Compliance measurements for assessing structural integrity

Abstract

The phenomenon of aging structures has focused attention on the problems of multiple-site damage (MSD) and widespread fatigue damage (WFD). In Australia, the problem was highlighted by the November 1990 failure of a Royal Australian 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 management 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 evaluate whether compliance measurements are useful in assessing continuing airworthiness. 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 described in this paper enables the displacement field, and hence the compliance, to be calculated at any given location within the structure. Initial results confirm that this technique produces correct displacements and is capable of determining the crack tip opening displacement to within ∼0.7% for semi-elliptical surface flaws. Earlier work conducted on two dimensional MSD problems found that when using compliance measurements to evaluate cracking there was an optimal sensor length for monitoring crack interaction effects. The present paper extends this study to three-dimensional flaws via a combined analytical and experimental research program. The experimental work focuses on specimens containing two interacting quarter elliptical cracks. Here the changes in compliance of the specimen under monotonic loading, and fracture load, were measured and were found to be in good agreement with those predicted using the newly developed MSD FEAT algorithm. Results of this analysis indicate that placement of sensors in an optimal position is crucial.