Fatigue behavior of Long and Short Cracks in Aluminum Alloys.

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., 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, this program was 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. Based on experiments in 2124, 7150 and 2090 alloys, it is shown that whereas the behavior of long cracks at near threshold levels is largely controlled by the magnitude of the crack tip shielding from crack closure and deflection mechanisms, short crack growth rates are invariably faster because of their inability to develop shielding due to their limited wake. Experiments whereby the wake material is physically removed confirm such notions and indicate that the predominant closure forces are located close within 500 um of the tip. Keywords Fatigue Defect-tolerant fatigue design Variable amplitude loading Fatigue in aluminum alloys Fatigue behavior of long and short cakes Fatigue cracks Crack closure.