## 2 Experimental Setup Using the Explosive Driven Shock Tube  
### Description of the Experimental Setup  
HME are improvised, they are extremely diverse in design and can come in many forms, ranging from a small pipe bomb (practical option for terrorists) to a sophisticated device capable of causing massive damage to people and infrastructure [23]. Most of IEDs used in terrorist attacks are either spherical or cylindrical in shape. The enhancements added to the explosive charges differ in mass, shape, and geometry from one configuration to another. The full range of possibilities in the shape and the mass of the charge, together with the geometry, shape and mass of the fragments is considered as one of the great difficulties in IED research [13; 19; 20; 21]. Therefore covering the whole range of possibilities in a laboratory-scale experiment is not realistic. In this paper, the choice is made to use a spherical steel projectile with a variable diameter attached to the front surface of a spherical explosive charge. Steel spheres with diameters of 5, 7, and 8 mm are half buried into the radial center of the front surface of the charges. C4 explosive charge masses of 10, 20, and 30 g are positioned at the entrance of the EDST. This choice is made to cover a specific configuration range that usually observe in real scenarios. The EDST is based on a cylindrical steel tube with a length of 1200 mm, a wall thickness of 4.5 mm and an inner diameter of 168.2 mm. The tube is reinforced at its entrance, over the first 200 mm, with a second steel tube. The latter has a 4.5 mm thickness and an inner diameter of 193.7 mm. Steel fiber-reinforced concrete is used to fill the radial space between the two tubes [24]. It is used to strengthen the region where the highest pressures occur. The EDST is mounted on a steel structure to allow for a controllable height from the ground as shown in Figure 1a. A steel frame with dimensions of 1000 mm \(\times\) 1000 mm \(\times\) 15 mm is placed at the exit of the tube. It is rigidly fixed to the ground and has an opening of 300 mm \(\times\) 300 mm as shown in Figure 1b. This opening will hold a witness plate that will be subjected to the combined effect of blast wave loading and projectile impact.  
A high-frequency pressure sensor [25] is placed at 20 mm from the exit of the EDST. It is used to record the incident pressure as well as the blast wave front arrival time. The pressure sensor is mounted on a special adaptor [25], which is used to attenuate the vibrations in the EDST wall resulting from the explosion. An image of the experimental setup showing all equipment is shown in Figure 2. An SA5 photron fastcam digital camera is positioned at a distance \(Z_{c}\) of 2350 mm from the center point of the tube. The camera is focused on a steel frame with plexiglas, which is positioned on the same optical axis at a distance \(Z_{b}\) of 1945 mm from the tube's center. A frame rate of 20,000 fps (frame per second) and a shutter speed of 6.42 \(\upmu\)s are chosen together with a picture resolution of 704 \(\times\) 520 pixels in order to measure the projectile velocity at the exit of the tube. A black and white speckle pattern, printed on a transparency film, is attached to the plexiglas [26]. Additionally, the speckle pattern is back-illuminated using two 575 W halogen lamps. A wall composed of thick rubber bricks is placed at a distance \(Y_{w}\) of 1885 mm from the tube's center. It serves to retain the steel spheres after the explosion and to record their impact points. The main aim is to extract conclusions concerning the projectile flight trajectory and the dispersion of impacts.  
Figure 1: (**a**) EDST mounted on its controllable steel structure showing: (**i**) the placement of the C4 charge with the steel ball bearing at the tube’s entrance, (**ii**) the high-frequency pressure sensor at the exit of the tube, (**b**) steel frame placed at the exit of the EDST to allow for the measurement of pressure, velocity, and arrival time.