In a nuclear weapon, many materials are subjected to impulsive loading-high- intensity, short-duration forces-caused by detonating high explosives. To understand how materials respond to such conditions, scientists must study the dynamic properties of plutonium metal and other materials over a wide variety of pressures and time scales.
Scientists can duplicate these extreme conditions of elevated temperature and pressure by creating shock waves and allowing them to propagate through materials. The size, speed, and shape of the shock wave is determined by the dynamic material behavior of the sample being studied, so careful measurements of the shock wave can be used to determine material properties.
Nuclear Materials Science (NMT-16) operates a Kolsky-bar apparatus to gather dynamic data on plutonium at low pressures and a relatively long time scale, and a gas gun to gather dynamic data at much higher pressures and shorter time scales. One of the simplest, most-controlled, and most-accurate tools used to create shock waves is a smoothbore gun. Researchers in Detonation Science and Technology (DX-1) and NMT-16 are conducting experiments on such a gun-the 40mm Launcher, so named because of its bore size.
Researchers are using the gas gun contained in this glove box in PF-4 to duplicate the extreme conditions of elevated temperature and pressure created by the high explosives in a nuclear weapon. Research on a delta phase plutonium alloy has resulted in the first dynamic data for the alloy. The gun can launch projectiles at speeds ranging from about 200 miles per hour to more than 4,000 miles per hour.
Photo by Paul Moniz
The work so far has focused on a particular delta phase plutonium alloy. Researchers have obtained a considerable amount of shock Hugoniot data that will allow the moderate- to low-pressure equation-of-state to be defined. In addition, new data on the phase diagram for delta plutonium have been obtained, including the location of solid-solid phase changes and dynamic melting. Information concerning the rate, or kinetics, of these transitions also has been obtained.
Probably the largest amount of research has focused on the dynamic strength of delta plutonium in tension: spall. Careful research has been done on the effect of impurities and peak stress on tensile strength. The research has resulted in the first dynamic data ever obtained on this plutonium alloy. The data are of very high quality, according to the researchers, and currently is being used by theorists to develop new physics models for the dynamic response of this alloy.
The Launcher, housed in Building PF-4 at TA-55, can be used with either a gas breech or a propellant breech to provide the projectile acceleration. It can launch projectiles at speeds ranging from about 0.1 kilometer per second to almost 2 kilometers per second, or from 200 miles per hour to more than 4,000 miles per hour.
The gun works by firing a projectile at a small plutonium sample, or target. When the projectile impacts the sample, shock waves are generated in both the projectile and the sample. In the target, material ahead of the shock wave is stationary until the shock wave passes; after the shock wave passes, the material is moving. A shock wave also moves back into the projectile, slowing down the projectile's initial velocity. High pressures are generated in the region between these two shock waves.
Higher projectile velocities lead to higher pressures and faster shock velocities. Shock waves may be viewed as wave disturbances that abruptly change the pressure, temperature, density, and internal energy of a substance from an initial value to a final state. The final state generated is at a higher pressure, temperature, internal energy, and density than the initial unshocked material. In other words, a shock wave compresses a material.
This simple diagram of a target assembly shows the electrical pins, one on each side of the target, from which researchers obtain the time at which impact occurred. The velocity interferometer (VISAR) detects the velocity history of the back of the target. This surface moves when a shock wave emerges from the sample, and the velocity interferometer senses this motion and sends back information about wave(s) generated by the impact.
Because NMT-16's gas gun is used to study plutonium, it is contained in a glove box. This greatly increases operational difficulties compared with guns used outside of TA-55, and special techniques had to be developed to perform well-controlled experiments.
The Launcher's projectile is a cylinder of plastic or metal. High-pressure gas in the breech is used to push the projectile down the barrel. The material inserted into the nose of the projectile, called the impactor, varies depending on the data researchers are trying to collect. Some materials are stiffer than others, and so produce higher pressures in the target for a given projectile velocity.
Two electrical pins, one placed on each side of the target, record the exact time of impact. Another diagnostic tool, a velocity interferometer, or VISAR, measures the velocity history of the back of the target. This surface moves when a shock wave emerges from the sample. The velocity interferometer senses the motion and sends back information about wave(s) generated by impact. Other pin arrival times are used to measure the angle at which the impactor hits the target plate; also known as impact tilt. This is important for researchers to know because large amounts of tilt can cause data quality to suffer. In addition, by combining impact time with the time-resolved velocity information, researchers can determine the velocity of the wave(s) moving through the target material.
This delta plutonium sample has been tested in a dynamic tension, or "spall," experiment. The sample was shock-compressed to a peak stress state of about 25 kilobar, released, and then recovered. Dynamic wave interactions have caused the sample to be split almost in half in a well-controlled manner. This sample has been sectioned and analysis of the tensile damage is under way. Photo by Mick Greenbank
VISAR data also may be used to obtain the size and shape of the wave(s) moving through the target, as well as the speed of the material just behind the shock wave-called the particle velocity. In general, faster projectiles generate higher pressures, higher shock velocities, and higher particle velocities.
A graph of shock velocity vs. particle velocity defines a curve called the shock "Hugoniot" of a material. The Hugoniot describes the locus of end states that may be achieved in a material through shock-wave compression and is different for different materials. The most basic understanding of how a material responds to shock compression is contained in the shock Hugoniot.
Data obtained from these kinds of experiments provide a wealth of additional information. Materials with limited strength typically have two distinct waves created in an experiment. The first wave propagates at the longitudinal wave speed and takes the material to the point where it plastically yields. A second wave, which moves more slowly than the first and in which plastic deformation occurs, follows the first wave. The velocity interferometer will clearly show this kind of wave structure.
Shock-wave experiments can produce information on how a material changes phase under increased pressure and temperature. This process must be well understood for scientists to develop physics models that correctly describe the dynamic response of phase changing materials. Shock-wave experiments also can be used to produce data about the dynamic strength of materials in tension. More complicated shock-wave techniques than those described above allow the tensile, or spall, strength of materials to be studied on very short time scales.
Ben Jacquez of Structure/Property Relations (MST-8), left, and Johnny Montoya of Nuclear Materials Technology (NMT-16) make adjustments to the target of the 40mm Launcher during its shakedown period in November 1995, before the windows and gloves were installed in the glove box. Researchers performed several experiments on inert samples this way to test the system before going hot. Part of the gun barrel is shown on the left; the round aluminum plate with the plastic cylinders is the target. Photo by Tom Baros
While the Launcher itself is owned and operated by NMT-16, the shock-wave experiments involve several divisions. Researchers in DX-1, with input from others in Applied Physics (X) Division, design the experiments. Members of Weapons Component Technology (NMT-5) prepare many of the samples. The data are collected and analyzed by DX-1 and sent to X and Theoretical (T) divisions for further analysis, physics model developments, and eventual inclusion in computer codes. The multidivisional research effort has resulted in a significant amount of new data on plutonium over the past few years.
Contributors to this article are: Robert Hixson and John Vorthman (DX-1), and Benjamin Lopez (NMT-16).
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