## 9 Results and Discussion  
Table 1 shows the comparison of the critical buckling load of (S-F-S-F) boundary conditions for different orientations when the plate is stiffened with one and two stiffeners. The critical buckling load is increased with the increasing of orientations and number of stiffeners. The critical buckling load results at the orientations (0/90/0/90) and (0/90/90/0) and are the same when the main plate is stiffened by one stiffener. When the main plate is stiffened by two stiffeners, the results of critical buckling load are not the same at the orientations (0/90/0/90) and (0/90/90/0). The numerical results of the critical buckling  
Figure 21: Power spectrum analysis of the experiment test of the bending deflection for (\(\
u_{\mathrm{f}}\) = 25%) and (a/b = 2.5). Figure 22: Power spectrum analysis of the numerical simulation of the bending deflection for (\(\
u_{\mathrm{f}}\) = 25%) and (a/b = 2.5). load were done using the ANSYS program while the experiment results were carried out using strain gauge through a strain meter device. The analytic results of the critical buckling load were completed using the Levy solution of classical laminate plate theory. Figures 23-25 show the deflection contour distribution for the un-stiffened plate, stiffened plate with one stiffener and stiffened plate with two stiffeners of fiber orientation (0/90). The frequency was 12.31 Hz, 20.96 Hz and 23.94 Hz for the un-stiffened plate, stiffened plate with one stiffener and stiffened plate with two stiffeners, respectively. The central point in the middle of the main plate was considered in the calculation of the bending deflection contour. The maximum bending deflection decreased with the increasing of number of stiffeners. The ANSYS program was used in the calculation of the bending deflection contour. Figure 24: Bending deflection contour for plate with one stiffener. Figure 25: Bending deflection contour for plate with two stiffeners. Figure 26 shows the critical buckling load against the aspect ratio of (S-C-S-C and S-F-S-F) boundary conditions for plate with one stiffener in the presence of in-plane compression mechanical load and shear force. The critical buckling load increased with the increasing of the aspect ratio. The critical buckling load value for (S-F-S-F) boundary conditions is greater than the value of the critical buckling load for (S-C-S-C) boundary conditions. The two clamped boundary conditions in (S-C-S-C) generate shear stress in the opposite direction, which reduces the value of critical buckling load. The ANSYS program is used in the calculation of the critical buckling load. Figure 27 shows the critical buckling load against the thickness ratio for (S-C-S-C and S-F-S-F) boundary conditions for plate with one stiffener in the presence of the in-plane compression mechanical load and shear force. The critical buckling load is varied sinusoidal with a thickness ratio in which it reaches a maximum value at thickness ratio (a/h = 20). The ANSYS program is used in the calculation of the critical buckling load. Figure 28 shows the critical buckling load against the lamination angle for the plate with one stiffener for (S-C-S-C and S-F-S-F) different boundary conditions in the presence of the in-plane compression mechanical load and shear force. The critical buckling load  
Figure 27: Critical buckling load against thickness ratio for (S-C-S-C) and (S-F-S-F) boundary conditions. is varied sinusoidal for (S-C-S-C) boundary conditions, while the critical buckling load is varied parabolic for (S-F-S-F) boundary conditions. The critical buckling load is the same at the lamination angle (40 and 60) degrees for (S-F-S-F) boundary conditions. The ANSYS program is used in the calculation of the critical buckling load. Figure 29 shows the critical buckling load against a number of stiffeners at (S-F-S-F, S-S-S-S-S-S-C) boundary conditions in the presence of in-plane compression mechanical load and shear force.