Armor/Anti-Armor

The Rand Corporation, E. I DuPont de Nemours, Honeywell, Inc., Battelle Washington Operation, Textron Defense Systems

Achief concern driving the cur-rent U.S. armor/anti-armorprogram is that the Sovietshave a significant lead over

the United States in tanks and antitankweapons (see “A Comment by GeneralStarry”). Moreover, simple solutions inarmor and anti-armor technology havealready been implemented, so it mayno longer be possible to find a “quickfix” that will catapult the conventionalU.S. forces to a decisive lead over theSoviets. The problems are complex andthe science sophisticated enough thatwe cannot depend on small, incrementalimprovements. We must base new solu-tions on a better scientific understandingof materials and their behavior underballistic conditions.

As a result the national Armor/Anti-Armor Program, although spearheadedby DARPA (Defense Advanced Re-search Projects Agency), is also a col-laborative effort with the U.S. Army,the U.S. Marine Corps, and 130 corpo-rations, laboratories, and universities.A key element in this collaboration isthe Advanced Technology AssessmentCenter created at Los Alamos to pro-vide the strong scientific base neededfor high-tech advances in armor and ar-mor Penetrators. ATAC serves as both atesting center for new developments anda scientific resource that all participantscan draw upon.

A major impetus for choosing LosAlamos as the Advanced TechnologyAssessment Center was history: therehas always been a synergistic and inti-mate relationship between the Labora-tory’s nuclear and conventional weaponstechnologies. In the early days of theManhattan Project, ordnance expertscame to Los Alamos to design essentialcomponents of the first nuclear devices.The overlap between the design of nu-clear and conventional weapons that wasestablished then, and which continuestoday, includes the hydrodynamics ofhigh explosives, firing systems (detona-

tors and electronics), materials proper-ties (especially at high strain rates andextreme pressures), and computer mod-eling. Further, the precision required fornuclear ordnance has forced the Labo-ratory to explore these technologies atvery detailed and precise levels.

Throughout the 1970s Los Alamoscontributed to Department of Defenseconventional munitions. For example,we developed a uranium alloy to serveas an armor-piercing round for the AirForce. The material proved so effectiveit became a standard for large-caliberPenetrators. We also collaborated withindustry and Navy laboratories to solvea propellant safety problem that threat-ened the Trident system. This last effortled to the joint development—by LosAlamos, Lawrence Livermore NationalLaboratory, and the Air Force RocketPropulsion Laboratory-of a method-ology for testing the safety of solidrocket propellant. One of our most no-table contributions was the long standoffpenetrator, a shaped-charge, chemical-energy weapon that was tested in 1979and shown to penetrate more deeply intoarmor than any other such weapon. In-terest in the design of this penetratorled to more extensive Los Alamos in-teractions with Department of Defensemunitions researchers and, ultimately, tothe choice of Los Alamos as ATAC.

Armor/Anti-ArmorProgram Goals

The long-term goals of the nationalArmor/Anti-Armor Program are to de-velop a broad base of expertise in pri-vate industry and make that expertiseavailable to the U.S. Armed Forces. Ona short-term basis the national programalso hopes to increase the rate at whichwe modernize armor and anti-armorsystems until the U.S. can outperformthe Soviet Union in the development ofseveral key technologies. The strategydevised to accomplish both goals is to

challenge industry in the key technolo-gies by making the research, develop-ment, test, and evaluation stages of thenational program highly competitive.

The strategy has been implementedby breaking the core of the Armor/Anti-Armor Program into three major ele-ments: the Blue Teams, the Red DesignBureau, and ATAC. The Blue Teamsconsist of contractors who are develop-ing armor and antitank weapons. Thesecontractors are large industrial corpo-rations that have enlisted universities,national laboratories, and other corpo-rations as subcontractors to help withtheir competitive efforts. The first phaseof the program involves about 130 ma-jor contractors selected from an originalfield of over 400 companies.

The Red Design Bureau—headed byBattelle Memorial Institute in Colum-bus, Ohio was created to design a “So-viet threat” for the competitive stages ofthe program. The threat is based on anindependent evaluation of available in-telligence data. In other words, the Bu-reau tries to “think Soviet, ” design whatthe Soviets might be designing, and thenfabricate actual prototypes. These futur-istic Soviet armors and Penetrators areused to test and assess the Blue Teamhardware. Members of the Blue Teamsdo not learn what the threat will be untilabout a month before competitive test-ing.

The roles of ATAC are, first, to stim-ulate the entire process by transferringtechnology from Los Alamos to indus-try and, second, for the Laboratory toserve as a neutral referee in the compet-itive stages. Specifically, ATAC helpsthe Blue Team members develop betterproducts, and then it tests those productsagainst the threat created by the RedDesign Bureau. Much of the help pro-vided by ATAC comes from two majorareas of Los Alamos research: mater-ials science (see “Armor/Anti-Armor—Materials by Design”) and computa-tional codes (see “Modeling Armor Pen-

Los Alamos Science Summer 1989

Armor/Anti-Armor

General Research Corporation, Nuclear Metals, Inc., Aerojet Ordnance Co., AAI Corporation, FMC Corporation, Physics International Company

etration”). ATAC is also responsiblefor ensuring that testing of Blue Teamproducts is performed and evaluated ac-curately, thoroughly, and without bias.Our evaluations include recommenda-tions for future funding of promisingtechnologies.

