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  • American Institute of Physics (AIP)  (18)
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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 86 (1999), S. 1749-1753 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An investigation has been carried out into the differences between the deflagration-to-detonation (DDT) process as it occurs in low density [∼30% theoretical maximum density (TMD)] columns of conventional grain size (∼180 μm) pentaerythritol tetranitrate (PETN) and in ultrafine PETN with a grain size ∼1 μm. The principle technique for observing the process utilized charges confined within a steel housing fitted with a polycarbonate slit window. This allowed direct recording of the transition using high speed streak photography. The explosive was thermally ignited using a pyrotechnic mixture with low gaseous emission to minimize any prepressurization of the charge. In addition to the photographic records of the events, the outputs of photodiodes along the length of the column were monitored in order to determine the rate at which the reaction proceeds. The results obtained show that the DDT process in the larger grain PETN at low density was similar in structure to the DDT process at higher densities. In contrast a different mechanism leads to detonation in columns composed of the smaller grain size PETN when packed to densities less than 50% TMD. After ignition hot gases propagate along the column both compacting and igniting material as they pass. After the gases have reached the downstream end of the column, the column continues to burn and the pressure and temperature increase. Some time later initiation takes place at a point along the burning column, and detonation waves propagate in both directions from this point. The detonation waves propagate from the initiation point at speeds that would normally be associated with material compacted to around 60% TMD. The process appears to be in effect a deflagration-to-localized thermal explosion detonation transition. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 73 (2002), S. 2185-2189 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A high power fiber delivery system has been developed for a Q-switched Nd:YAG laser. Multimode fibers with a core diameter of 400 μm were tested with a view to transmitting the maximum possible amount of optical energy. The importance of surface finish was investigated by employing a number of different polishing procedures. Atomic force microscopy and laser-induced damage threshold measurements were used to identify a clear correlation between surface finish and transmission capability. Surface roughness measurements as low as 3 nm were made and the transmission of up to 30 J/cm2 achieved. The front face of the fibers would be improved during laser testing due to plasma formation which acts to anneal the surface. The various damage mechanisms that limit the performance of the fiber have been studied and attributed to different optical and physical effects. The nature of the light spot emerging from the fiber was analyzed and quantified by beam profilometry. A number of different methods for altering the beam profile were tried but changing the length of the fiber was found to be the most effective. © 2002 American Institute of Physics.
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 78 (1995), S. 3736-3739 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A mechanism for the compressive failure of soda-lime and borosilicate glasses is proposed based upon high-speed photography of impact on glasses. Shock loading was produced by the impact of a 50 mm diameter projectile so inducing shock states of one-dimensional strain in glass targets. The shock waves and failure fronts were visualised using the shadowgraph technique. The failure appeared to occur at discrete nucleation sites and propagated out to form a continuous front. The velocity of this front increased with higher impact stresses and varied with the glass composition. © 1995 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 88 (2000), S. 3859-3864 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Laser-driven flyers were launched from substrate backed aluminum films. They were produced by single pulses from a Q-switched Nd:yttrium–aluminum–garnet laser, of 10 ns duration and typically up to a few hundred mJ energy, with a focused spot size of approximately 1 mm diameter. The aluminum films were between 2 and 6 μm thick and the flyers typically achieved average velocities of a few km s−1. The integrity of the flyers was studied using streak photography and microscopy of the impacted flyers. Threshold times and distances prior to plasma penetration were used as a measure of integrity and were calculated for films of different thickness, launched with various laser pulse energies. © 2000 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 86 (1999), S. 6707-6709 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Lateral stress measurements in a tungsten alloy, in combination with known Hugoniot data, have been used to find the shear strength of this material, and its variation with longitudinal shock stress, up to 14 GPa. Results show that the shear strength increases significantly with increasing stress. Prior to this work, there has been disagreement in the literature on the effect of shock stress on the shear strength of tungsten and its alloys. The present work agrees with the data obtained by Zhou and Clifton [J. Appl. Mech. 64, 487 (1997)] who used pressure shear. However, the range of stresses studied has been greatly extended. © 1999 American Institute of Physics.
