Fatigue crack initiation in Ti-6Al-4V alloy

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Fatigue crack initiation in Ti-6Al-4V alloy. Kristell Le Biavant - Guerrier directed by :. Claude Prioul Sylvie Pommier LMSS-Mat, Ecole Centrale Paris. Valérie Gros Bruno Brethes Snecma, Villaroche. Contributions of : M.Sampablo, S.Billard & V.Malherbe. Plan. Industrial issue - PowerPoint PPT Presentation

Text of Fatigue crack initiation in Ti-6Al-4V alloy

  • Fatigue crack initiation in Ti-6Al-4V alloy Kristell Le Biavant - Guerrierdirected by :Claude Prioul Sylvie Pommier

    LMSS-Mat, Ecole Centrale ParisValrie Gros Bruno Brethes

    Snecma, VillarocheContributions of : M.Sampablo, S.Billard & V.Malherbe

  • PlanIndustrial issue A ghost structure : the macrozonesMacrozones and fatigue failureModel for fatigue life predictionConclusions and perspectives

  • PlanIndustrial issue A ghost structure : the macrozonesMacrozones and fatigue failureModel for fatigue life predictionConclusions and perspectives

  • Industrial issue Fatigue tests on notched specimensApplied stress (MPa)Fatigue life

    -3s

  • Industrial issueThe materialTimeTemperatureb-forgingb-transusa+b-forgingrecrystallisationannealing950C700C

  • Industrial issueThe materialMicrostructure = 50% primary a grains (hcp)+50% lamellar grains (a lamellae (hcp) in b matrix (cc))Base

    Element

    Ti

    Al

    V

    C

    Fe

    O

    % min (weight)

    5.5

    3.5

    -

    -

    -

    % max (weight)

    6.75

    4.5

    0.08

    0.3

    2500ppm

  • PlanIndustrial issue A ghost structure : the macrozonesMacrozones and fatigue failureModel for fatigue life predictionConclusions and perspectives

  • A ghost structure : the macrozonesA contrast appears at a millimetric scale (after a 0,5 HF - attack)

  • A ghost structure : the macrozonesA strongly inhomogeneous strain at a millimetric scaleS Photoelastic analysisS=350MPaS=800MPa1cm

  • A ghost structure : the macrozones Vocabulary A 2 scale material

  • A ghost structure : the macrozones RX characterisation Basal pole figuresPrismatic pole figuresMacrozone 1Macrozone 2

  • A ghost structure : the macrozones Conclusions Existence of a millimetric structure : the macrozones

    Macrozones = areas where a-phase has a major crystallographic orientation+ minor secondary orientations

    The origin of the macrozones is still unclear

    The macrozones have a strong influence on the local mechanical response of the material

  • PlanIndustrial issue A ghost structure : the macrozonesMacrozones and fatigue failureObservationsCrack initiationCrack growthModel for fatigue life predictionConclusions and perspectives

  • Macrozones and fatigue failureObservationsA strongly inhomogeneous cracking process at a millimetric scale

  • Macrozones and fatigue failureObservationsS N=3000cyclesSpecimen surface after a cyclic bending test (Smax=800MPa, R=-1)

  • Macrozones and fatigue failureCrack initiationRelationship between the crystallographic orientation and crack initiation :

  • Macrozones and fatigue failureCrack initiationRelationship between the crystallographic orientation and crack initiation :

  • Macrozones and fatigue failureCrack initiationFatigue cracks initiate along slip bands :

  • Macrozones and fatigue failureCrack initiationSchmid factor calculations :Hypotheses :A single orientation within the macrozones local = S macroscopic

    Slip intensity :t = S . cos f . cos l

  • Macrozones and fatigue failureCrack initiation

    Major crystallographic orientation (measured) Maximum resolved shear stresses tmax (calculated)Within each of the 12 macrozones studied :

    Fatigue cracks observedcracks parallel to basal plane or cracks parallel to a prismatic plane or no cracks

  • Macrozones and fatigue failureCrack initiationMacrozones with basal cracksMacrozones withprismatic cracksMacrozone number

  • Macrozones and fatigue failureCrack initiationMacrozones with basal cracksMacrozones withprismatic crackstmax (MPa)prismMacrozone number

  • Macrozones and fatigue failureCrack initiationMacrozones with basal cracksMacrozones withprismatic crackstmax (MPa)Macrozone number

  • Macrozones and fatigue failureCrack initiationWithin each studied macrozone :

    Fatigue cracks observed Fatigue crack density (measured)

    Major crystallographic orientation (measured) Resolved shear stresses amplitude Dtmax (calculated)

