S. Pitt

A methodology for structural optimisation with damage tolerance constraints

Abstract This paper presents recent developments in the optimal design of structural components with fracture constraints. To minimise the computational effort it is suggested that an initial “near optimum” shape be used. One approach could be to begin with the optimal shape for the non-cracked geometry. This initial near optimum shape would then be used in conjunction with the alternating finite element method, for multiple cracks, which builds on existing CAD based finite element models and the resultant formulation then linked to available optimisation codes. This approach is illustrated by considering the problem of an optimum cut-out geometry for a square plate subjected to a 4:1 and a 2:1 biaxial stress field. When considering the problem of shape optimisation with static fracture constraints it was found that an initial “near optimal” shape, based on the optimal shape for the uncracked geometry, was in fact an excellent approximation to the optimal solution for the cracked problem. It was also found that, in each case, for a given crack length the stress intensity factors, for cracks emanating at any arbitrary point around the hole, were essentially constant along most of the circumference. This behaviour is intuitively expected for an optimised geometry, where it would be hoped that all locations around the hole would be equally critical. A similar behaviour was found when considering shape optimisation with durability, i.e. crack growth, constraints. In this case it was hypothesised that the optimum shape would be such that all locations around the cut-out would be equally fatigue critical. However, in this case it is believed that the final optimal shape will depend on: the initial flaw size, the critical crack length, the load spectrum and the overall geometry of the structure. The analysis also reveals that, as the geometry of the cut-out or hole changes, the location of the crack which determines the minimum fatigue life can also change. This makes it necessary to consider flaws at a range of locations around the hole.

Structural optimisation with damage tolerance constraints

Recent developments in cluster computing and structural optimisation offer the potential for significant improvements in the design of more durable structures, and for producing optimum fatigue life extension rework profiles. The present paper reveals the importance of non-destructive inspection (NDI) and the role it plays in determining optimum structural rework geometries. This finding is important since when performing stress based optimisation the interrelationship of NDI and optimum rework geometries is not immediately apparent. The problems studied also reveal the flatness of the solution space and, unlike stress based optimisation, the existence of multiple maxima. Such solution spaces represent a major challenge for any optimisation procedure. To this end we explore the advantages of using the NIMROD optimisation suite of programs in conjunction with A cluster computer architecture and the highlight the benefit of visualizing the solution space.

Assessment of multiple flat elliptical cracks with interactions

The failure of aging aircraft, pressure vessels and many welded structures is frequently due to elliptical cracks, which arise due to in service operation or the initial fabrication processes. Cracks have the potential to interact. Determination of the residual strength of structure requires an accurate knowledge of the stress intensity factor distribution around the crack front. When interacting cracks prevail, the lack of interaction effect in among multiple cracks may lead to an inadequate evaluation. A hybrid formulation capable of representing stiffness change is developed. This enables the accurate analysis of multiple load path structures containing multiple three dimensional cracks. This procedure complement the standard finite element alternating technique.

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.

Smart Structures Application to Airworthiness and Repair

The present paper summarizes recent work, undertaken as part of a collaborative research program between DSTO and the DSTO Centre of Expertise in Structural Mechanics (CoE-SM), for development and assessment of new techniques for the in-situ health monitoring of structures and any associated repairs.