Mechanisms for the retardation of fatigue cracks following single tensile overloads: behavior in aluminum-lithium alloys

Mechanisms for the transient crack growth retardation of fatigue cracks following single tensile overloads are examined in a high-strength aluminium-lithium alloy 2090-T8E41, with specific emphasis on the role of fatigue crack closure. Mechanically, it is found that following the application of the overload, an immediate acceleration in fatigue crack growth is observed, which results from a reduction in (far-field) crack closure levels due to crack tip blunting. Subsequent crack growth retardation, as the crack penetrates the overload plastic zone, is associated with enhanced crack tip shielding prompted by compressive residual stresses ahead of the crack tip, crack deflection and consequent (near-tip) crack closure induced by asperity wedging in the immediate crack wake. Aluminum-lithium alloys, by virtue of their marked anisotropy and planar slip characteristics which promote meandering crack paths and hence high levels of shielding, are found to show large load interaction effects, and thus rank superior to other high strength aluminum alloys under tension-dominated variable amplitude loading spectra. However, under conditions where specimen surface layers are removed, or where cycling is performed at high (positive) load ratios, or involves compressive overload cycles (all processes which tend to limit the development of crack closure), the superior performance of aluminum-lithium alloys under variable amplitude fatigue loading may be compromised.