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

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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.