Identification of symmetrical structures with fabrication and damage induced asymmetry

Experimental vibration testing is a common technique employed to identify modal frequencies, mode shapes, and at times, modal damping. The identified data can then be used for updating the structural dynamic models, e.g. those obtained by using approaches like the finite element method. It can also be used to identify any damage in the structure. For many symmetric structures like buildings, large shells such as cooling towers, bridges, and aircraft, conventional flexible modes are often separated into symmetric and anti-symmetric modes. This leads to significant reduction in computing both the natural modes and frequencies, and the resulting dynamic responses. However, in practice, during fabrication and manufacturing of these structures, or due to damage during their operation, it is often observed that the structure can develop significant asymmetry. As a result, for real structures, the often assumed separation into symmetric and antisymmetric modes no longer holds. The modes start to possess both symmetric and antisymmetric components in the same mode, which then poses a significant challenge for both modal identification and FEM modeling. Furthermore, it can significantly change the dynamic response of structures. This paper describes the process of ground vibration test, modal identification and FEM modeling for an asymmetrical aircraft. Details of the best practices for incorporating the asymmetry during the model updating process is discussed and the results of the GVT and FEM updating procedure for the aircraft are described. The FEM update procedure successfully identifies the source of the asymmetry. The suitability of the FEM update procedure for characterizing the source of asymmetry is discussed.