Effect of Temperature,Density and Confinement on Deflagration to Detonation Transition of an HMX‐Based Explosive

In order to obtain the characteristics of the deflagration‐to‐detonation transition (DDT) of PBX‐2 (an HMX‐based explosive) under different conditions, DDT tests were carried out as a function of charge density, temperature, and shell confinement. In these tests, the energetic materials were electrically ignited. The DDT response characteristics for PBX‐2 with 53 % and 99 % of theoretical maximum density (TMD) were evaluated by different shell thickness confinements at ambient temperature and at 85 °C. The test results with different densities, confinements and temperatures exhibited a wide range of reaction violence. Firstly, at both ambient temperature and at 85 °C under 10 and 20 mm shell thickness confinement, PBX‐2 did not undergo fully DDT at 99 % TMD, only a low velocity detonation (LVD) occurred. Secondly, PBX‐2 at 53 % TMD underwent DDT, and significant influence on the minimum run distance to detonation by the shell confinement thickness was observed. Strong confinement is favorable for the transition of DDT but the confinement does not influence reaction degree. Thirdly, the reaction degree of PBX‐2 at 85 °C was remarkably lower than that at ambient temperature. This insensitizing effect of temperature is induced by the melting and flowing of bonders which reduces the porosity and inhibits an important step of DDT, namely, high turbulent combustion.     

Divergent Detonation Wave Processes in PBX Based on RDX

In order to characterize the initial phase of the divergent detonation wave in PBX, a hemispheric explosive sample was initiated by a long cylindrical charge of the same explosive. The tested PBX is composed of 85 wt% of RDX and 15 wt% of binder based on HTPB. This PBX‐RDX presents an effective density of 1.57 g/cm³, and a detonation velocity of 7.90 mm/μs.     

Physical Properties of Conventional Explosives Deduced from Radio Frequency Emissions

Los Alamos National Laboratory collected broadband radio frequency (RF) electric field change measurements from multiple detonations of high explosives (HE). Three types of HE were used: small cylinders of flake TNT, solid TNT, and PBX‐9501. Low frequency signals (<80 MHz) were shot‐to‐shot repeatable and occurred within the first 100 μs at measured amplitudes of about 2 V m⁻¹ at 35 m distance. High frequency signals (>290 MHz) occurred later, were an order of magnitude lower in signal strength, and were not repeatable. There is a positive correlation between the maximum electric field change and the shock velocity of the HE. The amount of free charge produced in the explosion estimated from the first RF pulse is between 10 and 150 μC. This implies a weakly ionized plasma with temperatures between 2600 and 2900 K.     

Initiation Pressure Thresholds from Three Sources

Pressure thresholds are minimum pressures needed to start explosive initiation that ends in detonation. We obtain pressure thresholds from three sources. Run‐to‐detonation times are the poorest source but the fitting of a function gives rough results. Flyer‐induced initiation gives the best results because the initial conditions are the best known. However, very thick flyers are needed to give the lowest, asymptotic pressure thresholds used in modern models and this kind of data is rarely available. Gap test data are in much larger supply but the various test sizes and materials are confusing. We find that explosive pressures are almost the same if the distance in the gap test spacers are in units of donor explosive radius. Calculated half‐width time pulses in the spacers may be used to create a pressure‐time curve similar to that of the flyers. The very‐large Eglin gap tests give asymptotic thresholds comparable to extrapolated flyer results. The three sources are assembled into a much‐expanded set of near‐asymptotic pressure thresholds. These thresholds vary greatly with density: for TATB/LX‐17/PBX 9502, we find values of 4.9 and 8.7 GPa at 1.80 and 1.90 g/cm³, respectively.     

