Simplified method to predict elastic-plastic behaviour of aeroengine casings

The assessment of elastic-plastic damage and residual deformations in complex aeroengine casing structures is an important issue for structural integrity and safety. The availability of computational methods for prediction of the elastic-plastic behaviour of components and structures gives an opportunity for significant new insight and thus more optimised design. However, the complexity of typical casing geometries makes routine non-linear analyses expensive, both in terms of model development and analysis run-times. This paper is therefore concerned with the development of simplified but accurate methods for non-linear casing analyses. A simplified aeroengine casing structure is considered characterised by features such as a thin outer shell with flanges, a thick inner hub and solid radial spokes connecting the latter to the former. Typical aeroengine casings may experience axial force, bending moment and radial loading due to shaft bearings and attention is focussed here on static loading, including overload conditions due to an extreme event, such as the fan blade-off condition (FBO). The repeated local joints at the spoke-shell casing junctions are of particular interest as the most likely sites for plastic deformation and possibly crack initiation. Analogy with previous work on prediction of elastic-plastic force-displacement responses in isolated tubular joints has suggested the possibility of employing a similar technique. Therefore, a prediction procedure has been developed for the spoke to outer shell connection behaviour based on complementary energy level curves in the two-dimensional loadspace of spoke axial loads and bending moments.