456 



38 



Figure 22b - Low-Speed Streak Photograph 



downward, presumably under the Influence of gravity and air pressure, and re- 

 turns to the original level at about 75 milliseconds. 



Simultaneously with this return of the holding ring, the diaphragm 

 receives another blow which appears to be not quite so impulsive in nature 

 yet strong enough under the proper conditions to do considerably more damage 

 to the diaphragm. This contributes the major part of the "C-phase" damage, 

 inferred in Reference (10). Indeed, at charge distances less than l8 inches, 

 the diaphragms invariably rupture during the application of this second blow. 

 Because the time interval between the initiation of the motion and the second 

 blow agrees so closely with the period of oscillation of the explosion-gas 

 globe, it seems quite probable that the increase in damage is due mainly to 

 the pressure wave sent out from the gas globe on recompression. It is also 

 quite possible that the return of the boat to the water level Just in time 

 to receive the second pressure pulse might fortuitously have increased its 

 damaging effect. That is, the results obtained in tests of this kind almost 

 certainly depend upon the nature of the supporting structures. 



That little, if any, work is done on a diaphragm in the time inter- 

 val between 2 milliseconds after the initiation of the motion and the time t^ 

 75 milliseconds later Just before it receives the second blow is evident from 

 comparison of the central deflections Zc^, measured at the latter time and 

 plotted in Figure 33, with the central deflections Zc^ shown in Figure 32. 

 It is found possible to measure the central deflections Z^^ of a few of the 

 unruptured diaphragms after the second blow. These are plotted in Figure 3**; 

 because of the paucity of data and because of the screening effect of the sec- 

 ond blow, the data are not separated into two branches. 



After the tests were completed the diaphragms were cut from the 

 holding rings and their profiles were drawn. Measurements on these yielded 