Dead Zones in LX‐17 and PBX 9502

Pin and X‐ray corner turning data have been taken on ambient LX‐17 and PBX 9052, and the results are listed in tables as an aid to future modeling. The results have been modeled at 4 zones/mm with a reactive flow approach that varies the burn rate as a function of pressure. A single rate format is used to simulate failure and detonation in different pressure regimes. A pressure cut‐off must also be reached to initiate the burn. Corner turning and failure are modeled using an intermediate pressure rate region, and detonation occurs at high pressure. The TATB booster is also modeled using reactive flow, and X‐ray tomography is used to partition the ram‐pressed hemisphere into five different density regions. The model reasonably fits the bare corner turning experiment but predicts a smaller dead zone with steel confinement, in contradiction with experiment. The same model also calculates the confined and unconfined cylinder detonation velocities and predicts the failure of the unconfined cylinder at 3.75 mm radius. The PBX 9502 shows a smaller dead zone than LX‐17. An old experiment that showed a large apparent dead zone in Composition B was repeated with X‐ray transmission and no dead zone was seen. This confirms the idea that a variable burn rate is the key to modeling. The model also produces initiation delays, which are shorter than those found in time‐to‐detonation.     

Studies on an Improved Plastic Bonded Explosive (PBX) for Shaped Charges

This paper reports the characteristics and performance evaluation of a pressed plastic bonded explosive (PBX) composition based on hexanitrohexaazaisowurtzitane (HNIW, CL‐20) and polyurethane (PU) in comparison with PU‐coated cyclotetramethylene tetranitramine (HMX). PU‐coated compositions were prepared by slurry method. The processed CL‐20‐based composition exhibited a relatively higher sensitivity compared to that of the HMX composition. The measured velocity of detonation (VOD) of the CL‐20‐based composition was found to be higher than predicted. A theoretical approach was applied to assess the penetration capability of the CL‐20 formulation. Shaped charges of 32 mm caliber were prepared and penetration experiments were carried out at 37 mm standoff distance on mild steel blocks. The results established high penetration capability of CL‐20‐based formulation. An attempt was made to explain the trends obtained.     

Highly Thermal Stable TATB‐based Aluminized Explosives Realizing Optimized Balance between Thermal Stability and Detonation Performance

In this work, a series of TATB‐based aluminized explosives were formulated from 1, 3, 5‐triamino‐2, 4, 6‐trinitrobenzene (TATB), aluminum powders and polymeric binders. The thermal stability, heat of detonation, detonation velocity and pressure of the TATB based aluminized (TATB/Al) explosives were systematically investigated by cook‐off, constant temperature calorimeter, electrometric method and manganin piezo resistance gauge, respectively. The selected PBX‐3 (70 wt% TATB/25 wt% Al/5 wt% fluorine resin) achieved optimized balance between thermal stability and detonation performance, with the thermal runaway temperature around 583 K. The thermal ignition of TATB‐based aluminized explosive occurred at the edge of the cylinder according to the experimental and numerical simulations. Moreover, the critical thermal runaway temperature for PBX‐3 was calculated based on the Semenov's thermal explosion theory and the thermal decomposition kinetic parameters of the explosive, which was consistent with the experimental value.     

Characterization of Granular and Polymer‐Embedded RDX Grades: Floret Tests

In an attempt to further contribute to the characterization of explosive compositions, small scale Floret tests were performed using four RDX grades, differing in product quality. A Floret test provides a measure – by indentation of a copper block – of detonation spreading or the initiability and shock wave divergence and is applied in particular to explosives used in initiation trains. Both as‐received RDX and PBXs (based on the AFX‐757 composition, a hard target penetrator explosive) containing these RDX grades were tested in the Floret test set‐up. It was found that the Floret test method, when applied to granular, as‐received RDX, was not able to discriminate between the overall RDX product qualities on the basis of the resulting volume of the indentation in the copper block. For the Floret test data of the PBX samples, a division into two parts, where one of the RDX lots shows a lower dent volume compared to the other RDX lots tested. Based on the results presented in this paper with granular RDX and a PBX composition and earlier results with a different type of PBX (based on PBXN‐109, an insensitive high explosive used in a wide range of munitions), the Floret test could be developed into a screening test for shock sensitivity and product quality, without the need for complex and large volume casting of specific PBX compositions.     

