C. Stennett

Shock propagation in a cemented tungsten carbide

WC-based ceramic metal composites (cermets) are of great importance in both armor and munition design due to the combination of properties imparted by the presence of two different phases. WC-Co cermets are of interest in this area due to the hardness and strength imparted by the WC phase while the cementing Co matrix acts to increase plasticity and toughness. Here the dynamic response of G13 WC-Co manufactured by Kennametal Engineered Products B.V. was studied via a series of plate impact experiments involving both longitudinal and lateral gauges, which allowed determination of the Us - Up relationship, measurement of a Hugoniot elastic limit of 3.3±0.2 GPa, measurement of a spall strength of 4.38 GPa, and an investigation of the stress dependence of shear strength in such a strong material.

The high strain-rate behaviour of selected tissue analogues

The high strain-rate response of four readily available tissue simulants has been investigated via plate-impact experiments. Comparison of the shock response of gelatin, ballistic soap (both sub-dermal tissue simulants), lard (adipose layers) and Sylgard(®) (a potential brain simulant) allowed interrogation of the applicability of such monolithic tissue surrogates in the ballistic regime. The gelatin and lard exhibited classic linear Hugoniot equations-of-state in the US-uP plane; while for the ballistic soap and Sylgard(®) a polymer-like non-linear response was observed. In the P/σX-v/v0 plane there was evidence of separation of the simulant materials into distinct groups, suggesting that a single tissue simulant is inadequate to ensure a high-fidelity description of the high strain-rate response of complex mammalian tissue. Gelatin appeared to behave broadly hydrodynamically, while soap, lard and Sylgard(®) were observed to strengthen in a material-dependent manner under specific loading conditions at elevated shock loading pressures/stresses. This strengthening behaviour was tentatively attributed to a further polymeric-like response in the form of a re-arrangement of the molecular chains under loading (a steric effect). In addition, investigation of lateral stress data from the literature showed evidence of operation of a material-independent strengthening mechanism when these materials were stressed above 2.5-3.0GPa, tentatively linked to the generically polymeric-like underlying microstructure of the simulants under consideration.

INITIATION OF SECONDARY EXPLOSIVES MEASURED USING EMBEDDED ELECTROMAGNETIC GAUGES

There is considerable evidence that secondary explosive materials having a relatively large (10–12%) proportion of HTPB binder do not exhibit DDT under cook‐off. However, the understanding of the mechanisms controlling the growth of reaction in such experiments is incomplete. Most importantly, it is not known whether a mechanistic reason exists to preclude DDT; it is possible that existing techniques to explore cook‐off simply do not offer the correct conditions to allow DDT to occur. We present experiments in which impacts were made against a RDX/HTPB PBX using a single‐stage light gas gun. Electromagnetic particle velocity gauges were embedded within the targets at different distances from the impact face to record the onset of reaction, and in some cases detonation. These experiments were also performed against RDX/TNT targets. The time‐resolved particle velocity histories have allowed comparison of some of the factors governing growth of reaction, and have provided run‐to‐detonation distance data for ...

A sealed capsule system for biological and liquid shock-recovery experiments.

This paper presents an experimental method designed to one-dimensionally shock load and subsequently recover liquid samples. Resultant loading profiles have been interrogated via hydrocode simulation as the nature of the target did not allow for direct application of the diagnostics typically employed in shock physics (e.g., manganin stress gauges or Heterodyne velocimeter (Het-V)). The target setup has been experimentally tested using aluminium flyer plates accelerated by a 50-mm bore single-stage gas-gun reaching projectile impact velocities of up to ~500 ms(-1) (corresponding to peak pressures of up to ca. 4 GPa being experienced by fluid samples). Recovered capsules survived well showing only minor signs of damage. Modelled gauge traces have been validated through the use of a (slightly modified) experiment in which a Het-V facing the rear of the inner capsule was employed. In these tests, good correlation between simulated and experimental traces was observed.

On the shock response of the magnesium alloy elektron 675

Alloying elements such as aluminum, zinc or rare-earths allow precipitation hardening of magnesium (Mg). The low densities of such strengthened Mg alloys have led to their adoption as aerospace materials and (more recently) they are being considered as armor materials. Consequently, understanding their response to high strain-rate loading is becoming increasingly important. Here, the plate-impact technique was employed to measure longitudinal stress evolution in armor-grade wrought Mg-alloy Elektron 675 under 1D shock loading. The spall behavior was interrogated using a Heterodyne velocimeter (Het-v) system, with an estimate made of the materials Hugoniot elastic limit for both aged and un-aged materials.

SHOCK COMPRESSION AND RECOVERY OF MICROORGANISM‐LOADED BROTHS AND AN EMULSION

The microorganisms Escherichia coli, Enterococcus faecalis and Zygosaccharomyces bailii and an oil‐based emulsion, have been subjected to shock compression using the flyer‐plate technique to initial pressures of 0.8 GPa (in the suspension). In each experiment, a stainless steel capsule was used to contain the broths and allow for recovery without contamination. Where cavitation was mostly suppressed by virtue of simultaneous shock and dynamic compression, no kill was observed. By introducing an air gap behind the suspension, limited kill was measured in the yeast. Results also suggest that stable emulsification occurs in coarse oil‐based emulsions that are subjected to shock.

