SHOCK WAVE MEASUREMENTS 



A. Peak pressure and time constant. Despite the limitations of an 

 exponential decay as a description of shock waves, its analytical con- 

 venience is so great that parameters based on it are widely used as a 

 first approximation, and in many cases such parameters furnish a quan- 

 titatively reliable description of the earlier parts of the curve. In this 



6000 



4000 



CM 



? 2000 



\ 



CD 



UJ 



CC 1000 



CO 800 



U) 



LU 



Ct 600 



400 



200 



0.5 1.0 1.5 



TIME (msec) 



2.0 



25 



Fig. 7.1 Semilogarithmic plot of pressure against time (14 feet from an 80 

 pound 50/50 pentolite charge). 



approximation, the pressure P as a function of time t after arrival of the 

 shock front is expressed as 



(7.1) 



P = Pme-^/^ 



where Pm is the initial peak pressure and B is the time constant of ex- 

 ponential decay. The accuracy of this approximation in any particular 

 case can be tested by plotting log P as a function of t, which should give 

 a straight line of negative slope \/B. An example of such a plot is given 

 in Fig. 7.1 for the pressure 14 feet from an 80 pound charge of pentolite. 

 It is seen that the form of the actual curve is well represented by a 

 straight line out to about 0.3 msec, at which time the pressure is about 