Changes in Pore Size Distribution upon Thermal Cycling of TATB‐based Explosives Measured by Ultra‐Small Angle X‐Ray Scattering

Hot‐spot models of initiation and detonation show that voids or porosity ranging from nanometer to micrometer in size within highly insensitive energetic materials affect initiability and detonation properties. Thus, the knowledge of the void size distribution, and how it changes with the volume expansion seen with temperature cycling, are important to understanding the properties of the insensitive explosive 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB). In this paper, void size distributions in the 2 nm to 2 μm regime, obtained from small‐angle X‐ray scattering measurements, are presented for LX‐17‐1, PBX‐9502, and ultra‐fine TATB formulations, both as processed and after thermal cycling. Two peaks were observed in the void size distribution: a narrow peak between 7–10 nm and a broad peak between 20 nm and about 1 mm. The first peak was attributed to porosity intrinsic to the TATB crystallites. The larger pores were believed to be intercrystalline, a result of incomplete consolidation during processing and pressing. After thermal cycling, these specimens showed an increase in both the number and size of these larger pores. These results illuminate the nature of the void distributions in these TATB‐based explosives from 2 nm to 2 μm and provide empirical experimental input for computational models of initiation and detonation.     

Low-Sensitivity Explosive Compounds for Low Vulnerability Warheads

Looking for explosives for Low Vulnerability Ammunitions leads to an interest in explosive molecules less sensitive than the usual nitramines (RDX, HMX). If TATB is quite convenient in terms of sensitivity, its performance is too low. The researches described here are related to synthesis and use of NTO (nitrotriazolone), another insensitive molecule. The synthesis by nitration of TO (triazolone) is easy and the two steps from available starting materials have been optimized. A comparison of desensitivation of PBX either by TATB or by NTO have been made. The sensitivity levels were found equivalent while the detonation velocity of the NTO based PBX was slightly higher. Unfortunately in this case, the failure diameter would be larger. The last part relates to an extensive characterization in terms of performance and vulnerability to fast cook off, slow cook off, bullet impact, shock sensitivity and sympathetic detonation of a NTO and HMX based PBX. This PBX, B 2214, was one of the first examples of explosive composition showing no sympathetic detonation, even in 248 mm large diameter.     

FOX-7和RDX基含铝炸药的冲击起爆特性

为研究FOX-7和RDX基含铝炸药的冲击起爆特性,对其进行了冲击波感度试验和冲击起爆试验,结合冲击波在铝隔板中的衰减特性,确定了FOX-7和RDX基含铝炸药的临界隔板值和临界起爆压力,并通过锰铜压阻传感器记录了起爆至稳定爆轰过程压力历程的变化。结果表明,以Φ40mm×50mm的JH-14为主发装药时,FOX-7和RDX基含铝炸药临界隔板值分别为37.51和34.51mm,对应的临界起爆压力为10.91和11.94GPa;起爆压力为11.58GPa时,FOX-7炸药的到爆轰距离为25.49~30.46mm,稳定爆轰后的爆轰压力为27.68GPa,爆轰速度为8 063m/s;起爆压力为14.18GPa时,RDX基含铝炸药的到爆轰距离为17.27~23.53mm,稳定爆轰后的爆轰压力为17.16GPa,爆轰速度为6 261m/s。     

Analytic Model of Reactive Flow

A simple analytic model allows prediction of rate constants and size effect behavior before a hydrocode run, if size effect data exist. It utilizes detonation velocity, average detonation rate, pressure and energy at infinite radius. This allows the derivation of a generalized radius, which becomes larger as the explosive becomes more non‐ideal. The model is applied to near‐ideal PBX 9404, in‐between ANFO and most non‐ideal AN. The power of the pressure declines from 2.3, and 1.5 to 0.8 across this set. The power of the burn fraction, F, is 0.8, 0 and 0, so that an F‐term is important only for the ideal explosives. The size effect shapes change from concave‐down to nearly straight to concave‐up. Failure is associated with ideal explosives when the calculated detonation velocity turns in a double‐valued way. The effect of the power of the pressure may be simulated by including a pressure cut‐off in the detonation rate. The model allows comparison of a wide spectrum of explosives providing that a single detonation rate is feasible.     

Explosive Performance Properties of Erythritol Tetranitrate (ETN)

Erythritol tetranitrate (ETN) is a melt‐castable explosive with impressive performance, similar to the well‐known related nitrate ester, pentaerythritol tetranitrate (PETN). Though ETN has been known since 1849, its properties have not been thoroughly investigated. We report here the first 1/2′′ copper cylinder tests of ETN, compared with PETN. We discuss detonation and wall expansion velocity, along with diameter effect information in unconfined rate stick tests. The detonation velocity of ETN is 99 % that of PETN in the same test setup, showing that performance properties are very similar for the two nitrate esters.     

