Fatigue resistance of Al-Cu-Li alloys – effect of environment

Jean-Christophe EhrströmOlivier AndreauChristine Baudoux (Institut Pprime, ENSMA)

Constellium Technology Center, Voreppe, France

Contents

� General introduction on Al-Cu-Li alloys � AIRWARE®

� Fatigue crack growth rate of Al-Cu-Li alloys

� Overall behaviour very attractive

� Al-Cu-Li alloy fit in the framework of « Poitiers », J. Petit et al.

� Fatigue performance of Al-Cu-Li alloys

� Conventional samples

� Technological specimens

� Specific study: effect of environment on endurance fatigue

� With or without notch

� Air or vacuum

� Conclusion

1/27/2017 2

Airware® alloys can be used in the whole aircraft structure

1/27/2017 3

Alloys in Blue designate AIRWARE® = trade mark for Constellium’s Al-Cu-Li alloys

Upper Wing Skin

2195, 2075, 7449 & 7056

Upper & Lower Wing stringers

2195, 2196, 2296, 2076, 2075, 7449 & 2027

Wing Box & Inner Ribs

2050, 7140, 7040 & 7449

Wing Spars and Ribs

2050, 7140 & 7040Fuselage Stringers

2195, 2196 & 7349

Floor Beams, Seat Tracks

2196Main Frames, Fittings & Structures

2098, 2198, 2050, 2139, 6x56, 2022, 2027, 7449 & 7040

Hi-temp applications

2050, 2139 & 2xxx-HT

Fuselage Skin & Webs

2098, 2198, 2074, 2139, 6056, 6156 & 2022

Lower Wing Skin

2196, 2050, 2085, 2027 & 2024

2050 industrially applied for thick structures

A350 ribs is a key market

1/27/2017 4

2198 damage tolerance � high performance fuselage

Part 1: Properties of materials

27/01/2017 5

-

5.00

10.00

15.00

20.00

25.00

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

N pressurization

half

crac

k le

ngth

(in

)

2024 HDT

2198 T8

Initial crack (2a0)

frames

2024 HDT = A380 generation alloy 2198 T8

Longitudinal crack length (in)

Number of flights

2198 T8 = alloy of C-series fuselage

Contents

� General introduction on Al-Cu-Li alloys � AIRWARE®

� Fatigue crack growth rate of Al-Cu-Li alloys

� Overall behaviour very attractive

� Al-Cu-Li alloy fit in the framework of « Poitiers », J. Petit et al.

� Fatigue performance of Al-Cu-Li alloys

� Conventional samples

� Technological specimens

� Specific study: effect of environment on endurance fatigue

� With or without notch

� Air or vacuum

� Conclusion

1/27/2017 6

Method: fatigue crack propagation in wing panels

Long range spectrum

Lower wing Al-Cu-Li alloys performing better than A380 generation alloys (2027, 2024HDT > 2024)

Gain de densité de 5 – 6%

For high strength top wing alloys, FCG under spectr um is improved by a factor 2

� All alloys have similar yield stress

1/27/2017 9

0

20

40

60

80

100

120

140

160

180

-20000 -10000 0 10000 20000 30000 40000

number of flights

crac

k le

ngth

(m

m)

7449-T79

7056-T79

2195-T8

2195 T851

Conv. advanced 7000 alloys

10

FCG in metals can be described with 4 basic regimes� FCG curves for different Al alloys and other metals (steel, Ti) can be reduced to

master curves (e.g.. stage II)

� Effective stress intensity factor (divided by E) is the main parameter

Water adsorption controlled regime

Hydrogen assisted regime

Intrinsic stage II

Pseudo –stage I

[Henaff G. & Petit J. 1992 – 1995]

11

R= 0.1 Lab air and vacuum

stage II assisted by water adsorption

intrinsic stage II

pseudo stage I

H2 assisted propagation

Vac.

The 4 regimes positioning apply to AIRWARE ®

� In ∆Keff plots, all 3 alloys (sheet, plate, low and higher Li contents) behave similarly

� Environment effect amounts to x100 at ∆Keff= 3 MPa.m1/2

� No environment effect at ∆Keff> 15 MPa.m1/2

35 Hz

[S. Richard 2011]

12

Pseudo-stage I observed under vacuum

� Crystallographic fracture surface at ∆Keff < 7 MPa.m1/2

R= 0.1 Lab air and vacuum

stage II assisted by water adsorption

intrinsic stage II

pseudo stage I

H2 assisted propagation

Vac.

35 Hz2050 T8

[S. Richard 2011]

13

R= 0.1 Lab air and vacuum

stage II assisted by water adsorption

intrinsic stage II

pseudo stage I

H2 assisted propagation

Vac.

