ABSTRACT
Home-made explosive devices, as the ones used in the above incidents, consist simply of a small quantity of military high explosive detonator (usually semtex) surrounded by a fertiliser and a fuel. The detonation of such devices results in the generation of highly pressurised hot gases which expand violently displacing the surrounding air from the volume it previously occupied. A layer of compressed air, known as the blast wave, forms in front of the expanding gas and is characterised by an instantaneous rise from ambient pressure to a peak incident pressure (see Fig.4). This pressure increase, or shock front, travels away from the blast source in the form of a hemispherical wave with a diminishing velocity. The speed of the pressure wave of most home-made explosives rates around 2,700 m/s compared to 7,500 m/s for high explosives. As the gas expands the pressure decreases until it falls, eventually, to or below the atmospheric pressure. This negative pressure is associated with the over-expansion of the gas, the result of which is a reversal of flow towards the source (i.e., suction). This negative phase has a longer duration than that of the positive phase but has a much lower peak value. As the blast waves radiate out at supersonic speeds within the confined city streets they are reflected and refracted by adjacent buildings where greater pressures may result. If the explosion is close to, or on the ground surface, a small proportion of the energy will be transmitted through the ground as seismic waves, whereas most of the energy will be absorbed by the ground surface displacing it and forming a crater (see Fig. 1).
