Dynamic behaviour of ceramic armour systems

47 Pages·2015·4.32 MB·English

FFI-rapport 2015/01485
Dynamic behaviour of ceramic armour systems
Dennis B. Rahbek and Bernt B. Johnsen
Forsvarets
FFI
forskningsinstitutt
Norwegian Defence Research Establishment

FFI-rapport 2015/01485

Dynamic behaviour of ceramic armour systems
Dennis B. Rahbek and Bernt B. Johnsen
Norwegian Defence Research Establishment (FFI)
10 November 2015
FFI-rapport 2015/01485
1301

P: ISBN 978-82-464-2608-2
E: ISBN 978-82-464-2609-9

Keywords
Ballistikk
Beskyttelse
Keramikk
Materialteknologi
Mekanismer

Approved by
Rune Lausund Research Manager
Svein Rollvik Director of Research
Jon E. Skjervold Director

2 FFI-rapport 2015/01485
English summary
Protection against high-velocity impact from objects such as projectiles is of major concern for
bothmilitary and civilian purposes. Lightweight body armour systems for ballistic protection of
personnel can be designed by combining different materials with different properties. These
hybrid systems typically have a hard ceramic strike face that blunts, fractures and erodes the
projectile, and a soft backing, made from ballistic fibres, that absorbs the residual energy.

FFI has for many years been involved in the development of, and research on, body armour for
protection of military personnel. In the period from 2004 to 2007, the technology behind an
armour plate was developed in cooperation with industry. For future developments, however, it is
necessary to have knowledge about the mechanisms that are acting under ballistic impact. Today,
this topic is studied in several projects at FFI, and this report was written as a part of that work.

The most common ceramics for armour purposes are - in order of improved performance -
aluminium oxide, silicon carbide and boron carbide. The ceramics generally have relatively low
density and high hardness. These, and other properties, make them useful in armour applications.
The mechanisms by which a ceramic armour fails is quite complex, and involve failure
mechanisms such as radial cracking, cone cracking and comminution due to micro-cracking. To
what extent they appear and the relative timing of these mechanisms, depends on factors such as
the impact velocity, the ceramic and projectile properties, and the dimensions of the ceramic and
projectile.

To improve the ballistic performance of the ceramic armour system, it is desirable to delay the
failure of the ceramic for as long as possible. This allows more time for deformation and erosion
of the projectile. It has been shown in the literature, and by work conducted at FFI, that the
ballistic performance, both in terms of single-hit and multi-hit capacity, can be improved by
radially confining the ceramic tile, by covering it with a sheet of another material, and/or by
tuning the interfacial strength between the cover and the ceramic.

The main aim of this report is to give an overview of the mechanisms that are involved in
ballistic, or dynamic, failure of ceramics and ceramic-based hybrid armour. The failure
mechanisms, the factors that govern the failure mechanisms, and what can be done to delay the
failure of the ceramic, are discussed on the basis of some of the available literature on the topics.

FFI-rapport 2015/01485 3
Sammendrag
Beskyttelse mot anslag fra blant annet prosjektiler ved høy hastighet er et viktig tema innen både
militære og sivile anvendelser. Lettvektsløsninger for ballistisk beskyttelse av personell kan
designes ved å kombinere ulike materialer med ulike egenskaper. Disse hybridsystemene har
typisk en hard plate av keramikk som avrunder, bryter opp og eroderer prosjektilet, samt en myk
’backing’ laget av ballistiske fibre som absorberer den gjenværende energien.

FFI har i mange år vært involvert i utvikling av og forskning på ballistisk beskyttelse for militært
personell. I perioden 2004 til 2007 ble teknologien bak en beskyttelsesplate for soldater utviklet i
samarbeid med norsk industri. For videre utvikling av teknologien er det imidlertid nødvendig
med en bedre forståelse av hvilke mekanismer som finner sted ved ballistiske anslag. Dette er et
tema som for tiden studeres i flere prosjekter ved FFI, og denne rapporten ble skrevet som en del
av dette arbeidet.

Keramikk har generelt lav tetthet og høy hardhet. Disse, og andre, egenskaper gjør materialet
anvendbart for ballistisk beskyttelse. De vanligste keramikktypene som er benyttet i
beskyttelsesløsninger er – i stigende rekkefølge med tanke på økt ytelse - aluminiumoksid,
silisiumkarbid og borkarbid. Mekanismene som keramikk bryter sammen med ved høyhastighets
anslag er relativt komplekse. De involverer feilmekanismer som for eksempel dannelse av radielle
sprekker, dannelse av en kjegle gjennom keramikkplaten og fragmentdannelse som følge av
mikrosprekker under anslagspunktet. Hvor mye hver enkelt mekanisme bidrar, og i hvilken
rekkefølge de opptrer, avhenger av faktorer som anslagshastigheten, egenskapene til keramikken
og prosjektilet, samt dimensjonene til keramikken og prosjektilet.

