SHOCK WAVE MEASUREMENTS 



231 



30 per cent of its initial value, but at later times the "tail" of the shock 

 wave decays much more slowly than in the exponential approximation. 

 This result is typical of charges which do not differ too much from 

 spherical symmetry, and illustrates the extent to which the exponential 

 approximations of Eq. (7.1) can be expected to apply in simple cases. 

 Although it would be possible to devise auxiliary functions to fit the 

 later portions of the curve with additional parameters, increasing experi- 

 mental uncertainty as to the exact values of the smaller pressures ordi- 



.0.5 



1.0 



15 2.0 



TIME (msec) 



2.5 



3.0 



Fig. 7.2 Increase in impulse from later portions of the shock wave (pressure 

 20 feet from a 300 pound charge). 



narily does not justify the necessary labor, and other measures of the 

 pressure-time curve, which include these later portions less explicitly, 

 are used in preference. 



B. Impulse. For many purposes, the effectiveness of a shock wave 

 depends on the time-integral of pressure, or impulse, more significantly 

 than on the detailed form of the pressure versus time. This may be 

 true, for example, in cases of structural damage if the time of deflection 

 of the structure is much longer than the duration of pressure in the 

 incident wave. By definition, the impulse of unit area of the shock 

 wave f ront^ up to a time t after its arrival is given by : 



(7.2) 



Ii.1) 



jPii) 



dt 



2 The proper definition of impulse is the time integral of force, and the quantity 

 referred to as impulse in the text is strictly a specific impulse for unit area. The 

 term impulse for time integral of pressure is so general, however, that this usage is 

 followed. 



