Organic and inorganic explosives were first developed and put into service in the 19th century, before there was much understanding of how the energy release mechanisms differed from those of the long established gunpowder. Theoretical advances in the understanding of shock waves combined with improvements in photographic and electronic techniques led to the hypothesis that a detonation is a shock wave maintained by the rapid release of chemical energy. Studies of accidental ignitions/initiations showed that explosive events can occur even when the energy input is much less than that required to heat the bulk explosive to the deflagration temperature. Hence, the highly fruitful idea of the localised hot spot was conceived. Apart from electrical stimuli, the main hot spot mechanisms are currently accepted as being adiabatic asymmetric collapse of gas spaces (producing gas heating, jetting, viscoplastic work) and the rubbing together of surfaces as in friction or adiabatic shear. Initiation mechanisms are also connected with the anisotropy of plasticity and fracture in explosive crystals. Decomposition of molecules can take place as they are forced past each other in a deforming crystal. There is, however, still much to discover about reaction pathways. Novel optical and electron microscopy techniques have given a great deal of new and precise information about displacements and failure mechanisms when explosive crystals are bonded together using polymers. The deflagration-detonation transition (DDT) has been extensively studied in model one-and two-dimensional systems.
The secondary explosive cyclotetramethylene-tetranitroamine (HMX) exists in a variety of crystal structures; the most widely used being the β-phase which is stable at room temperature and pressure. On heating, a more impact sensitive form (δ phase) is produced. The nonlinear optical technique of second harmonic generation (SHG) can be used as a probe of phase since δ-phase HMX generates a second harmonic at 532nm when 1064nm laser light is incident upon it. We present high-speed photography of SHG in HMX samples during dropweight impact and show that this technique can provide good spatial information and time resolution. We find evidence for small areas of δ-phase HMX appearing in the period from 13μs before ignition to 10μs afterwards, demonstrating that the heating on impact is sufficient to overcome the loading conditions and cause the phase change.
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.
Particle size effect on strength, failure, and shock behavior in polytetrafluoroethylene-Al-W granular composite materials J. Appl. Phys.Measurement of intergranular stress and porosity during dynamic compaction of porous beds of cyclotetramethylene tetranitramine
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