For å bedre ytelsen til et ballistisk beskyttelsessystem basert på keramikk, er det ønskelig å
forsinke nedbrytningen av keramikken så lenge som mulig. Dette gir lenger tid til deformasjon og
erosjon av prosjektilet. Det har blitt rapportert i litteraturen, og erfart gjennom arbeid utført ved
FFI, at ulike metoder kan benyttes for å bedre den ballistiske ytelsen. Egenskapene ved ett eller
flere anslag kan bedres ved å legge et annet materiale (for eksempel et fiberkompositt) rundt
kanten av keramikken, ved å dekke keramikken med et lag av et annet materiale, og/eller ved å
endre heften i grenseflaten mellom dette laget og keramikken.

Hovedmålet med denne rapporten er å gi en oversikt over mekanismene som er involvert i
nedbrytning av keramikk, og beskyttelsesløsninger basert på keramikk, ved ballistiske anslag (det
vil si ved dynamiske påvirkninger). Feilmekanismene, hvilke faktorer som kontrollerer
feilmekanismene, og hva som kan gjøres for å forsinke nedbrytningen av keramikken, blir
gjennomgått med grunnlag i utvalgt litteratur om disse temaene.

4 FFI-rapport 2015/01485
Contents
List of abbreviations 6
1 Introduction 7
2 Armour ceramics 9
2.1 Materialparameters 9
2.2 Ceramicmicrostructure 10
2.3 Commonlyusedarmourceramics 11
3 Methods for ballistic testing 14
3.1 Completearmoursystems 14
3.2 Armourcomponents 15
4 Failure mechanisms of ceramics 19
4.1 Dynamicfailure 19
4.2 Quasi-staticfailure 22
4.3 Damageevolution 22
5 Confinement and covering of ceramics 25
5.1 Coveringoftheceramic 25
5.2 Longrodpenetrationinthickceramicdiscs 26
5.3 Impactwithregularprojectiles 28
5.4 Coveringwithfibre-composites 30
6 Effect of adhesive and interfacial bonding 36
6.1 Adhesionandbondlinethickness 36
6.2 Multi-hit 37
6.3 Surfacetreatment 38
6.4 Interlayer 38
7 Conclusion 39
Bibliography 41
FFI-rapport2015/01485 5
List of abbreviations
Al O aluminiumoxide.
2 3
AP armourpiercing.
B C boroncarbide.
4
BEV ballisticefficiencyvalue.
DOP depthofpenetration.
FEM finiteelementmethod.
FFI Forsvaretsforskningsinstitutt.
FOI Totalfo¨rsvaretsforskningsinstitut.
HP hotpressed.
HV Vickershardness.
PC polycarbonate.
PU polyurethane.
RB reactionbonded.
SAPI smallarmsprotectiveinsert.
SiC siliconcarbide.
SPS sparkplasmasintering.
TNO theNetherlandsOrganisationforAppliedScientificResearch.
UD uni-directional.
UHMWPE ultra-highmolecularweightpolyethylene.
WC tungstencarbide.
6 FFI-rapport2015/01485
1 Introduction
Protection against high-velocity impact from objects such as projectiles is a major issue in many
militaryandalsocivilianapplications. Inmanycases,hybridarmoursystemsconsistingofahard
ceramic strike face and a comparatively soft metal or composite backing are used for ballistic
protectioninbothvehiclesandbodyarmour. Theroleoftheceramicistoerodeandbluntthehardtip
oftheprojectile,whiletheroleofthebackingmaterialistoabsorbtheresidualenergyoftheeroded
projectileandthefragmentsofthefracturedceramic. Oneoftheadvantagesofemployingceramics
in hybrid armour, compared to materials such as steel and aluminium, is that more lightweight
systemscanbedesigned. Theloadonthevehicleorthesoldierisreducedandthemobilityisthereby
improved.
Forsvarets forskningsinstitutt (FFI) has for many years been involved in the development of, and
research on, lightweight body armour for ballistic protection of military personnel. In the period
from2004to2007,thetechnologyforaSAPIplatewasdevelopedinacooperationbetweenFFI
andindustry. Thistechnologyisnowusedinarmoursystemsthatareemployedbyarmedforces
bothnationallyandinternationally. Forfuturedevelopmentsofbodyarmour,itisnecessarytohave
knowledgeaboutthemechanismsthatareactingunderballisticimpactofthearmour. Knowledge
about how the ceramics functions as stand-alone materials, and also how ceramic-based armour
systemsfunction,isimportant. Today,thistopicisstudiedinseveralprojectsatFFI,andthisreport
iswrittenasapartofthatwork.
Themainaimofthisreportistogiveanoverviewofthemechanismsthatareinvolvedinballistic
