Fuellekrug U.

The first step in experimental modal analysis is usually the measurement of frequency response functions (FRF). The identification of modal parameters is then based on the measured FRFs and requires adequate identification methods. Most of those methods identify complex eigenvectors and deliver modes with real and imaginary parts. However, the analytical modelling with Finite Elements usually comprises only mass and stiffness properties and no damping characteristics. Thus the analytical models deliver real normal modes. A correlation with experimental data requires therefore the transformation of experimentally identified complex eigenvectors into real normal modes. In this paper, a method for the computation of real normal modes from complex eigenvectors is described. The method is based on the exact equations. The problem of using these equations consists in the modal truncation. A solution to this problem is achieved by a special transformation. The theory of the method is described in detail and the context to other published methods is explained. With the purpose to demonstrate the applicability, a simulated vibration system is employed. Also, experimental data from a ground vibration test (GVT) on an aircraft are used. It can be shown that the method is able to transform identified complex eigenvectors into real normal modes with good reliability and accuracy.