ATAC’S effectiveness in the Armor/Anti-Armor Program requires that LosAlamos be at the technical forefront ofarmor and anti-armor research. ThusATAC invests a significant portion ofits budget in research on materials sci-ence and technology, development ofdiagnostic techniques, and computa-tional research. In addition, Los Alamoshas contracted a number of consultantsand universities for help in disciplinesoutside the expertise of Los Alamos sci-entists.

A Scientific Challenge

The basic challenge in armor/anti-armor research and development is tounderstand each problem from soundphysical principles and the appropriatematerials properties. For example, ourthinking about the usefulness of ceram-ics as armor material changed drasticallywith an increase in our understandingof ceramic material properties and howceramics react physically to ballistic im-pact.

One of the most efficient ways forarmor to defeat a projectile is to turnmost of the kinetic energy back into de-struction of the projectile itself, say byfracture or plastic flow in the penetra-tor. Ceramics were considered possibleas armor material because they gen-erally have high compressive strengthand are lightweight, two qualities de-sirable for mobile weapons systemsthat need tough, light armor. Unfortu-nately, ceramics also tend to be brittleand break up easily on impact, which, itwas thought, would undermine the ma-terial’s ability to destroy the penetrator.When ceramic materials were tested,

however, they appeared much more ef-ficient than expected. In truth, no oneunderstood how the armor worked.

Eventually it was proposed that thebreakup created a mass of hard, abrasiveceramic chips that eroded the penetrator.Detailed computational and materialsresearch at both Los Alamos and Liv-ermore confirmed this hypothesis andmade us realize how to turn the appar-ent disadvantage to our favor. The goalwas then to keep fractured ceramic infront of the penetrator as long as possi-ble, causing the rod to erode as it forcedits way through the rubble. As a result,how the material was packaged becameas important as its strength and fracturecharacteristics.

Only by examining defeat mecha-nisms in this detailed way can scientistsoptimize both the material character-istics (abrasive chips) and product de-sign (how to confine those chips) toenhance the desired effects. (See both“Armor/Anti-Armor-Materials by De-sign” and “Studying Ceramic Armorwith PHERMEX” for a fuller discussionof ceramic armors.)

The same challenge of’ understandingthe physics of the ballistic event, thematerials involved, and the effect ofproduct design is true for Penetrators.An example is the metal liner used inchemical-energy weapons. The detona-tion of a shaped charge moves this linerat the target in such a way that the ma-terial transforms into a high-velocity jetof solid stretching material. To be ef-fective, the liner must have the propercombination of strength and ductility toallow it to stretch without breaking.

If one is to achieve a desired linerperformance, criteria—such as the ma-terial density, the tip velocity, and thecoherency of the jet—must first be de-termined. Then the key elements neces-sary to those criteria must be identified.For instance, work at Los Alamos hasshown that selection of liner material,design of the charge, and the crystalline

microstructure of the liner are criticallyimportant. Additionally, we have foundthat a specific “heat and beat” fabrica-tion process for each material has tobe followed to achieve the preferredcrystalline microstructure. We had toexplore a variety of these processesto select the correct one—an expen-sive, time-consuming task if done solelyon an experimental basis. We hope toshorten the task considerably by us-ing a computer model to predict linerperformance based upon various crys-talline microstructure. We will makethis information and the modeling codeavailable to others, including Blue Teamcontractors. In fact, industry has beenbriefed on the preliminary work.

We are hoping that the same processevolves for kinetic-energy Penetrators,which several Blue Team contractorshave undertaken to explore. At present,the idea that dominates development ofkinetic-energy Penetrators is simply thatthe heavier the penetrator and the fasterit hits the target, the better. However,Blue Team contractors are asking them-selves several questions: What mechan-ical properties should be considered?Does fracture toughness give the pene-trator its ultimate strength? What effectdoes chemical composition have on thisstrength? In other words, a better under-standing of the physics of high-velocityimpact needs to be acquired. This is thetype of challenge facing the Blue Team.

Current Research

Further advances in the develop-ment of the chemical-energy warheadis a tactically urgent problem that theArmor/Anti-Armor Program is currentlytackling with much vigor. Before re-active and spaced armors were intro-duced, chemical-energy weapons had anumber of clear-cut advantages. For in-stance, the chemical-energy penetratoris better at penetrating steel armor than

continued on page 56

I.os Alamos Science Summer 1989 53

A w e are behind the Sovietsin both armor and bullets.That simple declarative

Comment sentence is what makes the ratificationof the Intermediate-range Nuclear Force(INF) treaty a provocative action. It is

bvGeneralStarry

the raison d’etre for the new national in-terest in armor and anti-armor technolo-gies. And it was the principal finding ofthe 1985 Defense Science Board TaskForce on Armor/Anti-Armor, of which Iwas the chairman.

