STP868: Observations on a Fracture Mechanics Approach to Fatigue Crack Initiation in Ti-6Al-4V

    Yoder, GR
    Metallurgist, Metallurgist, and Mechanical Engineer, Mechanics of Materials Branch, Material Science and Technology Division, Naval Research Laboratory, Washington, DC,

    Cooley, LA
    Metallurgist, Metallurgist, and Mechanical Engineer, Mechanics of Materials Branch, Material Science and Technology Division, Naval Research Laboratory, Washington, DC,

    Crooker, TW
    Metallurgist, Metallurgist, and Mechanical Engineer, Mechanics of Materials Branch, Material Science and Technology Division, Naval Research Laboratory, Washington, DC,

    Pages: 14    Published: Jan 1985


    Abstract

    Resistance to the initiation of fatigue cracks in Ti-6Al-4V has been determined from blunt-notched (keyhole) and sharp-notched (V) compact specimens of the WOL type. The primary alloy studied was in the mill-annealed condition with an ultimate tensile strength of 1034 MPa (150 ksi). In the case of the blunt-notched specimen, the notch root radius was ρ = 1.59 mm (0.0625 in.); for the sharp-notched specimen, ρ = 0.25 mm (0.010 in.). The influence of surface roughness, 0.4 μm versus 1.8 μm (16 μin. versus 72 μin.), on fatigue crack initiation resistance has also been examined. Notch root surfaces were specially prepared to remove all machining marks in the thickness direction. Specimens were cycled under constant load amplitude conditions in air with a stress ratio of R = 0.10. Cycles-to-initiation data normalize very well for the two specimen types when analyzed in terms of the ratio of stress intensity factor range to the square root of the notch root radius, ΔKρ. The fatigue crack initiation resistance also appears to be independent of surface roughness in the notch root region. Moreover, fatigue crack initiation resistance from these notched compact-type specimens is found to correlate well with fatigue strength determined from smooth, rotating beam specimens of this very same material. Finally, fatigue crack initiation resistance has been measured with the blunt notch compact specimen for a number of beta-annealed Ti-6Al-4V alloys with a broad range of oxygen contents (0.06 to 0.20 wt%). Though oxygen content can have a pronounced influence on fracture toughness, the present data do not indicate a significant influence of oxygen content on fatigue crack initiation resistance.

    Keywords:

    fatigue (materials), crack initiation, titanium alloys, fatigue strength, fracture mechanics, surface finish, notch root radius, oxygen content


    Paper ID: STP34265S

    Committee/Subcommittee: E08.08

    DOI: 10.1520/STP34265S


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