Acoustic emission testing the F-111

Abstract

Significant portions of the F-111 aircraft were fabricated of D6AC steel, which is now known to have a fairly small critical crack size. To prove structural flight-worthiness the Air Force built a chamber at McClellan AFB, where all F-111 aircraft are periodically chilled to 40°C and stressed to 7.3g and3.0g. Recently the chamber was modernized, and Physical Acoustics Corporation was selected to supply an acoustic emission system to locate any sources of structural failure. The new F-111 Cold Proof Test Station Acoustic Emission Monitoring System has several innovative features, including a colour CRT which displays the exact location of AE events in real time on simultaneous overhead and side views of the F-111. The events are coloured green, yellow and red according to their severity, as calculated from their amplitude and energy. A monochromatic CRT is used concurrently with the colour CRT to display severity information on AE events which only arrive at one sensor. Alarms also audibly alert the operator to crucial events, using two tones to distinguish the degree of severity.