# Delamination Effect on the Buckling Behaviour of Carbon-Epoxy Composite Typical Aeronautical Panels
## 4 Results and Discussion
### CD5-CD8 Configurations  
In Table 4, the buckling loads and displacement obtained from the linear eigenvalue buckling analyses are reported for each configuration.  
According to Table 4, the global stiffness of the panels is again very similar (\(\sim 399,100\) N/mm). The loads related to the buckling of the delamination are very different (an order of magnitude) when the depth of the delamination changes, in this case doubling from two to four layers (from CD5, where it is 5.28 kN, to CD6, where it is 30.7 kN, and from CD7, where it is 9.2 kN, to CD8, where it is 53.7 kN). These configurations exhibit the buckling of the skin, coincident, or almost coincident, with the global buckling. Delamination initiation occurs before global buckling in all of the configurations, and specifically it is first triggered when the delamination is 0.66 mm deep (CD6 and CD8, where the onset load is 178 and 193 kN versus 200 kN of configurations CD5 and CD7). Figure 7 displays the out-of-plane displacement contour plot of all of the configurations.  
To summarise, in the configurations with delamination of both 40 mm and 30 mm of radius and depth of 0.33 mm (under two plies), the local buckling of the delamination occurs at very low loads (5.28 and 9.2 kN) and displacements (0.013 and 0.023 mm), but then propagation is triggered at elevated loads. On the contrary, in the case of delamination under four plies, of both radius 40 and 30 mm, the buckling of the delamination occurs at higher loads (30.7 and 53.7 kN) but propagation occurs earlier. According to Figure 8a,  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Configuration**} & \multirow{2}{*}{**Stiffness**} & \multicolumn{2}{c}{**Buckling Circular**} & \multirow{2}{*}{**Buckling Skin**} & \multirow{2}{*}{**Buckling Stringer**} & \multirow{2}{*}{**Delamination Initiation**} \\  & & **Delamination** & & & \\ \hline CD5 & 399,172 & 5.28 kN & 264 kN & 264.3 kN & 200 kN \\  & N/mm & 0.013 mm & 0.661 mm & 0.661 mm & 0.506 mm \\ CD6 & 399,105 & 30.7 kN & 255.1 kN & 259.3 kN & 178.4 kN \\  & N/mm & 0.077 mm & 0.639 mm & 0.649 mm & 0.45 mm \\ CD7 & 399,176 & 9.2 kN & 262.3 kN & 262.3 kN & 200.8 kN \\  & N/mm & 0.023 mm & 0.657 mm & 0.657 mm & 0.5065 mm \\ CD8 & 399,109 & 53.7 kN & 257.7 kN & 257.7 kN & 193.7 kN \\  & N/mm & 0.134 mm & 0.646 mm & 0.6457 mm & 0.4875 mm \\ \hline \hline \end{tabular}
\end{table}
Table 4: Buckling loads and displacements for CD5–CD8 configurations.  
Figure 6: CD1–CD4 configurations: (**a**) delaminated area vs. load curves; (**b**) delamination evolution at final analyses load (pink represent the delaminated area).  
representing the delaminated area evolution as a function of the load, CD6 and CD8 configurations present an unstable (sudden) delamination evolution, if compared against CD5 and CD7. Hence, if the delamination is more superficial, propagation occurs later and more gradually. In Figure 8b, the final status of delamination can be observed.  
Figure 8: _Cont_.  
Figure 7: Out-of-plane displacement contour plot: (1) buckling of circular delamination, (2) buckling of the skin, (3) global buckling including stringers and (4) delamination initiation, with (**a**) CD5, (**b**) CD6, (**c**) CD7 and (**d**) CD8.