21 



4000 



c 3000 



2000 



m 



a 1000 



-1000 



0.6 0.8 1.0 



Time in milliseconds 



Figure 5 - Curves Illustrating the Incidence of a Shock Wave 

 on a Plate when Cavitation Does Not Occur 



The curves are drawn for the wave from 300 pounds of TNT falling upon an air-backed 1-inch steel 



plate 50 feet away, T„ is the compliance time, at which the plate has acquired maximum 



velocity; 7"^ is the time constant of the wave or l/a in the formula, p- = P—*""'- 



In the last column of Table 1 is shown the value of e ""'""», or the 

 ratio of the incident pressure at the time t = T^ to the maximum incident 

 pressure. 



In many model tests conditions occur that are comparable in terms 

 of similitude to the wave from 300 pounds falling on a 1-inch plate. Curves 

 for this case, with the plate at 50 feet from the charge, as calculated from 

 the one-dimensional theory, are shown in Figure 5- 



The use of the one-dimensional formulas implies the tacit assump- 

 tion that during the time T„ diffraction effects may be neglected. This is 

 Justified provided the plate is sufficiently large in lateral dimensions. 

 Consideration of this condition leads to the introduction of a third charac- 

 teristic time, which may be called the diffraction time, T^. This can be de- 

 fined with sufficient precision for practical purposes as the time required 

 for a sound wave in the water to travel from the center of the plate to the 

 edge. Thus for a circular plate of radius o, T^ = a/c, where c is the speed 

 of sound in water. 



Diffraction can be regarded as a process acting to equalize the 

 pressure laterally, or in directions perpendicular to the direction of prop- 

 agation of a wave. Because of this process, a wave that has passed through 

 an opening in a screen spreads laterally, contrary to the laws of the recti- 

 linear propagation of waves. Similarly, when a wave of pressure falls on a 



