fatigue crack initiation in ti-6al-4v alloy kristell le biavant - guerrier directed by : claude...
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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 Gros
Bruno Brethes
Snecma, Villaroche
Contributions of : M.Sampablo, S.Billard & V.Malherbe
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Plan
Industrial issue A ghost structure : the macrozones Macrozones and fatigue failure Model for fatigue life prediction Conclusions and perspectives
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Plan
Industrial issue A ghost structure : the macrozones Macrozones and fatigue failure Model for fatigue life prediction Conclusions and perspectives
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Industrial issue Fatigue tests on notched specimens
App
lied
str
ess
(MP
a)
Fatigue life
<N>
<N> -3
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Industrial issue The material
Time
Temperature
-forging
-transus
-forgingrecrystallisation
annealing
950°C
700°C
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Industrial issue The material
Microstructure
=
50% primary grains (hcp)
+
50% lamellar grains
(lamellae (hcp) inmatrix (cc))
Element Ti Al V C Fe O
% min (weight) 5.5 3.5 - - -
% max (weight) 6.75 4.5 0.08 0.3 2500ppmBase
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Plan
Industrial issue A ghost structure : the macrozones Macrozones and fatigue failure Model for fatigue life prediction Conclusions and perspectives
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A ghost structure : the macrozones
A contrast appears at a millimetric scale
(after a 0,5 HF - attack)
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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
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A ghost structure : the macrozones Vocabulary
~1mm15µm
A 2 scale material
‘macrozones’ ‘grains’
nodules or lamellar grains
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A ghost structure : the macrozones RX characterisation
Basal pole figures Prismatic pole figures
Macrozone 1
Macrozone 2
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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
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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
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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)
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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)
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C.O.C.O. Average crack
orientation (C.O.)
Macrozones and fatigue failure Crack initiation
Relationship between the crystallographic orientation and crack initiation :
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Macrozones and fatigue failure Crack initiation
Relationship between the crystallographic orientation and crack initiation :
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Macrozones and fatigue failure Crack initiation
Fatigue cracks initiate along slip bands :
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Macrozones and fatigue failure Crack initiation
Schmid factor calculations :
Hypotheses : A single orientation within the macrozone
local = macroscopic
Slip intensity :
= . cos . cos
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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
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Macrozones and fatigue failure Crack initiation
Macrozones with ‘basal’ cracks
Macrozones with
‘prismatic’ cracks
c
max (MPa)basal
Macrozone number
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Macrozones and fatigue failure Crack initiation
Macrozones with ‘basal’ cracks
Macrozones with
‘prismatic’ cracks
max (MPa)prism
Macrozone number
Pc
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Macrozones and fatigue failure Crack initiation
Macrozones with ‘basal’ cracks
Macrozones with
‘prismatic’ cracks
max (MPa)
Macrozone number
Pc
c
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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
Pmax > P
c
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Macrozones and fatigue failure Crack initiation
max (MPa)basal
Crack density of the macrozone
(µm/mm2 for N cycles)
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Macrozones and fatigue failure Crack initiation
?
Crack density of the macrozone
(µm/mm2 for N cycles)
max (MPa)prism
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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, pp745-755.
‘uneasy’ initiation
Surface effect correction cos . cos . cos
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Macrozones and fatigue failure Crack initiation
Crack density of the macrozone
(µm/mm2 for N cycles)
max . cos (MPa)uncracked macrozones
crack density = f(cos , N)
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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
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Macrozones and fatigue failure Crack growth
Importance of crack coalescence in growth mechanism :
Cra
ck le
ngth
(µ
m)
Number of cycles
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Macrozones and fatigue failure Crack growth
Importance of crack coalescence in growth mechanism :
Example of coalescence process
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Macrozones and fatigue failure Crack growth
Importance of crack coalescence in growth mechanism :
Cra
ck le
ngth
(µ
m)
Number of cycles
Cra
ck le
ngth
(µ
m)
Number of cycles
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Macrozones and fatigue failure Crack growth
Two mechanisms are involved in fatigue crack growth :
• crack coalescence
• ‘pure’ crack growth
Crack initiationdensity
Macrozone crystallographic
orientation
not significant
Inhomogeneous cracking process
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Macrozones and fatigue failure Conclusions
Strong influence of macrozones on short cracks :
Cracks initiate along basal or prismatic slip bands
if max > c
Fatigue crack density = f (1,, 2,cos , N)
Short crack growth = crack coalescence + ‘pure’ crack growth
Long crack growth follows a Paris regime (a > 500µm)
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Plan
Industrial issue A ghost structure : the macrozones Macrozones and fatigue failure Model for fatigue life prediction
Model description Hypotheses control
Conclusions and perspectives
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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
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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
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Model for fatigue life prediction Model description
zone of influence of the crack(Kachanov, 1993)
Definition of a crack density
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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
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Initiation model description
N=1000
N=3000
N=2000
N=5000
crack density
N
.cos
crack density
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Initiation model description
crack density
.cos
N=1000
.cos
rc dc
Transition betweenshort / long cracks
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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
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Plan
Industrial issue A ghost structure : the macrozones Macrozones and fatigue failure Model for fatigue life prediction
Model description Hypotheses control
Conclusions and perspectives
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Model for fatigue life prediction Hypotheses control
N growth = C.Km daa0
af
?~ macrozone size
Crack growth model(Paris law)
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Model for fatigue life prediction Hypotheses control
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Model for fatigue life prediction Hypotheses control
1. Initiation located on the fracture surface Macrozone located at the initiation site electropolished
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Model for fatigue life prediction Hypotheses control
1. Initiation located on the fracture surface Macrozone located at the initiation site electropolished
4. Nf 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-
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Model for fatigue life prediction Hypotheses control
X 3,8
X 0,9
X 0,8
X 0,3
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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
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Plan
Industrial issue A ghost structure : the macrozones Macrozones and fatigue failure Model for fatigue life prediction Conclusions and perspectives
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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
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Comparison between notch size
and macrozone size
Conclusions
?Notch size ~ macrozone size Notch size >> macrozone size
Large scatter of NfLow scatter of Nf
(lower limit)
Distribution of
crystallographic orientations
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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
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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
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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