Nonlinear Damping Properties of Nanocomposite Beams via Frequency Responses and Laser Doppler Vibrometry

The nonlinear damping properties of nanocomposite samples, made of polybutylene terephthalate (PBT) hosting matrix, integrated by branched multi-walled carbon nanotubes (b-MWCNTs), are experimentally investigated by performing frequency sweep tests and Experimental Modal Analysis (EMA) by using the Siemens-TestLab acquisition system and the Polytec PSV-500-3D vibrometer, respectively. The frequency response curves (FRCs) for beams with different weight frac-tions of b-MWCNTs are obtained in cantilever con figuration by applying a base excitation with a shaker and measuring the free tip displacement with a triangulation laser (optoNCDT 1320 produced by Micro-Epsilon). The dependence of the damping ratio on the oscillation amplitude for the lowest mode is identifi ed with the half power bandwidth method applied to the FRCs (see. Fig. 1 (a)) acquired for increasing excitation levels. The observed trends of the damping ratio and resonance frequency can be associated to the geometric and material nonlinearities due to the cantielever con figuration (nonlinear curvature) and stick-slip interaction between the PBT matrix and b-MWCNTs. The PSV and the PolyWave software are employed to identify the lowest three modes (see. Fig. 1 (b)). Several identi fications are performed increasing the amplitude of the chirp excitation provided by the shaker with the aim of capturing the nonlinearities in the small oscillation range. This approach can be considered as an equivalent linearization of the response at di erent os- cillation amplitudes. The identi cations performed with the two methods are in agreement and show the reliability of this approach to characterize the mechanical properties of nanocomposite materials.