Model structure correction and updating of aeroengine casings using fictitious mass modifications

Abstract

In this paper the results of a finite element model updating exercise, carried out on closely axisymmetric aeroengine casings, are presented. The correction of the combustion chamber outer casing (CCOC) model is considered and, after assembly with the turbine casing (TC), the quality of the resulting combined model is investigated. The dynamics of both casings is characterized by pairs of close modes, which may be separated by fictitious point mass modifications. The natural frequencies and mode shapes of the fictitiously modified CCOC are determined from receptances obtained from the CCOC in its original (unmodified) configuration. The modifications are shown to improve the understanding of both the CCOC and the system formed by connecting the CCOC to the TC. A particular problem is revealed when model updating is applied to the CCOC. An analysis of the mode shapes locates a modelling error on an inner shell of the structure but it is found that the finite element model is unable to be parameterized for the correction of two pairs of wrongly ordered predicted modes. This can only be achieved by firstly correcting the ‘structure’ of the model itself. The main error is found to be a geometrical inaccuracy, and, when this is put right, the sequence of the modes is corrected. Model updating is then applied to the thickness of certain shell elements and the CCOC is found to be in excellent agreement with measured data, as is the complete model formed from the two models of the CCOC and the TC together.