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
ltNgt
Applied stress (MPa)
ltNgt -3s
Fatigue life
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? Industrial issue The material
Temperature
b-transus
950C
700C
Time
recrystallisation
b-forging
ab-forging
annealing
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? Industrial issue The material
Base

• Microstructure
• 50 primary a grains (hcp)
• 50 lamellar grains
• (a lamellae (hcp) in b matrix (cc))

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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
Photoelastic analysis
S
1cm
S350MPa
S800MPa

• A strongly inhomogeneous
• strain at a millimetric scale

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? A ghost structure the macrozones Vocabulary
A 2 scale material
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? A ghost structure the macrozones RX
characterisation
Macrozone 1
Macrozone 2
Prismatic pole figures
Basal pole figures
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? 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

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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

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? Macrozones and fatigue failure Observations
S

• Specimen surface after a cyclic bending test
• (Smax800MPa, R-1)

N3000cycles
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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
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
• s local S macroscopic

• Slip intensity
• t S . cos f . cos l

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? 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)

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? Macrozones and fatigue failure Crack initiation
Macrozone number
Macrozones with prismatic cracks
Macrozones with basal cracks
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? Macrozones and fatigue failure Crack initiation
prism
tmax (MPa)
Macrozone number
Macrozones with prismatic cracks
Macrozones with basal cracks
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? Macrozones and fatigue failure Crack initiation
tmax (MPa)
Macrozone number
Macrozones with prismatic cracks
Macrozones with basal cracks
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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
  Dtmax (calculated)

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

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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 tmax gt 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 gt
  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
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? Model for fatigue life prediction Model
description
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? Model for fatigue life prediction Model
description
Definition of a crack density
zone of influence of the crack (Kachanov, 1993)
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? Model for fatigue life prediction Model
description
Threshold short / long cracks
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Initiation model description
crack density
N1000
Dt.cosq
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Initiation model description
crack density
N1000
Dt.cosq
Dt.cosq
S,f1,F,f2
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? Model for fatigue life prediction Model
description
Number of cycles for short crack growth
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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
Crack growth model (Paris law)
macrozone size
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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
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? Model for fatigue life prediction Hypotheses
control
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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
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

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
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? Conclusions

• Comparison between notch size
• and macrozone size

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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