“Our Task Force study reported that,in armor and anti-armor systems, theU.S. has been behind the Soviets forperhaps fifteen to twenty years, and weare falling further behind at an alarm-ing rate (see Figure). Back in 1985,we considered the problem as one ‘ap-proaching a matter of national urgency.’Today we have crossed the threshold;the situation is now a matter for urgentnational priority.

“The problem is not a lack of tech-nology or intelligence data. Scientificjournals and other open literature collec-tively provide a fairly substantial bodyof data from which we can determine, atleast by inference, what they are doingin research and development.

“However, over time, we find infor-mation concerning a given technologydeclining in volume or even disappear-ing from their literature, Does this meanthat the Soviets have given up on atechnology? The U.S. has a tendencyto believe so. That may be true, but itis equally possible that they have movedthe technology into full-scale engineer-ing development. Eight to ten yearsmay pass. Then, all too frequently,we identify what we would call a newweapon system on a test track or, insome cases, being issued to the troops—a system that fields the so-called disap-peared technology.

“The Task Force called this decline ofinformation during full-scale engineer-ing the Bathtub of Ignorance, Histori-cally, it has taken us at least five years

to catch up and frequently as long as fif-teen years to apply the same technologyin our fielded systems.

“This is not an indictment of our in-telligence system. We do gather suf-ficient information on which to makefairly reliable estimates. In fact, threeyears ago we had the intelligence com-munity make some estimates of whatwas in the ‘bathtub;’ to no one’s sur-prise, those developments are now be-ginning to appear.

“The flaw, instead, is in our decision-making process. Our system reactspositively only when confronted withhard evidence—a photograph of fieldedequipment—and negatively to an in-telligence community ‘bathtub’ projec-tion. No one in Washington is willingto make a decision until shown a pictureof a fielded system incorporating newtechnology; then there will be all sortsof doomsday and ‘how could this havehappened’ reactions.

“So the first problem our country hasis how we look at the threat. The sec-ond problem is one of technology field-ing. We are fighting against a naturaltendency of laboratory scientists-evenat places like Los Alamos—to keep thetechnology at the workbench too long.Of course, they want to keep improv-ing the capabilities, But if you allowthe scientists more and more time andfunds, you may end up with a wait offive to ten years, an expenditure of mil-lions or billions of dollars, and only amarginal improvement in performance,In other words, a laboratory has no in-centive to get the technology out.

“It is vital to have a decision-makingmechanism to drive the technology offthe workbench and into the field. TheSoviets have such a mechanism: thefive-year planning process. Relentlessly,every five years the Soviets transfertechnology from the bench to the field.We have-no similar system. In fact, theTask Force examined thirty of our tech-nology developments and found at least

Armor/Anti-Armor

60

50

40

of Tanks(thousands

30

20

10

0Year

SOVIET AND U.S. TANK DEVELOPMENT

U.S./SOVIET

1948 1958 1968 1978 1988

* Classified or unknown

T-55 T-62 T-64 T-72 M-1 T-80 FST-I

Crew 4 5.,,,,.,,,,.,,,,,,,,,,,,,,,,,,4 4 4 4 3 3 4 3 ●

Combat Weight 35 41 57 42 *(metric ton)Power/Weight Ratio 14.4 17.5 18.0 14.2 184(horsepower/rnetric ton)

higher . *

Maximum Road Speed 48’ 48 42 48 50 50 80 60 66 9 0 *(kilometer/hour)Main Gun Diameter(millimeter)

100 90 90 105 100 115 125 125 105 125 *

Turret Front ArmorThickness (millimeters) 203 115 203 242 * 280 , *

Soviet tank development outpaces that of the U.S. both in total numbers and in the introduction of modern technology. The Soviets regardthe tank as the primary element of their ground combat power, and Soviet military theory emphaslzes the Importance of the tank in thecombined-arms team. As a result, the Soviets commit a major portion of their resources to their tank industry, achieving an Integrated,evolutionary program of tank development that produces thousands of main battle tanks each year. Long-term improvement can be seenIn all three Soviet armor subsystems-firepower, protection, and mobility. Modern tanks (T-64, T-72, and 1-60) now make up approximatelyforty per cent of the Soviet force in the field. (The information for this figure was compiled by the International Technology Division of theLos Alamos National Laboratory.)

Los Alamos Science Summer 1989

Armor/Anti-Armor

Aluminum Company of America (ALCOA), General Motors Delco Systems Operation, A.R.A.P. Division of California Research and Technology

But the situation started to reverseitself in the mid-to-late 1970s whenthe advent of spaced armor, then ce-ramic laminate armor, and finally re-active armor reduced the effectivenessof chemical-energy weapons. Todaythe combination of spaced or ceramicarmor and reactive appliques has prob-ably made every fielded system using achemical-energy warhead obsolete.

