QUANTIFICATION AND CORRELATION OF PIT PARAMETERS TO 'SMALL' FATIGUE CRACKS

摘要

The primary objective of this study was to correlate corrosion "damage" using pit depth as a controlling factor with the "small" fatigue crack growth rates of 2024-T3 aluminum alloy specimens including a specimen made from a dismantled JSTAR fuselage panel. In all fatigue experiments, the cycles to first detectable crack length were recorded. A video recording system was used to observe the specimen surface, to record real time video as well as to measure cracks during fatigue experiments. The initial detectable crack length recorded using this system was in the range 0.15 to 0.21 mm. From the plots of 'a' vs. N for all of the specimens, it can be observed that the smaller the range of pit depth, the longer the fatigue cycles to form the first detectable crack. Moreover, in general, the crack growth rates for the prior-corroded specimens and the JSTAR specimen were greater than the uncorroded specimen. At low ΔK values (for example@0.1 MPa√m and 0.25 MPa√m), the crack growth rates of the prior-corroded specimens in the "small" region were considerably faster than the uncorroded specimen. Finally, fractographic analysis revealed some interesting results. The origins of the cracks were as expected for the most part. On the uncorroded specimens the origins were at the corner of the rivet hole where the stress concentration was highest, due to no other damage on the surface. The prior corroded specimens cracked at the rivet hole also at the points of large corrosion pits. The most important result was the crack origin of the JSTAR specimen, which was inside the rivet hole. This shows the extent of the damage inside the rivet holes that may go unseen but that may result in failure.