Explosive Shocks in Air

A large explosion may inflict widespread damage and represent major disaster. To minimize such effects some knowledge of the mechanisms of explosion damage becomes essential. Only then is it possible to provide the best available planning, design, and construction for needed disaster resistant facilities. The material presented here on explosive blast and shock gives essential background concerning the physics of the blast wave. It describes in relatively simple manner generation of a blast wave with its shock front, its transmission through the atmosphere, its interaction with a variety of structures and objects, and typical structure responses. Preliminary to this study of explosions is a description of explosive blast and shock in the atmosphere.

Explosion products are complex in nature with materials being formed both in the direct explosion reaction and by subsequent reaction with the surrounding atmosphere. Direct products from a chemical explosive include the usual combustion products of carbon dioxide (CO₂), carbon monoxide (CO), and water vapor (H₂O), and molecular nitrogen (N₂). They may also include molecular hydrogen (H₂) and molecular oxygen (O₂), along with nitric oxide (NO). An explosion can also produce species that are not stable at room temperature. These include hydroxyl (OH), monatomic hydrogen (H), monatomic oxygen (O), and monatomic nitrogen (N), plus small amounts of ionized gases and associated electrons. These electrically charged particles, though present in small amount, make the initial combustion products mixture electrically conductive, a feature that has been utilized in the study of explosion reactions. (Reference 1.)

An explosion in air may cause widespread damage by means of the blast wave which it creates. The pressures generated in this blast wave, although sufficient to cause extensive damage, are quite modest when compared with those within the explosion itself, and for the most part are well within the range over which air acts as an ideal gas. The characteristic equation of state describing any ideal gas may be written as where P is the absolute pressure, V the volume of one mole of ideal gas, and T the absolute temperature. The molar gas constant R _m, in the metric units of the Système International d’Unités (SI units), has the value 8.31434 joules per mole-kelvin (J/mol-K). In this coherent system, pressures are to be expressed in pascals (newtons per square metre), volumes in steres (the stere, symbol m³, is the classic name for the cubic metre), and temperatures in kelvins. However, it is often convenient to use working metric units and express pressures in bars (one bar is 10⁵ pascals, or 14.5 psi, or 1/1.01325 standard atmospheres). The working value for the molar gas constant R _m then is 0.0831434 bar-steres per kilomole-kelvin. This is also its value in bar-litres per mole-kelvin. Table XVI gives these and other factors.

The shock front of a blast wave is in many ways a determining factor in its behavior. The concern here is with shock fronts in air, which for present purposes may be considered as conforming to the specification of an ideal gas.

Among the more complex, and in many respects the more puzzling, characteristics of an explosive shock front are its behavior when reflected. Reflection phenomena are of three types, (1) normal reflection, which occurs when the shock impinges head-on onto an unyielding surface with the plane of the shock front parallel to that of the surface, (2) oblique reflection where a shock impinges with a small angle between the plane of the shock and the plane of the reflecting surface, and (3) Mach stem formation, a spurt-type effect which occurs when a shock front impinges on a surface near grazing incidence. These three types are considered separately.

An important aspect of any large explosion in air is the blast wave that it forms. This blast wave is generated when the atmosphere surrounding the explosion is forcibly pushed back, as by the gases produced from a conventional chemical explosive, or as furnished by a volatilized container and components in a nuclear explosion, or from the atmosphere itself when a portion is subjected to a sudden energy release as in a lightning flash.

Characteristics of the blast wave generated in an explosion depend both on the explosive energy release and on the nature of the medium through which the blast propagates. These characteristics are readily defined quantitatively for any particular explosion, as for example, those described in Figure 6–4 or in Table XI. The problem is to find a corresponding description for other explosions as well. This is made possible by the scaling law for explosions, by which means the particular values of the figure or table are made to supply general solutions for the explosive blast wave problem.

Blast waves generated in a large variety of explosion situations can be described quantitatively by utilizing the facilities offered by the scaling law, the characteristics of a reference explosion, and the transmission behavior of the atmosphere. One method for organizing a description of some postulated explosion is by means of contour lines constructed to represent equal values of the peak overpressure; contours for equal durations and equal impulses per unit area for the positive pressure pulse of the blast wave system likewise can be constructed. The information there contained is needed for many military and nonmilitary purposes in connection with civil defense planning, with provisions for disaster relief, or for tactical problems.

Internal blast is the concussion effect of pressure rise caused by rapid combustion of fuel dispersed within a confined volume of air. Internal blast can be quite damaging, or even disastrous. Examples include destruction of grain elevators by grain dust explosions, loss of life in coal mine explosions, destruction of buildings as a result of natural gas leakage, and perhaps some of the unexplained losses of “empty” petroleum tankships at sea.

The blast wave generated in an explosion imposes a dynamic load on any object in its field. This dynamic load is characterized by a rapidly reached peak value which then decreases as the blast wave decays. The net effect of the load depends both on the nature of the blast wave and on the geometry and construction of the object (Reference 1).

The blast wave from an explosion damages a structure by causing it to deform, and these deformations may range all the way from the trivial to those corresponding to total destruction. There are several situations in which it is important to anticipate the extent of these deformations, and hence the possible blast damage which might be caused in an explosion. These include the design and construction of blast resistant structures and the planning of recovery efforts after some postulated catastrophe.