Understanding and predicting gun barrel erosion

48 Pages·0.499 MB·English

by  Johnston I.A.

Understanding and Predicting Gun Barrel Erosion
Ian A. Johnston
WeaponsSystems Division
Defence ScienceandTechnologyOrganisation
DSTO(cid:150)TR(cid:150)1757
ABSTRACT
·eAustralianDefenceForcewillsoonhavetocontendwithgunbarrelero-
sionissuesarisingfromtheuseofnewlow-vulnerabilitygunpropellants,the
acquisitionofnewammunitionandgunsystems,andpossiblemodi(cid:12)cations
toexistingpropellingchargedesigns. Acritical,technicalreviewofadvances
in gun barrel erosion research, mitigation, and assessment over the last (cid:12)f-
teenyearsispresented. Knownandpostulatederosionmechanisms,obtained
through recent experimental and numerical modelling work, are described
and contrasted. New approaches to erosion mitigation and updated knowl-
edge of existing methods are reviewed. Also included is an assessment of
theutilityofthevariouserosionmodellingandexperimentaltechniques,and
notesontheirpossibleusefordefenceapplicationsinAustralia.
APPROVEDFORPUBLICRELEASE
DSTO(cid:150)TR(cid:150)1757
Publishedby
WeaponsSystemsDivision
DSTODefenceScienceandTechnologyOrganisation
POBox1500
Edinburgh,SouthAustralia,Australia5111
Telephone: [phone]
Facsimile: [phone]
'CommonwealthofAustralia2005
ARNo. 013-473
August,2005
APPROVED FORPUBLICRELEASE
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Understanding and Predicting Gun Barrel Erosion
EXECUTIVESUMMARY
·eerosionofgunbarrelsinserviceleadstoreducedgunperformanceandavailabil-
ity, and the expense of barrel replacement over the lifetime of a gun system. It is partic-
ularly problematic for those guns which operate in high performance ballistic regimes.
AlthoughtheAustralianDefenceForcehaslonghadtocontendwiththeproblemofgun
barrelerosion,ithasrecentlyreceivedreneweda(cid:145)ention.Anewdefenceinstructionman-
dating the future use of low vulnerability (LOVA) propellants, the near- and medium-
termacquisitionofnewammunitionandweaponsystems,andthepossiblemodi(cid:12)cation
of existing propelling charge con(cid:12)gurations, all present the need for reliable prediction
andassessmentoftheassociatedbarrelerosionrisks.
·is report is a critical, technical review of advances in gun barrel erosion research,
mitigation, and assessment, over the last (cid:12)(cid:2)een years. Known and postulated erosion
mechanisms, obtained through recent experimental and numerical modelling work, are
describedandcontrasted.Newapproachestoerosionmitigationandupdatedknowledge
of existing methods are reviewed. Also included is an assessment of the utility of the
variouserosionmodelling andexperimental techniques,andnotesontheirpossibleuse
for defence applications in Australia. A summary of key topics covered in the review
follows.
Inthepastitishasbeencommonlyheldthatho(cid:145)er-burninggunpropellantsaremore
erosive,howeverthisisnotalwaystrue.Asigni(cid:12)cantnumberofcaseshavebeenreported
whereerosiondoesnotincreasewith(cid:8)ametemperature,andchemicala(cid:145)ackofthebore
bypropellantgasspecieshasbeentheprimarydeterminantoferosivity. Althoughthere
issomecon(cid:8)icting evidence inthe literature, it isgenerally accepted that the most com-
monLOVApropellantsaremoreerosivethanequivalentconventionalpropellants. Many
LOVApropellantformulationscontainRDX,andithasbeenconvincinglyshownbysev-
eralinvestigatorsthatRDXishighlychemicallyerosive.
New,experimentallow-erosivityLOVApropellantshavebeenproducedbyreducing
RDX content and introducing nitrogen-rich energetic binder or (cid:12)ller compounds. ·e
resulting propellant combustion gases, rich in nitrogen, act to re-nitride bore surfaces
during (cid:12)ring and inhibit erosive surface reactions. ·e result is increased bore hard-
ness,increasedresistancetomelting,andreducedchemicalerosion. ·eloweredhydro-
gen concentration in the combustion gas of some of these propellants may also reduce
hydrogen-assisted cracking of the bore surface. Of the high-nitrogen propellants under
development, the majority possess impetus and (cid:8)ame temperatures lower than RDX: a
compromisebetweenperformance,sensitivenessanderosivitymustbereachedinthese
cases.
