A. M. Milne

Experimental and Numerical Study of Free‐Field Blast Mitigation

The development of a fundamental understanding of the mechanisms governing the attenuation of explosives effects by a surrounding mitigant material or system would benefit many civilian and military applications. Current approaches rely almost exclusively on empirical data, few if any truly predictive models exist. Dstl has recently pursued an experimental programme investigating the mitigation of effects from detonating explosives in support of general requirements to attenuate blast and fragmentation. The physical properties of a range of mitigant materials have been studied at a more fundamental level, both experimentally and numerically. A preliminary numerical parameter study has been undertaken by FGE using two‐phase numerical simulations to complement the experimental studies. Initial work used idealised equations of state for generic mitigants but more recently material characterisation experiments have been undertaken at RMCS. Results confirm that porosity and particle density are dominant factor...

Shock to detonation transition in a plastic bonded explosive

It is important to define a methodology to assess the safe handling conditions of explosive materials. Such a route must be simple enough to capture relevant mechanisms in order to construct a modeling capability. To this end, a series of plate impact experiments has been undertaken in order to characterize the unreacted Hugoniot of a research, castable, plastic-bonded explosive (PBX) RF-38-22. A second program, using embedded particle velocity sensors, has been undertaken to investigate the shock to detonation transition (SDT) in the PBX. The experimental facility is described, the techniques used are discussed, the material is introduced, and results obtained are reported. The data obtained have been used to calibrate a hydrocode model for SDT based on viscoplastic ignition and subsequent grain burning, with pressure-dependent kinetics. This model is described, and results of calculations are presented. This represents the program in the construction of the methodology. It is hoped that further work wil...

The response of soda-lime glass-hydroxyterminated polybutadiene composites to shock loading

The effect of particle size on the shock response of three soda-lime glass-hydroxyterminated polybutadiene composites has been investigated. While the shock velocity–particle velocity relationship has been shown to be nearly identical in all three materials, thus indicating that the hydrodynamic response is particle size independent, the shock stresses have been shown to be strongly dependent upon particle size. It has been proposed that this be due to the nature of the microstructure, with the larger particles restricting flow, and thus increasing shear strength, while the finer microstructure can flow as a whole.

Modelling of complex blast

There is an extensive hierarchy of models for the prediction of free-field blast wave propagation and loading. As one begins to move to more complex scenarios (e.g. near-field blast, non-ideal explosives, internal blast or buried charges), there is a need for more detailed understanding and modelling. In this article, we discuss a range of sub-models for the components of complex blast which can be used in hydrocode modelling of the loading of targets or structures. Many explosives are fuel-rich and thus have the capacity to undergo secondary combustion after detonation (afterburn). Even in the free field, this mechanism affects the wave profile and we illustrate the errors associated with simpler models, even for the case of detonation of a sphere of 2,4,6-trinitrotoluene (TNT) in the free field. We identify why these effects should be accounted for in any analysis of later time reflections in more complex targets and discuss how the effects of complex blast are also important in assessing human injury. We also report some engineering models which we have developed based on parameter studies using hydrocode models. The effects of any casing surrounding the explosive charge also influence the loading on a target, both by taking energy out of the blast wave and by accelerating fragments which produce a heterogeneous loading on a structure. Some blast mitigation concepts also make use of the transfer of energy to powder, liquid or foam surrounds. Buried charges can also behave in a similar manner to powder-based blast mitigants and lead to fast moving fingers of solid material which can produce localised loadings on structures. We identify how recent studies in these areas are influencing complex blast calculations.

Explosive formation of coherent particle jets

A coherent jet of particles may be generated by accelerating a conical volume of particles by detonating a layer of explosive lining the outside of the cone. Experiments have been carried out to determine the dependence of the velocity history and coherency of the jet on the particle properties and the ratio of the masses of the particles and explosive. Steel particles form thin, coherent jets, whereas lighter glass particles lead to more diffuse jets. For steel particles, the cone angle had little effect on the coherency of the jet. The efficiency of the conversion of chemical to kinetic energy is explored by comparing the experimental jet velocity with the velocity predicted from a formulation of the Gurney method for a conical geometry. The effect of particle density and cone angle on the jet formation and development was also investigated using a multimaterial hydrocode. The simulations give insight into the extent of the deformation of the particle bed in the early stages of explosive particle dispersal.

High‐Rate Compaction of Aluminium Alloy Foams

The response of aluminium foams to impact can be categorised according to the impact velocity. Tests have been carried out at a range of impact velocities from quasi‐static to velocities approaching the speed of sound in the foam. Various experimental arrangements have been employed including pneumatic launcher tests and plate impact experimants at velocities greater than 1000 m s−1. The quasi‐static compression behaviour was approximately elastic, perfectly‐plastic, locking. For static and dynamic compression at low impact velocities the deformation pattern was through the cumulative multiplication of discrete, non‐contiguous crush bands. Selected impact tests are presented here for which the impact velocity is less than the velocity of sound, but above a certain critical impact velocity so that the plastic compression occurs in a shock‐like manner and the specimens deform by progressive cell crushing. Laboratory X‐ray microtomography has been employed to acquire tomographic datasets of aluminium foams b...

The Shock Response, Simulation and Microstructural Determination of an Inert Simulant

Assessing microstructural details in a polymer matrix composite is important in addressing safety issues in energetic materials. The generation of three‐dimensional microstructure, using a non‐invasive method of high resolution will advance knowledge in a range of fields. An inert composite analogous to plastic bonded explosives (PBXs) has been studied, and both X‐ray microtomography for microstructural investigation in 3‐D and a parallel series of shock experiments (with associated modelling) have been conducted. The experimental aims of this study lay in several areas. Firstly, to adequately define the bulk morphology, secondly, to determine the geometry of defects that might lead to sites for accidental ignition within the material and finally, to demonstrate a direct linkage into the finite element prediction of mechanical response. This work is the first step in providing a coordinated capability to understand accidental ignition within insensitive high explosives (IHEs).

The Burning Rate of Aluminium Particles in Cylinder Tests

Aluminium is a common fuel component in propellants and explosives. There is a wealth of literature on Aluminium combustion in gases at relatively low pressure but limited data on combustion at high pressure (as in explosive detonation products). In this work we have carried out and analysed cylinder tests with Aluminium loaded explosives with a view to assessing the applicability of low pressure burning rates in this regime. The analysis makes use of detailed numerical two phase flow modelling and a range of experiments used to validate other relevant aspects of the physics, such as drag laws. We conclude that the burning rate is significantly faster than that implied by extrapolating laws applicable at lower pressures.

On the Unreacted Hugoniots of Three Plastic Bonded Explosives

There is a continuing interest in determining the detonation characteristics of loaded plastic‐bonded explosives (PBXs). The UK licensing agency for explosives, DOSG, wishes to better understand the response of insensitive high explosives. This has required more detailed investigation of the transit of reaction from the unreacted state to products. The starting condition, before application of a kinetic scheme to describe reaction, is thus the unreacted Hugoniot for the material. In this work three PBXs, manufactured by BAE Land Systems, are investigated and modelled. All contain RDX in differing quantities in an HTPB binder. One of them contains aluminium. Two of the materials have the same weight percentage of filler and binder but differ in the grain size distribution entrained. The experimental Hugoniots are presented, and a composite equation of state is derived using an engineering model and shown to describe the measurements well. Further applications of the technique are described and future uses ...