Unusual softening-to-hardening switching in branched carbon nanotube nanocomposites

Abstract The unusual features of the nonlinear dynamic response of nanocomposite beams made of polybutylene terephthalate (PBT) and branched carbon nanotubes (bCNTs) are documented experimentally. The frequency response curves for cantilever specimens with different bCNTs weight fractions (wt%) are obtained under harmonic base excitations by measuring the free tip displacement via 3D scanning laser vibrometry. The steady-state response acquired for different excitation levels exhibits a surprising nonlinear softening trend which is switched into hardening for higher bCNTs wt% and increasing oscillation amplitudes. The stick-slip hysteresis, caused by the interaction of the bCNTs with the thermoplastic hosting matrix, determines a softening nonlinearity of the material that counteracts the well-known geometric hardening associated with the nonlinear curvature of the first mode of the cantilever. However, for bCNTs weight fractions greater than 1%, the bridging of the branched CNTs gives rise to the formation of a strong network which contributes to the hardening response at higher oscillation amplitudes. This mechanical behavior is detected by the trend of the nonlinear harmonic spectra and by the estimation of the equivalent damping ratio using the half power bandwidth method. A nonlinear mathematical model of the nanocomposite cantilever samples derived from a 3D mesoscale hysteretic model of the PBT/bCNT material is shown to predict the observed unusual experimental behavior. The presence of bCNTs in a thermoplastic matrix seems to be the main driver of the highly tunable nonlinear stiffness and damping capacity.