STP982: On the Role of Crack Closure Mechanisms in Influencing Fatigue Crack Growth Following Tensile Overloads in a Titanium Alloy: Near Threshold Versus Higher ΔK Behavior

    Ward-Close, CM
    Visiting scientist and professor, University of CaliforniaRoyal Aircraft Establishment, Farnborough, BerkeleyHants, CA

    Ritchie, RO
    Visiting scientist and professor, University of CaliforniaRoyal Aircraft Establishment, Farnborough, BerkeleyHants, CA

    Pages: 19    Published: Jan 1988


    Abstract

    A comparative study has been made of the transient fatigue crack growth rate behavior following tensile overloads at low (near-threshold) and high stress intensity ranges in an α/β-type titanium alloy IMI 550, with specific emphasis of the role of crack closure mechanisms. After tensile overloads, fatigue cracks in both coarse-grained β-annealed and fine-grained α/β microstructures were observed initially to accelerate, followed by significant retardation, before growth rates returned to their baseline levels. The initial acceleration was attributed to an immediate reduction in near-tip closure, as indicated by metallographic sectioning, and a slight decrease in far-field closure, as measured by back-face compliance methods. Subsequent retardation was not associated with marked changes in far-field closure, although there were indications on compliance curves of a second “closure point” at a higher load, suggesting an approximate 50% increase in near-tip closure. Load interaction effects were found to be most severe where specific mechanisms of crack closure were prominant. Thus, the maximum post-overload retardations were seen in the coarse-grained α microstructure, and when baseline stress intensity ranges were close to the threshold ΔKTH, or when the maximum overload stress intensities approached the fracture toughness.

    Keywords:

    fatigue (materials), crack (fracturing), crack propagation, variable amplitude loading, tensile overloads, crack closure, fatigue thresholds, titanium alloys


    Paper ID: STP27202S

    Committee/Subcommittee: E08.06

    DOI: 10.1520/STP27202S


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