# Nondestructive Evaluation of Aluminium Foam Panels Subjected to Impact Loading
## 3 Results and Discussion
### Repeated Impact Tests  
Figure 7 shows the load-displacement curves of the repeated impact tests on AF panels without and with skins made of aluminium and GFRP. Figure 8 shows the load-displacement curves for a Type IV specimen subjected to 10 repeated impact tests including stages of loading and unloading at impact energy equal to 35 J. The impact repeating was sustained until obtaining a displacement of 36 mm equal to the specimen thickness. \begin{table} \begin{tabular}{c c c c c} \hline \hline
**Sandwich Structures** & \begin{tabular}{c} **Test** \\ **Conditions** \\ \end{tabular} & \begin{tabular}{c} \(\rho_{\text{core}}\) \\ **[kg/m\({}^{3}\)]** \\ \end{tabular} & \
\begin{tabular}{c} **ttot** \\ **[mm]** \\ \end{tabular} &
\begin{tabular}{c} **Maximum SAE** \\ **[J m\({}^{3}\)/kg]** \\ \end{tabular} \\ \hline AFS Alulight with bonded skins & c-hi 1 & 906 & 11 & 0.140 \\ AFS Schunk with integral skins & c-hi & 674 & 11 & 0.131 \\ GFRP-AFS Alulight & c-hi & 1066 & 17.5 & 0.211 \\ GFRP-AFS Schunk & c-hi & 1032 & 17.5 & 0.218 \\ AHS d3 (cell diameter d = 3 mm) & c-hi & 674 & 11 & 0.131 \\ AHS d6 (cell diameter d = 6 mm) & c-hi & 639 & 11 & 0.138 \\ GFRP-AHS d3 & c-hi & 926 & 17.5 & 0.243 \\ GFRP-AFS d6 & c-hi & 910 & 17.5 & 0.313 \\ AHS d3 & ss-hi 2 & 130 & 11 & 0.126 \\ AHS d6 & ss-hi & 80 & 11 & 0.140 \\ AHS d3 double-layer & ss-hi & 130 & 22 & 0.312 \\ AHS d6 double-layer & ss-hi & 80 & 22 & 0.328 \\ AHS double-layer (d3 up layer & ss-hi & 130/80 & 22 & 0.288 \\ and d6 bottom layer) & ss-hi & 80/130 & 22 & 0.289 \\ AHS double-layer (d6 up layer & ss-hi & 80/130 & 22 & 0.289 \\ and d3 bottom layer) & c-hi & 370 & 70 & 0.811 \\
7L-TCAS 0\({}^{\circ}\)/90\({}^{\circ}\) & c-hi & 370 & 70 & 0.797 \\ GFRP-BCC & c-hi & 825 & 9.5 & 0.098 \\ AF (Type I) & ss-ci 3 & 477 & 20 & 0.147 \\ AF (Type II) & ss-ci & 583 & 34 & 0.256 \\ AFS with bonded skin (Type III) & ss-ci & 421 & 22 & 0.313 \\ AFS with bonded skin (Type IV) & ss-ci & 616 & 36 & 0.508 \\ \hline \hline \end{tabular}   
* 1: [FOOTNOTE:*]Footnote  
Figure 9 shows the load-displacement curves for a Type II specimen subjected to repeated impact tests, including stages of loading and unloading at an impact energy equal to 35 J. The impact repeating was sustained until the full panel perforation. Figure 10 shows the load-displacement curves obtained by three tests on Type IV specimens, namely, a single impact test at an impact energy equal to 350 J, a single impact test at an impact energy equal to 400 J and a repeated impact test at an impact energy equal to 35 J.  
Figure 8: Loading and unloading curves for a Type IV specimen subjected to repeated impacts at impact energy equal to 35 J.  
Figure 10: Load-displacements curves obtained by three tests on Type IV specimens. Figure 9: Loading and unloading curves for a Type II specimen subjected to repeated impacts at an impact energy equal to 35 J. According to the results (Figures 8 and 10), the behaviours of the AFS panels under repeated impacts were similar to those observed for the AHS panels [18; 20]. The unloading stiffness after an impact was almost equal to the loading stiffness of the next impact (Figures 8 and 9). As the impact number increased, the peak force and the loading stiffness increased, while the displacement decreased (Figure 8). With the increase in the impact number, the foam core compressed, gradually reaching the densification stage, and the foam panel was strengthened, resulting in an increase of the impact force and a reduction of the permanent deflection. The following results were obtained considering all ten repeated impact tests at an impact energy \(E_{i}\) equal to 35 J: \(E_{a}=308\) J, \(F_{MAX}=1.19\)E\(\times 10^{4}\) N, \(SAE=0.487\) J\(\cdot\)m\({}^{3}\)/kg (evaluated considering the initial value of specimen density \(\rho=632\) kg/m\({}^{3}\), while the core density \(\rho_{\text{core}}=510\) kg/m\({}^{3}\)) and \(SAE_{w}=3743\) J/kg.