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