Racci DeLuca

Time–Temperature Analysis, Tension and Compression in PBXs

Time–temperature principles are applied to a wide range of mechanical properties data for two highly-filled polymer composites. Compressive and tensile properties were measured for plastic-bonded explosives (PBX) 9501 and 9502, spanning a wide range of strain rates and temperatures. Stress–strain parameters were determined and carried forward in a time–temperature analysis. For PBX 9501, best-fit constants were −8.0 and −10.0 K per decade of strain rate, for compression and tension, respectively. For PBX 9502, the best-fit constant for all data was −8.0 K/rate decade. Dynamic mechanical analysis (DMA) data for both formulations adhere to the same or similar time–temperature shift factors.

Time-temperature superposition applied to PBX mechanical properties

The use of plastic-bonded explosives (PBXs) in weapon applications requires that they possess and maintain a level of structural/mechanical integrity. Uniaxial tension and compression experiments are typically used to characterize the mechanical response of materials over a wide range of temperatures and strain rates, providing the basis for predictive modeling in more complex geometries. After many years of data collection on a variety of PBX formulations, we have here applied the principles of time-temperature superposition to a mechanical properties database which includes PBX 9501, PBX 9502, PBXN-110, PBXN-9, and HPP (propellant). Consistencies are demonstrated between the results of quasi-static tension and compression, dynamic Split-Hopkinson Pressure Bar (SHPB) compression, and cantilever Dynamic Mechanical Analysis (DMA). Timetemperature relationships of maximum stress and corresponding strain values are analyzed, in addition to the more conventional analysis of modulus. The extensive analysis sho...

Taylor impact tests and simulations of plastic bonded explosives

Taylor impact tests were conducted on plastic bonded explosives PBX 9501 and PBXN-9 for impact velocities between 80 and 214 m/s. High-speed photography was used to image the impact event at a rate of one frame for every 25 μs. For early times, PBXN-9 showed large-deformation mushrooming of the explosive cylinders, followed by fragmentation by an amount proportional to the impact speed, was observed at all velocities. PBX 9501 appeared to be more brittle than PBXN-9, the latter demonstrated a more viscoelastic response. The post-shot fragments were collected and particle size distributions were obtained. The constitutive model ViscoSCRAM was then used to model the Taylor experiments using the finite element code ABAQUS. Prior to the Taylor simulations, ViscoSCRAM was parameterized for the two explosives using uniaxial stress-strain data. Simulating Taylor impact tests validates the model in situations undergoing extreme damage and fragmentation.

Taylor impact tests on PBX composites: imaging and analysis

A series of Taylor impact tests were performed on three plastic bonded explosive (PBX) formulations: PBX 9501, PBXN-9 and HPP (propellant). The first two formulations are HMX-based, and all three have been characterized quasi-statically in tension and compression. The Taylor impact tests use a 500 psi gas gun to launch PBX projectiles (approximately 30 grams, 16 mm diameter, 76 mm long), velocities as high as 215 m/s, at a steel anvil. Tests were performed remotely and no sign of ignition/reaction have been observed to date. Highspeed imaging was used to capture the impact of the specimen onto anvil surface. Side-view contour images have been analyzed using dynamic stress equations from the literature, and additionally, front-view images have been used to estimate a tensile strain failure criterion for initial specimen fracture. Post-test sieve analysis of specimen debris correlates fragmentation with projectile velocity, and these data show interesting differences between composites. Along with other quasi-static and dynamic measurements, Taylor impact images and fragmentation data provide a useful metric for the calibration or evaluation of intermediate-rate model predictions of PBX constituitive response and failure/fragmentation. Intermediate-rate tests involving other impact configurations are being considered.

CHARACTERIZING THE EFFECTS OF RATCHET GROWTH ON PBX 9502

Pressed composites of TATB (2,4,6‐trinintro‐l,3,5‐benzenetriamine) undergo irreversible volume change when subjected to thermal cycling. Using micro x‐ray computed tomography and ultra‐small angle neutron scattering, we have characterized the micro‐structure of as‐pressed and ratchet grown specimens of PBX 9502, a TATB‐based composite, thereby distinguishing the effects of ratchet growth from the effects of density alone. Porosity differences are shown to effect mechanical properties, presented here, with ongoing efforts to evaluate sensitivity and/or performance effects.