H2 assisted fracture � chevrons + smooth zones

� Chevrons associated to cracking of {111} planes

35 Hz

2198 T8

2050 T8 [S. Richard 2011]

Closure does not fully explain R effect

R=0.7 � H2O can access crack tip

An exposure based model fits with the data

1/27/2017 15

2050 T8 2050 T8

The exposure based model introduced in PREFFAS allows predicting overload effect

2050 T8

Contents

� General introduction on Al-Cu-Li alloys � AIRWARE®

� Fatigue crack growth rate of Al-Cu-Li alloys

� Overall behaviour very attractive

� Al-Cu-Li alloy fit in the framework of « Poitiers », J. Petit et al.

� Fatigue performance of Al-Cu-Li alloys

� Conventional samples

� Technological specimens

� Specific study: effect of environment on endurance fatigue With or withoutnotch

� Air or vacuum

� Conclusion

1/27/2017 17

Open hole fatigue: 2050 > 7050 or 7010

In many studies, 2050 shown to have better resistance than 7000 alloys

100

150

200

250

300

10000 100000 1000000 10000000

fatigue life

max

str

ess

(MP

a)

2050-T8

7050-T74

R=0.5 max stress = 225 MPa

1/27/2017 18

� Full surface preparation included before riveted assembly

0

50 000

100 000

150 000

200 000

250 000

300 000

350 000

7056-T79

2195-T8stringers

aver

age

fatig

ue li

fe (

cycl

es)

LLT trials, +/-130MPa

2195-T8plates

2195-T8stringers

7056-T79plates

x2x2

Low Load Transfer fatigue testing: Constant Amplitud eAIRWARE® products have increased lifetime compared to 7xxx a lloys

Peening/Picking/CAA/Primar/Rivets

AIRWARE® alloys have 2 times longer fatigue life compared to 7xxx alloys in a low load transfer assembly fatigue configuration

1/27/2017 19

0

10

20

30

40

50

60

7056-T79

7056-T79

2195-T8 2195-T8 2195-T8

ardrox + TSA

num

ber

of s

pect

rum

s

under spectrum LLT trials, with anti-buckling systemspectrum : A318 (inverted, modified)

sulfochromic + CAA

x2

Low Load Transfer fatigue testing: Variable Amplitu de AIRWARE® products have increased lifetime compared to 7xxx a lloys

� Specimen preparation: Ardrox + TSA

AIRWARE® alloys have 2 times longer fatigue life compared to 7xxx alloys: This gain is confirmed for Chromium free surface treatment

1/27/2017 20

High Load Transfer fatigue testingAIRWARE® 2050 has typical 2xxx alloys type behaviour

� Specimens preparation:

� Shot peening

� Pickling + Chromic Acid Anodizing

� Coating with primer

� Titanium rivet

AIRWARE® 2050 has a high fatigue performance in an assembled test configuration including the full surface preparation process rout e.

10000

100000

1000000

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

187213,20mm

215914,20mm

215908,30mm

215906,30mm,low Ag

187218,30mm,low Thomo

197279,10% cold-

rolling

197277 174676 174678 103000 184646180237 180243 180238 180240 145070 238557,EMC

2050 2196 2023 2027 2024

HLT60MPa +/-100MPa

2050 2196 2024

1/27/2017 21

Contents

� General introduction on Al-Cu-Li alloys � AIRWARE®

� Fatigue crack growth rate of Al-Cu-Li alloys

� Overall behaviour very attractive

� Al-Cu-Li alloy fit in the framework of « Poitiers », J. Petit et al.

� Fatigue performance of Al-Cu-Li alloys

� Conventional samples

� Technological specimens

� Specific study: effect of environment on endurance fatigue With or withoutnotch

� Air or vacuum

� Conclusion

1/27/2017 22

Tests selected to identify intrinsic behaviour

� Sample with and without a small slot at notch root

� Kt=2.15

� R=0.5 � no closure

� Vacuum (<2.10-5 mBar), air at f= 0.3 Hz, 15 Hz and 50 Hz

1/27/2017 23

Materials investigated

1/27/2017 24

Si Fe Cu Mn Mg Zn Ti Zr Ag Li

2050 0.08 max

0.10 max

3.2-3.9

0.20-0.50

0.20-0.6

- 0.10 max

0.06-0.14

0.20-0.7

0.7-1.3

7010 0.12 max

0.15 max

1.5-2.0

- 2.1-2.6

5.6-6.7

0.06 max

0.10-0.16

- -

2024 0.50 max

0.50 max

3.8-4.9

0.30-0.9

1.2-1.8

- 0.15 max

- - -

2024 T3 30 mm7010 T74 110 mm2050 T8 90 mm

2050: less constituents > 7 microns than 7010

1/27/2017 25

1

10

100

1000

2

2.5 3

3.5 4

4.5 5

5.5 6

6.5 7

7.5 8

8.5 9

9.5

10

Mor

e

Nb

part

icle

s/m

m²

Dcircle (µm)

