Bomb Blast Characteristics

When a high explosive, such as trinitrotoluene (TNT), is detonated a rapid decomposition of the condensed phase material occurs, resulting in the release

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FIG. 2—Blast pressure as function of time at a given location (Ta: Time of blast pres­sure arrival, Tso: Time of first return to atmospheric pressure, P0: atmospheric pressure, Pso: maximum blast pressure).

of a gas at extremely high temperature and pressure. Typically the pressure at the origin of the detonation is of the order of 20 GPa. The sudden release of a gas at high pressure results in an outward directed pressure wave in the air with an extremely steep pressure rise. This shock wave, consisting of highly compressed air, is traveling radially outward from the source at supersonic velocities (up to 10 000 m/s at the explosion origin) [10]. As the shock wave expands, the pressure decreases rapidly with distance (as a function of the cube of the distance). However, when it meets a surface that is in line-of-sight of the explosion, it is reflected and, for planar surfaces, typically amplified by a factor of 8-12. The amplification factor may be much higher for geometrically com­plex structured building surfaces at which double or even triple reflections occur [11]. The magnitude of the reflection factor is a function of the proximity of the explosion, the topology of the building envelope, and the angle of inci­dence of the shock wave on the building surface. It is not uncommon for a building structure in the path of the pressure wave to be subjected to blasts with peak pressures of the order of hundreds, if not thousands, of kilopascal (kPa). As the detonation wave passes over a given point, the pressure initially rises almost instantaneously to its peak, followed by a rapid, i. e., exponential, decay (see Fig. 2). The positive pressure phase has a very brief span of exis­tence, measured in milliseconds. Late in the explosive event, the shock wave becomes negative, creating suction. While the negative pressure phase can have a longer duration (typically at least twice as long as the positive pressure phase), it is limited to the maximum negative pressure of one atmosphere (100 kPa), and, therefore, represents only a small fraction of the initial positive impulse. The positive phase of the pressure trace is generally simplified to an equivalent triangular pulse and for engineering calculations only two param­eters, the peak pressure and effective duration, are used to characterize the load. The duration of the positive blast pressure phase is directly related to blast damage. For a given value of peak air blast pressure, damage increases as duration increases, because the air blast forces act for a longer period [12]. Alternatively, peak pressure and positive phase impulse are used to describe a blast wave. The positive phase load impulse is the integrated pressure load over time, from the pulse arrival time (Ta) to the time when the pressure first returns to atmospheric pressure (Ta + Tso), or the area under the pressure duration curve between those two time intervals.