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  • 6
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The temperature rise produced in iron and copper specimens by high strain deformation in a compression split Hopkinson pressure bar was measured simultaneously using two independent techniques: (i) small thermocouples (0.2 mm junction size) and (ii) an infrared (IR) camera system based on mercury cadmium telluride (spectral response between 8 and 12 μm). The response time of the thermocouple system was limited by the time taken for heat to diffuse into the junction. In order to obtain useful data with the IR camera system, the emissivity of the specimen surface needed to be modified by depositing a layer of soot. Even so, the measured emissivity was low (0.4), suggesting that the soot layer was semitransparent to IR radiation. The thermocouples, however, yielded temperature measurements that were consistent with all the mechanical work performed on the specimens being converted to heat. The main positive result obtained with the IR camera system is that heating of the iron specimens was spatially nonuniform. © 2000 American Institute of Physics.
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  • 7
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The detonation velocity following type II deflagration-to-detonation transitions (DDT) has been observed to be markedly different to that which would be expected were the charges directly initiated. In charges of pentaerythritol tetranitrate (PETN) velocities as high as 5.6 mm μs−1 have been observed compared to 3.7 mm μs−1 which is both predicted and measured for the charge were it to be directly initiated. Experiments have also been carried out that attempt to measure the temperature within columns of PETN prior to a type II DDT event. Other experiments and hydrocode simulations have been carried out that attempt to isolate the possible causes of the anomalous detonation velocity and as a result determine what the critical factors that determine the velocity are. These studies have led to a greater understanding not only of the reasons behind the change in detonation velocity, but also of the stages that occur in the build up to type II DDT. © 2002 American Institute of Physics.
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  • 8
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 84 (1998), S. 734-738 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The response to plate impact loading of three aluminas with varying glass content and porosity has been investigated. Spall strengths have been shown to be dependent on the amplitude and duration of the compression pulse which precedes the tensile loading, but insensitive to the rate of release. Some tensile strength is measured in impacts where the Hugoniot elastic limit has been exceeded. The effect of the material microstructure on the dynamic tensile strength has also been studied. Low porosity aluminas with many microstructural irregularities were found to possess the lowest spall strengths. Experimental values compare well with those predicted by an energy balance theory developed by Grady [J. Mech. Phys. Solids 36, 353 (1988)]. © 1998 American Institute of Physics.
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  • 9
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 3842-3847 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The interaction of laser radiation from a Nd/glass laser with single crystals of cyclotrimethylene trinitramine (RDX) secondary explosive and RDX slices (cleaved from larger crystals of RDX) has been studied by high-speed framing photography with microsecond interframe times. Decomposition and deflagration were recorded following ignition at localized regions or "hot spots.'' © 1996 American Institute of Physics.
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  • 10
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 59 (1986), S. 3945-3952 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: This paper describes a study of the decomposition of pentaerythritol tetranitrate (PETN) using a high-resolution time-of-flight mass spectrometer. The decomposition was induced by fracturing single crystals and by laser irradiation. In the fracture experiments, the energy input was varied from the lowest level necessary to produce smooth cleavage surfaces to high-energy loading which produced rough conchoidal fracture surfaces. In the laser experiments, a ruby laser was used in both normal and Q-switched modes, and again the energy input was varied. For all the various experiments, the reaction products were analyzed and reaction schemes are proposed. It is shown that low-energy fracture causes decomposition which follows the same reaction pathway as that induced thermally, with initial failure at the RO-NO2 bond. However, high-energy fracture results in the breaking of the C-C bonds. Two reaction pathways were observed with the laser irradiation. The first is the normal thermal process, but evidence was also found for failure at the R-ONO2 bond. The reaction continued for several milliseconds after the end of the laser pulse, suggesting a "partial'' ignition of the explosive. In other experiments, the conditions for laser initiation of PETN in vacuum were investigated. Explosion occurred when Q-switched pulses of 1 J energy were applied to a molten layer of PETN.
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