Willis Mock

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...

DYNAMIC PROPERTIES OF POLYUREA 1000

The shock response of the viscoelastic polymer material polyurea 1000 has been investigated. Sabots carrying Al or Cu metal disks were launched into target assemblies containing the polyurea material. The target consisted of a thin metal disk on the impact side, a 6.5‐mm‐thick polyurea disk, and a thick metal backup disk. 50‐Ω manganin gauges were epoxied between the metal/polymer and polymer/metal interfaces to measure the interface stresses and shock transit time. Measured longitudinal stresses ranged from 6 to 43 kbar. The measured shock velocity‐particle velocity relationship was linear over this stress range. Maximum volume compression was about 30% for the initial shock wave. Several reshock waves were also measured for each experiment.

Experimental and computational study of the impact deformation of titanium Taylor cylinder specimens

Abstract Gas-gun reverse-ballistic Taylor cylinder impact deformation experiments have been performed with specimens of 99.7% titanium. The flat-ended, 6.35 mm diameter, 25.4 mm long solid cylindrical specimens were impacted by sabot-mounted 34.0 mm diameter, 19.0 mm thick disks of maraging 350 steel. Experiments were performed at 225 and 294 m/s; a soft recovery system prevented post-impact damage to the specimens. Plastic deformation extended over approximately 57 and 64% of the deformed specimen length, respectively, with little lateral spreading. The specimen shapes were digitized via a toolmakers microscope connected to a computer. Metallographic examination of longitudinal sections through the specimens revealed extensive deformation twinning, with twinned grains appearing throughout the deformed regions. Distributions of twins as a function of distance from the impact end were determined for each specimen. The importance of twinning in the computational modelling of deformation in body centered cubic (BCC) iron was shown by Zerilli and Armstrong. It is also very important for hexagonal close packed (HCP) titanium which exhibits a BCC type of behavior. Deformation twinning can cause a reduction in the effective size of the twinned grains; an associated increase in the yield stress could explain the relatively small amount of lateral spreading observed for the titanium Taylor specimens. Computational simulations of the impact experiments performed without provision for twinning show significant differences between the simulations and the experimental shapes. When twinning effects are included, good fits to the experimental shapes are obtained.

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.

EFFECT OF ALUMINUM PARTICLE SIZE ON THE IMPACT INITIATION OF PRESSED PTFE/AL COMPOSITE RODS

A gas gun has been used to investigate the impact initiation of rods of a mixture of 72 wt% PTFE (polytetrafluoroethylene) and 28 wt% aluminum powders. The rods were sabot‐launched in vacuum into steel anvils at impact velocities ranging from 468 to 969 m/s. A framing camera was used to observe the time sequence of events following impact. At the lowest impact stress of 25 kbar no light was observed. Above the initiation threshold, the initiation time dropped from 74 μs just above threshold to 14 μs at 64 kbar. These results are compared with earlier rod impact experiments for a similar material in which the only major difference is a smaller aluminum particle size.

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.