(dynamic)failureofceramicsandceramic-basedhybridarmour. Hence,thereportisnotintendedto
giveacompleteoverviewoftheliteratureinthefieldofballisticfailureofprotectionmaterials,butit
discussessomeimportanttopicswithrespecttothedynamicbehaviourofceramicarmouronthe
basisoftheavailableliterature.
Ballistic protection systems
Depending on scenario, the soldiers role and the mission, protection can be provided by several
meansamongwhichaballisticprotectionsystemcanbeakeypart. Modernsystemsforballistic
protectionofpersonnelismostoftenmodularsincedifferentpartsofthebodyneeddifferentlevels
ofprotection,andsincenotallmissionsrequirethesamelevelofprotection. Forprotectionagainst
fragments and projectiles, the main focus is on protecting the torso and the head, as an injury to
these regions are generally more acute than injuries to other parts of the body. The protection
againstdifferenttypesofthreatsthatonemightencounterisdividedintoprotectionlevelsinballistic
teststandards. Theprotectionlevelisessentiallyrelatedtotheabilitytopreventaprojectilefrom
perforatingthearmour. Inverygeneralterms,abasiclevelofprotectionisavestandahelmetthat
protectsagainstfragmentsandmosthandgunprojectiles. Boththevestandthehelmetareusually
made from sheets of ballistic fibres (polymers), such as aramid and ultra-high molecular weight
polyethylene(UHMWPE).However,theyareproducedbydifferentmethodssothatthehelmetis
stiffwhereasthevestisrelativelyflexible. Toincreasethesizeoftheprotectedarea,protectionto
FFI-rapport2015/01485 7
the crotch might be added (also made from ballistic fibres). Unfortunately, when the threats are
projectiles from rifles, the ballistic fibres do not provide the necessary stopping power and other
materialsareneeded.
Figure1.1 Schematicofthecross-sectionofaSAPIplate. Ahardceramicstrikefaceiscovered
withastiffcompositematerial,oftenmadefromaglassfibre/epoxyfabric. Thecovered
ceramictileisbackedbyseverallayersofaballisticfibrematerial,usuallyaramidor
UHMWPE.Finally,thewholeplateiscoveredinatoughfabricclothtoprotectagainst
UV-lightandwear.
In order to stop armour piercing rifle rounds, a small arms protective insert (SAPI)-plate is often
used in combination with the vest. Modern SAPI-plates are usually made from a combination of
materialssuchasceramicsandballisticfibres/composites. AtypicallayoutofaSAPIplateisseenin
Figure1.1. Thespecificcombinationofmaterials,ceramicandfibres/composites,dependsonthe
typeofthreatstheplatehastobecapableofstopping. ThemaincomponentoftheSAPI-plateisa
thin(approximately8mmthick)tilemadefromaveryhardceramic. Theceramicblunts,erodesor
fracturesprojectiles(evenarmourpiercing)onimpact. Unfortunately,ceramicsareverybrittle,thus
thetilecracksandbreaksintopiecesonimpact. Tokeeptheceramicfromcompletelyshattering,
it is often covered in ballistic fibres or a composite material. Moreover, the ballistic fibres at the
backabsorbthemajorityofthekineticenergyandpreventthedeformedprojectileandtheceramic
fragmentsfromperforatingthearmour.
Outline of the report
Thereportisorganizedinsections. Thesectionscovertopicssuchasageneraldescriptionofarmour
ceramics,relevanttestmethods, failuremechanisms, andtheeffectofconfinement,coveringand
adhesivebondinginceramicarmour. Inthebeginningofeachsection,atextboxsummarisesthe
mainfindingsofthatparticularsection. Mostoftheliteraturethatisdiscussedisonpersonalarmour
systems. However, the materials and the dynamic failure mechanisms are also relevant for other
applications,suchasvehiclesandaircraft.
Inthereport,thereisafocusontheliteraturedealingwithballisticcharacterisation,i.e. dynamic
failure,ofceramic-basedarmourmaterials. Resultsfromquasi-staticcharacterizationarecovered
onlybriefly. Thereasonforthisisthatithasprovenverydifficulttopredicttheballisticbehaviourof
materialsbasedonquasi-staticmaterialproperties. Thus,resultsobtainedfromballistictestingare
muchmorerelevant.
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