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

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

3

Plan


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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


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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

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

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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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Relationship between the crystallographic orientation and crack initiation :

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Fatigue cracks initiate along slip bands :

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Schmid factor calculations :

Hypotheses : A single orientation within the macrozone

local = macroscopic

Slip intensity :

= . cos . cos

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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 with ‘basal’ cracks

Macrozones with

‘prismatic’ cracks

c

max (MPa)basal

Macrozone number

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Macrozones with


max (MPa)prism

Macrozone number

Pc

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Macrozones with


max (MPa)

Macrozone number

Pc

c

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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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max (MPa)basal

Crack density of the macrozone

(µm/mm2 for N cycles)

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?



max (MPa)prism

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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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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


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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Example of coalescence process

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Cra

ck le

ngth

(µ

m)

Number of cycles

Cra

ck le

ngth

(µ

m)

Number of cycles

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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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Large grain microstructure

Within the macrozone, equivalence between

crack density and a longer crack

Small grain microstructure Macrozones

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zone of influence of the crack(Kachanov, 1993)

Definition of a crack density

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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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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


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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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1. Initiation located on the fracture surface Macrozone located at the initiation site electropolished

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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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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


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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


Model for fatigue life prediction Model description Hypotheses control


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

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