7. Shock Wave Measurements 



7.1. The Form of the Shock Wave 



The underwater shock wave from high explosives is found in most 

 cases to be a highly reproducible phenomenon, which can often be 

 represented at a given point to a first approximation by a discontinuous 

 rise in pressure followed by an exponential decay in time. As the shock 

 wave spreads out from a charge, its peak value decreases and its du- 

 ration, as estimated for example from the time constant of exponential 

 decay, increases gradually. These broad general features are shown by 

 the reproductions in Plate V of pressure-time curves, obtained using 

 tourmaline gauges, at distances of 20 and 500 feet from a cylindrical 

 300 pound charge of TNT.i The peak pressure of 6,000 Ib./in.^ at 20 

 feet falls to 150 Ib./in.^ at 500 feet, and the time constant of 500 ^tsec. at 

 20 feet, defined here as time for pressure to fall to 1/e = 0.37 of its 

 initial value, increases to 900 /^sec. at 500 feet. (The irregularities at 

 later times are the result of reflections from the surface and bottom, not 

 characteristics of the pressure in an infinite body of water.) 



It is evident from Plate V that the description of the shock wave as a 

 simple exponential is only an approximation. For charges reasonably 

 close to a sphere in symmetry, the approximation is a reasonably good 

 one for pressures greater than about one-third the peak value, but two 

 types of departure are observed at later times, even for charges of high 

 symmetry. First, a hump or irregularity of variable magnitude and 

 shape, depending on the shape of charge and explosive, and second, a 

 very much more gradual decrease of pressure than the continuation of 

 the initial exponential decay. The gradual decrease is generally ob- 

 served, and has its explanation in the expansion of the gas sphere follow- 

 ing detonation and, in the incompressive approximation, a falling off of 

 pressure at points in the surrounding water with time as ^~^^^ This 

 characteristic of the pressure-time curve has been considered in section 

 3.8 according to the theory of Kirkwood and Bethe, and is further dis- 

 cussed in section 9.2. 



The "hump," or local region of pressure in excess of a smooth curve, 

 which frequently occurs somewhere near or after the time constant, has 

 been found for geometrically similar charges to be characteristic of the 

 explosive, and it is often possible to distinguish pressure-time curves and 

 identify the explosives responsible by the appearance of these humps. 



^ The experimental data quoted in this chapter were nearly all obtained at the 

 Underwater Explosives Research Laboratory, Woods Hole, by methods described in 

 Chapter 5. More complete compilations of data will be found in reports of the UE 

 series (114), and formal reports by J. S, Coles (23) and by Arons and Smith (5). 



