A fractographic study was carried out on fatigue crack closure and crack growth rates in an Al-Cu alloy under conditions of monotonic and cyclic yield at the notch root. Comparative tests were also conducted on smooth specimens subject to elastic loading conditions. A notch root fatigue crack can see substantial difference between crack closure and opening stress. This difference is considered responsible for local mean stress effects observed in notch fatigue response to complex load sequences. The difference between closure and opening stresses diminishes to negligible proportions as stress excursions reduce to elastic interval and is therefore hardly observed in long cracks. To support validation effort on predictions, notch root crack growth rates were determined from optical fractography under a programmed load adaptation of the FALSTAFF load sequence. The results of these measurements suggest that scatter in short crack growth data is similar to that in long cracks at comparable growth rates. Life to initiation of (naturally initiating) multiple notch root cracks is dissimilar. The crack having been in existence longest appears to assume the role of the dominant crack and accelerates others (smaller) cracks.

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The kinetics of short cracks at notches were studied in 2014-T6511 Al-Cu alloy extruded bar stock under modified versions of FALSTAFF and TWIST load spectra. Experiments were conducted on test coupons of different thickness and at different stress levels. The effects of these and other parameters on the density and growth rate patterns of multiple cracks were investigated. The quality of fractographic crack growth rate measurements was found to be superior to surface replica technique. It was also observed that artificially initiated fatigue cracks grow slower than naturally initiated ones. Interaction of naturally initiating multiple cracks was quantified.

Optical fractography was used to estimate growth of small cracks at notches under programmed FALSTAFF loading in an Al-Cu alloy. Crack sizes as low as 25 microns and growth rates over two orders of magnitude could be resolved using this technique. Randomized MiniFALSTAFF load sequence was modified into a programmed load equivalent with major loads either preceding or following marker loads. Crack growth rate under programmed FALSTAFF spectrum as estimated by optical fractography conformed to compliance based estimates on a SE(T) specimen. Long crack growth rates under programmed and randomized MiniFALSTAFF spectrum were essentially similar. Spectrum load fatigue crack growth was studied in central hole coupons under notch inelastic conditions. Scatter in growth rates for small notch cracks was found to be of the same magnitude as that of long cracks. Multiple fatigue cracks are observed at the notch root, ,and they appear to influence each other.

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