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Through the use of laterally mounted stress gauges the Hugoniot Elastic Limits (HELs) of various geological materials have been measured. These materials, kimberlite, basalt, siltstone, quartz/feldspathic gneiss and amphibolite show a range of HEL values, from 1.3 GPa to 5 GPa. The value of the HEL appears to be influenced strongly by the grain size of the material, whereas, for example, it would appear to be independent of both the elastic properties and the measured Hugoniot slope in σ-up space. The current experimental data has been supplemented with relevant literature data. [ABSTRACT FROM AUTHOR]

A problem has recently been reported concerning the use of manganin gauges as a diagnostic in certain geological materials in that excessive noise can limit usability. A number of plate impact experiments have been performed with un-powered gauges to examine the extent to which electromagnetic emission from piezoelectric materials within the rocks affects gauge performance. It has been shown that materials with higher quartz content produce significantly more noise in the gauge traces. Mineral analysis has also been conducted to identify possible gauge straining due to different impedances of minerals within the rock types investigated. [ABSTRACT FROM AUTHOR]

A series of plate impact experiments was performed using the single stage light gas gun facility at the Cavendish Laboratory, Cambridge University to determine the dynamic tensile, or spall strength of a hypabyssal kimberlite and a siltstone. Two distinct impact configurations were employed, over a range of impact velocities up to 185 m/s. VISAR and manganin gauges were used to monitor the spalling of the material. Thicker targets were found to be less suited for spall measurements as the signal appeared to be degraded by the increased amount of material traversed before reaching the measuring plane. A comparison with a theoretical scheme relating the spall strength to the Hugoniot elastic limit is performed, and while the agreement is not perfect it is of the same order of magnitude. [ABSTRACT FROM AUTHOR]

A series of plate impact experiments was performed using the single stage light gas gun facility at the Cavendish Laboratory, Cambridge University to determine the principal Hugoniot curves of a hypabyssal kimberlite and a siltstone. A reverse impact configuration was employed, over a range of impact velocities 244–927 m/s. VISAR was the main diagnostic and sample geometry allowed for release characteristics to be studied. The release measurements show a significant deviation from the Hugoniot at higher initial shock pressures, though this does not correspond specifically to the HEL. Combined with other data previously reported the resulting Hugoniots are compared to various static data and a good correlation is shown with elastic impedance. [ABSTRACT FROM AUTHOR]

A series of plate impact experiments were performed using the single stage light gas gun facility at the Cavendish Laboratory, Cambridge University to investigate the shear strengths of a hypabyssal kimberlite, a siltstone and a sandstone. The siltstone and the kimberlite show a linear dependence longitudinally in σ-up space up to pressures in excess of 10 GPa [l]. The lateral stress data, as measured using manganin gauges, shows Hugoniot elastic limits of 2.2±0.1 GPa for kimberlite and 4.8±0.2 GPa for siltstone. The sandstone does not appear to behave elastically at any point in the experimentally determined Hugoniot, which is expected due to its high porosity. This is reinforced by a lack of measurable elastic behavior in the lateral stress data, and no obvious HEL [ABSTRACT FROM AUTHOR]

A series of plate impact experiments was performed to determine the principal Hugoniot curves of geological materials. The experiments were carried out using the plate impact facility at the Cavendish Laboratory, Cambridge University. By means of conventional impacts, over a range of impact velocities 217–1038 m/s, the Hugoniot of a quartz feldspathic gneiss and an amphibolite have been obtained. Manganin stress gauges were used as the main diagnostic tool. The Hugoniots show that the materials are either behaving elastically in the range of interest, or that the elastic limit of the materials are not visible due to there being no significant change in slope at the HEL. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

