R. R. Alcon

In-situ magnetic gauging technique used at LANL-method and shock information obtained

Measuring techniques, including magnetic gauges, quartz gauges, manganin gauges, PVDF gauges, velocity interferometry, piezoelectric pins, shorting pins, flash gaps, etc., have been used over the years in shock experiments in condensed phase materials. The use of a particular technique depends on the measured parameter and the sample material properties. This paper concentrates on in-situ magnetic gauging which is particularly useful in high explosive (HE) shock initiation experiments. A short history of this technique will be given but the main discussion will concentrate on the multiple magnetic gauge technique developed at Los Alamos National Lab. (LANL). Vorthman and Wackerle started the technique development in 1980, concentrating on particle velocity and “impulse” gauges so that Lagrange analysis could be used to map the entire reactive field. Over the years, changes to the gauge design, fabrication, and experimental focus have led to the present LANL technique. During the past two years measurement...

Measurements of shock initiation in the tri-amino-tri-nitro-benzene based explosive PBX 9502: Wave forms from embedded gauges and comparison of four different material lots

We have completed a series of ambient temperature (23±2°C) shock initiation experiments on four lots (batches) of the insensitive high explosive PBX 9502. PBX 9502 consists by weight of 95% dry-aminated tri-amino-tri-nitro-benzene (TATB) and 5% of the plastic binder Kel-F 800, a 3∕1 copolymer of chloro-trifluoro-ethylene and vinylidene-fluoride. Two of the four lots were manufactured using the “virgin” process. Both of these lots had few fine TATB particles. One virgin lot was stored the majority of its life (>15yr) as a molding powder and pressed as a 240mm diameter by 130mm thick cylinder. The other virgin lot was stored the majority of its life as a hollow hemispherical pressing. Two lots were manufactured using the “recycle” process and had many fine TATB particles. One recycled lot was stored the majority of its life as a molding powder, while the other was stored as a pressed charge. Shock initiation experiments were performed using precisely characterized planar shocks generated by impacting an exp...

Particle velocity and stress measurements in low density HMX

Magnetic particle velocity gauges and PVDF stress rate gauges have been used to measure the shock response of low density HMX explosive (1.24) g/cm3. In experiments done at LANL, magnetic particle velocity gauges were located on both sides of the explosive. In nearly identical experiments done at SNL, PVDF stress rate gauges were located at the same positions. Using these techniques both particle velocity and stess histories were obtained for a particular experimental condition. Loading and reaction paths were established in the stress‐particle velocity plane for each input condition. This information was used to determine that compacted HMX has an impedance close to that of Kel‐F and also that a global reaction rate of ≊0.13 μs−1 was observed in HMX shocked to about 0.8 GPa. At low input stresses the transmitted wave profiles had long rise times (up to 1 μs) due to the compaction processes.

Detonation wave profiles in HMX based explosives

Detonation wave profiles have been measured in several HMX based plastic bonded explosives including PBX9404, PBX9501, and EDC-37, as well as two HMX powders (coarse and fine) pressed to 65% of crystal density. The powders had 120 and 10 μm average grain sizes, respectively. Planar detonations were produced by impacting the explosive with projectiles launched in a 72-mm bore gas gun. Impactors, impact velocity, and explosive thickness were chosen so that the run distance to detonation was always less than half the explosive thickness. For the high density plastic bonded explosives, particle velocity wave profiles were measured at an explosive/window interface using two VISAR interferometers. PMMA windows with vapor deposited aluminum mirrors were used for all experiments. Wave profiles for the powdered explosives were measured using magnetic particle velocity gauges. Estimates of the reaction zone parameters were obtained from the profiles using Hugoniots of the explosive and window.

Magnetic gauge instrumentation on the LANL gas-driven two-stage gun

The LANL gas-driven two-stage gun was designed and built to do initiation studies on insensitive high explosives as well as equation of state and reaction experiments on other materials. The preferred method of measuring reaction phenomena involves the use of in-situ magnetic particle velocity gauges. In order to accommodate this type of gauging in our two-stage gun, it has a 50-mm-diameter launch tube. We have used magnetic gauging on our 72-mm bore diameter single-stage gun for over 15 years and it has proven a very effective technique for all types of shock wave experiments, including those on high explosives. This technique has now been installed on our gas-driven two-stage gun. We describe the method used, as well as some of the difficulties that arose during the installation. Several magnetic gauge experiments have been completed on plastic materials. Waveforms obtained in some of the experiments will be discussed. Up to 10 in-situ particle velocity measurements can be made in a single experiment. T...

Shock initiation of the tri-amino-tri-nitro-benzene based explosive PBX 9502 cooled to −55 °C

We report a series of shock initiation experiments on PBX 9502 cooled to −55 °C. PBX 9502 consists of 95% dry aminated tri-amino-tri-nitro-benzene (TATB) and 5% poly-chloro-trifluoro-ethylene5 (Kel-F 800) binder. PBX 9502 samples were shock initiated by projectile impact from a two stage gas gun. Buildup to detonation was measured with 10 or more particle velocity gauges embedded at different depths in the sample. Three shock wave trackers measured the position of the shock front with time. Particle velocity vs. time wave-profiles and coordinates for onset of detonation were obtained as a function of the impact stress or pressure. PBX 9502 sample temperatures were monitored using type-E thermocouples, two inside the sample and two on the sample surface. Additional thermocouples were mounted on other parts of the cooling apparatus. Wave profiles from embedded gauges are qualitatively similar to those observed at 23 °C. However, at −55 °C, PBX 9502 is much less sensitive than at 23 °C. For example, at an inpact stress of 15.4 GPa, the distance to detonation at −55 °C is 7.8 mm. At 23 °C, the distance is 4.3 mm.

