Small fatigue crack detection using phased array technology

The objective of this research was to investigate the applicability of a non-destructive testing (NDT) technique to detect fatigue crack initiation and propagation. A special area of interest was to determine the capabilities of ultrasonic phased array to detect small fatigue cracks under fastener heads in a two layered lap joint configuration. Mock aircraft fuselage panels were created by overlapping two 0.063 inch thick 2024-T3 aluminum plates with three rows of fasteners. These test specimens were then subjected to a cyclic tension-tension loading between 3,276 lb and 327 lb. These specimens were also inspected periodically for fatigue crack growth. It was determined that although small fatigue cracks could be detected underneath the fastener heads, their actual length could not be quantified. It was also found that small fatigue cracks could be detected only when the specimen was under a load, but when the load was removed, the cracks became undetectable with phased array inspection. This shows that phased array can be used to monitor the very early stages of fatigue crack initiation and growth underneath the fasteners of a mock aircraft fuselage lap joint.