Multi-Scale Experimental Study of Blast Propagation Around a Hemi-Cylindrical Barrier

Abstract

Interaction between large blast and targets can rarely only be directly studied, due to cost and practicality considerations. Blast tests using reduced-scale high explosive charges represent an attractive alternative. The first necessary step consists in studying blast propagation in free-field at the considered scales. The second step focuses on the determination of the blast load around various types of reference obstacles, in order to provide a critical input for numerical simulation. This approach also aims to build simplified models allowing faster risk assessment for government agencies. Since 2017, the French Institute for Nuclear Safety (IRSN) and the French-German Research Institute of Saint-Louis (ISL) have been studying blast propagation in free-field and in front of a hemi-cylinder at two different reduced scales using Hexomax ® charges. IRSN developed a significant experience on hemispherical blast effect assessment using 42 g reference Hexomax ® charges detonated in contact with a planar surface supporting a hemi-cylindrical obstacle, both equipped with pressure sensors. Based on this experience, ISL developed its own outdoor blast-pad: 333 g Hexomax ® charges are detonated in a factor two up-scaled version of IRSN test configuration. Similar sensors are flush-mounted on the pad and the surface of an up-scaled version of the IRSN obstacle. In addition, the fine structure of the shock transmitted into the air and propagating along the obstacle surface is studied using high-speed imaging. Two respective series of charges were detonated at distances between 0.6 and 3.5 m/kg 1/3 from the hemi-cylinder, in order to assess its influence on the wave reflection structure and the resulting blast load. High-speed images enabled the triple and contact points tracking on the obstacle. Finally, this project illustrates an innovative methodology not only to assess the blast load on a convex structure, but also the potential downstream protective effects of such a structure used as a barrier.

Investigation on the Post-Processing of Blast Loading Characteristics Along a Convex Structure

“…2. Further angular positions pressure recordings are analysed in [5]: the wave propagating at 90° and above is strongly influenced by the expansion around the hemi-cylinder [4]- [6]. At 30 and 60°, as the flow is being compressed on the front face of the obstacle, shockwave type profiles (steep increase) are still being recorded.…”

“…Overpressure evolutions versus time for all sensors on the ISL hemi-cylindrical obstacle at 3.2 m (ISL scale) are presented in Fig. 11 in order to illustrate the different types of signal recorded [5]: on the front face (30 and 60°), the shock wave is being strengthened by the progressive compression consequent to the interaction with the obstacle. The pressure decay is consequently modified in comparison with the free-field classic profile [7], preventing the direct application of the KG processing method.…”

Towards a model to predict blast propagation around a hemicylindrical barrier

“…In a previous document, Trélat et al [18] proposed the innovative Model S describing the evolution of the maximum overpressure on the diffraction portion of a wooden hemicylindrical obstacle (0.4 m diameter and 1.6 m long) exposed to a 3D blast wave. In our approach, obstacles are supposed to be non-deformable.…”

“…Results were confronted to literature models (TM5-1300 [9] for reflected overpressures in full black lines and Kinney-Graham [8] for free-field overpressures in dashed black lines), leading to a good agreement between the experimental data and the prediction on the blast-exposed face of the hemicylinder. Trélat [18] addressed the expansion side of the obstacle by proposing a physical conjecture named Model S: the transmission coefficient is represented as a function of a geometrical parameter based on the obstacle dimensions (reduced shifted direct distance or RSDD on Fig. 1) as seen on Fig.…”

“…An alternative model could also be based on the use of the parameter µ. Based on IRSN blast table [1,2,14,18], ISL modified an existing concrete blast pad seen on Fig. 3 to conduct similar experiments at larger scales: explosive charges with a mass multiplied by 8 are ignited on a factor 2 up-scaled version of IRSN test table.…”