Detonation Measurements on Damaged LX‐04

We have applied thermal insults on LX‐04 at 185 °C and found that the material expanded significantly, resulting in a bulk density reduction of 12%. Subsequent detonation experiments (three cylinder tests) were conducted on the thermally damaged LX‐04 samples and pristine low‐density LX‐04 samples and the results showed that the fractions reacted were close to 1.0. The thermally damaged LX‐04 and pristine low‐density LX‐04 showed detonation velocities of 7.7–7.8 mm μs⁻¹, significantly lower than that (8.5 mm μs⁻¹) of pristine high‐density LX‐04. Detonation energy densities for the damaged LX‐04, low‐density pristine LX‐04, and hot cylinder shot of LX‐04 were 6.48, 6.62, and 6.58 kJ cm⁻³, respectively, lower than the detonation energy density of 8.11 kJ cm⁻³ for the high density pristine LX‐04. The break‐out curves for the detonation fronts showed that the damaged LX‐04 had longer edge lags than the high density pristine LX‐04, indicating that the damaged explosive is less ideal.     

Double Shock Experiments on PBX Explosive JOB‐9003

One‐dimensional plate impact experiments have been performed to study the double shocks to detonation transition and Hugoniot state in the HMX‐based explosive JOB‐9003. The flyer was a combination of sapphire and Kel‐F, which can pass two different pressure waves into PBX Explosive JOB‐9003 sample after the impact. The particle velocities at interface and different depths in the JOB‐9003 sample were measured with Al‐based electromagnetic particle velocity gauge technique, thus obtaining particle velocity‐time diagram. According to the diagram, the corresponding Hugoniot state can be determined based on the particle velocity and shock wave velocity in the sample. Comparing with sustained pulse experiments, PBX Explosive JOB‐9003 shows desensitization feature due to the pre‐pressed shock wave.     

The Development of a Confined Impact Test for Evaluating the Safety of Polymer‐Bonded Explosives During Warhead Penetration

During warhead penetration, which lasts several milliseconds, warheads undertake moderate pressures reaching hundreds of MPa. Previous methods have been unable to mimic such stimuli to evaluate the safety of ammunitions. Hence, new safety evaluation methods with moderate pressures and long durations to assess the stability of the explosive charges during actual penetrations are needed. Based on existing explosives safety estimation technologies and preliminary understanding of overload environments during penetration, a confined impact model was developed. The peak stimuli pressure is more than 0.3 GPa and the duration of the pressure is between 1 to 3 milliseconds. Various scales of a polymer‐bonded explosive (PBX‐9) were subjected to this confined impact test to study the response characteristics. The ignition mechanism of PBX‐9 was obtained by synthesizing results from X‐ray photoelectron spectroscopy (XPS), computed tomography (CT) scanning, differential scanning calorimetry (DSC) and thermalgravimetric analysis (TG). It was found that cracks formed on the surface during impact and the initial ignition of PBX‐9 is mainly due to the decomposition of Ammonium perchlorate (AP) and hydroxyl‐terminated polybutadiene (HTPB) on the interface at low temperature. Our study indicates that the designers of the PBX formulation should try to control the early decomposition of AP and HTPB in order to let the PBX survive the penetration.     

LX‐17 Corner‐Turning

We have performed a series of highly‐instrumented experiments examining corner‐turning of detonation. A TATB booster is inset 15 mm into LX‐17 (92.5% TATB, 7.5% kel‐F) so that the detonation must turn a right angle around an air well. An optical pin located at the edge of the TATB gives the start time of the corner‐turn. The breakout time on the side and back edges is measured with streak cameras. Three high‐resolution X‐ray images were taken on each experiment to examine the details of the detonation. We have concluded that the detonation cannot turn the corner and subsequently fails, but the shock wave continues to propagate in the unreacted explosive, leaving behind a dead zone. The detonation front farther out from the corner slowly turns and eventually reaches the air well edge 180° from its original direction. The dead zone is stable and persists 7.7 μs after the corner‐turn, although it has drifted into the original air well area. Our regular reactive flow computer models sometimes show temporary failure but they recover quickly and are unable to model the dead zones. We present a failure model that cuts off the reaction rate below certain detonation velocities and reproduces the qualitative features of the corner‐turning failure.     

