ABSTRACT
In the field of structural dynamics, probably the most widely-used method of visualizing the input–output properties of a system is to construct the frequency response function or FRF. This chapter describes how FRFs are defined and constructed for nonlinear systems. The interpretation of the FRFs is discussed and it is shown that they provide a representation of the system as it is linearized about a particular operating point. FRF distortions are used to provide information about nonlinearity. It is important to recognize that nonlinear systems do not respond to a monoharmonic signal with a monoharmonic at the same frequency. It has been shown that nonlinear systems can respond at multiples of the forcing frequency if the excitation is a pure sinusoid. Harmonic balance is a useful technique for deriving the describing functions or FRFs of nonlinear systems if the nonlinear differential equation of the system is known.