Reactive armor-used first by the Is-raelis in the 1970s and now estimatedto be on half of the Soviet tanks—is aformidable challenge (Fig. 2). It wasdeveloped primarily as a countermea-sure for chemical-energy weapons andconsists of a trilayered sandwich ofmetal, high explosive, and metal. Tile-like boxes of reactive armor are simplybolted to vulnerable areas of a tank out-side its existing armored shell.

Because of the ability of reactive ar-mor to destroy an incoming jet, some

REACTIVE ARMOR

Fig. 2. Reactive armor typically has a layer of

high explosive sandwiched between two lay-

ers of armor plate. When a high-energy jet col-

lides with the armor, the explosive detonates,

pushing the plates into the path of the jet to

deflect or deform it, thereby protecting the in-

ner layer of armor. Ideally, the reactive armor

plate will be moving at an angle to the path of

the jet, forcing it to drill a slot rather than a

hole through the plate and giving the plate a

larger effective thickness.

people in the weapons community fearthat fielded chemical-energy warheadsare now obsolete. However, prelimi-nary tests of some new chemical-energywarheads developed by Blue Team con-tractors show promising results. Signifi-cant improvements in performance seemattainable with existing technology. Un-fortunately, we cannot give more detailson these developments due to the pro-prietary nature of the new designs.

We can, however, talk about onepromising concept—tandem chemi-cal-energy warheads (Fig. 3). In theseweapons a small charge at the front ofthe warhead fires to activate the tank’sreactive armor. A time delay allows thereactive-armor plates to move out of theway, then a large shaped charge furtherback in the warhead fires a metal jet todefeat the base armor. Weight reductiontechnologies, various time delays be-tween the firing of the two charges, andinnovative designs and materials for theshaped-charge liners are currently beingdeveloped. Blast shields between theprecursor and the main charge are beingoptimized, and the most effective stand-off distance- the distance to the shapedcharge at the instant of its detonation—is being determined. Tandem chemical-energy warheads seem likely to play animportant role in the defeat of reactivearmor.

The national Armor/Anti-Armor Pro-gram has also taken up the challenges ofnew composite materials and advancedprocessing techniques. To achieve thelow weight and elevated mechanicalproperties needed for armors, the idealmaterial may actually be a compositeof ceramic and high-strength reinforce-ments. Ceramics possess low density,high hardness, dilatancy (the tendencyto expand when fractured), and highshear strength—all good properties forarmor-but they lack fracture tough-ness, the ability to resist crack propa-gation, which for some purposes, suchas multiple-hit resistance, may be im-

portant. To improve fracture toughnesswithout sacrificing the other properties,we are investigating the use of single-crystal whiskers or platelets as a rein-forcement in ceramics. Tailoring mate-rial properties for specific applications isone of the challenges of armor develop-ment.

Interface With Industry

How is Los Alamos helping industryto meet the challenges of the Armor/An-ti-Armor Program’? A recent incidentillustrates how ATAC’s presence at LosAlamos allows the program to tap theLaboratory’s experience and developedtechnology. Los Alamos has alwaysperformed nondestructive inspectionsof every nuclear weapon system tested.It was thus only natural to perform thesame tests on the chemical-energy war-heads sent to us by Blue Team contrac-tors. The evaluations revealed hereto-fore undetected cracks and voids insome of the warheads that could haveaffected performance. We informedthe contractors of the problems so theycould make substitutions, thus ensuringthat all participants had a fair chance inthe competition.

ATAC also provides direct help insolving contractor’s problems. Ourprogram managers, test directors, hy -drocode developers, and materials sci-entists deal on a one-to-one basis withour industrial counterparts. For exam-ple, last summer we worked with thecompany that produces the Joint Ser-vices hypervelocity missile to help solvea control problem. The missile con-sists of a large kinetic-energy penetra-tor mounted in the missile’s warhead.Although the warhead is very heavy,the missile is long range and able tomove fast— 1.7 kilometers per second.Unfortunately, the aluminum tins onthe missile-critical to its stability andcontrol-melted during flight. We even-tually solved the problem by suggesting

Los Alamos Science Summer 1989 57

A TANDEM WARHEAD

Fig. 3. One concept for defeating reactive ar-

mor is to use a chemical-energy warhead with

two explosive charges rather than one. As the

warhead approaches the armor (1 through 3)

the small explosive charge at the front of thewarhead is detonated. The resulting impact

activates the explosive in the reactive armor (4

through 6), causing the plates of armor to be

blown away. Later, the large explosive charge

at the rear of the warhead fires (7). However,

the angle of the armor and the gap between

the warhead’s initial and final charges causes

the plates to miss the penetrating jet formed

by the detonation of this second charge.

Armor/Anti-Armor

California Research & Technology, Kaman Sciences, Inc., Lawrence Livermore National Laboratory, Radkowski Associates, General Dynamics

both a particular ceramic coating and anapplication technique for the coating.

