Glass, in various formulations, may be useful as a transparent armor material. Fused quartz (SiO{sub 2}), modified with either B{sub 2}O{sub 3} (13 % wt.) or Na{sub 2}O (15 % wt.), was studied to determine the effect on the dynamic response of the material. Utilizing powder and two-stage light gas guns, plate impact experiments were conducted to determine the effect on strength properties, including the elastic limits and plastic deformation response. Further, the effect of glass modification on known transitions to higher density phases in fused quartz was evaluated. Results of these experiments will be presented and discussed.
While isentropic compression experiment (ICE) techniques have proved useful in deducing the high-pressure compressibility of a wide range of materials, they have encountered difficulties where large-volume phase transitions exist. The present study sought to apply graded-density impactor methods for producing isentropic loading to planar impact experiments to selected such problems. Cerium was chosen due to its 20% compression between 0.7 and 1.0 GPa. A model was constructed based on limited earlier dynamic data, and applied to the design of a suite of experiments. A capability for handling this material was installed. Two experiments were executed using shock/reload techniques with available samples, loading initially to near the gamma-alpha transition, then reloading. As well, two graded-density impactor experiments were conducted with alumina. A method for interpreting ICE data was developed and validated; this uses a wavelet construction for the ramp wave and includes corrections for the ''diffraction'' of wavelets by releases or reloads reflected from the sample/window interface. Alternate methods for constructing graded-density impactors are discussed.
Recent advances in magnetic loading techniques have permitted quasi-isentropes to be measured to unprecedented levels. However, the relevant equations for planar waves provide no information about transverse stresses, leaving the deviatoric (strength) behavior of an isentropically loaded material unknown. Because materials are much cooler under isentropic loading than under shock loading, they can remain solid and thus retain strength to very high pressures. Thus, to improve our ability to model material behavior under isentropic loading, techniques to measure strength are needed. In this paper, existing techniques for determining high-pressure strength will be discussed along with their limitations. A technique for assessing the strength of isentropically loaded materials will be presented and used to determine the strength of an aluminum alloy using data from the Z machine and gas gun experiments. These results will be compared to existing models for material strength. Finally, limitations of the technique and future work needed will be discussed.
The shock behavior of two varieties of the ceramic silicon carbide was investigated through a series of time-resolved plate impact experiments reaching stresses of over 140 GPa. The Hugoniot data obtained are consistent for the two varieties tested as well as with most data from the literature. Through the use of reshock and release configurations, reloading and unloading responses for the material were found. Analysis of these responses provides a measure of the ceramic's strength behavior as quantified by the shear stress and the strength in the Hugoniot state. While previous strength measurements were limited to stresses of 20-25 GPa, measurements were made to 105 GPa in the current study. The initial unloading response is found to be elastic to stresses as high as 105 GPa, the level at which a solid-to-solid phase transformation is observed. While the unloading response lies significantly below the Hugoniot, the reloading response essentially follows it. This differs significantly from previous results for B{sub 4}C and Al{sub 2}O{sub 3}. The strength of the material increases by about 50% at stresses of 50-75 GPa before falling off somewhat as the phase transformation is approached. Thus, the strength behavior of SiC in planar impact experiments could be characterized as metal-like in character. The previously reported phase transformation at {approx}105 GPa was readily detected by the reshock technique, but it initially eluded detection with traditional shock experiments. This illustrates the utility of the reshock technique for identifying phase transformations. The transformation in SiC was found to occur at about 104 GPa with an associated volume change of about 9%.
A suite of impact experiments was conducted to assess spatial and shot-to-shot variability in dynamic properties of tantalum. Samples had a uniform refined {approx}20 micron grain structure with a strong axisymmetric [111] crystallographic texture. Two experiments performed with sapphire windows (stresses of approximately 7 and 12 GPa) clearly showed elastic-plastic loading and slightly hysteretic unloading behavior. An HEL amplitude of 2.8 GPa (corresponding to Y 1.5 GPa) was observed. Free-surface spall experiments showed clear wave attenuation and spallation phenomena. Here, loading stresses were {approx} 12.5 GPa and various ratios of impactor to target thicknesses were used. Spatial and shot-to-shot variability of the spall strength was {+-} 20%, and of the HEL, {+-} 10%. Experiments conducted with smaller diameter flyer plates clearly showed edge effects in the line and point VISAR records, indicating lateral release speeds of roughly 5 km/s.