Experimental Study of Grit Particle Enhancement in Non‐Shock Ignition

The drop weight impact test is the most commonly used configuration for evaluating sensitivity of explosives to non‐shock ignition. Although developed 60 years ago and widely used both as a material compression test and as a test bed for understanding the ignition process itself, little is known about the flow mechanisms or involvement of grit particles as sensitizing agents. In this paper, we present the results of a series of experiments designed to study the flow mechanisms and events leading up to ignition. The experimental configuration used involves two pellet sizes, 3 and 5 mm in diameter, tested with three conditions: (1) smooth steel anvils, (2) standard flint sandpaper, and (3) shed grit particles loaded between the steel anvils and the pellet faces. Diagnostics include optical micrographs, and scanning electron micrographs. Un‐reacted samples show a variety of morphologies, including what appear to be quenched reaction sites, even at very low drop heights. Quasi‐static crushing experiments were...

Time-Temperature Analysis, DMA and Compression in PBXs

The mechanical response of plastic-bonded explosives (PBXs) is highly complex and depends on a number of factors including, but not limited to temperature, strain rate and binders. In this work, we have measured and analyzed the mechanical properties of LX-14, which contains 95.5 wt% HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) and 4.5 wt% Estane 5702, with no additional plasticizer. We are interested to compare LX-14 with PBX 9501 (95 wt% HMX, 2.5 wt% Estane 5703 and 2.5 wt% nitroplasticizer), which has a relatively high content of plasticizer. The measurements span a wide range of strain rates and temperatures. We have applied time-temperature analysis on the LX-14 mechanical properties, i.e. dynamic mechanical analysis (DMA) measurements and quasi-static uniaxial compression to form satisfactory master curves, and these data are compared with those from PBX 9501. A unique inhomogeneity is observed in the LX-14 compression data as a function of the specimen location.

Instrumented pressing of inert powders to study the effect of particle size

It is well-known that the detonation and mechanical properties of pressed high explosives (HEs) depend on density. Furthermore, specific particle size distributions have been shown to compact more easily and to effect some shock/detonation properties. Theoretically, particle size distributions can be optimized for compaction and performance. Here, in anticipation of future experiments on HE powders, we explore the role of inert particle-size characteristics on compaction properties and pellet integrity. An instrumented compaction instrument was used to press inert powders that have similar particle size distributions to 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX), as well as compounds that are being developed as mock materials for HMX. The measured force and displacement measurements provide information in the form of compaction curves and Heckel plots. The results of the compaction measurements and subsequent Instron compression tests identify 5-iodo-2’-deoxyuridine (IDOX) as a potential HMX mock for de...

Irreversible volume expansion of a TATB-based composite and compressive strength

It has long been known that compacted composites containing TATB (triaminotrinitrobenzene) crystals undergo “ratchet growth,” an irreversible volume expansion upon thermal cycling. A clear mechanism has not been established for this phenomenon, but is believed to arise from the highly-anisotropic CTE of TATB crystals and interactions caused by compaction. Explosive performance depends fundamentally on bulk density, so the effect may be important. PBX 9502 is a plastic bonded explosive containing 95 wt% TATB crystals. We have monitored uniaxial length changes of PBX 9502 specimens for various thermal cycles providing mechanistic insight. Post-cycled specimens were compression tested to determine if mechanical properties correlated with the detailed thermal history.

Coefficient of Thermal Expansion of Pressed PETN Pellets

The PETN single crystal coefficient of thermal expansion (CTE) values were measured and reported by Cady in 1972 [1] over the temperature range of -160 to 100°C. Measurements were made in the (001) and (100) crystallographic directions, see Figure 1 (a replicate of Figure 1 from the Cady paper). Cady used his single-crystal data to calculate the linear CTE for a randomly-oriented multi-crystal pressing of PETN, and his values ranged from 76.5 μe/°C (at 20°C) to 89.9 5 μe/°C (at 90°C).