## 1 Introduction  
In May 2020, over 264 incidents of explosive violence around the world, resulting in 1573 deaths and injured victims, were recorded in the English language media worldwide [1]. In today's society, the threat of terrorism is ever present. The number of terrorist attacks using Improvised Explosive Devices (IED) has increased [2], targeting both civilians and military personnel in many places but especially in high-density public areas such as airports, concert arenas, security check-points, and public transport stations. Shrapnel bombs, pipe bombs or nail bombs, which are very often used in terrorist attacks, pose a serious threat to people and structures in the vicinity of the detonation. They are easy to manufacture and differ significantly from conventional m Their lethality is based on both the velocity of the projected fragments and the intensity of the blast wave. A shrapnel bomb is often referred to as an IED based on Home Made Explosives (HME) combined with a collection of metallic parts such as nails, screws, bolts and other randomly shaped fragments. They are used to increase the harmful effect on surrounding human and material targets. On detonation, a shrapnel bomb can result in three types of loadings: (i) a sudden pressure increase, (ii) multiple fragment impacts, and (iii) a combined loading caused by both the blast wave and the fragments' impact. The assessment of this combined loading is challenging. Therefore, blast and fragment impact loading are commonly treated separately in the design of protective structures. Previous studies [3,4] showed that in a realistic detonation scenario, a synergetic effect exists, resulting from the combination of blast loading and fragment impacts. More explicitly, this means that the structural response caused by the combined effect of blast and fragments is more severe than the sum of the responses caused by the separate actions of both loadings. Moreover, the arrival time of the blast wave front and the fragments at the targeted structure is different. This is due to the different velocities of both loadings and the distance between the charge and the structure. At short stand-off distance (SOD), the blast wave front travels faster than the fragments. As of a certain distance, the fragments will strike the structure first due to the faster decay of the blast wave velocity. With the aim of reducing the combined effect of blast wave and fragment impact, two important challenges exist. The first challenge resides in the conception and design of protective (anti-blast and anti-fragment) materials and structures. The second one resides in how to accordingly test their combined ballistic and explosive protection level. On the one hand, different international ballistic standards are available to test structures under only fragment impact loading [5,6]. On the other hand, standards and testing methods to test structures under only blast loading are available as well [7, 8, 9]. However, few techniques are established for testing structures under the combined effect of blast wave and fragment impact loading [10, 11, 12]. Trying to simplify an actual shrapnel bomb test and maintain its repeatability, is a challenge. Mastering the different parameters that govern the synergetic effect of the blast wave and the fragment impact is required. An in-depth understanding of the phenomena accompanying the momentum transfer from the explosive to the fragments is needed. Therefore, the simplified test to be designed needs to accurately represent the influence of charge and projectile parameters on the dynamic behavior of the target. A number of research efforts addressing this issue have been reported. [PERSON] et al. [13] experimentally investigated the different failure mechanisms of a sandwich panel under combined blast and fragment impact loading. The combined loading was generated using prefabricated cemented tungsten carbide fragments attached to the bottom surface of a cylindrical explosive charge.