4.1. Rigidities of MWCNTs
The values of the tensile,
, bending,
, and torsional,
, rigidities of the MWCNTs, numerically obtained and using Equations (4)–(6), are represented as a function of the difference between the outer and the inner layers diameters,
, in the
Figure 3a–c, respectively. For each nanotube type, the evolutions of tensile rigidity,
, are clearly separated, depending on the diameter of the inner layer of the MWCNTs: (i) for the smallest inner layer,
=
for
armchair and
=
for
zigzag structures and (ii) for the largest inner layer diameter,
=
for
armchair and
=
for
zigzag MWCNTs. The same is true for the evolutions of the bending,
, and torsional,
, rigidities with
.
In order to clarify the trends shown in
Figure 3a–c, the values of the tensile rigidity,
, are plotted as a function of
, and the values of the bending,
, and torsional,
, rigidities are plotted as a function of
, in
Figure 4a–c. These plots are inspired by the expressions 10, 11, and 12 of the area and the moments of inertia of the MWCNTs.
Figure 4a–c show that, for this representation, for each type of nanotube, the results follow the same straight line, regardless of the diameter of the inner nanotube. Furthermore, the results are only slightly influenced by the type of nanotube, i.e., armchair or zigzag MWCNTs. This small difference in the rigidity behavior of the two types of MWCNTs can be attributed to the different interlayer spacing of these structures: armchair (
) and zigzag (
). The straight lines in
Figure 4a–c can be expressed as follows:
The fitting parameters for armchair and zigzag MWCNTs are given in
Table 4. The mean difference between the values of the
,
, and
rigidities calculated with Equations (19)–(21) and the values obtained directly from FE analysis are, respectively, 0.10%, 1.23%, and 0.45% for armchair nanotubes and 0.35%, 0.97%, and 0.31% for zigzag nanotubes.
4.2. Young’s and Shear Moduli of MWCNTs
The Young’s and shear moduli of the , , armchair and , , zigzag MWCNTs structures are analyzed in this subsection.
The MWCNTs Young’s modulus values can be calculated by Equation (7) or Equation (8) using the results of the numerical tensile and bending tests, respectively, and Equation (17), which uses the results of numerical tensile and bending tests. The values of the MWCNTs shear modulus can be calculated by Equation (9), using only the numerical torsional test results, and Equation (18), which uses the results of numerical tensile, bending, and torsional tests.
Equation (17) and relationships (19)–(21) as well as the knowledge of the values of the parameters
,
, and
in
Table 4 allow the easy determination of the Young’s modulus of the MWCNTs, as a function of the outer layer diameter,
, and the inner layer diameter,
, without resorting to the numerical simulation as follows:
In the same way, but using the Equation (18) and relationships (19)–(21), the shear modulus of the MWCNTs can be calculated as follows:
Figure 5 compares the Young’s and shear modulus results as a function of the difference between outer and inner layers diameters,
(
Figure 5a,c), and the number of layers,
, (
Figure 5b,d) in the MWCNT structure, for armchair and zigzag nanotubes. The Young’s and shear modulus values were obtained by Equation (7) (Equation (8) leads to results with a relative difference of 0.9%, on average) and (17) and by Equations (9) and (18), respectively, using numerical simulation results. The evolutions of the
and
values, calculated analytically by Equations (22) and (23), respectively, are also plotted for armchair and zigzag structures.
For both types, armchair and zigzag MWCNTs, the evolutions of the Young’s modulus, , and shear modulus, , are similar, regardless of the equation (used for its calculation (Equation (7) or Equation (17), for the Young’s modulus, and Equation (9) or Equation (18), for the shear modulus). Moreover, Equation (22) permits accurate analytical prediction of the value of the Young’s modulus of MWCNTs as well as Equation (23) for the shear modulus value.
The mean difference between the Young’s moduli calculated, e.g., from Equation (17), using numerical results, and those evaluated with Equation (22) is 0.32% and 0.53% for structures of armchair and zigzag, respectively. For the shear modulus, the mean difference between its values calculated by Equation (18) and those evaluated with Equation (23) is 1.55% and 1.62% for armchair and zigzag structures, respectively.
It can be seen from
Figure 5 that the Young’s modulus values of the armchair structure are slightly higher (in average 3.8%) than those for the zigzag structure. In the case of shear modulus, its values for armchair structure are in average 1.5% higher than those for the zigzag structure. The Young’s and shear modulus values of the armchair MWCNTs is about the same regardless the value of
(
Figure 5a,c) or the number of layers,
(
Figure 5b,d) constituting the MWCNT. In the case of zigzag MWCNTs, a slight decrease of the Young’s modulus value (at about 2.50%) and shear modulus value (at about 2.08%) is observed when the difference between outer and inner layers’ diameters (
Figure 5a,c) or the number of layers (
Figure 5b,d) increase. As pointed out above in relation to rigidity, the differences between the Young’s modulus of the armchair and zigzag MWCNTs are also certainly related with the different interlayer spacing for armchair and zigzag structures. The same is true for the differences between the shear moduli of the armchair and zigzag MWCNTs.
