Fatigue crack growth behavior of small cracks emanating from a corner notch

Fatigue crack growth rates of small cracks emanating from through-thickness and corner notches in 2124-T851 and 7050-T7451 aluminum alloys were determined. Fatigue tests were performed using Single Edge Notched Tension, SE(T) specimens. In case of corner-notched specimens, crack growth rates were determined both along the surface and along the thickness directions. Crack length was monitored by cellulose acetate replication method. Fatigue crack closure was determined in both through-thickness and corner cracks by near-tip strain gage method. In case of through-thickness cracks, fatigue crack growth rates of small cracks were observed to initially decrease and then increase with applied Delta K. Crack growth rates in corner cracks in the small crack range were observed to accelerate and decelerate both along the surface as well as along the thickness direction. Crack closure was found to build up gradually with crack length, before reaching a steady value in the long crack range. The difference in crack closure level between corner cracks and through-thickness cracks can explain the observed differences in crack growth rates.