Behaviour of a blast-driven ball bearing embedded in rear detonated cylindrical explosive

Highlights

• Flight characteristics of a blast-driven ball bearing embedded in a rear detonated cylindrical explosive.

• Novel experimental techniques to measure ball bearing velocities and blast impulse characteristics.

• Differences in detonation pressure profile and momentum transfer efficiency caused by different length-to-diameter ratios of the explosive charge.

• Plastic deformation of ball bearing from blast loading.

Abstract

This paper presents insights into the flight characteristics of a ball bearing embedded in a rear detonated cylindrical charge, which represents an idealised piece of shrapnel from an improvised explosive device. A novel experimental technique was developed to quantify the loading from a blast-driven ball bearing. The impulse contributions from the blast pressure and the ball bearing impact were separately identifiable in the experimental data. Computational simulations, validated using experimental data, were used to elucidate additional detail about the momentum transfer and damage in the ball bearings during the blast event. The results show the critical influence of charge mass and aspect ratio on the development of the detonation pressure profile, its interaction with the embedded bearing, and the flight characteristics of the bearing. Length-to-diameter ratios below a critical value were more efficient in transferring momentum to the embedded bearings. These findings provide unique and detailed insights that will prove valuable to blast protection engineers considering the effects of embedded projectiles in improvised explosive devices.

Introduction

Between 2011 and 2016, over 130 000 people [1] were injured or died during incidents involving improvised explosive devices (IED), 81% of whom were innocent civilians. IEDs remain a threat to the safety of the general public. As the name ‘IED’ suggests, this type of explosive weapon is improvised (often home-made) and can take many forms or shapes. In many cases, the explosives are embedded with other household items (such as nails or ball bearings) to enhance their lethality. One of the great difficulties in IED research is the wide variety of possible configurations. Hence, a fundamental understanding of the momentum transfer mechanisms to embedded solid particles in the explosive is needed. This will allow model validation at the fundamental level so that more complex models representing real IEDs can be developed with confidence.

The two simplest explosive charge shapes to model are spheres and cylinders, due to their symmetry. While spherical shaped explosives are simpler, the cylindrical charge shape is a more realistic and practical option for an IED (for example, pipe bombs). Additionally, most ordnance and warheads are basically cylindrical. Considerable work has examined the shock wave and impulse distributions resulting from the detonation of cylindrical charges [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Most work compares the development to that of spherical charge detonations. The effect of charge shape was found to be most significant in the near field, and of decreasing importance at longer stand-off distances. The Hopkinson-Cranz scaled distance, Z is given by the expression R/W^1/3, where R is the stand-off distance from the charge to the target and W is the TNT equivalent charge mass. For Z > 5 m/kg^1/3, the effect of charge shape on peak pressure was insignificant [2]. Similarly, Held [3] investigated the impulse distribution from cylindrical charges by tracking the displacement of metal cylinders. The radial impulse distribution was found to be distinctively different from that of a spherical charge. Rigby et al [4] measured the impulse distribution arising from the detonation of spherical and cylindrical charges. The work showed that cylindrical charges with a 1/3 length/diameter (L/D) aspect ratio produced similar axial impulse distributions on the target plates as spheres with a smaller stand-off distance.

Ismail and Murray [5] showed that a cylindrical charge generates complex blast waves, comprising multiple reflections. Spherical waves are generated in both the axial (end) and radial (side) directions, and these waves interact and constructively interfere [6]. The overlapping waves form a bridge wave reinforced by both the axial and radial waves. The distribution of pressure and impulse is extremely non-uniform, particularly in the near-field, demonstrated by Wu et al [7,8] when filming the fireball generated by an rear-detonated cylinder of Composition B. Xiao et al [9] reported results from a numerical study into the influence of detonation point and aspect ratio on the pressure and impulse characteristics of cylindrical TNT charges. Higher pressures were evident when the cylinders were detonated from one end, but the effect of shape could be neglected when Z > 3.9 m/kg^1/3 for both overpressure and impulse magnitude. In the near-field (at Z = 0.7 m/kg^1/3), the shock wave was highly focused in the axial direction with pressure up to four times that predicted for the same mass spherical charge.

The influence of charge aspect ratio (L/D=0.0262 to L/D=0.477) and charge diameter on axial impulse was investigated using ballistic pendula in Ref. [10], [11], [12]. The axial impulse increased with increasing charge diameter for a given charge mass. The charge aspect (L/D) ratio significantly influences the blast load distribution, with less energy in the axial direction at higher L/D ratios [6], [7], [8], [9], [10], [11], [12]. Kennedy [13] reported that the axial impulse from cylindrical charges varied considerably with the charge aspect ratio, as they are subject to side losses. Thus, the effective volume of a cylindrical charge was limited to the shape of a cone. A critical aspect ratio L/D = $\sqrt{3}/2$ was suggested to be the maximum effective aspect ratio for axial impulse.

Whilst there is a large body of literature on cratering damage due to impact, research into blast-induced projectile impacts are focused on fragmentation [14], [15], [16], [17], such as that in warhead, or blast driven metal [13]. A limited experimental study on the damage caused by incorporating a foreign object in an explosive charge is available in reference [18]. Kang and Chung Kim Yuen [18] did not measure the velocity of the explosively driven projectiles, but analysed the damage distribution in the walls of a surrounding steel cylinder. They used the penetration depth to infer information about the effect of packing pattern.

This paper reports the results of experimental and numerical studies on the influence of cylindrical charge geometry on the velocity characteristics of an explosively driven single ball bearing. The paper provides detailed insights into the flight characteristics of a ball bearing embedded in a rear detonated cylindrical charge, which is of enormous practical relevance to IED research. Charge mass and aspect ratio are both considered. The paper is arranged as follows. The explosive charge design and experimental arrangement are described in Sections 2 and 3 respectively. Section 4 reports the results of the blast tests while Section 5 describes the computational modelling approach. The axial impulse, ball bearing flight velocities and bearing damage are discussed, using the experimental and simulation results. Finally, the performance of the experimental arrangement, the verdict on the influence of charge geometry on the ball bearing velocity is presented in conclusions.