Malcolm D. Cook

The Role of Binders in Controlling the Cook‐Off Violence of HMX/HTPB Compositions

There is a clear difference in cook‐off vulnerability between highly‐loaded pressed compositions such as LX‐14 (pressed 95.5% HMX/4.5% binder), which yield violent responses, and cast compositions with low loadings, such as CPX 301 (85% RDX/15% HTPB), which yield relatively mild responses. These two classes of composition differ primarily in the quantity of binder, and in the manufacturing method used in production. An experimental study was conducted in an attempt to determine the filling proportion beyond which violent responses are observed. Here we describe a series of small‐scale cook‐off experiments which studied pressed compositions of 88%, 91%, 95% and 96% HMX, mixed with cured, cross‐linked HTPB. The experiments used a novel glass‐windowed test vehicle, instrumented internally with thermocouples. A trend of increasing event violence with increasing proportion of HMX was found, although in none of the experiments was mass reaction recorded. The results from these experiments are discussed.

DETONATION FAILURE IN IDEAL AND NON‐IDEAL EXPLOSIVES

In this paper we revisit and extend the classic treatment of detonation failure developed by Eyring et al. [1]. We recently published a development of this theory [2] in which a pressure dependant rate law was substituted for the Arrhenius temperature dependant law originally considered. Here we show that by assuming a 2‐component rate law based upon a temperature dependant ignition phase and a pressure dependant growth phase we are able to rationalise the very different failure characteristics (critical diameter and velocity decrement at failure) of ideal and non‐ideal explosives.

Ab‐Initio Prediction of Impact Sensitivity

An important goal of the energetic materials community has been to develop a first‐principles technique for the prediction of sensitivity. Here we discuss some previous work and propose alternative approaches. Using basic parameters we have attempted to obtain correlations with experimental drop weight impact data for a wide range of explosives. We compare and contrast the methods and draw conclusions with regard to the most important factors. Finally, we use the best correlations to make predictions for novel, as yet un‐synthesised, poly‐nitrogen materials.

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...

Observation of sub-detonation response in confined high density HMX based PBXs

This paper describes experiments and modelling aimed at understanding the behaviour of highly loaded (90%-95%) pressed HMX-based PBX compositions, when subjected to shock compression and ignition, by means of a propellant donor charge, under confinement. Such tests are routinely carried out in the UK on new formulations to determine their burn to violent reaction characteristics. The Bullseye propellant donor charge has been characterised in terms of pressure and temperature output. A range of tubes have been designed to examine the contribution of tube material properties (steel versus aluminium, 218.5 MPa) and to examine the effect of reduced confinement (120 MPa). For the reduced confinement scenario polycarbonate as well as steel and aluminium vessels have been designed which allow the reaction of the energetic material to be captured using a Phantom high-speed camera. In particular, tests carried out in the polycarbonate tubes have given a good insight of the processes occurring. Preliminary hydrocode modelling runs predicted an oscillating compressive wave in the explosive and considerable damage at either end of the explosive column. The latter leads to potential deconsolidation once the donor charge has burnt out allowing increased burning and violence.

ON DELAYED DETONATION (XDT) UNDER FRAGMENT IMPACT—AN ANALYSIS OF EXPERIMENTAL DATA AND A SIMPLE PHENOMENOLOGICAL MODEL

Fragment and bullet impact pose a serious threat for many weapon systems because of their potential to induce violent reactions in the energetic materials (explosives and propellants) contained within them. In some scenarios detonations have been observed under conditions insufficient to cause SDT or DDT. These events have been labelled XDT (X for unknown, Detonation Transition). It has generally been assumed that XDT arises as the result of some combination of damage to the energetic material and re‐shock or re‐compression of this damaged material. In this paper we review the results of our previous experimental studies aimed at understanding the conditions under which XDT may occur as the result of fragment impact. In addition, we describe some new experiments with improved instrumentation to help elucidate the key processes. Based on the experimental evidence and some simple modelling we then propose a phenomenological model for the XDT process.