It has been known for some time that batches of the secondary explosive RDX from different manufacturers show significant variation in their shock sensitivity. No clear correlation between shock sensitivity and either chemical composition or morphology has been identified. As yet no comprehensive study has been reported covering RDX from different manufacturers, different production methods, and different particle sizes. In this work we use a range of techniques to study the morphology of RDX grains more closely and to assess which hotspot mechanisms might be dominant. Crystals were characterised using mercury porosimetry, environmental scanning electron microscopy (ESEM) and optical microscopy. This range of methods yields quantitative data on internal void size and number and surface porosity. Shock sensitivity is quantified using small-scale gap tests and these demonstrate clear differences in sensitivity between batches from different manufacturers. The samples used are from three manufacturers, produced by both the Woolwich and Bachmann processes, and of two different sizes, so a comprehensive study of how morphology might affect hotspot formation is possible. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Failure fronts or “waves” in brittle materials have been the subject of extensive research. This is a phenomenon where a damage front propagates behind the main shock wave and reduces the strength. It is reported to be observed by means of embedded lateral gauges where it manifests itself as a two-step structure. Such profiles were obtained when impacts were performed using copper flyers. Later, it was shown that this second rise in lateral stress was not evident in ceramic targets when either a ceramic projectile plate was employed or a thin ceramic cover plate was mounted on the front of the target. We now extend the use of different projectile materials such as heat treated 4340 steel and tungsten. We find that the second step is not evident and may instead be an effect of the experimental parameters and not related to the failure front. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

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. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Plate impact experiments have been performed on the igneous diamond-bearing matrix kimberlite. Longitudinal and lateral stresses were measured in the uniaxial strain regime using manganin stress gauges. The shock Hugoniot of the kimberlite has been characterized at axial stresses between 1 and 9 GPa. The kimberlite has a low impedance response when compared with similar data for other geological materials. The data indicate that the rock behaves inelastically above shock stresses of 1 GPa. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Symmetric Taylor and reverse ballistics tests were used in conjunction with high-speed photography to examine the characteristics of failure in fused silica rods. Experiments were performed at impact velocities up to 800 m s-1 yielding impact pressures to approximately 7 GPa. Failure front velocities were strongly dependent on impact pressure consistent with results for borosilicate and soda-lime glasses. One-dimensional strain waves were observed in some experiments before a one-dimensional stress regime was reached. For high impact pressures (> 2 GPa) the failure fronts followed almost immediately behind the incident shocks at a velocity near [square_root_of_2] cS for fused silica, borosilicate and soda-lime glasses. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

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. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Room temperature stress-strain curves were measured for the polymer bonded explosive (PBX) EDC 37 at strain rates between 400 s-1 and 4000 s-1. The material was found to exhibit strong strain rate dependence in this region, the strength increasing linearly with increasing rate. Speckle photography was used to measure the two dimensional surface deformation of the specimens during the test. The behaviour was found to be uniform at most rates, however, the specimens tested at 4000 s-1 showed shear banding. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

The sensitivity of energetic materials can be tested under various stimuli, such as low velocity impact in a drop-weight, static electric discharge or friction. Gap testing uses an explosive loading as the stimulus. The shock pulse is moderated by a layer placed between the loading charge and a column of the energetic material of interest. This study shows how, with careful attention to the production of the column, the test can be made sensitive to very small changes in the gap thickness of <0.04 mm of polymethylmethacrylate (PMMA). In these tests there is little crossover. This is highlighted by a series of experiments on the secondary explosive PETN with 180 μm grain size compared to samples with sub-micron grain size. A parallel series of experiments using polyvinylidenedifluoride (PVDF) gauges directly measures the level of stimulus experienced by the energetic material. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Ammonium nitrate (AN) is a commonly used fertiliser and also one component of the most widely used explosive in the world AN: Fuel Oil mixtures. This study uses a combination of high-speed photography, UV/Visible spectroscopy and modelling. By using thin beds, <0.5 mm thick, with a porosity of 22%vol the number and evolution of hot-spots in ammonium nitrate are monitored directly under dynamic loading conditions. The critical conditions for ignition are defined in terms of energy localisation mechanisms, temperature rise and inter-communication between the hot-spots. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Earlier work found that lateral stress gauges were unusable when the longitudinal stress in the sample exceeded around 13 GPa in samples of steel. This pressure corresponds to a phase transition found in iron and its alloys. It was hoped that protective padding would allow measurement of stresses higher than this. It was therefore necessary to investigate the effect of the padding on the stress measured. A series of experiments were carried out using mild steel with different thicknesses of mylar or polycarbonate padding. This research has been supported by simulation studies using the Eulerian hydrocode GRIM, which has indicated some issues with the representation of the lateral gauge in the hydrocode. The simulations have confirmed the experimental trends observed with additional padding, but they have also indicated the potential need for additional analysis on the raw lateral gauge data. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

