  Experiments on Micro- and Nano-mechanical systems (M/NEMS) have shown that
their behavior upon bending loads departs in many cases from the classical
predictions using Euler-Bernoulli theory and Hooke law. This anomalous response
has usually been seen as a dependence of the material properties with the size
of the structure, in particular thickness. A theoretical model that allows for
quantitative understanding and prediction of this size effect is important for
the design of M/NEMS. In this paper, we summarize and analyze the five theories
that can be found in the literature: Grain Boundary Theory (GBT), Surface
Stress Theory (SST), Residual Stress Theory (RST), Couple Stress Theory (CST)
and Surface Elasticity Theory (SET). By comparing these theories with
experimental data we propose a simplified model combination of CST and SET that
properly fits all considered cases, therefore delivering a simple (two
parameters) model that can be used to predict the mechanical properties at the
nanoscale.
