326 T. L. Hill et al. 34.5 Conclusions This paper has introduced the concept of energy transfer rate for nonlinear normal modes. This describes the rate at which energy must be transferred between the underlying linear modes of a nonlinear system, given a time-delay perturbation to a perfect forcing (i.e., a forcing that allows the system to respond precisely on an NNM). It is demonstrated that the energy transfer rate relates to the significance of an NNM, and a high rate means that the response is unlikely to be observed in a typical forced response. This is verified by considering the time taken for an NNM response to converge to a steady-state response, given a time-delay perturbation in a perfect forcing. It is seen that responses with a high energy transfer rate are seen to correspond to high convergence times. The key feature of the energy transfer rate is the efficiency of its computation. 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