Compliance measurements for assessing structural integrity

The phenomenon of aging structures has focused attention on the problems of multiple-site damage (MSD) and widespread fatigue damage (WFD). In Austral ia, the problem was highlighted by the November 1990 failure of a Royal Aus tralian Air Force (RAAF) Macchi aircraft which suffered a port wing failure whilst in an estimated 6 g maneuvre and by the September 1998 explosion at the EXXON gas plant in Victoria. To assist in the understanding and the ma nagement of this problem the present paper uses the newly developed finite element alternating technique, for an arbitrary number of interacting three dimensional cracks, which we refer to as the MSD FEAT algorithm, to evalua te whether compliance measurements are useful in assessing continuing airwo rthiness. Traditionally the MSD FEAT and the FEAT analysis tools, i.e. the analysis methodology for a single crack, have been used only to analyse the stress intensity factor distributions around crack faces. The new work des cribed in this paper enables the displacement field, and hence the complian ce, to be calculated at any given location within the structure. Initial re sults confirm that this technique produces correct displacements and is cap able of determining the crack tip opening displacement to within similar to 0.7% for semi-elliptical surface flaws. Earlier work conducted on two dimen sional MSD problems found that when using compliance measurements to evalua te cracking there was an optimal sensor length for monitoring crack interac tion effects. The present paper extends this study to three-dimensional fla ws via a combined analytical and experimental research program. The experim ental work focuses on specimens containing two interacting quarter elliptic al cracks. Here the changes in compliance of the specimen under monotonic l oading, and fracture load, were measured and were found to be in good agree ment with those predicted using the newly developed MSD FEAT algorithm. Res ults of this analysis indicate that placement of sensors in an optimal posi tion is crucial. (C) 2001 Published by Elsevier Science Ltd.