Fatigue Damage Mechanism in Very High Cycle Regime Nian Zhou 2012.5.24
Fatigue Low cycle fatigue N f 10 5 cycles Strain-controlled fatigue High cycle fatigue N f f 10 5 cycles Stress-based fatigue Endurance limit 7 Surviving 10 cycles
Very High Cycle Fatigue Fracture failure N f f 10 cycles stress lower than the conventional fatigue limit Applied stress below the yield strength No fatigue limit 7 Mechanism for fatigue damage and crack initiation in the VHCF regime? The concept of gigacycle S N curve
Materials One-phase material Alloy 690 Two-phase material SAF 2205 SAF 2507 Martensite-austenite Multi-phase material Martensite-bainite Titanium Ti6Al4V
Experimental Fatigue test Amlser testing machine + giga cycle tester Slip bands investigation LOM + SEM Fatigue crack initiation and propagation SEM Giga cycle fatigue mechanism EBSD In-situ EBSD TEM
Results and Discussions Fatigue life and S-N characteristics Fatigue crack initiation and fracture surface study by SEM Fatigue pre-damage investigation Localized strain concentration, misorientation change Damage accumulation study
Results and Discussions Fatigue life and S-N characteristics Fatigue crack initiation and fracture surface study by SEM Fatigue pre-damage investigation Localized strain concentration, misorientation change Damage accumulation study
Fatigue Life and S-N Characteristics
Results and Discussions Fatigue life and S-N characteristics Fatigue crack initiation and fracture surface study by SEM Fatigue pre-damage investigation Localized strain concentration, misorientation change Damage accumulation study
Fatigue Crack Initiation and Fracture Surface Study in VHCF Regime Alloy 690 Duplex Martensite- Austenite Martensite- Bainite Ti6Al4V SNDFCO Sub-surface defect Fish-eye or SNDFCO SNDFCO Fish-eye or SNDFCO SNDFCO:Subsurface non-defect fatigue crack origins
Results and Discussions Fatigue life and S-N characteristics Fatigue crack initiation and fracture surface study by SEM Fatigue pre-damage investigation Localized strain concentration, misorientation change Damage accumulation study
Fatigue pre-damage investigation Slip bands -- pre-initiation of fatigue damage Alloy 690 Duplex Ti6Al4V Slip bands Slip bands Slip bands
Results and Discussions Fatigue life and S-N characteristics Fatigue crack initiation and fracture surface study by SEM Fatigue pre-damage investigation Localized strain concentration, misorientation change Damage accumulation study
Localized strain concentration, misorientation change EBSD technique Investigate damage process Samples Polish just down to the plane at the fatigue crack initiation area Presented maps Strain contouring and misorientation with relative values
Alloy 690 sample as received σ = 130MPa a N = 7.27 10 f 8 =200 µm; Map11; Step=2 µm; Grid329x261 Relative frequency 1 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0 Relative number of low-angle boundaries before and after fatigue tests for SAN 69 0,95 1,05 1,15 1,25 1,35 1,45 1,55 1,65 1,75 1,85 1,95 2,05 Misorientation ( ) as received 130MPa, 7.27E8 cycles 135MPA, 5.45E8 cycles =100 µm; Map11; Step=1,7 µm; Grid284x185 σ = 135MPa a N = 5.45 10 f 8 =100 µm; Map11; Step=1,5 µm; Grid314x211
as received sample Relative numbers of strains before and after fatigue tests for SAN 69 =200 µm; strain; Step=2 µm; Grid329x261 12 130MPa 7.27E8 cycle 10 8 aver age highest =100 µm; strain; Step=1,7 µm; Grid284x185 6 135MPa 5.45E8 cycles 4 2 =100 µm; strain; Step=1,5 µm; Grid314x211 0 As r eveived 130MPa, 7.27E8 135MPa, 5.45E8 140MPa,2.77E8
as received sample =20 µm; strain; Step=0,7 µm; Grid244x281 SAF 2205 Relative numbers of strains before and after fatigue tests for SAF 2205 8 280MPa 8.36E5 cycle 7 6 =50 µm; strain; Step=0,7 µm; Grid267x18 5 300MPa 1.2E5 cycles 4 3 2 average highest =50 µm; strain; Step=0,7 µm; Grid308x242 240MPa 2.17E9 cycles (Run out) 1 0 As received 280MPa, 8.36E5 300MPa, 1.2E5 240MPa, 2.17E9 run out =20 µm; strain; Step=0,7 µm; Grid217x2
SAF 2507 Sample as received Relative numbers of strains before and after fatigue test for SAF 2507 12 10 8 300MPa 2.29E9 cycles =50 µm; strain; Step=0,5 µm; Grid301x244 Relative stain 6 4 average highest ( Run out ) 2 0 as received 300MPa, 2.2898E9 =50 µm; strain; Step=0,5 µm; Grid301x244
Martensite-Austenite Strain contouring map at the subsurface non-defect fatigue crack origin (matrix), fatigue tested at σa =600MPa and Nf=1.19E8 cycles
Ti6Al4V Sample as received Ti6Al4V 4 3,5 3 2,5 380MPa 1.2E7 cycles ( Run out ) 2 1,5 1 aver age highest 0,5 0 as r eceived 380MPa, 1.2E7 r un out
Results and Discussions Fatigue life and S-N characteristics Fatigue crack initiation and fracture surface study by SEM Fatigue pre-damage investigation Localized strain concentration, misorientation change Damage accumulation study
Damage Accumulation Study In-situ EBSD analysis Alloy 690 Alloy 690 Stress ( MPa) 500 450 400 350 300 250 200 150 100 50 0 0 1 2 3 4 5 6 7 8 9 10 11 12 Strain (%)
As received =200 µm; Map5; Step=2 µm; Grid320x240 ε = 10,6% =200 µm; Map5; Step=2 µm; Grid320x240
Local Strain for Individual Grains 1 0 5 4 4 3,5 Local strain for individual grains 1 1 3 9 8 2 =200 µm; GB; Step=2 µm; Grid320x240 1 7 6 Local strain (%) 3 2,5 2 1,5 1 0,5 0 0 0,5 1 1,5 2 2,5 3 3,5 4 Applied strain (%) applied strain 1 2 3 4 5 6 7 8 9 10 11
Comparison of uniaxial and cyclic deformation
Conclusions Fracture will happen in the VHCF regime, no conventional fatigue limit can be defined With lowering the applied cyclic load, fatigue crack initiation shifts from surface defect to subsurface defect or subsurface non-defect fatigue crack origins (matrix) Localized strain concentration leads to crack initiation and fatigue damage in the VHCF regime Heterogeneity had been proved of the polycrystalline materials, even in the elastic region, plastic deformation can happen in some individual grains
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