4.1. Verification
A simply supported, rectangular FG sandwich plate, shown in
Figure 4a, was taken to verify the proposed numerical method, where the lengths of the two sides,
and
, were
and the thickness
was taken as variable. The two constituent materials were
and
, and their mechanical properties are given in
Table 1. The metallic
was used for the softcore while the ceramic
was used for the hardcore, respectively. To numerically implement the
hierarchical model, the mid-surface of the sandwich plate was uniformly discretized by an
set of grid points, as shown in
Figure 4b.
The simply supported constraint was implemented as follows: for all four sides of the mid-surface, for the two sides labelled ① and ③, and for the other two sides, ② and ④. The stiffness and mass matrices in Equations (18) and (19) were computed by combining the 2-D, in-plane Gaussian integration rule (7 points) and the thickness-wise trapezoidal rule (30 uniform segments). The lowest nine natural frequencies and natural modes were solved through the Lanczos transformation method and the Jacobi iteration method.
The fundamental frequencies,
, of the (2-2-1) FG sandwich plates with softcores (i.e.,
cores) were computed using the hierarchical models and calibrated according to
where
and
. The numerical experiments were parametrically carried out with respect to the width-to-thickness ratio
for the same volume fraction index,
, of unity. The computed calibrated fundamental frequencies are given in
Table 2 and represented in
Figure 5, where the solutions of 3-D elasticity were obtained by ANSYS [
35] using 10,000 3-D shell elements. All the hierarchical models showed the remarkable relative differences in
for thick plates, such that they overestimate the fundamental frequencies with non-negligible differences. However, for a specific value of
, the hierarchical models showed the spectral model accuracy such that their relative differences uniformly decreased, proportional to the model level,
. Furthermore, the relative differences of all the hierarchical models monotonically decreased in proportion to the width-to-thickness ratio
. In particular, it was observed that the relative difference became remarkably smaller when the model level equaled or exceeded (3,3,2), regardless of the width-to-thickness ratio. Meanwhile, both the (9,9,8) hierarchical model and the 3-D elasticity approach to the same limit equated to 1.6595, which was the solution provided by the classical laminated plate theory (CLPT) [
36]. Meanwhile, it was observed that the limit values,
, of other hierarchical models became slightly larger as the model level,
, decreased. However, the relative difference of the (1,1,0) model was 2.773% at
; hence, the lower-order models were not expected to cause inaccuracy problems in the free vibration analysis for thin FG sandwich plates with homogeneous cores. The generic constant,
, which was involved in the modified shear correction factor (MSCF), was evaluated as 3/2 through this limit experiment.
Next, the (1,1,0) hierarchical model was compared with the five-variable, refined plate theory of Bennoun et al. [
10]. The former was equivalent to the first-order shear deformation theory (FSDT) and the latter was refined from the FSDT.
Table 3 presents the comparison for the (2-2-1) FG sandwich with a softcore (i.e.,
layer). The relative error uniformly decreased in proportion to the width-to-thickness ratio,
, except for the volume fraction,
, which demonstrated the reverse trend. Here, the (2-2-1) FG sandwich resembled a pure homogeneous plate of
when
is 0. Regarding the volume fraction,
, the largest errors occurred at
for
,
for
, and
for
. Thus, the present method using the (1,1,0) hierarchical model was in good agreement with the FSDT, such that the overall relative error ranged from 0.895% to 5.843%, with a relatively coarse NEM grid.
Table 4 presents a comparison of (2-2-1) FG sandwiches with hardcores (i.e.,
layers), in which the sandwich resembles a pure homogeneous plate of
when
is 0. The relative error varied from 0.044% to 7.961%, so the range of relative errors was wider than in the softcore case. Contrary to the softcore, the relative error for hardcores uniformly increased in proportion to the width-to-thickness ratio,
, for all the volume fraction indices,
. Regarding the volume fraction index
, the largest error occurred at
for
, and
for
and
, which is similar to the softcore case. From the comparison for both the softcore and hardcore cases, it has been confirmed that present method using the (1,1,0) hierarchical model showed a reasonable agreement with the five-variable, refined plate theory, with a maximum relative error equal to 7.961%. Moreover, it is expected that the accuracy of the present method would be improved when higher-order hierarchical models are used.
