13. Failure Mode and Effects Analysis

Chapter13FailureModeandEffectsAnalysis13.1INTRODUCTIONFailuremodeandeffectsanalysis(FMEA)isatoolforevaluatingtheeffect(s)ofpotentialfailuremodesofsubsystems,assemblies,components,orfunctions.Itisprimarilyareliabilitytooltoidentifyfailuremodesthatwouldadverselyaffectoverallsystemreliability.FMEAhasthecapabilitytoincludefailureratesforeachfailuremodeinordertoachieveaquantitativeprobabilisticanalysis.Additionally,theFMEAcanbeextendedtoevaluatefailuremodesthatmayresultinanundesiredsystemstate,suchasasystemhazard,andtherebyalsobeusedforhazardanalysis.AmoredetailedversionoftheFMEAisknownasfailuremode,effectsandcri-ticalityanalysis(FMECA).TheFMECArequiresthatmoreinformationbeobtainedfromtheanalysis,particularlyinformationdealingwiththecriticalityanddetectionofthepotentialfailuremodes.TheFMEAmethodisadisciplinedbottom-upevaluationtechniquethatfocusesonthedesignorfunctionofproductsandprocessesinordertoprioritizeactionstoreducetheriskofproductorprocessfailures.Inaddition,theFMEAisatoolfordocumentingtheanalysisandcapturingrecommendeddesignchanges.TimeandresourcesforacomprehensiveFMEAmustbeallottedduringdesignandprocessdevelopment,whendesignandprocesschangescanmosteasilyandinexpensivelybeimplemented.235HazardAnalysisTechniquesforSystemSafety,byCliftonA.Ericson,IICopyright#2005JohnWiley&Sons,Inc.13.2BACKGROUNDTheFMEAtechniquefallsunderthedetaileddesignhazardanalysistype(DD-HAT)becauseitisadetailedanalysisdoneatthecomponentorfunctionallevel.ThebasichazardanalysistypesaredescribedinChapter3.AnalternatenameforthistechniqueisFMECA.FMECAisbasicallythesameasFMEAexceptitaddscriticalityevaluationtoeachfailuremode,aswellastheevaluationofpossiblefailuremodedetectionmethods.ThepurposeofFMEAistoevaluatetheeffectoffailuremodestodetermineifdesignchangesarenecessaryduetounacceptablereliability,safety,oroperationresultingfrompotentialfailuremodes.Whencomponentfailureratesareattachedtotheidentifiedpotentialfailuremodes,aprobabilityofsubsystemorcomponentfailurecanbederived.FMEAwasoriginallydevelopedtodeterminethereliabilityeffectoffailuremodes,butitcanalsobeusedtoidentifymishaphazardsresultingfrompotentialfailuremodes.TheFMEAisapplicabletoanysystemorequipment,atanydesiredlevelofdesigndetail—subsystem,assembly,unit,orcomponent.FMEAisgenerallyper-formedattheassemblyorunitlevelbecausefailureratesaremorereadilyavailablefortheindividualembeddedcomponents.TheFMEAcanprovideaquantitativereliabilitypredictionfortheassemblyorunitthatcanbeusedinaquantitativesafetyanalysis(e.g.,faulttree).FMEAtendstobemorehardwareandprocessorientedbutcanbeusedforsoftwareanalysiswhenevaluatingthefailureofsoft-warefunctions.Thetechniqueisthoroughforevaluatingpotentialindividualfailuremodesandprovidingreliabilityinformation.However,forsafetypurposes,anFMEAislimitedbecauseitconsidersonlysingleitemfailuresandnotthecombinationofitemsfail-ingtogether;generally,mishapsresultfromfailurecombinations.AlsoanFMEAdoesnotidentifyhazardsarisingfromeventsotherthanfailures(e.g.,timingerrors,radiation,highvoltage,etc.).Thetechniquecanbeeasilyperformedandmastered;however,abasicunder-standingoffailuresandfailuremodetheoryandhazardanalysistheoryisnecessaryaswellasknowledgeofsystemsafetyconcepts.Additionallyadetailedunderstand-ingofthesystemdesignandoperationisrequired.Themethodologyisuncomplicatedandeasilylearned.StandardFMEAformsandinstructionsareincludedinthischapter.TheFMEAtechniqueisavaluablereliabilitytoolforanalyzingpotentialfailuremodesandcalculatingsubsystem,assembly,orunitfailurerates.Severityandprob-abilityevaluationoffailuremodesprovidesaprioritizedlistforcorrectiveactions.FMEAcanalsobeextendedtoidentifyhazardsresultingfrompotentialfailuremodesandevaluatingtheresultingmishaprisk.Note,however,thatanFMEAwilllikelynotidentifyallsystemhazardsbecauseitisonlylookingatsinglecom-ponentfailuremodes,whilehazardscanbetheresultofmultiplehazardsandeventsotherthanfailuremodes.Forthisreason,FMEAisnotrecommendedasthesoletoolforhazardidentification.FMEAshouldonlybeusedforhazardanalysiswhendoneinconjunctionwithotherhazardanalysistechniques.236FAILUREMODEANDEFFECTSANALYSISAmodifiedFMEAforhazardidentificationisrecommendedforevaluationoffailuremodes,whendoneinsupportofotherhazardanalyses.However,theFMEAisnotrecommendedasthesolehazardanalysistobeperformed,sincetheFMEAprimarilylooksatsinglefailuremodesonly,whileahazardanalysiscon-sidersmanyadditionalsystemaspects.13.3HISTORYTheFMEAwasdevelopedfortheU.S.militaryasaformalanalysistechnique.MilitaryprocedureMIL-P-1629(nowMIL-STD-1629A)[1],titled“ProceduresforPerformingaFailureMode,EffectsandCriticalityAnalysis,”isoriginallydatedNovember9,1949.Itwasusedasareliabilityevaluationtechniquetodeter-minetheeffectofsystemandequipmentfailures.Failureswereclassifiedaccordingtotheirimpactonmissionsuccessandpersonnel/equipmentsafety.Thetermper-sonnel/equipment,takendirectlyfromanabstractofmilitarystandardMIL-P-1629,isnotablebecauseofthesignificancegiventopersonnel.Usedforaerospace/rocketdevelopment,theFMEAandthemoredetailedFMECAwerehelpfulinavoidingerrorsonsmallsamplesize