## Materials and Methods  
A single diaphragm shock tube with a circular section was used in the present investigation. The shock tube has a 50 mm inner diameter \(\times\) 2 m long driver section, and a 36.5 mm inner diameter \(\times\) 3 m long driven section, separated by 1.2 mm thick (t) aluminum diaphragm with a 't/3' deep cross notch at the centre. Helium gas was pressurized in the driver section until the diaphragm ruptured. Air at atmospheric pressure was maintained in the driven section, through which the shock wave propagated on rupture of the diaphragm. The shock tube was instrumented with piezoelectric pressure sensors of model 102B04 and 102B of PCB Piezotronics (Depew, NY, USA), which had a rise time of less than a microsecond. A high-speed data acquisition system (NI PCI-6115 S, National Instruments, Austin, TX, USA) was used to acquire the pressure data. Sensor-1 (102B04) was mounted flush to the inner diameter of the shock tube at 94 mm distance from the front surface of the foam to measure the incident as well as reflected pressures (see Figure 1a,b).  
Closed cell aluminum foams were prepared in the form of cylindrical blocks by the melt route described elsewhere [18]. Since shock-foam interaction is sensitive to foam density [12], foams of incrementally varying densities from 0.28 to 0.49 g/cc were selected to capture the differences in their response to incident shock. Due to the inherent variability of foam density within the starting blocks, the density varied from sample to sample. Consequently, one experiment was done for each density but collectively, the samples covered a wide range of densities. Foam samples having a diameter of \(40\pm 0.5\) mm and thickness \(25\pm 0.5\) mm (Figure 2a), were mounted within a Teflon bush resting against the end-wall such that the back face of the foam was in full contact with the end-wall (see Figure 1b). The diameter of the foam was kept larger (by about 10%) than the shock tube ID to ensure that no material other than the foam lies in the path of the shock.  
The end-wall of the shock tube was made of stainless steel having a centreline axial cavity of 9 mm dia running through the entire thickness of the flange. Sensor-2 (102B) was inserted through the axial cavity from behind the end-wall, resting about 2 mm away from the back surface of the foam. The hole allows the sensor to be recess-mounted such that it is not damaged during foam deformation on shock loading. Further, it is known that, if the end-wall has a hole of size smaller than a third of the shock tube diameter, the shock reflection at the end-wall is unaffected [19]. Due to this, and the fact that the sensor is placed very close to the exit (back/rear) face of the foam, the pressure-time history recorded by sensor-2 is expected to reveal the effects of compaction wave and flow filtration through the foam, on the ambience behind the foam [8,9]. A baseline experiment was carried out in the empty shock tube to measure the reflected pressure off the end-wall.  
In a separate experiment, the speed of the shock wave was measured by recording the incident wave pressures at two locations separated by a distance of 474 mm in the driven section, and was found to be 1000-1050 m/s (more than Mach 3).  
A scanning electron microscope (SEM) (Zeiss, GEMINI 300, Oberkochen, Germany) was used to examine the foam before and after the shock exposure. A 3-D X-ray microscope (3D CT) (Zeiss, VERSA 520, Oberkochen, Germany) was used to examine the internal structure of the foam before and after the shock tests.