Figure 6 compares the Young’s modulus of the MWCNTs with the Young’s moduli of SWCNTs corresponding to the inner and outer constituent layers, for selected armchair (
Figure 6a) and zigzag (
Figure 6b) MWCNTs with 2, 5, 10, 15, and 20 layers. In this figure, the comparison of the MWCNTs’ shear modulus with the shear moduli of the inner and outer constituent SWCNTs is also shown for armchair (
Figure 6c) and zigzag (
Figure 6d) structures. The Young’s and shear modulus values obtained by Equations (17) and (18), respectively, are used. The Young’s modulus values for the armchair MWCNTs are very close to the values of
obtained for the inner and outer layers. The Young’s modulus values for zigzag MWCNTs are lower than the Young’s moduli of the inner and outer constituent layers. The same trends are observed for the shear modulus values.
In order to further test the methodology for analytical evaluation of the Young’s and shear moduli of MWCNTs (Equations (22) and (23)), two sets of MWCNTs (armchair and zigzag), for which the innermost nanotube layer is still smaller in diameter than those analyzed so far, were considered. The configurations for both sets, armchair and zigzag, MWCNTs are shown in
Table 5.
Figure 7 compares the Young’s and shear moduli values obtained by Equations (22) and (23), in function of the difference between outer and inner layers diameters,
, (
Figure 7a,c), and the number of layers,
, (
Figure 7b,d) in the MWCNTs structure, for
(
Table 5) and
(
Table 2) armchair and
(
Table 5) and
(
Table 3) zigzag MWCNTs. The Young’s modulus values obtained are similar for two sets of armchair MWCNTs, as well as for two sets of zigzag MWCNTs. The same is true for shear modulus values.
4.3. Comparison with Literature Results
Table 6 summarizes the current elastic moduli results of MWCNTs and those from literature, which include numerical and experimental results.
With regard to experimental evaluations, in the work of Treacy et al. [
46], the MWCNT Young’s modulus was obtained by measuring the amplitude of the nanotube intrinsic thermal vibrations by transition electron microscopy (TEM). Kashyap and Patil [
47] evaluated the Young’s modulus of MWCNT from TEM bright field image of CNT/Al composite.
The Young’s shear moduli were calculated from the numerical results of the conventional tensile [
21,
24,
25,
27,
30,
31,
32] and torsion [
22,
24,
25,
28,
30] tests, respectively, using the respective definitions from the classical theory of elasticity. Santosh et al. [
22] evaluated the MWCNTs’ Young’s modulus from the numerical results of conventional compression test. With regard to the boundary conditions, the simulation of the MWCNT’s tensile test, in the works of [
24,
25,
32], was achieved by subjecting all nodes at one end to the same axial force, while all nodes at the other end were fixed. In the simulation of torsion tests, Kalmakarov et al. [
25], Fan et al. [
30], and Santosh et al. [
22] applied a torsional moment to all end nodes of multiwalled nanotube, but in the study of Li and Chou [
24] only the outer layer of MWCNT was subjected to torsion. Ghavamian et al. [
27,
28], in tensile and torsion tests, Nahas and Abd-Rabou [
31] and Hwang et al. [
21], in tensile tests, and Santosh et al. [
22], in compression tests, applied displacements, instead of forces or moments, to all nodes at one end of the MWCNT, leaving the other end fixed.
In order to facilitate the comparison of the current results with those available in the literature, the Young’s and shear moduli were represented as a function of the outer layer diameter,
, of the MWCNT and the number of layers,
, constituting the MWCNT structure, as shown in
Figure 8 (please see, designations in
Table 6). MWCNTs with
up to
were considered. This diameter corresponds to multiwalled structures containing up to five layers. The results from the works [
21,
23,
24,
25,
27,
30,
31], which permit appropriate comparison of the Young’s modulus evolution with
, were considered in the figures.
Most aforementioned studies share the same modelling approach for the simulation of the MWCNT structure, i.e., a NCM approach employing 3D beam elements [
24,
25,
27,
30,
31,
32], although Almagableh et al. [
32] used rectangular cross-section beams instead of circular ones. Regarding the simulation of the noncovalent van der Waals interactions between layers, truss rod elements [
24], spring elements [
25,
27,
31], nonlinear solid elements [
32], and beam elements [
31] were used. The model proposed by Tu and Ou-Yang [
23] does not take into account the van der Waals forces, and in the works of Hwang et al. [
21] and Santosh et al. [
22], the vdW forces were modelled in a frame of MD approaches used.
