| Nonlinear Energy Sink(NES)is a kind of pure nonlinear vibration absorber which possesses cubic stiffness and has advantages such as lightweight and robust,all these make NES possesses wide application prospect.Owing to the fact that NES exists no such a natural frequency,it can achieve broadband vibration absorption.However,there exists a threshold for the input energy for the NES,which makes its efficiency meets a reduction under the environment with its excitation greatly changes.This paper purposed to make improvements of the NES and make investigations of the vibration absorption efficiency of the improved absorbers under environment with changing excitation.The efficiency of the absorbers while main oscillator generates higher branch response are studied.Basic contents are as follows:Vibration absorption efficiency of a variable-stiffness nonlinear energy sink(NES)is investigated when the main oscillator is subjected to harmonic and impulse excitations.The slow flow equations of the coupled system are derived by using the complexification-averaging method,and the nonlinear equations which describe the steady-state response are obtained.With the harmonic excitation force increases,the system which comprise constant-stiffness NES generates higher branch responses,which greatly reducing the vibration absorption efficiency.The influence of nonlinear stiffness on the responses of the system is investigated.Results show that,with the increase of harmonic exciting force,a reduction of NES stiffness can eliminate the higher branch responses and even the frequency band of strongly modulated responses.The vibration absorption efficiency of variable-stiffness NES attached to the linear oscillator for different amplitudes of impulse excitation is investigated.Results show that the proper reduction of nonlinear stiffness under increasing impulse excitation can greatly increase the vibration absorption efficiency of NES.The vibration absorption efficiency of a single-degree-of-freedom(SDOF)NES and a2-degree-of-freedom(2-DOF)series NES are compared,and the suppression of higher branch response by the 2-DOF NES is analyzed.The slow flow equations of the system are derived from complexification-averaging method,and the frequency-response curves are obtained.The responses of the system attached with the SDOF NES are compared with that of the system with the series NES,under harmonic excitation with different amplitudes.The results show that the series NES can maintain a decent vibration absorption efficiency in a large range of changing excitation amplitudes.Furthermore,the parameter influences of the 2-DOF series NES on the vibration suppression effect are investigated both under small excitation and large excitation amplitude.The vibration absorption for the host oscillator is mainly influenced by the primary NES under the small amplitude excitation,but by the secondary NES under the large amplitude excitation.Vibration absorption of a variably nonlinear absorber with an electromagnetic repulsion device is investigated when the primary system is subjected to a harmonic excitation.Linear negative stiffness is used for counteracting the linear part of the stiffness generated by theelectromagnetic device to realize purely nonlinear stiffness.By controlling the current in the electromagnetic coil,the stiffness of the absorber can be alternated between purely nonlinear and weakly nonlinear.This enables us to construct a variably nonlinear absorber.With the excitation amplitude increasing,the variably nonlinear absorber performs better than NES and weakly nonlinear absorber.Furthermore,the duration of pure and weakly nonlinear stiffness and the amount of current can also affect performance of the absorber.When the system attached to NES generates higher branch responses,although the vibration absorption of the variably nonlinear absorber decreases under the same conditions,it can exhibit a better performance than NES and weakly nonlinear absorber. |
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