A Novel Experimental System for Blast Structure Interaction Research

Abstract

In the past two decades the need to better understand the interaction of shock and blast waves with structures has become increasingly important. The common way of conducting research on blast wave-structure interaction is by full- and small-scale tests. Those tests usually involve the use of energetic materials; another approach is by numerical means. Full scale experiments are very resource consuming, have poor repeatability and are limited by the availability of diagnostic methods. The vast majority of small scale tests utilize small charges, for example: Smith et al.[1] conducted an experimental study on the load developed within a structure using mini-charges. They measured the pressure history induced by small PE4 and PETN charges on the walls of small rigid models. In the work of Kleine et al.[2], silver azide mini-charges (milligram range) were used to generate the blast wave. In their work they found the TNT equivalent of the small silver azide charge for any distance. They conclude that the well validated scaling laws of Hopkinson [3] and Cranz [4] can be applied to charge masses ranging from a few milligrams to hundreds of tons. Those finding encouraged us to adopt a small-charge experimental approach together with numerical simulations to investigate blast-structure interactions. In this work we present a newly developed experimental apparatus which creates blast waves by means of exploding wire technique. The low cost, high repeatability and safe operation are the main advantages of this system. Furthermore, using this system under laboratory conditions permits the use of delicate diagnostic systems such as high speed schlieren and shadowgraph systems. To create the blast wave we adopted the exploding wire technique.When a large capacitor is discharged through a thin metal wire, the wire explosively burns out. The large current heats the wire which evaporates in a very short time (few micro-seconds or less). Prior to the shock wave creation around the heated wire the wire evolves as follows: (a) the wire is heated by the strong current passing through it, (b) a liquid column is formed and (c) development of instability forms small droplets from the liquid column, (d) arcs are created between the droplets and the overall resistance increases. This time interval is called “dwell time”, where the voltage remains constant across the wire. (e) Next the wire resistance drops again and a sudden flash is observed. (f) A shock front emerges from the hot plasma. The exploding wire physics is discussed in great detail in the proceedings of three conferences edited by Chace & Moore [5]. Bennett [6] described the strong blast wave that accompanies the exploding wire phenomena. In the next section the experimental apparatus with the accompanying diagnostic system will be presented followed by the description of the calibration procedure. An investigation of the blast wave evolution near a two story buildingmodel is presented followed by a comparison to numerical simulation under the same initial conditions.