1 / 21

7 th National Turbine Engine HCF Conference

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

xiu
Download Presentation

7 th National Turbine Engine HCF Conference

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 7th 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 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.

  2. Motivation • 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 • High Cycle Fatigue (HCF) • Low Cycle Fatigue (LCF) • Foreign Object Damage (FOD) • Fretting

  3. 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)

  4. Problem 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 mm) through-thickness cracks and small (<50 mm) surface cracks

  5. Material & Microstructures Investigated Alloy Composition (wt%) A Ti Al V Fe O N H Bal. 6.29 4.17 0.19 0.19 0.013 0.0041 bimodal (STOA) structure 64% primary a, grain size ~ 20 mm a lath spacing ~ 1-2 mm Uniaxial Tensile Properties Yield Strength Ultimate Tensile Reduction Fracture Toughness (MPa) Strength (MPa) in Area (%) KIc (MPam) A: 930 978 45 64 B: 975 1055 10 100 B -annealed lamellar structure prior-b grain size ~ 1 mm a colony size ~ 500 mm, a lath spacing ~ 1-2 mm

  6. Short cracks from notches Small cracks Large cracks Mode I large crack threshold Small Fatigue Cracks • Cracks that can be considered “small”: Ritchie and Lankford, Mater. Sci. Eng. A, 1986

  7. Large, Short and Small Fatigue Cracks • Large in all dimensions • Small in one dimension • Reduced crack-tip shielding • Small in all dimensions • Reduced crack-tip shielding • Biased microstructural sampling Ritchie and Lankford, Mater. Sci. Eng., 1986

  8. w M M Asymmetric Four-Point Bend Specimen • The offset s, from the load-line is used to control the degree of mode-mixity, DKII /DKI, and hence the phase angle,  = tan-1 (DKII /DKI) • Range of mixities studied: DKII/DKI from 0 to 7.1; b from 0 to 82 • Linear-elastic stress-intensity solutions from He and Hutchinson, J. Appl. Mech.,2000: • KI =

  9. Bimodal Ti-6Al-4V 25oC, Air =82o Growth No Growth =62o R=0.1 R=0.5 R=0.8 =26o =82o Lamellar Ti-6Al-4V 25oC, Air Growth No Growth =62o R=0.1 R=0.5 R=0.8 =26o Large Crack Thresholds • Lamellar microstructure shows superior resistance, especially at low phase angles • Load ratio, R, and mode mixity, b, can reduce DKI significantly for both microstructures Nalla, Campbell & Ritchie, Fat. Fract. Eng. Mater. Struct., 2002

  10. 11 11 Lamellar Ti-6Al-4V 25oC, Air Bimodal Ti-6Al-4V 25oC, Air 10 10 R=0.1 R=0.1 9 9 8 8 R=0.5 R=0.5 6 R=0.8 R=0.8 5 90 7 3 0 7 6 5 3 0 Single Parameter Characterization DG= (DKI2+ DKII2)/E′ • Lamellar microstructure shows superior resistance, especially at low phase angles • Threshold DGTH measured in pure mode I can be considered as “worst-case” Nalla, Campbell & Ritchie, Fat. Fract. Eng. Mater. Struct., 2002

  11. MTS = 39.7o exp ~ 39o applied = 26o Mode I MTS = 60.8o exp ~ 37o Mode I applied = 62o Large Fatigue Crack Profiles • 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 Campbell & Ritchie, Metall. Mater. Trans. A, 2001

  12. Correction for Crack-tip Shielding • 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: DKI,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: DKII,eff = DKII,maxtip - DKII,mintip Campbell & Ritchie, Eng. Fract. Mech., 2000

  13. Shielding Corrected Thresholds • 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 Nalla, M.S. Thesis, U.C. Berkeley, 2001

  14. 11 10 9 8 Bimodal Ti-6Al-4V 25oC, Air 7 R=0.1 6 5 R=0.1 R=0.5 3 R=0.5 R=0.8 0 Shielding-Corrected Large Crack Scatter band Short Crack R=0.8 11 Lamellar Ti-6Al-4V 25oC, Air Small Crack 10 9 8 7 R=0.1 6 R=0.1 5 Shielding-Corrected Large Crack Scatter band R=0.5 3 R=0.5 R=0.8 0 R=0.8 Short Crack Short-Crack Thresholds • 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 Nalla, Campbell & Ritchie, Fat. Fract. Eng. Mater. Struct., 2002

  15. KI k1 KII  k2 a KI k2 k1 KII  b a Definition of the Mixed-Mode Threshold • G calculation based on precrack direction of subsequent propagation G= (KI2 +KII2)/E′ where k1 = KIk2 = KII • G calculation based on infinitesimal kink Geff= (kI2 +kII2)/E′ where k1 = aII(a)KI + aI2(a)KIIk2 = a2I(a)KI + a22(a)KII b << a Nalla, Campbell & Ritchie, Int. J. Fatigue, 2002

  16. Definition of the Mixed-Mode Threshold • In general, the trend is to reduce the computed values of DKeq,TH somewhat, except at very high phase angles • At b = 26o, however, the large crack DKeq,TH threshold is reduced by as much as 40%; this translates into a reduction in threshold DKeq,TH values by between 1 and 2 MPam • Effects are far less significant for short cracks • Nalla, Campbell & Ritchie, Int. J. Fatigue, 2002

  17. 10 m Small Crack Thresholds in Mode I • 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 a-grains 1 to 3 revealed near-basal orientation of the fracture plane. (Courtesy: Dr. J.O. Peters) Nalla et al, Metall. Mater. Trans. A, 2002

  18. Mixed-Mode Small-Crack Testing KI – Newman & Raju, Eng. Fract. Mech., 1981 KII – He & Hutchinson, Eng. Fract. Mech., 2000 wide bend bar specimen • the tensile loading component, 22 induces the mode I contribution • the shear loading component, 12 induces the mode II and mode III components • the in-plane component, 11 makes no contribution. small “inclined-crack” specimen Nalla, Campbell & Ritchie, Fatigue Fract. Eng. Mater. Struct., 2002

  19.  ~ 50o  5 m initial precrack subsequent crack growth Inclined Semi-Elliptical Surface Crack • A typical crack path taken by a microstructurally-small crack under mixed-mode loading (R = 0.1,  ~ 28o, G ~ 20 J/m2, angle of inclination  ~ 50o) • Strong influence of local microstructure near the crack tip is evident on the crack path  Nalla, Campbell & Ritchie, Fatigue Fract. Eng. Mater. Struct., 2002

  20. Bimodal Ti-6Al-4V 25oC, Air Large Crack R=0.8 Shielding-Corrected Large Crack Scatter band 6 5 Short Crack Small Crack 4 3 2 0 Mixed-Mode Small-Crack Thresholds • 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 DKEQ,TH (up to ~7 times) and DGTH (up to ~50 times) with respect to large cracks seen for microstructurally-small cracks Nalla, Campbell & Ritchie, Fatigue Fract. Eng. Mater. Struct., 2002

  21. Conclusions • Marked effect of mode-mixity and load ratio on mixed-mode fatigue thresholds for large (> 4 mm) through-thickness cracks • Thresholds DGTH 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.

More Related