Blast Quantification Using Hopkinson Pressure Bars

Summary

This protocol details the use of Hopkinson pressure bars to measure reflected blast loading from near-field explosive events. It is capable of interpolating a pressure-time history at any point on a reflective boundary and as such can be used to fully characterize the spatial and temporal variations in loading produced.

Abstract

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.

Introduction

Characterizing the output of explosive charges has many benefits, both military (defending against buried improvised explosive devices in current conflict zones) and civilian (designing structural components). In recent times this topic has received considerable attention. Much of the knowledge gathered has aimed at the quantification of the output from charges to enable the design of more effective protective structures. The main issue here is that if the measurements made are not of high fidelity then the mechanisms of load transfer in these explosive events remain unclear. This in turn leads to problems validating numerical models which rely on these measurements f....

Protocol

1. Rigid Reaction Frame

1. Determine scaled distance at which testing will take place using Equation 1, where R is the distance from the center of the explosive, and W is the charge mass expressed as an equivalent mass of TNT.
   Z = R/W^1/3     (1)
2. Calculate approximate maximum impulse this arrangement will generate via numerical modelling (see Appendix A) or specific tools such as ConWep³.
   Note: The use of ConWep³ is only valid for free air blast, if an estimation of the pressures generated from buried charges is required the more advanced numerical modelling is required.
3. ....

Results

An effectively rigid reaction frame needs to be provided. In the current testing a total imparted impulse of several hundred Newton-seconds needs to be resisted with minimal deflection. An illustration of the rigid reaction frame used is given in Figure 1. In each frame a 50 mm steel 'acceptor' plate has been cast into the base of the cross beams. Whilst not explicitly required, this allows for easy fixing of the load cells / target plate and provides added protection to .......

Discussion

Using the protocol outlined above the authors have shown that it is possible to get high fidelity measurements of the highly varying loading from an explosive charge, using an array of Hopkinson pressure bars. Using the interpolation routine outlined the discrete pressure-time histories can be transformed into a continuous shock front which is usable directly as the loading function in numerical modelling or as validation data for the output of such models.

When using buried charges the method.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Load Cell	RDP	RSL0960	This is only indicative, the exact load cell should be able to resolve the required loading
Steel target plate / HPBs	Garratts		Fabricated to order
Strain gauge	Kyowa	KSP-2-120-E4	To use with steel HPBs
Cyanoacrylate	Kyowa	CC-33-A	Check with manufacturer depending on mar material to be used
Digital Oscilloscope	TiePie	HS4 16-bit Handyscopes	6 used in parallel in current testing
Leighton Buzzard sand	Garside sands	Garside 14/25	Uniform silica sand