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 PHOTOGRAPHYA 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][2][3][4][5][6][7][8][9][10][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][22][23] and dynamic studies [19,20,[24][25][26]. HOPKINSON BARSThe split Hopkinson pressure bar (SHPB) is one of the most widely used machines for obtaining the stress-strain properties of materials at high rates of deformation (10 3 -10 4 s -1 ) [27,28]. Our initial work in this area was to develop a miniaturised direct impact system (DIHB) allowing strain rates between 10 4 and 10 5 s -1 to be accessed [29,30]. A larger DIHB was later developed to obtain the high rate mechanical properties of polymer-bonded explosives (PBXs) and polymers [7, 31]. In recent years, we have constructed a suite of conventional SHPBs of various mechanical impedances so as to be able to measure the high rate properties of materials

Impact initiation of explosives and propellants via statistical crack mechanics

“…Furthermore, the interaction of hot spots is probably crucial to initiation, but is difficult to assess. However, experimental evidence that hot-spots will die out without interactions was given recently by Proud et al (2002Proud et al ( , 2004 and Field et al (2004).…”

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

“…This gives temperatures in the range 3800K within 160 ns of the reaction starting. The grain size of a material is intimately linked to its sensitivity [24,25]. Gap tests are widely used as a measure of shock sensitivity.…”

Temperature and strain rate effects on the mechanical properties of a polymer-bonded explosive

“…[12][13][14][15][16][17][18][19][20][21][22][23][24]. The main conclusions of such research were: (i) the strain rate and temperature response are generally governed by the properties of the polymer binder [15,16]; (ii) the absolute value of the stress supported for a given strain rate and temperature is governed by the size of the explosive crystals [12,14].…”