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Surface Crack Testing of Welded Titanium Sheet
Surface Crack Testing of Welded Titanium Sheet
Daniel B. Garcia, Peter C. McKeighanSouthwest Research Institute®
May 23, 2007
Daniel B. Garcia, Peter C. McKeighanSouthwest Research Institute®
May 23, 2007
OverviewOverview
Review of ATSM E740-03Review of ATSM E740-03
Annealed and “as-welded”Annealed and “as-welded” Ti-6Al-4V sheetTi-6Al-4V sheet
Developing surface cracks in thin sheetsDeveloping surface cracks in thin sheets
Testing surface cracksTesting surface cracks
Fractographic resultsFractographic results
Test resultsTest results
SummarySummary
ASTM E740-03ASTM E740-03
E740-03 is a standard E740-03 is a standard practicepractice
Measures residual Measures residual strength, not toughnessstrength, not toughness
• Based on original crack Based on original crack dimensiondimension
Questionable applicability Questionable applicability for aspect ratios less than for aspect ratios less than a/c<0.2a/c<0.2
• Newman-Raju curve fit Newman-Raju curve fit providedprovided
Only ASTM standard Only ASTM standard practice available to practice available to assess surface flawsassess surface flaws
… “… “no assurance of no assurance of meaningful information…”meaningful information…”
Results useful when test Results useful when test exactly matches service exactly matches service conditions …Structurally conditions …Structurally RelevantRelevant
2c
at
W
2c
at
W
Shallow Surface CracksShallow Surface Cracks
E740 uses curve fit to E740 uses curve fit to establish SIFestablish SIF
Curve fit diverges at low Curve fit diverges at low aspect ratiosaspect ratios
NASGRO solution uses NASGRO solution uses Newman-Raju FEA data and Newman-Raju FEA data and interpolates/extrapolates interpolates/extrapolates actual dataactual data
Aspect ratios of interest, Aspect ratios of interest, a/c=0.2, 0.13, 0.1a/c=0.2, 0.13, 0.1
NASGRO more conservative NASGRO more conservative at low aspect ratiosat low aspect ratios
Significance?Significance?a/t
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Kde
pth (
ksi
in)
for b
ulk=
140k
si
0
20
40
60
80
100
120
2c
at
W
2c
at
W
c=0.1
c=0.15
c=0.2
Newman Raju
NASGRO
t=0.030 inches
W=2.0 inches
NASGRO Limita=0.020 in
Ti-6Al-4V SheetTi-6Al-4V Sheet
Received 4 as-welded Received 4 as-welded sheets and 1 mill-annealed sheets and 1 mill-annealed virgin sheetvirgin sheet
Welded sheets 9-inches Welded sheets 9-inches high with 35 inch radius high with 35 inch radius bowbow
uu=142-148 ksi=142-148 ksi
yy=130-135 ksi=130-135 ksi
Material supplied for thru-Material supplied for thru-crack and surface crack crack and surface crack testingtesting
Creating Shallow Surface CracksCreating Shallow Surface Cracks
Needed to generate shallow Needed to generate shallow surface crackssurface cracks
Implemented EDM linear array Implemented EDM linear array plunger techniqueplunger technique
Started with 8 divots, later to 12Started with 8 divots, later to 12
S o u t h w e s t R e s e a r c h I n s t i t u t eS a n A n t o n i o, T e x a s
Title
Drawing Number
Filename
Scale
Sheet 4 of 5
none
Different Initial EDM Plunge Notch Configurations
18.11887 (JPL)
BasicDimension
under 6
6-24 incl
over 24
2 place
±0.01
±0.03
±0.06
±0° 30'
3 placeFrac-tions
angles
Decimals
±0.005
±0.010
±0.015
±1/32
±1/16
±1/6
±1° 0'
Tolerances (inches)
Drawn:
Date:
P. C. McKeighan
17 October 2005
Matl:
Finish:
QtyReqd: c:\pcm\drawings\spec-dwgs.prz
Ti-6Al-4V
64 RMS
n/a
weld and specimen dimensions exagerated for clarity
weld
2.001.00
weld bead center flaw(on crown of weld if crown present)
HAZ (offset) flaw
0.005
IMPORTANT: ONLY ONE FLAW TYPE PER SPECIMEN
0.030
CL
A
A
Section -AA-
CL
