Name: blast quantification using hopkinson pressure bars
Uploaded: 2016-07-05T14:00:00
Description: Watch this Scientific Journal Video about Blast Quantification Using Hopkinson Pressure Bars at JoVE.com

The authors wish to thank the Defence Science and Technology Laboratory for funding the published work.

1. Rigid Reaction Frame
<ol>
	<li>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/W1/3&#160;&#160;&#160;&#160;&#160;(1)</li>
	<li>Calculate approximate maximum impulse this arrangement will generate via numerical modelling (see Appendix A) or specific tools such as ConWep3. 
	Note: The use of ConWep...

<ol>
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M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+response+of+small+scale+rigid+targets+to+shallow+buried+explosive+detonations.">The response of small scale rigid targets to shallow buried explosive detonations.</a> Int. J. Impact Eng. 38 (11), 882-891 (2011).</li><li>Ehrgott, J. Q., Rhett, R. G., Akers, S. A., Rickman, D. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Design+and+fabrication+of+an+impulse+measurement+device+to+quantify+the+blast+environment+from+a+near-surface+detonation+in+soil.">Design and fabrication of an impulse measurement device to quantify the blast environment from a near-surface detonation in soil.</a> Experimental Techniques. 35 (3), 51-62 (2011).</li><li>Pope, D. 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D., Warren, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Validation+of+semi-empirical+blast+pressure+predictions+for+far+field+explosions+-+is+there+inherent+variability+in+blast+wave+parameters?.">Validation of semi-empirical blast pressure predictions for far field explosions - is there inherent variability in blast wave parameters?.</a> , (2014).</li><li>Rigby, S. E., Tyas, A., Bennett, T., Clarke, S. D., Fay, S. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+negative+phase+of+the+blast+load.">The negative phase of the blast load.</a> Int. J. of Protective Structures. 5 (1), 1-20 (2014).</li><li>Rigby, S. E., Fay, S. D., Tyas, A., Warren, J. A., Clarke, S. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Angle+of+incidence+effects+on+far-field+positive+and+negative+phase+blast+parameters.">Angle of incidence effects on far-field positive and negative phase blast parameters.</a> Int. J. of Protective Structures. 6 (1), 23-42 (2015).</li><li>Tyas, A., Warren, J., Bennett, T., Fay, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Prediction+of+clearing+effects+in+far-field+blast+loading+of+finite+targets.">Prediction of clearing effects in far-field blast loading of finite targets.</a> Shock Waves. 21 (2), 111-119 (2011).</li><li>Tyas, A., Watson, A. 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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...

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 for validation.
The term near-field is used to describe blasts with scaled distances, Z, less than ~1 m/kg1/3, where Z = R/W1/3, R is the distance from the center of the explosive, and W is the charge mass expressed as an equivalent mass of TNT. In this regime the loading is typically characterized by extremely high magnitude, highly spatial and temporally non-uniform loads. Robust instrumentation is hence required to measure the extreme pressures associated with near-field loading. At scaled distances Z &#60; 0.4 m/kg1/3, direct measurements of the blast parameters are either non-existent or very few1 and the semi-empirical predictive data for this range is based almost entirely on parametric studies. This involves using the semi-empirical predictions given by Kingery and Bulmash2, which is outside of the author&#39;s intended scope. Whilst tools based on these predictions3,4 allow for excellent first-order estimations of loading they do not fully capture the mechanics of near-field events, which are the focus of the current research.
Near-field blast measurements have in recent times focused on quantifying the output from buried charges. The methodologies employed vary from assessing the deformation caused to a structural target5-7 to direct global impulse measurement8-13. These methods provide valuable information for the validation of protective system designs but are not capable of fully investigating the mechanics of load transfer. Testing can be done at both laboratory scales (1/10 full scale), or at near to full scale (&#62; 1/4), with pragmatic reasons such as controlling burial depth or ensuring no inherent shape of the shock front is generated by the use of detonators rather than bare charges14. With buried charges the soil conditions need to be highly controlled to guarantee the repeatability of the testing15.
Independent of the whether the charge is placed in free air or is buried, the most fundamental issue in measuring the resulting blast is ensuring the validity of measurements made by the instrumentation deployed. In the designed test apparatus16 a fixed &#39;rigid&#39; target plate is used to shield the Hopkinson pressure bars17 (HPBs) whilst at the same time ensuring that the ends of the bars can only record the fully reflected pressures. The authors have previously shown that measurement of reflected pressure from a rigid target is more accurate and repeatable than incident, or &#39;free-field&#39; measurements18-20. The geometry of this plate is such that any pressure relief generated by clearing or flow around the target edge21 would be negligible. This new test apparatus has been constructed at 1/4 scale. At this scale tight control over the burial conditions and the explosives can be ensured, with the full scale charge size of 5 kg scaled down to 78 g, at a burial depth of 25 mm.

