Lowcyclefatigueofweldedjoints:newexperimentalapproachOriginalResearchArticle

Totakeintoaccountthereducedfatiguestrengthofweldedjoints,areductionlifefactorappliedonfatiguecurves(Jfvalue)wasintroducedintotheRCC-MR[DesignandConstructionRulesforMechanicalComponentsofFBRNuclearIslands,AFCEN,1993],forthedesignandconstructionoffastbreederreactors.Tobetterassessthisfactor,previousworkshowedthatmechanicalbehaviorofaweldedassemblyisinfluencedbythegeometryoftheweldandbytheinteractionofthedifferentcyclicplasticbehaviorofthetwomaterials:basemetal(BM)andweldmetal(WM).Anewprocedure(namedFFAST)wasperformedonweldedjointspecimensextractedfrombutt-weldedpipeconnections(uniaxialtensile–compressiveload).AninnovativeexperimentalapproachisproposedtostudythelocalmechanicalbehavioroftheweldedjointspecimensandthendeterminetheJfparameter.ThemainadvantageofthemethodistoavoidproblemsduetotherelativestiffnessofweldpartversustheBMpartofthespecimen.Acontinuousrecordingofthestressandstrainintheweldallowsanestimationofthemechanicalbehaviorandfinallythefatiguelifeofthejoint.Observationsofthecracksurfaceshowtwodifferentcrackinitiationzonesnearthewelddependingontheloadlevel.Calculationsofthetestsandcomparisonwithexperimentalresultsarepresented.ThesestudiesmakeitpossibletoassessinapracticalwaytheJfdesignmethod.ItappearsthatJfvaluecannotbeconsideredasasinglevalueforitisinfluencedbyseveralfactorsdependingontheweldmentandontheloadlevel.