Signi(cid:12)cante(cid:11)orthasrecentlybeendirectedatunderstandingtheerosionmechanisms
forbarrelscoatedwithprotectiverefractorymetals.·emostplausiblemechanismisthat
microcracksinthecoatings,presentfromthetimeofmanufacture,propagateduetopres-
sureandthermalstresscyclingandeventuallyreachthegunsteelsubstrate. ·roughnu-
mericalmodellingandanalysisoferodedbarrels,anumberofinvestigatorshaveshown
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thatoncecracksreachthesubstrate,chemicalerosion,gaswash,andhighinterfacialtem-
peraturescausepi(cid:145)ingofthesubstrateandeventuallyunderminethecoating. Segments
ofcoatingaresubsequentlyremovedbythe(cid:8)oworengagementwiththeprojectile,and
atthispointtheerosionrateofcoatedbarrelsmayexceedthatofsteelbarrels. Anumber
of ways to mitigate this erosion pathway have been suggested, including: development
of be(cid:145)er coating techniques to avoid the initial microcracks, pre-nitriding the gun steel
before coating to slow substrate erosion, introducing a protective interlayer, and con-
trolledbarrelstorageandpost-(cid:12)ringtreatmenttopreventoxidationofexposedsubstrate.
Modellingandexperimentshaveadditionallyshownthat,withthenotableexceptionof
chromium, the erosion resistance of refractory metal coatings varies amongst di(cid:11)erent
propellantgaschemistryenvironments.
Due to very good wear characteristics and thermal resistance, ceramic barrel liners
havebeenidenti(cid:12)edasapromisingtechnologyforsometime. Howeverthesusceptibil-
ity of ceramics to fracture, driven by stress induced by the di(cid:11)erent thermal expansion
propertiesofsteelandceramics,havepreventedtheirwidespreaduse. Newfunctionally
gradedceramic-to-metalliners,whichavoidanabruptmismatchofthermalexpansionat
theceramic/metalinterface,arebeingdevelopedtoaddressthisissue. Forsmallcalibres,
fabricationofentirebarrelsusingcompositereinforcedceramicshasbeendemonstrated.
Particularly for cooler propellants, it has been shown that charge arrangement can
a(cid:11)ecttheseverityanddistributionoferosionduetogaswash,andthatcombustiblecases
can reduce erosion through cooling-layer e(cid:11)ects. Several investigators have shown that
propellantgasblow-bymarkedlyincreasesheattransfertothebore,andtherebythermal
erosion.
Over the last ten years there have been signi(cid:12)cant advances in computational mod-
ellingoferosion,andtwocodescapableofsimulatingabroadrangeoferosionphenom-
enahavebeenreviewed. Modellingresultsshowreasonableagreementwiththeerosion
ofin-servicegunbarrelsandlaboratoryexperiments. Insomecases,however,signi(cid:12)cant
calibrationviainputofexperimentaldatawasrequiredtoachievethisagreement. Atruly
predictive and comprehensive erosion model, capable of supplanting experiment, does
not yet exist. Nevertheless, in combination with experiment the existing computational
erosion models have proved extremely useful in be(cid:145)er understanding how the various
erosionmechanismsact.
NeartermworkinAustraliawillmostlikelyfocusontheerosionassessmentofnew
propellants,LOVApropellants,newandmodi(cid:12)edchargedesigns,andnewweaponsys-
tems. Sincenumericalerosionmodelsrequireexperimentalvalidationanyway,itissug-
gested that the limited resources available for research in this area are best directed to-
wardsestablishingamodestexperimentalcapability. Ventedvesseltestinghaslongbeen
theprimarysmall-scaleerosionresearchtool,butthequestionableapplicabilityofresults
to full-scale gun barrel erosion has previously restricted their usefulness. New vented
vessel testing methods, methodologies for the selection of appropriate and realistic test
conditions,andempiricalrelationsdesignedtoreconcilevesselandgunresults,havesig-
ni(cid:12)cantlyalleviatedthisdi(cid:14)culty,however. ·usaproperlydesignedventedvesseltest
facility,togetherwithlimitedfull-scalegun(cid:12)rings,isrecommendedasthemoste(cid:14)cient
approachtoperformingerosionresearchandassessmentwithrestrictedresources.
