The fatigue crack propagation behavior of naturally-occurring, microstructurally-small (1–1000 μm), surface cracks is examined as a function of microstructure in commercial aluminum-lithium alloys 2090-T8E41, 8091-T351 and 2091-T351, and results compared with behavior in traditional high-strength aluminum alloys 2124 and 7150. Despite large differences in the fatigue crack propagation behavior of long (≳ 10 μm) cracks in these alloys, little difference is observed in the small crack growth resistance, with the small crack growth rates for all microstructures lying within a scatterband some 2–4 orders of magnitude higher than the near-threshold fatigue behavior of long cracks. Such results are attributed primarily to a lack of crack tip shielding (developed from crack deflection and resulting crack closure from asperity wedging) with small cracks of limited wake. Since the well-known superior fatigue crack growth resistance of aluminum-lithium alloys can be traced principally to such shielding, promoted by the branched and tortuous nature of their crack paths, the anomaly between long and small crack behavior appears to be most significant in these alloys.
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