Linear modal analysis in the time and frequency domains is well established. Yet, as mechanical systems become increasingly more complex, incorporating electromechanical components or biological and biomimetic elements, the likelihood exists that their dynamics will be strongly nonlinear and nonstationary. Examples of sources of strong nonlinearity, some of which are realized even for small amplitudes of vibration) include local buckling, plastic deformations, clearance and backlash, hysteresis, friction-induced oscillations, and vibro-impact motions. Such effects cannot be accurately addressed by linear modal analysis, since standard techniques, such as the classical FT, and well-established concepts such as normal mode, natural frequency, and modal space cannot be applied to the identification of nonlinear and nonstationary dynamic regimes.
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