Initiation and shape development of corrosion-nucleated fatigue cracking

There presently exists the need for a more complete understanding of initiation and early stage shape evolution of corrosion-nucleated fatigue cracking. These aspects are of critical importance in modeling efforts which are of increasing relevance in the current era of aging aircraft. The present work seeks to contribute to this need through interrupted fatigue testing of pre-corroded, bare, 2024-T3 aluminum sheet specimens. Several specimen orientations and stress levels were considered. Cycle counts at which the specimens were interrupted (overloaded) to expose crack development were set as percentages of total expected life as determined from previous fatigue-to-failure testing done under the same conditions. In this work it was conclusively deduced that crack initiation is essentially immediate upon the application of cyclic loading. Fatigue cracking was found on all specimens, including those tested for just 10% of their total expected life. The initial cracks observed were found to bear resemblance to the grain structure in terms of size, shape, and orientation, and were noted to originate from pitting damage at numerous locations. It was additionally found that surface breaking cracks did not consistently appear until 25% of the total expected life had been consumed. A detailed narrative of the entire damage evolution process in corrosion-nucleated fatigue is given. The utility of interrupted testing is demonstrated and conclusions of relevance to modeling endeavors are presented.