ARMOR Anti- ARMOR Materials by Design by Donald J
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Armor/Anti-Armor ARMOR anti- ARMOR materials by design by Donald J. Sandstrom magine tank armor that grains aligned in a sheet of ura- chews up a high-velocity nium that allow it to stretch into a projectile on impact . or long, lethal jet of unbroken metal. I composites of tungsten and These examples illustrate how uranium that lend an antitank Los Alamos is using its knowl- penetrator rod the stiffness of edge of materials to design and the tungsten. the density and py - fabricate new and stronger com- rophoric property of the uranium, ponents for both armor and pene- and the surprising strength of trators of armor. their mixture or tiny crystal Our interest in applying ma- 36 Los Alamos Science Summer 1989 terials research to conventional tive process of theory, design. There is also a complementarily weapons has its origins in the fabrication, and testing used to between the applications of mate- Laboratory’s nuclear weapons develop nuclear weapons serves rials in conventional and nuclear program. To deal with the unique as the basis for a similar process weapons-one that has a syner- materials used in nuclear weap- in developing conventional ord- gistic effect on both programs, A ons, such as actinides, special ce- nance. The attention to detail in nuclear weapon releases so much ramics, polymers, and so forth, material properties required for energy so rapidly that materials the Laboratory had to develop nuclear weapons is, perhaps, even behave much like isotropic fluids significant expertise in materi- more important for conventional and can usually be described by als research. Further, the itera- weapons. hydrodynamic equations. In addi- Los Alamos Science Summer 1989 37 Armor/Anti-Armor tion, the performance of a nuclear de- A KINETIC-ENERGY PENETRATOR vice is more dependent on the nuclear and atomic properties of its constituents Fig. 1. These x-ray pictures are orthogonal views of the U. S. Army’s M-833 standard round (a fin- than on material properties, In contrast, stabilized, sabot-discarding projectile for tanks) taken after the round had traveled about two and a a conventional munition subjects ma- half meters from the muzzle of the tank gun. The central rod, or core, is a kinetic-energy penetrator terials to less severe deformation rates, made from a dense, hard alloy of depleted uranium and titanium, and the tip is hardened steel. and the deformation processes are more The sabot is a device that allows the pressure of the expanding gas from the burning propellant dependent on the chemistry and prior to accelerate the core and sabot assembly out the barrel of the gun. The sabot is discarded after fabrication history of its constituents. the core exits. These pictures show the beginning of the sabot-core separation. Also, note that For example, the behavior of an armor- the lower view reveals a bent fin on the core. piercing projectile is strongly affected by variations in the chemical composi- tion. processing history. microstructure, Two Orthogonal Views and mechanical properties of the materi- als from which it was formed. Further, nuclear reaction times are extremely short, whereas the reaction times for conventional munitions are of the order of microseconds-sufficiently long to allow for many types of mea- surements. And generally. very little, if any, material is recoverable from a test of a nuclear weapon, whereas a test of a conventional weapon frequently leaves a considerable amount of material for post-mortem analysis, The philosophy underlying the design of nuclear weapons at Los Alamos is traditionally conservative (in the most positive sense), especially in regard to reliability and ease of production. Our approach to conventional weapons follows the same philosophy and pays the same close attention to detail. We strive to use well-characterized, wel1- understood starting materials, we care- fully control the synthesis and manu- facturing processes. and wc work to develop a complete understanding of the Kinetic-Energy Penetrators the simple spin-stabilized slug of a 30- experimental results. Only in this way mm cannon to fin-stabilized projectiles are we able to relate the performance of Weapons designed to penetrate armor that consist of a long, steel-tipped pen- armor and anti-armor systems to slight generally fall into two classes: kinetic- etrator rod and a sabot that falls free of and often subtle variations in material energy penetrators and chemical-energy the penetrator after it is tired (Fig. 1 ). properties or device design and fabri- penetrators. I will discuss the first class If the material strength and kinetic en- cation. I will point out many of’ those now and return to the second later, ergy of the projectile are sufficient, it subtleties as I discuss advances