Surface Crack Testing of Welded Titanium Sheet Daniel B. Garcia, Peter C. McKeighan Southwest Research Institute ® May 23, 2007

Overview Review of ATSM E Review of ATSM E Annealed and as-welded Ti-6Al-4V sheet Annealed and as-welded Ti-6Al-4V sheet Developing surface cracks in thin sheets Developing surface cracks in thin sheets Testing surface cracks Testing surface cracks Fractographic results Fractographic results Test results Test results Summary Summary

ASTM E E is a standard practice E is a standard practice Measures residual strength, not toughness Measures residual strength, not toughness Based on original crack dimensionBased on original crack dimension Questionable applicability for aspect ratios less than a/c<0.2 Questionable applicability for aspect ratios less than a/c<0.2 Newman-Raju curve fit providedNewman-Raju curve fit provided Only ASTM standard practice available to assess surface flaws Only ASTM standard practice available to assess surface flaws … no assurance of meaningful information… … no assurance of meaningful information… Results useful when test exactly matches service conditions …Structurally Relevant Results useful when test exactly matches service conditions …Structurally Relevant

Shallow Surface Cracks E740 uses curve fit to establish SIF E740 uses curve fit to establish SIF Curve fit diverges at low aspect ratios Curve fit diverges at low aspect ratios NASGRO solution uses Newman-Raju FEA data and interpolates/extrapolates actual data NASGRO solution uses Newman-Raju FEA data and interpolates/extrapolates actual data Aspect ratios of interest, a/c=0.2, 0.13, 0.1 Aspect ratios of interest, a/c=0.2, 0.13, 0.1 NASGRO more conservative at low aspect ratios NASGRO more conservative at low aspect ratios Significance? Significance?

Ti-6Al-4V Sheet Received 4 as-welded sheets and 1 mill- annealed virgin sheet Received 4 as-welded sheets and 1 mill- annealed virgin sheet Welded sheets 9-inches high with 35 inch radius bow Welded sheets 9-inches high with 35 inch radius bow u = ksi u = ksi y = ksi y = ksi Material supplied for thru- crack and surface crack testing Material supplied for thru- crack and surface crack testing

Creating Shallow Surface Cracks Needed to generate shallow surface cracks Needed to generate shallow surface cracks Implemented EDM linear array plunger technique Implemented EDM linear array plunger technique Started with 8 divots, later to 12 Started with 8 divots, later to wide, high, later high wide, high, later high Close spaced EDM notches Close spaced EDM notches Notches on Weld or HAZ Notches on Weld or HAZ HAZ notched approx from weld interfaceHAZ notched approx from weld interface HAZWeld Crown

Precracking 4 point bend 4 point bend 108 ksi, R= ksi, R=0.1 Problems Problems Edge crackingEdge cracking Weld interface crackingWeld interface cracking No cracking on weld crownNo cracking on weld crown Crack front scallopingCrack front scalloping Visually monitor notches throughout test Visually monitor notches throughout test Wait for linking of notchesWait for linking of notches Cycles 40, ,000Cycles 40, ,000

Precracking Iterations – Lessons Learned No problem precracking parent material No problem precracking parent material Edge cracking at weld interface noted on welded specimens Edge cracking at weld interface noted on welded specimens Solution required multiple iterations Solution required multiple iterations Blending the weld edgeBlending the weld edge Shot PeeningShot Peening Removing weld crownRemoving weld crown Method for determining sufficient crack size Method 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 Cracking Shot Peen Edges Polish Weld Crown at Edge

Toughness Testing Multiple gages Multiple gages Dual Probe Potential Drop (PD)Dual Probe Potential Drop (PD) Clip GagesClip Gages Strain GagesStrain Gages LVDTLVDT LoadLoad Anti-Bucklers used Anti-Bucklers used Actually a measure of residual strength Actually a measure of residual strength

First Specimen Fracture Surface EDM (8) fatigue crack growth dimple rupture (slow stable tearing) fast fracture EDM (8) front of TL-1

Comparison of Notch Configurations 8 Notches High Wide Cracking on Mutliple Planes 12 Notches High Wide More Uniform Surface Crack

Surface Crack in Weld

Surface Crack in HAZ

Results Bending less than 5% of gross stress Bending less than 5% of gross stress Use PD to correlate rupture Use PD to correlate rupture PD indicated when surface crack broke through sheetPD indicated when surface crack broke through sheet Parent tests – generally ductile Parent tests – generally ductile HAZ – half brittle, other half exhibit stable growth HAZ – half brittle, other half exhibit stable growth Weld – generally brittle Weld – generally brittle Parent HAZWeld

Results in Context of E Fracture behavior quantified by residual strength Fracture behavior quantified by residual strength Cannot track crack growth during test Cannot track crack growth during test Possibly implement surface crack PD methodPossibly implement surface crack PD method Parent Metal – net / flow ~1.0 Parent Metal – net / flow ~1.0 Tensile properties dictate fractureTensile properties dictate fracture Similar behavior with HAZSimilar behavior with HAZ No influence of specimen width observed in surface crack experiments No influence of specimen width observed in surface crack experiments Structural relevance is key to interpretation of results Structural relevance is key to interpretation of results Fracture surfaces exhibited different morphology dependent upon weld, HAZ, or parent Fracture surfaces exhibited different morphology dependent upon weld, HAZ, or parent

Effect of Crack Depth Study K depth as a function of crack depth Study K depth as a function of crack depth Crack depth after precracking usedCrack depth after precracking used Shallow cracks more critical Shallow cracks more critical Deeper cracks exhibit higher apparent toughness Deeper cracks exhibit higher apparent toughness Pop-in behavior appears to exhibit valid toughness Pop-in behavior appears to exhibit valid toughness

Summary Performed surface crack fracture experiments on thin annealed Ti-6Al-4V sheets according to ASTM E740 Performed surface crack fracture experiments on thin annealed Ti-6Al-4V sheets according to ASTM E740 Developed robust method for generating shallow surface flaws in thin walled specimens ( deep and 0.30 wide) Developed robust method for generating shallow surface flaws in thin walled specimens ( deep and 0.30 wide) Results must be interpreted in the context of structural significance Results must be interpreted in the context of structural significance i.e. if thin walled short panels are similar to application, then tests are valid, if other application … use with cautioni.e. if thin walled short panels are similar to application, then tests are valid, if other application … use with caution