## 3 Results and Discussions  
Figure 3 shows sensor-1 data for foams of various densities. A certain scatter in pressure values is seen, which is partly attributed to experimental variations in incident pressure (related to diaphragm rupture) and to morphological variations in the cell structure of the foam. In each case, pressure rises to its incident value, stays constant for about 0.3 ms, rapidly jumps to a higher value, and then rises at  
Figure 1: (**a**) Stainless steel flange of the shock tube for mounting foam samples. (**b**) Details of foam and sensor placement inside the flange. Figure 2: Aluminum foam samples (**a**) before and (**b**) after shock exposure. a reducing rate till reaching the maximum before slowly decreasing till the end of the measurement period. The incident pressure marks the arrival of the incident shock at sensor-1. As discussed by [PERSON] et al. [8], the initial steep rise in reflected pressure is due to the arrival of the shock reflected from the foam front face, while the subsequent rise is attributed to additional shock reflections from the contact surface (driver-driven contact surface). For the empty tube (no foam) condition, the shock reflects from the end-wall of the tube. Table 1 gives numerical values of the incident pressures (the \"plateau\" values) and the reflected pressures measured by sensor-1 for foams of various densities and the empty shock tube. The shock exposed foams were seen to deform plastically, as shown in Figure 2b, where the total deformation (change in foam thickness) decreases with increasing foam density (Table 1 and Figure 4). Interestingly, the deformation in the foam samples was non-uniform along the length and always occurred from an intermediate location along the length (which was dependent on the foam density) extending up to the exit face. Another observation, being reported for the first time in shock-foam interaction literature, is the formation of perforations in cell walls due to shock exposure. The SEM image of the shock exposed foam in Figure 5b, when compared with 5a (unexposed) (both foams are of density 0.31 g/cc) clearly illustrates the perforation phenomenon. Although similar perforations have been observed in all the shock exposed foams, the perforation severity reduces with increasing foam density since the thickness of the cell wall increases with increasing density. The observation of the cell wall perforation is important since it creates the possibility of flow filtration through an otherwise closed cell foam. Figure 5c,d shows the 3D CT images of an unexposed and shock exposed foam sample, respectively, of 0.31 g/cc foam. The cell wall puncturing (Figure 5d) can be seen to continue to the subsurface region, which remains uncollapsed. The 3D CT images confirm that densification occurs towards the far end of the foam because the exit face is fully supported by the rigid end-wall (but for \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Density (g/cc)**} & **Plastic** & \multirow{2}{*}{**Incident Pressure (Sensor-1) (bar)**} & **Reflected Pressure (Sensor-1) (bar)** & **Pressure in the End-Wall Cavity** \\  & & & **(Sensor-2) (bar)** & **(Sensor-2) (bar)** \\ \hline No Foam & - & 10.3 & 49.5 & 51.5 \\
0.28 & 9.3 & 10.3 & 52.1 & 52.5 \\
0.31 & 5.9 & 10.3 & 51.0 & 36.0 \\
0.32 & 5.78 & 10.7 & 52.2 & 35.1 \\
0.36 & 4.44 & 9.7 & 46.6 & 38.8 \\
0.46 & 1.75 & 9.7 & 48.8 & 3.3 \\
0.47 & 0.75 & 9.8 & 50.1 & 2.7 \\
0.49 & 0.55 & 9.7 & 54.4 & 2.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Effect of foam density on foam plastic deformation and shock pressures. Figure 3: Variation of incident and reflected shock pressures with time (sensor-1). the cavity part). Unlike the observation of reference [13], no disintegration of the central portion of the foam nor radial cracks around the disintegrated region were observed in the foams after the tests, possibly because the foams were fully supported in the present work, as opposed to being simply supported in the referred work.