The Role of Aluminum in the Detonation and Post‐Detonation Expansion of Selected Cast HMX‐Based Explosives

In order to improve understanding of how aluminum contributes in non‐ideal explosive mixtures, cast‐cured formulations have been analyzed in a series of cylinder tests and plate‐pushing experiments. This study describes the contribution of 15 % aluminum (median size of 3.2 μm) vs. lithium fluoride (an inert substitute for aluminum; <5 μm) in cast‐cured HMX formulations in different temporal regimes. Small cylinder tests were performed to analyze the detonation and wall velocities (1–20 μs) for these formulations. Near‐field blast effects of 58 mm diameter spherical charges were measured at 152 mm and 254 mm using steel plate acceleration. Pressure measurements at 1.52 m gave information about free‐field pressure at several milliseconds. While the observed detonation velocities for all formulations were within uncertainty, significantly higher cylinder wall velocities, plate velocities, and pressures were observed for the aluminum formulations at ≥2 μs. Additionally, hydrocode calculations were performed to determine how non‐ideal behavior affected the plate test results. Collectively, this work gives a clearer picture of how aluminum contributes to detonation on timescales from 1 μs to about 2 ms, and how the post‐detonation energy release contributes to wall velocities and blast effects. The experiments indicate that significant aluminum reactions occur after the CJ plane, and continue to contribute to expansion at late times.     

Effect of Scale and Confinement on Gap Tests for Liquid Explosives

The factors influencing initiation of detonation in gap tests for liquid explosives are investigated experimentally. A calibrated donor charge (nitromethane) and PMMA attenuator disk arrangement are used to transmit shocks of known strength (2–10 GPa) into a test explosive of nitromethane sensitized with 5% diethylenetriamine. The test explosive is contained in capsules of different wall materials (PVC, Teflon, aluminum), and the dimensions of the charges vary from 25 mm to 100 mm in diameter. For the small‐scale charges, the presence of the confining wall of the test capsule is seen to have a pronounced effect on the detonation initiation. Certain wall materials (PVC, Teflon) exhibit a multi‐valued critical gap thickness, meaning that a weaker shock may result in initiation while a stronger shock does not. The effect of the wall materials could not be correlated with their acoustic or shock impedance, and the only way to eliminate these effects was to make the diameter of the test charge larger than the donor charge. When the size of the donor charge was increased, the critical pressure required for initiation decreased. These results could be correlated to “ideal” shock initiation experiments that use flyer plates as shock sources assuming that lateral rarefactions quench detonation initiation if they reach the central axis of the charge before the onset of detonation is complete.     

Energetic Residues from the Detonation of IMX‐104 Insensitive Munitions

The development of insensitive munitions by NATO countries is an ongoing effort. Less‐sensitive ingredients in both explosives and propellants will ensure the protection of deployed troops against an unwanted reaction to an external stimulus on the munitions stockpile. In the US Army, current efforts are directed towards the development of melt cast insensitive explosive formulations. Various formulations, mainly based on DNAN and NTO, have been developed and are now being fielded. Our research goal is to measure the deposition rate of energetics compounds from various insensitive munitions detonation scenarios. Our hypothesis is that the relative insensitiveness of these formulations leads to slightly higher deposition rates than conventional explosive formulations. This paper describes detonation residues research on mortar rounds containing IMX‐104 explosive. Analyses indicate that high‐order detonation residues are slightly greater for this formulation than for conventional munitions. However, blow‐in‐place detonations (BIPs) resulted in much higher residues deposition, indicating that a larger donor charge is required for efficient detonation. The highly soluble compound NTO was particularly problematic, with BIP deposition approaching 95 % of the original load. Toxicological studies of NTO are not finalized, leaving considerable uncertainty regarding the feasibility of approving these rounds for distribution.