ATAC Facilities

ATAC, in its role of testing and eval-uating the competing antitank systemsand armors, uses Laboratory expertise,technologies, and capabilities. For ex-ample, we are testing chemical-energywarheads using the 1000-foot monorailrocket sled track at Los Alamos (Fig. 4).The sled can reach Mach-1 speeds, andthe track can be extended to 2000 feetif higher speeds become necessary. Thesled track is very useful for carryingout realistic tests of tandem-warheaddesigns. Most tandem designs have asignificant time delay between firing thefirst and second warheads, during whichtime the second warhead moves consid-erably. The rocket sled can be used totest the effect of missile motion underprecisely controlled conditions.

We are also building a new intenseflash x-ray machine next to the sledtrack, This machine should allow ar-mor and anti-armor developers to “see”the penetration process through eightinches of steel (Fig. 5). The sourcein this system will operate at 8 to 10mega-electron-volts (MeV) and generatean x-ray dose greater than 500 roent-gens at 1 meter. This device will easilytrack both kinetic-energy Penetratorsand chemical-energy jets well inside thetargets.

Los Alamos also designed and helpeddevelop a state-of-the-art test range inSocorro, New Mexico, at the TerminalEffects Research and Analysis (TERA)branch of the New Mexico Instituteof Mining and Technology. The rangeoccupies 1 square mile and features ahighly instrumented target area that fol-lows the incoming trajectory and targetresponse optically. A continuous recordof the test is provided by an advancedvideo system that operates at speeds upto 2000 frames per second. All data are

Los Alamos Science Summer 1989

ROCKET SLED

Fig. 4. A chemical-energy warhead is shown being tested on the rocket sled at Los Alamos.Mounted on a 1000-foot monorail, the sled can reach Mach-1 speeds and allows scientists to testwarheads under controlled but realistic conditions.

FLASH X-RAY MACHINE

Fig. 5. A flash x-ray machine being constructed at Los Alamos capable of “viewing” ballisticevents through eight inches of steel. When the 8 to 10 million electron volts of energy stored inthe Marx generator are released into the diode envelope, electrons accelerated from the cathodeto the anode will generate an x-ray fluence greater than 500 roentgens 1 meter from the end of

logged and processed automatically bycomputers, which allows Los Alamospersonnel to control the firing timesof munitions and to measure time andspace information relative to the arrivalof the devices. Technical calculationscan be done directly from the video.There is also a four-camera. single-image system with a ten-nanosecondshutter speed. The multiple cameras,combined with electronic imaging, per-mit Los Alamos personnel to locate theposition of munitions within the targetarea and not interfere with the muni-tions control and guidance equipment.

ATAC built the range at TERA becausesufficient land was not available at LosAlamos for safe firing of full-size livemiss iles and projectiles.

A variety of other diagnostic ca-pabilities are available for gatheringthe maximum data from each test per-formed at Los Alamos, These includefour portable 2-MeV x-ray systems,twelve 450-keV flash x-ray machines.five rotating-mirror streak cameras withwriting speeds of 20 millimeters per mi-crosecond. four image-intensifier cam-eras with X)-nanosecond shutter times,a laser velocitmeter and a microwave

59

Armor/Anti-Armor

General Electric Company, Ordnance Systems Division, Science & Technology Associates, New Mexico Institute of Mining & Technology

Fig. 6. One of the goals of the national Armor/Anti-Armor program is to establish a useful flow ofinformation between the military, industry, and various research institutions. The interactions ofATAC with this network continue beyond the research phase into the testing, development, andperhaps even production phases of the weapon systems.

velocimeter to record transit velocities,time-interval meters with nanosecondresolution, 200-megahertz analog-to-digital signal converters, and a widerange of more conventional instrumen-tation, We also can use PHERMEX,a 30-MeV flash x-ray machine, andECTOR, a 3-MeV machine. Both ofthe machines have access to impressivedigital-enhancement capabilities for flashradiographs, and they allow us to deter-mine the internal structure of anti-armorand armor devices at the time of impact(see “Studying Ceramic Armor withPHERMEX”),

An Evolving Process

At ATAC we can see a new processevolving among industry. the military,and the Laboratory in which a naturalinterplay of needs, research, testing, pro-totyping, evaluating, developing, andprocuring guides the development ofarmor and anti-armor systems (Fig. 6).To illustrate how the process works,consider the case of ceramic-filled poly-mer armor (described in “Armor/Anti-

Armor-Materials by Design”). Whenthe military told the Laboratory aboutthe need for a less expensive ceramicarmor, our materials scientists testedvarious ideas and developed a processfor fabricating a less expensive butequally effective ceramic armor. Wethen initiated transfer of the technolog-ical concepts to industry, and AlliedSignal used the ideas to develop theirown armor package. Allied Signal isnow busy producing prototypes of thearmor, which they will submit for test-ing and evaluation at ATAC. If that ar-mor is successful in the competition, itwill eventually become a new productavailable for military use.