4.2. Free Vibration Characteristics
Next, the free vibration characteristics of FG sandwich plates with homogeneous cores were investigated with respect to the major parameters of the plates. For this parametric experiment, the (3,3,2) hierarchical model was used because its convergence to the 3-D elasticity was verified through the previous verification experiment.
Figure 6a,b represent the calibrated fundamental frequencies versus the thickness-to-width ratios of (2-2-1) and (6-6-1) FG sandwich plates with homogeneous softcores. The bottom curve of the metal is the case of
, while the top curve of the ceramic corresponds to
. It was observed that the fundamental frequency increased in proportion to the volume fraction index,
. Referring to
Figure 2, the relative amount of ceramic present in the plate increased in proportion to the volume fraction index,
. Furthermore, the ceramic was much stiffer than the metal, even though its density was greater than the metal’s, as given in
Table 1. For this reason, the sandwich plate became stiffer with the increasing value of the volume fraction index,
. Meanwhile, from
Figure 6b, it may be seen that the frequencies of the center five curves became lower. This is because the relative amount of ceramic became smaller as the pseudo thickness ratio was changed from (2-2-1) to (6-6-1). In other words, the relative thickness ratio of metal core increased from 2/5 (= 40%) to 6/13 (= 46%).
Figure 7a,b represent the curves of FG sandwich plates with homogeneous hardcores. Because the core was ceramic, the bottom and top curves correspond to
and
, respectively. Contrary to the previous softcore, the fundamental frequency decreased in proportion to the volume fraction index,
. This is because the sandwich plate with a hardcore became softer with the increasing value of the volume fraction index,
. The reason for this can be explained in the opposite manner to the previous softcore. Meanwhile, from
Figure 7b, one can see that the frequencies of five curves in the middle became higher. This is because the relative amount of ceramic became larger as the pseudo thickness ratio was changed from (2-2-1) to (6-6-1).
Figure 8a,b represent the calibrated fundamental frequencies versus the volume fraction index,
, for (2-2-1) FG sandwich plates with homogenous cores. It is seen that the curves converged to the limits, both the fundamental frequencies of ceramic plates for softcore cases and those of metal plates for hardcore cases. The curves were upper-bounded for the softcore cases while those of the hardcore cases are lower-bounded. This limit characteristic, due to the material gradient in the FGM layers, could be confirmed from
Figure 6a and
Figure 7a. With the increasing value of volume fraction index,
, two FGM layers in the softcore sandwich plate became ceramic-rich, while those in the hardcore sandwich plate became metal-rich. This fact can be confirmed from the volume fraction distributions shown in
Figure 2. However, in both softcore and hardcore cases, it was observed that the calibrated fundamental frequency increased in proportion to the width-to-thickness ratio,
.
Table 5 and
Table 6 present the calibrated fundamental frequencies versus the relative thickness
of the top facesheet of softcore and hardcore FG sandwich plates, respectively. The 8-8-1 and 16-16-1 configurations were included to examine the limits of
as
tends to zero. The width-to-thickness ratio,
, of the sandwich plates was set by 10. The results are comparatively represented in
Figure 9a,b, where the value,
, of the
-
bi-material plate is 1.259. The curves of the softcore cases were lower-bounded because the amount of
slightly increased as
became smaller. Conversely, those of hardcore cases were upper-bounded as
became smaller, because the slight increase of the amount of
. However, in both softcore and hardcore cases, the calibrated fundamental frequencies tended to 1.259 of
-
bi-material plate as the volume fraction index
tend to
. This is because the
of the FG sandwich plates approached that of the
-
bi-material plates as
and
tended to
, regardless of the material type of core layer.
Table 7 presents the calibrated fundamental frequencies,
, of simply supported FG sandwich plates with respect to the aspect ratio
for various values of
. The volume fraction index,
, and the width-to-thickness ratio,
, were set at 1.0 and 10, respectively. The aspect ratio,
, was variable; changes to
were applied, while
was kept unchanged because the fundamental frequency was calibrated with respect to
, as given in Equation (22). It was found that the calibrated fundamental frequency increased, reversely proportional to the aspect ratio, regardless of the type of core, because the plate became stiffer as the aspect ratio decreased. This effect of the aspect ratio on the calibrated fundamental frequency is well represented in
Figure 10a,b. Meanwhile, both softcore and hardcore cases show variations to the thickness,
, which are similar to
Figure 9a,b, respectively, regardless of the value of the aspect ratio.