Non-linear Resonant Decay method (NL-RDM) addresses the identification of multi-degree of freedom non-linear systems. This method offers a practical approach to the identification of lumped parameter and continuous systems by producing a non-linear extension of the classical linear modal model. The method is introduced and its potential as a practical identification approach explained. This paper is concerned with the inclusion of residual modes, above and below any region of interest. Any structure that is not supported to earth will have rigid body modes, being modes that have a natural frequency at zero Hertz; a good example is an aircraft. These modes are sometimes ignored but for a complete mathematical model obtained from NL-RDM the effects of rigid body modes must be analysed. NL-RDM relies on a method of curve fitting to generate modal characteristics, in terms of force, displacement, velocity and acceleration in modal space. The lower residual region was observed to contain rigid body modes, and these were observed to affect significantly the results in the region of interest. Monitoring this effect was shown to be too difficult currently for the NL-RDM, given technological restrictions. A Mass Substitution method was generated to model the system response more accurately. Its accuracy was demonstrated through case studies. Most systems also contain an indefinite number of residual modes, these being modes that occur at frequencies greater than the frequency range of interest. The section will attempt to determine the effects of excluding residual modes on the accuracy of NL-RDM. The effect of the upper residual area on the modelling of the system within the region of interest was observed through several case studies. Simple validation methods have been considered to preserve the accuracy of the model, taking into account both harmonic and coupled residual modes and their effects.
