Blast and shock wave

Detonation (and Explosion), Effects of Blast and Shock Wave on Structures. As this subject was not discussed in Vol 2 of Encycl, under "BLAST EFFECTS IN AIR, EARTH AND WATER , pp B180-L to B184-R, there is given here a brief description as taken from the book of Robinson (1944), where it is described in detail on pp 45-53... 

Effects of Blast and Shock Waves of Detonation (or Explosion) Produced on Structures,... 

Detonation (and expin), effects of blast and shock waves on structures 4 D256... 

Effects of blast and shock waves of detonation produced on structures 4 D256—D257... 

PA Tech Div Lecture, 29pp) 14) G. Taylor, PrRoySoc 201A, 159-74(1950) 15) H.G. Snay J.H. Rosenbaum, Shockwave Parameters in Fresh Water for Pressures up to 95 Kilobars, NAVORD Rpt 2383(1952) 16) Armament Engrg (1954), 185 97 17) Coll, Symposium on Blast and Shock Waves, HA, England (1955) 18) F. Berry et al, PrRoySoc 227A, 258-70(1955) ... 

Explosion, Effects of Blast and Shock Waves on Structures. See Vol 4, pp D256 D257... 

In the extreme case of a detonation in air from, for example, an aluminium based composition, the shock wave caused in the air by the explosion is termed airblast. Blast and sound waves in air are longitudinal meaning that the vibration of the air particles and the movement of the wave are in the same direction. 

As a general rule, fireworks do not involve detonation conditions and so their effects are restricted to blast and sound waves. Pressures in the shock front of blast waves are much lower than detonation pressures and blast pressures are normally quoted as overpressures. 

Break up of drops accelerated by air blasts (including shock waves) can occur by an inverted bag mechanism similar to that described in Section A above, for = UApd /a between about 16 and 10 (HI, H2, H4, L6). Reichman and Temkin (R7) give a detailed description of four stages of bag-type breakup. Under some circumstances, deformation preceding breakup appears more like a parasol than an inflating bag (SI2). The distance x moved by the drop is given approximately by... 

Atmospheric effects of large-scale TNT expins have also been studied in depth both practically and theoretically. Factors considered include pressure and impulse effects, decay characteristics and travel and duration times, all as a function of distance, and for both free-field and reflection situations (Refs 3,9,15,16, 17,24,32, 33,34,35,36,44, 53,75,76,115 116). A distinction is made between the blast area dose to the source, comprising air and the products of expln, and that farther away involving air only (Ref 53). Double-burst conditions (fireball and shock wave interaction, and torus formation) have been studied (Ref 149), as have also the dynamics of dust formation and motion (Refs 25,26 117). Performance tests were run on a naval blast valve (Ref 92), and on aircraft wing panels (Ref 4)... 

The mechanical explosion damage is caused by the blast or shock wave. The explosion starts acoustic waves in the air which travel with the acoustic velocity, c, superposed on the velocity u of the mass motion with which material is convected out from the centre. Since c ylr where Tis the absolute temperature and since both u and c are greater farther back in the wave disturbance it follows that the back of the wave overtakes the front and thus builds up a sharp front. This is essentially discontinuous in both pressure and density. It has been shown that in such a wave front the density just behind the front rises abruptly to six times its value just ahead of the front. In back of the front the density falls down essentially to zero. 

If the combustion process within a gas explosion is relatively slow, then expansion is slow, and the blast consists of a low-amplitude pressure wave that is characterized by a gradual increase in gas-dynamic-state variables (Figure 3.7a). If, on the other hand, combustion is rapid, the blast is characterized by a sudden increase in the gas-dynamic-state variables a shock (Figure 3.7b). The shape of a blast wave changes during propagation because the propagation mechanism is nonlinear. Initial pressure waves tend to steepen to shock waves in the far field, and wave durations tend to increase. 

The pressure vessel under consideration in this subsection is spherical and is located far from surfaces that might reflect the shock wave. Furthermore, it is assumed that the vessel will fracture into many massless fragments, that the energy required to mpture the vessel is negligible, and that the gas inside the vessel behaves as an ideal gas. The first consequence of these assumptions is that the blast wave is perfectly spherical, thus permitting the use of one-dimensional calculations. Second, all energy stored in the compressed gas is available to drive the blast wave. Certain equations can then be derived in combination with the assumption of ideal gas behavior. 

If, on the other hand, blast modeling is a starting point for structural analysis, the TNT-blast model is less satisfactory because TNT blast and gas explosion blast differ substantially. Whereas a TNT charge produces a shock wave of very high amplitude and short duration, a gas explosion produces a blast wave, sometimes shockless, of lower amplitude and longer duration. In structural analysis, wave shape and positive-phase duration are important parameters these can be more effectively predicted by techniques such as the multienergy method. 

Blast A transient change in the gas density, pressure, and velocity of the air surrounding an explosion point. The initial change can be either discontinuous or gradual. A discontinuous change is referred to as a shock wave, and a gradual change is known as a pressure wave. 

Pressures of deflagration or detonation shock waves build upon the existing system pressure at the time of the initial blast. When a deflagration starts and then builds to a detonation, the resulting peak pressure can be quite high because the final pressure of the detonation builds on the peak pressure of the deflagration. 

The Universal Hopkinson-Cranz and Sachs Laws of Blast Scaling have both been verified by experiment. These laws state that self-similar blast (shock) waves are produced at idendcal scaled distances when two explosive charges of similar geometry and the same explosive composition, but of different size, are detonated in the same atmosphere [49]. 

In general, a reflected shock wave of 55 psi on a human for 400 milliseconds would be just about the tolerance limit [41] (see Table 7-25B). For a more detailed discussion of blast scaling and overpressure, see Ref [40]. 

Explosive Properties. It undergoes an expl reaction with H2, but concn and temp limits of the expin were not reproducible in Pyrex or stainless steel reactors, probably due to the presence or absence of Initiating radicals on the walls. The results became more reproducible after the walls were coated with silicone oil. Addn of tetrafluorohydrazine to H2/difluoramine or H2/N trifluoride mixts caused immediate explns (Ref 9). It also can expld on contact with reducing agents or from high press produced by shock wave or blast (Ref 11)... 

The reflected pressure wave amplitude and impulse for shock waves associated with detonations are well documented, as shown in Figure A. 3 (Ref. 7, Volume II). Less information is available on reflected overpressure and impulse resulting from deflagration pressure waves. Reference 67 documents approaches for evaluating reflected overpressure from weaker blast pressure waves. Forbes (Ref. 71) suggests the following approximate relation to model the more complex relations in Reference 64 ... 

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