<table border="0" cellpadding="0" cellspacing="0" width="573">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Load Cell</td>
			<td style="width:71px;">RDP</td>
			<td style="width:119px;">RSL0960</td>
			<td style="width:167px;">This is only indicative, the exact load cell should be able to resolve the required loading</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Steel target plate / HPBs</td>
			<td style="width:71px;">Garratts&#160;</td>
			<td style="width:119px;">N/A</td>
			<td>Fabricated to order</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Strain gauge</td>
			<td style="width:71px;">Kyowa</td>
			<td>KSP-2-120-E4</td>
			<td>To use with steel HPBs</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Cyanoacrylate</td>
			<td style="width:71px;">Kyowa</td>
			<td>CC-33-A</td>
			<td>Check with manufacturer depending on mar material to be used</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Digital Oscilloscope</td>
			<td style="width:71px;">TiePie</td>
			<td>HS4 16-bit Handyscopes&#160;</td>
			<td>6 used in parallel in current testing</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Leighton Buzzard sand</td>
			<td style="width:71px;">Garside sands</td>
			<td style="width:119px;">Garside 14/25</td>
			<td>Uniform silica sand&#160;</td>
		</tr>
	</tbody>
</table>

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.

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 &#39;acceptor&#39; 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 protec...

Watch this Scientific Journal Video about Blast Quantification Using Hopkinson Pressure Bars at JoVE.com

Blast Quantification Using Hopkinson Pressure Bars

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.

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 ...