2050

7010

2024

Air, no slot: 2050 ~ 2024 > 7010

� No frequency effect

1/27/2017 26

50000

100000

150000

200000

250000

300000

350000

0 10 20 30 40 50 60

Nb

of

cycl

es

to f

ail

ure

Test frequency (Hz)

Air - no slot - 225 MPa

2024 T351

2050 T851

7010 T7451

Air, with the slot: 2024 > 2050 > 7010

� No frequency effect

1/27/2017 27

50000

100000

150000

200000

0 10 20 30 40 50 60

Nb

of

cycl

es

to f

ail

ure

Test frequency (Hz)

Air - with 0.3 mm slot - 150 MPa

2024 T351

2050 T851

7010 T7451

Vacuum, no slot: smaller difference between alloys

� No frequency effect

1/27/2017 28

0

200000

400000

600000

800000

1000000

1200000N

um

be

r o

f cy

cle

s to

fa

ilu

re

Vacuum - 225 MPa

2024 T351

2050 T851

7010 T7451

2050 T851 with slot

7010 T7451 with slot

With slot

No slot

Most initiation occurs on constituent particlesfor 7050 (similar to 7010) and 2050

� Analysis of crack initiation by FEG-SEM (after failure)

� � Detection of the origin of fatigue failure

� � in most cases : presence of insoluble constituent particle

alloy 7050, 150MPa, 105986 cyclesAl7Cu2Fe

alloy 2050, 220MPa, 32000 cycles(Al, Cu, Fe, Mn) particle

1/27/2017 29

Some initiation occurs on slip bands, for 2050-T8 o nly

� Analysis of crack initiation by FEG-SEM (after failure)

� At low stress level:� No constituent particle

� Slip bands are present

Alloy 2050, 180MPa, 327960 cycles

1/27/2017 30

Initiation on slip bands corresponds to low stress level and high fatigue life

� FEG-SEM of fatigue initiation� Initiation on slip bands obtained only for 2050, for stress levels≤200MPa

100

120

140

160

180

200

220

240

260

280

10000 100000 1000000 10000000

fatigue life

max

str

ess(

MP

a)

2050-T87050-T74

Slip bands

� The fatigue life is higher when initiation occurs on slip bands compared to initiation on constituent particles

1/27/2017 31

Inter-striation distance is higher for 7050 than fo r 2050

� Measure of inter-striation by FEG-SEM on failed samples at 1mm from the hole

� 1 striation = 1 cycle

2050, 200MPa7050, 200MPa

1/27/2017 32

Striation distance is consistent with surface crack growth rate ( � slower rate for 2050)

�da/dN obtained by:

�Measurement of small cracks by microscopy on the surface

�Inter-striation distance on fractographs in the bulk

�For both methods:

�da/dN (2050)<da/dN (7050)

0

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

0.0007

0.0008

0 2 4 6a (surface) (mm)

da/d

N (

mm

/ctc

le)

7050-T74, surface crack, 160MPa2050-T8, surface crack, 160MPa7050-T74, striation, 160MPa2050-T8, striation, 160MPa

Note: presentation as a function of a rather than ∆K avoids theoretical issues with K definition of small cracks in open hole specimens

1/27/2017 33

2050 performance: slower FCGR, (smaller constituents)

1/27/2017 34

0

50

100

150

200

250

50 100 150 200 250 300

stre

ss f

or

3.1

0^

5 c

ycl

es

in a

ir (

MP

a)

Stress for 3 10^5 cycles in vacuum (MPa)

2050 T8

7010 T76

2024 T3

Slot

No slot

vacuum effect

Propagation faster in 7010

Propagation faster in 7010 + larger particles

In air

FCGR difference even larger in vacuum

Initiation lives similar in vacuum

Conclusion

� Al-Cu-Li alloys offer can fulfill the requirements on the whole aircraft

� Very high strength, higher than 7000 can be combined with 2000 type fatigue and damage tolerance properties

� Some very high damage tolerance versions show better DT than best conventional

� The effect of environment is significant and needed to understand the propagation behaviour

� Better fatigue properties than 7000 are mainly due to FCG improvement

1/27/2017 35