William H. Holt

Fragmentation behavior of Armco iron and HF‐1 steel explosive‐filled cylinders

The fragmentation response of explosive‐filled cylinders of Armco iron and HF‐1 steel with two heat treatments has been determined. The cylinders were filled with cast‐in‐place composition B explosive and detonated. At least 99% of the metal mass was recovered in all the experiments. The recovered fragments had both brittle and shear fracture surfaces. The average fragment mass for the Armco iron and HF‐1 steel experiments was 6.3 and 0.9 g, respectively. Average engineering radial strain values for the fragments were 0.35 and 0.12 for Armco iron and HF‐1 steel, respectively. A scheme was devised for characterizing the recovered fragments according to their cylindrical and fracture surfaces. Fragment mass distributions are presented for the characterized fragments.

Pulse charging of nanofarad capacitors from the shock depoling of PZT 56/44 and PZT 95/5 ferroelectric ceramics

Gas‐gun impact techniques have been used to pulse charge nanofarad capacitors from the shock depoling of PZT 56/44 and PZT 95/5 ferroelectric ceramics. The PZT materials were depoled in the normal mode. Pulse powers of hundreds of kilowatts were produced in a few microseconds. The PZT 56/44 material was impacted in the stress range from 4.4 to 11.8 GPa. A maximum load voltage of 55 kV was produced at 7.9 GPa, resulting in a charge release of about 78%. Shock‐induced electrical breakdown in the PZT material occurred at 11.8 GPa. The PZT 95/5 material was impacted at 1.4 and 2.9 GPa stress levels. A maximum load voltage of 81 kV was produced at the higher stress, resulting in a charge release of about 74% before electrical breakdown occurred in the PZT material.

Reverse‐ballistic impact study of shear plug formation and displacement in Ti6Al4V alloy

Gas‐gun reverse‐ballistic experiments have been performed in which Ti6Al4V alloy disks were impacted onto smaller diameter, hardened steel rods to push out shear plugs from the disk material. The range of disk velocities was 219–456 m/s. For each experiment, the disk, plug, and rod were soft recovered after impact. Below 290 m/s, the plugs were pushed only partway through the disks, but localized shear bands outlining the plug shapes were easily recognized in metallographic sections. Optical and scanning electron microscopies were used to determine shear zone widths and to describe microstructural details associated with the primary shear zones. There is evidence for appreciable adiabatic heating and consequent thermal softening and melting of material in the main shear zone. A simple model is used to relate the observed plug displacements to the impact velocities and to provide estimates of several features: the shear zone strength, the threshold energy for shear plug displacement, and the threshold ener...

Axial‐current‐mode shock depoling of PZT 56/44 ferroelectric ceramic disks

Gas‐gun impact techniques have been used to investigate the electrical response of shock‐depoled PZT 56/44 ferroelectric ceramic disks in the stress range rom 0.7 to 8.8 GPa. External short‐circuit current pulses were measured as the shock wave propagated axially through the disk in a direction either parallel or antiparallel to the remanent polarization vector. Complete charge release was not observed in any of the shots. About 82% of the available charge was released for both orientations in the stress range from about 1.5 to 5.6 GPa. At stress levels of 6.8 and 8.8 GPa the charge release for the antiparallel orientation shots was less than for the parallel orientation shots. A series of electrical polarization reversal measurements was performed under nonshock conditions on PZT disks to estimate the electrical properties in the region ahead of the shock front. Maximum electric fields for the regions ahead of and behind the shock front were estimated to be 1–6 and 4–24 kV/mm, respectively, as the shock ...

Impact Initiation of Rods of Pressed Polytetrafluoroethylene (PTFE) and Aluminum Powders

A gas gun has been used to investigate the shock initiation of rods of a mixture of 74 wt% PTFE and 26 wt% aluminum powders. The rods were sabot‐launched into 4340 steel anvils at impact velocities ranging from 104 to 963 m/s. A framing camera was used to observe the time sequence of events. At low velocity, no initiation occurred. Above an initiation threshold, the initiation time dropped abruptly from 56 μs just above threshold to 4 μs at the highest impact velocity. Several high velocity experiments were performed for pure PTFE material for comparison with the PTFE/Al rods.

