35 



453 



-I millisecond • 



Bending Wave 

 Reoches Center 



Center Spot- 



Bending Wove 

 Traveling Inward — I 



-Bulging Possibly Due to 

 Cavitation Closing Up 



effect mentioned in the previous para- 

 graph. The central deflection Z^j at 

 this time, varies from 1 .2 inch at a 

 Charge distance of 7 Inches to 0.26 

 inch at a charge distance of 52 Inches. 

 Z(- is plotted in Figure 28 and shows 

 the characteristic splitting Into a 

 U- and L-graph. 



The splitting is not quite 

 as obvious in the graphs of central 

 deflection against charge distance as 

 It is In the graphs of time against 

 charge distance. However, it should 

 be emphasized that the double-branch 

 phenomenon is not due simply to a ran- 

 dom scatter. This is quite clear from 

 an examination of the displacement- 

 time curves. Moreover, the diaphragms 

 which appear in the U-branch of one 

 graph also appear in the U-branch of 

 the other graphs. The same statement 

 applies to the diaphragms of the 

 L-class. 



Just before, or, more frequently, a few tenths of a millisecond 

 after the time (j at which the bending wave reaches the center, an additional 

 Impulse is received by the diaphragm. This takes the appearance, as in Fig- 

 ure 29, of a sharp increase in velocity near the center, which region has al- 

 ready come to rest when the charge distance exceeds 7 inches. The impulse is 

 accompanied by -a bulging of the outer portions of the diaphragm. This phe- 

 nomenon Is thought to be due to a kind of water-hammer effect when the cavi- 

 tation in the water closes up under the Influence of the pressures in the 

 tall of the shock wave. The time t^ of occurrence of the blow at the center 

 Is plotted in Figure 30 which again exhibits the splitting of the data into 

 the two groups, U and L. The trends are quite definite in this graph and 

 show, as the charge distance is increased, a comparatively rapid increase in 

 the time at which the blow Is received. 



At about 1.1 millisecond after the Initiation of the motion, the 

 diaphragm finally comes to rest relative to the holding ring save for a 

 slight elastic motion of very small amplitude. The central deflection Z^.^ 

 at this time t^ is shown in Figure 3T plotted against charge distance. The 



Figure 29 - An Enlarged Streak Record 



This record was traced from a photograph. It 

 shows the bending wave traveling to the center 

 of the diaphragm, and the effect of the blow 

 received at the center as cavitation closes. 



