Martin Greenaway

An Investigation into the Initiation of Hexanitrostilbene by Laser‐Driven Flyer Plates

An investigation into the shock sensitivity of hexanitrostilbene (HNS) has been carried out. A Q‐switched Nd:YAG laser was used to launch miniature flyer plates from substrate‐backed aluminium films. The impact produces a shock with duration of the order of 1 ns and pressure of the order of 10 GPa. The explosive samples were pressed into PMMA cylinders to 65–78 % theoretical maximum density. The threshold laser pulse energy required to produce a flyer with sufficient velocity to cause detonation was found. A high‐speed camera was used to record the entire event. Initial curvature of the streak record, for impacts just below the detonation threshold, showed that reaction started inside the column. This feature was not seen in a previous study. It was found that conventional HNS, with a mean particle size of approximately 25 μm, could not be detonated while fine grained HNS (sub‐micron particle size) would detonate.

Study of Sensitivity and Repeatability of Piezoelectric Sensors

The sensitivity and repeatability of stress and density measurements obtained using commercially available piezoelectric probes have been studied for high‐velocity (> 450 m s−1) gas gun‐driven spray experiments. The probes used are Dynasen Piezopins, in which the sensor element is a small ((0.4 ± 0.05) mm thick, (1.2 ± 0.1) mm diameter) PZT (Lead Zirconate Titanate) disk. The probe gives an output voltage V(t) proportional to the time derivative of the force normal to the poled axis of the PZT. The stress level is obtained using the time‐integrated voltage. It is assumed that there is complete momentum transfer between the spray and the Piezopin, therefore the spray density can be found from the stress level. The spray is produced by accelerating aluminum powder (10 μm grain size) in a gas gun. Spray density measurements are compared with values measured from x‐ray images, and stress measurements are compared with extrapolated values from the spray densities obtained from the x‐ray images.

The Development of a Laser‐Driven Flyer System

This paper describes recent advances to a laser‐driven flyer system. In this technique, laser‐induced plasma is used to drive miniature flyer plates at velocities approaching 10 km/s. The flyers are launched from substrate‐backed metal films and are typically less than 1 mm in diameter and a few microns thick. The system has found application in detonics, high‐strain rate testing and micrometeorite simulation. Recent advances described here are concerned with manipulating the flyer profile and enhancing performance. A fiber‐optic delivery system is used to alter the spatial intensity distribution of the launch pulse. High‐speed photography was used to verify the effectiveness of this technique as illustrated by the excellent correlation between beam profile and flyer shape. A technique using bi‐layered films was developed with a view to improving the energy efficiency of the system. The kinetic energy of flyers launched with the additional layer was found to be enhanced by a factor of near three.

The Shock Initiation and High Strain Rate Mechanical Characterization of Ultrafine Energetic Powders and Compositions

ABSTRACT This paper reviews the techniques that have been developed at the Cavendish Laboratory for the study of the mechanical and ignition properties of energetic materials. HIGH-SPEED PHOTOGRAPHY A number of techniques have been developed and applied in our laboratory for the investigation of the properties of energetic materials. One method we have used in a wide range of such studies has been high-speed photography e.g. refs [1-11]. The advantage is that it is possible to see directly what is going on in, for example, hot spot initiation of energetic materials. In recent years, it has increasingly been desired to model the impact response of structures containing energetic materials using numerical methods. If meaningful numerical results are going to be obtained for, say, munitions or rockets, it is of vital importance that constitutive relations be constructed which describe the mechanical response of unreacted energetic materials over the temperature and strain rate ranges of interest. With this in mind, we have developed a range of techniques for obtaining the mechanical properties of energetic materials over a wide range of strain rates and temperatures. Examples of publications where such data have been published include refs [4, 7, 12-20]. One problem with conventional mechanical testing methods is that they only allow the measurement of the global response of a specimen. Energetic materials usually consist of a viscoelastic binder heavily loaded with explosive crystals. In order to develop realistic and physically-based constitutive models for such unusual composites, it is vital to determine how these materials deform on the mesoscale. To this end, we have developed a range of optical and microscopy techniques. These have been used for both quasistatic [14, 15, 18, 21-23] and dynamic studies [19, 20, 24-26].

Characterising the Response of Energetic materials and Polymer-Bonded Explosives (PBXs) to High-Rate Loading.

Polymer-bonded explosives (PBXs) are being increasingly used as energetic fillings and components in many systems. They are perceived as more chemically and mechanically stable than traditional fillings such as RDX/TNT. They are castable into predetermined shapes, machinable and can be used as structural components. However, along with all these undeniable advantages, as a class, these materials are now undergoing extensive characterisation to ensure they comply with both the legal and technical requirements in energetic systems. It is well-known that polymers display non-linear behaviour and are much more complex than, for example, simple metal systems at any rate of strain. The understanding of PBX systems involves areas as diverse as polymer chemistry, chemical compatibility, mechanical properties, impact tests, and thermal stability. In this paper, aspects of energetic material response are outlined which are relevant to the understanding of PBX sensitivity.

Effect of surface finish on the high-power transmission characteristics of fused-silica optical fibers

At high optical power densities, materials that would normally be classed transparent, break down. The mechanisms by which high purity fused silica optical fibers fail are discussed in this paper. Multimode fibers with a core diameter of 400 micrometers have been tested with a Nd:YAG laser with a view to transmitting the maximum amount of energy. The importance of surface finish has been verified by implementing polishing schedules of varying degrees. The front face of many of the fibers would be improved during laser testing, due to plasma formation which acts to anneal the surface. It has been found that the energy level at which this effect first occurs gives a good indication of the initial surface roughness. Atomic force microscopy has been used to confirm surface roughness measurements as low as 3 nm and excellent agreement between high power transmittance and surface quality has been found.

Monitoring Phase Change in HMX during Dropweight Impact

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 non‐linear optical technique of second harmonic generation (SHG) can be used as a probe of phase since δ‐phase HMX generates a second harmonic when 1064 nm laser light is incident upon it. In this paper, simultaneous high‐speed photography of ignition and SHG is presented. With this system we found no evidence for the presence of δ‐phase prior to ignition of an impacted β‐phase sample.