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

1. 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

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

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

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

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

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

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

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

9.  A ghost structure : the macrozones 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 Photoelastic analysis A strongly inhomogeneous strain at a millimetric scale S 1cm S=350MPa S=800MPa

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

11.  A ghost structure : the macrozones RX characterisation Macrozone 1 Macrozone 2 Prismatic pole figures Basal pole figures

12.  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

13. 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

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

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

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

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

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

19.  Macrozones and fatigue failure Crack initiation Schmid factor calculations : Hypotheses : • A single orientation within the macrozone • slocal = Smacroscopic Slip intensity : t = S. cosf. cosl

20.  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)

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

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

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

24.  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)

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

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

27.  Macrozones and fatigue failure Crack initiation Surface effect ‘easy’ initiation ‘uneasy’ 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.

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

29. 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

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

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

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

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

34.  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, cosq , N) • Short crack growth = crack coalescence + ‘pure’ crack growth • Long crack growth follows a Paris regime (a > 500µm)

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

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

37.  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

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

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

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

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

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

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

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

45.  Model for fatigue life prediction Hypotheses control

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

47.  Model for fatigue life prediction Hypotheses control 2.Major crystallographic orientation at the initiation site measured (EBSD) S Ti-6Al-4V 3. tBmax and tPmax calculated 3D-analysis elastic calculation Ti-a 1. Initiation located on the fracture surface Macrozone located at the initiation site electropolished 4. Nf calculated

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

49.  Model for fatigue life prediction Conclusions Threshold short / long cracks = macrozone size 1. 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

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