Blast Quantification Using Hopkinson Pressure Bars

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.