The true value of the national Armor/Anti-Armor Program may not lie in asimple leapfrogging of Soviet armor andbullets by U.S. technology. Rather itmay lie in the way this uniquely struc-tured program has opened fresh inter-actions between our nation’s military,industries, laboratories. and universitiesthat will allow us to constantly maintainan edge over the Soviets. ■

Richard Mah is the director of ATAC and a formcr

group leader in the Materials Science Division al

Los Alamos. Prior to coming to the Laboratory, heworked as a senior research engineer at Dow Chem-

ical and C. F. Braun & Co. Hc holds dcgrccs in

theoretical and applied mechanics and metallurgical

cngineering from the Universiy of Illinois and haspublished over twenty papers on materials science

topics.

Phyllis Martell is the technical writer and public re-

lations manager for ATAC at Los .Alamos and pub-

lishcs a newsletter, The BULLET, on Armor/Anti-

Armor Program activities. Before joining the Lab-

oralory, she was the public information officer for

the Los Alamos School District. She holds a de-

gree in French and English from the University ofWisconsin.

60 Los Alamos Science Summer 1989

Armor/Anti-Armor

StudyingCeramicArmor with

by Ed Cort

The ballistic impact of penetratoragainst armor is a brief momentof violence and shock hidden in a

confusion of smoke and debris (Fig. l,).If we are to learn what material prop-erties are relevant to the outcome, wemust pierce the veil and freeze in placethe key aspects of this event. Large x-ray machines are ideally suited to thistask. A short flash of intense x-radiation Fig. 1, Live fire test of the M1A1 Abrams tank at Aberdeen Proving Ground. (Photograph taken by

can penetrate the debris and armor and U.S. Army Combat Systems Teat Activity and provided to Los Alamos by the U.S. Army Ballisticetch an instantaneous image of deforma- Research Laboratory.)

tion and material flow.We are currently using an x-ray ma-

chine called PHERMEX (Fig. 2) tostudy the internal structure of ceramicarmor during impact with both penetrat-ing jets from chemical-energy weaponsand long-rod kinetic-energy penetra-tors. The machine uses a 30-MeV high-current linear accelerator to generate very intense but short-duration bursts of x rays from a thin tungsten target. Al-though built in the early 1960s, PHER-MEX is still unequaled at producing high-resolution radiographs of large, fast. objects. We are particularly interested inusing PHERMEX to study ceramic ar-mor because the mechanisms by which

mors. We have only recently begun to penetrators (fired from the gun at the right).

Los Alamos Science Summer 1989 6 1

Armor/Anti-Armor

,’

,,

,,

,. ,,

undmtand what tiwse differences are. of four to six nominally identical shotsThe lqqg~f@~ti~a@r pi~mes a W- produces a time-resolved penetration

~et,”~W@@@cor Q~rwiae, by history for one ceramic material and onedepoiiiti&hr&~ amounts df kiinetic ,en- ef set of engagement conditions (velocity,e~, in ~:,~~rt~ed region,” The *, obliquity, and yaw) in one plane. In fu-Whicll ~itt’ & $i$@&qj ‘~ a“right Cir” ture tests, we hope to flash PHERMEX

Our current test series ranges overthree ceramic materials (boron carbide,aluminum oxide, and titanium diboride),two impact velocities, two obliquities,a number of confinement geometries,and both kinetic-energy rods and jetsfrom chemical-energy weapons. Wealso look at the flight characteristicsof the penetrator (velocity, yaw in twoorthogonal planes, rate of change ofyaw, and fiducial time at impact).

We are modeling the tests with exist-ing hydrocode models (see “ModeiingArmor Penetration”). The code predictsthat because the ceramic is relatively in-compressible, even when fractured, andbecause there is no free volume for therubble to expand into except the pene-tration hole itself, the ceramic defeatsthe penetrator. Although the predic-tions of ‘the model are reasonably closeto actual events (Fig. 4), our materialmodel for the ceramic, at the moment, isbased more on experimental data fromprior tests rather than on principles ofphysics. Consequently, if the rod’s ve-locity, say, were to change significantly,we would not be able to extrapolatewith confi~ence.

At the end of our current series ofttppr@xirnat63y thirty shots, an advi-SO~ panpl ‘{f ex@@~ ‘will review the~@ts and,itel~ inkttp~t @ data. How-tiv~r, preliminary rkk@s confirm that$%@tsncy (the teni%h$y of the fracturedmamic k sxpand) is an important gen-@# femti~ for the c@ft3at of jets fired*W e~@i@*!@ ~e-. ~ this*~~@ *&*~,~*&~fills meitiqxict hole, constantly’fmcing the jetto penetrate new material, and, as the

Armor/Anti-Armor

CERAMIC PENETRATION

Fig. 4. (a) A PHERMEX radiograph of a tung-

sten-alloy penetrator colliding with the eeramlc

target of Fig. 3. (b) The same event at the

same moment in time es simulated with the

HULL hydrocodes. (See “Modeling Armor Pen-

etration” for a dlscussion of the hydrocodes.)The iight blue areas in the computer simuia-

tion are regions of faiied ceramic that do not

appear in the radiograph because of Iack of

resolution and the tight conflnement of the ce-

ramic by the target holder.

rubble flows from the impact hole, itpushes inward and attacks the jet fromthe sides. In the case of long-rod pen-etrators, material flows out the hole butdoes not appear to attack the sides ofthe penetrator as it goes.

