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. 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

3. 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

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

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

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

7. 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

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
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

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

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

14.  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

15.  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

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

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
s local = S macroscopic
Slip intensity :
t = S . cos f . cos l

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
tmax (MPa)
basal
tBc
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
‘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, pp

28.  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)

29. 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

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

33.  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

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, cos q , N)
Short crack growth = crack coalescence + ‘pure’ crack growth
Long crack growth follows a Paris regime (a > 500µm)

35. 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

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.  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
Threshold short / long cracks
Short cracks
1mm
500µm
Long cracks
Crack length
Crack density
evolution law
macrozone size
Paris law

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

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

42.  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

43. 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

44.  Model for fatigue life prediction Hypotheses control
N growth = C.DKm da a0 af ?
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
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

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
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. 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

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. ?  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

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. 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

55. 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