# Nondestructive Evaluation of Aluminium Foam Panels Subjected to Impact Loading
## 3 Results and Discussion
### Nondestructive Evaluation  
Nondestructive evaluation by means of DR allowed damage detection in the AF specimens without skins subjected to an energy impact of 35 J (Figure 11). Thus, considering the very low level of the impact energy, the detected foam damage involved only the foam under the indenter nose without provoking perforation. The impact damage could be assessed by analysing the grey levels, which describe densification in the darker regions and cell failure in the lighter ones. The damage depth could be observed only in the side projection, which is not possible to perform in onsite application with portable equipment. Nevertheless, due to a high content of air with a composite-specific, near-wall probe, ultrasonic waves could not penetrate the specimens. Thus, UPA did not allow inspection of the specimens without external skins (Types I and II).  
In the double-layer specimens (Type VI), UPA did not allow obtainment of reliable results, while damage detection was evaluated by the DR technique. As depicted in Figure 12a, the core damage (lighter area in the centre of the specimen) could be easily observed also in the case in which upper skin fracture did not occur. Obviously, such damage could not be detected by visual inspection (Figure 12b). In the side projection, global bending without perforation produced the damage of the inner skin was observed.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Test Type** & **F\({}_{\text{MAX}}\) [N]** & **w [mm]** \\ \hline Repeated impact tests & \(1.19\times 10^{4}\) & 36.45 \\ \hline Single impact test at 350 J & \(1.52\times 10^{4}\) & 30.57 \\ \hline Variation [\%] & +27.73 & \(-16.13\) \\ \hline \hline \end{tabular}
\end{table}
Table 4: Comparison between repeated impact tests and single impact test at 350 J for the Type IV panel.  
Figure 11: Radiographic images for Type I (**upper**) and Type II (**below**) AF specimens subjected to an energy impact of 35 J.  
Thus, the energy absorbing mechanism was mainly due to the global compression strain, which involved the second layer. A similar behaviour was detected for double-layer AHS panels [10].  
By analysing the load-displacement curves reported in Figure 4d-f for Type VI, an abrupt decrease in load, at about 8-9 mm in depth, was seen. This load drop was probably due to the collapse mechanism of double-layer AF panels. Moreover, a global densification damage, due to the collapse of the AF cell walls, occurred in the bottom layer of Type VI panels for impact energies over 150 J, as demonstrated by the radiographic and visual inspections, shown in Figure 12a,b. A similar behaviour was reported in [10].  
UPA was successfully applied for damage evaluation of Type IV specimens (as reported in a previous study [13]) also in the case in which the damage was provoked by a second impact at the same energy (35 J, Figure 13). Indeed, ultrasonic waves easily detected both the densification areas in about the mid-depth of the specimen (Figure 13c) and the debonding between skin and core. The latter was barely detected by DR due to superimposition of all projections (Figure 13a,b). Core densification was detected in the darker zone in the mid-depth of the specimen. Different considerations can be drawn for the Type V specimens, since UPA was well-suitable for damage evaluation of the skin only (Figure 14c,d). Indeed, due to the high beam attenuation of the skin, ultrasonic waves could not reach the foam bottom without allowing damage detection of the core. As demonstrated in a previous research paper [13], UPA allows the detection of skin delamination and fibre failure. Thus, it remains the most suitable technique for this kind of specimens.  
Figure 12: (**a**) Radiographic and (**b**) visual inspections for Type VI specimen subjected to an energy impact of 300 J.