FATIGUE CRACK INITIATION AND PROPAGATION IN SANDWI

FATIGUECRACKINITIATIONANDPROPAGATIONINSANDWICHSTRUCTURESbyMagnusBurmanDepartmentofAeronauticsDivisionofLightweightStructuresKungligaTekniskaHögskolan(RoyalInstituteofTechnology)S-10044StockholmSwedenReportNo.98-29ISSN0280-4646AcademicdissertationwhichbypermissionfromKungligaTekniskaHögskolan(RoyalInstituteofTechnology)inStockholmispresentedtopublicreviewforpassingthedoctoralexamination10.15amThursday29thofOctober1998inKollegiesalen,KTH,Valhallavägen79,Stockholm.Whenmodernmanbuildslargeload-bearingstructures,heusesdensesolids:steel,concrete,glass.Whennaturedoesthesame,shegenerallyusescellularmaterials:wood,bone,coral.Theremustbeagoodreasonforit!Prof.M.F.Ashby,CambridgeUniversityiPREFACEThisinvestigationwasapartoftheBrite/EuRamprojectDAMTOS,DamageToleranceofFRPSandwichStructures,CECContractNo.CT92-0297,ProjectNo.BE-5781.ThefinancialsupportforKTHwasprovidedbyTheSwedishBoardforTechnicalandIndustrialDevelopment,NUTEK.TheworkcompiledinthisdoctoralthesishasbeencarriedoutattheDepartmentofAeronauticsbetween1993and1998.During1997IwasgiventheopportunitytoworkwiththeDynamicsGroupatIndustrialResearchLtd.(IRL),NewZealandandthecontentsofChapter11isaresultfromthisco-operation.IwouldhereliketoexpressmysinceregratitudetoMr.GraemeFinchandallmyfriendsatIRL,foranenjoyabletimeandafruitfulcollaboration.Aworkofthistypewouldhardlybepossiblewithouthelpandco-operation-sometimesanonymous-fromanumberofpeople.AmongthoseIwanttothankareProfessorsJanBäcklund,DanZenkertandUlfRingertzforcontributingandmakingwayfor”theopendoorpolicy”atthedepartment.Thispolicyprovidepossibilitiesforeasycommunicationwhichistheveryessenceoffruitfulcollaboration,ofcreativediscussionsandnewideas.IamingreatdepttoOlleSchubert,LarsFalkandBoMagnussonfortheirassistancewiththetimeconsumingtestingandIwouldfurtherliketothankAndreyShipsaandLindaJohanssonwhomIhavehadthepleasuretoworktogetherwith.FinallyIwishtothankeachandeverybodyattheDepartmentofAeronauticsforcreatinganinspiringandtrulyenjoyableatmosphere,everyday.Totheguyswhogottheurge,theconsideratenessandenormousgenerosity;StefanHallström,JakobKuttenkeulerandthekiwiMarkBattley.Whoneedenemieswithfriendslikeyou?TomylovelywifeTina,sinceoneandahalfyearback,mybiggestjoyinlife,Johanna,mydearparentsMartinandMargaretha,sisterMariaandeverybodythatIamhappytocallmyfamily:Thanksforthecontinuousloveandinspiration.ThisthesiswouldnotbeenpossiblewithoutDanZenkertsinfinitewellofpatience,encouragement,guidanceandsupport.Cheersmate!ÅrstainOctober1998MagnusBurmaniiiiiABSTRACTThefocusthroughoutthisthesisisonthefatiguecharacteristicsofcorematerialsusedinsandwichstructures.Threesandwichconfigurationsareinvestigated,twowithcellularfoamsandonewithhoneycombcorematerialThesecorrespondstotypicalmaterialsanddimensionsusedinthemarineandaeronauticalindustry.Amodifiedfour-pointbendingrig,whichenablesreversedloading,issuccessfullyusedforconstantamplitudefatiguetestsofallmaterialconfigurations.Thecorematerialsaretestedasusedincompositesandwichbeamsandthroughthedesignofthespecimensthedesiredfailureisinshearofthecore.Analysesandinspectionsduringandafterthetestssupportsthetheorythatthefractureinitiationandfatiguefailureoccursinalargezoneofthecorewithwelldistributedmicrocracksratherthanasinglepropagatingcrack.ThefatiguetestresultsareplottedinstresslifediagramsincludingaWeibulltypefunctionwhichprovidesagoodaccuracycurvefittotheresults.Thefatiguelifeofthecorematerialsisfoundtobereducedwithaincreasedloadratio,R.Theinfluenceonthestrengthandfatigueperformanceonsandwichbeamswithtwotypesofcoredamages,aninterfacialdisbondandaflawedbutt-joint,areexperimentallyinvestigated.Thefatiguefailureinitiatesatthestressintensitylocationswhicharepresentduetothepre-damage.Thespecimenswithflawedbutt-jointsdisplayafatiguecrackpropagationintheinterfacebetweenthecoreandfaceofthesandwichwhilethecrackpropagatesthroughthethicknessofthebeamswhereaninitialinterfaceflawispresent.Afatiguefailurepredictionmodelissuggestedwhichutilisesthefatigueperformanceofundamagedbeamsandthestrengthreductionduetothedamages.Theapproachiscorrelatedwithresultsfromfatiguetestingandsatisfactorycorrelationisfound.Auni-axialfatiguetestsmethodisdevelopedwhichsimplifiestherigandspecimenscomparedtothefourpointbendmethod.Acomparisonbetweentheresultsfromuni-axialtension/compressionfatiguetestsandshearfatiguetestsshowsgoodcorrelation,althoughtheR-dependencydiffersinsomecases.Thefatiguecrackpropagationratesareinvestigatedfortwoconfigurations:crackspropagatinginpurefoamcorematerialandcrackspropagatinginthecorematerialnearandalongasandwichface/coreinterface.TherateatwhichacrackpropagatesstableinthesocalledParis’regimeisextractedforbothModeIandModeIIloading.TheagreementbetweentheModeIcrackpropagationrateinthepurefoamandinthecore/facesandwichinterfacelayersupportsthetheorythatthecrackactuallypropagatesinthesandwichcorebeneathastiffenedresinrichlayerpresentintheface/coreinterface.Thestressintensitythresholdsandthelimitsatwhichthecrackgrowthbecomesunstablearefurtherestablished.AcousticEmission(AE)isusedtomonitorcrackinitiati