This paper describes a simple optical technique for simultaneous photographic recording of high-speed events in stereo, using a translated lens stereoscopic system. An optical wedge is used to refract two spatially separated images together and the result is photographed as a split-frame. The technique is applied to the bulging of a plate on impact by a cylindrical rod. Methods of analysing in-plane movements in each stereoscopic image include use of a grid and a speckle pattern. The different in-plane movements between the two images, owing to perspective differences, can be used to extract topological information. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

The technique of Digital Speckle Radiography (DSR) has been shown to be widely applicable to the field of ballistics through its ability to measure internal displacement fields to a high precision. In a DSR experiment, a flat plane, typically 500 μm thick, of X-ray opaque filings are introduced into the sample during manufacture. Flash X-rays can then be used to image the plane, both before and during an impact event, to produce random speckled images on an X-ray film. Small sub-regions of the images can be tracked relative to each other using a Digital Speckle Photography (DSP) correlation algorithm. This produces a displacement field within the specimen on the seeded plane. By repeating an experiment with identical specimens, but varying the delay of the X-ray exposure, a full time history of the impact event can be obtained. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

An extensive study has been carried out into the relationships between the particle size of a charge, the density to which it is packed, the presence of inert additives and the sensitivity of the charge to different initiating shocks. The critical parameters for two different shock regimes have been found. The long duration shocks are provided by a commercial detonator and the short duration shocks are imparted using laser-driven flyer plates. It has been shown that the order of sensitivity of charges to different shock regimes varies. In particular, ultrafine materials have been shown to be relatively insensitive to long duration low pressure shocks and sensitive to short duration high pressure shocks. The materials that have been studied include HNS, RDX and PETN. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Digital Speckle X-ray Flash Photography is a technique which combines Digital Speckle Photography with Flash X-Ray Photography to measure 2- and 3-D displacement fields within dynamically deforming specimens. Measurements are made throughout a plane within the specimen seeded with X-ray opaque particles. This technique has already been successfully applied to the study of polyester, cement¹ and sand², and is used here to study the influence of water on a sand bed under impact from a hemispherical-tipped copper rod travelling at 100 m s[sup -1]. Significant differences in the response of dry and wet sand beds were detected, and examples of the deformation fields measured are illustrated here. These results may be applicable in many spheres, for example, in the design of mechanisms to destroy buried ordnance. [ABSTRACT FROM AUTHOR]

A series of plate impact experiments was performed on Dolerite (diabase) igneous rock. Longitudinal stresses were measured using embedded manganin stress gauges up to ca. 11 GPa. In addition, lateral stresses were also measured up to ca. 7 GPa. In combination with the longitudinal stresses, these results have been used to obtain the material shear stress under shock compression. Results indicate that the longitudinal behaviour is elastic for the stress range involved although shear stresses indicate deviation from elastic loading for longitudinal stresses higher than ca. 4.3 GPa. The results are then compared and contrasted to data for other geologic materials. [ABSTRACT FROM AUTHOR]

A series of plate impact experiments on cement paste (grout) has been performed to assess the dynamic strength of this material. Lateral stresses have been directly measured by means of embedded manganin stress gauges. In combination with longitudinal stresses, measured previously [1], these results have been used to obtain shear strength under shock loading. Results indicate that the material is behaving in an inelastic manner with the shear strength increasing with increasing pressure. [ABSTRACT FROM AUTHOR]