Boron carbide displays a rich response to dynamic compression that is not well understood. To address poorly understood aspects of behavior, including dynamic strength and the possibility of phase transformations, a series of plate impact experiments was performed that also included reshock and release configurations. Hugoniot data were obtained from the elastic limit (15-18 GPa) to 70 GPa and were found to agree reasonably well with the somewhat limited data in the literature. Using the Hugoniot data, as well as the reshock and release data, the possibility of the existence of one or more phase transitions was examined. There is tantalizing evidence, but at this time no phase transition can be conclusively demonstrated. However, the experimental data are consistent with a phase transition at a shock stress of about 40 GPa, though the volume change associated with it would have to be small. The reshock and release experiments also provide estimates of the shear stress and strength in the shocked state as well as a dynamic mean stress curve for the material. The material supports only a small shear stress in the shocked (Hugoniot) state, but it can support a much larger shear stress when loaded or unloaded from the shocked state. This strength in the shocked state is initially lower than the strength at the elastic limit but increases with pressure to about the same level. Also, the dynamic mean-stress curve estimated from reshock and release differs significantly from the hydrostate constructed from low-pressure data. Finally, a spatially resolved interferometer was used to directly measure spatial variations in particle velocity during the shock event. These spatially resolved measurements are consistent with previous work and suggest a nonuniform failure mode occurring in the material.
To address known shortcomings of classical metal plasticity for describing material behavior under shock loading, a model which incorporates a distribution in the deviatoric stress state is developed. This distribution will translate in stress space under loading, and growth of the distribution can be included in the model as well. This proposed model is capable of duplicating the key features of a set of reshock and release experiments on 6061-T6 aluminum, many of which are not captured by classical plasticity. The model is relatively simple, is only moderately more computationally intensive, and requires few additional material parameters.
In the current study we are developing an experimental fracture material property test method specific to dynamic fragmentation. This test method allows the study of fracture fragmentation in a reproducible laboratory environment under well-controlled loading conditions. Motion and fragmentation of the specimen are diagnosed using framing camera, VISAR and soft recovery methods. Fragmentation properties of several steels, nitinol, tungsten alloy, copper, aluminum, and titanium have been obtained to date. The values for fragmentation toughness, and failure threshold will be reported, as well as effects in these values as the material strain-rate is varied through changes in wall thickness and impact conditions.
Sandia National Laboratories has developed a unique method for a hyper-velocity launch (HVL), the three-stage gun. The three-stage gun is a modified two-stage light-gas gun, consisting of a piston used in the first stage, an impactor in the second stage, and a flyer plate in the third stage. The impactor is made up of different material layers that are increasing in shock impedance. The graded or pillowed layers allow the flyer to be launched at velocities up to 16 km/s without the formation of a single shock wave in the flyer plate and without it melting. Under certain experimental conditions the flyer velocity cannot be measured by standard means, X-rays and VISAR. Also, there is a need to know the flyer velocity prior to a launch in order to calibrate instruments and determine the appropriate shot configuration. The objective of HVL{_}CTH is to produce an accurate forecast of the flyer plate velocity under different launch conditions. CTH is a Eulerian shock physics computational analysis package developed at Sandia National Laboratories. Using CTH requires knowledge of its syntax and capabilities. HVL{_}CTH allows the user to easily interface with CTH, through the use of Fortran programs and batch files, in order to simulate the three-stage launch of a flyer plate. The program, HVL{_}CTH, requires little to no knowledge of the CTH program and greatly reduces the time needed to calculate the flyer velocity. Users of HVL{_}CTH are assumed to have no experience with CTH. The results from HVL{_}CTH were compared to results of X-ray and VISAR measurements obtained from HVL experiments. The comparisons show that HVL{_}CTH was within 1-2% of the X-Ray and VISAR results most of the time.