Relationship between the morphology of granular cyclotrimethylene-trinitramine and its shock sensitivity

The sensitivity of explosives has been shown to be critically dependent on their morphology as well as their chemical properties. A critical hotspot can be seen as one end of the temperature distribution produced in a heterogeneous material by a shock. Studying shock sensitivity in a granular explosive thus allows insight into the temperature distribution produced by a shock in a granular material. In the case of cyclotrimethylene-trinitramine (RDX), a commonly used explosive, no consensus exists on the dominant features which promote critical hotspots. We present an investigation of the morphology and sensitivity of seven batches of RDX in two size classes and with a range of morphologies. Both the individual crystal morphology and the bulk granular morphology were studied and the features likely to interact with a shock in a granular material to form a critical hotspot are identified. We find that the internal void count does not correlate with the sensitivity of a granular bed and that crystals with defects can be insensitive in such a charge. Sensitivity in our tests was correlated with the presence of surface “dimples” for particles which are 10–30??m in size. For larger particles (100–300??m in size), more angular crystals were more sensitive.<br/

A comparison of the quasistatic and dynamic compressibilities of wet and dry vermiculite

Wet and dry vermiculite granules were compressed both dynamically and quasistatically in a steel powder-compression cell. The compressibility was found to depend both on strain rate and water content. The wet vermiculite was less compressible than the dry. Vermiculite (both wet and dry) was also less compressible under dynamic loading. The stress supported by discs of vermiculite pre-compressed to a density of 2445 kg m−3 was found to be around 90 MPa at a strain rate of around 4000 s−1 using a split Hopkinson pressure bar

Shock properties of diamond and kimberlite

Plate impact experiments have been performed on diamond and the igneous diamond-bearing matrix kimberlite. Longitudinal and lateral stresses within kimberlite have been measured in the uniaxial strain regime using embedded manganin stress gauges. The shock Hugoniot of the kimberlite has been characterised, and the elastic limit and shear strength have been studied. An experimental technique has been developed to observe the shock loading of transparent particles of dimension ~5 mm embedded in a matrix of lower shock impedance. The destruction of a diamond sample has been observed using high-speed photography with an inter-frame time of 100 ns. A crack within the diamond was found to propagate at 10 $\pm$ km s$^{-l}$, which is close to the Rayleigh wave velocity in diamond

Shock-wave stability in quasi-mono-disperse granular materials

This study investigated the stability of shock-waves in brittle granular materials and thus determined whether materials, such as sand, could be represented by a Hugoniot equation to describe their shock response. A series of plate impact experiments on quasi-mono-disperse soda-lime glass microspheres was conducted with the aim of measuring the shock-wave profile to determine if there was any change over time/run distance. Granular bed thickness and input stress were varied to study the effects of dispersion on shock-wave velocity (Csh). It has been shown that it is possible to sustain steady shock-waves in brittle granular materials due to no measurable wave dispersion occurring in the samples tested. This study also highlighted an unsteady precursor wave that is present in the wave profiles at low stresses

Classic and symmetric Taylor impact Tungsten alloys

Classic and symmetric Taylor tests were carried out on sintered Tungsten alloys in order to determine the failure modes, deformation profiles and strain histories during impact. Two material types were considered ; one that exhibited brittle fracture, the other more ductility. Impact velocities of 200 m/s were used in classic Taylor experiments and 300–370 m/s in the symmetric Taylor experiments. It was found in symmetric experiments that the rods bent ; fracture strains of up to 7.5% were attained, much higher than those found in quasi-static experiments. Both materials exhibited brittle intergranular fracture. Angled impact faces were also considered

Mechanical behaviour of polymers at high rates of strain

The stress-strain behaviours of polycarbonate (PC) and polyvinylidene difluoride (PVDF) have been measured over a range of strain rates at room temperature and a range of temperatures at high strain rate. Both materials show an approximately bilinear dependence of yield stress on strain rate over the rates examined. The experiments at different temperatures allow the high strain rate glass and $\beta$ transitions to be identified in PC, and the melting point and glass transition to be identified in PVDF. These can be confirmed by comparison to Dynamic Mechanical Analysis (DMA) measurements on the materials. Applying a time-temperature superposition to the data shows that these transitions are the cause of the bilinearity in the strain rate dependence of the materials. The behaviour of nominal (or engineering) stress in PC is also examined

Beta-delta phase change during dropweight impact on cyclotetramethylene-tetranitroamine

The secondary explosive cyclotetramethylene-tetranitroamine (HMX) exists in a variety of crystal structures; the most widely used being the ?-phase which is stable at room temperature and pressure. On heating, a more impact sensitive form (? phase) is produced. The nonlinear optical technique of second harmonic generation (SHG) can be used as a probe of phase since ?-phase HMX generates a second harmonic at 532?nm when 1064?nm 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 ?-phase HMX appearing in the period from 13??s before ignition to 10??s afterwards, demonstrating that the heating on impact is sufficient to overcome the loading conditions and cause the phase change.<br/

The dynamic temperature measurement of split Hopkinson bar specimens using small thermocouples and infrared streak photography

Two independent methods (embedded thermocouples and a commercial infrared detection system) were used to determine the temperature rise due to dynamic deformation of copper and iron specimens in a compression split Hopkinson pressure bar system. A paint with a quoted emissivity of 0.95 was used to improve the accuracy of the infrared measurements. Using this value, the two measurement methods were found to agree to within 1 K

Use of 3D simulations to investigate the effect of grooves on the surface of a plate impact experiment

Recent studies have demonstrated that the surface finish for a plate impact specimen has little influence on the Hugoniot stress attained in it, as obtained from a uniaxial strain loading scenario [1]. However, the surface finish did clearly affect the rise time of the gauge trace due to local inhomogenieties in the wave structure. The Eulerian hydrocode GRIM3D has been used to investigate the detailed wave profiles obtained in copper to explain and quantify these effects. The simulations have been used to compare the Hugoniot stress, rise times and general wave structure in the specimen and compared with idealised an equivalent 1D loading case. An attempt is also made to identify the critical groove depth and width before the wave profile is significantly affected compared to the 1D case. This is important since some specimens are difficult to prepare to very high tolerances. The results are discussed in terms of material response, numerics and with a recommendation for further work