Abstract : The fatigue behavior of short cracks, which are small compared to the scale of the microstructure, small compared to the scale of local plasticity or simply physically small (i.e., or = 1 mm), must be considered as one of the major factors limiting the application of defect-tolerant fatigue design for airframe and engine components. Accordingly, the current program is aimed at identifying factors which govern the growth of such short cracks (in contrast to long cracks) in a series of commercial aluminum alloys, with specific reference to behavior at near-threshold levels (below approx. .000001)mm/cycle. In this annual report, the status of the program is described in terms of i) a description of results on the role of compression overloads in influencing fatigue crack growth in a new aluminum-lithium alloy (2090) and a comparison of behavior with results in 2124 and 7150, ii) an evaluation of the role of crack tip shielding in controlling the growth of short (50 to 400 micrometers) through-thickness cracks and small (10 to 400 micrometers) surface cracks in 2124, and iii) a general assessment of the small crack problem. It is concluded that the near-threshold behavior of long cracks and the near- and sub-threshold behavior of small cracks is strongly influenced by considerations of crack tip shielding, specifically from crack deflection and crack closure mechanisms. Other factors responsible for anomalous small crack behavior, however, can be identified and are discussed in the report.
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