made A kinetic-energy penetrator is a solid penetrates the armor, In addition, the at Los Alamos in the design of armor projectile, usually fired from a gun, that shock wave generated by the impact penetrators and armor, including some uses high-velocity impact (typically, at may travel through the armor plate and surprising properties of a new type of about 1 to 2 kilometers per second) to blow off a portion of its backside. Frag- ceramic armor. defeat the armor. Examples range from ments both from this spall and from the 38 Los Alamos Science Summer 1989 Armor/Anti-Armor penetrator itself can cause considerable can be heat-treated easily (by water- STRESS-STRAIN CURVE damage to people and equipment behind quenching and subsequent aging in a the armor. high-vacuum furnace) to eliminate the cracking problem, and its properties are Depleted uranium. Materials research not sensitive to precise composition. Yield has made particularly noteworthy con- These last two features help give the Plastic Flow tributions to the design and develop- alloy low manufacturing costs. Point ment of the kinetic-energy penetrator. The alloy was originally developed The most effective armor-piercing ma- and evaluated at Los Alamos for the terial to date is an alloy developed at U.S. Air Force’s GAU-8 system, a 30- Los Alamos—an alloy of depleted ura- mm gatling gun system mounted on the nium (most of the fissionable isotope A-10 close support aircraft. The gun has been removed) and a small amount can fire a thousand armor-piercing pen- of titanium (0.75 per cent). etrator rounds per minute and is said to Depleted uranium was considered an be the most effective antitank system o 20 attractive material for kinetic-energy in the world. The uranium-titanium al- Strain (per cent) penetrators for a number of reasons. Its loy was so successful that it has been high density (almost twice that of steel) adopted as the standard for large-caliber Fig. 2. Many material properties, such as hard- makes it easy to produce a penetrator penetrators (such as the one shown in ness and strength, are determined from the re- that delivers high momentum and ki- Fig. 1). lationship between stress (the force per unit netic energy to a small volume of target area applied to the material) and strain (the re- armor. Uranium is highly pyrophoric, sulting deformation of the material). The ini- Dynamic Deformation and its impact against steel targets at tial, approximately linear part of a stress-strain and Fracture velocities as low as 30 meters per sec- curve is called the elastic region because ma- ond produces burning fragments that can The penetrating ability of armor- terial stressed in this region will not suffer any ignite fuel or propellants. In addition, piercing rounds improves with the hard- permanent deformation when the stress is re- depleted uranium is readily available ness and strength of the material used. laxed (in other words, the stress-strain curve in large quantities and is considerably Mechanical properties of this nature are returns to the origin). The point at which the cheaper than alternative materials. normally determined from the stress- curve leaves the elastic region by bending to- Uranium, however, is more reactive strain curve for that material (Fig. 2). ward the horizontal indicates the onset of per- than most other penetrator materials, Stress is the force per unit area applied manent deformation and is a measure of the and its reactivity can result in corro- to a sample, and strain is the relative material’s yield strength. Beyond that point sion problems, particularly in moist deformation of the sample as a result is the inelastic, or plastic-flow, region of the air. In addition, some uranium alloys of that stress. Various kinds of defor- curve. The slope of the curve in the elastic re- are susceptible to delayed cracking due mation can occur (elongation, compres- gion is the elastic modulus, a measure of the to residual stresses induced by fabri- sion, bending, etc.) depending on the material’s stiffness. The slope in the plastic- cation and heat treatment of the rods. nature of the applied force. If stress to flow region is a measure of work hardening The cracking can be avoided if care is the material is kept below the so-called since a steeper slope means more stress must taken in the heat treatment to reduce yield point, or proportional limit, the be applied to create a given amount of defor- such stresses and to reduce entrapped material will spring back to its origi- mation. hydrogen gas to levels less than a few nal undeformed state—in other words, parts per million. the response is elastic. Once this yield Extensive testing at Los Alamos of strength has been exceeded, however, stress needed to achieve a given amount uranium alloyed with various metals at plastic flow occurs, and the material re- of plastic flow).