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Contents
Nomenclature vii
1 Introduction 1
2 ErosionMechanisms 3
2.1 ChemicalErosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 ·ermalErosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3 MechanicalErosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3 ErosionMitigation 12
3.1 AlternativePropellantFormulations . . . . . . . . . . . . . . . . . . . . . 12
3.2 Additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.3 SurfaceCoatingsandLiners . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.4 NovelErosionMitigation . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4 ErosionModellingandPrediction 19
4.1 Empirical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.2 Computational . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5 ExperimentalAssessmentTechniques 25
6 Conclusion 28
Appendices
A WearCalculationsforthe5(cid:148)/54Gun 37
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Nomenclature
APFSDS ArmourPiercingFinStabilizedDiscardingSabot
CAB CelluloseAcetateButyrate
CAN CelluloseAcetateNitrate
CAZ ChemicallyA(cid:11)ectedZone
CFD ComputationalFluidDynamics
EFC E(cid:11)ectiveFullCharge
HAC HydrogenAssistedCracking
HAZ HeatA(cid:11)ectedZone
IWTC In-Wall·ermocouple
KE KineticEnergy
LC LowContractile
LOVA LowVulnerabilityAmmunition
MOCVD Metal-OrganicChemicalVapourDeposition
OR OriginofRi(cid:8)ing
RAVEN SonicRarefactionWaveLowRecoilGun
RDX CyclotrimethyleneTrinitramine
A
WearCoe(cid:14)cient[m]or[m/s]
c
v Speci(cid:12)cHeatatConstantVolume[J/(kgK)]
d
BoreDiameter[m]
D
E
MolarActivationEnergy[J/mol]
f
SpeciesVolumeFraction[%]
h 2
ConvectionHeatTransferCoe(cid:14)cient[W/(m K)]
I
PropellantImpetus[J/kg]
k
·ermalConductivity[W/(mK)]
m
DynamicViscosity[Pas]
m
Mass[kg]
M
MolecularWeight[kg/mol]
Nu NusseltNumber
P
Pressure[Pa]
q 2
HeatFlux[W/m ]
r 3
Density[kg/m ]
R
Speci(cid:12)cGasConstant[J/(kgK)]
R
UniversalGasConstant[J/(molK)]
Re ReynoldsNumber
T
Temperature[K]
t
Time[s]
T
max MaximumBoreSurfaceTemperature[K]
u
Speed[m/s]
V 3
Volume[m ]
w
DiametralWear[m]
x
Axialdistance[m]
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1 Introduction
·eerosionofgunbarrelsinserviceleadstotwoproblemsfortheAustralianDefence
Force: (i)barrelreplacementcostsoverthelifespanof(cid:12)eldedweaponsystems,andpar-
ticularly those guns frequently operating in high performance ballistic regimes, and (ii)
reducedoperationale(cid:11)ectivenessduetovariablegunperformanceandavailability.
·e erosion of a gun barrel under normal (cid:12)ring conditions is typically manifested
in damage to the bore surface, and a bore diameter which progressively increases [1].
Typicalerosionratesareintherangeof0.1(cid:150)200(cid:181)mper(cid:12)ring[2],withtheworstdamage
usually occurring near the origin of ri(cid:8)ing (OR) position or, for smooth-bore barrels, at
the analogous location. Erosion of the bore near the muzzle end is also o(cid:2)en reported,
thoughitisusuallylessseverethanthatoccurringattheOR[3].
In some cases the rated fatigue life of a gun barrel, in terms of number of (cid:12)ring cy-
cles, may be reached before the barrel is eroded past condemning limits, obviating ero-
sion concerns. ·e possibility of immediate catastrophic fatigue failure, rather than the
morebenigne(cid:11)ectsofprogressiveerosion,raisesevengreaterconcernsinthissituation.
Normally, however, the rate of erosion exceeds the fatigue crack propagation rate [2],
and erosion is the driving factor in barrel retirement. An example of an erosion-limited
barrel is the M199 howi‚er cannon, which has a normal wear life of 2700 e(cid:11)ective full
charge(EFC)roundsandafatiguelifeof10000EFCrounds,usingatriple-basepropelling
charge[1]. Incomparison,theM126E1howi‚erhasanexpected wearlifeof30000EFC
rounds and a fatigue limit of 7500 EFC rounds, using a slightly cooler single-base pro-
pellant [1]. ·e wear limits for these 155 mm guns are 2.5 and 2.0 mm respectively. ·e
condemningerosionlimitcanvaryconsiderablybetweenguns,primarilydependingon
accuracyandperformancerequirements: forsomeindirect(cid:12)reweapons,erosionofupto
8%ofborediametermaybetolerable,butthetolerancefortankgunsistighterandtyp-
icallyintherange0.5(cid:150)1%[2]. Aswouldbeintuitively expected, highperformance guns
with high muzzle velocities usually wear fastest [4]. For example the 105 mmM68 tank
gun, operating at a 1600 m/s muzzle velocity, has a normal wear life as low as 100 EFC
rounds[1]. Ithasalsolongbeenassumedando(cid:2)enobservedthathotpropellants cause
moreerosionthandosimilarly-performingcooler-burningpropellants. Althoughthisis
o(cid:2)entrue,therearesigni(cid:12)cantexceptionswhichwillbediscussedlaterinthisreport.
