J.J. Reay

Observations from Preliminary Experiments on Spatial and Temporal Pressure Measurements from Near-Field Free Air Explosions

It is self-evident that a crucial step in analysing the performance of protective structures is to be able to accurately quantify the blast load arising from a high explosive detonation. For structures located near to the source of a high explosive detonation, the resulting pressure is extremely high in magnitude and highly non-uniform over the face of the target. There exists very little direct measurement of blast parameters in the near-field, mainly attributed to the lack of instrumentation sufficiently robust to survive extreme loading events yet sensitive enough to capture salient features of the blast. Instead literature guidance is informed largely by early numerical analyses and parametric studies. Furthermore, the lack of an accurate, reliable data set has prevented subsequent numerical analyses from being validated against experimental trials. This paper presents an experimental methodology that has been developed in part to enable such experimental data to be gathered. The experimental apparatus comprises an array of Hopkinson pressure bars, fitted through holes in a target, with the loaded faces of the bars flush with the target face. Thus, the bars are exposed to the normally or obliquely reflected shocks from the impingement of the blast wave with the target. Pressure-time recordings are presented along with associated Arbitary-Langrangian-Eulerian modelling using the LS-DYNA explicit numerical code. Experimental results are corrected for the effects of dispersion of the propagating waves in the pressure bars, enabling accurate characterisation of the peak pressures and impulses from these loadings. The combined results are used to make comments on the mechanism of the pressure load for very near-field blast events.

Experimental studies of the effect of rapid afterburn on shock development of near-field explosions

Many conventional high explosives do not contain sufficient internal oxygen to fully combust the gaseous products which result from detonation of the explosive material. Because of this, under-oxygenated explosives continue to burn after detonation. This process, called afterburn, is known to influence the late-time pressure and energy released by the explosive, which has particular significance for confined explosives. Recent experimental work at the University of Sheffield, along with a small number of previous studies, has shown that some afterburn occurs at timescales commensurate with the development of the shock wave. This article presents the results from a series of tests measuring the reflected pressure acting on a rigid target following the detonation of small explosive charges. High-speed video is used to capture the emerging structure of the detonation products and air shock, while the spatial and temporal distributions of the reflected pressure are recorded using an array of 17 Hopkinson pressure bars set flush with an effectively rigid target. Tests are conducted in inert atmospheres and oxygen-rich atmospheres in order to assess the contribution of rapid afterburn on the development of the shock front and interaction with a rigid target situated close to the explosive charge. The results show that early-stage afterburn has a significant influence on the reflected shock parameters in the near-field.

Displacement timer pins: An experimental method for measuring the dynamic deformation of explosively loaded plates

Abstract The measurement of dynamic deformation of an explosively loaded plate is an extremely onerous task. Existing techniques such as digital image correlation are expensive and the equipment may be damaged by explosively driven debris/ejecta, particularly if it is necessary to locate such equipment close to loaded elements which are likely to fail. A new, inexpensive and robust measurement technique for use in full-scale blast testing is presented, which involves the placement of displacement timer pins (DTPs) at pre-defined distances from the rear surface of the centre of a plate. A strain gauge on the perimeter of each pin records the time at which the plate comes into contact with the end of each DTP and hence has deformed to that value of displacement, giving a direct measure of the time-varying deformation at a discrete point on the plate. An experimental proof-of-concept was conducted and the results are compared with numerical displacements determined using LS-DYNA. The numerical and experimental results were in very good agreement, which suggests that the proposed experimental method offers a valuable means for determining the full-scale response of structures subjected to blast loads in aggressive environments. Further improvements to the experimental procedure are outlined, along with applications where the DTPs are particularly suited.

Blast quantification using Hopkinson pressure bars

Near-field blast load measurement presents an issue to many sensor types as they must endure very aggressive environments and be able to measure pressures up to many hundreds of megapascals. In this respect the simplicity of the Hopkinson pressure bar has a major advantage in that while the measurement end of the Hopkinson bar can endure and be exposed to harsh conditions, the strain gauge mounted to the bar can be affixed some distance away. This allows protective housings to be utilized which protect the strain gauge but do not interfere with the measurement acquisition. The use of an array of pressure bars allows the pressure-time histories at discrete known points to be measured. This article also describes the interpolation routine used to derive pressure-time histories at un-instrumented locations on the plane of interest. Currently the technique has been used to measure loading from high explosives in free air and buried shallowly in various soils.

Numerical Modelling of Honeycomb Energy Dissipating Materials

Cardboard honeycomb energy dissipating material (EDM) is used extensively forprotection of cargo that is air dropped. Whilst air dropped containers are deployed withparachutes, a significant amount of energy still requires dissipating on impact with the ground.The specification of cardboard EDM in dynamic applications has tended to be ad-hoc withno thorough understanding of the energy absorbing characteristics of the material. This paperseeks to address this gap in knowledge.