7 th National Turbine Engine HCF Conference. Role of Crack Size and Microstructure in Influencing Mixed-Mode High Cycle Fatigue Thresholds in Ti-6Al-4V. R.K. Nalla, J.P. Campbell and R.O. Ritchie Department of Materials Science and Engineering, University of California, Berkeley, CA 94720 - PowerPoint PPT Presentation
7 th National Turbine Engine HCF Conference Role of Crack Size and Microstructure in Role of Crack Size and Microstructure in Influencing Mixed-Mode High Cycle Fatigue Influencing Mixed-Mode High Cycle Fatigue Thresholds in Ti-6Al-4V Thresholds in Ti-6Al-4V R.K. Nalla, J.P. Campbell and R.O. Ritchie Department of Materials Science and Engineering, University of California, Berkeley, CA 94720 May 15, 2002 Work supported by the U.S. Air Force Office of Scientific Research under Grant No. F49620-96- 1-0418 under the auspices of the Multidisciplinary University Research Initiative (MURI) on High Cycle Fatigue to the University of California.
Transcript
7th National Turbine Engine HCF Conference
Role of Crack Size and Microstructure in Influencing Role of Crack Size and Microstructure in Influencing Mixed-Mode High Cycle Fatigue Thresholds in Ti-6Al-4VMixed-Mode High Cycle Fatigue Thresholds in Ti-6Al-4V
R.K. Nalla, J.P. Campbell and R.O. Ritchie
Department of Materials Science and Engineering, University of California, Berkeley, CA 94720
May 15, 2002
Work supported by the U.S. Air Force Office of Scientific Research under Grant No. F49620-96-1-0418 under the auspices of the Multidisciplinary University Research Initiative (MURI) on High Cycle Fatigue to the University of California.
MotivationMotivation
• High Cycle Fatigue (HCF) has been identified as the single biggest cause of failures in military turbine engines. Such failures result in costly engine damage/loss and related down-time, in addition to loss of human life
• A successful “solution” would save ~$2 billion over the next 20 years
• A “damage-tolerant approach” may offer an alternative over the combination of Goodman Diagram/ “Safe Life” (S/N) based approach used now
• The basis of the MURI has been to seek a physical understanding behind the development of such a damage- tolerant approach
time
0 < R < 1
HCF/LCF Interactions
Foreign Object Damage
Fretting at Dovetail/Fir Tree Attachment
time
kt
residual
time
Edge of contact
Center of Contact
time
{• High Cycle Fatigue (HCF)
• Low Cycle Fatigue (LCF)
• Foreign Object Damage (FOD)
• Fretting
Why Study Multiaxial Fatigue?Why Study Multiaxial Fatigue?
• Common in turbine engines - e.g., in association with fretting in the dovetail/disk contact region
• High frequencies involved (1-2 kHz) may necessitate a threshold-based methodology incorporating mode-mixity effects
• Presence of shear loading known to dramatically reduce mode I threshold (John et al, in: Mixed-Mode Crack Behavior, ASTM STP 1359, 1999)
• No information on HCF mixed-mode thresholds for small cracks
• Multiaxial fatigue research goes back to only 1969 (Iida and Kobayashi, J. Bas. Eng., 1969), while fatigue research goes back well over a century (Albert, Archive für Minerlogie, Geognosie, Bergbau und Hüttenkunde, 1838)
• Only two studies on Ti-6Al-4V in the archival literature - by Pustejovsky (Eng. Fract. Mech., 1979) and Gao et al (Multiaxial Fatigue, ASTM STP 853, 1985)
Problem Statement and ObjectiveProblem Statement and Objective
• Very little data have been reported on the role of mode-mixity in influencing fatigue thresholds in Ti-6Al-4V alloys
• Similarly, little information is available on how microstructure can affect such mixed-mode thresholds
• There is no information on the role of crack size on mixed-mode thresholds in any material
• Hence, our objective is to:
- compare the mixed-mode HCF threshold behavior for two microstructures in Ti-6Al-4V with widely differing micro-