0.005(12 places)
0.010 0.015
0.0075
0.025 B B
Section -BB-
0.0100.005
12 (or 8 in a limited number of cases) Total EDM Divots
0.010” wide, 0.005” high, later 0.010” wide, 0.005” high, later 0.0035” high0.0035” high
Close spaced EDM notchesClose spaced EDM notches
Notches on Weld or HAZNotches on Weld or HAZ
• HAZ notched approx 0.005” from HAZ notched approx 0.005” from weld interfaceweld interface
HAZ
Weld Crown
PrecrackingPrecracking
4 point bend4 point bend
• 108 ksi, R=0.1108 ksi, R=0.1
ProblemsProblems
• Edge crackingEdge cracking
• Weld interface crackingWeld interface cracking
• No cracking on weld No cracking on weld crowncrown
• Crack front scallopingCrack front scalloping
Visually monitor notches Visually monitor notches throughout testthroughout test
• Wait for linking of notchesWait for linking of notches
• Cycles 40,000 - 80,000Cycles 40,000 - 80,000
Precracking Iterations – Lessons Learned
Precracking Iterations – Lessons Learned
No problem precracking parent materialNo problem precracking parent material
Edge cracking at weld interface noted on welded specimensEdge cracking at weld interface noted on welded specimens
Solution required multiple iterationsSolution required multiple iterations
• Blending the weld edgeBlending the weld edge
• Shot PeeningShot Peening
• Removing weld crownRemoving weld crown
Method for determining sufficient crack sizeMethod for determining sufficient crack size
• Link-up of EDM notchesLink-up of EDM notches
• Growth of surface flaw beyond 15 mils of last EDM notchGrowth of surface flaw beyond 15 mils of last EDM notch
• Large change in PD that correlates to back face plastic zoneLarge change in PD that correlates to back face plastic zone
Solutions to Edge CrackingSolutions to Edge Cracking
Shot Peen Edges
Polish Weld Crownat Edge
Toughness TestingToughness Testing Multiple gagesMultiple gages
• Dual Probe Potential Drop (PD)Dual Probe Potential Drop (PD)
• Clip GagesClip Gages
• Strain GagesStrain Gages
• LVDTLVDT
• LoadLoad
Anti-Bucklers usedAnti-Bucklers used
Actually a measure of residual strengthActually a measure of residual strength
S o u t h w e s t R e s e a r c h I n s t i t u t eS a n A n t o n i o, T e x a s
Title
Drawing Number
Filename
Scale
Sheet 5 of 5
none
Instrumentation Positions on JPL Fracture Specimens
18.11887 (JPL)
BasicDimension
under 6
6-24 incl
over 24
2 place
±0.01
±0.03
±0.06
±0° 30'
3 placeFrac-tions
angles
Decimals
±0.005
±0.010
±0.015
±1/32
±1/16
±1/6
±1° 0'
Tolerances (inches)
Drawn:
Date:
P. C. McKeighan
25October 2005
Matl:
Finish:
QtyReqd:
2.000
c:\pcm\drawings\spec-dwgs.prz
Ti-6Al-4V
64 RMS
n/a
as received(8.875 typical)
NO SCALE
dogbone specimen shown -- same setup for plane sided specimen
crack (or divots)centerline of
specimen/crack
DESCRIPTIONSYMBOL
voltage probes (2 x 2, inner and outer probe sets)
current input position (2)
strain gage (4, front and back)
insulated clip gage knife edges (2 x 2, front and back)
0.100
0.075
1.20
1.60
3.20
Dimensions referenced from notch position
First Specimen Fracture SurfaceFirst Specimen Fracture Surface
EDM (8)
fatigue crack growth
dimple rupture (slow stable tearing) fast fracture
EDM (8) front of TL-1
Comparison of Notch ConfigurationsComparison of Notch Configurations
Fractograph Description
Specimen ID/Test No.:TL2-PS-2
Crack Location: Parent
Crack Type: Surface
Fractograph Description
Specimen ID/Test No.:F1B-PS-4
Crack Location: Parent
Crack Type: Surface
8 Notches0.005” High 0.010” Wide
Cracking on Mutliple Planes
12 Notches0.0035” High 0.010” Wide
More Uniform Surface Crack
Surface Crack in WeldSurface Crack in Weld
Fractograph Description
Specimen ID/Test No.:C1D-WS-1
Crack Location: Weld
Crack Type: Surface
