## 3 Edge-On Impact Tests  
The edge-on impact (EOI) test aims to investigate the damage modes under impact loading [19]. The test is based on the impact of a metallic cylindrical projectile (generally made of steel or an aluminium alloy) that hits a target (concrete, rock, or ceramic tile) on its edge. This test can be performed in two different configurations: the open configuration and the sarcophagus configuration. In the free (open) configuration, the target is visualized using an UHS camera. In the second configuration, the target is encased in a metallic box. In this way, all of the fragments are kept in the same position after impact, making it possible to study the fragmentation pattern (Figure 7a). However, a small gap (a few tens of mm) is imposed between the metallic box and the target to ensure that the sarcophagus has no influence on the damage process within the impacted target. Results of EOI experiments conducted in open configuration with an ultra-high performance concrete (UHPC) and in sarcophagus configuration with 3 types of concrete--an UHPC, a microconcrete, and a common concrete--tested in wet and dry conditions are reported in [7]. In these experiments, multiple fragmentation with numerous oriented cracks resulting from tensile hoop stresses was observed (Figure 7b,c). Figure 7: Edge-On Impact testing method applied to concrete. (**a**) Cut-view of EOI set-up in sarcophagus configuration [7], (**b**) Dry microconcrete tested in sarcophagus configuration [20], (**c**) Dry common concrete tested in sarcophagus configuration [9]. A dynamic confinement system is used in both open and sarcophagus configurations to restrain the compressive damage near the impact point and to favour the propagation of the incident wave leading to the tensile damage. This dynamic confinement system can be composed of two half-cylinders and two half-crowns placed in contact with the lateral surfaces near the impact area. The set-up used as an open configuration in the present work is illustrated in Figure 8. The dimensions of each target are reported in Table 3. The target is impacted by a cylindrical projectile, 20.1 mm in diameter and 40 mm in length, made of a high-strength aluminium alloy (elastic limit of about 450 MPa). The dynamic confinement system is composed of two half-cylinders, each 30 mm in diameter and 50 mm in length, and two half-crowns, 31 mm in inner diameter, 60 mm in outer diameter, and 52 mm in length, all made of steel. When the projectile strikes the target, a compressive divergent wave propagates from the impact area through the target, leading to a complex transient dynamic loading made of radial compression stresses and tensile hoop stresses. In the present work, the projectile velocity was set to \(v=147\) m/s in the EOI test performed in open configuration and to \(v=162\) m/s in the EOI test performed in sarcophagus configuration. A Kirana ultra-high-speed camera providing 180 pictures with a resolution of 924 \(\times\) 768 pixels was used in an open configuration with a recording frequency set to 400 kfps. The camera is triggered with a laser interferometer directed to the centre of the target's rear-edge, with a triggering level set to 2 m/s. Two pictures of the specimen are shown in Figure 9a before impact and in Figure 9b after fragmentation in an open configuration. The fragmentation process has a typical duration of 10 \(\upmu\)s. Already before impact a number of structural defects can be observed. These structural defects seem to favour certain failure paths after impact. Smoke was also ejected from a few fracture surfaces located in the centre of the target. The smoke would correspond to very fine powder induced by frictional mechanisms when pre-existing structural defects are loaded in mode II under compression loading, generating a slip on the crack lips. Figure 8: Edge-on impact set-up in open configuration considered in the present study.