From these experiments, we shouldobtain radiographs of the dilatancymechanism in action and accurate ma-terials data on such things as the hard-ness of the ceramic, One of the mainpoints of the tests is to accumulate moreaccurate experimental data to validatecode-modeling parameters for armor andanti-armor designers.

Although the PHERMEX experimentsprovide valuable data, several funda-mental questions about the dynamic be-havior of ceramic armor are more easilyaddressed in laboratory experiments.One question concerns the sequence ofevents-does fracture occur at the rearof the ceramic (Fig. 4) during the pas-sage of the initial shock wave or lateras the penetrator forces its way throughthe material? In addition, scientists mustdetermine what factors dictate the sizeand shape of the individual fracturedparticles and then understand how tomodel penetration of the resulting pul-verized material.

To address such questions, Los Ala-mos scientists have designed two exper-iments that complement the PHERMEX

Los Alamos Science Summer 1989

Armor/Anti-Armor

Modeling

by Ed Cort

A rmor and anti-armor technologyis becoming increasingly com-

ers to rely more and more on computermodeling. For years, computer simula-tions of armor penetration contributedonly modestly to armor developmentcompared, say, to the role of computa-tional fluid dynamics in the aircraft andaerospace industries. However, com-puter modeling is becoming a major toolin the study of armor-penetrator inter-actions by offering weapons designersa number of distinct advantages in theirquest of an essential understanding ofthe processes.

For example, the destruction and thespeed of ballistic penetration make ex-perimental diagnostics expensive, dif-ficult to interpret, and, in many cases,impossible to gather. In comparison,a computer simulation, when bench-marked against even limited test data,can “replay” the experiment in slowmotion. Computer modeling can alsoresolve velocity and stress and straincomponents in the target and penetratorin fine detail and pinpoint the relativeinteraction between armor components.

The role of penetrator velocity, platespacing in multilayered armor, and yaw(the angle of the penetrator’s axis withrespect to its velocity vector) can be as-sessed easily, and armor designers cantest their understanding and arrive atnew insights by changing and optimiz-ing such parameters. The results of acomputation. done before the experimentcan be used to guide test design by an-swering questions about the most advan-tageous locations for the instruments,the proper scale ranges for recordingdata, and the important experimental

variables.The goals of the computational re-

search being carried out under ATAC’sdirection are to validate and benchmarkcodes and methods, to pinpoint areas ofneeded research, and to improve exist-ing codes-especially the ability to dealwith a three-dimensional modeling ofimpact and penetration.

The hydrocodes used in the simu-lations are grounded in classical con-tinuum mechanics, which attempts todescribe the dynamics with a set of dif-ferential equations bed on the con-servation of mass, momentum, and en-ergy. An equation of state relates thematerial’s density, internal energy, andpressure. Finally, a constitutive equationdescribes the stress-strain relationshipin the material and reflects changes in,,, ,the properties of the material, such aswork hardening that result from severedistortion. In fact, there is a frequent,,need to model the material after it hasfailed, a need that may sometimes dis-tort the usual assumptions of continuummechanics beyond simple extrapolation.

From a practical point of view, theideal design code should have a user in-terface that allows problems to be setup conveniently, standardized materialmodels and properties that can be ex-panded or modified easily, and powerfulgraphics and post-processing that candepict results quickly and in a man-ner that is easy to interpret. The code

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should be accurate in the physics andmaterial behavior it intends to modelas well as in the numerical implemen-tation and programming that translateequations into code. The code mustbe adaptable to a wide variety of prob-lems, efficient in memory use and run-ning time (although, here, the defini-tion of what is unacceptable constantlychanges), and robust enough that thecode does not fail when it encounters anunexpected situation.

The bulk of the computer codes usedon a production basis fall into two cat-egories: Eulerian and Lagrangian. Sim-ply stated, Eulerian methods move thematerial through a fixed mesh as theproblem progresses whereas Lagrangianmethods have a computational grid at-tached to the material that distorts withmovement of the material. (Euleriancodes are frequently used in fluid dy-namics whereas Lagrangian methods aremore often used in structural analysis,)Each method has its peculiar advantagesand disadvantages. For instance, La-grangian methods tend to be faster, canimplement sophisticated material mod-els more easily, are efficient with largeproblems, and treat material interfacesaccurately. However, they also deal in-accurately with large shear flows, aremore complex to set up, and are not ro-bust with large distortions such as thosethat occur when armor penetration issignificant. Eulerian methods are almosta mirror image of Lagrangian methodssince they are robust, easy to set up,and capable of handling large shears anddistortions. On the other hand, Eule-rian codes tend to be less accurate in thetreatment of material interfaces, ineffi-cient in the use of computer memory,difficult to implement with more sophis-ticated material models, and generallyslower in running.