Some authors [
22,
24,
27,
30] pointed out that Young’s modulus of MWCNTs did not change significantly with the increase of the outer layer diameter,
, and the number of layers,
, composing the MWCNT structure. A substantial increase of the Young’s modulus with the increase of the outer layer diameter or the number of layers was reported by Kalmakarov et al. [
25]. Nahas and Ab-Rabou [
31] noted slight increase of the Young’s modulus values with the number of layers for DWCNTs and TWCNTs. A considerable decrease of the Young’s modulus with the increase of the outer layer diameter was reported by Almagableh et al. [
32] for zigzag DWCNTs, and Hwang et al. [
21] reported slight reduction of the Young’s modulus upon transition from armchair DWCNT (
) to TWCNT (
). Tu and Ou-Yang [
23] predicted a substantial reduction in Young’s modulus (from 4.70 TPa for SWCNT to 1.05 TPa for MWCNT with
) with the increase of the number of layers in the MWCNT structure. Some authors [
24,
30,
31] pointed out that Young’s modulus of MWCNTs is slightly higher than that SWCNTs, but Young’s modulus values for MWCNTs, which are very close to the values obtained for SWCNTs constituting the MWCNT, were also reported [
27].
The current results show particularly good agreement with the results of: (i) Fan et al. [
30] (
Figure 8a,b), for zigzag MWCNTs; (ii) Ghavamian et al. [
27] (
Figure 8a,b), for armchair and zigzag MWCNTs, where the spring elements for simulation of the vdW interactions were considered; (iii) Li and Chou [
24] (
Figure 8a,b), for armchair MWCNTs, who used the truss rod elements for simulation of the van der Waals forces; (iv) Santosh et al. [
22] (
Figure 8b) for armchair DWCNTs; and (v) Hwang et al. [
21] (
Figure 8a,b), for armchair DWCNTs and TWCNTs, where both covalent and vdW interactions between carbon atoms were modelled with recourse of MD approach. The smallest difference of 0.79% occurs for the Young’s modulus calculation performed by Fan et al. [
30] for (15, 0)(24, 0)(33, 0) zigzag TWCNTs with
. Differences of 1.39% and 1.62% occur for the results of Ghavamian et al. [
27] for armchair and zigzag MWCNTs, respectively. The comparison with the results reported by Li and Chou [
24] shows differences of 1.41% and 8.48% for armchair and zigzag MWCNTs, respectively. The Young’s modulus values obtained by Nahas and Abd-Rabou [
31] (
Figure 8b) show differences of 3.4% and 9.5% for armchair and zigzag MWCNTs, respectively, when compared with the current results. The Young’s modulus calculated by Nahas and Abd-Rabou [
31] is also lower than those obtained in the other studies [
21,
22,
23,
24,
25,
27,
30]. Differences of 3.54% and 4.82% were observed with the results of Santosh et al. [
22] for armchair DWCNTs with
up to
and Hwang et al. [
21] for armchair TWCNTs with
, respectively.
Substantial differences (36.9% for armchair and 46.0% for zigzag MWCNTs with
) were found with the Young’s modulus results predicted by Kalamkarov et al. [
25] (
Figure 8b). The biggest differences, in the range of 39.2% for (5, 0)(14, 0) DWCNTs with
to 50.70% for (21, 0)(30,0) DWCNT with
, were observed with the results obtained by Almagableh et al. [
32] (
Figure 8a) and from 60.17% (for MWCNT with
) to 16.45% (for MWCNT with
) with the Young’s modulus values calculated by Tu and Ou-Yang [
23] (
Figure 8b).
Figure 9 compares current results of the shear modulus in function of the outer layer diameter,
(
Figure 9a) and the number of layers,
, constituting the MWCNTs (
Figure 9b), with the results available in the literature (see,
Table 6). As in the case of the Young’s modulus, MWCNTs with
up to
, which corresponds to up to five layers in the structure, were considered [
22,
24,
25,
28,
30].
Kalmakarov et al. [
25] observed a substantial increase of the shear modulus with the increase of the outer layer diameter or the number of layers, and Santosh et al. [
22] and Ghavamian et al. [
28] pointed out an almost constant shear modulus with the increase of
and
. Li and Chou [
24] and Fan et al. [
30] reported lower shear modulus values for MWCNT than for SWCNTs, and in both studies, a decreasing trend was observed for the shear modulus with the increase of the outer layer diameter or the number of layers.
When compared with current results of the shear modulus, the smallest differences of 2.85% and 2.57% occur for the results of Ghavamian et al. [
28] for armchair and zigzag MWCNTs, respectively. The comparison with the results reported by Li and Chou [
24] shows substantial differences of 23.27% and 19.85% for armchair and zigzag MWCNTs, respectively. The differences of 22.28% for armchair and 30.75% for zigzag structures with current results are observed for shear modulus values calculated by Kalmakarov et al. [
25]. The shear modulus obtained by Fan et al. [
30] for (15,0)(24,0)(33,0) TWCNT with
shows the difference of 29.18% when compared with current results. The biggest difference of 44.42% is observed with the results of Santosh et al. [
22] evaluated for armchair DWCNTs. The shear modulus calculated by Santosh et al. [
22] is also lower than those obtained in the other studies [
24,
25,
28,
31].
In summary, the current results show that reliable values of the Young’s and shear moduli were obtained when compared with several of the literature results.