Methods using Frequency Domain Data

The methods using measured FRF data optimise a penalty function involving the FRF data directly. Extracting natural frequencies and mode shapes for structures with close modes or a high modal density can be difficult. The FRF data may be used directly to update the finite element model without extracting the natural frequencies and mode shapes (Sestieri and D’Amrogio, 1989). Friswell and Penny (1992) discussed the use of a typical algorithm for structures with close modes. One problem with any method using the FRFs directly is that damping must be included in the finite element model. Methods using modal data, Chapters 7 and 8, are able to use undamped models because the measured natural frequencies and damping ratios may be separated. The inclusion of damping is vital to obtain a good correpondence between the measured and predicted FRFs. Since damping is so difficult to model accurately, proportional damping is often assumed (see Chapter 2). The rest of this chapter will assume a proportional viscous damping model, in which the constants of proportionality are unknown. Other damping models may be easily incorporated.