Surface Crack in HAZSurface Crack in HAZ
Fractograph Description
Specimen ID/Test No.:C2D-HS-5
Crack Location: HAZ
Crack Type: Surface
ResultsResults
Bending less than 5% of gross stressBending less than 5% of gross stress
Use PD to correlate ruptureUse PD to correlate rupture
• PD indicated when surface crack broke through sheetPD indicated when surface crack broke through sheet
Parent tests – generally ductileParent tests – generally ductile
HAZ – half brittle, other half exhibit stable growthHAZ – half brittle, other half exhibit stable growth
Weld – generally brittleWeld – generally brittle
Test ID: F1A-PS-3
Average CMOD Displacement, mils
0 2 4 6 8 10 12
Ap
pli
ed L
oad
, k
ips
0
1
2
3
4
5
6
7
8
Po
ten
tial
Dro
p O
utp
ut,
vo
lts
5
6
7
8
9
10
11
loadPD (100x)
Test ID: C1F-HS-1
Average CMOD Displacement, mils
0 2 4 6 8 10
Ap
pli
ed L
oa
d,
kip
s
0
1
2
3
4
5
6
7
8
Po
ten
tial
Dro
p O
utp
ut,
vo
lts
4.1
4.2
4.3
4.4
4.5
loadPD (50x)
Test ID: C1D-WS-1
Average CMOD Displacement, mils
0 2 4 6 8 10 12
Ap
pli
ed L
oad
, k
ips
0
1
2
3
4
5
6
7
Po
ten
tial
Dro
p O
utp
ut,
vo
lts
3
4
5
6
7
8
9
10
loadPD (50x)
ParentHAZ Weld
Results in Context of E740-03Results in Context of E740-03
Fracture behavior quantified by residual strengthFracture behavior quantified by residual strength
Cannot track crack growth during testCannot track crack growth during test
• Possibly implement surface crack PD methodPossibly implement surface crack PD method
Parent Metal – Parent Metal – netnet/ / flowflow ~1.0 ~1.0
• Tensile properties dictate fractureTensile properties dictate fracture
• Similar behavior with HAZSimilar behavior with HAZ
No influence of specimen width observed in surface No influence of specimen width observed in surface crack experimentscrack experiments
Structural relevance is key to interpretation of resultsStructural relevance is key to interpretation of results
Fracture surfaces exhibited different morphology Fracture surfaces exhibited different morphology dependent upon weld, HAZ, or parentdependent upon weld, HAZ, or parent
Effect of Crack DepthEffect of Crack Depth
solid points: maximum loadopen points: pop-in load
Normalized Crack Depth, a/t
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Kd
epth
(m
ax o
r p
op
-in
), k
si i
n
30
40
50
60
70
80 parentweldHAZForman (NASA TM 102165)
Simulation:W = 2-inch 2c = 0.320-inchcrack depth a varied
=
flow
(142
ksi
) =
0.8
flow
= 0
.67
flow
lower exte
nt of d
ata
Surface Crack Tests Study KStudy Kdepthdepth as a function as a function
of crack depthof crack depth
• Crack depth after Crack depth after precracking usedprecracking used
Shallow cracks more Shallow cracks more criticalcritical
Deeper cracks exhibit Deeper cracks exhibit higher apparent higher apparent toughnesstoughness
Pop-in behavior appears Pop-in behavior appears to exhibit valid toughnessto exhibit valid toughness
SummarySummary
Performed surface crack fracture experiments on thin Performed surface crack fracture experiments on thin annealedannealed Ti-6Al-4V sheets according to ASTM E740Ti-6Al-4V sheets according to ASTM E740
Developed robust method for generating shallow Developed robust method for generating shallow surface flaws in thin walled specimens (0.015-0.025” surface flaws in thin walled specimens (0.015-0.025” deep and 0.30” wide)deep and 0.30” wide)
Results must be interpreted in the context of Results must be interpreted in the context of structural significancestructural significance
• i.e. if thin walled short panels are similar to application, i.e. if thin walled short panels are similar to application, then tests are valid, if other application … use with then tests are valid, if other application … use with cautioncaution