structural dimensions, i.e., bimodal (STOA) and lamellar
- characterize the effect of mode-mixity and load ratio on mixed-mode thresholds for cracks with widely
differing dimensions, i.e., large (>4 mm) and short (~200 m) through-thickness cracks and small (<50 m) surface cracks
Material & Microstructures InvestigatedMaterial & Microstructures Investigated
Yield Strength Ultimate Tensile Reduction Fracture Toughness (MPa) Strength (MPa) in Area (%) KIc (MPam)
A: 930 978 45 64
B: 975 1055 10 100
Ti Al V Fe O N H
Bal. 6.29 4.17 0.19 0.19 0.013 0.0041
Uniaxial Tensile Properties
Alloy Composition (wt%)
bimodal (STOA) structure 64% primary grain size ~ 20 m lath spacing ~ 1-2 m
-annealed lamellar structureprior- grain size ~ 1 mm colony size ~ 500 m, lath spacing ~ 1-2 m
A
B
Small Fatigue CracksSmall Fatigue Cracks
Ritchie and Lankford, Mater. Sci. Eng. A, 1986
Small cracks
Short cracks from notches
Large cracks
Mode I large crack threshold
Cracks that can be considered “small”:
Large, Short and Small Fatigue CracksLarge, Short and Small Fatigue Cracks
Ritchie and Lankford, Mater. Sci. Eng., 1986
• Large in all dimensions
• Small in one dimension
• Reduced crack-tip shielding
• Small in all dimensions
• Reduced crack-tip shielding
• Biased microstructural sampling
Asymmetric Four-Point Bend SpecimenAsymmetric Four-Point Bend Specimen
• The offset s, from the load-line is used to control the degree of mode-mixity, KII /KI, and hence the phase angle, = tan-1 (KII /KI)
• Range of mixities studied: KII/KI from 0 to 7.1; from 0 to 82
• Linear-elastic stress-intensity solutions from He and Hutchinson, J. Appl. Mech., 2000:
KI =
waFa
wM
I26 KII =
waF
wa
wawQ
II2/1
2/3
)/1(
)/(
wM M
Large Crack ThresholdsLarge Crack Thresholds
0 1 2 3 4 5 6 7 8M O DE I STRESS-INTENSITY R ANG E AT TH RESH O LD
K I,TH (M Pam )
0
2
4
6
8
10
12
MO
DE
II S
TRE
SS
-INTE
NS
ITY
RA
NG
E A
T TH
RE
SH
OLD
KII,
TH (M
Pa
m)
0
2
4
6
8
10
KII,
TH (k
si in
)
0 1 2 3 4 5 6 7K I,TH (ksiin )
Bimodal Ti-6Al-4V25oC, Air
Growth
No Growth
=26o
=62o
=82o
R=0.1R=0.5R=0.8
Lamellar Ti-6Al-4V25oC, Air
0 1 2 3 4 5 6 7 8M O D E I S TR ESS-INTEN SITY R AN G E AT TH R ES H O LD
K I,TH (M P am )
0
2
4
6
8
10
12
MO
DE
II S
TRE
SS
-INTE
NS
ITY
RA
NG
E A
T TH
RE
SH
OLD
KII,
TH (M
Pa
m)
0 1 2 3 4 5 6 7K I,TH (ksiin )
0
2
4
6
8
10
KII,
TH (k
si in
)
Growth
No Growth
=26o
=62o
=82o
R=0.1R=0.5R=0.8
• Lamellar microstructure shows superior resistance, especially at low phase angles
• Load ratio, R, and mode mixity, can reduce KI significantly for both microstructures
Large Fatigue Crack ProfilesLarge Fatigue Crack Profiles
Campbell & Ritchie, Metall. Mater. Trans. A, 2001
• Observed crack paths follow a path of maximum tangential stress (MTS), i.e., one of KII = 0, for the bimodal microstructure
• For the coarser-grained lamellar microstructure, significant deviations were observed from MTS predictions – the role of microstructure becomes critical, especially in the precrack wake
Mode I applied = 26°
exp
~39°
(a)
400 m
MTS =60.8°
Mode I applied = 62°
exp
~ 37°
(b)
200 m
a
Mode I applied = 26o
MTS = 39.7o
exp ~ 39o
Mode I applied = 62o
MTS = 60.8o
exp ~ 37o
Correction for Crack-tip ShieldingCorrection for Crack-tip Shielding
Campbell & Ritchie, Eng. Fract. Mech., 2000
• Mode I shielding, in the form of crack closure, determined from the compliance curve for the opening displacements from the first deviation from linearity on unloading: KI,eff = KI,max – Kcl
• Mode II shielding, in the form of asperity rubbing and interlock, determined in an analogous fashion from the compliance curve for shear displacements: KII,eff
(b) 0 10 20 30 40 50 60 70 80 90PH ASE ANG LE, (o )
0
200
400
600
800
1000
THR
ES
HO
LD S
TRA
IN E
NE
RG
Y R
ELE
AS
E
RA
TE R
AN
GE
, G
TH (J
/m2 )
M ode I M ode II
0
3
5
6
7
8
9
10
11
THR
ES
HO
LD E
QU
IVA
LEN
T S
TRE
SS
-INTE
NS
ITY
