6.1. Braced-truss model
For the braced-truss model, the AMBER force constants used
were kr = 32.6 nN °A
−1 and kθ = 0.438 nN A° rad−2 [18, 19].
A modified version of the linearized Morse potential [40]
(kr = 42.3 nN °A
−1 and kθ = 0.45 nN A° rad−2) was adopted
to accommodate the total mechanical strain energy of the
truss as indicated by Odegard et al [18]. The C–C lengths
and thickness of the rods used in the analytical calculations
have been the ones determined through the finite element
minimization technique. For the AMBER case, the loading
along direction 1 leads to the equilibrium C–C bond length
of 1.39 A° , while for the case of mechanical loading along 2
the same length is 1.35 A° , also for the pure shear case. The
Morse case provides a C–C length for loading along direction
1 of 1.45 A° , while for the case on direction 2 the length is
1.35 A° , and for pure shear 1.36 A° . The thickness distribution
provides very different values for the rods a–f and struts g–
n. For loading along direction 1, rods a–f have thickness
ds = 0.565 °A, regardless of the force model used. The same
is valid also for loading along direction 2 and pure shear, with
a thickness value 0.55 A° . Rods g–n thickness have a different
behaviour. For the AMBER case with loading along 1 axis,
the thickness db is 2.24 A° , while for the Morse force model
the same thickness is 1.81 A° . For loading along direction 2,
the two force models provide a thickness of 2.22 A° , while for
pure shear both AMBER and Morse potentials give a db value
of 6.4 ° A. All these thickness values are lower than the 6.9 °A
indicated in [18], although in the same reference the maximum
thickness for the braced unit cell under pure shear is reported
as 5.7 A° .
The analytical expressions for the Young’s and shear
modulus (6) and (7) provide a conservative estimation
compared to the values from the finite element simulations
(table 1). The variation in Young’s modulus between the
AMBER analytical and FE results is around 10%, while a
more significant discrepancy is recorded for the shear modulus
(32%). The degree of anisotropy (E1/E2) 0.94 is in line with
the one observed in finite size graphene sheet [23]. On the
other hand, theMorse linearized potential provides a far greater
anisotropy degree (0.71). All force models provide in-plane
Poisson’s ratio around 0.56, higher than the analytical 0.33, as
well as the 0.44 reported in [19].