# Delamination Effect on the Buckling Behaviour of Carbon-Epoxy Composite Typical Aeronautical Panels
## 4 Results and Discussion
### Cd1-CD16 Configuration Comparison  
In this last subsection, the results of all of the configurations are compared. The comparisons were made according to different criteria, distinct from the previous ones, for a better understanding of the results.  
Figure 13, for example, compares four configurations that share the radius and depth of delamination, but differ in geometric characteristics (skin thickness, stringer thickness and bay width). In detail, the delamination radius is 40 mm and the depth of delamination is 0.33 mm (two layers) in all of the configurations, but the stringer foot increased from 2.64 mm to 3.96 mm when passing from CD1 to CD5, while the bay reduced from 176 mm  
Figure 12: CD11–CD16 configurations: (**a**) delaminated area vs. load curves; (**b**) delamination evolution at final analyses load (pink represent the delaminated area).  
to 146 mm when passing from CD1 to CD9 and CD11. The latter differs from CD9 because the foot stringer increased from 2.64 mm to 3.96 mm.  
It may be observed that increasing the stringer and skin thicknesses by 1.32 mm (CD5) results in a significant increase in the overall panel stiffness. On the other hand, varying the width of the bay, assuming the same thicknesses, does not change the stiffness of the panel, but changes the propagation of the delamination, which increases as the bay is reduced and becomes unstable and sudden (CD9 configuration compared to CD1). Finally, the foot stringer thickening at the same bay width (CD9 to CD11) does not cause significant changes in either stiffness or delamination propagation.  
For the CD3, CD7, CD10 and CD12 configurations, displayed in Figure 14, which all have a delamination of radius 30 mm under the second lamina (0.33 mm), similar considerations apply. Indeed, the stringer foot increased from 2.64 mm to 3.96 mm when passing from CD3 to CD7, while the bay reduced from 176 mm to 146 mm when passing from CD3 to CD10 and CD12. The latter differs from CD10 because the foot stringer increased from 2.64 mm to 3.96 mm.  
Figures 15 and 16 show configurations with a delamination depth of 0.66 mm (four laminae) and a radius of 40 mm and 30 mm (CD2, CD6 and CD15 versus CD4, CD8 and CD16, respectively). In both cases, the change is from a 16-layer skin and foot stringer to a 24-layer skin and foot stringer (the stacking sequence remains quasi-isotropic, symmetrical and balanced) with a high increase in panel stiffness and a very different delamination behaviour; the first does not propagate as the global buckling is reached, while in the second, the initial delamination increases more than 50% (see CD2-CD6 and CD4-CD8). On the contrary, by increasing only the thickness of the stringer foot from 16 to 24 layers and by moving the stringers closely together to reduce the width of the bay, no major changes in either delamination or loading occur (CD15 and CD16 configurations).  
Figure 14: Delaminated area vs. load and load vs. displacement charts. Comparison against CD3, CD7, CD10 and CD12 configurations.  
Figure 13: Delaminated area vs. load and load vs. displacement charts. Comparison against CD1, CD5, CD9 and CD11 configurations.