Response to Blast Waves

Response to Blast Waves The effect of blast waves upon eqmp-ment and people is difficult to assess because there is no single blast wave parameter which can fully describe the damage potenti of the... 

Response to Blast Waves The effect of blast waves upon equipment and people is difficult to assess because there is no single blast wave parameter which can fully describe the damage potential of the blast. Some targets respond more strongly to the peak incident overpressure and others to the impulse (J p dt) of the blast. The blast parameters are usually based on the conservative assumption that the... 

D.R. Richmond et al, "The Relationship between Selected Blast Wave Parameters and the Response of Mammals Exposed to Air Blast", See Ref 8, pp 103-121)... 

Figure 53.3 summarizes injury mechanisms associated with torso impact deformation. For low speeds of deformation, the limiting factor is crush injury from compression of the body (C). This occurs at C = 35-40% depending on the contact area and orientation of loading. For deformation speeds above 3 m/sec, injury is related to a peak viscous response of VC = 1.0 m/sec. In a particular situation, injury can occur by a compression or viscous responses, or both, as these responses occur at different times in an impact. At extreme rates of loading, such as in a blast-wave exposure, injury occurs with less than 10 15% compression by high-energy transfer to viscous elements of the body. 

In [66] it is proposed to calculate the blast waves from the depressurizations of vapour and liquid separately in the first place and to add them together thereafter. The latter approach is considered to be conservative. Even if the experiments suggest that the vapour energy alone is responsible for the blast wave it cannot be discarded that the flash vaporization contributes to the blast wave in the far field. Resulting differences are illustrated by Example 10.31. 

For the purposes of structural design or evaluation, the variation or dechne of both the incident and dynamic pressures with time should be established, since the response of a structure subjected to a blast loading depends upon the time history of the loading as well as the dynamic response characteristics of the structure. The idealized form of the incident blast wave is characterized by an abrupt rise in pressure to a peak value, a period of decay to ambient pressure and a period in which the pressure is below ambient (the negative pressure phase). A simpler representation of only the positive blast pressure phase including reflected wave effects is usually sufficient for the purposes of engineering evaluation. 

The overall load on a structure is also a function of the size of the structure. The effect of the lateral distribution of load local to an obstruction (front wall of a structure facing the blast) should be carefully analysed. For structures with depths less than about 50 m parallel to the direction of travel of the blast wave, the blast load would have largely passed the structure before the structure had time to respond to the blast wave since the wave is always travelling at or above the speed of sound. For a 50 m deep structure, the peak blast wave front would only engage the structure for about 0.02 s, which is typically well below the global fundamental natural period of the structure but not necessarily the local element response period. 

When a detonation occurs, a blast wave is produced which spreads out from the point of detonation like the ripples produced by dropping a stone into a pond. The blast wave travels faster than the speed of sound in air, compressing the air in front of it and producing an almost instantaneous rise in pressure lasting a fraction of a millisecond before falling away to a negative pressure (see Fig. 3.3). Atmospheric pressure is then restored as the blast wave passes. The blast wave is responsible for primary blast injuries (pages 95-96). 

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