Jamie Stull

In Situ Imaging during Compression of Plastic Bonded Explosives for Damage Modeling

The microstructure of plastic bonded explosives (PBXs) is known to influence behavior during mechanical deformation, but characterizing the microstructure can be challenging. For example, the explosive crystals and binder in formulations such as PBX 9501 do not have sufficient X-ray contrast to obtain three-dimensional data by in situ, absorption contrast imaging. To address this difficulty, we have formulated a series of PBXs using octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) crystals and low-density binder systems. The binders were hydroxyl-terminated polybutadiene (HTPB) or glycidyl azide polymer (GAP) cured with a commercial blend of acrylic monomers/oligomers. The binder density is approximately half of the HMX, allowing for excellent contrast using in situ X-ray computed tomography (CT) imaging. The samples were imaged during unaxial compression using micro-scale CT in an interrupted in situ modality. The rigidity of the binder was observed to significantly influence fracture, crystal-binder delamination, and flow. Additionally, 2D slices from the segmented 3D images were meshed for finite element simulation of the mesoscale response. At low stiffness, the binder and crystal do not delaminate and the crystals move with the material flow; at high stiffness, marked delamination is noted between the crystals and the binder, leading to very different mechanical properties. Initial model results exhibit qualitatively similar delamination.

Development of inert density mock materials for HMX

ABSTRACT Inert surrogates or mocks for high explosives are commonly used in place of the real material for complex experiments or in situations where safety is a concern. Here, several materials were tested as potential mocks for HMX in terms of density, thermal stability, and processability. Selection criteria were developed and a literature search was conducted primarily using the Cambridge Structural Database. Out of over 200 potentially acceptable materials, six were chosen for crystallization experiments and a suite of analytical characterization. Of these six, 5-iodo-2ˊ-deoxyuridine, N,Nˊ-bis(2,3,4,5,6-pentafluorophenyl)oxamide, and 2,3,4,5,6-pentafluorobenzamide all were found to be thermally stable at 150°C, matched HMX density as a pressed pellet, and could be crystallized to appropriate particle sizes. These three materials are considered suitable inert density mocks for HMX and will be the subject of future testing.

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...