“…Pioneered in the 1960s by Rosenberg [1], nonlinear normal modes (NNMs) are a rigorous extension of linear normal modes (LNMs) to nonlinear systems. The concept was then further developed [2,3] and enjoyed various applications in nonlinear structural dynamics, see, e.g., [4,5,6,7,8,9]. First defined as families of vibration in unison of the undamped, unforced system [1], NNMs were latter defined as (non-necessarily synchronous) periodic oscillations in order to account for modal interactions [10,11].…”