Effects observed when using metallic flyers and barriers with the embedded particle velocity gauge technique

A number of experiments were carried out using a modified version of the standard particle velocity gauge technique in plate impact experiments with inert targets. Unusually these utilised dynamic metallic elements. Traditional methodology advises against the use of metallic flyers/barriers with this technique as conductive objects moving in the magnetic field produce perturbations in the output gauge voltage leading to inaccuracies in the derived particle velocities. This body of work investigated the causes of the perturbation effect, methods of minimising its magnitude and possible post-processing correction methods. In experiments with Al flyers, perturbations on the order of 10-15% of signal strength were observed. While the magnitude of the voltage traces were distorted, key features such as shock impact could still be observed, and shock tracker gauges were still effective. The case of metallic barriers was also examined and similar effects observed. This study has indicated that while a coarse empirical correction is possible, uncertainty in the validity of the correction would preclude the use of dynamic metallic elements in experiments where high fidelity data is required.

Developments in a small scale test of violence

Utilising recent advances in diagnostic technologies, a test is under development to obtain a numerical value for the violence of response to thermal stimuli on a small explosive sample. The assembly is designed to accept pressed explosive pellets which enables the test to be conducted at the small-scale development stage and thus is anticipated to be of use in the screening of new materials. Building on previously published work describing the test development, twenty-three new cook-off experiments have been conducted. Eleven explosive compositions were subjected to the same slow heat input profile. As a sample rapidly decomposed, part of the steel confinement was designed to rupture, producing a pellet whose velocity was measured using a Heterodyne Velocimeter (Het-V). Temperatures of the confinement unit were also recorded. A development aim is to interpret this data to provide useful information on the violence of decomposition. This is discussed in the paper and leads to the data from these experimen...

Live Decomposition Imaging of HMX/HTPB Based Formulations During Cook-Off in the Dual Window Test Vehicle.

Thin, cylindrical samples of HMX/HTPB formulations with solids loadings from 85-95% by mass have been heated at 1°C and 10°C minute until a reaction occurred in the new dual window cook-off test vehicle. The test vehicle has captured the response of these formulations, and shown the influence of variables such as confinement, heating rate and sample size. Live imaging of the heated samples revealed that, as with pure nitramine samples, three distinct stages of change take place during heating; phase changes, melting and slow, flameless decomposition with production of gaseous intermediates and finally burning with a luminous flame of the gaseous intermediates. In addition, the binder appears to undergo decomposition before the HMX, darkening along the edge closest to the thermal input before the HMX melts. No apparent volume change has been detected during the beta-delta phase change in HMX, however this could be due to the low-density hand pressed samples, allowing possible occlusions to be present. Prior to violent reaction, flame speeds were measured at approximately 30m/s for high confinement, which reduces by 2-3 orders of magnitude when confinement is lowered.Thin, cylindrical samples of HMX/HTPB formulations with solids loadings from 85-95% by mass have been heated at 1°C and 10°C minute until a reaction occurred in the new dual window cook-off test vehicle. The test vehicle has captured the response of these formulations, and shown the influence of variables such as confinement, heating rate and sample size. Live imaging of the heated samples revealed that, as with pure nitramine samples, three distinct stages of change take place during heating; phase changes, melting and slow, flameless decomposition with production of gaseous intermediates and finally burning with a luminous flame of the gaseous intermediates. In addition, the binder appears to undergo decomposition before the HMX, darkening along the edge closest to the thermal input before the HMX melts. No apparent volume change has been detected during the beta-delta phase change in HMX, however this could be due to the low-density hand pressed samples, allowing possible occlusions to be present. Prio...

Development of a Dual Windowed Test Vehicle for Live Streaming of Cook-Off in Energetic Materials.

A modular, axially connected test vehicle for researching the influence of various heating rates (cook-off) on energetic materials and how they fundamentally decompose, leading to a violent reaction has been developed and tested. The vehicle can accommodate samples measuring up to 50 mm in diameter, with thicknesses variable from 0.5 mm up to 50 mm long. A unique feature of this vehicle is the ability to have a live high speed camera view, without compromising confinement during the cook-off process. This is achieved via two special windows that allow artificial backlighting to be provided at one end for clear observation of the test sample; this has allowed unprecedented views of how explosives decompose and runaway to violent reactions, and has given insight into the reaction mechanisms operating, and challenges current theories. Using glass windows, a burst pressure of 20 MPa has been measured. The heating rate is fully adjustable from slow to fast rates, and its design allows for confinement to be varied to study the influence on the violence of reaction during cook-off. In addition to being able to view the test sample during cook-off, embedded thermocouples provide detailed temperature records and the ability to use Photon Doppler Velocimetry (PDV) instrumentation is also incorporated.A modular, axially connected test vehicle for researching the influence of various heating rates (cook-off) on energetic materials and how they fundamentally decompose, leading to a violent reaction has been developed and tested. The vehicle can accommodate samples measuring up to 50 mm in diameter, with thicknesses variable from 0.5 mm up to 50 mm long. A unique feature of this vehicle is the ability to have a live high speed camera view, without compromising confinement during the cook-off process. This is achieved via two special windows that allow artificial backlighting to be provided at one end for clear observation of the test sample; this has allowed unprecedented views of how explosives decompose and runaway to violent reactions, and has given insight into the reaction mechanisms operating, and challenges current theories. Using glass windows, a burst pressure of 20 MPa has been measured. The heating rate is fully adjustable from slow to fast rates, and its design allows for confinement to be var...