Real - time detection of fatigue cracks at multiple holes in 7075 aluminum using acoustic emission.

Abstract

In the E-2C Hawkeye's wing center section, stress fractures have been identified in the main beam web section. The cracks occurred in several web section attachment holes. Previous work, concerned with fatigue data, used a finite element code to design a specimen which modeled the actual section of the beam at one particular fastener hole near wing station 49. Acoustic emission testing was used to determine if crack initiation could be identified. In this work, acoustic emission techniques were applied to the monitoring of multiple cracks. The E-2C fatigue spectrum was used to load the specimens but fatigue testing was not an objective. Specimens were modified by drilling holes and attaching angles that represented the structural shapes used to attach the section to the wing skin. The original one-hole configuration and the new multiple hole specimen configuration were tested. The cap angles were found to create a great deal of noise containing frequency components below 400kHz. Special high pass filters were fabricated which eliminated most of this noise. It was shown that linear location could be used to discriminate between crack growth signals and the filtered noise signals.