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

A ghost structure : the macrozones Macrozones and fatigue failure Plan Industrial issue A ghost structure : the macrozones Macrozones and fatigue failure Model for fatigue life prediction Conclusions and perspectives

A ghost structure : the macrozones Macrozones and fatigue failure Plan Industrial issue A ghost structure : the macrozones Macrozones and fatigue failure Model for fatigue life prediction Conclusions and perspectives

 Industrial issue Fatigue tests on notched specimens Applied stress (MPa) <N> -3s Fatigue life

 Industrial issue The material Temperature b-transus 950°C 700°C Time b-forging recrystallisation a+b-forging annealing

 Industrial issue The material Base Microstructure = 50% primary a grains (hcp) + 50% lamellar grains (a lamellae (hcp) in b matrix (cc))

A ghost structure : the macrozones Macrozones and fatigue failure Plan Industrial issue A ghost structure : the macrozones Macrozones and fatigue failure Model for fatigue life prediction Conclusions and perspectives

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

 A ghost structure : the macrozones Photoelastic analysis Plastic strain + cyclic bending test Specimen surface after : a tensile test conducted up to a plastic strain of 1% a cyclic bending test (Smax=800MPa, R=-1) S S 1cm S=350MPa S=800MPa A strongly inhomogeneous strain at a millimetric scale

 A ghost structure : the macrozones Vocabulary A 2 scale material ~1mm 15µm ‘macrozones’ ‘grains’ nodules or lamellar grains

 A ghost structure : the macrozones RX characterisation Basal pole figures Prismatic pole figures

 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

A ghost structure : the macrozones Macrozones and fatigue failure Plan Industrial issue A ghost structure : the macrozones Macrozones and fatigue failure Observations Crack initiation Crack growth Model for fatigue life prediction Conclusions and perspectives

 Macrozones and fatigue failure Observations Specimen surface after a cyclic bending test (Smax=800MPa, R=-1) S Within each macrozones cracks are parallel one to another S A strongly inhomogeneous cracking process at a millimetric scale

 Macrozones and fatigue failure Observations Specimen surface after a cyclic bending test (Smax=800MPa, R=-1) N=4000cycles N=3000cycles Fatigue cracking and interfaces between neighbouring macrozones N=5000cycles

 Macrozones and fatigue failure Crack initiation Relationship between the crystallographic orientation and crack initiation : C.O. Average crack orientation (C.O.)

 Macrozones and fatigue failure Crack initiation Relationship between the crystallographic orientation and crack initiation :

 Macrozones and fatigue failure Crack initiation Fatigue cracks initiate along slip bands :

 Macrozones and fatigue failure Crack initiation Schmid factor calculations : Hypotheses : A single orientation within the macrozone s local = S macroscopic Slip intensity : t = S . cos f . cos l

 Macrozones and fatigue failure Crack initiation Within each of the 12 macrozones studied : Fatigue cracks observed  cracks parallel to basal plane or cracks parallel to a prismatic plane or no cracks Major crystallographic orientation (measured)  Maximum resolved shear stresses tmax (calculated)

 Macrozones and fatigue failure Crack initiation tmax (MPa) basal tBc Macrozone number Macrozones with ‘prismatic’ cracks Macrozones with ‘basal’ cracks

 Macrozones and fatigue failure Crack initiation tPc prism tmax (MPa) Macrozone number Macrozones with ‘prismatic’ cracks Macrozones with ‘basal’ cracks

 Macrozones and fatigue failure Crack initiation tPc tBc tmax (MPa) Macrozone number Macrozones with ‘prismatic’ cracks Macrozones with ‘basal’ cracks

 Macrozones and fatigue failure Crack initiation tBmax > tBc Fatigue crack initiation if or tPmax > tPc Within each studied macrozone : Fatigue cracks observed  Fatigue crack density (measured) Major crystallographic orientation (measured)  Resolved shear stresses amplitude Dtmax (calculated)

 Macrozones and fatigue failure Crack initiation Crack density of the macrozone (µm/mm2 for N cycles) basal Dtmax (MPa)

 Macrozones and fatigue failure Crack initiation Crack density of the macrozone (µm/mm2 for N cycles) ? prism Dtmax (MPa)

 Macrozones and fatigue failure Crack initiation Surface effect ‘uneasy’ initiation ‘easy’ initiation  Surface effect correction cos  . cos  . cos  M.W. Brown , K. J. Miller, (1973). A theory for fatigue failure under multiaxial stress-strain conditions. Proc.Instn.Mech.Engrs, Vol. 187, pp745-755.