Barrelerosionisunlikelytocausecatastrophicfailure,andthuscondemninglimitsare
primarilysettoensurethee(cid:11)ectsoferosionongunperformancedonotbecomeexcessive.
·ee(cid:11)ectsofanerodedboremayinclude[3,5]:
(cid:15)
rangeandrangeaccuracyloss,
(cid:15)
directionalstabilitylossandresultantdispersion,
(cid:15)
fuzemalfunctions,
(cid:15)
excessivetorsionalimpulse(ri(cid:8)edbarrels),
(cid:15)
propellantgasblow-by,
(cid:15)
reductioninbarrelfatiguelife,
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DSTO(cid:150)TR(cid:150)1757
(cid:15)
excessivemuzzle(cid:8)ash,and
(cid:15)
increasedblastoverpressure.
Someoftheseconsequenceshavethepotentialtomarkedlyreduceoperationale(cid:11)ective-
ness.
AlthoughtheAustralianDefenceForcehaslonghadtocontendwiththeproblemof
barrelerosion,ithasrecentlyreceivedreneweda(cid:145)entionforseveralreasons. First,anew
defence instruction, InsensitiveMunitions [6], has mandated the use of low vulnerability
ammunition(LOVA)forallnewexplosiveordnanceprocurement,unlessawaiverisob-
tained. In addition, an implementation plan will be developed to address the issue of
sensitiveness for munitions already in service. ·e resulting move to LOVA propelling
chargesmeansthatitislikelythatnewpropellantswillbeintroducedtoservice. ·edif-
ferent chemicalcomposition, (cid:8)ametemperature and(usuallyhigher)erosivity ofLOVA
propellantsplacesincreasedemphasisonaddressingtheproblemoferosion. Second,the
imminentupgradeoftheADIMulwalapropellantmanufacturingfacilitymayleadtothe
production of new propellants with di(cid:11)erent erosive behaviour to those already in ser-
vice. ·ird, procurementactivities suchasLand 17(replacement orenhancement of the
Army howi‚er (cid:8)eet) and MARAP (medium artillery replacement ammunition project)
will result in new barrel, propelling charge and projectile con(cid:12)gurations. ·ese new or
upgraded systems will likely not exhibit the same erosive wear characteristics as is cur-
rentlyencounteredinexistingsystems.
Inthecontextofthesegunandpropellingchargereplacementsandupgrades,theca-
pabilitytomodel,predict,test,measureandunderstandtheassociatederosionprocesses
becomesimportant. Unfortunately, though, therehasbeenli(cid:145)lerecentworkinthesear-
eas at DSTO. ·ere was no active Australian participation in the Technical Cooperation
Program(TTCP)2000(cid:150)2003GunTubeWearandErosionresearchactivities[7],forexample.
Onepossiblereasonforthe dearthofAustralianworkisalackofresourcestoapproach
the complex, cross-discipline nature of gun barrel erosion research: it crosses the (cid:12)elds
of material science and metallurgy, solid mechanics, compressible gas dynamics, chem-
istry, interior ballistics, heat transfer, and statistical mechanics. Nevertheless, there will
likelybeanear-termrequirementtoestablishatleastbasicerosionresearchcompetence
tosupporttheactivitiesnotedabove.
·e aim of this report, then, is to describe and assess the current state of analytical,
numerical, empirical and experimental approaches to gun barrel erosion research, with
aviewtotheir practical usebyDefenceinAustralia. Research prior to 1988hasalready
beenthoroughlyreviewed(cid:151)byAhmad[1]andBracuti[5]forexample(cid:151)anditisnotthe
intentionofthisreporttore-examinethesameground. ·isreportwillfocusonmaterial
omi(cid:145)ed from these reviews, and work that has been conducted since their publication.
Section 2 beginswithareviewofknownandpostulated erosionmechanisms,andade-
scriptionofinsightsobtainedthroughthemostrecentexperimentalandmodellingwork.
Disagreementsintheliterature,includingtherelativeerosivityofthevariouspropellant
gas species, the existence of protective species, and the e(cid:11)ect of (cid:8)ame temperature on
erosion, will be addressed. Section 3 continues with a discussion of erosion mitigation
methods. Withthebackgroundestablished,Sections4and5goontocriticallyassessthe
utility of modern approaches to erosion modelling, prediction and experimental assess-
ment.
2