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