation. In order to overcome this barrier and allow evaluation of the susceptibility of the different formulations to ignition, a new test was recently developed. This test is called Hybrid Hopkinson Bar Drop Weight test, which overcomes shortcomings of two systems. When an explosive sample is compressed between two rigid flat surfaces and the material freely slides on the rigid plates without pinning, the mechanical energy is uniformly dissipated throughout the sample. In absence of any pressure gradient, pure-shear conditions apply throughout the sample. The ignition in this case, will not be localized at the edges, which is the basis for obtaining ignition condition in the new apparatus. Using this method, hydroxy-terminated poly butadiene (HTPB) bonded explosives, PBXN-110 and PBXW-128, cast TNT and Comp B are compared for their ignition thresholds. This method uses novel approach in diagnostics techniques, data acquisition and reduction methods to simultaneously quantify mechanical properties and ignition conditions. [Preview Abstract] |
Tuesday, June 26, 2007 11:30AM - 11:45AM |
G6.00004: Pressure Dependence of Crack Growth and Plastic Flow Processes in Composite Plastic Bonded Explosives Donald Wiegand, Kevin Ellis The stress-strain curves of composite plastic bonded explosives can be divided into two regions as a function of pressure. In a low pressure region stress as a function of increasing strain exhibits a maximum followed by work softening while in the higher pressure region yield followed by work hardening is observed. The results indicate that the work softening is due to crack growth damage and the work hardening is associated with plastic flow. In the low pressure region the compressive strength increases linearly with pressure and in the higher pressure region the flow stress (yield strength) increases approximately linearly with pressure but with a much lower slope. Therefore, the results indicate that in the low pressure region the stress required for plastic flow is higher than the stress for crack growth so crack processes dominate while in the higher pressure region the stress required for plastic flow is lower than the stress required for crack growth and so plastic flow dominates. [Preview Abstract] |
Tuesday, June 26, 2007 11:45AM - 12:00PM |
G6.00005: Individual contributions of friction and impact on non-shock initiation of high explosives Paul Peterson, Gabe Avilucea, Robert Bishop, John Sanchez Throughout the years a variety of tests have been designed which provide insight into the sensitivity of high explosives (HE) to non-shock initiation. Various standard tests such as the LANL drop weight impact, LLNL drop hammer, drop tower and skid tests have been developed to measure energetic response of explosives subjected to a combination of friction and oblique impact. In addition, the BAM test (for HE powders on roughened ceramic) and ABL friction test (powders or solids on roughened metal) have been developed for testing HE under frictional loading. In an effort to understand first principles of non-shock initiation, we have designed a series of tests to try to isolate friction and impact during the insult of HE. An initial series of tests have been completed with PETN, HMX, and as-pressed pellets of PBX 9501 (95 wt. percent HMX, 5 wt. percent inert binder), PBX 9502 (95 wt. percent TATB, wt. percent inert binder), Cyclotol (75 wt. percent RDX/25, wt. percent TNT), and Comp B3 (60 wt. percent RDX, 40 wt. percent TNT). The results suggest that some types of high explosives are relatively insensitive to pure impact and pure friction but relatively sensitive to insults involving a combination of impact and friction. [Preview Abstract] |
Tuesday, June 26, 2007 12:00PM - 12:15PM |
G6.00006: Grain Scale Discrete Element Simulation of Shock Responses of Explosives Wenqiang Wang, Jidong Yu, Hua Fu, Cangli Liu, Feng Zhao, Chengwei Sun The bulk mechanical and chemical responses of explosives are largely determined by microstructures. Here we present some recent progress in grain-scale discrete element simulation of shock responses of granular explosives and PBXs. The discrete element models can be built based on Voronoi tessellation as well as real micrographs. Our studies have been focused on the damage and fracture issues and the evolution of hot spots. The results are roughly in agreement with those from mesh-based simulation techniques. We will discuss the newly developed discrete element force models. Some preliminary 3D work will also be presented. [Preview Abstract] |
Tuesday, June 26, 2007 12:15PM - 12:30PM |
G6.00007: Measurements of Two-Dimensional Shock Initiation Process of Condensed Explosive Fenglei Huang, Xiangyu Hu In two-dimensional shock initiation, great differences exist between the axial center and the surface of shock profile. A two-streak high-speed photography measuring system is designed to record the reaction behavior within the run distance of shock initiation. The PMMA attached to the output surface of the explosive acts as the first streak. The second streak is produced by the opaque aluminum foil and paper. As the wave arriving time to the PMMA gap is different, the refractive index around the vicinity of the incline shock wave in the PMMA gap has been abruptly changed. The light changes its direction where the profile of the reactive shock wave can be recorded by high-speed camera. As the reactive wave has strong reaction, the light will pass the second streak and the reflection mirror to enter the camera. The reactive wave at different locations can be obtained by designing different length of the acceptor. The experiment results show that the delay-time of strong reaction decreases and the strength of reaction increases when the run distance of shock wave increased. The reaction rate and strength are smaller near the input shock zone within the initiation distance. [Preview Abstract] |
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