Investigations into the dynamic properties of alumina ceramic have been carried out for several years at the Cavendish Laboratory [1,2,3]. Previous work has demonstrated a reduction in spall strength with an increased width of the compression pulse using either thicker fliers, or a flier with longer double transit time for the shock wave. Here the variation in spall strength of an alumina ceramic as part of a multilayer system is investigated. Results indicate that the spall strength decreases with increasing time for which the target is under compression. There is some indication that spall strength may decrease faster under shock ring-up than a single shock taking the sample to the same ultimate pressure. [ABSTRACT FROM AUTHOR]

In-material longitudinal and lateral stress histories in three dense, silica-based glasses were directly measured by embedded manganin stress gauges during plate impact experiments. Lateral stress profiles in all of the materials show evidence of failure fronts that behave in a similar manner to those observed in open-structured glasses. The measured stress histories were used to calculate the deviatoric responses and results indicate that ahead of the failure front the shear stress increases linearly along the estimated elastic response. Behind the failure front, however, the shear stress appears to first decrease and then increase as the pressure increases, contrary to a previous interpretation [1]. [ABSTRACT FROM AUTHOR]

Type II DDT has been observed in low density charges of ultrafine PETN and RDX. The compressive burning regime that mediates the final stages of type I DDT has been shown to be absent in this mechanism. Convective burning controls the propagation of reaction throughout the column and detonation breaks out at some point along the column. The exact features of the detonation vary between PETN and RDX. The study described here gives new details on the pressure and temperature regimes that operate within the column during the build-up to detonation. Velocity measurements of the waves (in particular the detonation waves) found in these systems have been made. It has been shown that the detonation wave velocities are anomalous for materials at the initial pressing densities, with PETN having an enhanced detonation velocity following a type II DDT and RDX having a retarded detonation velocity. [ABSTRACT FROM AUTHOR]

Hydrogen Peroxide has been known as an explosive for many years and has been used as such in several limited applications. It is also notorious for its instability with regard to organic materials, metal powders and temperature. It does, however, offer the opportunity to study shock-induced reaction in a very simple chemical system. This paper outlines the data gained from ultra-violet / visible emission studies when shock induced reaction occurs in pure hydrogen peroxide. [ABSTRACT FROM AUTHOR]

Plate impact experiments have been performed on two igneous rocks of differing grain size. Shock stresses have been measured by means of embedded manganin stress gauges, up to ca. 12 GPa. In the coarser grained material data was obtained from both copper impactors on rock and the reverse situation. Results are compared and contrasted to the existing data for other geological materials. [ABSTRACT FROM AUTHOR]

A series of plate impact experiments has been performed to determine the shock loading behaviour of cement paste (grout) with water to cement ratio of 0.35 by weight and no aggregates. Hugoniot stress data were collected by means of embedded manganin gauges up to 5 GPa. Results are then compared to those obtained in previous studies on concrete varied on aggregate size using a plate reverberation technique and velocity interferometry. Results indicate that the average loading of cement and concrete are comparable. In addition, the in-material gauge technique has the advantage of estimating the cement Hugoniot directly and provides a measurement of the rise times, which were found to be inversely related to impact stresses. [ABSTRACT FROM AUTHOR]

Research has been carried out into the mechanisms by which columns of ultrafine granular explosives undergo the deflagration-to-detonation transition (DDT). Unusual results have been obtained in columns of pentaerythritol tetranitrate (PETN) and cyclotrimethylene trinitramine (RDX) when the column density is below 50% of theoretical maximum density (TMD). Experimental evidence shows that these columns undergo a transition which does not include the compressive deflagrating stage that is a usual precursor to DDT. Instead a channel is created in the column by the gas given off during the initial burning. The eventual transition to detonation takes the form of a localized 'thermal' explosion occurring at some point on the column. [ABSTRACT FROM AUTHOR]