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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.

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Presentation on theme: "Fatigue crack initiation in Ti-6Al-4V alloy Kristell Le Biavant - Guerrier directed by : Claude Prioul Sylvie Pommier LMSS-Mat, Ecole Centrale Paris Valérie."— Presentation transcript:

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 2 Plan ¬Industrial issue ­A ghost structure : the macrozones ®Macrozones and fatigue failure ¯Model for fatigue life prediction °Conclusions and perspectives

3 3 Plan ¬Industrial issue ­A ghost structure : the macrozones ®Macrozones and fatigue failure ¯Model for fatigue life prediction °Conclusions and perspectives

4 4 Industrial issue Fatigue tests on notched specimens Applied stress (MPa) Fatigue life -3

5 5 Industrial issue The material Time Temperature -forging -transus -forging recrystallisation annealing 950°C 700°C

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

7 7 Plan ¬Industrial issue ­A ghost structure : the macrozones ®Macrozones and fatigue failure ¯Model for fatigue life prediction °Conclusions and perspectives

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

9 9 A ghost structure : the macrozones A strongly inhomogeneous strain at a millimetric scale Plastic strain + cyclic bending test Specimen surface after : ¬ a tensile test conducted up to a plastic strain of 1% ­ a cyclic bending test ( max=800MPa, R=-1) Photoelastic analysis =350MPa =800MPa 1cm

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

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

12 12 A ghost structure : the macrozones Conclusions Existence of a millimetric structure : the macrozones Macrozones = areas where -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 13 Plan ¬Industrial issue ­A ghost structure : the macrozones ®Macrozones and fatigue failure GObservations GCrack initiation GCrack growth ¯Model for fatigue life prediction °Conclusions and perspectives

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

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

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

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

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

19 19 Macrozones and fatigue failure Crack initiation Schmid factor calculations : Hypotheses : ¬A single orientation within the macrozone local = macroscopic Slip intensity : =. cos. cos

20 20 Macrozones and fatigue failure Crack initiation ­Major crystallographic orientation (measured) Maximum resolved shear stresses max (calculated) 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

21 21 Macrozones and fatigue failure Crack initiation Macrozones with basal cracks Macrozones with prismatic cracks c max (MPa) basal Macrozone number

22 22 Macrozones and fatigue failure Crack initiation Macrozones with basal cracks Macrozones with prismatic cracks max (MPa) prism Macrozone number P c

23 23 Macrozones and fatigue failure Crack initiation Macrozones with basal cracks Macrozones with prismatic cracks max (MPa) Macrozone number P c c

24 24 Macrozones and fatigue failure Crack initiation Within each studied macrozone : ¬Fatigue cracks observed Fatigue crack density (measured) ­Major crystallographic orientation (measured) Resolved shear stresses amplitude max (calculated) max > c Fatigue crack initiation if or P max > P c

25 25 Macrozones and fatigue failure Crack initiation max (MPa) basal Crack density of the macrozone (µm/mm 2 for N cycles)

26 26 Macrozones and fatigue failure Crack initiation ? Crack density of the macrozone (µm/mm 2 for N cycles) max (MPa) prism

27 27 Macrozones and fatigue failure Crack initiation Surface effect easy initiation M.W. Brown, K. J. Miller, (1973). A theory for fatigue failure under multiaxial stress-strain conditions. Proc.Instn.Mech.Engrs, Vol. 187, pp uneasy initiation Surface effect correction cos. cos. cos

28 28 Macrozones and fatigue failure Crack initiation Crack density of the macrozone (µm/mm 2 for N cycles) max. cos (MPa) uncracked macrozones crack density = f( cos, N)

29 29 Plan ¬Industrial issue ­A ghost structure : the macrozones ®Macrozones and fatigue failure GObservations GCrack initiation GCrack growth ¯Model for fatigue life prediction °Conclusions and perspectives

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

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

32 32 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

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

34 34 Macrozones and fatigue failure Conclusions Strong influence of macrozones on short cracks : G Cracks initiate along basal or prismatic slip bands if max > c Fatigue crack density = f ( 1,, 2, cos, N) G Short crack growth = crack coalescence + pure crack growth Long crack growth follows a Paris regime (a > 500µm)

35 35 Plan ¬Industrial issue ­A ghost structure : the macrozones ®Macrozones and fatigue failure ¯Model for fatigue life prediction GModel description GHypotheses control °Conclusions and perspectives

36 36 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

37 37 Model for fatigue life prediction Model description Large grain microstructure Within the macrozone, equivalence between crack density and a longer crack Small grain microstructure Macrozones

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

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

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

41 41 Initiation model description crack density.cos N=1000.cos rcrc dcdc Transition between short / long cracks

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

43 43 Plan ¬Industrial issue ­A ghost structure : the macrozones ®Macrozones and fatigue failure ¯Model for fatigue life prediction GModel description GHypotheses control °Conclusions and perspectives

44 44 Model for fatigue life prediction Hypotheses control N growth = C. K m da a0a0 afaf ? ~ macrozone size Crack growth model (Paris law)

45 45 Model for fatigue life prediction Hypotheses control

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

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

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

49 49 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

50 50 Plan ¬Industrial issue ­A ghost structure : the macrozones ®Macrozones and fatigue failure ¯Model for fatigue life prediction °Conclusions and perspectives

51 51 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

52 52 Comparison between notch size and macrozone size Conclusions ? Notch size ~ macrozone size Notch size >> macrozone size Large scatter of N f Low scatter of N f (lower limit) Distribution of crystallographic orientations

53 53 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

54 54 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

55 55 Plan ¬Industrial issue GFatigue on notched specimen GThe material of the study ­A ghost structure : the macrozones ®Macrozones and fatigue failure GObservations GCrack initiation GCrack growth ¯Model for fatigue life prediction GModel description GHypotheses control °Conclusions and perspectives


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