Arrival Time and Duration

There is a finite time interval required for the blast wave to move out from the explosion centre to any particular location. This time interval is dependent upon the energy yield of the explosion and the distance involved. At 1.6 km from a 1 megaton burst, the arrival time would be about 4 s. Initially, the velocity of the shock front is quite high, many times the speed of sound, but as the blast wave progresses outwards, so it slows down as the pressure at the front weakens. Finally, at long ranges, the blast wave becomes essentially a sound wave and its velocity approaches ambient sound velocity. 

The positive phase duration is shortest at close ranges and increases as the blast wave moves outwards. At 1.6 km from a 1 megaton explosion, the duration of the positive phase of the blast is about 2 s. The period of time over which the positive dynamic pressure is effective may be taken as essentially the positive phase of the overpressure. 

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In Fig. 7.4.1, an idealized representation of tlie blast wave, the pressure pulse is shown as a function of tlie distance from tlie explosion center. Tlie sliape of tlie curve at A is not shown tlie cun-es at B tluougli D and times 2 tluough 4 show tlie decrejise in peak oi erpressure as tlie wave moves outward. Botli positive and negative pressures are observed in tlie curve at point D and time 4. Figure 7.4.2 illusUates tlie variation of overpressure witli time at point D t, and td are tlie peak overpressure arrival time and duration time, respectively. The modified Friedlander equation is most conunonly used to describe tlie oveqiressure curve. 

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