R
AN
GE
, K
eq,T
H (
MP
am
)
Bimodal Ti-6Al-4V25oC, Air
R=0.1
R=0.5
R=0.8
R=0.1
R=0.5
R=0.8
Shielding-CorrectedLarge Crack data
0 10 20 30 40 50 60 70 80 90PH AS E AN G LE , (o )
0
200
400
600
800
1000
TH
RE
SH
OLD
ST
RA
IN E
NE
RG
Y R
ELE
AS
E
RA
TE
RA
NG
E,
GT
H (
J/m
2 )
M ode I M ode II
8
0
3
5
6
7
9
10
11
TH
RE
SH
OLD
EQ
UIV
ALE
NT
ST
RE
SS
-IN
TE
NS
ITY
R
AN
GE
, K
eq,T
H (
MP
am
)
• Effects of mode-mixity, load ratio and microstructure markedly reduced after taking account of crack-tip shielding from mode I closure and mode II crack-surface interference
• The role of crack-tip shielding is evident from the substantially lower thresholds
• The technique for estimating the mixed-mode shielding by Campbell et al gives reasonable, though slightly overestimated, values for the thresholds
Definition of the Mixed-Mode ThresholdDefinition of the Mixed-Mode Threshold
• G calculation based on precrack
where
k1 = aII() KI + aI2() KII k2 = a2I() KI + a22() KII
b << aa
KII
KI
b
k2
k1
G = (KI2 + KII
2)/E′
Geff = (kI2 + kII
2)/E′
• G calculation based on infinitesimal kink
direction of subsequent propagation
where
k1 = KI k2 = KII
a
KII
KI
k2
k1
Nalla, Campbell & Ritchie, Int. J. Fatigue, 2002
Mode I applied = 26°
exp
~39°
(a)
400 m
MTS =60.8°
Mode I appl ied = 62°
exp
~ 37°
(b)
200 m
a
Nalla, Campbell & Ritchie, Int. J. Fatigue, 2002
Definition of the Mixed-Mode ThresholdDefinition of the Mixed-Mode Threshold
• In general, the trend is to reduce the computed values of Keq,TH somewhat, except at very high phase angles
• At = 26o, however, the large crack Keq,TH threshold is reduced by as much as 40%; this translates into a reduction in threshold Keq,TH values by between 1 and 2 MPam
• Effects are far less significant for short cracks
Bimodal Ti-6Al-4V25oC, Air
R = 0.1
R = 0.5
R = 0.8
Large Crack Data
0
200
400
600
800
1000
1200
Bimodal Ti-6Al-4V25oC, Air
Short Crack DataR = 0.1
R = 0.5
R = 0.8
Large Crack DataR = 0.8
THR
ES
HO
LD E
QU
IVA
LEN
T S
TRE
SS
-INTE
NS
ITY
RA
NG
E,
Keq
,TH (M
Pa
m)
0 10 20 30 40 50 60 70 80 90PHASE ANGLE, (o)
0
200
400
600
800
1000
1200
Mode I Mode II
8
0
3
5
6
7
9
10
11
0
12
8
0
3
5
6
7
9
10
11
0
12
PHASE ANGLE, (o)
THR
ES
HO
LD S
TRA
IN E
NE
RG
Y R
ELE
AS
E R
ATE
, G
TH (J
/m2 )
Small Crack Thresholds in Mode ISmall Crack Thresholds in Mode I
Nalla et al, Metall. Mater. Trans. A, 2002
• Optical micrograph showing a typical initiation site for the bimodal microstructure - Initiation predominantly occurs in the primary- grains.
• SEM image of crack initiation and early growth along planar slip bands leading to facet type fracture surface - EBSD analysis of fractured -grains 1 to 3 revealed near-basal orientation of the fracture plane.
• Thresholds for small cracks (<50 m) are significantly lower than for large (>4 mm) and short (~200 m) cracks, especially under shear-dominant loading
• Large reductions in KEQ,TH (up to ~7 times) and GTH (up to ~50 times) with respect to large cracks seen for microstructurally-small cracks
ConclusionsConclusions
• Marked effect of mode-mixity and load ratio on mixed-mode fatigue thresholds for large (> 4 mm) through-thickness cracks
• Thresholds GTH values measured in pure Mode I represent a “worst-case” condition
• Lamellar structure generally exhibited higher large-crack thresholds
• Thresholds for short (~200 m) through-thickness cracks were considerably lower and were relatively insensitive to load ratio, mode-mixity and microstructure. This was attributed to a reduced role of crack-tip shielding
• Thresholds for microstructurally-small (< 50 m) surface cracks in the bimodal microstructure were similarly insensitive to load ratio and mode-mixity, and were substantially lower than those for large cracks. This was related to limited crack-tip shielding and biased microstructural sampling associated with the small cracks.