The overall goal of this experiment is to accurately measure the spatial and temporal distribution of pressure in the extremely aggressive environment generated close to an explosive charge. This method can help answer the key questions in the field of blast protection engineering such as the exact form of the imparted load and how factors such as explosive type and shape influence the imparted load. The main advantage of this technique is that it allows us to record pressures that are beyond the limits of traditional measuring approaches.Though this method can provide insights into free air explosions, it can also be applied to other events, such as buried or underwater charges. We first trialed the idea for this method using a single Hopkinson pressure bar, and soon realized a large array is needed to accurately capture the data. To begin, calculate the approximate maximum impulse the test frame arrangement will generate using software analysis, such as with ConWep.For buried charges, this process is less straightforward, cause it requires more advanced numerical techniques to model the interaction between the soil, explosives, and the target plate. Details on the production of the test frame and the load cells are each provided in the text protocol. Choose the position on the Hopkinson pressure bars that the strain gauge will be positioned, being as close as possible to the loaded face to minimize dispersion.In this setup, the thickness of the target plate and the maneuverability required to fit the bars results in the gauges being installed 250 millimeters from the loaded face. The calculated bar radius required to capture the event is in this case five millimeters. Use the tightest spatial resolution for the bars that does not compromise structural integrity.In this case, the distance is 25 millimeters. Further details are provided in the text protocol. To begin, using cyanoacrylate, attach semiconductor strain gauge to the Hopkinson pressure bars, then to the load cells.Fit the target plate to the rigid reaction frame using the load cells if needed. Make certain that all of the cabling is well grounded for improved signal quality. The wiring must also be long enough to connect to an oscilloscope outside the blast area.Any shielded wire should carry enough signal. Now, hang Hopkinson pressure bars from the bar assembly receiver. Pass the loaded end through the correct hole in the target plate, and hang the Hopkinson pressure bars freely from the nut screwed onto their distal ends.Using a level, adjust the nuts to position the bars vertically and to make their faces level with the target plate. Now, use trial and error to set the trim on the variable resistor in the conditioning circuit to keep the voltage within the limits of the oscilloscope. Zero the out of balance reading on each channel as reported by the amplifier boxes.Next, connect the amplified gauge output to a suitable digital oscilloscope. Configure the oscilloscope to a 1.56 megahertz sampling frequency with a 28.7 millisecond recording duration and set the pre-trigger duration to 3.3 milliseconds. 22 total gauges should be connected, 17 from Hopkinson pressure bars, four from load cells, and the one break wire.Record the voltage and time from each gauge. Set the recording to trigger when the voltage in the break wire exceeds an out-window value, such as plus or minus 100 millivolts. In the case of a free air charge test, use a thin strip of wood to suspend the charge below the target plate at the correct stand-off, in this case 200 millimeters.Position the charge co-axially with the measurement array to ensure valid readings. The critical element in the buried charge test is in the preparation of the soil bed and the burial process. Precision is required to ensure repeatable results are attained.Next, close the range. Deploy sentries to ensure the range is clear during the firing. Now, just before firing a free air charge, attach the break wire to the detonator and insert an electrical detonator halfway into the charge from the base.Now, move to the firing point and confirm that the instrumentation is operative. Then, supply power to the break wire. Now, make certain to check with the sentries that it is safe to proceed with the firing.Then, initiate the explosives. After the detonation, make the test area safe and download and back up the data. Whilst a protocol is being written to describe the steps required in this stage, a developed Matlab script is also being made available to allow the data processing to be done quickly using the exact methodology.Import the data from the raw data files into Matlab by double clicking on the file name and then clicking Finish in the Import Wizard. Next, open the interpolation Matlab script. In the meshing section of the code, define a regular grid over which the interpolation will run by changing the mesh.Use the same resolution in any future numerical modeling. This crucial step transforms the discreet data into a 2D map. The script will time shift all Hopkinson pressure bar pressure traces.The time shift is required to allow the interpolation routine to successfully locate the shock front at any given time. Now, align the data from each radial array so all the maximum pressures are synced. Next, calculate the radius, r, and angle, beta, for a given point of interest on the grid.Apply the 1D interpolation to the two Hopkinson pressure bar arrays closest to the point of interest of the current radius. For example, at 45 degrees, the interpolation would use the X, X and Y, Y arrays. Now, interpolate the linearity between the two pressures based on the angle.For example, at 45 degrees, use 50%X, X and 50%Y, Y.Then, time shift the pressure time history for each location based on cubic interpolation of the shock arrival time. Ultimately, the result is a fully interpolated pressure time history. An effectively rigid reaction frame capable of resisting several hundred Newton-seconds with minimal deflection was conceived using a 100 millimeter mild steel target plate.This frame withstood tests up to 500 Newton-seconds. A single test was made with 17 Hopkinson pressure bars configured in a 2D array utilizing 3.25 meter long bars with five millimeter radii. Spacing was set to 25 millimeters.For this test, the strain gauge was attached 0.25 meters from the loaded face. A charge buried in saturated soil was detonated. Data from each of the four radial arrays with a central Hopkinson pressure bar common to all the plots shows the very clear shock front, with the pressure decaying slowly with radial distance.The recorded pressure time histories were then run through the 2D interpolation routine. The interpolated pressure acting on the target plate shows a 20 millisecond delay in the arrival of the shock front. The shock front is the time taken for the shock wave to cover the distance between the charge and the target plate.The asymmetric nature of the loading is especially clear at 0.22 milliseconds. By 0.3 milliseconds after detonation, the shock front was almost symmetric along all axes. Once the apparatus is commissioned, up to six free air tests per day can be conducted.This number is greatly reduced with a test using buried charges due to the added complexity of preparing the soil. This is the first time that such high resolution measurements have been possible. As a result, we're now able to measure the difference in the form of the loading caused by variations in test geometry.The numerical routine developed offers a very powerful way to visualize the loading and then to apply this loading directly in numerical models to act as a first step in modeling the response of structures to explosive detonations. The data produced from the current test has provided unique validation data to enhance the next generation of numerical models, improving our understanding of the problem and our ability to protect ourselves against explosive blasts.

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