 Macrozones and fatigue failure Crack initiation Crack density of the macrozone (µm/mm2 for N cycles) crack density = f(Dt, cos q , N) uncracked macrozones Dtmax . cos q (MPa)

A ghost structure : the macrozones Macrozones and fatigue failure Plan Industrial issue A ghost structure : the macrozones Macrozones and fatigue failure Observations Crack initiation Crack growth Model for fatigue life prediction Conclusions and perspectives

 Macrozones and fatigue failure Crack growth Importance of crack coalescence in growth mechanism : Crack length (µm) Number of cycles

 Macrozones and fatigue failure Crack growth Importance of crack coalescence in growth mechanism : Example of coalescence process 

 Macrozones and fatigue failure Crack growth Importance of crack coalescence in growth mechanism : Crack length (µm) Number of cycles Crack length (µm) Number of cycles

 Macrozones and fatigue failure Crack growth Two mechanisms are involved in fatigue crack growth : crack coalescence ‘pure’ crack growth Crack initiation density Inhomogeneous cracking process Macrozone crystallographic orientation not significant

 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 > 500µm)

A ghost structure : the macrozones Macrozones and fatigue failure Plan Industrial issue A ghost structure : the macrozones Macrozones and fatigue failure Model for fatigue life prediction Model description Hypotheses control Conclusions and perspectives

 Model for fatigue life prediction Model description Number of cycles for short crack growth Number of cycles for long crack growth Number of cycles for fatigue failure

 Model for fatigue life prediction Model description Small grain microstructure Macrozones Large grain microstructure Within the macrozone, equivalence between crack density and a longer crack

 Model for fatigue life prediction Model description Definition of a crack density zone of influence of the crack (Kachanov, 1993)

 Model for fatigue life prediction Model description Threshold short / long cracks Short cracks 1mm 500µm Long cracks Crack length Crack density evolution law macrozone size Paris law

Initiation model description crack density N=3000 N=2000 N=5000 crack density N N=1000 Dt.cosq

Initiation model description crack density Transition between short / long cracks S rc dc N=1000 Dt.cosq Dt.cosq S,f1,F,f2

 Model for fatigue life prediction Model description Crack density evolution law Number of cycles for short crack growth Threshold short / long cracks Number of cycles for long crack growth Number of cycles for fatigue failure

A ghost structure : the macrozones Macrozones and fatigue failure Plan Industrial issue A ghost structure : the macrozones Macrozones and fatigue failure Model for fatigue life prediction Model description Hypotheses control Conclusions and perspectives

 Model for fatigue life prediction Hypotheses control N growth = C.DKm da a0 af ? Crack growth model (Paris law) ~ macrozone size

 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 2. Major crystallographic orientation at the initiation site measured (EBSD) S 3. tBmax and tPmax calculated 3D-analysis elastic calculation Ti-6Al-4V Ti-a 4. Nf calculated

 Model for fatigue life prediction Hypotheses control X 3,8 X 0,9 X 0,8 X 0,3

 Model for fatigue life prediction Conclusions 1. Initiation model based on fatigue crack density within the macrozone 2. Crack growth model Threshold short / long cracks = macrozone size 3. Fatigue life prediction Good understanding of life scatter on notched specimen

A ghost structure : the macrozones Macrozones and fatigue failure Plan Industrial issue A ghost structure : the macrozones Macrozones and fatigue failure Model for fatigue life prediction Conclusions and perspectives

 Model for fatigue life prediction Conclusions 1. Main aim of this study achieved :  Fatigue life scatter explained 2. New result :  Macrozone existence exhibited 3. Influence of macrozones on fatigue failure : Crack initiation  Crystallographic orientation Crack growth  Macrozone size 4. Fatigue life model proposed

?  Conclusions Comparison between notch size and macrozone size Notch size ~ macrozone size Notch size >> macrozone size Large scatter of Nf Low scatter of Nf (lower limit) Distribution of crystallographic orientations

 Perspectives Model improvements 1. Normal stress 2. Stress calculation within the macrozone 3. Improvement of evolution law of crack density 4. Distribution of crystallographic orientations

Material improvements  Perspectives Material improvements 1. Understanding of thermo-mechanical treatment 2. Reduce macrozone size 3. Control of crystallographic orientation ? Fatigue life for each zone of the disk

A ghost structure : the macrozones Macrozones and fatigue failure Plan Industrial issue Fatigue on notched specimen The material of the study A ghost structure : the macrozones Macrozones and fatigue failure Observations Crack initiation Crack growth Model for fatigue life prediction Model description Hypotheses control Conclusions and perspectives