In our work at Los Alamos, we firstexplored an existing three-dimensionalEulerian code, HULL. We wanted totest its ability to accurately predict pene-

tration of spaced armor, which has mul-tiple layers of armor separated by gapsand set at oblique angles to the penetra-tor’s line of flight. The intent of sucha configuration is for the obliquity ofthe plates to deflect, bend, or break thelong rod so that later plates can stop theresidual pieces more easily. Reactivearmor is another type of multilayered ar-mor that also attempts to interfere withthe rod’s trajectory. In this case yaw iscreated on impact when a layer of ex-plosive ignites, shoving a plate of armortoward the penetrator to knock it askew.

A computer simulation of the pene-tration of spaced armor plate will be re-alistic only if the code deals accuratelywith (1) the erosion of the front of therod as it penetrates a plate, (2) the lossin velocity of the residual rod, (3) anychanges in the orientation of the rod,and (4) the yielding and failure in theplate. We tested the ability of HULL tomodel armor penetration accurately byhaving it simulate a set of experimentscarried out in the late 1970s using thePHERMEX machine. In these experi-ments, long-rod uranium-alloy Penetra-tors impacted steel-alloy plates set atvarious angles to the flight of the rod.Comparison of a PHERMEX radiographand the corresponding computer simu-lation (Figure) illustrates how well thecode predicted the interaction betweenpenetrator and target.

These benchmark experiments gaveus confidence that the code had thepotential to provide useful informa-tion about similar experiments withmore complex targets, such as ceramics,whose interaction with the penetratorwas more difficult to model, But a com-putation of this type pushed HULL tothe limit of its capability—it had a run-ning time in a CRAY X-MP computerof 11 hours, and the computer mem-ory would not hold enough informationto model a second target plate with anintervening space. Even if larger com-puter memories were available, realistic

targets-up to 10 times as thick as thepreliminary example—would requireconsiderably more computer time tomodel. Our evaluation was that HULLis a useful but limited code.

The evaluation, coupled with manyother code comparisons, motivated us todevelop a new three-dimensional codedesigned specifically for simulationsof armor and anti-armor systems. Thecode, called MESA, is Eulerian andtreats hydrodynamic flow and the dy-namic deformation of solid materials.Because it uses state-of-the-art numer-ical methods, it runs faster and is lessaffected by spurious numerical problemsthan existing Eulerian codes. The ver-sion of MESA now being tested incor-porates several of the standard strengthmodels that take into account both theelastic and the plastic regions of thestress-strain relationship of the materi-als. There is also a programmed-bummodel for the explosives. We havedeveloped ‘a number of such models,which should increase our ability tosimulate a variety of interactions formodern armor systems. In future ver-sions of MESA we will include moreadvanced materials models.

One such model, called the Mechan-ical Threshold Stress model, will incor-porate the physical deformation mech-anisms needed to simulate conditionsnot easily achieved in the laboratorybut important to this type of research.Specifically, the model will allow us toextrapolate better into regimes of highdeformation rate, high temperature, andlarge amounts of strain. The model sep-arates the kinetics of strain hardening(that is, dependencies on temperatureand strain rate) from the kinetics relatedto the strength at a given instant. So farwe have demonstrated the model onlyfor certain well-characterized metal-lic systems, but we are extending it tothe more complicated materials usedin armor and anti-armor applications.We also hope to combine the defor-

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Table

Current application of MESA (funded by the Department of Defense and theArmy Missile Command as well as DARPA, the Defense Advanced ResearchProject Agency).

Los Alamos Science Summer 1989 67

!

This report was prepared as an account of worksponsored by the United States Government. Nei-ther the United States Government, nor the UnitedStates Department of Energy, nor any of their em-ployees makes any warranty, expressed or implied,or assumes any legal liability or responsibility forthe accuracy, completeness, or usefulness of any in-formation, apparatus, product, or process disclosed,or represents that its use would not infringe privatelyowned rights. Reference herein to any specific com-merical product, process, or service by trade name,mark, manufacturer, or otherwise, does not neces-sarily constitute or imply its endorsement, recom-mendation, or favoring by the United States Gover-nment or any agency thereof. The views and opinionsof authors expressed herein do not necessarily stateor reflect those of the United States Government orany agency thereof.

Los Alamos National Laboratory is an Equal Op-portunity Employer conducting a variety of nationaldefense programs and energy-related programs, aswell as a broad spectrum of basic scientific re-search. For scientific and technical employment op-portunities, individuals and academic placement of-ficers are encouraged to contact the Personnel Divi-sion Leader, Los Alamos National Laboratory, LosAlamos, New Mexico, 87545.

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