Low pressure shock initiation of porous HMX for two grain size distributions and two densities

Shock initiation measurements have been made on granular HMX (octotetramethylene tetranitrainine) for two particle size distributions and two densities. Samples were pressed to either 65% or 73% of crystal density from fine ({approx} 10 {mu}m grain size) and coarse (broad distribution of grain sizes peaking at {approx} 150 {mu}m) powders. Planar shocks of 0.2--1 GPa were generated by impacting gas gun driven projectiles on plastic targets containing the HMX. Wave profiles were measured at the input and output of the {approx} 3.9 mm thick HMX layer using electromagnetic particle velocity gauges. The initiation behavior for the two particle size distributions was very different. The coarse HMX began initiating at input pressures as low as 0.5 GPa. Transmitted wave profiles showed relatively slow reaction with most of the buildup occurring at the shock front. In contrast, the fine particle HMX did not begin to initiate at pressures below 0.9 GPa. When the fine powder did react, however, it did so much faster than the coarse HMX. These observations are consistent with commonly held ideas about bum rates being correlated to surface area, and initiation thresholds being correlated with the size and temperature of the hot spots created by shock passage. For each size, the higher density pressings were less sensitive than the lower density pressings.

Preshock desensitization of PBX explosives

Preshocking delays initiation of PBX‐9404 and PBX‐9501, relative to unshocked material. In PBX‐9404 preshock experiment, a first shock of 2.3 GPa was followed 0.65 μs later by a second shock of 5.6 GPa. Both PBX explosives show clear desensitization while the preshock persists. In PBX‐9404, initiation of detonation occurs nearly as anticipated for the material, after coalescence of the preshock and main shock into a single wave. Multiple embedded magnetic gauges were used to measure the shock histories. Our data indicates a slighly longer run to detonation than expected, even though a single wave is initiating the material. A slight stress reduction at coalescence, as required by the shock dynamics, may be responsible for the overrun. A reactive wave is clearly evident while the preshock persists. The long run to detonation indicates that this reactive wave is not driving the initiation. A set of four preshock experiments were performed on PBX−9502, which is unreactive at these pressures to investigate th...

High Pressure Hugoniot and Reaction Rate Measurements in PBX9501

Single‐stage and two‐stage gas gun experiments have been completed to measure the unreacted Hugoniot of PBX9501 high explosive (HE). Two types of experiments were done: 1) PBX9501 was impacted with a higher impedance projectile and the interface particle velocity history was measured using a magnetic gauge glued to the HE front; 2) a PBX9501 disc was mounted in the front of a projectile that impacted a LiF window and velocity interferometers (VISAR) were used to measure the impact interface particle velocity history. Inputs to the PBX9501 ranged from 3 to 15 GPa in these experiments. Particle velocity waveforms show an induction time followed by a particle velocity change (the nature of the change depends on the type of experiment) corresponding to shock‐induced reaction in the PBX9501. The induction part of the waveform provided unreacted Hugoniot information so several new high‐pressure Hugoniot points were generated. These data do not indicate a softening in the unreacted Hugoniot at high pressures; more experiments will be necessary to determine this. By using an estimate for the reaction product EOS, it was possible to estimate the average PBX9501 initial reaction rate for each experiment. The induction time decreases with pressure and the reaction rate increases with pressure.Single‐stage and two‐stage gas gun experiments have been completed to measure the unreacted Hugoniot of PBX9501 high explosive (HE). Two types of experiments were done: 1) PBX9501 was impacted with a higher impedance projectile and the interface particle velocity history was measured using a magnetic gauge glued to the HE front; 2) a PBX9501 disc was mounted in the front of a projectile that impacted a LiF window and velocity interferometers (VISAR) were used to measure the impact interface particle velocity history. Inputs to the PBX9501 ranged from 3 to 15 GPa in these experiments. Particle velocity waveforms show an induction time followed by a particle velocity change (the nature of the change depends on the type of experiment) corresponding to shock‐induced reaction in the PBX9501. The induction part of the waveform provided unreacted Hugoniot information so several new high‐pressure Hugoniot points were generated. These data do not indicate a softening in the unreacted Hugoniot at high pressures; mo...

Magnetic Particle Velocity Measurements of Shocked Teflon

A series of shock compression experiments have been undertaken on Teflon using single‐ and two‐stage gas‐guns. Peak pressures in these experiments range from a few kbars to over 10 kbars, as well as one shot completed at 117 kbar. Multiple particle velocity wave profiles, at a number of Langrangian positions, are obtained for each experiment using in‐situ magnetic gauges. Shock velocity is calculated from arrival times at both the particle velocity gauges and at embedded shock trackers. These direct measurements of particle and shock velocity are compared to previous shock compression results on Teflon. Particular attention is focused in the region below 10 kbar where evidence of a shock induced phase transition has been reported, based upon a cusp in the Hugoniot. The volume change for this transition is only ∼ 2.2 % making its observation difficult. A two‐wave structure on the shock front would be strong evidence of the shock‐induced transition, but has not been observed in these initial low‐pressure ex...