The characterization of multiple-site damage by use of laboratory test specimens is an important precursor to the understanding of multiple-site damage in aging aircraft. As the number of damage sites in the laboratory specimen increases, it becomes more time-consuming to measure crack initiation and growth from the various sites. Typically the crack initiation and crack growth would be measured either visually or with a low-power microscope. This technique is accurate, but it is labor-intensive. To decrease the time necessary to obtain multiple-site crack growth rate data, an automated crack length measuring technique, AC potential drop, was investigated.
This AC potential drop technique was applied to aluminum test specimens under fatigue cycling conditions as well as monotonic R-curve testing. Aluminum ALCLAD 2024 test specimens of three different specimen geometries were examined. Specimens with a single-hole were used to examine the effects of lead placement on the sensitivity of AC potential drop. Three-hole specimens were used to simulate multiple-site cracking from several holes. Riveted specimens containing one row of three rivets were used to examine multiple-site cracking from fasteners. Optimum current and potential lead geometries were determined for each specimen geometry. Relationships between the AC potential and crack length were determined for AC current frequencies of 3, 10, and 30 kHz. The sensitivity of AC potential drop in measuring crack initiation was investigated. The sensitivity was found to increase substantially with increasing AC current frequency.