Spectrum history effects for small fatigue cracks in aluminium alloy 7050-T7451

This study investigates the influence of spectrum history on small fatigue cracks in AA7050-T7451 aluminium alloy, a material commonly used in aerospace applications. The research utilised quantitative fractography (QF) to measure near-threshold fatigue crack growth rates and examine the role that load sequence effects can have on such rates. The QF measurements indicated significant load history effects, especially for the growth of small cracks where growth slowed or even stopped when large numbers of small amplitude constant amplitude cycles were applied. The investigation indicated this behaviour was influenced by the crack tip growing through the monotonic plastic zone associated with the application of prior larger loads. For these test conditions, crack growth retardation increased until it peaked at approximately 8 % of the way through the estimated plastic zone. However, the addition of underloads in the load spectrum was found to counteract this retarding influence. The load history effects noted may affect the accuracy of fatigue predictions for small fatigue cracks experiencing variable amplitude spectrum loading. Moreover, they could be particularly important for in-service cracks in aircraft structures that experience load spectra with similar characteristics, such as those experienced in airframes of helicopters and those produced by aerodynamic buffet in agile aircraft.