Moisture dependence of positron annihilation spectra in nylon-6

Abstract Positron annihilation time spectra have been measured in nylon-6 specimens as a function of their moisture content. The measured average longlife component lifetime values are: 1722 ± 47 ps (dry), 1676 ± 40 ps (14.6% saturation value), 1719 ± 26 ps (29.3% saturation value), 1720 ± 35 ps (50% of saturation value), 1857 ± 35 ps (78.1% of saturation value) and 1936 ± 57 ps (saturated). It is noted that the longlife component lifetime at first decreases and then increases with increasing moisture content in the specimens. This behavior is quite different from that observed in earlier studies of various epoxy, polyamide, and polyimide materials, where the longlife components lifetime decreased linearly with increasing moisture content. The longlife component intensity on the other hand, decreases steadily as the moisture content increases from 0 to 100% of the saturation value. A possible explanation for these anomalous features is discussed.

Determination of dynamic fracture parameters for HF‐1 steel

Dynamic fracture parameters have been determined for two heat treatments of HF‐1 steel. A gas gun was used for the experiments. Different amounts of fracture damage were produced in HF‐1 steel specimens under known impact conditions. The specimens were soft recovered, sectioned, and polished to reveal any internal microscopic fracture. The fracture cracks were then digitized. The velocities of the 1.15‐, 1.59‐, and 2.37‐mm‐thick impactor disks range from 0.120 to 0.276 km/s. The specimen disks were 3.18‐ and 6.35‐mm thick. An SRI stress wave propagation computer program with a brittle fracture model was used for calculating the dynamic fracture parameters. A series of Hugoniot experiments was performed for HF‐1 steel to determine equation of state input data for the computer program. The Hugoniot elastic limits were 2.2 and 2.4 GPa for the two heat treatments.

X-ray photoelectron spectroscopy indication of decomposition species in the residue of shocked polytetrafluoroethylene powder

Forty-eight percent porous polytetrafluoroethylene (PTFE) powder placed inside a steel closed container has been impact shock loaded with a gas gun and soft recovered. The initial stress in the powder is calculated to be 7.2 kbar. The residue in the container showed dark regions where the originally white powder had decomposed to form black soot. X-ray photoelectron spectroscopy (XPS) was used to analyze in situ a portion of a dark region. The resulting spectrum showed a large amorphous carbon peak that was not observed in the unshocked powder spectrum. In addition, the shocked material showed several peaks containing hydrogen and/or oxygen, suggesting reactions of dissociation products with ambient air and/or water vapor in the polymer pores or possibly with residual water in the polymer. (The residual gas analyzer in the XPS system detected water vapor in the unshocked specimen.) Both spectra showed peaks corresponding to the PTFE linear polymer chain F–C–F.

Shock‐wave compression of an alumina‐filled epoxy in the low gigapascal stress range

The shock‐wave equation of state for a commerically available alumina‐filled epoxy has been determined in the stress range from 0.4 to 3.7 GPa via gas‐gun‐impact experiments. A linear fit to the shock‐velocity–particle‐velocity data yields U=2.78+1.63u. This curve lies between the U‐u curves for an unfilled epoxy and for an alumina‐filled epoxy of higher density.

Effects of transition metal ions on positron annihilation characteristics in epoxies

Abstract Effects of several 3d-transition metal ions on positron annihilation characteristics in MY-720 epoxy resin have been investigated. All metal complexes were added at the mole ratio of 1 metal complex for every 10 MY-720 repeat units. In all cases, it appears that the 3d magnetic electrons are localized on their respective ions. Cr 3+ -ions make the strongest impact — both on the probability of formation of positronium atoms as well as their subsequent decay. The effects of Co 2+ and Co 3+ ions are not greatly different, indicating that the final oxidation states of Co x + -ions in the host epoxy may be the same, namely 2 + . Ni 2+ -ions are unique because their coordination geometry remains unchanged when Ni(acac) 2 is introduced in the MY-720 epoxy. In this respect, Ni(acac) 2 is similar to Co(aeac) 2 and both have comparable impacts on positron annihilation characteristics. The effects of Fe 3+ -ions are consistent with coordination symmetry change from sp 3 d 2 to d 1 sp 3 when Fe(acac) 3 is introduced in the epoxy.