The overall goal of this procedure is to evaluate the detonation velocity and detonation pressure for a novel explosive formulation, using piezoelectric pins, and a photonic doppler velocimetry window. This is accomplished by first preparing the piezoelectric pins and a photonic doppler velocimetry window for the test procedure. The second step is to measure and load the explosive samples into the machined acrylic tubes.
Next, the detonator is affixed. The experiment is placed in the enclosed test chamber, and the test charge is detonated. The final step is collecting and analyzing the time of arrival and pressure data.
Ultimately, combined detonation velocity and detonation pressure tests were used to characterize and optimize explosive formulations. As explosive formulators, we are not afforded the luxury of multiple runs and nondestructive tests. Therefore, accurate and repeatable measurements of detonation velocity and detonation pressure is extremely important.
Here, explosive engineers Erik Wrobel and Rodger Cornell will be demonstrating how to measure detonation pressure and detonation velocity. The main advantage with photonic doppler velocimetry is the extremely accurate measurement of detonation pressure. First, prepare bundles of six BNC cables to be used with the piezoelectric pins.
The cable lengths should be adjusted to the test site's geometry. Now, using a high accuracy caliper, measure the test sample and booster pellet's diameter and length. The test fixtures can be machined for any size pellets.
Also, measure the masses of the pellets. Now, load the explosive pellets one by one into the plastic fixture. Record the number of pellets loaded, and their locations within the fixture.
Next, load the booster pellet into the tube from the top of the fixture. On top of the booster pellet, place an acrylic detonator holder. Now, insert the piezoelectric pins through the holes and down the length of the plastic fixture.
Secure the pins using a five minute epoxy. After the epoxy has cured, position the acrylic tube containing the explosive pellets on top of the steel witness plate. Secure the test fixture to the steel plate with either a weight or some tape.
There should not be any air-gap between the last explosive pellet and the steel plate. Next, epoxy the edge of the test fixture to secure it to the plate. After the epoxy has fully cured, place the detonator in the holder and secure with tape.
Transport the test fixture to the test chamber. There, connect the piezoelectric pin cables to the pins in the chamber and to a BNC multiplexer box in the camera room. Then, connect the Mux box to an oscilloscope.
A one gigahertz bandwidth is more than sufficient. Connect the firing line to the detonator. Following the local standard operating procedures, lock down the test site.
Next, connect the trigger out of the high voltage fire set to one channel on the oscilloscope. Check that the trigger threshold is three volts. Then, connect the summing box to a second channel on the oscilloscope.
On the oscilloscope, set both channels to five volts per division, and the timebase to five microseconds per division. Set the delay to negative 20 microseconds. Now, with the high energy fire set, execute the detonation.
Analysis of the data is covered in the text protocol. Begin by machining a PMMA disc. Cut the disc from a quarter inch optically clear sheet of cast PMMA to ensure the faces of the disc are free of imperfections.
Match the disc diameter to the diameter of the explosive. Once machined, inspect the faces of the disc for physical defects. Clean and polish small defects in the surface to restore its optical clarity.
If there are large defects, such as deep scratches or cavities, discard the disc and start over. Now, tape very thin aluminum foil to the disc, diffuse side down, using optically clear tape. Smooth out the foil against the disc to remove any ripples or bubbles.
Just like the previous procedure, measure the explosive sample pellet diameters, lengths, and masses. Form the explosive charge from these sample pellets. At each explosive interface, apply a silicone-based elastomer to minimize the formation of air-gaps.
Use an elastomer tested to be compatible, which does not facilitate a chemical reaction. Now, mount the time of arrival piezoelectric pins into an acrylic holder. Then, attach the loaded holder near the bottom of the charge to capture the steady-state detonation velocity.
Before proceeding, ensure the holder has the pins travelling parallel to the axis of the explosive billet. Continue the photo doppler velocimetry test by attaching an acrylic PDV probe holder to the free surface of the PMMA window. Then, insert the PDV probe into the holder, and align it to the aluminum foil using a one milliwatt back reflection meter.
So even though PMMA transmits about 90%of the laser light that we're working with, the free surface of the disc that we're working with are very specular. So if the probe that we're working with aligns perfectly to that free surface, we're actually gonna get a very strong back reflection level. And if we pick up that back reflection level and we mistake it for the aluminum that we're looking at, it can give us a false alignment positive, and we can end up missing a lot of the data that we're trying to capture.
Once aligned for optimal back reflection, epoxy the PDV probe in place. Then, attach a booster and EBW detonator to the charge to complete the assembly. Now, place the test item into the chamber and wire the TOA pins and PDV fiber.
Then, connect the firing line to the RP-80 detonator. Now, secure the testing site, and conduct an area lockdown operation. Ensure all intralocks are activated and all personnel are accounted for.
The final preparation is to the check the PDV signal, and reference power levels to ensure the desired beat frequency will be captured. Now, detonate the item using the high energy fire set. Save the oscilloscope traces for both the PDV and the TOA data.
Using the described protocol, PAX-30 was compared to a traditional PBXN-5 high explosive. Upon detonation, the dent plates from traditional detonation velocity dent shots were analyzed. This graph shows the detonation velocity of PAX-30 compared to a traditional high energy explosive, PBXN-5.
The PAX-30, even with 20%less high explosive in the formulation, possesses nearly the same detonation velocity, pressure, and total energy as PBXN-5. This arises due to the uniquely designed aluminum additive. The photonic doppler velocimetry trace of the particle's velocity from the bottom of the explosive shows that it rapidly accelerated to approximately three kilometers per second.
The detonation, or Chapman-Jouget, pressure was calculated from modeling the product's gas, Hugoniot, with Cooper's approximation, and then extrapolating the CJ point, once the aluminum explosive, Hugoniot, was matched. The calculations slightly underestimated the pressure, as evidenced by the results. Work is ongoing to develop new equations to fit the early particle acceleration.
So after watching this video, you should have a good understanding of how to measure detonation velocity and detonation pressure for new explosive formulation.