  • Macrozones and fatigue failureCrack initiationDtmax (MPa)basalCrack density of the macrozone(m/mm2 for N cycles)

  • Macrozones and fatigue failureCrack initiationCrack density of the macrozone(m/mm2 for N cycles)Dtmax (MPa)prism

  • Macrozones and fatigue failureCrack initiationSurface effecteasy initiationM.W. Brown , K. J. Miller, (1973). A theory for fatigue failure under multiaxial stress-strain conditions. Proc.Instn.Mech.Engrs, Vol. 187, pp745-755.uneasy initiation Surface effect correction cos . cos . cos

  • Macrozones and fatigue failureCrack initiationCrack density of the macrozone(m/mm2 for N cycles)Dtmax . cos q (MPa)uncracked macrozonescrack density = f(Dt, cos q , N)

  • PlanIndustrial issue A ghost structure : the macrozonesMacrozones and fatigue failureObservationsCrack initiationCrack growthModel for fatigue life predictionConclusions and perspectives

  • Macrozones and fatigue failureCrack growthImportance of crack coalescence in growth mechanism :Number of cycles

  • Macrozones and fatigue failureCrack growthImportance of crack coalescence in growth mechanism :Example of coalescence process

  • Macrozones and fatigue failureCrack growthImportance of crack coalescence in growth mechanism :Number of cyclesCrack length (m)Number of cycles

  • Macrozones and fatigue failureCrack growthTwo mechanisms are involved in fatigue crack growth :

    crack coalescence

    pure crack growth

  • Macrozones and fatigue failure Conclusions Strong influence of macrozones on short cracks :

    Cracks initiate along basal or prismatic slip bands if tmax > tc

    Fatigue crack density = f (1,, 2,Dt, cos q , N)

    Short crack growth = crack coalescence + pure crack growth

    Long crack growth follows a Paris regime (a > 500m)

  • PlanIndustrial issue A ghost structure : the macrozonesMacrozones and fatigue failureModel for fatigue life predictionModel descriptionHypotheses controlConclusions and perspectives

  • Model for fatigue life predictionModel descriptionNumber of cycles for short crack growth

  • Model for fatigue life prediction Model description

  • Model for fatigue life prediction Model descriptionzone of influence of the crack(Kachanov, 1993)Definition of a crack density

  • Model for fatigue life prediction Model descriptionThreshold short / long cracks

  • Initiation model descriptionN=1000Dt.cosqcrack density

  • Initiation model descriptioncrack densityDt.cosqN=1000S,f1,F,f2Dt.cosq

  • Model for fatigue life predictionModel descriptionNumber of cycles for short crack growth

  • PlanIndustrial issue A ghost structure : the macrozonesMacrozones and fatigue failureModel for fatigue life predictionModel descriptionHypotheses controlConclusions and perspectives

  • Model for fatigue life predictionHypotheses control~ macrozone sizeCrack growth model(Paris law)

  • Model for fatigue life prediction Hypotheses control

  • Model for fatigue life prediction Hypotheses control 1. Initiation located on the fracture surface Macrozone located at the initiation site electropolished

  • Model for fatigue life prediction Hypotheses control 1. Initiation located on the fracture surface Macrozone located at the initiation site electropolished 4. Nf calculated

  • Model for fatigue life prediction Hypotheses control

  • Model for fatigue life prediction Conclusions1. Initiation model based on fatigue crack density within the macrozone

    2. Crack growth model 3. Fatigue life prediction Good understanding of life scatter on notched specimen

  • PlanIndustrial issue A ghost structure : the macrozonesMacrozones and fatigue failureModel for fatigue life predictionConclusions and perspectives

  • Model for fatigue life prediction Conclusions1. Main aim of this study achieved :

    Fatigue life scatter explained 2. New result :

    Macrozone existence exhibited3. Influence of macrozones on fatigue failure :

    Crack initiation Crystallographic orientation

    Crack growth Macrozone size4. Fatigue life model proposed

  • ConclusionsComparison between notch size and macrozone size

  • PerspectivesModel improvements1. Normal stress

    2. Stress calculation within the macrozone

    3. Improvement of evolution law of crack density

    4. Distribution of crystallographic orientations

  • PerspectivesMaterial improvements1. Understanding of thermo-mechanical treatment

    2. Reduce macrozone size

    3. Control of crystallographic orientation ? Fatigue life for each zone of the disk

  • PlanIndustrial issue Fatigue on notched specimenThe material of the studyA ghost structure : the macrozonesMacrozones and fatigue failureObservationsCrack initiationCrack growthModel for fatigue life predictionModel descriptionHypotheses controlConclusions and perspectives