  The elastic properties of 1D nanostructures such as nanowires are often
measured experimentally through actuation of the nanowire at its resonance
frequency, and then relating the resonance frequency to the elastic stiffness
using elementary beam theory. In the present work, we utilize large scale
molecular dynamics simulations to report a novel beat phenomenon in [110]
oriented Ag nanowires. The beat phenomenon is found to arise from the asymmetry
of the lattice spacing in the orthogonal elementary directions of the [110]
nanowire,i.e., [-110] and [001] directions, which results in two different
principal moments of inertia. Because of this, actuations imposed along any
other direction are found to decompose into two orthogonal vibrational
components based on the actuation angle relative to these two elementary
directions, with this phenomenon being generalizable to <110> FCC nanowires of
different materials (Cu, Au, Ni, Pd and Pt).The beat phenomenon is explained
using a discrete moment of inertia model based on the hard sphere assumption,
the model is utilized to show that surface effects enhance the beat phenomenon,
while the effect is reduced with increasing nanowires cross-sectional size or
aspect ratio.Most importantly, due to the existence of the beat phenomena, we
demonstrate that in resonance experiments only a single frequency component is
expected to be observed, particularly when the damping ratio is relatively
large or very small.Furthermore, for a large range of actuation angles, the
lower frequency is more likely to be detected than the higher one,which implies
the experimental predictions of Young's modulus obtained from resonance may in
fact be under predictions. The present study therefore has significant
implications for experimental interpretations of Young's modulus as obtained
via resonance testing.
