Basedontheobservations, afailuremechanism hasbeenpostulated. Itstartswiththecoldweldingoffoil wrapintheoverlapregionduringcoilflexing.AsexplainedinReference 1,thecoldweldingisa molecular-level bondingbetweentwoatomicallycleansmoothsurfaceswhentheyareinclosecontact. Silverishighlysusceptible tocoldweldinginavacuumenvironment becausesilveroxideisunstableand tendstobreakdowninsuchanenvironment. Asaresult,thereisnooxidebarrierbetweenthetwo contactsurfacesinthefoiloverlapregion.Duetoappreciable contactstressduringcoilflexureanddither inavacuum,coldweldingisthoughttooccurinthefoiloverlapregion.Thiseffectivelyincreasesthefoil thicknessintheoverlapregionandthuscreatesastressconcentration alongtheedgeoffoiloverlap duringcoilflexure.Withcontinuedcyclesofflexure,crackswillinitiate.Thesecrackswillthenpropagate alongtheedgeoffoiloverlap,whichisessentiallyinthedirectionnormaltotheprincipalstress.Torsional stressesmayplayaminorroleherealso.Whenthelocalbendingstressisrelativelysmall,itwilltake manyflexurecyclestocausefoilfailure.Thisfailuremechanism isbelievedtypicalforhigh-cyclefatigue. Whenmassivecoldweldingformsatthewirebraid-to-foil interfaces,theseelementswillbondtogetherin anumberofplaces.Thustherewillberegionswherethelocalbendingstiffnessincreases.Theclassical strengthofmaterialanalysismethodology asdescribedinAppendixAwasusedtoestimatetheeffectof wirebondingontheeffectivestiffnessandbendingmomentofthecable.Sincecoilflexingisa displacement controlledmotionatthesupportpoints,theincreaseoflocalstiffnesswillcausethefoilin adjacentregionstobehighlystrained.Figure8showsaplotofthenumberofwirebraidsinalayerofthe 190Acablevs.thebendingmomentpredictedbytheanalysis.Thebendingmomentneededtobendthe cabletoanincreasedradiusofcurvatureresultsinhigherbendingstress,whichmayalsocontributetoan increaseincrackpropagation rateinthefoil.Thewirebraidcanexperience somefailureswithout significantRFimpacts,butthefoilmustretainitsdimensional integrity. Fromlinearelasticfracturemechanics (LEFM)theory,foragivencracksize,"a",itsfatiguecrackgrowth ratecanbeexpressedbyParisLaw(Reference 3): da/dN=C(,&K)n=C(Ao4_a)" where"a"isthecrackdimension; "N"isthenumber offatiguecycles; "A_"istheremotely applied tensile stressrange; "C"and"n"arethePariscrackgrowthratecoefficient andexponent respectively. Forcopper material, theexponent nis3.2(Reference 4).Thusifthereare20wiresperlayer(28%of totalwiresperlayer)bonded tothefoil,therewillbea40%increase ofapplied bending moment (bending stress)whichwillcausecrackpropagation ratetobeincreased byafactorof(1.4)m2=3,foracrackwith identical initialsize.Ifthereare50wiresperlayer(70%oftotalwiresperlayer)bonded tothefoil,an increase ofapplied bending moment (bending stress) willbeafactoroftwothatcouldcausethecrack growthratetoincrease of(2)3.2=9. Whenapredominant crackpropagates toitscriticalsizeandcauses breakage ofthefoil,theload induced bytheflexingmovement willberedistributed andthuspossibly causebreakage ofthewirebraid andeventually theouterjacketasseenintheA190A failure. Thesequence offailure isnotwell understood. Thelowhardness valuesassociated withthefoilwrapandthewirebraidofA190A/CS arethought toplay averyimportant roleinitsearlyfailure.Because thehardness ofthewirebraidandespecially thefoilare low,theyieldstrength oftheseelements isalsolow(Reference 2).Perhaps thelowyieldstrength increases theas-built conformity ofthewirebraidandthefoil,thusincreasing thecontact areaandthe chance forcoldwelding todevelop. Oncethewirebraidandthefoilarebonded together bycoldwelding, thecracksthathaveinitiated fromthefoilstressconcentration regionmaypropagate veryfast,asseenin specimen A190A. Tofurther theunderstanding ofthesephenomena, consideration isbeinggivento measuring certainmechanical properties ofthefoilduringthemanufacturing process. NASA/CPm2002-211506 57